Download Routing in Packet Switching Networks Contd.

CSE3153 Network Administration
Semester-2- 2006
This is an elective unit and aims to develop understanding of
the theory and practice in administration and management of
modern computer networks. Local area networks, connected
by wide area links, form the infrastructure on which many
distributed systems are constructed, and a deeper
understanding of the installation, operation and management
of this infrastructure has become an important area of
specialisation within the larger discipline of Information
Technology.
Standardised specifications of this unit is also available in the
Monash University handbook and in the FIT Unit Information
pages.
Unit Information on Unit web page
http://
Unit Book
On-line resources on the web
Lectures:
– 2 hrs per week, Wednesday
6.00pm~8.00pm
• Tutorials/Practical Sessions (2 hrs per week):
– Wednesday 8:00pm ~10:00pm [3Tutes]
– Thursday
9:00pm ~11:00pm
• Reading:
– Text Book
– Other resources on the unit page
Unit Details
Presented by: Mr. Pravin Shetty
6 point unit
2 hours Lecture
2 hours Tutorial
upto 8 hours additional private study
Each week for 13 weeks
Caulfield campus
Unit Objectives
The unit is intended to enable you to understand:
New Networking Technologies and Administration of:
 various techniques to transmit data over a transmission
medium
 characteristics of various transmission media
 various techniques for sharing a communication channel
 design issues of various flow and error control in data
communication
 identify hardware and software used in developing a Local Area
Network (LAN)
 design issues involved in developing various protocols for
Local Area Network
 analysis, design and implementation of a LAN for a given
communication need
 methods of connecting LAN with other LANs or connecting LAN
with Wide Area Network (WAN)
 architecture of several switching networks
Assessment
•
•
•
•
40% in two Assignments
50% by examination
10%from Tutorials and Practical Sessions
To pass the unit:
– both assignments must be attempted
– must pass in assignment assessment
– must pass the final examination
– Final mark according to the following formula
final mark = min(A+10, E+10, E*R+A*(1-R))
where A = overall assignment percentage,
E = exam percentage,
R = exam weighting (50% = 0.5)
Assessment-Assignments
• Assignment 1
– Due Date: TBA
– Weighting15 %
• Assignment 2
– Due Date: TBA
– Weighting 25 %
• Assignment details will be posted on the web
• Late submission ONLY with prior permission
and VALID reasons
Assessment-Examination
• 3 hour examination
• 50 % of total marks
• will test your knowledge in the unit matter
Objectives
This unit will develop student knowledge of the techniques and systems
for network administration. On completion of this unit, the student
should have acquired the knowledge needed to identify the tasks or
roles required of network administrators, understand current
developments and standards for network management, define the
principles involved in system and network administration and be able to
apply these to practical situations, analyse and classify the
requirements for management of a network particularly when it is a
critical part of the structure of an organisation, design and implement
network management policies, identify and compare different network
management techniques and strategies. Students should also have
developed practical skills in network administration, including
experience of various network management tools, their interface,
capabilities and operation, familiarity with typical methods of
documenting and modelling networks,be able to effectively and
efficiently setup networks and confirm correct operation,be able to
monitor networks and diagnose common network faults, be able to
construct test strategies and acceptance tests for networks.In addition,
students will Experience the need for cooperative management of
networks and computer equipment Work effectively in groups to achieve
a system implementation
Synopsis
The unit will provide students with fundamentals and theoretical
foundations of network administration. Specific topics include:
Introduction to Network Administration Scope, Goals, Philosophy &
Standards Challenges and common practice Network Administrators
Role Review IT System Components Network Structures Technology
(Sockets, Cables, etc)
Protocols (TCP/IP, X.25, ATM, etc) Network Operating Systems
Network System Management Hosts and Users, System Configuration
and Maintenance
Administration of Network Services TCP/IP Networks TCP/IP Toolkit
Methods of Network Administration Managing devices using SNMP
Remote Management using RMON DeskTop Management Network
Fault Diagnosis and Recovery Network Performance and Tuning
Network Security and Administration Analytical System Administration
Network Simulation Network Documentation Future of Network
Administration
Recommended texts
Burgess M
Principles of Network &
System Administration
Second Edition, Wiley 2004
Subramani
Network Management: Principles and Pra
Addison Wesley
ISBN 0-201-35
ISBN: 0-470-86807-4
Support material
Stallings W.
SNMP, v2, v3 & RMON I
and II
3rd Edition
Addison Wesley 1998
Stallin
Data and Computer Communica
7th Edition, Prentice Hall
ISBN 0-13-100
Co
ISBN: 0-201-48534-6
Hunt C.
TCP/IP Network
Administration
3rd Edition, O'Reilly Associates
2002
ISBN: 0-596-00297-1
Mikalsen A., Borgesen P.
Local Area Network
Cernick P., Degner M., Kruep
Cisco IP Routing Hand
M&T, IDG Books
ISB
Burke
Network Management: concepts and pra
Pearson - PrenticeHall,
ISB
Lecture 1
Introduction to Communication
─
Data & Information
─
─
─
Communication Model
Key Communication Tasks
Networking Concept
Communications
 Derived from Latin
 communis - means common
 communicate - the act of making it known to
many (what is known to one)
 Communication simply means the
transmission (transfer) of information
 Telecommunications
 communications using electronics, fibre optics
and other specialized circuits
Data & Information
 Data - representation of facts, concepts or
instructions in a formalized manner suitable for
communication and processing by human
beings
 Information is born when data is interpreted
 Information exchange implies involvement of at
least two parties
 Communication suggest a path and media
through which information flows
Why Study Data Communications?
 Information is a primary source for
 decision making
 optimum production
 keeping in line with the technological
developments
 many other things …
 And we need to exchange information
Types of Communication
 Voice communication
 human voice (telephone)
 Video communication
 pictures, diagrams, images, ...
 Data communication
 Numeric and text data, …
 The distinction between various types of
communication are not clear-cut and the
domains overlap
Computer Communications
The Computer-Communications revolution
has resulted in
– no fundamental difference between data
processing and data communication
– no fundamental differences among data,
voice, video, ..., communications
– the boundary between different systems
heavily overlap
A Communications Model

Source


Transmitter


Carries data
Receiver


Converts data into transmittable signals
Transmission System


generates data to be transmitted
Converts received signal into data
Destination

Takes incoming data
A Communications Model
• Source
– generates data to be transmitted
• Transmitter
– Converts data into transmittable signals
• Transmission System
– Carries data
• Receiver
– Converts received signal into data
• Destination
– Takes incoming data
Communications Tasks
Transmission system
utilization
Addressing
Interfacing
Routing
Signal generation
Recovery
Synchronization
Message formatting
Exchange management
Security
Error detection and
correction
Network management
Flow control
Simplified Communications
Model - Diagram
Simplified Data
Communications Model
Networking
• Point to point communication not usually
practical
– Devices are too far apart
– Large set of devices would need impractical
number of connections
• Solution is a communications network
– Wide Area Network (WAN)
– Local Area Network (LAN)
Wide Area Networks
•
•
•
•
Large geographical area
Crossing public rights of way
Rely in part on common carrier circuits
Alternative technologies
– Circuit switching
– Packet switching
– Frame relay
– Asynchronous Transfer Mode (ATM)
Circuit Switching
• Dedicated communications path
established for the duration of the
conversation
• e.g. telephone network
Packet Switching
• Data sent out of sequence
• Small chunks (packets) of data at a time
• Packets passed from node to node
between source and destination
• Used for terminal to computer and
computer to computer communications
Frame Relay
• Packet switching systems have large
overheads to compensate for errors
• Modern systems are more reliable
• Errors can be caught in end system
• Most overhead for error control is stripped
out
Asynchronous Transfer Mode
•
•
•
•
•
•
ATM
Evolution of frame relay
Little overhead for error control
Fixed packet (called cell) length
Anything from 10Mbps to Gbps
Constant data rate using packet switching
technique
Local Area Networks
• Smaller scope
– Building or small campus
• Usually owned by same organization as
attached devices
• Data rates much higher
• Usually broadcast systems
• Now some switched systems and ATM are
being introduced
LAN Configurations
• Switched
– Switched Ethernet
• May be single or multiple switches
– ATM LAN
– Fibre Channel
• Wireless
– Mobility
– Ease of installation
Metropolitan Area Networks
•
•
•
•
•
MAN
Middle ground between LAN and WAN
Private or public network
High speed
Large area
Networking
Configuration
Protocols
Used for communications between
entities in a system
Must speak the same language
Entities
User applications
e-mail facilities
terminals
Systems
Computer
Terminal
Remote sensor
What is a protocol?
human protocols:
• “what’s the time?”
• “I have a question”
• introductions
network protocols:
• machines rather than humans
• all communication activity in
Internet governed by protocols
… specific msgs sent
… specific actions taken when
msgs received, or other events
protocols define format, order of
msgs sent and received among
network entities, and actions taken
on msg transmission, receipt
What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
req.
Hi
TCP connection
reply.
Got the
time?
2:00
Get http://gaia.cs.umass.edu/index.htm
time
<file>
Protocol
Its characteristics
1. Monolithic VS Structured
2. Standard VS Non-standard
Overview
•
•
•
•
What is protocol?
Protocol Families
Function
Characteristics
– Monolithic VS Structured
– Standard VS Non-standard
What is protocol?
• A protocol is a convention or standard that
controls or enables the connection,
communication and data transfer between two
computing endpoints
In its simplest form, a protocol can be defined as
the rule governing the syntax, semantics and
synchronization of communication.
What is protocol?
• A protocol is varied in purpose and
sophistication
– Detection of underlying physical connection
(wired or wireless) or the existence of the
other endpoint or node
– How to start and end message
– How to format a message
– Termination of the session or connection
Protocol Families
• Most protocols are layered together into protocol
stacks where various tasks listed above are
divided among different protocols in the stack
Open Standard
Proprietary standard
• Internet protocol suite
• Open Systems
Interconnect
•
•
•
•
•
AppleTalk
DECnet
ISX/SPX
SMB
System Network
Architecture
• Distributed Systems
Architecture
Functions
• Segmentation &
Reassembly
• Encapsulation
• Flow Control
• Error Control
• Addressing
• Application
Synchronisation
• Multiplexing
• Transmission
Services
Characteristics
1. Monolithic VS Structured
• Monolithic Protocol
Application package includes communication
protocols. Modification is difficult with high
risk of introducing bugs
Characteristics
1. Monolithic VS Structured
•
Structured Protocol
Protocols separated out from the application
package using structured design techniques
to form a set of layers
- A Network architecture or protocol stack
Characteristics
Standard VS Non-standard
• Standard protocol
– It conforms to
recognised standard
– It provides more open
network environment
• Non-standard
– Built for specific
communication
situation
– Proprietary,
implemented by a
company for use
within its networks
– Problem of
interconnection
Protocol Functions
• Not all protocols have all functions; this
would involve a significant duplication of
effort
• Many instances of the same type of
function being present in protocols at
different levels.
Protocol Functions
•
•
•
•
•
•
•
•
•
Encapsulation and Delineation
Segmentation and reassembly
Connection control
Ordered Delivery
Flow Control
Error Control
Addressing
Multiplexing
Transmission services
Encapsulation
•
•
•
Each Protocol Data Unit (PDU) contains
not only data but control information
Some PDU consists solely of control
information
Control Information falls into 3
categories:
– Address
– Error detection code
– Protocol control
Encapsulation of PDU in TCP/IP
Delineation
• Trace the outline of the PDU
• Indicates the control information and the
actual data.
Segmentation and Reassembly
• packet-switched telecommunication network,
segmentation and reassembly (SAR, sometimes
just referred to as segmentation)
• is the process of breaking a packet into smaller
units before transmission and reassembling
them into the proper order at the receiving end
of the communication
• Packets are made smaller to speed them
through the network and specifically because of
specified packet size restrictions in a given path
Segmentation and Reassembly
Protocol Architecture
• Task of communication broken up into
modules
• For example file transfer could use three
modules
– File transfer application
– Communication service module
– Network access module
Two Types of Data Transfer:
Connectionless data
transfer:
Connection Oriented
Transfer:
– Every PDU is sent to
receiver as an
independent unit.
– ‘Connection’ is opened
between
sender/receiver
– No guarantee of loss,
error, misdelivery etc
– Preferable and
advantageous when
sending large data
Connection Control –
Connection-Oriented Transfer:
• Multiple PDU’s sent in a single session
• Ability to logically order multiple PDU’s
(sequencing)
• Consists of three main phases:
– Connection Establishment
– Data Transfer
– Connection Termination
• Further phases may exist for error
detection/recovery
Connection Establishment
•
•
•
•
Creates a connection with other party
Both must be using identical protocols
A connection request is sent
Connection must be accepted before any
exchanges can proceed
• Other features may be added to protocol
as required
Data Transfer
• Occurs once connection is established
• Data is transferred
• Control information transferred, e.g:
– Flow control
– Error control
– Acknowledgements of data sent/received
Data Termination
• Connection is terminated when request is
sent to other party
• May also be terminated by a central
authority (e.g. security reasons)
A typical Connection-Oriented
Session
• Source: Stallings 7E fig 18.1 p575
Sequencing
•
•
•
•
An ability of connection-oriented transfer
HDLC, IEEE 802.11 make use of this
Each PDU is numbered sequentially
Incoming/Outgoing numbers are
monitored
• Three main functions:
– Ordered delivery
– Flow Control
– Error Control
Ordered Delivery
• A function of sequencing
• Every PDU takes a different path, leading
to varying arrival times
• Each unique PDU can be sequenced
according to its sending order
Flow Control
• Receiving entity can limit amount/rate of
incoming data
• Helps prevent overflow/overburden
• Stop-and wait
• Credit system/sliding window techniques
• Implemented in several protocols
Briefly on TCP/IP
• Transmission Control Protocol (TCP) is
connection-oriented
– Logical created connection is between entities
– Employs sequencing devices
• Internet Protocol (IP) is connectionless
– PDUs delivered to destination as single,
stateless packets
– Combines with TCP to form basis of internet
transfers
Why the need for addressing?
• Addressing allows for the identification of
devices on the network, so that data can
be sent to the right device
• To further that end, part of the information
sent in a PDU (Protocol Data Unit) is the
address of the intended recipient of the
packet.
A problem…..
• Will is sitting at his computer, and wishes
to send an email to Jane. How can this be
achieved?
High Level Address
• For each device on the network assign a
unique logical address to the device.
• For TCP/IP this is an IP address:
– 130.194.15.119
• For the OSI Model this is a ‘Network
Access Service Point’ (NSAP) located
within the Network layer
High Level Address 2
• The logical Address (whether it is TCP/IP
or NSAP) allows data to be sent from one
device on the network to another device
with a guarantee that the PDU wont go to
the wrong device.
• Thus Will’s email will get through to Jane’s
computer.
Another problem
• Will has sent his email, but Jane is surfing
the web, receiving a file transfer of some
music, and talking online through an IM
program.
• How can Will’s email get to her email client
so she will read it?
Service Access Points
• Within a device that has a single logical
Address lies Service Access Points that
applications can attach themselves to in
order to receive the data they need.
An Example of SAPs
Picture by
Kieran Simpson
Ports
• Within the TCP/IP Model SAP are referred
to as ports.
• Some common ports are:
– 80 or 8080 for HTTP
– 22 for SSH
– 23 for Telnet
Still another problem
• How can Will’s and Jane’s computers be
on the network to begin with?
Device Identification
• Each device on the network has a
‘Network Attachment Point’ through it’s
Network Interface Card (NIC)
• This is usually a 48bit long number
represented as hexadecimal
– 00-02-B3-24-5A-51
• Therefore Will and Jane’s NICs will have
MAC addresses and be able to get onto
the network.
Diagrammatic Representation
Taken from:
“Data and
Computer
Communications”,
William Stallings
Page 41
How are these levels achieved?
Taken from:
“Data and
Computer
Communications”,
William Stallings
Page 42
Addressing Scope
• There are two types of scope for address.
– Local
– Global
Local Addressing Scope
• A local address within the LAN
• It is the name/address through which a
device is identified within it’s own system.
• Typically the MAC address.
Global Addressing Scope
• The name/address through which a device
can be known outside the system.
• Typically the logical Address (eg IP
address)
• Must be unique.
An example of Scope
• Outside the Monash network, the devices
can only be reached through an IP
address of:
– 130.194.xxx.xxx
– Example of global scope
• Within the Monash network devices are
located through an internal local address
scheme => local scope
Bit-oriented protocols
Bit-oriented protocols interpret a transmission
frame or packet as a succession of individual
bits, made meaningful by their placement in
the frame and by their juxtaposition with other
bits.
Bit-Oriented
protocols
SDLC
HDLC
LAPs
LANs
THREE TYPES OF STATIONS
• Primary station
– Controls operation of link
– Frames issued are called commands
– Maintains separate logical link to each secondary station
• Secondary station
– Under control of primary station
– Frames issued called responses
• Combined station
– May issue commands and responses
Tow link configurations
• Unbalanced
– One primary and one
or more secondary
stations
– Supports full duplex
and half duplex
• Balanced
– Two combined stations
– Supports full duplex
and half duplex
Transfer Modes
• Normal Response Mode (NRM)
– Unbalanced configuration
– Primary initiates transfer to secondary
– Secondary may only transmit data in
response to command from primary
– Used on multi-drop lines
– Host computer as primary
– Terminals as secondary
Transfer Modes
• Asynchronous Balanced Mode (ABM)
– Balanced configuration
– Either station may initiate transmission
without receiving permission
– Most widely used
– No polling overhead
Transfer Modes
• Asynchronous Response Mode (ARM)
– Unbalanced configuration
– Secondary may initiate transmission without
permission form primary
– Primary responsible for line
– rarely used
When we say it is a Bit-oriented
Protocol
• individual bits for
control information
and are the preferred
method for
transmitting data
• control codes are
used to control
another device or
provide information
about the status of
the session
• for local area
networks
• data is transmitted as
a steady stream of
bits
• Before date
transmission begins ,
special synchronism
characters are
transmitted by the
sender so the
receiver can
synchronize itself with
the bit stream
• transfer data frames
regardless of frame
contents.
• provide full-duplex
operation and are
more efficient and
reliable
Each piece of data is encapsulated in an HDLC frame by
adding a trailer and a header
HDLC address and an
HDLC control fields
The frames are separated by HDLC flag
sequences which are transmitted between each
frame and whenever there is no data to be
transmitted.
at the end of the
frame, and contains
a cyclic redundancy
check
01111110
Frame sent
Flag
Address
Control
Address
FCS
Flag
Stuffed and
unstuffed bits
Frame received
Flag
011111010
Control
011111010
FCS
Flag
01111110
J
• Start of the flag is 01111110that identifies
both beginning and end of frame and
services for receiver.
• May be misread by receiver
• The station finds a flag on line determines
that the frame is addressed to it and
begins reading the transmission.
• Watching for the next flag that signifies the
end of frame.
