Lucent Technologies MERLIN LEGEND Release 6.1 System Manager's Manual page 410

MERLIN LEGEND Communications System Release 6.1
System Manager's Guide 555-661-118
B About Telecommunications
Signaling
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:
Direct current (dc)
Inband tone
Out-of-band tone
Digital
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 (dial tone, 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.
The enhanced CCIS system in use today is called common channeling signaling
(CCS) system and supports advanced features, such as calling-number
identification (Caller ID). The calling party's number is transmitted from switching
office to office. This and other advanced services are available on the "intelligent"
network that has evolved because of the use of computers to control signaling.
Telephone Numbering Plans
Unlike the verbal communication of the called party's name in manually switched
systems, automated switching systems require that each telephone extension be
identified by a unique address that is convenient, readily understandable, and
similar in format to other extensions connected to the network.
In the early days of telecommunications, when a maximum of 10,000 lines could
be serviced by a telephone exchange, a 4-digit alphanumeric "address" was used
to specify the called party, for example, WA51 (Waverly 51).
2
2
Issue 1
August 1998
Page B-9