1.5.4 Protocols

Organising interfaces in a layered fashion

For a computer connected to a printer by a cable, there are many different interfaces, or layers, for this connection:

• • • • • • The physical design of the plugs and sockets used to connect the cable to the computer and equipment.

          • The electrical signals that need to be used, for example, the voltage to be used.

          • The software used to control how an application sends a file to be printed.

Each one of these areas needs an interface to other products.

Why are different layers needed?

If you laid down rules for the physical design of the plugs and sockets.

• A printer manufacturer could design any printer they liked, as long as it had that interface for the plugs and sockets.

• A cable manufacturer could also design any cable they liked, as long as it conformed to the interface.

• A software house could make any application they liked, as long as they conformed to the rules of how files were to be sent to the printer.

The important idea behind designing an interface is to understand that there are quite a few different aspects to consider.

Each aspect (electrical signals, physical connections, software, data communications etc) is in itself a complicated interface that needs clearly defining.

The different interfaces together are known as the OSI model or Open Systems Interchange' model.

There are 7 interface layers in the OSI model.

These are split into the Application Layers and Transport Layers

NB You do not need to learn these by heart.

BUT Ensure you understand that concept that

• messages transmitted over a network is split into layers for different types of information

In Summary:

• There are different identifiable aspects for interfacing any product with another in computing.

• Because there are so many different aspects to consider, the problem has been clearly described in a model known as the OSI model.

• Each 'different aspect' is commonly known as a 'layered interface'.

• Often, different manufacturers are responsible for a different part of a system.

• So that manufacturers can do what they like with their own products but can be confident it combines without any problem to other manufacturers' products, the rules that make up an interface are fixed.

• It is the interface between one manufacturer's product and another manufacturer's product that is important if those products are to be able to work together!