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COMP2221
Networks
in
Organisations
Richard Henson
February 2012
Open Systems Interconnect
(OSI)

Bold aim:
– produce a set of communication protocols
that would allow any system to exchange
information with any other system

Many proprietary models
– IBM biggest and most influential, major say

Open systems?
– Internet & TCP/IP
Open Systems Interconnect
(OSI)
All manufacturers wished to have their
own communication models
represented
 Principle:

– “A new layer must be created for each
new level of abstraction”

Result:
– 7 software layers!!!
Open Systems Interconnect
(OSI)

OSI layers universal - cover all
types of networks
– client/server
– peer-peer

Define standards for:
– interconnecting (linking data)
» physical -> network
– interworking (presenting data)
» transport -> application
OSI Model – Benefits

All manufacturers…
– target to aspire towards

Benefits of OSI compatible products:
– other manufacturers products would be
able to communicate with their own
– consumer would no longer be “locked in”
to specific vendor products
– vendors would be able to produce products
that work at specific layers only
» specialise and hence produce better products
Layer Communication
(Sending)

Each layer in the OSI model considers itself to
be talking to a peer layer in another computer
– adds/removes its own “header” (formatting info)

e.g. application layer
– adds a header to the user data on screen
– passed to the presentation layer as a single block
e.g. presentation layer
– adds its header to the block of data
– passed on to session layer as a single block…

and so on…
The OSI reference model
Transmit
Station
AH DATA
Application
Layer
Application
Layer
DATA
AH
PH AH DATA
Presentation
Layer
Presentation
Layer
DATA
AH PH
Session
Layer
Session
Layer
DATA
AH
PH
SH
TH SH PH AH DATA
Transport
Layer
Transport
Layer
DATA
AH
PH
SH
NH
TH SH PH AH DATA
Network
Layer
Network
Layer
DATA
AH
PH
SH
NH
TH SH PH AH DATA
Data link
Layer
Data link
Layer
DATA
AH
PH
SH
Physical
Layer
Physical
Layer
SH PH AH DATA
LH
Receive
Station
LT
Link
LT
TH
TH
TH
NH
NH
LH
Layer Communication
(Receiving)


Each layer in the OSI model strips away its
own header
e.g. physical layer
– removes header from data block
– passed to the data link layer

e.g. data link layer
– removes header to the block of data
– passed on to network layer

and so on…
The OSI reference model
Transmit
Station
AH DATA
Application
Layer
Application
Layer
DATA
AH
PH AH DATA
Presentation
Layer
Presentation
Layer
DATA
AH PH
Session
Layer
Session
Layer
DATA
AH
PH
SH
TH SH PH AH DATA
Transport
Layer
Transport
Layer
DATA
AH
PH
SH
NH
TH SH PH AH DATA
Network
Layer
Network
Layer
DATA
AH
PH
SH
NH
TH SH PH AH DATA
Data link
Layer
Data link
Layer
DATA
AH
PH
SH
Physical
Layer
Physical
Layer
SH PH AH DATA
LH
Receive
Station
LT
Link
LT
TH
TH
TH
NH
NH
LH
Simplifying The OSI model

Layers can be sub-divided into two groups
– The top 3 layers (interworking layers)
» user applications and support services
– The lower 4 layers (interconnection layers)
» the network (and navigation of packets)

Memory aids:
– PDNTSPA
– Please Do Not Throw Sausage Pizza Away!
Interworking Layers

All about servicing needs of users
– support for the application layer…
– includes presentation layer
– and session layer
Application Layer

Interface for applications to use to gain
access to network services:
– Networked file transfer
– Message handling
– Database query processing

Controls generalised network access:
– supports applications which exchange data
– provides error & status information for applications

If network is peer-peer…
– authenticates peer partners
– determines if peers are ready to communicate
Presentation Layer

Responsible (sending) for converting data from
– application-specific format
– to a generic (machine-independent) format that can be
passed across a network

Receiving…
– for converting incoming data from a generic format to one
that makes sense to the receiving application


Also responsible for protocol conversion, encryption
& decryption, and graphics commands
The redirector (software for handling service
requests) also operates at this layer:
– If a service cannot be resolved locally, it sends the request
out to the network resource that can offer the required
service
Session Layer





Sets up a logical connection between machines
called a “session”, which allows networked resources
to communicate
Manages the setting up of a user “session”, exchange
of information, and “tear down” as the session ends
Manages issues such as who may transmit data at a
certain time, and for how long, also ensuring that the
system doesn’t “time out” after inactivity
Ensures data is routed to the correct application on
the local machine
Synchronises services between tasks at each end of
the communications channel in half duplex
communications
Interconnection Layers

Concerned with packets of data
– and navigating them through the network
Transport
 Network
 Data Link
 Physical

The Four Layers Model

Introduced with Unix (mid-1970s, pre-OSI)
– based on Internet protocols…
“application”
“transport”
“network”
“physical”
TCP/IP

Evolved with the Unix four layers…
Application,
presentation, session
TCP
IP
Connecting with
physical medium
Transport Layer (from Unix)

Manages the transmission of level 4 data
from sender to corresponding layer in
receiver
– segments data streams into chunks of a given
packet size for the medium being used
– checks for errors due to corruption, requests
retransmission etc.

