- Communication networks have changed a lot over the years, but many aspects of them are still the same.
Communication networks elaborate the Fundamental Model of Communications. The model shown in Fig. 6.33.1 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.
Fig. 6.33.1 The prototypical communications network—whether it be the postal service, cellular telephone, or the Internet—consists of nodes interconnected by links. Messages formed by the source are transmitted within the network by dynamic routing. Two routes are shown. The longer one would be used if the direct link were disabled or congested.
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).
Develop the network model for the telephone system, making it as analogous as possible with the postal service-communications network metaphor.
The network entry point is the telephone handset, which connects you to the nearest station. Dialing the telephone number informs the network of who will be the message recipient. The telephone system forms an electrical circuit between your handset and your friend's handset. Your friend receives the message via the same device—the handset—that served as the network entry point.
What is most interesting about the network system is the ambivalence of the message source and sink about how the communications link is made. What they do care about is message integrity and communications efficiency. Furthermore, today's networks use heterogeneous links. Communication paths that form the Internet use wireline, optical fiber, and satellite communication links.
The first electrical communications network was the telegraph. Here the network consisted of telegraph operators who transmitted the message efficiently using Morse code and routed the message so that it took the shortest possible path to its destination while taking into account internal network failures (downed lines, drunken operators). From today's perspective, the fact that this nineteenth century system handled digital communications is astounding. Morse code, which assigned a sequence of dots and dashes to each letter of the alphabet, served as the source coding algorithm. The signal set consisted of a short and a long pulse. Rather than a matched filter, the receiver was the operator's ear, and he wrote the message (translating from received bits to symbols).
Because of the need for a comma between dot-dash sequences to define letter (symbol) boundaries, the average number of bits/symbol, as described in Subtleties of Coding, exceeded the Source Coding Theorem's upper bound.
Internally, communication networks do have point-to-point communication links between network nodes well described by the Fundamental Model of Communications. However, many messages share the communications channel between nodes using what we call time-domain multiplexing: Rather than the continuous communications mode implied in the Model as presented, message sequences are sent, sharing in time the channel's capacity. At a grander viewpoint, the network must route messages—decide what nodes and links to use—based on destination information—the address—that is usually separate from the message information. Routing in networks is necessarily dynamic: The complete route taken by messages is formed as the network handles the message, with nodes relaying the message having some notion of the best possible path at the time of transmission. Note that no omnipotent router views the network as a whole and pre-determines every message's route. Certainly in the case of the postal system dynamic routing occurs, and can consider issues like inoperative and overly busy links. In the telephone system, routing takes place when you place the call; the route is fixed once the phone starts ringing. Modern communication networks strive to achieve the most efficient (timely) and most reliable information delivery system possible.