• Guaranteed a flag lines not appear
inadvertently.
• Tell the receiver that the current sequence
is not a flag
Start
After 1 Zero and 5
continuous onec
0
Unstuff
zero
It is part of
the data
1
7th bit
0
8th bit
1
Continue
counting
ones
unitlthe
next zero
IT IS A
Flag
TOTAL
<15
It is an
abort
stop
>=15
It means
an idle
channel
Bit stuffing is the process of
adding one extra 0 whenever
there are five consecutive 1s in
the data so that receiver does
not mistake the data for flag
In summary, a protocol is….
• An agreement about communication between
two or more entities
• It specifies
– Format of messages
– Meaning of messages
– Rules for exchange
– Procedures for handling problems
Simplified File Transfer
Architecture
A Three Layer Model
• Network Access Layer
• Transport Layer
• Application Layer
Network Access Layer
• Exchange of data between the computer
and the network
• Sending computer provides address of
destination
• May invoke levels of service
• Dependent on type of network used (LAN,
packet switched etc.)
Transport Layer
• Reliable data exchange
• Independent of network being used
• Independent of application
Application Layer
• Support for different user applications
• e.g. e-mail, file transfer
The OSI Reference Model
OSI – Open Systems Interconnect
What is the OSI model?
• A model for structuring communications
software to provide an open
communication service that’s independent
of manufacturers’ equipment and
conventions
• A framework for standardization
• Split into 7 layers
OSI layers
•
•
•
•
•
•
•
Application
Presentation
Session
Transport
Network
Data Link
Physical
Basis of OSI design
• Layered Design
– Layers can me made independently and
simultaneously
– Changes to 1 layer should not affect others
• Lower levels need to know a great deal of detail.
Higher levels should not be concerned with
these details
• The design is based heavily on having a high
level of cohesion, a minimal amount of coupling
and using information hiding
The Benefits
• A high level of cohesion
– The operations performed in each layer are all
related by their functionality
– Reduces complexity – more manageable
code
• A minimal amount of coupling
– The looser the coupling, the easier it is to
change 1 module without affecting the others
– Keeps interface between modules simple
The Benefits
• Information hiding
– Abstraction allows higher levels to be kept
simpler
– Also makes the layer reusable in a wider
range of situations
• All of this leads to interchangeable,
reusable and manageable layers
Design Principles
• Keep it short and simple so the model
remains manageable
• Not so short as to require grouping of
unrelated functions into a single layer
• Each layer should follow International
standards for the protocol
– Interchangeable layers
Design Principles
• Boundaries between layers should be
placed so that the amount of data sent
between layers is minimal
• Group similar operations into layers
• Separate dissimilar operations into
different layers
• Boundaries should be placed where a
different level of abstraction is required
Design Principles
• Allow changes to a layer without affecting
other layers
– Adapt to changes in technology.Layers should
interface only with the layers directly above or
below
– eg. Layer 5 only interfaces with layers 4 and
6. Not with 3.
– Good cohesion + loose coupling + information
hiding = neat, manageable, maintainable &
interchangeable design. This is the basis of
the OSI model
Overview
•
•
•
•
Standards and Standards organizations
Multivendor model
Main emphasis - INTERCONNECTION
This implies an active relationship between
systems
• OSI is a set of standards, providing many
alternative choices
• Divided into 7 different functional layers
• Each layer: service provided + protocol
A Communications Analogy Business Correspondence
Manager
Manager
Secretary
Secretary
Mail Clerk
Mail Clerk
Letter Box
Letter Box
Post Office
Transport
Post Office
The OSI reference model
• The OSI reference model has 7 layers
• Each layer is defined so that some easily
described operation is performed, and the
software or hardware that is responsible can be
identified
• Each layer thinks it is communicating with an
identical layer on another machine and has little
concern for what occurs within adjacent layers.
The OSI reference model
• Each layer must know, of course, what form the adjacent
layers use when passing data back and forth, but they
don’t need to know what goes on inside.
• The direction of passing depends on whether the
message is being sent (down through the layers) or
received (up).
• On sending, each layer will perform some function, add
an identifying header to the incoming data, and pass the
modified message to the lower layer.
• On receiving, the process is reversed.
The OSI reference model
• In theory, we could have up to six added
headers.
• The physical layer does not add a header.
Added headers
OSI Model - Overview
OSI Model
• Application Layer
•
• What happens at this layer
• What is the justification for having this layer
• How does it differ from TCP/IP Model's Application layer
Application Layer
What happens in this layer?
File Transfer
- Different file systems have different file
naming conventions and ways of
representing data. Application Layer
handles t
Application Layer
How does it differ from TCP/IP Model's
Application layer
Application Layer - TCP/IP Vs OSI
Model
TCP/IP Application Layer contains the
equivalent of OSI Model layers
• Application Layer
• Presentation Layer
• Session Layer
Application-to-Application
Communication
•
•
•
•
•
•
•
Set application message in standard form
Convert data representation (syntax)
Set up session, synchronise data transfer
Transfer data in packets
Control/organise route
Low-level frames transfer, error handling
Transfer bits via physical link
Seven layers or subsystems
Seven layers or subsystems
• Each layer accepts services from layer below across an
interface
• Each layer provides services to layer above
• Each layer requires to exchange information with its
corresponding (peer) layer in the remote system, to do
what is required of it.
• The (N)-layer provides services to the (N+1)-layer;
• An (N+1)-entity requests the service of an (N)-entity
(below it) in order to communicate with its corresponding
(peer) (N+1)-entity.
Seven layers or subsystems
• The (N+1)-protocol provides an apparent horizontal link
between corresponding (N+1) layers in 2 systems. It is
established via an (N)-connection provided by the (N)layer.
Data Encapsulation
(Enveloping)
• Communication between layers uses protocols.
• The data unit increases in size as it is transferred down
through the layers. Each layer attaches header and
trailer information - an 'envelope'.
• These contain control information required by
corresponding target (peer) layer
• After transfer across the physical medium, the layers in
the receiving system; successively strip the protocol
control information, and pass the data package to the
layer above.
Data Encapsulation (Enveloping)
standards
• What is the problems without standards?
Hp
Dell
Microsoft
standards
• Advantage
–
–
lower cost
Purchaser more flexibility in equipment
selection and use
• Disadvantage
– Freeze the technology
– Multiple standards for the same thing
Standards Organizations
•
•
•
•
•
IAB - Internet Architecture Board
ISO - International Standards Organisation
ITU - International Telecommunication Union
IEC - International Electrotechnical Commission
IEEE - Institute of Electrical and Electronic
Engineers
• ….
different organizations
different standards
Internet Society
• Internet Society (ISOC) - An international
organization concerned with the growth and
evolution of the worldwide Internet, and the
social, political, and technical issues that arise
from its use.
• ISOC is an organization with individual and
organizational members, and is managed by a
Board of Trustees elected by the worldwide
individual membership
Internet Society
•Homepage
Internet Society
•
•
•
•
Over 20,000 individual members
more than 150 organizational members
over 180 countries
It can provide the same standards for
the world
Internet Society
• Is made up of
– IAB - Internet Architecture Board: responsible for
defining the overall architecture of the internet,
providing guidance and board direction to the IEEE
– IETF- Internet Engineering Task Force : the
protocol engineering and development arm of the
internet
– IESG – Internet Engineering Steering Group:
response for technical management of IETF and the
Internet standards process
RFC
• An RFC (Request for Comments ) is a
document describing the standards that
make the Internet work.
• The document series, begun in 1969,
which describes the Internet suite of
protocols and related experiments
• Not all (in fact very few) RFCs describe
Internet standards, but all Internet
standards are written up as RFCs
RFC
•RFC Database
RFC
• Example of RFCs:
– RFC 620 - Request for monitor host table
updates Mar 1974
– RFC 3869 - IAB Concerns and
Recommendations Regarding Internet
Research and Evolution August 2004
RFC Publications
Developing an ISO standard
1.
2.
3.
4.
5.
6.
Proposal
Preparatory
Consensus
Voting
Approval
publication
RFC Publications
•
An RFC starts life as an Internet Draft. Before it can be
published as an RFC, a document must first be
published as an Internet Draft (I-D). All RFCs have been
I-Ds, but not all I-Ds become RFCs.
•
In addition, the RFC Editor publishes as independent
submissions some RFCs that are outside the IETF
process but are relevant to the Internet community.
RFCs must first be published as Internet Drafts.
RFC Publications
RFC Publications
• The official specification documents of the Internet
Protocol suite that are defined by the Internet
Engineering Task Force (IETF) and the Internet
Engineering Steering Group (IESG ) are recorded and
published as standards track RFCs.
• IETF –recommends & publishes RFC
• IESG - approves
•
RFC Submission Process
– RFCs from the IETF
– Independent Submissions
• Final Review Period
RFC Publications
6
4
IETF –recommends & publishes RFC
IESG - approves
RFC Publications
Flow chart
Standards for physical interface
between devices
• Voltages, pulses, connectors, switches.
• Rules for bits transfer.
• Mechanical specifications, e.g. RS232C connector (9-pin
or 25-pin); RJ-45; BNC
• Electrical specifications, e.g. voltage levels, timing
• Functional specifications, e.g. what signal each pin is
used for
• Procedural specifications, - response and sequencing of
signals
Service Primitives and
Parameters
• Interaction between layers
– primitives specify the functions to be performed
– parameters are used to pass data and control
information
– four primitives define interaction between adjacent
layers
• REQUEST, INDICATION, RESPONSE, CONFIRM
• may be conformed or not conformed
Service Primitives and
Parameters
• Enable communication between layers in OSI model.
• Independent Layers system.
• Allow changes of functions or protocols to be made
within a layer without affecting other layers.
What are The Services
Primitives?
REQUEST
Invoke some service, pass the parameters.
INDICATION
A primitive issued by a service provider either to:
indicate that a procedure has been invoked, provide
the associated parameters, or notify the service user
of a provider-initiated action.
RESPONSE
A primitive issued by a service user to acknowledge
or complete some procedure.
CONFIRM
Complete some procedure previously invoked by a
request by the service user.
Sequences in Services
Primitives
•
Source: http://williamstallings.com
Layer 1 = Physical Layer
• This is the only layer with DIRECT
communication with another
corresponding subsystem.
• Physical Layer: physical and electrical
characteristics of essential hardware
Physical Layer
• This layer defines:
– Physical Media – bounded, unbounded
– Hardware devices; Mechanical interfaces,
electrical interfaces,
• e.g., NICs - Network Interface Cards, repeaters, hubs
– Interconnections: Physical topology (Logical
topology is Layer 2 MAC)
• linear bus, star, ring, etc.
– Techniques to transfer bit stream to medium,
• i.e., Signalling method, transmission technique (how bits
are transmitted)
– Interface to Layer 2 = Data Link Layer
Introduction
Asynchronous protocols: treat each character in a big stream independently.
Synchronous protocols: take the whole bit stream and chop it into character
of equal size.
Asynchronous Protocols
•
•
•
•
•
•
Long, long… time ago
Not complex and easy to implement
Slow
Required start/stop bit and space
Now mainly used in modem
Replaced by high speed synchronous
Synchronous Protocols
•
Character-oriented protocol
–
–
–
–
Based on one byte (8-bit)
Use ASCII for control character
Frames are interpreted as a sequence of characters
Not efficient because additional DLE character
needed
– Example: Binary Synchronous Communication
(BSC)
•
Bit-oriented protocol
– Going to be presented by Tutor in next question
Control characters for BSC
Control characters: is used to convey information about the transmission
Character
STX
ASCII
Code
STX
Binary
0000010
SOH
SOH
0000001
DLE
DLE
0010000
Function
Start of
text
Start of
header
Data link
escape
IBM’s Binary Synchronous
Communication (BSC)
• Character-oriented protocol
• Specifies half-duplex transmission with
stop-and-wait ARQ
• BSC_p divides a transmission into frames
1. Data frame (for transmission of data)
2. Control frame (connect/disconnect and
flow/error control)
A simple BSC data frame
A BSC frame with a header
S
Y
N
S
Y
N
S
O
H
Header
S
T
X
…Data…
E
T
X
B
C
C
B
C
C
• Header Fields:
– Include the address of the sending/receiving
device
– Identifying NO of the frame (0/1 for stop-andwait ARQ)
Multiblock frame
Block
Block
S S S
Y Y O
N N H
Header
S
T
X
…Data…
I B
T C
B C
B
C
C
S
T
X
ITB = Intermediate text block
• Reason to divided into blocks
…Data…
E
T
X
B
C
C
B
C
C
Multiframe transmission
S
Y
N
S
Y
N
S
O
H
B
C
C
B
C
C
A S
C0 Y
K N
S
Y
N
S
Y
N
S
O
H
B
C
C
B
C
C
A S
C1 Y
K N
Header
S
T
X
…Data…
I
T
B
B
C
C
B
C
C
S
T
X
…Data…
I
T
B
B
C
C
B
C
C
S
T
X
…Data…
E
T
B
B
C
C
B
C
C
…Data…
E
T
X
B
C
C
B
C
C
S
Y
N
Header
S
Y
N
S
T
X
Control frame
A control frame is used by one device to send commands to another device.
It contains only control characters but no data
Data Transparency
• BSC is designed for text message
• Now, non-text message like
video…graphics..
• Problem?
– BSC control character problem
• Data transparency: in data communication
means we should be able to send any
combination of bit as data
Byte Stuffing
Errors
Single-Bit
Burst
Errors
• Interference from heat, magnetism and other forms of
electricity.
• Errors can alter the meaning of data in binary-encoded
data
Single Bit error
• Only one bit of a given data unit is changed from 1 to 0 or from 0 to
1
• Such as byte, character, data unit or packet
• Occurs least likely in serial data transmission
• For example, if we send 1mbps of data. It means that each bit last
only 1/1000 000 second or 1microsecond. So, for the single Bit error
to occur, the sound must have duration of only 1microsec.
0 changed to 1
0 0 0 0 1 0 1 0
Received
0 0 0 0 0 0 1 0
sent
Burst error
• Two or more bits in the data unit have changed from 1 to 0 or
from 0 to 1.
• Occurs Most likely in a serial transmission.
• For example if we send 1kbps of data. If we have 10 error
bits, the noise will have a duration of 1/100seconds. So if we
are sending 1mbps, the same noise can affect
10 000bits
Burst Length of 4
0 changed to 1
0 1 1 0 1 0 1 0
Received
0 0 0 0 0 0 1 0
sent
a.
Given the following string of bits was terminated:
Transmitted Message : 11011011110111100011111011
Received Message : 11111111110101111111111011
Transmitted Message
1 1 0 11 0 1 1 1 1 0 1 1 1 1 0 0 0 1 1 1 1 1 0 1 1
Received Message
1 1 1 1 1 1 1 1 1 1 0 1 0 1 1 1 1 1 1 1 1 1 0 1 1
16 bits length of burst error
Error Control
• It happened in data link layer and
transport layer. The sending transport
layer makes sure that the entire message
arrives at the receiving transport layer
without ERROR (damage, loss or
duplication). It is performed end to end
rather than across a single link.
• In the data link layer, the term error
control refers primarily to methods of
error detection and retransmission.
Error Detection
• Despite the best prevention techniques, errors
may still happen.
• To detect an error, something extra has to be
added to the data/signal. This extra is an error
detection code.
• There are two basic techniques for detecting
errors:
1.
Parity checking
2.
cyclic redundancy checksum (CRC).
Error correction
Once an error is detected, what is the
receiver going to do?
• Do nothing
• Return an error message to the transmitter
• Fix the error with no further help from the
transmitter
Parity Checks
• If performing even parity, add a parity bit such
that an even number of 1s are maintained.
• If performing odd parity, add a parity bit such
that an odd number of 1s are maintained.
• For example, if the character 1001010 is to be
sent, using even parity, a parity bit = 1 would be
added to the character.
• If the character 1001011 is to be sent, using
even parity, a parity bit = 0 would be added to
the character.
• Can be defined as two ways VRC and LRC
Vertical redundancy check
(VRC)
In vertical redundancy check, a parity bit is added to every data unit so that
the total number of 1s becomes even.
Longitudinal redundancy check
(LRC)
In longitudinal redundancy check, a block of bits is divided into rows and
a redundant row of bits is added to the whole block.
VRC and LRC
Both simple parity and longitudinal parity do not catch all errors.
VRC can detect all single-bit errors. But VRC cannot detect errors where the
total number of bits changed is even
LRC is better at catching errors but requires too many check bits added to a
block of data.
Example
• Given the following frame is to be
transmitted:
row1
0
0
0
0
0
0
0
row2
0
1
0
1
0
0
0
row3
1
0
0
0
1
1
0
row4
0
1
0
0
0
0
0
row5
0
1
0
1
1
0
1
Example (con.)
i) Determine the VRC (row) odd parity and LRC
(column) even parity bits for the frame
0
0
1
0
0
0
1
0
1
1
0
0
0
0
0
0
1
0
0
1
0
0
1
0
1
0
0
1
0
0
0
0
0
0
1
1
1
0
0
1
1 1 0 0 0 1 1 1
Example (con.)
ii) Give an example of combination of error bits
occurring in the frame that would be
undetectable.
0
0
1
0
0
1
0
1
0
1
1
1
0
0
0
0
0
0
0
10
01
0
1
0
0
01
10
0
1
0
0
0
1
0
0
1
0
0
0
0
1
1
1
1
0
0
1
1
Local Area Network (LAN)
Fundamentals
Reference:
Chapter 15 -Stallings
Introduction
• LANs are usually owned by the organisation that uses a
network to interconnect equipment
• LANs have much greater capacity than wide area
networks (WANs), to carry what is generally a greater
internal communications load
• A LAN can be used for a variety of applications
– A common LAN is one that supports personal computers
Introduction Contd.
• LANs for the support of personal computers and work stations have
become nearly universal in organisations of all sizes
• Even the sites that still depend heavily on the mainframe have
transferred much of the processing load to networks of personal
computers
• For personal computer networks, a key requirement is low cost
– In particular, the cost of attachment to the network must be significantly
less than the cost of the attached device
– That is, the data rate of the network may be limited; in general, the
higher the data rate, the higher the cost
Introduction Contd.
– Backend networks are used to interconnect large systems such
as mainframes, supercomputers, and mass storage devices
• The key requirement here is for bulk data transfer among limited
number of devices in a small area
• Typically, backend networks are found at sites of large companies of
research installations with large data processing budgets
• A concept related to that of the backend network is the storage area
network (SAN)
– The SAN detaches storage tasks from specific servers and creates a
shared storage facility across a high-speed network
– The collection of networked storage devices can include hard disks,
tape libraries, and CD arrays
Introduction Contd.
– New demands of office environments require high-speed office
networks
• One reason is that desktop image processors have increased
network data flow by an unprecedented amount.
– Even with compression techniques, this will generate a tremendous
load
• These new demands require LANs with high-speed that can support
the larger numbers and greater geographic extent of office systems
as compared to backend systems
Introduction Contd.
