Transmission Facilities - AT&T MERLIN LEGEND Release 3.1 System Manager's Manual

About Telecommunications
Identifying which telephone to ring (that is, supplying the identity of the called
party) was communicated verbally from the caller to the operator when human
operators handled call connections. After automatic switches were in place,
telephone numbers were assigned to telephone service subscribers. The caller
identified the called party to the switch by dialing the called party's number. The
numbers were originally dialed by using a mechanical device called a rotary
dialer with a spring that was wound up by turning it in one direction; on its return
to normal position, it created interruptions in the flow of current, generating dial
pulses recognized by the switch.
Although there are still some rotary-dial telephones in use today, most modern
telephones have touch-tone dialing that involves the creation of unique tones
produced by pressing buttons on the dialpad. Touch-tone dialing is faster and,
with the advent of services available from touch-tone phones, more versatile.
Today's telephones still consist of the components described above. But, with
continued innovations, these basic elements have been enhanced to include
many other features and components, for example, built-in speakerphones,
programmable features and buttons, and even the capability to transmit and
receive digital signals.

Transmission Facilities

The telephone network can transmit various types of information which
originates in either of two forms: analog (continuously variable physical signals,
for example, speech or video signals) or digital (representation of signals in
discrete elements such as zero and one, for example, signals from computers).
This information is conveyed from one place to another in the network over
communications paths provided by transmission facilities. These facilities
involve different types of media as well as electronic equipment.
There are various types of media, including:
Open Wire. Strung on poles, uninsulated copper wire was used in the
early days of telecommunications until physical congestion became a
problem. It's still found, though rarely, in rural areas.
Paired Wire. Commonly called twisted pair . Consists of two copper
wires, individually insulated with wood pulp or plastic, twisted together.
Paired Cable. Combines many twisted pairs (from 6 to 3600) into a single
cable, originally sheathed in lead but now insulated with plastic. Cable
can be strung on poles, buried underground, or installed in a conduit of
either long blocks of concrete or plastic pipe. The first transoceanic
undersea cable was laid by AT&T in 1958.
A problem encountered, however, with many wire pairs running parallel
to each other is crosstalk , that is, the leaking of the electric signal from
one pair to another so that you can hear noise or intelligible speech.
System Manager's Guide
B–3