The Iso - Osi Model

8/14/2019 The Iso - Osi Model

1/46

November 9, 2009 NITW/ECE/LA 1

Established in 1947, the International StandardsEstablished in 1947, the International Standards

Organization (Organization (ISOISO) is a multinational body) is a multinational body

dedicated to worldwide agreement on internationaldedicated to worldwide agreement on international

standards. An ISO standard that covers all aspectsstandards. An ISO standard that covers all aspects

of network communications is the Open Systemsof network communications is the Open Systems

Interconnection (Interconnection (OSIOSI) model. It was first introduced) model. It was first introducedin the late 1970s.in the late 1970s.

THE ISO - OSI MODELTHE ISO - OSI MODEL


2/46


Layered Architecture

Networks are organized aslayersor levels.

Reduces design complexity. Each network architecture has

different:- number of layers- names of layers- functions of each layer

Between adjacent layers there

is an interface. Reference models exist to

architect a network.

OSI Protocols

Physical

Data

Network

Transport

SessionPresentation

Application


3/46


Protocols

Rules and conventions used by two communicatingentities are collectively known as protocol.

Each layer of network architecture has its ownprotocol.

Two communicating entities at the same layer arecalled peers.

A list of protocols used by a system is calledprotocol stack.

A set of layers and protocols is referred to asnetwork architecture.


4/46


THE THEORETICAL NETWORK - OSI MODEL

Peer communication between two computers


5/46


OSI Model OverviewOSI Model Overview

Data FlowLayers

Transport Layer

Data Link

Network Layer

Physical

Application(Upper)Layers

Session

Presentation

Application


6/46


Keeping differentapplicationsdata separate

User Interface

How data is presentedSpecial processingsuch as encryption

TelnetHTTP

ASCIIEBCDICJPEG

Operating System/Application Access

SchedulingTransport Layer

Data Link

Network Layer

Physical

EXAMPLES

Session

Presentation

Application

Role of Application LayersRole of Application Layers


7/46


TCPUDPSPX

802.3 / 802.2HDLC

EIA/TIA-232

V.35

IPIPX

Presentation

Application

SessionEXAMPLES

Role of Data Flow LayersRole of Data Flow Layers

Reliable or unreliable delivery Error correction before retransmit

Combines bits into bytes and

bytes into frames Access to media using MAC address Error detection not correction

Move bits between devices Specifies voltage, wire speed and

pin-out cables

Transport

Data Link

Physical

NetworkProvide logical addressing whichrouters use for path determination


8/46


Encapsulating DataEncapsulating Data

Transport

Data Link

Physical

Network

Upper Layer Data

Upper Layer DataTCP Header

DataIP Header

DataLLC Header

0101110101001000010

DataMAC Header

Presentation

Application

Session

Segment

Packet

Bits

Frame

PDU

FCS

FCS


9/46


Upper Layer Data

De-encapsulating DataDe-encapsulating Data

LLC Hdr + IP + TCP + Upper Layer Data

MACH

eader

IP + TCP + Upper Layer Data

LLCHe

ader

TCP+ Upper Layer Data

IPHead

er

Upper Layer Data

TCPH

eader

0101110101001000010

Transport

Data Link

Physical

Network

Presentation

Application

Session


10/46


An exchange using the OSI model

Osi.swf


11/46


The interaction between layers in the OSI model


12/46


The physical layer is responsible for movements of

individual bits from one hop (node) to the next.

Note


13/46


Physical layer


14/46


The data link layer is responsible for movingframes from one hop (node) to the next.

Note


15/46


Data link layer


16/46


The network layer is responsible for thedelivery of individual packets from

the source host to the destination host.

Note


17/46


Hop-to-hop delivery


18/46


Network layer


19/46


Source-to-destination delivery


20/46


The transport layer is responsible for the deliveryof a message from one process to another.

Note


21/46


Transport layer


22/46


Reliable process-to-process delivery of a message


23/46


The session layer is responsible for dialogcontrol and synchronization.

Note


24/46


Session layer


25/46


The presentation layer is responsible for translation,compression, and encryption.

Note


26/46


Presentation layer


27/46


The application layer is responsible forproviding services to the user.

Note


28/46


Application layer


29/46


Summary of layers


30/46


The layers in theThe layers in the TCP/IP protocol suiteTCP/IP protocol suite do not exactlydo not exactly

match those in the OSI model. The original TCP/IPmatch those in the OSI model. The original TCP/IP

protocol suite was defined as having four layers:protocol suite was defined as having four layers: host-host-to-network, internet, transport,to-network, internet, transport, andand application.application.

However, when TCP/IP is compared to OSI, we can sayHowever, when TCP/IP is compared to OSI, we can say

that the TCP/IP protocol suite is made of five layers:that the TCP/IP protocol suite is made of five layers:

physical, data link, network, transportphysical, data link, network, transport, and, andapplication.application.

TCP/IP PROTOCOL SUITETCP/IP PROTOCOL SUITE


31/46


TCP/IP Reference Model

PRESENTATION

SESSION

TRANSPORT

NETWORK

DATA LINK

PHYSICAL

APPLICATION APPLICATION

TRANSPORT

INTERNET

HOST-TO-NETWORK

(Data Link/

Physical)

OSI

Notpresent

In themodel

TCP/IP


32/46


TCP/IP and OSI model


33/46


Physical LayerPhysical Layer

MAC LayerMAC Layer

LLC LayerLLC Layer

Network LayerNetwork Layer

Transport LayerTransport Layer

AddressingAddressing

SchemesSchemes

Network AddressingNetwork Addressing

Addressingis used to

ensure that messagesbetween any two of themachines are notreceived by other users.

Addressing schemesexist at multiple layersof the OSI Model. Atypical TCP/IP packetwill contain addressesthat are designed to beused at the MAC Layer,the Network Layer and

the Transport Layer.


34/46


MAC Layer Addresses

Network Layer Addresses

Transport Layer Addresses

The addresses are stored in specifically definedparts of the IP packet and the LAN frame.

The consistent position of addresses is a keyfactor that allows software to interpret addressinginformation correctly. In other words, if we putaddresses in the wrong format, or in the wrong

place, our communication software will not workcorrectly.


35/46


Addresses in TCP/IP


36/46


Relationship of layers and addresses in TCP/IP


37/46


A node with physical address 10 sends a frame to a node

with physical address 87. The two nodes are connectedby a link (bus topology LAN). As the figure shows, the

computer with physical address 10 is the sender, and the

computer with physical address 87is the receiver.

Example


38/46


Most local-area networks use a 48-bit (6-byte) physicaladdress written as 12 hexadecimal digits; every byte (2

hexadecimal digits) is separated by a colon, as shown

below:

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address.


39/46


Figure next shows a part of an internet with two routers

connecting three LANs. Each device (computer or

router) has a pair of addresses (logical and physical) for

each connection. In this case, each computer is

connected to only one link and therefore has only one

pair of addresses. Each router, however, is connected to

three networks (only two are shown in the figure). So

each router has three pairs of addresses, one for eachconnection.

Example


40/46


IP addresses


41/46


42/46


Port addresses


43/46


The physical addresses will change from hop to hop,but the logical addresses usually remain the same.

Note


44/46


A port address is a 16-bit address represented by onedecimal number as shown.

753

A 16-bit port address represented

as one single number.


45/46


The physical addresses change from hop to hop,but the logical and port addresses usually remain the same.

Note


46/46


The Iso - Osi Model

Documents

Transcript of The Iso - Osi Model