THE OSI MODEL

Presented By:Amit Pandey

History Of Communication Networks

Initially networks had their own set of standards.

They were more hardware oriented. Each manufacturer developed their own

communication protocols for their networks.

EXAMPLES

IBM’s SNA(System network architecture) DEC’s DNA(Digital network architecture) Burroughs’s BNA(Burroughs Network

Architecture) Hewlett Packard’s DSN(Distributed

systems network)

LIMITATIONS

Data communication protocol of one network was not compatible with any other.

Standards of same networks were also not compatible with their own earlier versions.

Incompatibilities creates bottleneck in efficient and proper utilization of network resources.

Emergence Of OSI Model

ISO(International Standards Organization) recognized the problem.

It establishes the subcommittee to develop an international standard on network architectures.

The result was Open System Communication(OSI Model)

DEFINITION

An Open system is a model that allows any two different systems to communicate regardless of their underlying structure.

Purpose Of OSI Model

To open communication between different systems without requiring changes to the logic of underlying hardware and software.

OSI is not a protocol , it is a model for understanding and designing a flexible, robust and interoperable network architecture.

OSI is designed in a highly structured way.

It is a seven layer architecture. Each layer has a separate set of defined

protocols. Each layer has an independent function

and deals with one or more specific aspects of the communication.

First 3 Layers – in hardware Next 2 Layers - in operating system Next 1 Layer – in library subroutines Last Layer – in user’s address space

The OSI Model

APPLICATION LAYER

PRESENTATION LAYER

SESSION LAYER

TRANSPORT LAYER

NETWORK LAYER

DATA LINK LAYER

PHYSICAL LAYER

NODELINK LINK

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

DATALINK

PHYSICAL

APPLICATION

PRESENTATION

SESSION

TRANSPORT

NETWORK

DATALINK

PHYSICAL

NETWORK

DATA LINK DATA LINK

PHYSICAL PHYSICAL

COMMUNICATION PATTERN

Within the single machine, each layer calls upon the services of the layer just below it.

Between machines, layer x on one machine communicate with layer x on the another machine.

This communication is governed by protocols.

The processes on each machine that communicate at a given layer are called peer-to-peer processes.

Contd..

At physical layer communication is direct. Each layer in the sending machine adds

its own information to the message it receives from the layer just above it and passes the whole package to the layer just below it.

This information is added in the form of headers and trailers.

Headers are added at layers 6,5,4,3,2. A trailer is added at layer 2.

Contd..

At layer 1 the entire package is converted to a form that can be transferred to the receiving machine.

At receiving machine, the entire message is unwrapped layer by layer.

An interface between each pair of adjacent layer facilitates passing of data and information down through the layers.

Sub grouping of layers

Physical, Data and Network layers – Network Support Layers.

Session, Presentation and Application – User’s support layers.

Transport layer – end-to-end transmission.

PHYSICAL LAYER

It coordinates the functions required to transmit a bit stream over a physical medium.

Defines functions and procedures for physical devices and interfaces to perform transmission.

Physical layer data consists of a stream of bits to be transmitted and also their type of encoding.

It also defines the data transmission rate i.e. number of bits sent each second.

Contd..

It is also concerned with the connection of devices to the medium i.e. point-to-point configuration or multipoint configuration.

It also defines the physical topology. It defines the direction of transmission

between two devices i.e. simplex, duplex.

Data Link Layer

It transforms the physical layer to a reliable link and is responsible for node to node delivery.

It makes the physical layer appear error free at the network layer.

It divides the stream of bits received from the network layer into manageable data units called frames.

If the frames are to be distributed to different systems on the network , it adds a header on the frame to define source address and destination address.

Contd..

If the frame is meant for a system outside network, it adds the address of receiver’s device that connects the network.

If the rate of receiving data is less than the rate of data sent then it imposes a flow control mechanism.

It helps the physical layer to detect and retransmit the damaged or lost frames and prevents duplication of frames.

It provides access control to two or more devices connected to the same link.

NETWORK LAYER

It is responsible for the source to destination delivery of a packet across a multiple network.

It ensures that each packet gets from its point of origin to its final destination.

If two systems are connected to same network there is no need of network layer.

Of two systems are attached to the different networks with connecting devices between them , then network layer is responsible for source to destination delivery of the data.

Contd..

The physical addressing done by data link layer handles the problem locally.

If the data packet passes the network boundary, network layer adds a header to the packet coming from upper layer which consists of the logical addresses of the sender and the receiver.

Network layer provides the mechanism to route the packets to their final destination through the routers.

TRANSPORT LAYER

Network layer oversees the delivery of individual packets, it does not recognize any relationship between those packets.

Transport layer is responsible for source to destination delivery of the entire message in an intact and orderly manner.

It looks after the error control and flow control at the source to destination level.

It creates a logical connection between the source and destination associated with all packets in a message.

Contd..

Transport layer header includes a service point address which gets the entire message to the correct process on the destination computer.

A message is divided into transmittable segments containing a sequence number. These numbers enables the transport layer to reassemble the message correctly.

It is also responsible for flow control which is performed end to end rather than across a link.

SESSION LAYER

This layer is dialog controller . It establishes , maintains and synchronizes the interaction between communicating systems.

It allows two systems to enter into a dialog and allows communication between two processes to take place either in fill duplex or half duplex mode.

It allows a process to add checkpoints to the data. Ex:- If a file of 2000 pages is being transmitted , insert checkpoints after every 500 pages.

PRESENTATION LAYER

It is concerned with the syntax and semantics of the information exchanged between two systems.

To exchange the information every message has to be changed to bit streams before being transmitted. This layer provides interoperability between different encoding systems.

It is also responsible for the encryption and decryption of the message for the security purpose.

Contd..

This layer is also responsible for the data compression. It reduces the number of bits to be transmitted. It is important for the transmission of multimedia.

APPLICATION LAYER

It enables the user, whether human or software to access the network.

It provides user interface and support for services like e-mail , shared database etc.

A virtual terminal is a software version of the physical terminal and allows a user to log on to a remote host.

The application layer creates a virtual terminal to facilitate the communication between the remote terminal and host.

Contd..

This layer allows the user to access files, retrieve files and to manage and control files on a remote computer. This is called FTAM(File transfer, access, and management).

It provides basis for e-mail forwarding and storage.

It provides distributed database sources and access for global information about various objects and services.

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