Cn osi model

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H. Vignesh RamamoorthyM.Sc (Softw Engg), MCA, Assistant ProfessorUG – Computer Science

• OSI: Open System Interconnection .

• Developed by the International Organization for Standardization in 1974.

• There are seven layers in OSI reference Model .

• process or role

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OSI Reference 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

Upper Layer or

Software Layer

Lower Layer or

Hardware Layer

Heart of OSI

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Application Layer

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

Ports are Entry and Exit Points to the Layer

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



SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical


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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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2180 25 6753 69

Data flow from Application Layer

Data


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



SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical


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Data flow from Presentation Layer

Data

DataApplication Application


SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

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Session Layer

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

Examples :

RPC Remote Procedure Call SQL Structured Query LanguageNFS Network File System



SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

SessionSession

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Data flow from Session Layer

Data

Data

Data



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 the heart of OSI Layers. Following tasks are performed at the Transport Layer : -

• Identifying Service

• Multiplexing & De-multiplexing

• Segmentation

• Sequencing & Reassembling

• Error Correction

• Flow Control



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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SessionSession

Multiplexing & De-multiplexing

2180 25 6753 69

Transport Transport

TCP - 6 UDP - 17

Network Network

Data LinkData Link

PhysicalPhysical

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Data flow from Transport Layer

Data

Data

Data

DataTHSegment



SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

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Network Layer

Network Layer is responsible for providing best path for data to reach the destination. Logical Addressing works on this layer. Router is a Network Layer device.

It is divided into two parts

• Routed Protocols

e.g. IP, IPX, Apple Talk.

• Routing Protocols

e.g. RIP, IGRP, OSPF, EIGRP



SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

Network Network

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Data flow from Network Layer

Data

Data

Data

Segment

Segment

NH

e.g. Router

Packet



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 a 48 bit address i.e. 12 digit Hexadecimal Number.

It is also responsible for Error DetectionDevices working on Data Link Layer are Switch, Bridge, NIC.



SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

Data LinkData Link

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e.g. Switch

Data flow from Data Link Layer

Data

Data

Data

Segment

DH

Packet

DT



SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

PacketPacketFrame

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Physical Layer

Physical Layer is responsible for electrical, mechanical and procedural checks. Data will be converted into Binary (i.e.) 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.



SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysicalPhysicalPhysical

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Data flow from Physical Layer

Data

Data

Data

Segment

Packet

Frame

Bits

e.g. Hub



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



SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical



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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Transport Transport

InternetInternet

Network Access

Network Access

Comparing OSI with TCP/IP Layers

OSI Layers TCP/IP Layers



SessionSession

Transport Transport

Network Network

Data LinkData Link

PhysicalPhysical

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Thank You

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Cn osi model

Education

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