01 01 OSI Reference Model

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CIT 513: Introduction

to ComputerCommunications

Mohammed A. Salehhttp://ifm.ac.tz/staff/msaleh/CIT513.html

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http://ifm.ac.tz/staff/msaleh/CIT742.htmlhttp://ifm.ac.tz/staff/msaleh/CIT742.html


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The OSI Reference Models

What is the OSI reference model?

In the 1970 the International Standards Organization

(ISO) developed the Open Systems Interconnection (OSI)

reference model to define the basic standards for networkcommunication

It sets up the rules that all computers have to use no

matter whatprotocolthey are using

The OSI model is made up seven (7) layers.

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Cont

Easy way to remember the 7 layers of the OSI model

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Application

Presentation

SessionTransport

Network

Data Link

Physical

All

People

Seem

To

Need

Data

P

rocessing

Away

Pizza

Sausage

Throw

Not

Do

P

lease


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Cheat Sheet

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Cont This is what I would like to call the cheat sheetof the OSI

model.

The sheet gives out what the OSI model is comprised of,

not everything but covers what you need to know at thislevel

The sheet clearly shows that there are two OSI models,

one on the sending computer and the other on the

receiving computer

From the sending computer the arrows are pointing

downwards whereas on the receiving side the arrows are

pointing upwards

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Cont The reason for the upward and downward direction is to

show how data travels through the OSI model

This is how data travels through logically.

Even though data travels down travels in the upward anddownward directions, the dashed lines represents the

virtual communication that takes place.

Virtual communication means once data travels across

the network, the receiving physical layer onlyacknowledges what the sending physical layer did to the

data and then passes it up to the data link layer.

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Cont As data travels down the layers, each layers adds its own

piece of information to the data

Each layer adds a slice to one side of the data, except

data link layer that also adds information to the oppositeon the other side.

Whatever is added to the data by the layers can also be

said that the layers add header information to the data

(a) (b)

(a) Original data from the application layer moving down

the other layers

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Cont the additional data added by the data link layer is known

as trailer information

The reason is to make sure the data sent is not damaged

by the air Once the data arrives on the receiving side, it is stripped

off as it moves up the layers, leaving only the original sent

data

Video on the OSI Layers

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About the OSI Model The OSI model divides network architecture into seven

layers.

The term layerrefer to a collection of similar functions that

provide service to the layer above it and requests servicefrom the layer below it.

Each layer is responsible for a very specific task.

The function of one layer is not known by other layers,

which makes the OSI model a suitable model to be usedby network communications.

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Functions of the OSI Layers

Physical Layer

It identifies the physical characteristics of the network,

including the following specifications:

Hardware: The type of media used on the network such as typeof cable, type of connector, and pin out format for cables.

Topology: The physical layer identifies the topology to be used in

the network. Common topologies include ring, mesh, star, and

bus.

Data-Link Layer Is responsible for getting data to the physical layer so that

it can be transmitted over the network

Also responsible for error detection, error correction, and

hardware addressing 10


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Cont The term frame is used to describe the logical grouping of

data at the data-link layer.

The data-link layer has two distinct sub-layers the Media

Access Control (MAC) sub-layer and the Logical LinkControl (LLC) sub-layer. MAC layer: The MAC address is defined at this layer. The MAC

address is the physical or hardware address burned into each

NIC.

LLC layer The LLC layer is responsible for the error and flow-control mechanisms of the data-link layer

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Cont

Network Layer

The primary responsibility of the network layer is routing

providing mechanisms by which data can be passed from

one network system to another It does not specify how the data is passed, but rather

provides the mechanisms to do so.

Functionality at the network layer is provided through

protocols Protocols at the network layer are also responsible for

route selection, which refers to determining the best path

for the data to take throughout the network

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Cont In contrast to the data-link layer, which uses MAC

addresses to communicate on the LAN, network protocols

use software configured addresses and special routing

protocols to communicate on the network.

The main protocol operating at this layer is the Internet

Protocol (IP)

The termpacketis used to describe the logical grouping

of data at the network layer.

Transport Layer

It provides mechanisms to transport data between

network devices

Primarily it does this in three ways:

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Cont Error checking Protocols at the transport layer ensure that data

is sent or received correctly.

Service addressing Protocols such as TCP support many

network services. The transport layer makes sure that data is

passed to the right service at the upper layers of the OSI model. Segmentation: To traverse the network, blocks of data need to

be broken down into packets that are of a manageable size for

the lower layers to handle

Session Layer

Is responsible for managing and controlling the

synchronization of data between applications on two

devices

It does this by establishing, maintaining, and breaking

sessions. 14


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Cont

Presentation Layer

Its basic function is to convert the data intended for or

received from the application layer into another format

Transforms data into the form that only the applicationlayer can accept

Some common data formats handled by the presentation

layer include the following:

Graphics files JPEG, TIFF, GIF. Text and data: translate data into different formats such as

American Standard Code for Information Interchange (ASCII) and

the Extended Binary Coded Decimal Interchange Code (EBCDIC).

Sound/video MPEGs, QuickTime video, and MIDI files

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Cont A very important function of the presentation layer is

encryption, which is the scrambling of data so that it can't

be read by anyone other than the intended recipient.

Application Layer The function of the application layer is to take requests

and data from the users and pass them to the lower layers

of the OSI model

Incoming information is passed to the application layer,which then displays the information to the users

The most common misconception about the application

layer is that it represents applications that are used on a

system such as a Web browser, word processor, or a

spreadsheet 16


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Cont Instead, the application layer defines the processes that

enable applications to use network services, like web,

email, file transfer, terminal emulation programs

Each of the services has is represented by acorresponding protocol;

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Service Protocol

Web Hyper Text Transfer Protocol (HTTP)

Email Simple Mail Transfer Protocol (SMTP)

File Transfer File Transfer Protocol (FTP)

T E programs telnet


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Questions

Documents

01 01 OSI Reference Model