– Backbone LANs are attractive means of supporting increasing
use of distributed processing applications and personal
computers of local networking
• They employ lower cost, lower-capacity LANs within buildings or
departments and interconnect them with a higher-capacity
(Backbone) LAN
• They are a better alternative than having a single LAN due to:
– Better reliability
– More scalable
– A typical LAN with low cost equipment will not be able to provide overall
requirement.
Topologies
• In the context of a communication network, the term
topology refers to the way in which the end points, or
stations, attached the network are interconnected
• The common topologies for LANs are bus, tree, ring, and
star
– The bus is a special case of the tree, with only one trunk and no
branches
Topologies Contd.
Topologies Contd.
• Bus and Tree topologies:
– Both these topologies are characterised by the use of a
multipoint medium
• For the bus, all stations attach directly to a linear transmission
medium, or bus, through appropriate hardware interfacing known as
a tap
– Full-duplex operation between the station and the tap allows data to be
transmitted onto the bus and received from the bus
– A transmission from any station propagates the length of the bus in
both directions and can be received by all other stations
Topologies Contd.
• For the tree topology, the transmission medium is a branching cable
with no closed loops
– The tree layout begins at a point known as the headend
– One or more cables start at the headend, and each of these may have
branches
– Two problems present themselves in these topologies:
• As a transmission from any one station can be received by all other
stations, there needs to be some way of indicating for whom the
transmission is intended
• A mechanism is needed to regulate transmission
Topologies Contd.
– To solve these problems, stations transmit data in small blocks, known
as frames
• Each frame consists of a portion of the data that a station wishes to transmit,
plus a frame header that contains control information
• Each station on the bus is assigned a unique address, or identifier
• The destination address for a frame is included in its header
– With the bus or tree, no special action needs to be taken to remove
frames from the medium
• When a signal reaches the end of the medium, it is absorbed by the
terminator
Topologies Contd.
Topologies Contd.
• Ring Topology:
– The network consists of a set of repeaters joined by point-topoint links in a closed loop
– The repeater is a comparatively simple device, capable of
receiving data on one link and transmitting them, bit by bit, on
the other link as fast as they are received
– The links are unidirectional
• Data are transmitted in one direction only (clockwise or counterclockwise)
Topologies Contd.
– As with the bus and tree, data are transmitted in frames
• As a frame circulates past all the other stations, the destination
station recognises its address and copies the frame into a local
buffer as it goes by
• A frame continues to circulate until it returns to the source station,
where it is removed
– As multiple stations share the ring, medium access control is
needed to determine at what time each station may insert frames
Topologies Contd.
Topologies Contd.
• Star Topology:
– Each node is directly connected to a common central node
• Typically, each station attaches to a central node via two point-topoint links, one for trans mission and one for reception
– In general, there are two alternatives for the operation of the
central node:
• In one approach the central node operates in a broadcast fashion
– A transmission of a frame from one station to the node is retransmitted
on all of the out going links
» This transmission is received by all the other stations, and only one
station at a time may successfully transmit
Topologies Contd.
– In this case , the central element is referred to as a hub
• In the second approach, the central node acts as a frame-switching
device
– An incoming frame is buffered in the node and then retransmitted on an
outgoing link to the destination station
Choice of Topology
• The choice of topology depends on a variety of factors,
including reliability, expandability, and performance
– This choice is part of the overall task of designing a LAN and
cannot be made in isolation, independent of the choice of
transmission medium, wiring layout, and access control technique
• For a bus topology, baseband coaxial cable has achieved
widespread use, primarily for Ethernet systems
– Comparatively, bus topology using baseband coaxial cable is
difficult to work with
– Although there is a considerable installed base, a few new
installations are being attempted
Choice of Topology Contd.
• The ring topology is used for very-high-speed links over
considerable distances
– Hence, the ring has the potential for providing the best
throughput of any topology
– One disadvantage of the ring is that a single link or repeater
failure could disable the entire network
• The star topology is generally best for short distances
and can support a small number of devices at high data
rates
Choice of Transmission Medium
• The choice of transmission medium in a LAN is
determined by a number of factors:
– Topology
– Capacity
– Reliability
– Types of data supported
• Depends on the application
– Environmental scope
Choice of Transmission Medium
Contd.
• Typically, office buildings are wired to meet the
anticipated telephone system demand
– Voice-grade unshielded twisted pair (UTP) (Category 3) can be
used as it is inexpensive and there is no cable installation costs
– However, the data rate that can be supported is generally quite
limited
– Shielded twisted pair and baseband coaxial cabal are more
expensive that Category 3 UTP but provide greater capacity
Choice of Transmission Medium
Contd.
– However, in the recent years, the trend has been toward the use
of high-performance UTP, especially Category 5 UTP
• Category 5 UTP supports high data rates for a small number of
devices
• Larger installations can be supported by the use of star topology
– Optical fibre has a number of attractive features, such as
electromagnetic isolation, high capacity, and small size
• However, the market penetration of fibre LANs is low as yet,
primarily due to high cost of fibre components and lack of skilled
personnel to install and maintain fibre systems
LAN Protocol Architecture
• The architecture of a LAN is best described in terms of
layering of protocols that organise the basic functions of
a LAN
• The standardised protocol architecture for LANs
encompasses physical, medium access control (MAC),
and logical link control (LLC) layers
• The physical layer encompasses topology and
transmission medium, which we have discussed so far
• The next section provides an overview of the MAC and
LLC layers
IEEE 802 Reference Model
• Protocols defined specifically for LAN and MAN
transmission address issues relating to the transmission
of blocks of data over the network
• In OSI terms, higher layer protocols ( layers 3 or 4 and
above) are independent of network architecture and are
applicable to LANs, MANs, and WANs
• A discussion of LAN protocols is concerned principally
with lower layers of the OSI model
IEEE 802 Reference Model
Contd.
IEEE 802 Reference Model
Contd.
• The figure in the previous slide relates the LAN protocols
to the OSI architecture
• This architecture was developed by the IEEE 802
committee and has been adopted by all organisations
working on the specification of LAN standards
– It is generally referred to as IEEE 802 reference model
• The lowest layer of the model (physical layer) is
responsible for encoding/decoding, preamble generation/
removal, and bit transmission /reception
IEEE 802 Reference Model
Contd.
• The layer above the physical layer is associated with
providing service to LAN users; these services include:
– On transmission, assemble data into a frame with address and
error-detection fields
– On reception, disassemble frame, and perform address
recognition and error detection
– Govern the access to the LAN transmission medium
– Provide an interface to higher layers and perform flow and error
control
IEEE 802 Reference Model
Contd.
• The set of functions in the last bullet item are grouped
into a logical link control (LLC) layer
• The functions in the first 3 bullet items are treated as a
separate layer, called medium access control (MAC)
• Above separation is done for the following reasons:
– The logic required to manage access to a shared-access
medium is not found in traditional layer 2 data link control
– For the same LLC, several MAC options may be provided
IEEE 802 Reference Model
Contd.
Logical Link Control
• The LLC layer for LANs is similar in many respects to
other link layers in common use
• Like all link layers, LLC is concerned with the transmission
of a link level PDU between 2 stations, without the need of
an intermediate switching node
• LLC has 2 characteristics not shared by most other link
control protocols:
– It must support the multiaccess, shared-medium nature of the link
– It is relieved of some details of link access by the MAC layer
Logical Link Control Contd.
• LLC specifies the mechanisms for addressing stations
across the medium and for controlling the exchange of
data between two users
• The operation and format of this standard is based on
HDLC
• Three services are provided as alternatives for attached
devices using LLC
– Unacknowledged connectionless service:
Logical Link Control Contd.
• A very simple service that does not involve any of the flow- and errorcontrol mechanisms
• Thus the delivery of data is not guaranteed
– However, in most devices, there will be some higher layer of software that
deals with reliability issues (there by avoids duplication)
• Used for instances in which the overhead of connection establishment
and maintenance is unjustified or even counter-productive
– For example, data collection activities that involve periodic sampling data
sources, such as sensors and automatic self-test reports from security
equipment or network components
Logical Link Control Contd.
– Connection mode service:
• Similar to the service offered by HDLC.
• A logical connection is set up between 2 users exchanging data,
and flow control and error control are provided
• Could be used in very simple devices, such as terminal controllers,
that have little software operating above this level
• In this mode, the logical link control software must maintain some
sort of table for each active connection, to keep track of the status
of the connection
Logical Link Control Contd.
– Acknowledged connectionless service:
• This is a cross between the previous two services
• If the user needs guaranteed delivery but there are a large number
of destinations, this mode is preferred
– An example is a process control or automated factory environment
where central site may need to communicate with a large number of
processors and programmable controllers
– Another use of this is the handling of important and time-critical alarm
or emergency control signals in a factory
Logical Link Control Contd.
• The basic LLC protocol is modelled after HDLC and has
similar functions and formats
• The differences between the 2 protocols are:
– LLC makes use of asynchronous balanced mode of operation of
HDLC, to support connection mode LLC service
• This is referred to as type 2 operation
• The other HDLC modes are not employed
– LLC supports an unacknowledged connectionless service using
the unnumbered information PDU
• This is known as type 1 operation
Logical Link Control Contd.
– LLC supports an acknowledged connectionless service by using
two new unnumbered PDUs
• This is known as type 3 operation
– LLC permits multiplexing of the use of LLC service access points
(LASPs)
• All three LLC protocols employ the same PDU format,
which consists of 4 fields
– The DSAP (destination services access point) and SSAP (source
service access point) fields each contain a 7-bit address, which
specify the destination and source uses of LLC
Logical Link Control Contd.
– One bit of DSAP indicates whether the DSAP is an individual or
group address
– One bit of the SSAP indicates whether the PDU is a command or
response
– The format of LLC control field is identical to that of HDLC, using
extended (7-bit) sequence numbers
Logical Link Control Contd.
Medium Access Control
• All LANs and MANs consist of collections of devices that
must share the network’s transmission capacity
• The function of the MAC protocol is providing some
means of controlling access to the transmission medium
for an orderly and efficient use of the above capacity
• The control can be exercised in a centralised or
distributed manner
– In the former, a controller is designated that has the authority to
grant access to the network
Medium Access Control Contd.
– In the latter, the stations collectively perform a medium access control
function to determine dynamically the order in which stations transmit
• How the access control is accomplished is constrained by the
topology and is a tradeoff among competing factors, including
cost, performance, and complexity
• In general, access control techniques are categorised as
being either synchronous or asynchronous
– With the former, a specific capacity is dedicated to a connection
• Such techniques are generally not optimal in LANs and MANs as the
needs of the stations are unpredictable
Medium Access Control Contd.
– In the latter approach, capacity is allocated in an asynchronous
(dynamic) manner, more or less in response to immediate
demand
– The asynchronous approach can be further subdivided into three
categories:
• Round Robin:
– Each station in turn is given the opportunity to transmit
– During that opportunity, the station may decline to transmit or may
transmit subject to a specified upper bound
» The bound is usually expressed as a maximum amount of data
transmitted or time for this opportunity
Medium Access Control Contd.
– When a station has finished, it relinquishes its turn, and the right to
transmit passes to the next station in logical sequence
» The control of the sequence may be centralised or distributed
» Polling is an example of a centralised technique
– When many stations have data to transmit over an extended period of
time round-robin techniques can be very efficient
– If only a few stations have data to transmit over an extended period of
time, then there is a considerable overhead in passing the turn from
station to station
» Under these circumstances, other techniques may be preferable,
largely depending on whether the data traffic has stream or burst
characteristics
Medium Access Control Contd.
» Stream traffic is characterised by lengthy and fairly continuous
transmissions – examples are voice communications, bulk file
transfer
» Bursty traffic is characterised by short, sporadic transmissions –
interactive traffic
• Reservation
– Well suited for stream traffic, and reservation can be made in
centralised or distributed manner
– In general, for these techniques, time on the medium is divided into
slots , much as with synchronous TDM
– A station wishing to transmit reserves future slots for an extended or
even an indefinite period
Medium Access Control Contd.
• Contention
– Usually appropriate for bursty traffic
– No control is exercised to determine whose turn it is, all stations
contend for time
– These techniques are of distributed in nature
– Their principal advantage is that they are simple to implement and,
under light to moderate load, efficient
» For some of these techniques, performance tend to collapse
under heavy load
• In LANs, round-robin and contention techniques are the
most common
Medium Access Control Contd.
• As with other protocol layers, MAC implements its
functions making use of a PDU at its layer
– In this case, the PDU is referred to as a MAC frame
• The exact format of the MAC frame differs somewhat for
the various MAC protocols in use
• In general, all of the MAC frames have a format similar
to that was shown in the previous figure
Medium Access Control Contd.
• The fields of this frame are:
– MAC control
• Contains any protocol control information needed for the functioning
of the MAC protocol
– For example, a priority level could be indicated here
– Destination MAC address
• The destination physical attachment point on the LAN for this frame
– Source MAC address
– LLC
• The LLC data from the next higher layer
Medium Access Control Contd.
– CRC
• The cyclic redundancy check field – also known as the frame check sequence
(FCS) field
• In most data link control protocols, the data link protocol
entity is responsible not only for detecting errors using
CRC, but for recovering from those errors by retransmitting
• In LAN protocol architecture, these two functions are split
between MAC and LLC layers
– The MAC layer is responsible for detecting errors and discarding any
frame that are in error
– The LLC layer optionally keeps track of which frames have been
successfully received and retransmit unsuccessful ones
Local Area Network (LAN)
Developments
Reference:
Chapter 16 -Stallings
Introduction
• Recent years have seen rapid changes in technology,
design, and commercial applications for LANs
– A major feature of this evolution is the introduction of a variety of
new schemes for high-speed LANs
• The most important commercial products available are:
– Fast Ethernet and Gigabit Ethernet
– Fibre Channel
– High-speed wireless LANs
Introduction Contd.
• Until relatively recently, office LANs provided basic
connectivity services- connecting PCs and terminals to
mainframes and midrange systems
– It provided workgroup connectivity at the departmental level
– The traffic pattern was relatively light, with an emphasis on file
transfer and electronic mail
– The LANs that were used for this type of workload were primarily
Ethernet and token ring
• In recent years, two significant trends have altered the
role of the PC and therefore the requirements of the
LAN:
Introduction Contd.
– The speed and computing power of PCs have continually
increased
• Today’s more powerful platforms support graphics intensive
applications and elaborate graphical user interfaces to the operating
system
– MIS organisations have recognised the LAN as a viable and
essential computing platform, resulting the focus on network
computing
• The trend began with client/server computing, which has become a
dominant architecture in the business environment
• These approaches involve frequent transfer of large volumes of
data in a transaction-oriented environment
Introduction Contd.
• The following are examples of requirements that call for
higher-speed LANs:
– Centralised server farms
– Power workgroups
• A small number of cooperating users who need to draw massive
data files across the network
– Examples are software development groups and CAD companies that
run simulations regularly
Ethernet
• Most widely used high-speed LANs are based on Ethernet,
which is developed by the IEEE 802.3 standards committee
• The access method used by Ethernet is CSMA/CD (carrier
sense multiple access with collision detection)
• CSMA/CD and its precursors can be termed random
access, or contention, techniques
– There is no predictable or scheduled time for any station to transmit
– They exhibit contention in the sense that stations contend for time
on the shared medium
Ethernet Contd.
• The earliest of these techniques, known as ALOHA
(sometimes pure ALOHA), was developed for packet
radio networks
– However, it is applicable to any shared transmission medium
– In ALOHA, a station may transmit a frame at any time
– It then listens for an amount of time equal to the maximum
possible round-trip propagation delay on network plus a small
fixed time increment
– If the station hears an acknowledgment during that time, fine;
otherwise it resends the frame
Ethernet Contd.
– If the station fails to receive an acknowledgment after repeated
transmissions, it gives up
– A receiving station determines the correctness of an incoming
frame by examining a FCS field
– If the frame is valid and the destination address in the frame
address matches the receiver’s address, the station immediately
sends an acknowledgment
– A frame may be invalid due to noise on the channel or because
another station transmitted a frame at about the same time
• The latter case is known as a collision
Ethernet Contd.
• ALOHA is as simple as can be, but the number of
collisions rises rapidly with increased load
– The maximum utilisation of the channel is only about 18%
• To improve efficiency, a modification of ALOHA, known
as slotted ALOHA was developed
– Time on channel is organised into uniform slots whose size
equals the frame transmission time
– Some central clock or other technique is needed to synchronise
all stations
Ethernet Contd.
– Transmission is permitted to begin only at a slot
boundary
• Thus , frames that overlap will do so totally
– This increased the maximum utilisation of the system
to about 37%
• Both ALOHA and slotted ALOHA exhibit poor
utilisation
– Both fail to take advantage of one of the key properties of both
packet radio networks and LANs
Ethernet Contd.
• That is propagation delay between stations may be very small
compared to frame transmission time
– A short propagation delay provides the stations with better
feedback about the state of the network
• This information can be used to improve efficiency
• The above observations led to the development of
CSMA
– A station wishing to transmit first listen to the medium to
determine if another transmission is in progress (carrier sense)
Ethernet Contd.
– If the medium is in use, the station must wait
– If the medium is idle, the station may transmit
– It may happen that two or more stations attempt to transmit at
about the same time
• Then, there will be a collision
• To account for this, a station waits a reasonable amount of time
after transmitting for an acknowledgment
• If there is no acknowledgment, the station assumes a collision has
occurred and retransmits
Ethernet Contd.
• The maximum utilisation achievable using CSMA can far
exceed that of ALOHA or slotted ALOHA
– It depends on the length of the frame and propagation time
• The longer the frames or shorter the propagation time, the higher the
utilisation
• With CSMA, an algorithm is needed to specify what a
station should do if the medium is found busy
– One algorithm is nonpersistent CSMA
• If the medium is idle transmit
• If the medium is busy, wait an amount of time drawn from a probability
distribution (retransmission delay) and repeat the previous step
Ethernet Contd.
• A problem with nonpersistent CSMA is that capacity is wasted
because the medium will generally remain idle following the end of a
transmission even if there are stations waiting to transmit
– To avoid the above limitation, 1-persistent protocol can be used
• If the medium is idle, transmit
• If the medium is busy, continue to listen until the channel is sensed
idle; then transmit immediately
• If two or more stations are waiting to transmit, a collision is
guaranteed
– Things get sorted out only after the collision
Ethernet Contd.
– A compromise that attempts to reduce collisions and idle time is ppersistent
• If the medium is idle, transmit with probability p, and delay one time
unit with probability (1-p)
– The time unit is typically equal to the maximum propagation delay
• If the medium is busy, continue to listen until the channel is idle and
repeat the previous step
• If transmission is delayed one time unit, repeat the first step
• CSMA has one glaring inefficiency
– When two frames collide, the medium remains unusable for the
duration of transmission of both damaged frames
Ethernet Contd.
