In order to allow two or more computers to communicate it is necessary to establish sets of rules for the communication. A computer network protocol is the set of rules needed to allow communication over the network to take place, agreements having to be made on the format and relative timings of the messages to be exchanged. Obviously in the area of network compatibility advantages are to be gained by the use of standard protocols. It is useful to think of a hierarchy of protocols, each of which relies on tasks performed by lower-level protocols. This can be seen by many of the protocols currently in use.

The principal standard that will be considered is the model developed by the International Standards Organisation (ISO). The model is known as ISO OSI (Open Systems Interconnection) Reference Model. This model achieves two objectives. First of all, it provides a useful and commonly agreed method of understanding and examining the various functions of a complex communication system. Secondly, it provides a convenient framework for internationally agreed communication standards.

The method chosen was to view the total set of functions of a system as being divided into seven "layers". The hierarchy of the layers moves downwards from the general, the highest layers, to the specific, the lowest layer. Below are the criteria used to design the model:

1. A layer should be created where a different level of abstraction is needed
2. Each layer should perform a well defined function
3. Each layer has a unique and specific task to perform
4. The layer boundaries should be chosen to minimise the information flow across the interfaces
5. Functions that are similar or highly inter-related are collected together within one layer
6. A layer should only know about its immediately adjacent layers
7. A layer should use the services of the layer below
8. A layer should provide services to the layer above

Before examining the specific layer model of a network architecture, the concepts associated with a layer approach should be considered. It is apparent that network architecture is complex yet it is important to point out that the word "architecture" refers to the organisation of the components which make up a network.

The idea of a layered approach should be a familiar one. When contemplating a solution to a problem it is common to refine the problem into various stages. Using this approach there is a connection between any layer and the layers above - each layer communicates directly only with the layers immediately above or below it. As a consequence of adopting a layered approach only one layer might only have to be refined, thus saving copious time devising a completely new structure.

However, although there are obvious advantages of adopting a layered approach, various disadvantages also exist. As regards the ISO OSI reference model the following are the advantages and disadvantages associated with it.

Advantages of the ISO OSI Model

• If a network conforms broadly to agreed standards, users are insulated against some of the adverse effects of technological change - equipment does not become obsolete quickly
• It promotes modularisation of network support software
• Each module takes the form of a layer in the model and is responsible for providing selected services to the layer above
• In theory any layer can be replaced by a new layer that provides the same services but in a different way, without affecting the user's view of the framework

Disadvantages of the ISO OSI Model

• Due to the complexity of the system poor performance is obtained, especially in some real time applications
• Direct substitution of layers is not always possible e.g. if a LAN with broadcast capability is inserted below a network protocol that did not support this facility, then this service would be lost to the upper layers
• Although protecting equipment from becoming obsolete it simultaneously hinders technological advancement

The seven layers of the OSI Reference Model are as follows:

1. The Physical layer
2. The Data Link layer
3. The Network layer
4. The Transport layer
5. The Session layer
6. The Presentation layer
7. The Application layer

The three lower layers compose part of the whole communications environment and are rudimentary to the way in which a communications network operates and the hardware and software driving it. On the other hand the top three layers are part of the applications or user environment and are concerned about the method in which the network is used, rather than on the technical way it operates. thus the transport layer is fundamental to the ISO OSI Reference Model - it acts as a bridge between the layer elements controlled by the common carrier and the higher level controlled by the user.