lecture 2 - osi model

8/7/2019 Lecture 2 - OSI Model

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COMPUTER NETWORKS-CS65 DEPT. OF EEE, MEPCO

PREPARED BY Dr.S.MURALIDHARAN 1

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OSI MODEL

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OSI OPEN SYSTEM INTERCONNECTION

DEVELOPED BY INTERNATIONAL STANDARDORGANIZATION(ISO)

SALIENT FEATURE :

Layered Architecture

OPEN SYSTEM is a model that allows any two differentsystems to communicate regardless of their underlyingarchitecture

PEER-TO-PEER PROCESS :

The process on each machine that communicate ata given layer are called peer-to-peer processes

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WeWe useuse thethe concept concept of of layerslayers inin our our daily daily lifelife.. As As anan example,example, let let ususconsider consider twotwo friendsfriends whowho communicatecommunicate throughthrough postal postal mail mail.. TheThe process processof of sending sending a a letter letter toto aa friend friend would would bebe complex complex if if therethere werewere nono servicesservicesavailableavailable fromfrom thethe post post officeoffice..

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The philosopher-translator-secretaryarchitecture.


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Layers 1,2 & 3 (Physical, Data & Network) can begrouped to call as Network Support Layer Normallyimplemented by a combination of hardware andsoftware except the physical layer which is purelyhardware.

Layers 5,6, & 7(Session, Presentation andApplication) can be grouped to call as User Support Layer Normally implemented bysoftware

Each interface defines whatinformation and services a layer mustprovide for the layer above it.

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FUNCTIONS OF PHYSICAL LAYERS :Physical layer coordinates the functions required totransmit a bit stream over a physical medium.

It deals with the mechanical and electrical specifications of theinterface and transmission medium.

Data entering into physical layer are 0s & 1s. To be transmitted,bits must be encoded into signals electrical or optical. Physicallayer defines this encoding.

It determines the transmission rate bits per second, and alsothe duration of the bit (how long bit should stay in the network).

The sender and receiver must be in synchronization.

Line configuration : point-to-point/ multipoint

Physical topology : how the devices are connected to make anetwork

Transmission mode : direction of transmissionfull duplex two devices can send and receive at the same timehalf duplex two devices can send and receive but not at the sametime.

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DATA LINK LAYER :

Data link layer transforms the physical layer to areliable link and is responsible for node-to-nodedelivery. It makes the physical layer appear error freeto the network layer.

Framing : The data link layer divides the stream of bits receivedfrom the network layer into manageable data units called frames.

Physical addressing : If the frames are to be distributed todifferent systems on the network, the data link layer adds a header tothe frame to define t he physical address of the sender and/or receiver of the frame. If the frame is intended for a system outside thesenders network, the receiver address is the address of the devicethat connects one network to the next.

Flow control : to avoid queue

Error control : The data link layer adds reliability to t he physicallayer by adding mechanisms to detect and retransmit damaged or lost frames.

Access control : when two or more devices are connected to thesame link, data link layer protocols are necessary to determine whichdevice has control over the link at any given time.

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NETWORK LAYER :This layer is responsible for the source to destinationdelivery of a packet possibly across multiple networks. TheData link layer oversees the delivery of the packet betweentwo systems on the same network, the network layer ensures that each packet gets from its point of origin to itsfinal destination.

Logical addressing : The physical addressing implemented by thedata link layer handles the addressing problem locally. If a packetpasses the network boundary, we need another addressing system tohelp distinguish the source and destination systems. A network layer address a header the packet coming from the upper layer thatincludes the logical addresses of t he sender and receiver.

Routing : When independent networks are connected together tocreate an internetwork (a network of networks), the connectingdevices (called router or gateways) route the packets to their finaldestination.

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TRANSPORT LAYER :

This layer is responsible for source-to-destinationdelivery of the entire message. Network layer oversees the end-to-end delivery of the individualpackets, it does not recognize any relationshipbetween those packets.

Transport layer is also involved in establishingconnection between two end ports. A connection isa single logical path between the source anddestination that is associated with all packets in amessage. Creating connection involves three steps: connection establishment, data transfer andconnection release.

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Contd

SERVICE POINT ADDRESSING : this layer is al so important for handing over the data to the specific process (program) running in anysystem. For this transport layer header include a type of address calledservice point address (or port address).

SEGMENTATION & REASSEMBLY : Providing sequence nu mber for each message packets before transmission and re-assembling them atthe receiver end.

