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

OSI Model LayersApplication PresentationSessionTransport Network Data LinkPhysical

Application LayerApplication 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 65535Application

Example of HTTP requesthttp://www.zoomgroup.com

Example of HTTP requestHTTP Request http:// www.zoomgroup.comWebpageHTTP Request Listen on Port 80Sending HTTP ReplyWebpageReceived HTTP Reply http://www.zoomgroup.comWebpage

Example of FTP requestftp://ftp.microsoft.com

Example of FTP requestFTP Request ftp://ftp.microsoft.comFTP Request Listen on Port 21Sending FTP Reply

Received FTP Reply ftp://ftp.microsoft.com

Examples of Networking ServicesService Port No.HTTP80FTP21SMTP 25TELNET23TFTP69

Application How data flows from Application LayerData

Presentation LayerPresentation 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 Presentation

How data flows from Presentation LayerDataData

Session LayerSession 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 SystemSession

How data flows from Session LayerDataDataData

Transport LayerTransport 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 ServiceMultiplexing & De-multiplexingSegmentationSequencing & ReassemblingFlow ControlError CorrectionTransport

Identifying Service

Multiplexing & De-multiplexingTransport

SegmentationHello! How are you ?Hello!HowAreYou? Hello! How are you ?

Sequencing & ReassemblingHello! How are you ?Hello!HowAreYou? Hello! How are you ?

You Hello! How ? AreHello!HowAreYou?

Sequencing & ReassemblingHello! How are you ?Hello!1/5How2/5Are3/5You4/5?5/5 Hello! How are you ?

Hello! How are you ?Hello!1/5How2/5Are3/5You4/5?5/5Hello!1/5How2/5Are3/5You4/5?5/5

Error CorrectionHello! How are you ?Hello!1/5How2/5Are3/5You4/5?5/5 Hello! How are you ?

Hello!1/5Are3/5You4/5?5/5Hello!1/5Are3/5You4/5?5/5Segment MissingHow2/5How2/5Hello! How are you ?

Flow Control - WindowingSending 3-windowReceived 3-windowAck-Rec-3Sending5-windowReceived 4-windowAck. Received-4PC-A can send 4 Segment at a time to PC-B

How data flows from Transport LayerDataDataDataDataSegment

Network LayerNetwork 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 Protocolse.g. IP, IPX, Apple Talk.Routing Protocolse.g. RIP, IGRP, OSPF, EIGRPNetwork

Routed Protocols Hello! How are you ?

SegmentSource IP 192.168.1.1

Destination IP192.168.1.2Source IP 192.168.1.1

Destination IP192.168.1.2

Routing Protocols www.zoomgroup.com

How data flows from Network LayerDataDataDataSegmentSegmentPacket

Data link LayerDatalink 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.

Data Link

Error Detection CRC Check Hello! How are you ?

PacketSource MAC 00-20-18-C0-07-71

Destination MAC00-20-18-C0-07-72000003243422222896965543499323434349943446323111143333300000324342222289696554349932343434994344632311114Error DetectedPC-A CRC No. 33333 PC-B CRC No. 11114Error Detected For Error Correction Contact Source Transport layer

Error Detection CRC CheckPacketSource MAC 00-20-18-C0-07-71

Destination MAC00-20-18-C0-07-7200000324342222289696554349932343434994344632311114202020000032434222228969655434993234343499434463231111420202Source MAC 00-20-18-C0-07-71

Destination MAC00-20-18-C0-07-72

How data flows from Data Link LayerDataDataDataSegmentPacketPacketPacketFrame

Physical LayerPhysical Layer is responsible for electrical, mechanical or procedural checks. Data will be converted in Binary that is 0s & 1s. 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.Physical

Physical Layer ExampleFrame1010101010101010101010101010101001010110101010101010101011010101101010101010101010010101101010101010101010110101011010101010101010100101011010101010101010101

How data flows from Physical LayerDataDataDataSegmentPacketFrameBits

SegmentPacketFramePacketDataDataDataBitsDataSegmentData Encapsulation & De-capsulation PacketDataDataDataBitsSegmentPacketFrame

Application Transport InternetNetwork AccessComparing OSI with TCP/IP Layers OSI LayersTCP/IP Layers

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