– For long frames, compared to propagation time, the amount of
wasted capacity can be considerable
• This waste can be reduced if a station continues to listen to the
medium while transmitting
– The above leads to CSMA/CD
• If the medium is idle, transmit
• If the medium is busy, continue to listen until the channel is idle,
then transmit immediately
• If a collision is detected during transmission, transmit a brief
jamming signal to assure that all stations know that there has been
a collision and then cease transmission
Ethernet Contd.
• After transmitting the jamming signal, wait a random amount of time,
referred to as the backoff, then attempt transmit again
– An important rule followed in most CSMA/CD systems is that
frames should be long enough to allow collision detection prior to
the end of transmission
• If shorter frames are used, then collision detection does not occur
– CSMA/CD will exhibit the same performance as the less efficient CSMA
protocol
Ethernet
Contd.
Ethernet Contd.
• The MAC frame format for 802.3 protocol consists of the
following fields:
– Preamble
• A 7-octet pattern of alternating 0s and 1s used by the receiver to
establish bit synchronisation
– Start Frame Delimiter (SFD)
• The sequence 10101011, which indicates the actual start of the
frame and enables the receiver to locate the first bit of the rest of
the frame
– Destination Address (DA)
Ethernet Contd.
– Source Address (SA)
– Length/Type
• Length of LLC data field in octets, or Ethernet Type field, depending
on whether the frame conforms to IEEE 802.3 standard or the
earlier Ethernet specification
– In either case, the maximum frame size, excluding the Preamble and
SFD, is 1518 octets
– LLC data
– Pad
• Octets added to ensure that frame is long enough for proper CD
operation
Ethernet Contd.
– Frame Check Sequence (FCS)
• A 32-bit CRC, based on all fields except preamble, SFD, and FCS
Ethernet Contd.
• A traditional Ethernet is half-duplex
– A station can either transmit or receive a frame, but it cannot do
both simultaneously
– If a 100-Mbps Ethernet ran in full-duplex mode, the theoretical
transfer rate becomes 200 Mbps
• The attached stations must have full-duplex rather than half-duplex
adapter cards
• The central point in the star wire cannot be a simple multipoint
repeater but rather must be a switching hub
– In this case each station constitutes a separate collision domain
Ethernet Contd.
– In fact, there are no collisions and the CSMA/CD algorithm is no longer
needed
– However, the same 802.3 MAC frame format is used and attached
stations can continue to execute the CSMA/CD algorithm, even though
no collisions can ever be detected
• One of the strengths of the Fast Ethernet approach is
that it readily supports a mixture of existing 10-Mbps
LANs and newer 100-Mbps LANs
– For example, the 100-Mbps technology can be used as a
backbone LAN to support a number of 10-Mbps hubs
• These hubs are in turn connected to switching hubs that conform to
100BASE-T and that support both 10-Mbps and 100-Mbps links
Fast Ethernet
• Fast Ethernet refers to a set of specifications developed
by the IEEE 802.3 committee to provide a low-cost,
Ethernet compatible LAN operating at 100 Mbps
– The blanket designation for these standards is 100BASE-T
– The committee defined a number of alternatives to be used with
different transmission media
– All of the 100BASE-T options use IEEE 802.3 MAC protocol and
frame format
Gigabit Ethernet
• In late 1995, the IEEE 802.3 committee formed a High-Speed
Study Group to investigate means for conveying packets in
Ethernet format at speeds in gigabits per second range
– A set of 1000-Mbps standard have now been issued
• While defining a new medium and transmission specification,
Gigabit Ethernet retains the CSMA/CD protocol and Ethernet
format of its 10-Mbps and 100-Mbps predecessors
• As more organisations move to 100BASE-T, putting huge
traffic loads on backbone networks, demand for Gigabit
Ethernet has intensified
Gigabit Ethernet Contd.
10-Gbps Ethernet
• The principal driving requirement for 10 Gigabit Ethernet is
the increase in Internet and intranet traffic
• A number of factors contribute to the explosive growth in
both Internet and intranet traffic
– An increase in the number of network connections
– An increase in the connection speed of each end-station
• E.g., 10 Mbps users moving to 100 Mbps, analog 56Kbps user moving
to DSL and cable modems
10-Gbps Ethernet Contd.
• An increase in the deployment of bandwidth-intensive applications
such as high-quality video
– An increase in Web hosting and application hosting traffic
• Initially network managers will use 10-Gbps Ethernet to
provide high-speed, local backbone interconnection
between large capacity switches
– As the demand for bandwidth increases, 10-Gbps Ethernet will
be deployed throughout the entire network and will include
server farm, backbone, and campuswide connectivity
Token Ring
• The IEEE 802.5 token ring standard is an outgrowth of
IBM’s commercial token ring LAN product
• A ring consists of a number of repeaters, each
connected to two others by unidirectional transmission
links to form a single closed path
– Data are transferred sequentially, bit by bit, around the ring from
one repeater to the next
• Each repeater regenerates and retransmits each bit
Token Ring Contd.
• For a ring to operate as a communication network, three
functions are required: data insertion, data reception,
and data removal
– These functions are provided by the repeaters
• Each repeater, in addition to serving as an active
element on the ring, serves as a devise attachment point
• Data are transmitted in packets, each of which contains
a destination address field
Token Ring Contd.
• As a packet circulates past a repeater, the address field
is copied
– If the attached station recognises the address, the remainder of
the packet is copied
• Repeaters perform the data insertion and reception
functions similar to that of taps, which serve as devise
attachment points on a bus or tree
• Data removal is, however, is more difficult on a ring
– As a ring is a closed loop, a packet will circulate indefinitely
unless it is removed
Token Ring Contd.
• A packet may be removed by the addressed repeater
– Alternatively, each packet could be removed by the transmitting
repeater after it has made one trip around the loop
• This approach is more desirable as it permits automatic
acknowledgment and permits multicast addressing
• A repeater can be seen to have 2 main functions:
– To contribute to the proper functioning of the ring by passing on
all data that come its way
– To provide an access point for attached stations to send and
receive data
Token Ring Contd.
• Corresponding to the above two purposes, there ate two
states:
– The listen state
– The transmit state
• In the listen state, each received bit is retransmitted with
a small delay, required to allow the repeater to perform
required functions
– Scan passing bit stream for pertinent patterns
• Chief among these is the address of addresses of attached stations
• Another pattern indicates the permission to transmit
Token Ring Contd.
– Copy each incoming bit and send it to the attached station while
continuing to retransmit each bit
• This will be done for each bit of each packet addressed to this station
– Modify a bit as it passes by
• In certain control strategies, bits may be modified to, for example, indicate
that the packet has been copied
• This would serve as an acknowledgment
– When a repeater’s station has data to send and when the repeater has
permission to send, the repeater enters the transmit state
Token Ring Contd.
• The token ring technique is based on the use of a small
frame, called a token, that circulates when all stations
are idle
• A station wishing to transmit must wait until it detects a
token passing by
• It then seizes the token by changing one bit in the token,
which transforms it from a token to a start-of-frame
sequence for a data frame
• The station then appends and transmits the remainder of
the field needed to construct a data frame
Token Ring Contd.
• When a station seizes a token and begins to transmit a
data frame, there is no token on the ring
– So the stations wishing to transmit must wait
• The frame on the ring will make a round trip and be
absorbed by transmitting station
• In the default operation, the transmitting station will insert
a new token on the ring when
– The station has completed transmission of its frame
– The leading edge of the transmitted frame has returned
Token Ring
Contd.
Layer 2 = Data Link Layer
• Applies to the transfer of data frames
between locally, directly connected (linked)
devices via the physical layer (wire, fibre,
radio).
• A set of rules for exchanging messages.
Layer 2 = Data Link Layer
• Several important tasks:
– Delineation of data. Data link framing uses standard
fields, each with a specific task.
– Addressing. The source and destination addresses
are MAC addresses, 6 bytes = 48 bits long. (NOT the
IP address, which is 4 bytes long). The MAC is
usually burned into the NIC Network Interface Card.
Supposedly unique world-wide.
Layer 2 = Data Link Layer
• Error Control. The FCS Frame Check
Sequence, or CRC Cyclic Redundancy Check is
a check sum, generated using polynomials with
coefficients 0 or 1, that gives a very high
probability that errors will be detected.
• Flow Control. The flow of data from the sender
must not overwhelm the receiver. The receiver
must be able to inform the sender when some
limit is reached. Synchronisation, sequencing of
data frames.
Layer 2 = Data Link Layer
• Medium Access Control. E.g., 802.3, 802.4, 802.5.
• Transparency. The Start-of-frame and End-of-frame bit
patterns 01111110 are not confused with bit patterns in
the message itself. A 0 bit is inserted whenever five
consecutive 1 bits occur in the data.
• Code Independence. Any character code, e.g., ASCII or
EBCDIC, can be transmitted. In Ethernet, data is sent in
octets = groups of 8 bits.
Layer 2 = Data Link Layer
• Two Sublayers:
– Upper is 802.2 LLC Logical Link Control.
– Lower is MAC
Layer 2 = Data Link Layer Upper
layer (cont’d)
• Upper is 802.2 LLC Logical Link Control.
– Activate, maintain, release link. Make physical link
reliable. (The Physical Layer can lose bits and is
ignorant of this).
– Main services are error detection and control.
– Independent of MAC.
– The data unit is called a Protocol Data Unit (PDU);
three types:
• I-PDU = Information PDU;
• S-PDU = Supervisory;
• U-PDU = Unnumbered.
Layer 2 = Data Link Layer
Upper layer (cont’d)
• Based on the High level Data Link Control
(HDLC) protocol.
• 2 classes of service:
– Type 1:
• Type 1a – Unacknowledged Connectionless service
• Type 1b – Acknowledged Connectionless service
• Use only U-PDUs.
– Type 2:
• Connection-oriented service: (e.g., telephone call)
• Uses all 3 types of PDU
• maintain connection using special packets.
Layer 2 = Data Link Layer
Lower layer (cont’d)
• Lower is MAC
– Ethernet, CSMA/CD is IEEE 802.3
– Token passing bus is 802.4
– Token passing ring is 802.5
– Wireless is 802.11
Layer 2 = Data Link Layer
• Hardware: Bridges, Switches
– Switches: Cut-through, Modified cut-through,
Store-and-forward.
• ARP Address Resolution Protocol (Know
IP address, want MAC address).
(Broadcast) Who has 130.194.75.44? Tell
130.194.75.89.
Layer 3 = Network Layer
• Handles data transfer across communications
network - routing, relaying, switching logical
connections. Has view of entire network (Data
Link, Physical layers address subnetworks only).
• Provides the means to set up, maintain and
clear a connection path.
• If direct link between stations - network layer not
needed
Layer 3 = Network Layer
• Responsible for delivery of a packet
between source and destination.
• Packets are called datagrams.
Layer 3 = Network Layer
• Tasks:
– Internetworking – linking heterogeneous physical
networks.
– Addressing. Each device on the Internet must be
uniquely identified.
– Routing – determine optimal path through network.
Often there is a choice of pathways.
– Packetizing – encapsulates packets from the upperlayer protocols.
– Fragmenting. Different physical networks may have
different protocols, different frame sizes.
Layer 3 = Network Layer
Addressing
• Unique IP Internetworking protocol
address, e.g., 130.194.99.44.
• 4 bytes, written as 4 decimal values, 0 <=
N <= 255, separated by periods.
• 32 bits, hence 232 possible values,
4,294,967,296.
Layer 3 = Network Layer
Addressing
• 5 classes of network:
– Class A. First byte is 0 to 127, binary 00000000 to
01111111
• 128 such networks; each can support about 16,777,216
devices.
– Class B. First byte is 128 to 191, binary 10000000 to
10111111
• First 2 bytes define the network, 64 x 256 = 16384; each can
support about 65,536 devices. Typically allows 256 ‘subnets’.
Layer 3 = Network Layer
Addressing
– Class C. First byte is 192 to 223, binary 11000000 to
11011111
• First 3 bytes define the network, 32 x 256 x 256 = 2,097,152;
each can support about 256 devices.
– Class D. First byte is 224 to 239, binary 11100000 to
11101111.
• Designed for multicasting.
– Class E. First byte is 240 to 255, binary 11110000 to
11111111.
• Reserved
Layer 3 = Network Layer Routing
• Hardware: Routers (also routing switches, switching
routers).
• Routing table at A:
– Destination:
– Go via:
– Hop count:
B
B
1
A
C
B
2
B
E
D
B
3
C
E
E
1
D
F
F
E
2
Layer 3 = Network Layer Routing
• Destination address in routing table shows
next address along the path
– From A to B, C or D – go via B
– From A to E or F – go via E
A
B
E
C
D
F
Layer 3 = Network Layer Routing
• Routing can be:
– Fixed/static/non-adaptive
– Entered manually by administrator, no automatic
updating.
– For a small or experimental internet.
• Alternative path routing
– Alternative routes provided, as well as the first choice.
– Can be used if first choice fails.
Layer 3 = Network Layer Routing
• Dynamic routing, adaptive routing
– Route is dynamically generated from moment to moment,
using RIP or OSPF protocols optimising use of network
resources
– Routes change as the traffic loads change.
• RIP Routing Information Protocol.
– Routers periodically exchange information with their
neighbours using RIP broadcast packets.
– Workstations can query the nearest router using RIP request
packet.
– Uses an abstract distance measure called distance-vector.
Layer 3 = Network Layer Routing
• Open Shortest Path First (OSPF) protocol
– Uses link-state(usually cost) is minimized
– more complicated than RIP, becoming more
popular.
Layer 4 = Transport Layer
• Objective: provide reliable and efficient data
transport, medium independent, from a source
machine to a destination machine. Enhance the
quality of service provided by the network layer.
• Delivers a complete message (within a running
process) from source to destination, as
compared with individual Network Layer
packets.
Layer 4 = Transport Layer
• Tasks:
– Port addressing. There can be several simultaneous network
processes. Different ‘port addresses’ are used to distinguish these.
– Segmentation and reassembly. As in TCP Transmission Control
Protocol.
• Sequence numbers for consecutive segments enable reassembly.
– Connection control. TCP is connection oriented (machines
cooperating). UDP is connectionless (send and hope).
– Flow control. End-to-end flow control across the whole network, (rather
than across a single link as in the Data Link layer).
– Error control. Like flow control, end-to-end control across the whole
network.
Layer 4 = Transport Layer
• Optimise resources, according to requested quality of
service.
– Quality here means resilience to errors.
• Five classes of transport layer protocol, depending on
reliability of network layer:
– Simple class, no enhancement to network service. Do nothing.
– Basic error recovery class
– Multiplexing (increase throughput using several network
connections)
– Error recovery and multiplexing
– Error detection and recovery
Layer 4 = Transport Layer
• Controls data flow rates
– Cuts messages into packets, reassembles packets into
messages.
– Optionally, full end-to-end error checking.
• Border between providers of services (layers 1 to 3) and
users of services (layers 5 to 7).
• Similar to network layer, but:
– Network layer can lose packets. Transport layer detects
lost/damaged packets.
– Transport layer is visible to applications. Network layer is not.
Layer 4 = Transport Layer
• Some protocols:
– TCP Transmission Control Protocol (not all layer 4 OSI, but
close)
– UDP User Datagram Protocol (‘Unreliable’ Datagram Protocol).
– SPX Sequenced Packet Exchange (Novell)
– XNS Xerox Network System; Early internet protocol now
replaced entirely by TCP/IP
Layer 4 = Transport Layer
• Two types of transport service:
connection-oriented and connectionless.
– TCP is connection-oriented. A stream of
messages delivered in sequence with any
loss or failure signalled to both end systems.
– UDP is connectionless. Individual messages
might or might not get through.
Layer 4 = Transport Layer
• TCP Transmission Control Protocol:
– Data is organised as a stream of bytes. Usually full
duplex, two byte streams in opposite directions.
– Reliable delivery using sequence numbers, that count
bytes in the data stream. Each packet has the starting
sequence number of the data in the packet, and the
acknowledgement (sequence) number of the last byte
received.
– Flow Control using Sliding Window: Remote host is
informed of current buffer size (window). Can stop
sending, or send multiple packets.
Layer 5 = Session Layer
• Application programs do not bombard each
other with continuous unstructured streams of
data. They interact in a structured manner – the
basis of the Session layer.
• Basic Services:
– Session connection: Establish session liaison
between 2 applications e.g. flow of data, PC to printer
– Data transfer: Manage/monitor session dialogue
– Disconnection: Manage orderly release.
Layer 5 = Session Layer
• Telephone call analogy:
– Check phone is working
– Obtain number of person being called
– If answered, have conversation – a “session”
– When finished, sign off appropriately.
• Checkpointing is used for recovery/restart
Layer 6 = Presentation Layer
• A set of data transformation services
–
–
–
–
–
–
Transformation of syntax
Selection of syntax
Provide independence from character representation
Provide independence from machine characteristics
Compression (improve transmission rates)
Encryption (improve security)
Layer 6 = Presentation Layer
• Not needed on LANs with machines of the same type
– Presentation may be a “pass-through” layer. May have direct
mapping between Application and Session.
• Data conversion - syntax (representation), not semantics (meaning).
– Presentation layer negotiates and selects the appropriate
transfer syntaxes to be used during a transmission.
– Need a common representation of information in order to
preserve its meaning – a universally understood language must
be defined to allow the transfer language to be negotiated.
Layer 6 = Presentation Layer
• Data conversion - syntax (representation),
not semantics (meaning).
– Need a common representation of information
in order to preserve its meaning – a
universally understood language must be
defined to allow the transfer language to be
negotiated.
Layer 6 = Presentation Layer
• What differences are there?
• Hardware manufacturers never quite agree about the best way to do
things.
– Big-Endian vs Little-Endian – can affect ALL data formats
– Different word sizes – 16, 32, 36, 60, 64 … bits
– Different character codes,
• e.g.: ASCII, ANSI, EBCDIC, BCL(Burroughs Common
Language)
– Different number representations – Binary, BCD, 1’s/2’s
complement
– Different alignment rules for data within words, longwords e.g.
PACKED records in Pascal
Layer 7 = Application Layer
• Some protocols:
– FTAM File Transfer Access and Management:Remote file access and manipulation
– MHS Message Handling System X.400:-Electronic
mail
– Directory (X.500)
– ODA Office Document Architecture:- Interchange of
generic forms of documents
– VT Virtual Terminal
– RDA Remote Database
Some protocols(cont’d)
– Systems Management:
• SNMP Simple Network Management Protocol
• CMISE Common Management Information Service
Element
• CMIP Common management Information Protocol
Layer 7 = Application Layer
• NOT the application programs themselves.
• Provides services to Applications/Users
–
–
–
–
–
–
File Transfer Protocol (FTP)
Record transaction server
E-mail
Virtual terminal - screen display, keyboard reading
Network management
Remote systems job control
Layer 7 = Application Layer
• Application Service Elements ASE are
modules which support commonly
required services.
– Identifies communicators
– Authenticates, synchronises communicators
– Defines error recovery, flow control protocols,
etc.
Transmission Media
Signals a media dependant.
The OSI Model/Internet Model
Basic Protocol Functions Contd.
