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• OSI means Open System Interconnect model.

• Developed by the International Organization for Standardization in 1974.

• It consists of seven layers.

• Each layer has a different but specific processing function.

OSI Model

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OSI Model Layers

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

Layer - 7

Layer - 6

Layer - 5

Layer - 4

Layer - 3

Layer - 2

Layer - 1

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Application Layer

Application Layer is responsible for providing Networking Services to user. It also known as Desktop Layer. Identification of Services is done using Port Numbers.

Ports are nothing but Socket i.e. Entry and Exit Point to the Layer

Total No. Ports 0 – 65535 Reserved Ports 0 – 1023 Open Ports 1024 – 65535

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

Application Application

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Examples of Networking Services

Service Port No.

HTTP 80

FTP 21

SMTP 25

TELNET 23

TFTP 69

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Application Application

2180 25 6753 69

How data flows from Application Layer

Data

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

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Presentation Layer

Presentation Layer is responsible for converting data into standard format.

Examples : ASCII, EBCDIC, JPEG, MPEG, BMP, MIDI, WAV, MP3

Following tasks are perform at Presentation layer :

Encoding – DecodingEncryption – DecryptionCompression – Decompression

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

PresentationPresentation

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How data flows from Presentation Layer

Data

DataApplication Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

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Session Layer

Session Layer is responsible establishing, maintaining and terminating session. Session ID also works at Session Layer.

Examples :

RPC Remote Procedure Call SQL Structured Query languageNFS Network File System

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

SessionSession

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How data flows from Session Layer

Data

Data

Data

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

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Transport Layer

Transport Layer is responsible for end-to-end connectivity. It is also known as heart of OSI Layers. Following task are performed at Transport Layer : -

• Identifying Service

• Multiplexing & De-multiplexing

• Segmentation

• Sequencing & Reassembling

• Flow Control

• Error Correction

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

Transport Transport

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Identifying Service

• Transmission Control

Protocol

• Connection Oriented

• Acknowledgement

• Reliable

• Slower

• Port No. 6

• e.g. HTTP, FTP, SMTP

• User Datagram

Protocol

• Connection Less

• No Acknowledgement

• Unreliable

• Faster

• Port No. 17

• e.g. DNS, DHCP, TFTP

TCP UDP

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Application Application

PresentationPresentation

SessionSession

Multiplexing & De-multiplexing

2180 25 6753 69

Transport Transport

TCP - 6 UDP - 17

Network Network

Data LinkData Link

PhysicalPhysical

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How data flows from Transport Layer

Data

Data

Data

DataTHSegment

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

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Network Layer

Network Layer is responsible for providing best path to data to reach destination. Logical Addressing sits on this layer. Device working on Network Layer is Router.

It is divided into two parts

• Routed Protocols

e.g. IP, IPX, Apple Talk.

• Routing Protocols

e.g. RIP, IGRP, OSPF, EIGRP

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

Network Network

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How data flows from Network Layer

Data

Data

Data

Segment

Segment

NH

e.g. Router

Packet

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

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Datalink Layer

Datalink Layer is divided into two Sub Layers :

• LLC – Logical Link ControlIt talks about Wan protocols e.g. PPP, HDLC, Frame-relay

• MAC – Media Access ControlIt talks about Physical Address. It is 48 bit Addressing i.e. 12 digit Hexadecimal No. It is also responsible for Error DetectionDevice working on Data Link Layer is Switch, Bridge, NIC.

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

Data LinkData Link

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e.g. Switch

How data flows from Data Link Layer

Data

Data

Data

Segment

DH

Packet

DT

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

PacketPacketFrame

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Physical Layer

Physical Layer is responsible for electrical, mechanical or procedural checks. Data will be converted in Binary that is 0’s & 1’s. Data will be in the form of electrical pulses if it is Coaxial or Twisted Pair cable and in the form of Light if it is Fiber Optic Cable.

Devices working at Physical Layer are Hubs, Repeaters, Cables, Modems etc.

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysicalPhysicalPhysical

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How data flows from Physical Layer

Data

Data

Data

Segment

Packet

Frame

Bits

e.g. Hub

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

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Segment

Packet

Frame

A B

Packet

Data

Data

Data

Bits

Data

Segment

Data Encapsulation & De-capsulation

Packet

Data

Data

Data

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysicalBits

DH DT

NH

THSegment

Packet

DT PacketDH DTDH

NH Segment NH

TH Data TH

Frame Packet

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Application Application

Transport Transport

InternetInternet

Network Access

Network Access

Comparing OSI with TCP/IP Layers

OSI Layers TCP/IP Layers

Application Application

PresentationPresentation

SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

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