Seven layers of the OSI model

The OSI model is a set of guidelines that describes the process that should take place when two computers communicate on a network.  It also shows how network hardware and software should be designed to make sure that they are able to work properly on a network.

Developed by the International organisation for standardisation, it quickly became the international standard for network communication, providing a structure that illustrates network communication as a string of seven layers, which are shown below;

·         Application

·         Presentation

·         Session

·         Transport

·         Network

·         Data Link

·         Physical

Layer 7 – application

This layer provides applications with network services, like things such as file transfer, database queries, e-mail and other network software services. Communication partners are identified through this layer.

Layer 6 – Presentation

The presentation layer works to alter into a form that the application layer can read, it will format and encrypt the data to be sent over the network, without hindrance from compatibility issues, and is sometimes called the syntax layer.

Layer 5 – Session

This layer establishes, manages and ends communication connections between applications. It controls whose turn it is to communicate, and if it is allowed to communicate simultaneously, it will manage things between each node. It offers three different types of communication, and they are;

Simplex - Transmits data in only one direction, meaning only one computer can transmit at any one time, in a certain direction, so it would have to pass through other nodes to get to the computer that needs it.

Half-duplex - Allows communication in two directions but in only one direction at a time, same as simplex, except it can travel anyway round the network instead, like a walkie talkie, can transmit either way, but only one at a time.

Full-duplex - Allows communication in both directions at the same time. Meaning any computer can communicate at any time, or all at the same time if required.

Layer 4 – Transport

The transport layer accepts data from the session layer, ensures complete data transfer by breaking it down into things called packets which are essentially the data broken into smaller chunks, and then passes it onto the network layer.

Layer 3 – Network

The network layer supplies a method of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer.

Layer 2 – Data Link

Data Link layer encodes and decodes data packets into bits. The encapsulated data (packet) is actually placed inside a data frame. The frame type depends on the network architecture, e.g. an Ethernet frame will be used in Ethernet networks and a Token-ring frame will be used in Token-ring networks. The Data Link layer is also responsible for data movement across the actual physical link to the destination node.

Layer 1 – Physical

The frames are converted into a single bit stream as they are passed to the physical layer form the data link layer. This means that bits are queued up one by one as they move onto the network media. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects.

The OSI model is important because it divides a communications system into smaller layers, allowing for an organised communication between computers on a network, working through the layers, meaning the whole communications procedure becomes a lot easier on the networker.