Open Systems Interconnection Communication (OSI) Model

Developed by the ISO (International Organization for Standards) in 1983, the Open Systems Interconnection (OSI) reference model divides communication functions into seven-layer standard protocol system. The OSI model is a framework for developing a coordinated system of standards. The seven layers are described as follows:

1 Physical Layer

This layer describes the means for bit transmission to and from physical components of the network. This is where the data bits are equated to electrical, optical or other signals.  It deals with hardware issues such as the types of cable and connectors to be utilized, synchronizing the data transfer and signal levels. Examples of systems defined at physical layer include Ethernet, Token ring, RS-422, RS-485, RS-232, and Bell 202.

Related: RS-232, RS-422 and RS-485 Networks

2 Data Link Layer

This layer defines the protocols for sending and receiving messages, error detection and correction, and the proper sequencing of transmitted data. It is focused on packing data into packets and placing them on the cable and then taking them off the cable at the receiving end. Examples include Token passing, CSMA/CD, Ethernet, etc.

3 Network Layer

This layer deals with communication paths and the addressing, routing and control of messages on the network and thus making certain that the messages get to the right destinations. The transport layer protocols commonly employed are Internet Transmission Control Protocol (TCP) and Novell’s Sequenced Packet Exchange (SPX).

4 Transport Layer

This layer provides for reliable end-to-end message transport. The transport layer is focused on establishing and maintaining the connection between the transmitter and receiver. Some of the transport layer protocols are: Internet Transmission Control Protocol (TCP), Novell’s Sequenced Packet Exchange (SPX), User Datagram Protocol (UDP), etc.

5 Session Layer

This layer is concerned with the establishment of dialogues between application processes which are connected together by the network. It is concerned with determining when to turn a communication between two stations ON or OFF. Examples include: Sockets, NetBIOS, etc.

Also read: What is the difference between Broadband and Baseband Transmission?

6 Presentation Layer

This layer defines how the encoded data that is transmitted is presented in an appropriate form for the user manipulation.  In other words, this is where data gets converted between different formats. Examples include: ASCII, EBCDIC, MPEG, JPG, MP3, etc.

7 Application Layer                                   

This layer is concerned with the actual user information processing function and application-specific services, that to say, this is where the digital data takes on practical meaning in the context of some human or overall system function.

It provides functions like file transfer (FTP) or electronic mail which a station can use to communicate with other systems on the network. Other examples include: HTTP, Telnet, SSH.

Also read: Network Access Control Techniques