OSI model

OSI model

Compatible interconnection of network devices is fundamental to reliable network communications. Developing a set of standards that equipment manufacturers could adhere to went a long way towards providing an open environment for network communications.

In the late 1970s the International Organization for Standardization (ISO) worked on a seven layer model for LAN architectures by defining the Open Systems Interconnection Basic Reference Model (OSI). Alongside this The ISO developed a set of protocols that fit within this model. Since then, other models such as the 5 layer TCP/IP model were developed, however the OSI model is still used to map and categorise protocols because of its concise and clear way of representing network functions.

The IEEE formed the 802 committee in February 1980 with the aim of standardising LAN protocols. This resulted in the IEEE 802 series of committees that sit to develop worldwide standards for communications. Within the OSI model, the Data Link layer was split into two, the Media Access Control (MAC) sub-layer and the 802.2 Logical Link Control (LLC) sub-layer.

Application

It is employed in software packages which implement client-server software. When an application on one computer starts communicating with another computer, then the Application layer is used.

Presentation

This provides function call exchange between host operating systems and software layers. It defines the format of data being sent and any encryption that may be used.

Session

The Session layer defines how data conversations are started, controlled and finished.

Transport

This layer is resonsible for the ordering and reassembly of packets that may have been broken up to travel across certain media

Network

This layer is responsible for the delivery of packets end to end and implements a logical addressing scheme to help accomplish this.

Datalink

This layer deals with getting data across a specific medium and individual links by providing one or more data link connections between two network entities.

Physical

This layer deals with the physical aspects of the media being used to transmit the data.

References: http://www.webopedia.com/quick_ref/OSI_Layers.asphttp://compnetworking.about.com/cs/designosimodel/a/osimodel.htm

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