CONNECTION CONTROL : Transport layer can be either connectionless or connection oriented. A connection less transport layer treats each segment as an independent packet and delivers it to thetransport layer at the destination machine. A connection orientedtransport layer makes a connection with the transport layer at thedestination machine before delivering packets.

FLOW CONTROL : Like data link layer, this layer is responsible for flow control. However, flow control at this layer is performed end to endrather than across a single link.

ERROR CONTROL : Like data link layer, this layer is responsible for error control. As in flow control it is performed end to end rather thanacross a single link. Sending transport layer makes sure that the entiremessage arrives at the receiving transport layer without error.

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SESSION LAYER :The session layer is the network dialog

controller. It establishes, maintains andsynchronizes the interaction between communicatingsystems.

DIALOG CONTROL : This layer allo ws two systems to enter into a dialog. It allows the communication between twoprocesses to take place either in half-duplex or full-duplex.

SYNCHRONIZATION : This layer is responsible for addingcheckpoint(synchronization point)into a stream of data.


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PRESENTATION LAYER

This layer is concerned with the syntax and semantic sof the information exchanged between two systems.

TRANSLATION : Process of changing the information availablein character strings, numbers, so on..(in various process) to bitstream there should be a common encoding procedure. Sincedifferent computers follow different encoding systems, it is theresponsibility of the presentation layer to change the sender dependent format to common format. The receiving endpresentation layer will again convert this common format intoreceiver dependent format.

ENCRYPTION : It is essential for maintaining the privacy of data transfer

COMPRESSION : It is essential for transmitting data likemultimedia related informations like audio and video.

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APPLICATION LAYER :This layer enables the user, whether human or software, toaccess the network. It provides user interfaces and supportfor services such as electronic mail, remote file access andtransfer, shared database management, and other distributed information services.

NETWORK VIRTUAL TERMINAL :A network virtual terminal is a software version of a physical terminal andallows a user to log on to a remote host. To do so, the applicationcreates a software emulation of a terminal at the remote host. The userscomputer talks to the software terminal, which in turn , talks to the host,and vice versa. The remote host believes it is communicating with on of its own terminals and allows you to log on.

FILE TRANSFER, ACCESS AND MANAGEMENT : This applicati onallows a user to access files in a remote computer, to retrieve files from aremote computer and to manage or control files in a remote computer.

MAIL SERVICES : This is the basis for email forwarding and storage

DIRECTORY SERVICES : This appli cation provides distributeddatabase sources and access for global information about variousobjects and services.


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APPLICATION LAYER

PRESENTATION LAYER

SESSION LAYER

TRANSPORT LAYER

NETWORK LAYER

DATA LINK LAYER

PHYSICAL LAYER

TO ALLOW ACCESSTO NETWORKRESOURES

TO TRANSLATE,ENCRYPT ANDCOMPRESS DATA

TO PROVIDERELIABLE END-TO-END MESSAGEDELIVERY ANDERROR RECORERY

TO ESTABLISH,MANAGE, ANDTERMINATE SESSIONS

TO MOVE PACKETS FROMSOURCE TO DESTINATION;TO PROVIDEINTERNETWORKING

TO ORGANIZE BITSINTO FRAMES; TOPROVIDE NODE-TO-NODE DELIVERY TO TRANSMIT BITS OVERA MEDIUM; TO PROVIDE

MECHANICAL ANDELECTRICALSPECIFICATIONS

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Application All Analyzer P resentation P eople P eripheralS ession S eem S ystemTransport To TouchNetwork Need NotData Link Data DoP hysical P rocessing P lease

Remembering the Layers

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Encapsulation

Application

Presentation

Session

Transport

Network

Data Link

Physical

DATA

DATA

DATA

DATA

DATA

DATA

Presentation Header

Session Header

Transport Header

Network Header

Data Link Header Data Link Trailer

PH

SH

TH

NH

DLH DLT

PH

PH

PH

PH

SH

SH

SH

TH

THNH

Bits

To Destination Device


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OSI as Framework for Standardization

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Elements of Standardization required in eachlayer of protocol

Protocol specification Operates between the same

layer on two systems May involve different

operating system Protocol specification must

be precise Format of data units Semantics of all fields allowable sequence of

PDUs Service definition

Functional description of what is provided

Addressing Referenced by SAPs

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lecture 2 - osi model

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