» Typically, a unique address is associated with each end system
(e.g., workstation or server) and each intermediate system (e.g.,
router)
» Such an address, in general, is a network-level address
» In TCP/IP architecture, this is known as an IP address, or simply
an internet address
» In OSI architecture, this is referred to as a network service access
point (NSAP)
» The network-level address is used to route a PDU through a
network or networks to a system indicated by a network-level
address in the PDU
» Once data arrive at a destination system, they must be routed to
some process or application in a system
Basic Protocol Functions Contd.
» Typically, a system will support multiple applications and an
application may support multiple users
» Each application and, perhaps, each concurrent user of an
application, is assigned a unique identifier, referred to as a port in
the TCP/IP architecture and as a service access point (SAP) in the
OSI architecture
– Addressing Scope
» The internet address or NSAP address referred to previously is a
global address
» A global address identifies a unique system (global nonambiguity)
» It is possible at any global address to identify any other global
address, in any system, by means of the global address of the other
system (global applicability)
Basic Protocol Functions Contd.
» Because a global address is unique and globally applicable, it
enables an internet to route data from any system attached to any
network to any other system attached to any other network
» Each network must maintain a unique address for each device
interface on the network
» Examples are MAC address on an IEE 802 network and an ATM
host address
» This address enables the network to route data units (e.g., MAC
frames, ATM cells) through the network and deliver them to the
intended attached system
» Such an address is referred to as a network attachment point
address
Basic Protocol Functions Contd.
» The issue of addressing scope is generally only relevant for
network-level addresses
» A port or SAP above the network level is unique within a given
system but need not be globally unique
– Connection identifiers
» The concept of connection identifiers comes into play when we
consider connection-oriented data transfer (e.g., virtual circuit)
rather than connectionless data transfer
» For connectionless data transfer, a global identifier is used with
each data transmission
» For connection-oriented transfer, it is sometimes desirable to use
only a connection identifier during data transfer phase
Basic Protocol Functions Contd.
– Addressing mode
» Most commonly, an address refers to a single system or port; in
this case it is referred to a s an individual or unicast address
» It is also possible for an address to refer to more than one entity or
port; such an address identifies multiple simultaneous recipients
for data
» An address for multiple recipients may be broadcast, intended for
all entities within a domain, or multicast, intended for a specific
subset of entities
– Multiplexing
• One form of multiplexing is supported by means of multiple
connections into a single system
Basic Protocol Functions Contd.
– For example, there can be multiple data link connections terminating in
a single end system
» We can say that these data link connections are multiplexed over
the single physical interface between the end system and the
network
• Multiplexing can also be accomplished via port names, which also
permit multiple simultaneous connections
– For example, there can be multiple TCP connections terminating in a
given system, each connection supporting a different pair of ports
• Multiplexing is used in another context as well, namely, mapping of
connections from one level to another
Basic Protocol Functions Contd.
– In a network, for each process to process connection established at the
higher level, a data link connection could be created at the network
access level
» This is one-to-one relationship, but need not be so.
– Multiplexing can be used in on of two directions
» Upward multiplexing, or inward multiplexing, occurs when multiple
higher-level connections are multiplexed on, or share, a single
lower-level connection
» Downward multiplexing, or splitting, means that a single higherlevel connection is built on top of multiple lower-level connections,
the traffic on the higher connection being divided among the
various lower connections
Basic Protocol Functions Contd.
– Transmission Service
• A protocol may provide a variety of additional services to the entities
that use it
• Three common examples are:
– Priority
» Certain messages, such as control messages, may need to get
through to the destination entity with minimum delay
» Thus, priority could be assigned on a message basis, or on a
connection basis
– Quality of service
» Certain classes of data may require a minimum throughput or a
maximum delay threshold
Basic Protocol Functions Contd.
– Security
» Security mechanisms, restricting access, may be invoked
• All of these services depends on the underlying
transmission system and any intervening lower-level
entities
Principles of Internetworking
• Packet-switching and packet-broadcasting networks grew
out of a need to allow the computer user to have access to
resources beyond that available in a single system
– Resources of a single network are often inadequate to meet user’s
needs
• As the networks that might be interest exhibit so many
differences, it is impractical to consider merging them into
a single network
– Rather, what is needed is the ability to interconnect various
networks so that any 2 stations on any of the constituent networks
can communicate
Principles of Internetworking Contd.
• An interconnected set of networks, from a user’s point of
view, may appear simply a large network
– However, if each of the constituent networks retain its identity and
special mechanisms are for communicating across multiple networks,
then the entire configuration is often referred to as an internet
• Each constituent network in an internet supports
communication among the devices attached to the network
– These devices are referred to as end systems (ESs)
Principles of Internetworking Contd.
• In addition, networks are connected by devices referred to in
the ISO documents as intermediate nodes (ISs)
– ISs provide a communications path and perform the necessary relaying
and routing functions so that data can be exchanged between devices
attached to different networks in the internet
– Two types of ISs of particular interest are bridges and routers
• A bridge operates at layer 2 of the OSI 7 layer architecture and acts as a
relay of frames between similar networks
• A router operates at layer 3 of the OSI architecture and routes packets
between potentially different networks
Principles of Internetworking Contd.
• The overall requirements for an internetworking facility are:
– Provide a link between networks
• At minimum, a physical and link control connection is needed
– Provide for routing and delivery of data between processes on
different networks
– Provide an accounting service that keeps track of the use of various
networks and routers and maintains status information
– Provide the services just listed in such a way as not to require
modifications to the networking architecture of any of the constituent
networks
Principles of Internetworking Contd.
• This means that the internetworking facility must accommodate a
number of differences among networks:
– Different addressing schemes
» The networks may use different endpoint names and address and directory
maintenance schemes
» Some form of global network addressing must be provided, as well as a
directory service
– Different maximum packet size
» Packets from one network may have to be broken up into smaller pieces for
another; this process is referred to as fragmentation
– Different network access mechanisms
» The network access mechanism between station and network may be
different for stations on different networks
Principles of Internetworking Contd.
– Different timeouts
» Typically, a connection-oriented transport service will await an
acknowledgment until a timeout expires, at which it will retransmit
its block of data
» In general, longer times are required for successful delivery across
multiple networks
» Internetwork timing procedures must allow successful
transmission that avoids unnecessary retransmissions
– Error recovery
» Network procedures may provide anything from no error recovery
up to reliable end-to-end (within the network) service
Principles of Internetworking Contd.
» The internetwork service should not depend on nor be interfered
with by nature of the individual network’s error recovery capability
– Status reporting
» Different networks report status and performance differently
» It must be possible for the internetworking facility to provide such
information on internetworking activity to interested and authorised
processes
– Routing techniques
» Internetwork routing may depend on fault detection and
congestion control techniques peculiar to each network
» The internetworking facility must be able to coordinate these to
route data adaptively between stations on different networks
Principles of Internetworking Contd.
– User access control
» Each network will have its own user access control technique
» These must be invoked by the internetwork facility as needed
» Further, a separate internetwork access control technique may be
required
– Connection, connectionless
» Individual networks may provide connection-oriented or
connectionless service
» It may be desirable for the internetwork service not to depend on
the nature of the connection service of the individual network
Principles of Internetworking Contd.
• A key characteristic of an internet architecture is whether
the mode of operation is connection oriented or
connectionless
– Connection-oriented operation
• It is assumed that each network provides a connection-oriented
form of service
– That is, it is possible to establish a logical network connection between
any two end systems attached to the same network
• ISs are used to connect 2 or more networks
– Each IS appears as an ES to each of the network to which it is
attached
Principles of Internetworking Contd.
• When ES A wishes to exchange data with ES B, a logical
connection is set up between them
– This connection consists of the concatenation of a sequence of logical
connections across networks
• The individual network logical connections are spliced together by
ISs
– Any traffic arriving at an IS on one logical connection is retransmitted
on a second logical connection and vice versa
• A connection oriented IS performs the following key functions
– Relaying
» Data units arriving from one network via the network layer protocol
are relayed (retransmitted) on another network
Principles of Internetworking Contd.
– Routing
» When an end-to-end logical connection consisting of a sequence logical
connections, is to be set up, each IS in the sequence must make a routing
decision that determines the next hop in the sequence
• Thus, at layer 3, a relaying operation is performed
– It is assumed that all of the end systems share common protocols at layer 4
and above for successful end-to-end communication
– Connectionless Operation
• Connectionless-mode operation corresponds to the datagram mechanism
of a packet-switching network
• Each network protocol data unit is treated independently and routed from
source ES to destination ES through a series of routers and networks
Principles of Internetworking Contd.
• For each data unit transmitted by A, A makes a decision as to which
router should receive the data unit
• The data unit hops across the internet from one router to the next
until it reaches the destination network
– At each router a routing decision is made (independently for each data
unit) concerning the next hop
» Thus, different data units may travel different routes between
source and destination ES
• All Ess and routers share a common network-layer protocol known
generally as the internet protocol
• An Internet Protocol (IP) was initially developed for the DARPA
internet project and published as RFC 791 and has become an
Internet Standard
Connectionless Internetworking
• In this section we refer specifically to the Internet
Standard IP, but it applies any connectionless Internet
Protocol, such as IPv6
• IP provides connectionless, or datagram, service
between end systems
• The advantages of this approach are:
– Connectionless internet facility is flexible
• It can deal with a variety of networks, some of which are themselves
connectionless
• In essence, IP requires very little from the constituent networks
Connectionless Internetworking Contd.
– A connectionless internet service can be made highly robust
• This is basically the same argument made for a datagram network
service versus a virtual circuit service
– A connectionless internet service is best for connectionless
transport protocols, as it does not impose unnecessary overhead
Connectionless
Internetworking
Contd.
Connectionless Internetworking Contd.
• The figure in the previous slide depicts a typical example
using IP, in which two LANs are interconnected by a frame
relay WAN
• End System A has a datagram to transmit to end system B
– The datagram includes the internet address of B
• The IP module in A recognises that the destination B is on
another network
– So the first step is to send the data to a router, in this case router X
Connectionless Internetworking Contd.
• To send data to router X, IP passes the datagram down to
the next lower layer ( in this case LLC) with instruction to
send it to router X
• LLC in turn passes this information down to MAC layer,
which inserts the MAC-level address of router X into the
MAC header
• When the packet reaches router X, the router removes MAC
and LLC fields and analyse the IP header to determine the
ultimate destination of the data – in this case B
Connectionless Internetworking Contd.
• The router must now make a routing decision; there are
3 possibilities
– The destination station B is connected directly to one of the
networks to which the router is attached
• If so, the router sends the datagram directly to the destination
– To reach the destination, one or more additional routers must be
traversed
• If so, a routing decision must be made: to which router the datagram
must be sent?
Connectionless Internetworking Contd.
• In both above cases, the IP module in the router sends the
datagram down to the next lower layer with the destination network
address
– The router does not know the destination address
• Router returns an error message to the source of the datagram
• In the above example, the data must pass through router
Y before reaching the destination
– So the router X constructs a new frame by appending a frame
relay header and trailer to the IP data unit
– The frame relay header indicates a logical connection to router Y
Connectionless Internetworking Contd.
• When the frame arrives at router Y, the frame header
and the trailer are stripped off
– The router determines that this IP data unit is destined for B,
which is connected directly to a network to which this router is
attached
– The router therefore creates a frame with layer-2 destination
address of B and sends it out onto LAN 2
• The data finally arrive at B, where the LAN and IP
headers can be stripped off
Connectionless Internetworking Contd.
• At each router, before the data can be forwarded, the
router may need to fragment the data unit
– This is done to accommodate a smaller maximum packet size
limitation on the outgoing network
• The data units split into two or more fragments, each of
which becomes an independent IP data unit
• Each new data unit is wrapped in a lower-layer packet
and queued for transmission
Connectionless Internetworking Contd.
• The process described above continues through as
many routers as it takes for the data unit to reach its
destination
• As with routers, the destination end systems recovers
the IP data unit from its network wrapping
• If fragmentation has occurred, the IP module in the
destination end system buffers the incoming data until
the entire original data field can be reassembled
Connectionless Internetworking Contd.
• The service offered by IP is an unreliable one
– That is, IP does not guarantee that all data will be delivered or
that the data that are delivered will arrive in the proper order
• It is the responsibility of the next higher layer (e.g., TCP) to recover
from any errors that occur
– This approach provides a great deal of flexibility
• As the sequence of delivery is not guaranteed,
successive data units can follow different paths through
the internet
– This allows the protocol to react to both congestion and failure in
the internet by changing routes
Internet Protocol
• In this section, we will look at version 4 of IP, officially
defined in RFC 791
• Although it is intended that IPv4 will eventually be
replaced by IPv6, it is currently the standard IP used in
TCP/IP networks
• As with any protocol standard, IP is specified in two
parts:
– The interface with higher layer (e.g., TCP), specifying the
services that IP provides
– The actual protocol format and mechanisms
Internet Protocol Contd.
• The services to be provided across adjacent protocol
layers (e.g., IP and TCP) are expressed in terms pf
primitives and parameters
– A primitive specifies the function to be performed
• The actual form of a primitive is implementation dependent
• An example is a subroutine call
– Parameters are used to pass data and control information
• IP provides two service primitives at the interface to the
interface to the next higher layer
Internet Protocol Contd.
– The send primitive is used to request transmission of a data unit
– The delivery primitive is used by IP to notify a user of the arrival
of data unit
• The parameters associated with the two primitives are as
follows:
– Source address
– Destination address
– Protocol
• Recipient protocol entity (such as TCP)
Internet Protocol Contd.
– Type of service indicators
• Used to specify the treatment of the data unit in its transmission
through component networks
– Identification
• Used in combination with the source and destination addresses and
user protocol to identify the data unit uniquely
• This parameter is required for reassembly and error reporting
– Don’t fragment identifier
– Time to live
Internet Protocol Contd.
– Data length
– Option data
– Data
• The identification, don’t fragment identifier, and time to
live parameters are present in the Send primitive but not
in the Deliver primitive
– These 3 parameters provide instructions to IP that are not of
concern to the recipient IP user
Internet Protocol Contd.
• The options parameter allows for future extensibility and
inclusion of parameters that are usually not invoked
– The currently defined options are
• Security
– Allow a security label to be attached to a datagram
• Source routing
– A sequenced list of router addresses that specifies the route to be
followed
• Route recording
• Stream identification
• Timestamping
Internet Protocol Contd.
Internet Protocol Contd.
• The protocol between IP entities is best described with
reference to IP datagram format, shown in the previous
slide
• The fields are:
– Version
• Indicates version number, to allow evolution of the protocol; the
value is 4
– Internet Header Length (IHL)
• The length of header in 32-bit words
• The minimum value is 5, for minimum header length of 20 octets
Internet Protocol Contd.
– Type of Service
• Specifies reliability, precedence, delay, and throughput parameters
• This field is rarely used
– Total length
• Total datagram length, in octets
– Identification
• A sequence number that, together with the source address,
destination address, and user protocol, is intended to identify a
datagram uniquely
• Thus this number should be unique for the datagram’s source
address, destination address, and user protocol for the time during
which the datagram will remain in the internet
Internet Protocol Contd.
– Flags
• Only 2 bits are currently used
– The more bit is used for fragmentation and reassembly
– The Don’t fragment bit prohibits fragmentation when set
– Fragment Offset
• Indicates where in the original datagram this fragment belongs,
measured in 64-bit units
• This implies that fragments other than the last fragment must
contain data field that is a multiple of 64 bits in length
– Time to Live
• Specifies how long, in seconds, a datagram is allowed to remain in
the internet
Internet Protocol Contd.
– Protocol
• Indicates the next higher level protocol that is to receive the data
field at the destination
– Header Checksum
• An error-detecting code applied to the header only
• Because some header fields may change during transit, this is
reverified and recalculated at each router
– Source Address
– Destination Address
– Options
Internet Protocol Contd.
– Padding
• Used to ensure that the datagram header is a multiple of 32 bits in
length
– Data
• Must be an integer multiple of 8 bits in length
• The maximum length of that datagram is 65,535 octets
• The source and destination address fields in the IP header
each contain a 32-bit global internet address, generally
consisting of a network identifier and a host identifier
• The address is coded to allow a variable allocation of bits
to specify network and host, as shown in the next slide
Internet Protocol Contd.
Internet Protocol Contd.
• This encoding provides flexibility in assigning addresses to hosts
and allows a mix of network sizes on an internet
• The 3 principal network classes are best suited to the following
conditions:
– Class A
• Few networks, each with many hosts
– Class B
• Medium number of networks, each with a medium number of hosts
– Class C
• Many networks, each with a few hosts
Internet Protocol Contd.
• A mixture of classes is appropriate for an internetwork
consisting of a few large networks, many small networks,
plus some medium-sized networks
• IP addresses are usually written in what is called dotted
decimal notation, with a decimal number representing
each of the octets of 32-bit address
– For example, the IP address 11000000 11100100 00010001
00111001 is written as 192.228.17.57
• All class A network addresses begin with a binary 0
Internet Protocol Contd.
• Network addresses with a first octet of 0 (00000000) and
127 (01111111) are reserved
– So there are 126 potential Class A network numbers, which have
a first decimal number in the range 1 to 126
• Class B network addresses begin with a binary 10
– So the range of first decimal numbers in a class B address is
128 to 191 (binary 10000000 to 10111111)
– The 2nd octet is also part of the Class B address
• So there are 214 = 16,384 Class B addresses
Internet Protocol Contd.
• For Class addresses, the first decimal number ranges
from 192 to 223 (11000000 to 11011111)
• The total number of Class C addresses is 221= 2,097,152
Routing
in
Switched Networks
Reference:
Chapter 12 -Stallings
Introduction
• A key design issue in switched networks is that of routing
– These networks include circuit switching, packet switching,
frame relay, and ATM networks
• In general terms, the routing function seeks to design
routes through the network for individual pairs of
communicating end nodes such that network is used
efficiently
Routing in Circuit Switching
Networks
• In a large circuit-switching network, many of the circuit
connections will require a path through more than one
switch
• When a call is placed, the network must devise a route
through the network from calling subscriber to called
subscriber
– This route passes through some number of switches and trunks
• There are 2 main requirements for the network’s
architecture that bear on the routing strategy:
Routing in Circuit Switching
Networks Contd.
– Efficiency
• It is desirable to minimise the amount of equipment (switches and
trunks) in the network, subject to the ability to handle that amount of
load
• The load requirement is usually expressed in terms of a busy-hour
traffic load
– This is simply the average load expected over the course of the busiest
hour of use during the course of a day
• From a functional point of view, it is necessary to handle that
amount of load
• From a cost point of view, we would like to handle that load with
minimum
Routing in Circuit Switching
Networks Contd.
– Resilience
• Although a network may be sized for the busy hour load, it is
possible for the traffic to surge temporarily above that level (during a
major storm)
• It will also be the case that, from time to time, switches and trunks
will fail and be temporarily unavailable
• We would like the network to provide a reasonable level of service
under such conditions
• The key design issue that determines the nature of the
tradeoff between efficiency and resilience is the routing
strategy
Routing in Circuit Switching
Networks Contd.
