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“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 combi-
nations 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 M odems
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. T his 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 informa- tion 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
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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 T V 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 appro-
priate for the transmission medium. T here 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 situa-
tions affect the amplitude of the signal such as driving under a bridge or near electrical lines, lightning,
etc ..
Modem Modem
Telephone Line
Figure 9-1 Communication over a telephone line with modems.
Digital Signal AM
FM PM
1 1
1
Figure 9-2 Three common forms of modulation.
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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. T he 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
. T he use of phase offers a degree of freedom in addition to frequency, and is appropriate when the number of available frequen-
cies 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. T he original signal is sampled at twice the rate of the highest significant frequency, producing values at discrete intervals. T he 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. T here 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:
Amplitude
000 001
010 011
100 101
110 111
Time PCM sequence = 011 110 011 001 100 111 101
Figure 9-3 Conversion of an analog signal into a PCM binary sequence.
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data rate = bandwidth
×
log1 + SN
where S
is the signal and N
is the noise. Since a digital signal can be made to use arbitrarily noisy channels in which
SN is large because of its noise immunity,
higher data rates can be achieved over the same channel. T his is one of the driv- ing forces in the move to digital technology in the telecommunications industry.
T he transition to all-digital has also been driven by the rapid drop in the cost of digital circuitry.
9.2 Transmission M edia
