COMMUNICATION
“Communication” is the process of transferring information from a source to a destination. Communication systems for the most part cover distances between computers, and may involve the public telephone system, radio, and television. Wide-area communication systems have become very complex, with all combinations of voice, data, and video being transferred by wire, optical fiber, radio, and microwaves. Communication routes may traverse distances over land, under water, through local radio cells, and via satellite. Data that originates as analog voice signals may be converted to digital data streams for efficient routing over long distances, and then converted back to an analog signal, without the aware- ness of those communicating.
In this chapter we focus on communication between entities located at distances ranging within a kilometer for a local area network (LAN), and across much larger distances for wide area networks (WANs) as typified by the Internet. We start by discussing a few principles of communication.
9.1 Modems
People communicate over telephone lines by forming audible sounds that are converted to electrical signals, which are transmitted to a receiver where they are converted back to audible sounds. This does not mean that people always need to speak and hear in order to communicate over a telephone line: this audible medium of communication can also be used to transmit non-audible information that is converted to an audible form.
Figure 9-1 shows a configuration in which two computers communicate over a telephone line through the use of modems (which is a contraction of “modulator / demodulator”). A modem transforms an electrical signal from a computer into
an audible form for transmission, and performs the reverse operation when receiving. (Modems are not only used for telephone communication: they are also used in other communication systems as well, such as data-over-cable TV networks.)
Modem communication over a telephone line is normally performed in serial fashion, a single bit at a time, in which the bits have an encoding that is appropriate for the transmission medium. There are a number of modulation schemes used in communication, which are encodings of data into the medium. Figure 9-2 illustrates three common forms of modulation.
Amplitude modulation (AM) uses the strength of the signal to encode 1’s and 0’s. AM lends itself to simple implementations that are inexpensive to build. However, since there is information in the amplitude of the signal, anything that changes the amplitude affects the signal. For an AM radio, a number of situations affect the amplitude of the signal (such as driving under a bridge or near electrical lines, lightning, etc.).
Frequency modulation (FM) is not nearly as sensitive to amplitude related problems because information is encoded in the frequency of the signal rather than in the amplitude. The FM signal on a radio is relatively static-free, and does not diminish as the receiver passes under a bridge.
Phase modulation (PM) is most typically used in modems, where four phases (90 degrees apart) double the data bandwidth by transmitting two bits at a time (which are referred to as dibits). The use of phase offers a degree of freedom in addition to frequency, and is appropriate when the number of available frequencies is restricted.
In pulse code modulation (PCM) an analog signal is sampled and converted into binary. Figure 9-3 shows the process of converting an analog signal into a
PCM binary sequence. The original signal is sampled at twice the rate of the highest significant frequency, producing values at discrete intervals. The samples are encoded in binary and catenated to produce the PCM sequence.
PCM is a digital approach, and has all of the advantages of digital information systems. By using repeaters at regular intervals the signal can be perfectly restored. By decreasing the distance between repeaters, the effective bandwidth of a channel can be significantly increased. Analog signals, however, can at best be guessed and can only be approximately restored. There is no good way to make analog signals perfect in a noisy environment.
Shannon’s result about the data rate of a noisy channel applies here:
where S is the signal and N is the noise. Since a digital signal can be made to use arbitrarily noisy channels (in which S/N is large) because of its noise immunity, higher data rates can be achieved over the same channel. This is one of the driving forces in the move to digital technology in the telecommunications industry. The transition to all-digital has also been driven by the rapid drop in the cost of digital circuitry.