© N. Ganesan, Ph.D. , All rights reserved.

Chapter

OSI Reference Model

ISO and OSI Defined

• ISO– International Standards Organization

• OSI– Open Systems Interconnect/interface

OSI Model Background

• OSI model Introduced in 1978 and revised in 1984 by ISO

• It is first standardization of protocol of N/W.

• It is also called OSI because it connect open system.

• Open system means a system that is open for communication with other system.

OSI Model Background

• It give set of rules , how to different n/w communicate with each other.

• It used layered approach because it divide work and assign work to each layer.

• Protocol is used to implement specific task.

• Protocol is set of rules.

OSI Model Background

• OSI model is called reference model because it provide reference to implement network communication.

• It tell just what each layer should do.

• OSI is not a physical model, it is set of guideline that application developer can use to create and implement applications that can run on network.

The Layered Approach to Communication

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data Link

1. Physical

Division of Layers

Upper Layers

Lower Layers

Middle Layer

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data Link

1. Physical

The Function of a Layer• Each layer deals with one aspect of

networking– Ex. Layer 1 deals with the communication

media• Each layer communicates with the

adjacent layers– In both directions– Ex: Network layer communicates with:

• Transport layer• Data Link layer

• Each layer formats the data packet– Ex: Adds or remove header

Role of Layers

7. Application

6. Presentation

1. Physical

Node A

Data Out

Data In

To/from Node B

The Role of Layers in Point-to-point

Communication

7. Application

1. Physical

7. Application

1.Physical

Node a Node b

7. Application Layer function

• This layer provide user interface and support services.

• It allow user to direct access to application and n/w services.

• Services include web service(HTTP), mail service(SMTP,POP3, IMAP), telnet services(remote login), file service(FTP, TFTP)

6. Presentation Layer function

• Data formatting is done by this layer

• Presentation layer is responsible for data encoding/decoding, encryption/decryption, compression/decompression

• It is also responsible for user authentication

• It is also responsible for translation services

5. Session Layer Function

• Session layer is dialog controller.• Session layer establish, maintain,

and terminate session between two systems.

• It is also provide synchronization service.

• Session layer decide transmission type like.. Simplex, half-duplex, full duplex.

4. Transport Layer Function

• It provide transport( transport provider).

• It provide end to end connection via virtual circuit.

• It is responsible for connection less and connection oriented communication.

• It is also responsible for error detection, error correction(data recovery), flow control.

4. Transport Layer Function

• Message is divided into transmittable segment and reassemble.

• Connection oriented communication is reliable, secure and provide ack. Ex. TCP

• Connection less communication is not reliable, secure and does not provide ack.

Ex.UDP •

3. Network Layer Function

• This layer is responsible for routing.

• Routing means communication between different network.

• It provide logical addressing which router use for path determination.

• Using IP, it manage device addressing, track the address of the devices on the network and determine the best path to data transfer.

• It is also responsible for IP to MAC and Mac to IP Mapping.

2. Data Link Layer Function

• Data link layer is responsible for taking packet from network layer and put it on network media .

• It convert packet into frame and vise versa.

• Data link layer has two sub layers. 1). Logic link control.(LLC) 2). Media access layer(MAC).

Data Link Layer Subdivision

• Logical Link Control (LLC) work on IP– It identify network layer protocol and

tells what to do once frame received.– Error and flow control

• Media Access Control (MAC) work on MAC– It provide access to media using Mac

address.– It define how to put frame on the

media.– Error detection and error correction ,

flow control.

1. Physical Layer Function

• It is responsible for transmit bits over media. – Carry data from the h higher layers

• It define following characteristics.– Electrical– Mechanical– Functional– Example: specify voltage, pin out of

cable.

Media Access Control Application

• Network Interface Card driver

NETWORKSOFTWARE

NETWORKCARD

NIC Driverfacilitates data transfer

Summary of layers

Layer Operations

• At each layer, additional information is added to the data packet

• An example would be information related to the IP protocol that is added at Layer 3

Formatting of Data Through the Layers

Application Header Presentation Header Session Header

Transport HeaderNetwork Header

Data Link Header and Trailer Physical Frame Preamble

Data formatting

Packet : General Format

Header Trailer

Data

A general concept of packets serves as a prerequisite tothe understanding of the ISO-OSI model.

Some Header Information Added at Various Layers

• Packet arrival information• Receiver’s address• Sender’s address• Synchronization character

Data

• Actual data• May contain error correction code

– Performed on individual characters of the data

– Example: Parity• Size may vary

– Depending on the protocol– Example

• 802.3 specifies range of data packet length

Tasks involved in sending letter

7 Layers

7. Application Layer

6. Presentation Layer

7. Session Layer

8. Transport Layer

9. Network Layer

10. Data Link Layer

11. Physical Layer

All People Seem

To Need Data

Processing

How Does It All Work Together

• Each layer contains a Protocol Data Unit (PDU)– PDU’s are used for peer-to-peer contact

between corresponding layers.

– Data is handled by the top three layers, then Segmented by the Transport layer.

– The Network layer places it into packets and the Data Link frames the packets for transmission.

– Physical layer converts it to bits and sends it out over the media.

– The receiving computer reverses the process using the information contained in the PDU.

2

Layer 7 Application All user access and net services

Layer 6 Presentation People data formatting

Layer 5 Session Seem Dialogues controller

Layer 4 Transport To Computer Segment TCP and UDP,IPX end to end transport ation and Quality of Service,

Layer 3 Network NeedRouter, brouter, layer 3 switch

Packet

Routable Protocols. (IP, IPX, AppleTalk) routing protocol(rip, igrp) IGMP ICMP, ARP, RARP

ip addressind Path Selection, routing

Data Protocols Words to RememberOSI Model Layer

Pneumonic devicesOSI Model layer and

har

Layer 1 Physical ProcessingRepeater, Hub (Multi-port), Cabling

Bit

Layer 2 DataBridge (2 Ports) or Switch and NIC, intelligent hub

FrameData Link -MAC -LLC

protocol defined by underlying network

bit transmission over media

Computer Data

FTP, Telnet, SMTP, SNMP, DNS

making bits to frame and host to host delieveryprotocol defined by underlying network