Communication is the conveyance of a message from one entity, called the source ortransmitter, to another, called the destination or receiver, via a channel3 of some sort. A simple example of such a communication system is conversation; people commonly exchange verbal messages, with the channel consisting of waves of compressed air molecules at frequencies which are audible to the human ear.4 This is depicted in Figure 0.1.
The conveyance of a message could be followed by a reciprocal response message from the original destination (now a source) to the original source (now a destination) to complete one cycle in a dialogue between corresponding entities. Depending on the application or need for the information exchange, either atomic one-way transactions or a two-way dialogue could be appropriate.
The only way that a message source can be certain that the destination properly received the message is by some kind of acknowledgment response from the destination. Conversing people might say "I understand" or nod their head in response to a statement made by their peer. This acknowledged form of dialogue is the basis of reliable communications-somehow the source must get feedback that the destination correctly received the message.5