2.3.1 Physical Concepts

What is a message?

• A piece of information which needs to move from one process to each other eg a request to open a file, an email message, the results of a remote print request.

For the network, this is a just a sequence of bits in memory.

Need to communicate this sequence of bits across communications system to other host.

Signal to other side whether each bit in sequence is 0 or 1.

To communicate, need each end to each access a common substrate, and then for the sender to change the value of a physical characteristic of the substrate.

Examples - voltage level of a piece of wire, light level in optical fibre, frequency of radio wave in air