• Traditionally, the routing function in public
telecommunications networks has been quite simple
– In essence, the switches of a network were organised into a tree
structure, or hierarchy.
– A path is constructed by starting at the calling subscriber, tracing up
the tree to the first common node, and then tracing down the tree to
the called subscriber
– To add some resilience to the network, additional high-usage trunks
were added that cut across the tree structure to connect exchanges
with high volumes of traffic between them
Routing in Circuit Switching
Networks Contd.
• In general, the above mentioned is a static approach
– As the routing scheme is not able to adapt to changing
conditions, the network must be designed to meet some typical
heavy demands
• It is difficult to analyse varying demands, which leads to
oversizing and therefore inefficiency
• In terms of resilience, the fixed hierarchical structure with
supplemental trunks any respond poorly to failures
Routing in Circuit Switching
Networks Contd.
• To cope with the growing demand on public
telecommunications networks, virtually all providers have
moved away from static hierarchical approach to a
dynamic approach
• A dynamic routing approach is one in which routing
decisions are influenced by current traffic conditions
• Typically, the circuit switching nodes have a peer
relationship with each other rather than a hierarchical
one
Routing in Circuit Switching
Networks
Contd.
– All nodes are capable of performing the same function
• In such an architecture, routing is both more complex
and more flexible
– It is more complex because the architecture does not provide a
‘natural’ path or set of paths based on hierarchical structure
– It is more flexible because more alternative routes are available
• A form of routing used in circuit-switching networks is
known as alternate routing
– In this routing scheme, the possible routes to be used between
two end offices are predefined
Routing in Circuit Switching
Networks
Contd.
– It is the responsibility
of the originating
switch to select the
appropriate route for each call
– Each switch is given a set of preplanned routes for each
destination, in order of preference
• If a direct trunk connection exists between two switches, this is
usually the preferred choice
• If this trunk is unavailable, then the 2nd choice is to be tried, and so
on
– The routing sequences reflect an analysis based on historical
traffic patterns and designed to optimise the use of network
resources
Routing in Circuit Switching
Networks Contd.
Routing in Packet Switching
Networks
• The primary function of a packet-switching network is to
accept packets from a source station and deliver them to
a destination station
• To accomplish this, a path or route through the network
must be determined
– Generally more than one route is possible
• Thus, a routing function must be performed
• The requirements for this function include
– Correctness, simplicity, fairness, optimality, robustness, stability, and
efficiency
Routing in Packet Switching
Networks Contd.
» Robustness has to do with the ability of the network to deliver
packets via some route in the face of localised failures and
overloads
» The designer who seeks robustness must cope with competing
requirements for stability
» A tradeoff also exists between fairness and optimality;
» Some performance criteria may give higher priority to the exchange
of packets between nearby stations compared to an exchange
between distant stations
» Routing techniques involves some processing overhead at each
node and often a transmission overhead as well, both of which
impair network efficiency
Routing in Packet Switching
Networks Contd.
• Various design elements that contribute to a routing
strategy are:
– Performance Criteria
• The selection of a route is generally based on some performance
criterion
– The simplest criterion is to choose the minimum-hop route through the
network
– A generalisation of the minimum-hop criterion is least-cost routing
• In either the minimum-hop or least-cost approach, the algorithm for
determining the optimum route for any pair of stations is relatively
straightforward, and the processing time would be about the same for
either computation
Routing in Packet Switching
Networks Contd.
Routing in Packet Switching
Networks Contd.
– Decision Time and Place
• Two key characteristics of a routing decision are the time and place
that the decision is made
• Decision time is determined by whether the routing decision is made
on a packet or virtual circuit basis
– When the internal operation of the network is datagram, a routing
decision is made individually for each packet
– For internal virtual circuit operation, a routing decision is made at the
time the virtual circuit is established
» In the simplest case, all subsequent packets using that virtual circuit
follow the same route
Routing in Packet Switching
Networks Contd.
– The term decision place refers to which node or nodes in the network
are responsible for the routing decision
» Most common is distributed routing, in which each node has the
responsibility of selecting an output link for routing packets as they
arrive
» For centralised routing, the decision is made by some designated
node, such as a network control centre
» A third alternative, used in some networks, is source routing
• The decision time and decison place are independent design
variables
Routing in Packet Switching
Networks Contd.
– Network Information Source and Update Timing
• Most routing strategies require that decisions be based on
knowledge of the topology of the network, traffic load, and link cost
• However, some strategies use no such information and yet manage
to get packets through
• With distributed routing, in which the routing decision is made by
each node, the individual node may make use of only local
information from adjacent nodes, such as the amount of congestion
experienced at that node
• In the case of centralised routing, the central node typically makes
use of information obtained from all nodes
Routing in Packet Switching
Networks Contd.
• The concept of information update timing is a function of both the
information source and the routing strategy
• If no information is used, there is no information to update
• If only local information is used, the update is essentially continuous
– That is, an individual node always knows its local condition
• For all other information source categories, update timing depends
on the routing strategy
– For a fixed strategy, the information is never updated
– For an adaptive strategy, information is updated from time to time to
enable the routing decision to adapt to changing conditions
Routing in Packet Switching
Networks Contd.
• A large number of routing strategies have evolved for
dealing with the routing requirements of packet-switching
networks
• Many of these strategies are also applied to internetwork
routing
• Four key routing strategies are:
– Fixed Routing
• A single permanent route is configured for each source-destination
pair of nodes in the network
Routing in Packet Switching
Networks Contd.
• The routes are fixed, or at least only change when there is a change
in the topology of the network
– Thus, the link costs used in designing routes cannot be based on any
dynamic variable such as traffic
– They could, however, be based on expected traffic or capacity
• Fixed routing can be implemented using a central routing matrix, to
be stored perhaps at a network control centre
– In a routing matrix, it is not necessary to store the complete route for
each possible pair of nodes
» Rather, it is sufficient to know, for each par of nodes, the identity
of the first node on the route
Routing in Packet Switching
Networks Contd.
Routing in Packet Switching
Networks Contd.
• From the above routing matrix, routing tables can be developed and
stored at each node
– Each node needs only t store a single column of the routing directory
» A node’s directory shows the next node to take for each destination
• With fixed routing, there is no difference between routing for datagrams
and virtual circuits
– All packets from a given source to a given destination follow the same route
• The advantage of fixed routing is its simplicity, and it should work well in a
reliable network with stable load
• Its disadvantage is its lack of flexibility – does not react to network
congestion or failures
Routing in Packet Switching
Networks
Contd.
– Flooding
• A packet is sent by a source node to every one of its neighbours
• At each node, an incoming packet is retransmitted on all outgoing links
except for the link on which it arrived
• Eventually, a number of copies of the packets will arrive at the destination
• The packet must have a some unique identifier so that the destination
node knows to discard all but the first copy
• This technique requires no network information whatsoever
• Unless something is done to stop the incessant retransmission of packets,
the number of packets in circulation grows without bound
Routing in Packet Switching
Networks Contd.
• One way to prevent the above situation is for each node to remember
the identity of those packets it has already transmitted
– When a duplicate copies arrive they are discarded
• A simpler technique is to include a hop count field with each packet
– The count originally be set to some maximum value, such as the diameter
of the network
– Each time a node passes on a packet, it decrements the count by one
» When the count reaches zero, the packet is discarded
• The flooding technique has 3 remarkable properties:
Routing in Packet Switching
Networks Contd.
– All possible routes between source and destination are tried
» If at least one path between source and destination exists, a
packet will always get through
– Because all routes are tried, at least one copy of the packet arrive at
the destination will have used a minimum-hop route
– All nodes are directly or indirectly connected to the source node are
visited
• Because the flooding technique is highly robust, it could be used to
send emergency messages
• Flooding can also be useful for the dissemination of important
information to all nodes
• The principal disadvantage of flooding is the high traffic load that it
generates, which is directly proportional to the connectivity of the
network
Routing in Packet Switching
Networks Contd.
Routing in Packet Switching
Networks Contd.
– Random Routing
• A node selects only one outgoing path for retransmission of an
incoming packet
• The outgoing link is chosen at random, excluding the link on which
the packet arrived
• If all links are likely to be chosen, then a node may simply utilise
outgoing links in a round-robin fashion
• Random routing has the simplicity and robustness of flooding, with
far less traffic load
Routing in Packet Switching
Networks Contd.
• A refinement of the above technique is to assign a probability to
each outgoing link and to select the link based on that probability
– The probability could be based on data rate, in which case we have
Pi 
Ri
 Ri
Where Pi = probability of selecting link I
Ri = data rate on link I
• Like flooding, random routing requires the use of no network
information
• Because the route taken is random, the actual route will typically not
be the least cost route nor the minimum-hop route
Routing in Packet Switching
Networks Contd.
– Adaptive Routing
• The routing decisions that are made change as conditions on the
network change
– The conditions that influence routing decisions are:
» Failure
» Congestion
• In virtually all packet-switching networks, some sort of adaptive
routing techniques is used
• For adaptive routing to be possible, information about the state of
the network must be exchanged among the nodes
Routing in Packet Switching
Networks Contd.
• The drawbacks associated with adaptive routing, compared to fixed
routing are:
– The routing decisions are more complex
» Therefore the processing burden on the network nodes increases
– In most cases, adaptive strategies depend on status information that is
collected at on place but used at another
» There is a tradeoff between the quality of the information
exchanged and the amount of the overhead
» The more information that is exchanged, and the more frequently
it is exchanged, the better will be the routing decisions
» On the other hand, this information is itself a load on the
constituent networks, causing a performance degradation
Routing in Packet Switching
Networks Contd.
– An adaptive strategy may react too quickly, causing congestionproducing oscillations, or too slowly, being irrelevant
• Despite the above drawbacks, adaptive routing strategies are by far
the most prevalent, for 2 reasons:
–
An adaptive routing strategy can improve performance, as
the network user
seen by
– An adaptive routing strategy can aid in congestion control
» As it tends to balance loads, adaptive routing can delay onset of
severe congestion
• By and large, adaptive routing is an extraordinarily complex task to
perform properly
Routing in Packet Switching
Networks Contd.
• A convenient way to classify adaptive routing strategies is on the basis of
information source:
– Local
» A node routes each packet to the outgoing link with the shortest queue length
» This would have the effect of balancing the load on outgoing links; however, some
outgoing links may not be headed in the correct general direction
– Adjacent nodes
– All nodes
» Both strategies, adjacent and all nodes, are commonly used
» They take the advantage of information that each node has about delays
and outages
» Such adaptive strategies can be either distributed or centralised
Routing in Packet Switching
Networks Contd.
• First Generation Routing
– The original routing algorithm, designed in a969, was a distributed
adaptive algorithm using delay as the performance criterion
– For this algorithm, each node maintains two vectors:
d i1 
d 
 i2 
Di  . 
 
. 
d iN 
 i
 si1 
s 
 i2 
S i  . 
 
. 
 siN 
 
– Where Di = delay vector for node I
dij = current estimate of minimum delay from node I to node j
N = Number of nodes in the network
Si = successor node vector for node I
sij = the next node in the current minimum-delay route from I to j
Routing in Packet Switching
Networks Contd.
– Periodically (every 128ms), each node exchanges its delay vector with
all of its neighbours
– On the basis of all incoming delay vectors , a node k updates both of its
vectors as follows:
d kj 
skj = i
–

min
d ij  lki
i A

using i that minimises the preceding expression
Where A = set of neighbour nodes for k
lki = current estimate of delay from k to i
Routing in Packet Switching
Networks Contd.
– The estimated link delay is simply the queue length for that link
• Thus, in building a new routing table, the node will tend to favour outgoing
links with shorter queues
• This tends to balance the load on outgoing links
– However, as the queue lengths vary rapidly with time, the distributed perception of
the shortest route could change while a packet is en route
– This could lead to a thrashing situation in which a packet continues to seek out
areas of low congestion rather than aiming at the destination
– The major shortcomings of the above algorithm were:
• It did not consider line speed, merely queue lengths
– Thus higher-capacity links were not given the favoured status they deserved
Routing in Packet Switching
Networks Contd.
– Queue length is , in any case, an artificial measure of delay, because
some variable amount of processing time elapses between the arrival of a
packet at a node ad its placement in an outbound queue
– The algorithm was not very accurate
» In particular, it responded slowly to congestion and delay increases
• Second Generation Routing
– The new algorithm was also a distributed one, using delay as the
performance criterion, but the difference was significant
• Rather than using queue length as a surrogate for delay, the delay
was measured directly
Routing in Packet Switching
Networks Contd.
– At a node, each incoming packet was timestamped with an arrival time; a
departure time was recorded when the packet was transmitted
» If a positive acknowledgment is returned, the delay for the packet was recorded as
the departure time minus the arrival time plus transmission time and propagation
delay
» The node must therefore know the link data rate and propagation time
» If a negative acknowledgement comes back, the departure time is updated and
the node tries again, until a measure of successful transmission delay is obtained
– Every 10 seconds, the node computes the average delay on each outgoing
link
» If there are any significant changes in delay, the information is sent to all other
nodes using flooding
» Each node maintains an estimate of delay on every network link; when information
arrives, it recomputes its routing table
Routing in Packet Switching
Networks Contd.
– Experience with this second strategy indicated that it was more
responsive and stable than the previous one
– However, as the load on the network grew, a shortcoming in the
new strategy began to appear, and it was revised in 1987
– The problem was the assumption that the measured packet delay
on a link is a good predictor of the link delay encountered after all
nodes reroute their traffic based on this reported delay
• Thus, it is an effective routing mechanism only if there is some
correlation between the reported values and those actually
experienced after re-routing
• This correlation tends to be rather high under light and moderate
traffic loads, but there is little correlation under heavy loads
Routing in Packet Switching
Networks
Contd.
• Therefore, immediately after all nodes have made routing updates,
the routing tables are obsolete
• The ARPANET designers concluded that the essence of the
problem was that every node was trying to obtain the best route for
all destinations, and these efforts conflicted
• It was concluded that under heavy loads, the goal of routing should
be to give the average route a good path instead of attempting to
give all routes the best path
• The designers decided that it was unnecessary to change the
overall routing algorithm
– Rather, it was sufficient to change the function that calculates link costs
Routing in Packet Switching
Networks Contd.
• The calculation begins with measuring the average delay over the last 10
seconds
• The value is then transformed with the following steps:
– Using a simple single server queueing model, the measured delay is transformed
into an estimate of link utilisation
» From queueing theory, utilisation can be expressed as a function of a delay as
follows:

2(Ts  T )
Ts  2T
Where ρ = link utilisation
T = measured delay
Ts= service time
» The service time was set at network-wide average packet size (600bits)
divided by the data rate of the link
Routing in Packet Switching
Networks Contd.
– The results was then smoothed by averaging it with the previous
estimate of utilisation
U(n+1) = 0.5 * ρ(n+1) + 0.5* U(n)
Where U(n) = average utilisation calculated at sampling time n
ρ(n) = link utilisation measured at sampling time
» Averaging increases the period of routing oscillations, thus
reducing routing overhead
– The link cost is then set as a function of average utilisation that is
designated to provide a reasonable estimate of cost while avoiding
oscillations
Wide Area Networks
(WANs)
Reference:
Chapter 10 -Stallings
Introduction
• Traditional approaches to wide area network design are
circuit-switching and packet switching
• Since the invention of the telephone, circuit switching has
been the dominant technology for voice communications,
and has remained so well into the digital era
• Around 1970, research began on a new form of
architecture for long-distance digital data communications
known as packet switching
– Although the technology of packet switching has evolved
substantially, it is remarkable that:
Introduction Contd.
• The basic technology of packet switching is fundamentally the same
today as it was in early stages:
• Packet-switching remains one of the few effective technologies for
long-distance data communications
– Many advantages of packet-switching, such as flexibility, resource
sharing, robustness, and responsiveness, come with a cost
• The packet-switching network is a distributed collection of packetswitching nodes
• Ideally, all packet-switching nodes would always know the state of the
entire network
Introduction Contd.
• Unfortunately, there is a time delay between a change in status in
one portion of the network and the knowledge of that change
elsewhere
• Further, there is overhead involved in communicating status
information
• As a result, a packet-switching network can never perform
“perfectly”, and elaborate algorithms are used to cope with the time
delay and overhead penalties of network operation
Switched Communications
Networks
• For transmission of data beyond a local area, communication
is typically achieved by transmitting data from source to
destination through a network of intermediate switching nodes
• The switching nodes are not concerned with the content of the
data
– Rather, their purpose is to provide a switching facility that will move the
data from node to node until they reach their destination
– The end devices that wish to communicate may be referred to as
stations
– The switching nodes whose purpose is to provide communication are
referred to as nodes
Switched Communications
Networks Contd.
• Each station attaches to a node, and the collection of
nodes is referred to as a communication network
• The types of networks that are discussed in this lecture are
referred to as switched communication networks
– Data entering the network from a station are routed to the
destination by being switched from node to node
• In switched communication networks, some nodes connect
only to other nodes
– Their sole task is the internal (to the network) switching of data
Switched Communications
Networks Contd.
– Other nodes have one or more stations attached as well
• In addition to their switching functions, such nodes accept data from
and deliver data to the attached stations
– Node-node links are usually multiplexed, using either frequency
division multiplexing (FDM) or time division multiplexing (TDM)
– Usually, the network is not fully connected; that is , there is not a
direct link between every possible pair of nodes
• However, it is always desirable to have more than one possible path
through the network for each pair of stations
Switched Communications
Networks Contd.
Circuit Switching Networks
• Communication via circuit switching implies that there is
a dedicated communication path between two stations
– That path is a connected sequence of links between network
nodes
– On each physical link, a logical channel is dedicated to a
connection
– Communication via circuit switching involves 3 phases:
• Circuit establishment
– Before any signals can be transmitted, an end-to-end (station-tostation) circuit must be established
Circuit Switching Networks
Contd.
• Data transfer
– Information can be transferred from the source to destination, once a
connection is established
– The data may be analog or digital, depending on the nature of the
network
– Generally the connection is full duplex
• Circuit disconnect
– After some period of data transfer, the connection is terminated, usually
by the action of one of the two stations
– Signals must be propagated through the path to deallocate resources
Circuit Switching Networks
Contd.
• In circuit switching, the switches must have intelligence
to make resource allocations and to devise a route
through the network
• Circuit switching can be rather inefficient
– Channel capacity is dedicated for the duration of a connection,
even if no data are being transferred
– For a voice connection, utilisation may be rather high, but still
does not approach 100%
– For a terminal-to-computer connection, the capacity may be idle
during most of the time of the connection
Circuit Switching Networks
Contd.
– In terms of performance, there is a delay prior to signal transfer for
call establishment
• However, once the circuit is established, the network is effectively
transparent to the users
– Information is transmitted at a fixed data rate with no delay other
than the propagation delay through the transmission link
– The delay at each node is negligible
• Circuit switching was developed to handle voice traffic but
is now also used for data traffic
Circuit Switching Networks
Contd.
