AT&T MERLIN LEGEND Release 3.1 System Manager's Manual page 47

Background
Although there are still some rotary-dial telephones in use, most modern
telephones have touch-tone dialing, which is faster and, with the advent of
services available from touch-tone phones, more versatile.
The terms tip and ring , however, still describe any telephone equipment that
involves only one line, for example, a single-line telephone (such as those in
most homes), an answering machine, or a fax machine. These are referred to as
tip/ring (T/R) devices .
You can use several different types of telephones with the MERLIN LEGEND
Communications System, including single-line telephones, analog multiline
telephones, and MLX digital telephones. The terms analog and digital refer to
the type of signal the telephone produces:
Analog Signal. A signal that represents a range of frequencies, that is,
continuously variable physical qualities such as amplitude; for example,
the human voice.
Digital Signal. Information transmitted in a coded form (from a computer)
represented by discrete signal elements; for example off and on or zero
and one.
Switching Equipment
As described earlier, the telephone network is composed of a number of
centralized switching locations, called central offices (COs), where a telephone
circuit is connected, or switched, to another circuit. That is, the caller's line is
connected to the called party's line so the two can hold a conversation.
Telephone operators, who supplied the first manual switching, were slow and
costly but afforded some special functionality: calls could be forwarded,
messages taken, and calls interrupted. Electromechanical switching automated
that manual labor and made telephone service universally affordable, but the
technology was inflexible and did little more than switch calls. Now, with
electronic, computer-controlled switches, both flexibility and functionality are
affordable to everyone.
The Evolution of Switches
The method, type, capabilities, and capacities of switches have evolved as
geographic areas expanded and technological advances became available.
The following list describes each of these progressive innovations. Figure 2 3
illustrates this evolution of switching equipment.
Private-Line Service. In the first telephone installations, communication
was directly from one telephone to another, as in Bell's demonstration.
Thus, one telephone could communicate with only one other telephone.
2–6
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