6.32 Communication networks

Communication networks elaborate the Fundamental Model of Communications . The model shown in [link] describes point-to-point communications well, wherein the link between transmitter and receiver is straightforward, andthey have the channel to themselves. One modern example of this communications mode is the modem that connects a personalcomputer with an information server via a telephone line. The key aspect, some would say flaw, of this model is that thechannel is dedicated : Only one communications link through the channel is allowed for all time. Regardless whetherwe have a wireline or wireless channel, communication bandwidth is precious, and if it could be shared without significantdegradation in communications performance (measured by signal-to-noise ratio for analog signal transmission and bybit-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 postalservice. 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 thepostal 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 corruptthe message while doing so.
• The message arrives at one of the network's exit points, and is delivered to the recipient (what we havetermed the message sink).

What is most interesting about the network system is the ambivalence of the message source and sink about how thecommunications link is made. What they do care about is message integrity and communications efficiency. Furthermore, today'snetworks use heterogeneous links. Communication paths that form the Internet use wireline, optical fiber, and satellitecommunication links.

The first electrical communications network was the telegraph. Here the network consisted of telegraph operators who transmitted themessage efficiently using Morse code and routed the message so that it took the shortest possible path to its destination while taking into account internalnetwork failures (downed lines, drunken operators). From today's perspective, the fact that this nineteenth centurysystem handled digital communications is astounding. Morse code, which assigned a sequence of dots and dashes to each letter ofthe alphabet, served as the source coding algorithm. The signal set consisted of a short and a long pulse. Ratherthan a matched filter, the receiver was the operator's ear, and he wrote the message (translating from received bits tosymbols).

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 communicationschannel between nodes using what we call time-domain multiplexing : Rather than the continuous communications mode implied in the Model as presented, message sequences aresent, 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 somenotion of the best possible path at the time of transmission. Note that no omnipotent router views the networkas a whole and pre-determines every message's route. Certainly in the case of the postal system dynamic routing occurs, and canconsider 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 themost efficient (timely) and most reliable information delivery system possible.