Lucent Technologies 5 manual Telephone Numbering Plans

MERLIN LEGEND Communications System Release 5.0	Issue 1
System Manager’s Guide 555-650-118	June 1997

BAbout Telecommunications

Signaling Page B-9

■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 2

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).

Subsequently, 3-digit area codes were introduced to specify the area of the country to be reached, and country codes were introduced for international dialing. Finally, the individual telephone numbers expanded to the 7-digit numbers in use today: a 3-digit central office code and a 4-digit extension number.

NOTE:

In the past, the North American Numbering Plan has used area codes that have only a 0 or 1 for the middle digit, for example, 908 or 215. Currently, the numbering plan is being changed so that any number (0 through 9) can be used for the middle digit. This system has already been designed to take that change into account.