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COMPUTER NETWORKS
UNIT -1
DATA COMMUNICATIONS
The term telecommunication means communication at a
distance. The word data refers to information presented in
whatever form is agreed upon by the parties creating and
using the data. Data communications are the exchange of
data between two devices via some form of transmission
medium such as a wire cable.
 Components of a data communications system
 Data Flow
1.2
5 Basic Components of Communication
Simplified Communications Model Diagram
Simplified Data Communications
Model
Transmission Media Speed
•Bandwidth:The amount of data which can be transmitted on a
medium over a fixed amount of time (second). It is measured on Bits per
Second or Baud
•Bits per Second (bps): A measure of transmission
speed. The number of bits (0 0r 1) which can be transmitted in a second
•Baud Rate: Is a measure of how fast a change of state occurs
(i.e. a change from 0 to 1)
Data Representation
 Text
Email, articles, etc
Coding (Unicode, ASCII)
 Numbers
Direct conversion
 Images
Pixels, resolution
 Audio
Continuous, signal conversion
 Video
Movie, continuous/discrete
1.7
Data can be of two types: analog and digital.
Analog data
take on continuous values on some interval. Typical
examples of analog data are voice and video. The data
that are collected from the real world with the help of
transducers are continuous-valued or analog in nature.
Digital data
take on discrete values. Text or character strings can be
considered as examples of digital data. Characters are
represented by suitable codes, e.g. ASCII code, where
each character is represented by a 7-bit code.
Data flow
• Communication between two devices can be simplex, halfduplex, or full-duplex (as shown in next slide).
• Simplex
– The communication is unidirectional, as on a one-way street. An
example of simplex is Television, or Radio.
• Half-duplex
– Each station can both transmit and receive, but not at the same
time. It's like a one-lane bridge where two way traffic must give
way in order to cross. Only one end transmits at a time, the
other end receives.
• Full-duplex
– Both stations can transmit and receive simultaneously. It's like a
one-lane bridge where two way traffic must give way in order to
cross.
1.9
Transmission Direction
-
simplex: One direction only
Half Duplex Transmission
half duplex: Both
directions but
only one
direction at a
time
Full Duplex Transmission
full duplex:
send and
receive both
directions at
once
Data Flow
Figure : Data flow (simplex, half-duplex, and full-duplex)
Signaling : It is an act of sending signal over communication
medium.
Transmission: Communication of data by propagation and
processing is known as transmission.
Analog Signal: The sound waves that your mouth produces when
you speak are analogue - the waves vary in a smooth way. These
waves can be converted into an electrical signal by a microphone.
This electrical signal is also analogue:
Packets
• Transmissions are broken up into smaller
units or data transmissions called packets
Example
This file has now been broken into four packets
A data file is divided into packets.
It doesPACKET
not matter what the PACKET
transmission is. It could
be Word
PACKET
document, a PowerPoint or an MP3.
PACKET
DATA COMMUNICATIONS
 What do we need?
Hardware
Software
 Four fundamental characteristics:
1.
2.
3.
4.
Delivery: correct destination
Accuracy: correct data
Timeliness: fast enough
Jitter: uneven delay
Centralized Data Processing
• A Centralized Data Processing Centre may consists of:
–
–
–
–
–
Centralized computers,
Centralized processing,
Centralized data,
Centralized control,
Centralized support staff
• The Basic advantages:
– Economies of scale (equipment and personnel)
– Lack of duplication
– Ease in enforcing standards, security
17
Centralized Data Processing
Centralized Data Processing
Example of:
Distributed Data Processing
Facility is a WLAN that
supports both data traffic
and VoIP.
The WLAN connects to
the outside world via a
satellite
link
that
connects to the Internet,
to Carnivals private wide
area network (WAN), and
to the public switched
telephone
network
(PSTN) in the US.
Carnival Valor Wireless LAN
Distributed Data Processing
Distributed Data Processing
• Computers are dispersed throughout
organization with some means of
interconnection among them
• Allows greater flexibility in meeting individual
needs
• More redundancy
• More autonomy
22
Why is DDP Increasing?
Means and Motive:
• Dramatically reduction in hardware costs
• Dramatically increased distributed processing
capabilities (Hardware capabilities)
• Dramatically increased need for new applications
and shorter development times
• Ability to share data across multiple servers
23
DDP Pros & Cons
• There are no “one-size-fits-all” solutions
• Key issues
– How does it affect end-users?
– How does it affect management?
– How does it affect productivity?
– How does it affect bottom-line?
