The terminology used by the telephone company has changed somewhat since the beginning. This glossary contains some of the terms used throughout this history.

Historical Terms

Some of the following terms have fallen out of use but some are still used.

In North America, an exchange is a geographical area served by one or more central offices, usually associated with a city or town. Calling within an exchange is local. Calling from one exchange to another usually incurs a long distance charge unless EAS (Extended Area Service) is offered. In the U.K. and most of the rest of the world, the term exchange is equivalent to central office in North America.

Central Office
In North America, a central office — or just office— has two meanings. It can be a building containing switching equipment serving one or more Office Codes. For example, the Buckhead Central Office serves the 233, 237 and 261 Office Codes. It can also refer to a switching facility within a building. In the early years of manual service, Office Names were used instead of Office Codes. In this usage, each Office Name was referred to as an office, even though you could have more than one in the same building. For example the Walnut, Ivy and Main offices were in the Auburn Avenue building. In later years, with dial equipment, the telephone company personnel would also use the term this way. They might talk about the "261 office" in the Buckhead CO building. So it would be better to refer to the building as the Central Office Building.

Office Code
Today, this means the first 3 digits of a 7-digit telephone number. The term prefix is sometimes used but this is not really a telephone company official term. With the original Atlanta 6 digit dialing plan with office names, the term office code was not used. With 7 digit dialing, the office code would have been the office name plus a single digit ("TRinity-2", "TRinity-4", "Jackson-3").

In the 1970s, telephone company personnel would sometimes refer to the office code as the "NNX". This was referring to the fact that the office code was always in a specific format. The letter N means any digit from "2" through 9 and the letter X means any digit "0" through "9". So "234" is a valid NNX but "123" and "204" are not. During this time, Area Codes had a "0" or "1" as the middle digit and Office Codes did not. In the 1990s, when this restriction was removed, you could have "NXX" local office codes like "204".

Extended Area Service (EAS)
The extension of local calling beyond the caller's exchange to nearby exchanges. EAS was introduced in the 1950s in many major metropolitan areas of the U.S. after they had been fully converted to dial service. Prior to EAS, calling between nearby exchanges was handled as a short-haul toll call, involving an operator. DDD had not yet been introduced. EAS eliminated the toll charge and eliminated the operator handling of these calls. The telephone company was sometimes able to justify a slight rate increase to all subscribers in the area.

An older term that usually referred to a physical machine serving one or more office codes in a central office building. This might have been used for manual, Panel, No. 1 Crossbar, and ESS offices. In manual and Panel, each unit only served a single office name, which covered about 10,000 subscriber lines. No 1. Crossbar and ESS could serve more than 10,000 lines out of a single unit. In No. 1 and No. 5 Crossbar, the term marker group was usually used instead of unit. In Step-by-Step, there is no real concept of a unit.

Marker Group
This is a unit of No.1 or No. 5 Crossbar. It means a complete switching machine, which consists of markers, registers, senders, link frames, and auxilliary equipment. Each marker group serves one or more office codes. Multiple marker groups may be installed in the same central office building. Placing a call to another subscriber served by the same marker group is an intra-office call. Placing a call to a subcriber served by another marker group, even in the same building, is a inter-office call.

Switching Entity
This is the same thing as a Unit or a Marker Group.

A common abbreviation for No. 1 Crossbar.

A common abbreviation for No. 5 Crossbar.

A common abbreviation for No. 1 ESS.

A common abbreviation for No. 1A ESS.

A common abbreviation for Step-by-Step.

Post 1980s Terms

These terms were mostly created out of the 1980s divestiture of AT&T and competing local exchange carriers.

Local Access and Transport Area (LATA)
A LATA represents an area within which a divested Regional Bell Operating Company (RBOC) is permitted to offer exchange telecommunications and exchange access services. The RBOCs are generally prohibited from providing services that originate in one LATA and terminate in another. Traffic between LATAs must be carried by IECs (Inter-Exchange Carriers).

Rate Center
A geographical area comprised of one or more central offices. Calling within the rate center is free of long distance charges. Calling between rate centers usually incurs a long distance charge. In North America, this is the same as an exchange.

Wire Center
This is the common term used for a Central Office in the post-divestiture years.

Switching Entity
A physical machine serving one or more office codes in a central office building. Since the 1980s, each switching entity has a unique identifier code called a CLLI (Common Language Location Identifier), pronounced "SILLY".
Prior to the 1980s, the Bell companies would more often use the term Unit. When referring to No. 1 or No. 5 Crossbar they would use the term Marker Group.

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