Handout 6 - Network Protocols

Network Protocols and the OSI Model

Information is transmitted from one communications device to another over a communications medium. The information is sent over this medium in what is called packets or frames, each packet or frame represents part of (or a piece of) the information to be sent – the information to be sent is broken up into packets or frames, sent over the medium, received at the other end, re-assembled, and then reconstructed to form the original information that was sent.

Protocols govern how this information is broken up into packets or frames, transmitted over the communications medium, and re-constructed at the other end – A protocol is a formal set of rules and conventions that governs how computers exchange information over a network medium. They fall into one of the following categories:

• » LAN Protocols - These define communication over the various LAN media (Ethernet).
• » WAN Protocols - These define communication over the various wide area media.
• » Routing Protocols - These are responsible for determining the paths the packets or frames take in reaching their destination.
• » Network Protocols - These are protocols that exist in a protocol suit (TCP/IP).

The Open Systems Interconnection (OSI) Model

The OSI Model is a conceptual model, or framework, describing how two computing devices establish and maintain communications in order to transfer information between them. It is the international standard for inter-computer communication. It is important to note that the OSI Model is just a concept, and only provides a conceptual framework for communication between computing devices. Communication is implemented via protocols (TCP/IP).

The OSI Model is divided into seven layers and is arranged like a stack – it divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task, or group of tasks, is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained, so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers.  This is shown below (diagram taken from HSdataline):

From the diagram it is seen that the model is divided into two main categories:

• » Application - This consists of the upper three layers and is primarily concerned with application issues. It is implemented in software.
• » Data Transport - This consists of the lower four layers and is concerned with transporting data over the communications medium. It is implemented in both software and hardware.

Communication Between Computing Devices Using The OSI Model

Using the OSI Model, a computing device operating at one layer can only communicate with another device operating at the same layer. This is shown in the diagram below (diagram taken from HSdataline):

A given layer in the OSI layers generally communicates with three other OSI layers: the layer directly above it, the layer directly below it, and its peer layer in other networked computer systems. The data link layer in System A, for example, communicates with the network layer of System A, the physical layer of System A, and the data link layer in System B.

One OSI layer communicates with another layer to make use of the services provided by the second layer. The services provided by adjacent layers help a given OSI layer communicate with its peer layer in other computer systems.

Further Reading

Chapter 5, pp. 154 – 159 of the course text (4th Edition)
Chapter 5, pp. 160 – 167 of the course text (5th Edition)