24
Benefits of DDP
• Responsiveness
• Availability
• Correspondence to
Organizational Patterns
• Resource Sharing
• Incremental Growth
• Increased User
Involvement & Control
• End-user Productivity
• Distance & location
independence
• Privacy and security
• Vendor independence
• Flexibility
25
Drawbacks of DDP
• More difficulty test & failure diagnosis
• More components and dependence on
communication means more points of failure
• Incompatibility of components
• Incompatibility of data
• More complex management & control
• Difficulty controlling information resources
• Suboptimal procurement
• Duplication of effort
26
Reasons for DDP
• Need for new applications
– On large centralized systems, development can
take years
– On small distributed systems, development can
be component-based and very fast
• Need for short response time
– Centralized systems result in contention among
users and processes
– Distributed systems provide dedicated resources
27
What is a Network?
A network is a number of computers and
peripheral devices connected together so
as to be able to communicate (i.e. transfer
data)
Each device in a network is called a
node.
Terminals are data entry points which
can also display.
Classification Based on Transmission Technology
Computer networks can be broadly
categorized into two types based on
transmission technologies:
Broadcast networks
Point-to-point networks
Broadcast Network have a single
communication channel that is shared by all
the machines on the network.
Example of a broadcast network based on satellite
communication
This system generally also allows possibility of addressing the packet to all
destinations (all nodes on the network). When such a packet is transmitted and
received by all the machines on the network. This mode of operation is known as
Broadcast Mode.
Point-to-point Communication
The end devices that wish to communicate are
called stations. The switching devices are called
nodes. Some Nodes connect to other nodes and
some to attached stations.
There may exist multiple paths between a sourcedestination pair for better network reliability.
The switching nodes are not concerned with the
contents of data. Their purpose is to provide a
switching facility that will move data from node to
node until they reach the destination.
Types of connections: point-to-point and multipoint
PMP is usually used for establishing private enterprise
connectivity to offices in remote locations,
NETWORKS: categorized by size
LAN – a network that connects computers in a limited
geographical area.
MAN – a backbone that connects LANs in a metropolitan
area such as a city and handles the bulk of communications
activity across that region.
WAN – covers a large geographical area such as a city or
country. Communication channels include telephone lines,
Microwave, satellites, etc.
An isolated LAN connecting 12 computers to a hub in a closet
• In a building,
office, or school
• Share hardware,
software and data
• Client-server, hub
• LAN normally uses
only one type of
transmission
medium
• Bus, ring and star
LAN typically used transmission technology consisting of single cable to
which all machines are connected.
more reliable as compared to MAN and WAN.
Knowing this bound makes it possible to use certain kinds of design
that would not otherwise be possible.
Simplifies network management.
WANs: a switched WAN and a point-to-point WAN
WAN provides long-distance transmission of
data, voice, image and information over large
geographical areas that may comprise a
country, continent or even the whole world.
A heterogeneous network made of four WANs and two LANs
A WAN that is wholly owned and used by a single company is
often referred to as enterprise network.
Metropolitan Area Network
Interconnection of networks in a city into a single larger
network (which may then also offer efficient connection to a
wide area network).
It is also used to mean the interconnection of several local
area networks by bridging them with backbone lines.
For example, a company can use a MAN to connect the LANs
in all its offices in a city.
Topologies
3 different types of LANS are:
–Ring
–Bus
–Star
Ring
Uses an empty data packet
called a token and a special
protocol called “token ring”.
Packets travel around the ring
in a clockwise direction.
Clients require an empty
token to transmit data.
Advantages
- no collisions because all
data travels in same direction.
Disadvantages
- fails if an individual node in
the network fails
BUS TOPOLOGY
A bus is a form of Ethernet. Nodes linked by a cable known as the bus. Bus transmits in
both directions and uses CSMA/CD protocol
Advantages
Disadvantages
- Easy to set up and maintain
failure of one node does not affect
network
-Higher rate of data collision than
with a bus network
-fails if there is any damage to the
bus
Star Topology
All data is sent from one client
to another through the server.
Advantages
- If one client fails no other
clients are affected.
Disadvantages
- If central file server fails the
network fails.
Mesh Topology
Example: telephone regional
offices
1
2
4
5
3
A fully connected mesh topology (five devices)
Advantages:
•
no traffic problems
•
Robust. No link failure no
effect on others.
•
Privacy security
•
Easy to detect the abnormal
situation.
Disadvantages:
•
Amount of cables, i/o ports
•
Efficiency and effectiveness
•
Space
•
Cost
Why a layered model?
– Easier to teach communication process.
– Speeds development, changes in one layer does not
affect how the other levels works.
– Standardization across manufactures.
– Allows different hardware and software to work
together.
– Reduces complexity
The OSI Model
• OSI “ Open Systems Interconnection".