– The best-known example of a circuit-switching network is the
public telephone network
• This is actually a collection of national networks interconnected to
form the international service
• Although originally designed and implemented to service analog
telephone subscribers, it handles substantial data traffic via modem
and is gradually being converted to a digital network
– Another well-known application of circuit switching is the private
branch exchange (PBS), used to connect telephones within a
building or office
– Circuit-switching is also used in private networks
Circuit Switching Networks
Contd.
• A public telecommunications network can be described
using four generic architectural components:
– Subscribers
• The devices that attach to the network
• It is still the case that most subscriber devices to public
communications networks are telephones
– But the percentage of data traffic increases year by year
– Subscriber line
• The link between the subscriber and the network, also referred to as
the subscriber loop or local loop
• Almost all local loop connections use twisted-pair wire
• The length of a local loop is typically in a range from a few kilometres
to a few tens of kilometres
Circuit Switching Networks
Contd.
– Exchanges
• The switching centres in the network
– A switching centre that directly supports subscribers is known as an
end office
» Typically, an end office will support many thousands of subscribers
in a localised area
– In addition, intermediate switching nodes are used
– Trunks
• The branches between exchanges
• Trunks carry multiple voice frequency circuits using either FDM or
synchronous TDM
• Earlier these were referred to as carrier systems
Circuit Switching Networks
Contd.
Circuit Switching Networks
Contd.
Circuit-Switching Concepts
• A network built around a single circuit-switching node
consists of a collection of stations attached to a central
switching unit
– The central switch establishes a dedicated path between any
two devices that wish to communicate
• The heart of a modern system is digital switch
– The function of the digital switch is to provide a transparent
signal path between any pair of attached devices
– The path is transparent in that it appears to the attached pair of
devices that there is a direct connection between them
Circuit-Switching Concepts
Contd.
Circuit-Switching Concepts
Contd.
– The network interface element represents the functions and hardware
needed to connect digital devices, such as data processing devices
and digital telephones, to the network
– Analog telephones can also be attached if the network interface
contains the logic for converting to digital signals
– Trunks to other digital switches carry TDM signals and provide the
links for constructing multiple-node networks
– The control unit performs 3 general tasks:
• First, it establishes connections
– This is generally done on demand, that is, at request of an attached device
Circuit-Switching Concepts
Contd.
– To establish the connection, the control unit must handle and acknowledge the
request, determine if the intended destination is free, and construct a a path
through the switch
• Second, The control unit must maintain the connection
– Because the digital switch uses time division principles, this may require
ongoing manipulation of the switching elements
– However, the bits of communication are transferred transparently
• Third, the control unit must tear down he connection, either in response to
a request from one of the parties or for its own reasons
• An important characteristic of a circuit-switching device is
whether it is blocking or nonblocking
Circuit-Switching Concepts
Contd.
– Blocking occurs when the network is unable to connect two stations
because all possible paths between them are already in use
• A blocking network is one in which such blocking is possible
– A nonblocking network permits all stations to be connected (in pairs)
at once and grant all possible connection requests as long as the
called party is free
– When a network is supporting only voice traffic, a blocking
configuration is generally acceptable, because it is expected that
most phone calls are of short duration and that therefore only a
fraction of the telephones will be engaged at any time
Circuit-Switching Concepts
Contd.
– However, when data processing devices are involved, these
assumptions may be invalid
• For example, for a data entry application, a terminal may be
continuously connected to a computer for hours at a time
• Hence, for a data applications, there is a requirement for a
nonblocking or nearly nonblocking configuration
• One of the switching techniques internal to a single cirswitching node is space division switching
– It was originally developed for the analog environment and has
been carried over into the digital realm
– As the name implies, a space division switch is one which the
signal paths are physically separate from one another
Circuit-Switching Concepts
Contd.
– Each connection requires the establishment of a physical path
through the switch that is dedicated solely to transfer of signals
between the two end points
– The basic building block of the switch is a metallic cross-point or
semiconductor gate that can be enabled and disabled by a
control unit
– The crossbar switch has a number of limitations:
• The number of crosspoints grows with the square of the number of
attached stations
– This is costly for a large switch
Circuit-Switching Concepts
Contd.
Circuit-Switching Concepts
Contd.
• The loss of a crosspoint prevents connection between the two
devices whose lines intersect at that crosspoint
• The crosspoints are inefficiently utilised;
– even when all of the attached devices are active, only a small fraction of
the crosspoints are engaged
– To overcome these limitations, multiple-stage switches are
employed
• This type of arrangement has two advantages over a single-stage
crossbar matrix
– The number of crosspoints is reduced; in the example, the total number
of crosspoints for 10 stations is reduced from 100 to 48
Circuit-Switching Concepts
Contd.
Circuit-Switching Concepts
Contd.
– There is more than one path through the network to connect two
endpoints, increasing reliability
• However, a multistage network requires a more complex control
scheme
• Another consideration with a multistage space division switch is that
it may be blocking
– A single-stage crossbar matrix is nonblocking; that is a path is always
available to connect an input to an output
• With the advent of digitised voice and synchronous time
division multiplexing techniques, both voice and data
can be transmitted via digital signals
Circuit-Switching Concepts
Contd.
– This has led to a fundamental change in the design and
technology of switching systems
– Instead of relatively dumb space division systems, modern digital
systems rely on intelligent control of space – and time division
systems
– Virtually all modern circuit switches use digital time division
techniques for establishing and maintaining circuits
– Time division switching involves the partitioning of a lowerspeed bit stream into pieces that share a higher-speed stream
with other bit streams
Packet-Switching Principals
• When circuit switching networks began to be used
increasingly for data connections, two shortcomings became
apparent:
– In typical user/host data connection, much of the time the line is idle
• Thus, with the data connections, a circuit-switching approach is inefficient
– In a circuit-switching network, the connection provides for transmission
at a constant data rate
• Thus, each of the two devices that are connected must transmit and
receive at the same data rate as the other
– This limits the utility of the network in interconnecting a variety of host
computers and workstations
Packet-Switching Principals
• In packet switching, data are transmitted in short packets
– A typical upper bound on packet length is 1000 octets
• If a source has a longer message to send, the message is
broken up into a series of packets
• Each packet contains a portion (or all for a short message) of
the user’s data plus some control information
• The control information, at a minimum, includes the information
that the network requires to be able to route the packet through
the network and deliver it to the intended destination
Packet-Switching Principals
Contd.
Packet-Switching Principals
Contd.
• At each node en route, a packet is received, stored briefly, and
passed on to the next node
• The packet-switching approach has a number of advantages over
circuit-switching:
– Line efficiency is greater, because a single node-to-node link can be
dynamically shared by many packets over time
• The packets are queued up and transmitted as rapidly as possible over the
link
– By contrast, with circuit switching, time on a node-to-node link is preallocated using
synchronous time division multiplexing
Packet-Switching Principals
Contd.
– A packet-switching network can perform data-rate conversion
• Two stations of different data rates can exchange packets because
each connects to its node at its proper data rate
– When traffic becomes heavy on a circuit-switching network,
some calls are blocked
• On a packet-switching network, packets are still accepted, but
delivery delay increases
– Priorities can be used
• If a node has a number of packets queued for transmission, it can
transmit the higher-priority packets first
Packet-Switching Principals
Contd.
• A network uses two approaches to handle a stream of
packets as it attempts to route them through the network
and deliver them to the intended destination
– Datagram
• Each packet is treated independently, with no reference to packets
that have gone before
• Each node chooses the next node on a packet’s path, taking into
account information received from neighbouring nodes on traffic, line
failures, and so on
Packet-Switching Principals
Contd.
• So the packets, each with the same destination address, do not all follow
the same route, and they may arrive out of sequence at the exit point
– It is up to the exit node or the destination to restore the packets to original
order
– Further, it is up to the exit node or destination to detect the loss of a packet
and decide how to recover it
– Virtual circuits
• A preplanned route is established before any packets are sent
• Once the route is established, all the packets between a pair of
communicating parties follow this same route through the network
Packet-Switching Principals
Contd.
• Because the route is fixed for the duration of the logical connection, it
is somewhat similar to a circuit in a circuit-switching network and is
referred to as a virtual circuit
– This does not mean that there is a dedicated path, as in circuit switching
» A packet is still buffered at each node, and queued for out put over
a line, while other packets on other virtual circuits may share the
use of the line
• Each packet contains a virtual circuit identifier as well as data
– Each node on the preestablished route knows where to direct such
packets; no routing decisions are required
• At any time, each station can have more than one virtual circuit to
any other station and can have virtual circuits to more than one
station
Packet-Switching Principals
Contd.
Packet-Switching Principals
Contd.
Comparison of Circuit Switching
and Packet Switching
• When a comparison of performance between the two
types is done, we are concerned with 3 types of delay:
– Propagation delay
• The time it takes a signal to propagate from one node to the next
• This time is generally negligible
– Transmission time
• The time it takes for a transmitter to send out a block of data
• For example, it takes 1s to transmit a 10,000-bit block of data onto a
10-kbps line
Comparison of Circuit Switching
and Packet Switching
– Node delay
• The time it takes for a node to perform necessary processing as it
switches data
• In circuit switching, once a connection is established, a
constant data rate is provided to the connected stations
• In the case of packet switching, a variable delay is
introduced and packets arrive in a choppy manner
• For packet switching, analog data must be converted to
digital before transmission
Comparison of Circuit Switching
and Packet Switching
Asynchronous Transfer Mode
(ATM)
Reference:
Chapter 11 -Stallings
Introduction
• ATM is the transmission technology that is the foundation
of broadband ISDN (Integrated Services Digital Network)
• ATM is also finding widespread application beyond its use
as part of ISDN
• ATM is, in essence, a packet switching technology, but is
far more streamlined and efficient than traditional packet
switching
– It is designed to support very high data rates
Protocol Architecture
• Asynchronous Transfer Mode (ATM), also known as cell
relay, takes advantage of the reliability and fidelity of modern
digital facilities to provide faster packet switching than X.25
• Like packet switching and frame relay, ATM involves the
transfer of data in discrete chunks
• Also like packet switching and frame relay, ATM allows
multiple logical connections to be multiplexed over a single
physical interface
• In ATM, the information on each logical connection is
organised into fixed-size packets, called cells
Protocol Architecture Contd.
• ATM is a streamlined protocol with minimal error- and flow
control capabilities
– This reduces the overhead of processing ATM cells and reduces
the number of overhead bits required with each cell
• Thus ATM is able to operate at high data rates
• Use of fixed-size cells simplifies the processing required at
each ATM node
– This also supports the use of ATM at high data rates
Protocol Architecture Contd.
• The standards issued for ATM by ITU-T govern the basic
architecture for interface between user and network
• The physical layer involves the specification of a
transmission medium and a signal encoding scheme
• The data rates specified at the physical layer range from
25.6Mbps to 622.08Mbps
• Two layers of the protocol architecture relate to ATM
functions
Protocol Architecture Contd.
– ATM layer
• Common to all layers that provides packet transfer capabilities
• Defines the transmission of data in fixed-size cells and defines the
use of logical connections
– ATM adaptation layer (AAL)
• This layer is service dependent
• Use of ATM creates the need for an adaptation layer to support
information transfer protocols not based on ATM
– The AAL maps higher-layer information into ATM cells to be
transported over an ATM network
– It also collects information from ATM cells for delivery to higher layers
Protocol Architecture Contd.
Protocol Architecture Contd.
• The ATM protocol reference model involves 3 separate
planes:
– User plane
• Provides for user information transfer, along with associated controls (e.g.,
flow control error control)
– Control plane
• Performs call control and connection control functions
– Management plane
• Includes plane management, which performs management functions
related to a system as a whole and coordination between all planes
• Also includes layer management, which performs management functions
relating to resources and parameters residing in its protocol entities
ATM Logical Connections
• Logical connections in ATM are referred to as virtual
channel connections (VCCs)
• A VCC is the basic unit of switching in an ATM network
• A VCC is set up between two end users through the
network and a variable-rate, full-duplex flow of fixed-size
cells is exchanged over the connection
• VCCs are also used for user-network exchange (control
signalling) and network-network exchange ( network
management and routing
ATM Logical Connections Contd.
• For ATM, a second sublayer of processing has been
introduced that deals with the concept of virtual path
– A virtual path connection (VPC) is a bundle of VCCs that have
the same endpoints
• Thus, all of the cells flowing over all of the VCCs in a single VPC
are switched together
• The virtual path concept was developed in response to a
trend in high-speed networking
– In that the control cost of the network is becoming an
increasingly higher proportion of the overall network cost
ATM Logical Connections Contd.
– The virtual path technique helps contain the control cost by
grouping connections sharing common paths through the
network into a single unit
• Network management actions can then be applied to a small
number of groups of connections instead of a large number of
individual connections
– The advantages of using virtual paths are:
• Simplified network architecture
– Network transport functions can be separated into those related to an
individual logical connection ( virtual channel) and those related to a
group of logical connections (virtual paths)
• Increased network performance and reliability
– the network deals with fewer, aggregated entities
ATM Logical Connections Contd.
ATM Logical Connections Contd.
• Reduced processing and short connection setup time
– Much of the work is done when virtual path is setup
» By reserving capacity on a virtual path connection in anticipation of later call
arrivals, new virtual channel connections can be established by executing
simple control functions at the endpoints of the virtual path connection
» No call processing is required at transit nodes
» Thus addition of new virtual channels to an existing virtual path involves
minimal processing
• Enhanced network services
– The virtual path is used internal to the network but is also visible to the end user
» Thus, the user may define closed user groups or closed networks of virtual
channel bundles
ATM Logical Connections Contd.
ATM Logical Connections Contd.
• The process of setting up a virtual path connection is
decoupled from the process of setting up an individual
virtual channel connection:
– The virtual path control mechanisms include calculating routes,
allocating capacity, and storing connection state information
– To set up a virtual channel, there must first be a virtual path
connection to the required destination node
• Further, connection must have sufficient available capacity to support
the virtual channel with the appropriate quality of service
• virtual channel is setup by storing the required state information
VCC Uses
• The endpoints of a VCC may be end users, network
entities, or an end user and a network entity
– In all cases, cell sequence integrity is preserved within a VCC
• Examples of uses of a VCC are:
– Between end users
• Can be used to carry end-to-end user data
• Can also be used to carry control signalling between end users
• A VPC between end users provides them with an overall capacity
VCC Uses Contd.
– Between an end user and a network entity
• Used for user-to-network control signalling
• A user-to-network VPC can be used to aggregate traffic form an end
user to a network exchange or network server
– Between two network entities
• Used for network traffic management and routing functions
• A network-to-network VPC can be used to define a common route
for the exchange of network management information
VCC Characteristics
• ITU-T recommendation I.150 lists the following as
characteristics of VCCs
– Quality of Service
• A user of a VCC is provided with a quality of service specified by
parameters such as cell loss ratio and cell delay variation
– Switched and semipermanent virtual channel connections
• A switched VCC is an on-demand connection, which requires call
control signalling for setup and tearing down
• A semipermanent VCC is one that is of long duration and is setup
by configuring or network management action
VCC Characteristics Contd.
– Cell sequence integrity
• The sequence of transmitted cells within a VCC is preserved
– Traffic parameter negotiation and usage monitoring
• Traffic parameters can be negotiated between a user and network for
each VCC
• The input of cells to the VCC is monitored by the network to ensure that
negotiated parameters are not violated
• The types of traffic parameters that can be negotiated include average
rate, peak rate, burstiness, and peak duration
– The network may need a number of strategies to deal with congestion and
manage existing and requested VCCs
VCC Characteristics Contd.
» At the crudest level, the network may simply deny new requests for VCCs
to prevent
» Additionally, cells may be discarded if negotiated parameters are violated
or if congestion becomes severe
» In extreme situations, existing connections might be terminated
VPC Characteristics
• I.150 also lists characteristics of VPCs
– The first four characteristics listed are identical to those for VCCs
• That is, those listed in the last 2 slides apply for VPCs as well
– There are a number of reasons for this duplication:
• Provides some flexibility in how the network service manages the
requirements placed upon them
• The network must be concerned with the overall requirements for a
VPC, and within a VPC may negotiate the establishment of virtual
channels with given characteristics
VPC Characteristics Contd.
• Once a VPC is setup, it is possible for the end users to negotiate the
creation of new VCCs
– In addition, a fifth characteristic is listed for VPCs:
• Virtual channel identifier restriction within a VPC
– One or more virtual channel identifiers, or numbers, may not be available to
the user of the VPC but may be reserved for network use
» Examples include VCCs used for network management
Control Signaling
• In ATM, a mechanism is needed for the establishment and
release of VPCs and VCCs
• The exchange of information involved in this process is
referred to as control signaling and take place on separate
connections from those that are being managed
• For VCCs, I.150 specifies 4 methods for providing an
establishment/release facility
– Semipermanent VCC
• May be used for user-to-user exchange
• No control signalling is required
Control Signaling Contd.
– If there is no preestablished call control signalling channel, then
one must be setup
• For this purpose, a control signalling exchange must take place
between the user an network on some channel
• Hence, a permanent channel is required, probably of low data rate
– This can be used to setup a VCCs that can be used for call control
– Such a channel is called a meta-signaling channel, as the channel is
used to setup signaling channels
– The meta-signaling channel can be used to set upa VCC
between the user and network for call control signaling
• This user-to-network signaling virtual channel can then be used to
set up VCCs to carry user data
Control Signaling Contd.
– The meta-signaling channel can also be used to set up a userto-user signaling virtual channel
• Such a channel must be set up with in a preestablished VPC
• It can then be used to allow the 2 end users, without network
intervention, to establish and release user-to-user VCCs to carry
user data
• For VPCs, three signaling methods are defined in I.150:
– A VPC can be established on a semipermanent basis by prior
agreement
• No control signaling is required in this case
Control Signaling Contd.
– VPC establishment/release may be customer controlled
• A customer uses a signaling VCC to request the VPC from the
network
– VPC establishment/release may be network controlled
• Network establishes a VPC for its own convenience
• The path may be network-to-network, user-to-network, or user-touser
ATM Cells
• ATM makes use of fixed-size cells, consisting of a 5octet header and a 48-octet information field
• There are several advantages to the use of small, fixedsize cells:
– Reduce the queueing delay for a high-priority cell, because it
waits less if it arrives slightly behind a lower-priority cell
– It appears that fixed-size cells can be switched more efficiently
• This is important for very high data rates of ATM
ATM Cells Contd.
ATM Cells Contd.
– It is easier to implement the switching mechanism in hardware with
fixed-size cells
• In the cell header format, generic flow control (GFC) field
does no appear in the cell header internal to the network
– It only appears at the user-network interface
– Hence, it can be used for control of cell flow only at the local- usernetwork interface
– The field could be used to assist the customer in controlling the flow of
traffic for different qualities of service
– In any case, GFC mechanism is used to alleviate short-term overload
conditions in the network
ATM Cells Contd.
