Communication Networks

Communication networks elaborate the Fundamental Model of Communications. The model shown in Figure 1 describes point-to-point communications well, wherein the link between transmitter and receiver is straightforward, and they have the channel to themselves. One modern example of this communications mode is the modem that connects a personal computer with an information server via a telephone line. The key aspect, some would say flaw, of this model is that the channel is dedicated: Only one communications link through the channel is allowed for all time. Regardless whether we have a wireline or wireless channel, communication bandwidth is precious, and if it could be shared without significant degradation in communications performance (measured by signal-to-noise ratio for analog signal transmission and by bit-error probability for digital transmission) so much the better. The idea of a network first emerged with perhaps the oldest form of organized communication: the postal service. Most communication networks, even modern ones, share many of its aspects. A user writes a letter, serving in the communications context as the message source. This message is sent to the network by delivery to one of the network's public entry points. Entry points in the postal case are mailboxes, post offices, or your friendly mailman or mailwoman picking up the letter. The communications network delivers the message in the most efficient (timely) way possible, trying not to corrupt the message while doing so. The message arrives at one of the network's exit points, and is delivered to the recipient (what we have termed the message sink).