Open Systems Interconnection (OSI) model

OSI Reference Model - Introduction & Advantages

In 1980s, the International Standards Organization (ISO) recognized the need for a standard network model.

The Open Systems Interconnection (OSI) reference model, released in 1984, addressed this need.
The OSI model describes how information makes its way from application programs through a network medium to another application program in another computer

The seven layers of the OSI model are:-

Application
Presentation
Session
Transport 
Network
Data-link
Physical

The acronym used to remember these layers is: All People Seem To Need Data Processing.
The lower two OSI model layers are implemented with hardware and software.

The upper five are generally implemented only in software.

OSI Layers & Its Functions

The application layer

The application layer of the OSI model is the layer that acts as users interface. it provides services to application programs outside the scope of the OSI model.

Main functions are:-
• identifies and establishes the availability of the intended communication partner.
• synchronizes in sending and receiving applications.
• Establishes agreement on procedures for control of data and integrity error recovery.
• Determines whether sufficient resources for the intended communications exist.

Devices:-

• Browsers,Search engines,E-mail programs,Newsgroup and chat programs,Telnet,SNMP, Etc.,


The presentation layer

It ensures that the information sent by the application layer of one system will be readable by the application layer of another system. It provides a common format for transmitting data across various systems.
The presentation layer concerns itself not only with the format and representation of actual user data, but also with data structure used by programs.

Devices:-

• Encryption

• EBCDIC and ASCII

• GIF & JPEG

Top

The Session Layer

The main function of the OSI model's session layer is to control "sessions", which are logical connections between network devices. A session consists of a dialog, or data communications conversation, between two presentation entities.

• simplex (one-way)
• half-duplex (alternate)
• full-duplex (bi-directional)
Simplex conversations are rare on networks. Half-duplex conversations require a good deal of session layer control, because the start and end of each transmission need to be monitored.

Most networks are of course capable of full-duplex transmission, but in fact many conversations are in practice half-duplex.
Devices:-

• Network File System (NFS)
• Concurrent database access
• X-Windows System
• Remote Procedure Call (RPC)
• Digital Network Architecture

The Transport Layer

The transport layer of the OSI model as a boundary between the upper and lower protocols.The transport layer provides a data transport service that shields the upper layers from transport implementation issues such as the reliability of a connection.

The transport layer provides mechanisms for:-
• multiplexing upper layer applications
• the establishment, maintenance, and orderly termination of virtual circuits
• transport fault detection and recovery

Devices:-

• TCP, UDP, SPX and Sliding Windows.


The Network Layer

The network layer sends packets from source network to destination network.It provides consistent end-to-end packet delivery services to its user, the transport layer. Routers are special stations on a network, capable of making complex routing decisions.The network layer is the domain of routing.One of the functions of the network layer is "path determination".
Path determination enables the router to evaluate all available paths to a destination and determine which to use. It can also establish the preferred way to handle a packet.After the router determines which path to use it can proceed with switching the packet. It takes the packet it has accepted on one interface and forwards it to another interface or port that reflects the best path to the packet's destination.

Devices:-

• IP, IPX, Routers, Routing Protocols (RIP, IGRP, OSPF, BGP etc), ARP, RARP, ICMP.


The Data-Link Layer

This layer is responsible for providing reliable transit of data across a physical link. The data-link layer is concerned with

• physical addressing; Bridges, Transparent Bridges, Layer 2 Switches
• network topology; CDP
• line discipline (how end systems will use the network link)
• error notification
• ordered delivery of frames
• flow control
• Frame Relay, PPP, SDLC, X.25, 802.3, 802.3, 802.5/Token Ring, FDDI.

The Physical Layer

Layer one of the OSI model is the physical layer. The physical layer is concerned with the interface to the transmission medium. At the physical layer, data is transmitted onto the medium (e.g. coaxial cable or optical fiber) as a stream of bits.
So, the physical layer is concerned, not with networking protocols, but with the transmission media on the network.
The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. This layer puts 1's & 0's onto the wire.
Characteristics specified by the physical layer include

• voltage levels
• timing of voltage changes
• physical data rates
• maximum transmission distances
• physical connectors

Devices:-

• Hubs, FDDI Hardware, Fast Ethernet, Token Ring Hardware