• The virtual path identifier (VPI) constitutes a routing field for
the network
– It is 8 bits at the user-network interface and 12 bits at the networknetwork interface
• The latter allows support for an expanded number of VPCs internal to the
network, to include supporting subscribers and those required for network
management
• The virtual channel identifier (VCI) is used for routing to and
from the end user
• The payload type (PT) field indicates the type of information in
the information field
ATM Cells Contd.
– A value of 0 in the first bit indicates user information
• In this case, the 2nd bit indicates whether congestion has been
experienced
• The 3rd bit, known as the service data unit )SDU) type bit, is a onebit field that can be used to discriminate two types of ATM SDUs
associated with a connection,
– The term SDU refers to the 48-octet payload of the cell
– A value of 1 in the first bit of the payload type field indicates that
this cell carries network management or maintenance information
• This indication allows the insertion of network-management cells
onto a user’s VCC without impacting the user data
– Thus, the PT field can provide inband control information
ATM Cells Contd.
• The cell loss priority (CLP) bit is used to provide
guidance to the network in the event of congestion
– A value 0 indicates a cell of relatively higher priority, which
should not be discarded unless no other alternative is available
– A value of 1 indicates that this cell is subject to discard within the
network
• The user might employ this field so that extra cells (beyond the
negotiated rate) may be inserted into the network, with a CLP of 1,
and delivered to the destination if the network is not congested
• The header error control field is used for both error
control and synchronisation
ATM Service Categories
• An ATM network is designed to be able to transfer many
different types of traffic simultaneously
– These include real-time flows such as voice, video, and bursty
TCP flows
• Each such traffic is handled as a stream of 53-octet cells
travelling through a virtual channel
– However, the way in which each data flow is handled within the
network depends on the characteristics of the traffic flow and the
requirements of the application
ATM Service Categories Contd.
• The following service categories have been defined by ATM
Forum:
– Real-Time Services
• The most important distinction among applications concerns the amount
of delay and variability of delay (jitter) that the applications can tolerate
• Real-time applications typically involve a flow of information to a user
that is intended to reproduce that flow at a source
– A user expects a flow of audio or video information to be presented in a
continuous, smooth fashion
– Applications that involve interaction between people have tight constraints on
delay
ATM Service Categories Contd.
» Typically, any delay above a few hundred milliseconds become
noticeable and annoying
• Constant Bit Rate (CBR)
– Used by applications that require a fixed data rate that is continuously
available during the connection lifetime and a relatively tight upper bound
on transfer delay
– Commonly used for uncompressed audio and video information
– CBR applications include:
»
»
»
»
Videoconferencing
Interactive audio (e.g., telephony)
Audio/Video distribution (e.g., television)
Audio/Video retrieval (e.g., video on demand)
ATM Service Categories Contd.
• Real_Time Variable Bit Rate (rt-VBR)
– Intended for time sensitive applications
» That is, those requiring tightly constrained delay and delay
variation
– The main difference with CBR traffic is that rt-VBR applications transmit
at a rate that varies with time
» Equivalently, an rt-VBR source can be characterised as somewhat
bursty
– The rt-VBR service allows the network more flexibility than CBR
» The network is able to statistically multiplex a number of
connections over the same dedicated capacity and still provide the
required service to each connection
ATM Service Categories Contd.
– Non-Real-Time Services
• Intended for applications that have bursty traffic characteristics and
do not have tight constraints on delay and delay variation
– The network has greater flexibility in handling such flows and can make
greater use of statistical multiplexing to increase network efficiency
• Non-Real-Time Variable Bit Rate (nrt-VBR)
– For some non-real-time applications, it is possible to characterise the
expected traffic flow so that the network can provide substantially
improved quality of service in the areas of delay and loss
» Such applications can use the nrt-VBR service
ATM Service Categories Contd.
– With this service, the end system specifies a peak cell rate, a
sustainable or average cell rate, and a measure of how bursty or
clumped the cells may be
» With this information, the network can allocate resources to
provide relatively low delay and minimum cell loss
• Unspecified Bit Rate (UBR)
– At any given time, a certain amount of capacity of an ATM network is
consumed in carrying CBR and the two types of VBR traffic
– Additional capacity is available for one or both of the following reasons:
» Not all of the total resources have been committed to CBR and
VBR traffic
» The bursty nature of VBR traffic means that at some times less
than the committed capacity is being used
ATM Service Categories Contd.
» All of this unused capacity could be made available for the use of
UBR service
– This service is suitable for applications that can tolerate variable delays
and some cell losses, which is typically true of TCP-based traffic
– With UBR, cells are forwarded on a FIFO basis using the capacity not
consumed by other services
» No initial commitment is made to a UBR source and no feedback
concerning congestion is provided
» This is referred to as a best-effort service
• Available Bit Rate (ABR)
– To improve the service provided to bursty sources that would otherwise
use UBR, the ABR service has been defined
ATM Service Categories Contd.
– An application using ABR specifies a peak cell rate (PCR) that it will
use and a minimum cell rate (MCR) that it requires
– The network allocates resources so that all ABR applications receive at
least their MCR capacity
» Any unused capacity is then shared in a fair and controlled fashion
among all ABR sources
– The ABR mechanism uses explicit feedback to sources to assure that
capacity is fairly allocated
– Any capacity not used for ABR sources remains available for UBR
traffic
ATM Service Categories Contd.
• Guaranteed Frame rate (GFR)
– The most recent addition to ATM service categories
– Designed specifically to support IP backbone subnetworks
– GFR provides better service than UBR for frame-based traffic, including
IP and Ethernet
– The major goal of GFR is to optimise the handling of frame-based
traffic that passes from a LAN through a router onto an ATM backbone
network
– Such ATM networks are increasingly being used in large enterprise,
carrier, and Internet service provider networks to consolidate and
extend IP services over the wide area
ATM Service Categories Contd.
– ABR is also an ATM service meant to provide a greater measure of
guaranteed packet performance over ATM backbones
» However, ABR is relatively difficult to implement between routers
over an ATM network
– With the increased emphasis on using ATM to support IP-based traffic,
especially traffic that originates on Ethernet LANs, GFR may offer the
most attractive alternative for providing ATM service
– One of the techniques use by GFR to provide improved performance
compared to UBR is to require the network elements be aware of frame
or packet boundaries
» Thus, when congestion requires the discard of cell, network
elements must discard all the cells that comprise a single frame
ATM Service Categories Contd.
ATM Adaptation Layer
• The use of ATM creates the need for an adaptation layer
to support information transfer protocols not based on
ATM
– Two examples are PCM voice and the IP
• PCM voice is an application that produces a stream of bits from a
voice signal
• To employ this application over ATM, it is necessary to assemble
PCM bits into cells for transmission and to read them out on
reception in such a way to produce a smooth constant flow of bits
• When IP-based networks interconnect with ATM networks, a
convenient way of integrating the two is to map IP packets into ATM
cells
ATM Adaptation Layer Contd.
– This will usually mean segmenting 1 IP packet into a number of cells on
transmission and reassembling the frame from cells on reception
» By allowing the use of IP over ATM, all the existing IP
infrastructure can be used over an ATM network
• ITU-T I.362 lists the following general examples of
services produced by AAL:
– Handling transmission errors
– Segmentation and reassembly, to enable larger blocks of data to
be carried in the information field of ATM
ATM Adaptation Layer Contd.
– Handling of lost and misinserted cell conditions
– Flow control and timing control
• In essence, the AAL layer provides the mechanisms for
mapping a wide variety of applications onto the ATM
layer
– It provides protocols that are built on top of the traffic
management capabilities of the ATM layer
– Accordingly, the design of the AAL protocols must relate to the
service categories discussed earlier
ATM Adaptation Layer Contd.
• The types of applications that AAL and ATM together can
support include:
– Circuit emulation
• Refers to the support of synchronous TDM transmission structures
over an ATM network
– VBR voice and video
• Real-time applications that are transmitted in compressed format
• One effect of the compression is that a variable bit rate can support
the application, which requires a continuous bit-stream delivery to
the destination
ATM Adaptation Layer Contd.
– General data services
• These include messaging and transaction services that do not
require real-time support
– IP over ATM
• Transmission of IP packets in ATM cells
– Multiprotocol encapsulation over ATM (MPOA)
• Supports a variety of protocols other than IP (e.g., IPX, Apple Talk)
over ATM
– LAN emulation (LANE)
• Supports LAN-to-LAN traffic across ATM networks, with emulation of
LAN broadcast capability
ATM Adaptation Layer Contd.
• AAL layer is organised in two logical sublayers:
– Convergence sublayer (CS)
• Provides the functions needed to support specific applications
uising AAL
• Each AAL user attaches to AAL at a service access point (SAP),
which is simply the address of the application
• This sublayer is service dependant
– Segmentation and reassembly sublayer (SAR)
• Responsible for packaging information received from CS into cells
for transmission and unpacking the information at the other end
ATM Adaptation Layer Contd.
– Thus, SAR must pack any SAR headers and trailers plus CS
information into 48-octet blocks
• General protocol architecture for ATM and AAL typically
encapsulate a higher-layer block of data into a single
protocol data unit (PDU)
– This PDU consists of the higher-layer data and possibly a
header and trailer containing protocol information at the CS level
– This CS PDU is then passed down to the SAR layer and
segmented into a number of blocks
• Each of these blocks is encapsulated into a single 48-octet SAR
PDU
ATM Adaptation Layer Contd.
OSI Model Layer 1 – Physical.
• What media can be used to transmit
messages in a network?
• What are their characteristics?
• What criteria are important when selecting
the right medium and connection
structure?
Conducting Media - Bounded
• Coaxial Cable
– Thick
– Thin
• Twisted Pair
– Shielded
– Unshielded – Category 1 – 6 (mostly 5)
• Optical Fibre
Radiating (Electromagnetic)
Media - Unbounded
•
•
•
•
Broadcast Radio
Microwave
Infrared
Laser (Specialised infrared linking building
LANs)
Medium Selection Criteria
•
•
•
•
•
Speed
– Aggregate data rate – capacity: Kbps, Mbps, Gbps = 1000, 106, 109
bits per second; ‘b’ = ‘bit’, ‘B’ = Byte
– Response time (less than c = speed of light = 300,000 Km/Sec)
Distance
– How far can the signal be propagated?
Security
– Radiation, tapping, interception
Reliability
– Interference, noise
Cost
– Materials and Equipment
– Installation and Labour
– Operation
Medium Selection
Criteria(cont’d)
•
•
•
•
•
Availability
Expansibility, Adaptability
– Additional buildings, extensions
Environmental scope:
– Office, manufacturing, city,…
– Harsh, clean,…
Maintenance
– Manageable infrastructure
Accessibility
– Easy access for maintenance
• Safety
– Conform to safety standards and legal requirements
Medium Selection
Criteria(cont’d)Conducting
Medium: Twisted Pair TP
• Shielded (STP) and Unshielded (UTP)
– Structure: Two copper wires, twisted in helix (at a constant
rate).
– One at zero V, the second carries the signal.
– Usually, bundle several pairs together, each with a different rate
of twist
– Twisting averages out the interference equally wires in the pair.
Conducting Medium: Twisted
Pair TP
• Installation: relatively simple, flexible, easy to
configure.
• Electrical characteristics:
– Considerable radiation (hence easily eavesdropped).
– Susceptible to interference (electric motors,
fluorescent lights).
– STP (shielded) is resistant to interference, but more
expensive.
– Crosstalk between adjacent cables can occur; this is
reduced by using different twist rates.
Conducting Medium: Twisted
Pair TP
• Performance:
– Commonly, 10 Mbps is used (older Category 3, VG = Voice
grade).
– 100 Mbps baseband per pair for quality cable (Category 5).
– 380 MHz => 150 Mbps for glued/plasticised manufacture.
– STP usually 16Mbps in token ring.
– Good over short distances. Maximum length 100 metre. For
long distance, need repeaters, amplifiers.
• Costs: relatively low. Higher technology UTP is more expensive.
• Reliability: Good.
Conducting Medium: Coaxial
Cable
Conducting Medium: Coaxial
Cable
• Structure:
– Central copper conductor; concentric
dielectric (PVC, teflon); concentric solid/mesh
screen; insulation.
– Two forms, thin and thick coax.
• Installation:
– Usually fairly simple.
– Thick coax 1 cm diameter is difficult to bend
(“frozen yellow garden hose”)
Conducting Medium: Coaxial
Cable
• Electrical characteristics:
– High frequency signals
– Reduced radiation
– Almost immune to interference.
– Little crosstalk between adjacent cables.
• Performance:
– High transmission rates 10 Mbps
– Ordinary cable TV coax is similar, but not for use in LAN.
– Equipment and expertise, amplifiers and taps readily available.
– For long distance, need repeaters, amplifiers.
– Thin coax length 185 metre maximum.
– Thick coax length 500 metre maximum.
Conducting Medium: Coaxial
Cable
• Baseband:
– Copper mesh, 50 ohm. 10 Mbps easily.
– Unmodulated. Bit = discrete signal level.
– Passive, easy to tap. But security problem.
– Unmodulated, radiates more, making eavesdropping easier.
• Broadband:
– Aluminium screen, 75 ohm. 300 MHz, 150 Mbps.
– Analogue Modulation.
• Reliability: Good
• Cost: Moderate
Conducting Medium: Fibre
Optic = Optical Fibre
•
•
•
•
•
•
•
•
•
Structure: Total internal reflection, along a filament of glass or plastic.
– Higher bandwidth and transmission rate than Copper.
– 2 - 125 M (= 0.002 to 0.125 mm)
Electrically isolated (needed between buildings)
Not affected by external electromagnetic fields (magnets or static
electricity).
Less attenuation, so longer transmission distances
Higher data capacity, Gbps
Physically, smaller and lighter than Twisted Pair, Coax.
High reliability
Excellent security
Most expensive
Conducting Medium: Fibre Optic = Optical Fibre
• Infrared and (not quite) visible light.
• Three preferred wavelengths; 850, 1300, 1550 nm =
3.5, 2.5, 2.0 × 1014 Hz
• Longer wavelengths better for longer distances,
higher data rates
Three types of optical fibre
•
Multimode step-index
– Thickest 125 M
– Can use LED source
– Multipath propagation
– 20 Mbps for 1 km
– Wavelength 850 nm
Three types of optical fibre
•
Multimode graded-index
– Commonest 125 M
– Multipath propagation
– Varying refractive index – slow in centre, faster at outside.
– So outside ‘catches up’ to inside.
– Can use LED source
– 50 times higher data rate than step-index. 100, 155 Mbps
Three types of optical fibre
•
Single mode
– Very fine central filament 2 - 8 M
– Long distances
– Needs laser source
– Wavelength 1550 nm (more infrared)
– Up to 2 Gbps
Radiating Media (Unbounded)
• Speed = c = 299,792.458 km/sec =
299,792,458 m/sec ≈ 3  108 m/sec
• c = f  λ where f = frequency and λ =
wavelength.
The Electromagnetic Radiation Spectrum
Radiating Media
• Different frequencies have different behaviours.
• Some properties:
– Reflection: An object is visible because it reflects light.
– Refraction: Change of direction at interface of two
media.
– Diffraction: Can bend around corners.
– Penetration: Can pass through walls
Radiating Media
• Penetration:
– In general, the lower the frequency, the better
the penetration
– e.g. Radio, low frequency microwave.
– (But: Light can penetrate glass – special case;
– Xrays, Cosmic rays penetrate most solids
[inter-atomic distances])
Radiating Media
• General Positives:
– Can achieve high data rates
– Cost effective
– Easy to implement
• General Negatives:
– Susceptible to interference
– May need line-of-sight, see below
– Low security
Radiating Media
• Typical Uses:
– Wireless network 802.11 (microwave)
– Local LAN, e.g., within a room (infrared)
– Connecting LANs between buildings (infrared)
– Long distance (20 Km) high data rate
connections (microwave)
Radio Waves
• Mostly, omnidirectional. Antenna radiates in
all directions.
– Therefore, good for multicast transmissions –
radio, TV, paging.
– But, inverse square law: Power proportional to 1 /
distance2.
• Can travel long distances.
• Lower frequencies can penetrate walls.
– Advantage: Can receive signals inside a building.
– Disadvantage: Insecure, cannot isolate
transmissions within a building.
Microwave
•
•
•
•
•
Frequency 1 – 300 GHz
High data rates
For some frequencies, need special licence
Easy installation
Used in both unidirectional and omnidirectional
applications.
• Less directional than laser and infrared, and
hence easier to eavesdrop, intercept, interfere.
Microwave
• Less sensitive to external interference, e.g. rain, fog
• Long distance MAN, WAN connections.
• Concentrated line-of-sight directional transmission, parabolic dish
antenna and receiver.
• Typical frequencies 7 to 38 GHz licensed.
• Data Rates 34, 155 Mbps
• Wireless networks (802.11) use unlicensed frequencies 2.4 – 2.4835
GHz, 5.725 – 5.85 GHz, omnidirectional.
• 2.4 GHz is microwave ISM Industrial, Scientific, Medical band, and
microwave oven frequency (water molecule resonates).
Security in Wireless Networks; Spread Spectrum
Transmission
• Two common methods; both use pseudo-random sequences.
• DSSS Direct Sequence Spread Spectrum
– Multiply the radio frequency carrier with pseudo-random noise.
– This spreads the signal over a wider band
• FHSS Frequency Hopping Spread Spectrum
– Jump from narrow band to narrow band across a wide range of
frequencies
– Spend Less than 10 milliseconds at each frequency
– Pre-arranged sequence, known only to sender and receiver.
Infrared
• Common use: TV, VCR, etc., remote controller
(directional)
• Frequency 300 GHz to 400 THz
• Very directional. Difficult to intercept/interfere with.
• Reflects off surfaces.
• Mostly, used inside buildings (sun interferes outside)
• Can be used for internal networks. Use ceiling
access point, or rely on reflection.
Infrared
• Adjacent rooms could have separate LANs.
These can be connected by wired medium.
• Easy installation
• Inexpensive – cost effective
• Capable of high bandwidth, typically, 100 Kbps
to 16 Mbps
• Externally, susceptible to external interference,
e.g. rain, fog, sun.
• But see infrared laser, below.
Infrared Laser
• http://www.canon.com/bctv/canobeam/p
df/dt100.pdf
Infrared Laser
• Easy installation.
• Essentially for links between buildings, not between
individual workstations.
• Frequency: borderline visible red / infrared 3.82 x 10 14 ,
wavelength 785 nm.
• Infrared frequency allows high transmission rates, 25
Mbps – 1.25 Gbps.
• No radio interference caused or received.
• Parallel beam - bi-directional.
• Limited to line-of-sight.
Infrared Laser
• Susceptible to external interference, e.g. rain,
fog, birds, building cranes, …
• Sensitive to atmospheric attenuation, building
movement (hence auto-tracking).
• Class 1M laser, safe for human eye and skin at
output.
• Security good, e.g., 4m diameter footprint at
500m.