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DATA

COMMUNICATION

Lecture-8 & 9

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Recap of Lecture 7

Roles of Computer in Network Peer

Client

Server

Network Models Peer to Peer (Workgroup)

Server Based (Domain Environment)

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Overview of Lecture 8 & 9

The OSI Model

Function of Layers

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The layered model that dominated data communications and

networking literature before 1990 was the Open SystemsInterconnection (OSI) model.

Everyone believed that the OSI model would become the ultimate

standard for data communications, but this did not happen.

The TCPIIP protocol suite became the dominant commercial

architecture because it was used and tested extensively in theInternet; the OSI model was never fully implemented.

Established in 1947 by International Standards Organization (ISO)

An ISO standard, that covers all aspects of network

communications is the Open Systems Interconnection (OSI) model.

It was first introduced in the late 1970s.

No physical existence, but it is conceptual that helps to

understand the complex methods of data transmission among

various devices.

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How physically computers communicate and connect.

How bits are presented in a network medium.

How this ensure that data will reach on the target

error free and it will be received by correct computer. How network devices communicate, and how data is

converted from one format to another.

How computer comes to know that when it should

transmit the data and when not. How network devices maintain proper rate of data flow.

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Layered Tasks

Sender, Receiver and Carrier

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Layered Architecture

The OSI model is composed of seven layers ;

Physical (layer1), Data link (layer2), Network (layer3)

Transport (layer4), Session (layer5), Presentation (layer6)

Application (layer7) Layer

Designer identified which networking functions had relateduses and collected those functions into discrete groups thatbecame the layers.

The OSI model allows complete interoperability betweenotherwise incompatible systems.

Each layer uses the services of the layer immediately below it.

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Seven layers of the OSI model

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Peer-to-peer Processes

Layer x on one machine communicates with layer x onanother machine - called Peer-to-Peer Processes.

Interfaces between Layers

Each interface defines what information and

services a layer must provide for the layer aboveit.Well defined interfaces and layer functions

provide modularity to a network Organizations of the layers

Network support layers : Layers 1, 2, 3User support layer : Layer 5, 6, 7It allows interoperability among unrelated software systems

Transport layer (Layer 4) : links the twosubgroups

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The interaction between layers in the OSI model

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Figure 2.4An exchange using the OSI model

The data portion of a packet at level N-1 carries the wholepacket from level N.The concept is called encapsulation.

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Physical Layer

Physical layer coordinates the functions required to transmit

a bit stream over a physical medium.

The physical layer is responsible for movements of individual

bits from one hop (node) to the next.

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Physical Layer

Physical layer is concerned with the following:

(deal with the mechanical and electrical specification of theprimary connections: cable, connector) Physical characteristics of interfaces and medium

Representation of bits / Encoding Data rate : transmission rate

Synchronization of bits

Line configuration

Physical topology

Transmission mode baseband and broadband transmissionsMultiplexing /De multiplexing

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Data Link Layer

The data link layer is responsible for moving

frames from one hop (node) to the next.

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Data Link Layer

Sends data frames from the Network layer to the Physicallayer.

On the receiving end, it packages raw bits from the physicallayer into the data frames.

A data frames is an organized, logical structures in whichdata can placed.

The data link layer is responsible for providing the error freetransfer of these frames from one computer to anotherthrough the physical layer.

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Data Link Layer

waits for an acknowledgement

The Cyclical Redundancy Check (CRC) represents errorcorrection and verification

Data Link Layer is sub-divided into two sub-layers, i.e. Logical Link Control (LLC),

Media Access Control (MAC).

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Data Link Layer

LOGICAL LINK CONTROL

Responsible for establishing and maintaining links (i.e.either logical or physical) between the communicatingdevices.

MEDIA ACCESS CONTROL

controls the way in which multiple devices share the samemedia channel.

It communicated directly with the NICs. All NICs has a unique 12-digit hexadecimal MAC

addresses that are used to establish logical linksbetween computers on the same LAN.

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Data Link Layer

Major duties

Responsible for Node-To-Node Delivery

Framing

Physical addressing

Flow control

Error control

Access control

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Data Link Layer

Hop-to-hop (node-to-node) delivery

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Network Layer

Responsible for the delivery of individual packets from thesource host to the destination host.

Ensures that each packet gets from its point of origin to its final

destination responsible for

Logical addressing

and translating it to physical addresses.

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Network Layer

break large packets into smaller chunks.

Route determination.

Also manages traffic problems on the network

packets switching Routing

Controlling the congestion of data.

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Network Layer

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Network Layer Logical addressing

Routing

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Transport Layer

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

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Transport Layer

This layer ensures that packets are delivered error free, in

sequence, and with no losses or duplications.

This layer repackages messages, dividing long messages into

several packets to be transmitted efficiently over the network. At the receiving end, the transport layer unpacks the

messages, resembles the original messages and typically sends

an acknowledgement of receipt.

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Transport Layer

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Transport Layer

Service port addressing

Segmentation and reassembly

Connection control

Flow control

Error control

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Session Layer The session layer is responsible for dialog control and

synchronization.

Session Creation, Maintenance and termination.

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Session Layer

allows two applications on different computers to

establish, use and end a connection called a session.

performs name recognition and the function, such as

security, needed to allow two applications tocommunicate over the network.

provides synchronization between user tasks by placing

checkpoints in the data stream.

Also implements dialog control between communicating

processes, regulating which side transmits, when, for

how long, and so on.

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Presentation Layer

The presentation layer is responsible for translation,

compression, and encryption

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Presentation Layer

determines the format used to exchange data amongnetworked computers.

It can be called the networks translator.

At the sending computer this layer translates data from aformat sent down from the Application layer into acommonly recognized, intermediary format.

At the receiving computer, this layer translates theintermediary format into a format useful to that computers

Application layer.

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Presentation Layer

Responsible for

protocol conversion

translating the data

encryption

Changing on converting the character set

and expanding graphics commands

data compression to reduce the number of bits that needs to betransmitted.

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Application Layer

The application layer is responsible for providing services

to the user.

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Application Layer

serves as the window for the application processes toaccess network service.

The major duties of the application Network virtual terminal

File transfer, access, and management Mail services

Directory services

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Summary

The OSI Model

Function of the Layers

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Suggested Reading

Section

2.1,

2.2, 2.3,

Data Communications and Networking 4th Edition by

Behrouz A. Forouzan