1.4 Moving data around

Hello. In this final part of Session 1 we’re going to look at how data’s moved from source to destination. By the end of it you should have an insight into the layered model of communications networks and you’ll have met the term ‘protocol’ again and you’ll have been introduced to the TCP/IP protocol suite.

Civilisations have been moving goods around for centuries and often over very long distances – say, from one side of the world to the other. This usually involves a complex network of components and processes all working together to pass the goods to the required destination. It’s generally the case that individual parts of this network have no understanding of the roles of the other parts or of the tasks performed by them. Computer networking is no different in this respect.

It’s often useful to organise the separation of these roles and tasks in terms of layers with each layer being responsible for different aspects of the communication process. To help you appreciate the concept of different layers associated with moving stuff from one place to another, let’s think about the system for transporting goods sealed inside shipping containers.

The shipping containers are made in standard sizes so that a system that’s set up to deal with one standard size, can deal with any. Also they aren’t fussy about what goods are placed within them (providing that the necessary safety procedures are observed, of course) nor what quantity. So shipping containers provide a uniform and secure capsule for the transportation of goods. What’s more, this capsule isn’t concerned with how it will reach its destination: it doesn’t need to know that. Its role has been fulfilled simply by containing the goods.

Once the goods are sealed inside them, the shipping containers can be loaded onto a carrier – often in multiples if required – and then conveyed to their destination. The medium for conveyance could be water, rail or road, or indeed all three of these during the course of the journey. As long as the carrier is appropriate for the medium, the exact details of the carrier aren’t important. For example, if the medium’s water, it doesn’t matter what type or size of cargo ship is used. The water doesn’t need to ‘know’ what type of vessel is moving through it. Likewise with road, the type, size and capacity of the vehicle transporting the goods isn’t of relevance.

So the system of conveying goods in shipping containers can be thought of as having layers, with each layer being responsible for a particular aspect of the journey. This provides us with a conceptual model to help us organise, explain and design the services needed.

At the bottom of the stack, which we’ll call layer 1, there’s the physical medium for conveyance: water, rail or road in our example. Above that, layer 2, there is the means to access the physical medium: the ship, train or lorry in our example. Then at the top, layer 3, there are the containers in which the goods travel. Each of these three layers is responsible for a different service and each layer has its own set of rules and procedures.

Computer networks also use a layered approach to the delivery of data from source to destination. At the bottom layer there’s the physical layer, the medium – for example cable, radio waves or light pulses – and at the top layer are the applications that provide the interface between the network and the end user. When you use your browser you’re using one of these applications. Between the top and bottom layer, different layers provide different and distinct services.

Like the shipping analogy, the layers in computer networks have their own set of rules and procedures. These are known as protocols and the collection of protocols from each layer is often called a protocol suite, a protocol family or a protocol stack.

The TCP/IP protocol suite is the name given to the suite of protocols used on the internet and most other computer networks. It derives its name from two of the most important protocols used in the suite: Transport Control Protocol, abbreviated as TCP, and Internet Protocol, abbreviated IP. It has a four-layer protocol stack with each layer taking responsibility for different tasks and using different protocols.

Layer 1 at the bottom is the Network Access layer. Above that at layer 2 is the Internet layer. Next at layer 3 is the Transport layer and at the top layer 41 is the Application layer.

In later sessions you’ll be learning about the role of each of these layers and the protocols that reside within them. This part has just provided you with a brief introduction.

This concludes Part 3 of Session 1. In this session you’ve met the term ‘protocol’ again, being a set of rules and procedures for doing something. You’ve been introduced to the way computer communication is modelled and designed in layers, and you know that the TCP/IP protocol suite is the name given to the suite of protocols used on the internet.