carry data and image information (e.g., television). In the United States the connection to
the PSTN may be via a local exchange carrier (LEC) or by a competitive local exchange
The personal computer (PC) is beginning to take on a role similar to that of the
telephone--namely, being ubiquitous. Of course, as we know, the two are becoming
married. In many situations, the PC uses telephone connectivity to obtain Internet and
e-mail services. Cable television (CATV) offers another form of connectivity providing
both telephone and Internet service. In the case of Internet access, CATV can be shown
to be more efficient than a telephone line for data rate capacity. Then there are the radio
adjuncts to the telephone, typically cellular and PCS, which are beginning to offer similar
services such as data communications (including Internet) and facsimile (fax) as well
as voice. The popular press calls these adjuncts wireless. Can we consider wireless in
opposition to being wired?
Count the number of devices one has at home that carry out some kind of control-
ling or alerting function. They also carry out a personal communication service. Among
these devices are television remote controls, garage-door openers, VCR and remote radio
and CD player controllers, certain types of home security systems, pagers, and cordless
telephones. We even take cellular radios for granted.
In some countries, a potential subscriber has to wait months or years for a telephone.
Cellular radio, in many cases, provides a way around the problem, where equivalent
telephone service can be established in an hour--that is, the amount of time it takes to
buy a cellular radio in the local store and sign a contract for service.
The PSTN has ever-increasing data communications traffic where the network is used
as a conduit for data. PSTN circuits may be leased or used in a dial-up mode for data
connections. Of course the Internet has given added stimulus to data circuit usage of the
PSTN. The PSTN sees facsimile as just another data circuit, usually in the dial-up mode.
Conference television traffic adds still another flavor to PSTN traffic and is also a major
growth segment. The trend for data is upwards where today data connectivity greatly
exceeds telephone usage on the network.
There is a growing trend for users to bypass the PSTN partially or completely. The
use of satellite links in certain situations is one method for PSTN bypass. Another is to
lease capacity from some other provider. Other provider could be a power company with
excess capacity on its microwave or fiber-optic system. There are other examples such as
a railroad with extensive rights-of-way which may be used for a fiber-optic network.
Another possibility is to build a private network using any one or a combination
of fiber optics, copper wire line, line-of-sight microwave, and satellite communications.
Some private networks take on the appearance of a mini-PSTN.
INTRODUCTORY TOPICS IN TELECOMMUNICATIONS
An overall telecommunications network (i.e., the PSTN) consists of local networks inter-
connected by one or more long-distance networks. The concept is illustrated in Figure 1.1.
This is the PSTN, which is open to public correspondence. It is usually regulated by a gov-
ernment authority or may be a government monopoly, although there is a notable trend
toward privatization. In the United States the PSTN has been a commercial enterprise
since its inception.
End-Users, Nodes, and Connectivities
End-users, as the term tells us, provide the inputs to the network and are recipients of
network outputs. The end-user employs what is called an I/O, standing for input/output