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MERLIN LEGEND Communications System Release 5.0
System Manager’s Guide
555-650-118
Issue 1
June 1997
About Telecommunications
Page B-8Signaling
B
more powerful telephone network applications and services. It can operate in PBX
mode, along with two other modes that define how the system works.
Signaling 2
Telephone service involves a vast network of transmission and switching
equipment whose status and operation must somehow be controlled. This is done
by means of various types of signals.
Originally, a caller alerted the operator that he or she wanted service by turning
the crank on the telephone, which caused a lamp to flash for that line on the
switchboard at the exchange office. The operator plugged in on that line, the caller
verbally gave the number of the called party, and the operator visually checked
the lamp of the called party to see whether the person was available. If not, the
operator told the caller that the line was busy. If the line was available, the
operator rang the called party’s telephone and connected the parties. When the
call was over, the operator could observe that both lamps went out.
If the call was between switching offices, the two operators completed the steps of
the process.
Thus, the functions of signaling are:
■ Alerting for a request for service
■ Transmitting the address information (the telephone number) of the called
party.
■ Supervising the status of circuits and lines
■ Transmitting information such as dial tone to indicate service is available,
busy signal indicating that the called party is not available, and various
announcements
As switching systems were developed that automated much of the network’s
operation, an additional realm of signaling was required: machine-to-machine.
Because a primary objective of the telephone industry is for operation of the
telephone to be simple, universal, and practical, a relatively small number of
standard signals are required. On the other hand, since interoffice signaling is
between machines, the emphasis is on efficiency and flexibility rather than
usability. This has resulted in a large variety of signaling arrangements.
Signal Transmission 2
There are basically five types of signaling systems:
■ Direct current (dc)
■ Inband tone
■ Out-of-band tone