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Wire and Wireless Communication Applications Chap. 8
Results are presented for telephone systems, satellite systems, fiber-optic systems, cellular
telephone systems, personal communication service (PCS), television systems (including
digital TV), and link budget analysis for wireless systems. Link budget analysis is concerned
with the design of a system to meet a required performance specification as a function of tradeoff
in the transmitted power, antenna gain, and noise figure of the receiving systems. This performance
specification is the maximum allowed probability of error for digital systems and the
minimum allowed output SNR for analog systems.
8–2 TELEPHONE SYSTEMS
Modern telephone systems have evolved from the telegraph and telephone systems of the
1800s. Telephone companies that provide services for a large number of users over their
public switched telephone networks (PSTN) on a for-hire basis are known as common
carriers. The term is applied to widely diverse businesses, such as mail, airline, trucking,
telephone, and data services. The common carriers are usually regulated by the government
for the general welfare of the public, and, in some countries, certain common carrier
services are provided by the government. The information from multiple users is transmitted
over these systems, primarily by using time-division multiplex (TDM) transmission or
packet data transmission.
Historically, telephone systems were designed only to reproduce voice signals that
originated from a distant location. Today, modern telephone systems are very sophisticated.
They use large digital computers at the central office (CO) to switch calls and to monitor the
performance of the telephone system. The modern CO routes TDM PCM voice data, video
data, and computer data to remote terminals and to other central offices.
Digital service is provided to the customer in any of three ways: (1) a dedicated leased
circuit, such as a T1 circuit, that is available for use at all times without dialing or switching,
(2) a circuit-switched service that is available on a dial-up basis, and (3) a packet-switched
service such as DSL, discussed in Section 8–3, that is “always on” and is used only when
packets are exchanged.
Historical Basis
Modern telephone systems have evolved from the relatively simple analog circuit that was
invented by Alexander Graham Bell in 1876. This circuit is shown in Fig. 8–1, where two telephone
handsets are connected together by a twisted-pair (i.e., two-wire) telephone line and the
telephone handsets are powered by a battery located at the CO. (Historically, the telephonewire
connection between the two parties was made by a telephone switchboard operator.) The
battery produces a DC current around the telephone-wire loop. A carbon microphone element
is used in each telephone handset. It consists of loosely packed carbon granules in a box that
has one flexible side—the diaphragm. As sound pressure waves strike the diaphragm, the carbon
granules are compressed and decompressed. This creates a variable resistance that causes
the DC loop current to be modulated. Thus an ac audio current signal is produced, as shown in
the figure. The handset earphone consists of an electromagnet with a paramagnetic diaphragm
placed within the magnetic field. The ac current passing through the electromagnet causes the
earphone diaphragm to vibrate and sound is reproduced.