Chapter 3 OSI Model

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Open System
Interconnection

ISO


ORG FOR STANDARDISATION


The

International Organisation for Standardisation (ISO) is an
International standards organisation responsible for a wide
range of standards, including many that are relevant to
networking


In 1984 in order to aid network interconnection without
necessarily requiring complete redesign, the Open Systems
Interconnection (OSI) reference model was approved as an
international standard for communications architecture


OSI Model



An ISO standard that covers all aspects of
Network Communication is the
Open System
Interconnection


An open system allows two different systems to
communicate with each other regardless of their
underlying architecture


ISO is an Organization


OSI is a model



THE NEED FOR STANDARDS


Previously Networks used different Hardware &
Software implementations


Resulting in incompatibilities


With different specifications nodes were unable to
communicate with each other


To address problem ISO researched various network
schemes


Need to create a NETWORK MODEL


Help vendors create interoperable network
implementations

THE OSI REFERENCE MODEL


OSI is short for Open System Interconnection



Task : Moving info between computers over a network


Divide the task into SEVEN smaller / more
manageable tasks


Separation into smaller more manageable functions
is known as layering


WHAT needs to be done to send data from one
computer to another and not HOW it should be done


OSI Model


A LAYERED NETWORK MODEL


Each of 7 layers has particular Network functions


Each layer provides a service to the layer above it


Each layer correspond with the layers above and
below it and also the Peer layer.


Through agreed upon rules called Protocols


Peer to Peer process


Each layer add info to message it receives from layer
above it called Header & Trailer (Data link layer only)


By breaking up tasks into layers reduces complexity


Enhance professionalism


Organization of Layers


Lower 3 layers are Network Support Layers


Concerned with flow of data from end to end through Network


Combination of HW & SW


Physical layer always implemented in Hardware


Upper 3 layers are User Support Layers


Always implemented in Software.


Layer 4 ensures end to end reliable data transmission


Layer 2 ensures reliable transmission on a single link


Layers glued together by interfaces


Each interface defines what info & services it must provide for
the above layer


Headers & Trailers are added as data moves from Application
to Physical and removed vice versa


Data is
Encapsulated with the necessary protocol information
as it moves down the layers before network transit.



OSI Layers


Only last 3 layers involved as data moved through Intermediate nodes

Exchange Using the OSI Model

Header & Trailer being added & removed

LAYER 7: APPLICATION


The application layer is responsible for providing services to the
user


Closest to the user and provides user interface


Establishes the availability of intended communication partners


Does not provide services to any other OSI layer


Examples: Spreadsheet programs, word processing programs,
electronic mail, remote file access & transfer and share
database management and other distributed information
services


Examples of Application layer protocols are:

Telnet, SMTP,
FTP, SNMP



LAYER 6: PRESENTATION


Presentation layer is concerned with the syntax and
semantics of the information exchanged between two
systems


This layer is primarily responsible for the translation,
encryption and compression of data


It ensures that the information that the application layer of
one system sends out is readable by the application layer of
another system.
Defines coding and conversion functions


This

layer

also

manages

security

issues

by

providing

services

such

as

data

encryption

and

data

compression


Examples of these formats and schemes are:

MPEG,
QuickTime, ASCII, EBCDIC, GIF, TIFF, JPEG





LAYER 5: SESSION


The session layer defines how to start, control and end
conversations (called sessions) between applications


Establishes dialog control between the two computers in a
session, regulating which side transmits, plus when and how
long it transmits (Full duplex)


Synchronization:

Allows processes to add check points. E.g.
Insert check point at every 100 page of 2000 page file to ensure
that each 100
-
page unit is received & acknowledged


Transmits Data



LAYER 4: TRANSPORT


It regulates information flow to ensure process
-
to
-

process
connectivity between
host applications

reliably and accurately


Adds service point address or Port address


Segmentation & Re
-
assembly: SEGMENTS data from sending
node and reassembles data on receiving node


Flow control / Error control at Source to destination level


Connection oriented transport service ensures that data is
delivered error free, in sequence with no losses or duplications


Establishes, maintains and terminates virtual circuits


Connection oriented / Connectionless: TCP (Reliable, provides
guaranteed delivery), UDP (Unreliable, less overhead, reliability
can be provided by the Application layer)



Provides multiplexing; the support of different flows of data to
different applications on the same host


LAYER 3: NETWORK


Defines source to destination delivery of packets across NWs


Defines logical addressing


No need if 2 systems on same Link


Treat each packet independently


Defines how routing works and how routes are learned


Converts frames to packets


Routed protocols ( encapsulate data into packets) and Routing
protocols (create routing tables) work on this layer


Examples of Routed protocols are:

IP, IPX, AppleTalk

and

Routing protocols are OSPF, IGRP/EIGRP, RIP, BGP


Relieves higher layers from knowing about underlying data
transmission and switching technologies used to interconnect
systems


May use packet switched / circuit switched networks



Routers

operate

at

Layer

3
.



LAYER 2: DATA LINK


Packages raw bits from the physical layer into
FRAMES


The data link layer provides reliable transit of data across a
physical link by using the Media Access Control (MAC)
addresses


Source & Destination ( address of device that connects one
Network to next) address


Flow Control: Prevent overwhelming of Receiving Node


Error Control: Through Trailer


Access Control: Which device to have control


Data Link LAN specifications:

F
ast Ethernet, Token Ring, FDDI



Data Link WAN specifications are:

Frame Relay, PPP, X.25


Bridges and Switches operate at this layer




LAYER 1: PHYSICAL


Deals with the physical characteristics of the Transmission
medium


Transmits

bits

from

one

computer

to

another



Electrical
:

Voltage

levels

/

timing

of

voltage

changes



Mechanical
:

Physical

connectors

(DB
-
25
)


Procedural
:

Sequence

of

events

by

which

bit

stream

is

exchanged

across

Physical

medium


Data rates / Maximum transmission distances


Physical Topology


Synchronization of bits: Tx & Rx sync


Line configuration: Pt

to

Pt / Multi Point


Transmission modes

&
Repeaters operate at this layer



Data Encapsulation


Data Encapsulation is the process of adding a header to
wrap

the data that flows down the OSI model.



The 5 Steps of Data Encapsulation are:


1. The Application, Presentation and Session layers create
DATA from users' input.


2. The Transport layer converts the DATA to SEGMENTS


3. The NW layer converts the Segments to Packets (datagram)


4. The Data Link layer converts the PACKETS to FRAMES


5. The Physical layer converts the FRAMES to BITS.



Important


Data
-
Link: Communication between nodes on same network


Network: Comm between nodes on possibly different NWs


Transport: Communication between processes (running on
machines on possibly different networks)


Connecting Networks Devices


Repeater:

Physical layer


Bridge:


Data link layer


Router:


Network layer


Gateway:

Transport layer and above


SUMMARY


No standard for networks in the early days


Difficult for Networks to communicate with each other


ISO recognised this and researched various network schemes,
and in 1984

introduced OSI reference model


The OSI
ensure
greater compatibility and interoperability
between various types of network technologies


Organizes network functions into seven numbered layers


Each layer provides a service to the layer above it and
communicates with the same layer on other computers


Layers 1
-
4 are concerned with the flow of data from end to end
through the network and Layers 5
-
7 are concerned with
services to the applications