by Kewoni Berkley

Created on: June 02, 2008

Since computer networks can be very complex in their design, a model was created to show network protocol in a series of layers which are each built upon its predecessor. The OSI model (Open Systems Interconnection) is a protocol stack which was designed by the ISO (International Standards Organization) in 1984 as a replacement for the then existing protocols such as SNA from IBM. This was a reference model which the ISO developed for worldwide communication between end users. The model is a seven layer protocol stack which passes control downward from top to bottom. At each layer, the specific duties are carried out while the layer below prepares to receive information from the above layer. The OSI model consists of seven layers. Each layer from top to bottom is a collection of related functions that provide the above layer with services. The upper four layers are used when a message is passed to or from a user; these layers of the protocol handle matters related to software such as encryption, format, etc. The lower three layers are used to pass messages through the host computer. These layers deal more with the physical communication between the systems such as network connections with an Ethernet cable, etc.

The seven layers of the OSI model are Application, Presentation, Session, Transport, Network, Data Link, and Physical.

At the application layer, communication partners are identified, user authentication is considered and the quality of service and any errors in data syntax are identified. When communication partners are identified, the application layer must find out if the sufficiency of network resources exists for communication to take place. The application layer is the coordinator for synchronization and other processes and manages the communication between applications. Application services such as emailing and file transfer such as FTP (File Transfer Protocol) exists entirely at this layer.

The next layer in the model is the presentation layer. The presentation level is usually a part of the OS and converts in and outgoing data from presentation format to another which is ordered and meaningful. Here, data is converted to a form which the application layer can accept. Data compression schemes allow for compressed or encrypted data to be easily decompressed or decrypted upon arrival at its destination. A variety of changes such as encryption and ACSII conversion occur at this layer so that information can be sent across a network with ease then

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