• OSI model was first introduced in 1984 by the International Organization
for Standardization (ISO).
– Outlines WHAT needs to be done to send data from one computer to another.
– Not HOW it should be done.
– Protocols stacks handle how data is prepared for transmittal (to be
transmitted)
• In the OSI model, The specification needed
– are contained in 7 different layers that interact with each other.
What Each Layer Does
2
•
Gives end-user applications access to
network resources
•
Where is it on my computer?
– Workstation or Server Service in
MS Windows
3
OSI Model
Application Layer
1.
Application layer interacts with application programs and is the
highest level of OSI model.
2.
Application layer contains management functions to support
distributed applications.
3.
Examples of application layer are applications such as file
transfer, electronic mail, remote login etc.
OSI Model
Presentation Layer
Presentation layer defines the format in which the data is to be
exchanged between the two communicating entities.
Also handles
(cryptography).
data
compression
and
data
encryption
Session Layer
•
Allows applications to maintain
an ongoing session
•
Where is it on my computer?
– Workstation and Server
Service (MS)
– Windows Client for
NetWare (NetWare)
3
OSI Model
Session Layer
Session layer provides mechanism for controlling the dialogue between
the two end systems. It defines how to start, control and end
conversations (called sessions) between applications.
This layer requests for a logical connection to be established on an enduser’s request.
Any necessary log-on or password validation is also handled by this
layer.
Session layer is also responsible for terminating the connection.
This layer provides services like dialogue discipline which can be full
duplex or half duplex.
Session layer can also provide check-pointing mechanism such that if a
failure of some sort occurs between checkpoints, all data can be
retransmitted from the last checkpoint.
Basic Functions for the Session Layer
Protocols
• Managing multiple sessions
– A computer can establish multiple sessions with several
other computers
• e.g., session 1: exchanging information over the World Wide Web
with www.yahoo.com
• session 2: exchanging information over the World Wide Web with
www.google.com
• session 3: exchanging information over the World Wide Web with
www.espn.com
– Two computers can also establish multiple sessions,
• e.g., function 1: exchanging information over the World Wide Web;
• function 2: exchanging information over the FTP;
• function 3: exchanging information over the email
Transport Layer
•
•
Provides reliable data delivery
It’s the TCP in TCP/IP
•
Receives info from upper
layers and segments it into
packets
•
Can provide error detection
and correction
3
OSI Model
Transport Layer
Purpose of this layer is to provide a reliable mechanism for the
exchange of data between two processes in different computers.
Ensures that the data units are delivered error free.
Ensures that data units are delivered in sequence.
Ensures that there is no loss or duplication of data units.
Provides connectionless or connection oriented service.
Provides for the connection management.
Multiplex multiple connection over a single channel.
Network Layer
•
Provides network-wide addressing and a
mechanism to move packets between
networks (routing)
•
Responsibilities:
– Network addressing
– Routing
•
Example:
– IP from TCP/IP
3
The Network Layer
csie.ndhu.edu.tw
cs.berkeley.edu
routing: path selection
different network translation
congestion control
network accounting
The Network Layer
Network Layer Design Issues
Services Provided to the Transport Layer
OSI Model
Network Layer
Implements routing of frames (packets) through the network.
Defines the most optimum path the packet should take from the
source to the destination
Defines logical addressing so that any endpoint can be identified.
Handles congestion in the network.
Facilitates interconnection between heterogeneous networks
(Internetworking).
The network layer also defines how to fragment a packet into
smaller packets to accommodate different media.
5. The Network Layer
5.1 Network Layer Design Issues
5.1.2 Internal Organization of the Network Layer
OSI Model
Data Link Layer
Data link layer attempts to provide reliable communication over
the physical layer interface.
Breaks the outgoing data into frames and reassemble the
received frames.
Create and detect frame boundaries.
Handle errors by implementing an acknowledgement and
retransmission scheme.
Implement flow control.
Supports points-to-point as well as broadcast communication.
Supports simplex, half-duplex or full-duplex communication.
Physical Layer
• Determines the specs for all
physical components
– Cabling
– Interconnect methods (topology /
devices)
– Data encoding (bits to waves)
– Electrical properties
• Examples:
– Ethernet (IEEE 802.3)
– Token Ring (IEEE 802.5)
– Wireless (IEEE 802.11b)
3
OSI Model
Physical Layer
Provides physical interface for transmission of information.
Defines rules by which bits are passed from one system to another
on a physical communication medium.
Covers all - mechanical, electrical, functional and procedural aspects for physical communication.
Such characteristics as voltage levels, timing of voltage changes,
physical data rates, maximum transmission distances, physical
connectors, and other similar attributes are defined by physical
layer specifications.