Gateways can operate at this layer
Transport Layer (from Unix)

Other roles:
– managing flow control
– providing acknowledgement of successful
transmission of chunks of data
– software multiplexing
– routing in an Internetwork

Manages OSI levels 1-4 so messages travel
between network nodes via pairs of “sockets”
socket A
(sender)
socket B
(receiver)
Transport layer
Socket
A
End User
End User
Upper
OSI
Layers
Upper
OSI
Layers
Transport
Layer
Peer-to-Peer communications
Transport
Layer
Network
Layer
Network
Layer
Network
Layer
Network
Layer
Data link
Layer
Data link
Layer
Data link
Layer
Data link
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Physical
Layer
Network A
Network B
Socket
B
End-end v logical neighbour
communications

Top four OSI layers communicate logically
with remote peer…
– regardless of topology or distance

The lower layers all communicate physically
with their nearest neighbour in a network
– dependent on topology and routing to get the
packets through
Network Layer

Responsibilities:
– packet (IP) addressing and sequencing
– determining to route from source to
destination computer
– Routers operate up to this level
Network Layer Functions





Provides messages with an address for
delivery (e.g. IP address)
Translates logical network addresses/names
into physical equivalents
Handles packet switching and routes packets
to their destination on the local network
Controls network packet congestion
Ensures packets conform to the network's
format
Network Layer
User Specifies
Service
Transport
Layer
Network
Service
Network
Layer
Network provides
Service
Network layer service definitions
Data Link Layer



Responsible for error free transmission, using data
frames
A frame is a basic unit for network traffic, and has a
highly structured format
Mechanism:
– data from the upper layers (ie the network layer) is converted
by the data link layer into frames
– groups raw data bits received via the physical layer into
frames, for passing on to the upper layers
– may include an error recovery mechanism and also a flow
control mechanism, although this may be done at the
transport layer

Bridges operate up to this level
Physical Layer





Responsible for communicating with the network
media
Bits are converted into electrical signals and vice
versa
Issues include modulation of signals and timing
Manages the interface between a computer and the
network medium, but cable type and speeds of
transmission are deliberately omitted to allow future
technology to be easily included
Repeaters work only at this level
Standards and the
OSI reference model

OSI designed to promote the
development of protocols…
– that support open systems interconnection

Become an agreed standard in 1984
– ISO 7498 (the International Standard)
– BSI 6568 (the identical British Standard)
– CCITT recommendation X.200
The IEEE 802 Specifications
and the OSI model

Emerged from IEEE-OSI meeting: February ‘80
– applied mainly to lower level OSI layers (1/2)
– found it necessary to extend the data link layer into
two parts
– Essential for development of LANs

IEEE layer 1/2 definitions used by manufacturers
when defining specifications for hardware such
as network interface cards
Effect of IEEE 802
on the OSI model

To cover all different types of interfaces,
IEEE divided the Data Link Layer into two
sub-layers:
– Layer 2 (upper) Logical Link Control used by 802.1 & 802.2
– Layer 2 (lower) Media Access Control 802.3, 4, 5, 11, 12, etc.
Layer 2 (upper) Logical Link
Control

Focuses on IEEE 802.1 & 802.2

Controls transfer of data to the network layer

Uses logical interface points called SAPs
(service access points)
Layer 2 (lower) Media
Access Control

Direct communication with the network card
– provides packets with MAC address



Focuses on IEEE 802.3, 4, 5, 11, 12…
Provides shared access for multiple network
interface cards to the physical layer
Responsible for ensuring error-free
communication across the network
OSI layer software and
Network cards


Layer 1 and 2 software supplied with the
network card
– card itself should contain software (on ROM)
that conforms to one of the sixteen IEEE 802
specifications
Cards for wired connections have connectors
for cables:
– usually IEEE 802.3
– more rarely… IEEE 802.5

Wireless Cards
– usually based on IEEE802.11
“Binding” Network Card Software

OSI Level 3
software
binding

OSI Level
1/2
software

Data received by the
network card needs to be
passed on to level 3
software
Normally held on the
computer hard disk
Configuration:
– level 2 software needs to
combine with level 3
– achieved through “binding”
Now for the practical…
P.S. Homework: check out the
IEEE 802 standards and decide
which are most important
in 2012