Instruction manual

About Telecommunications
Thus, the functions of signaling are:
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Alerting for a request for service
■
Transmitting the address information (the telephone number) of the
called party.
■
Supervising the status of circuits and lines
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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
There are basically five types of signaling systems:
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Direct current (dc)
■
Inband tone
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Out-of-band tone
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Digital
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Common-channel interoffice
The signals for alerting and supervisory functions are generated by the flow (or
absence of flow) of the direct current between the telephones and the switching
office.
Address information is communicated either by interruptions in the direct
current produced by rotary-dial telephones, or by the unique tones produced by
touch-tone telephones. Information signals (dialtone, busy signal, and so on)
are provided by the transmission of certain combinations of tones. For touch-
tone telephone service, dual-tone multifrequency (DTMF) signaling is used.
The supervision of trunks between COs also occurs by the flow (or absence of
flow) of direct current. However, because long-distance trunks carry only
alternating current (AC), a different kind of frequency is used. In 1976, AT&T
introduced a new interoffice signaling scheme called common channel
interoffice signaling (CCIS) in which a separate circuit between the offices is
dedicated to digital signaling transmissions between the computers that control
the switches.
System Manager's Guide
B–
9