Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

A CO is part of the telephone network in your area. It is a building in which the phone lines in your home or office terminate and connect to a much larger switching system. In large metropolitan areas, COs are more appropriately LOs (local offices), because they serve a local area. The term "CO" is from the early days of the telephone system when the local telephone company really did have only one central office in each area.

The telephone cable from the CO to your home or office is called the local loop and is, in most cases, the last remaining part of the telephone network that uses analog voice signaling over copper cables. The maximum distance of a CO-to-home/office cable is about 5 kilometers. A CO in a metropolitan area may provide service to over 100,000 local loops.

The CO is connected to a much broader switching system in which switching offices are interconnected with trunk lines that may be coax cable, fiber-optic cable, or microwave transmission systems. While the local loop is an analog circuit for voice calls, once a call gets to the CO, it is converted to a 64-Kbit/sec stream of digital data for routing through the switching system. A trunk line will carry hundreds of these digitized voice calls using multiplexing techniques. See "Multiplexing and Multiplexers" and "T Carriers."

Telephone switching systems perform the task that was historically handled by telephone operators. They connect one phone line to another. There is a hierarchy of switches as pictured under the topic "Circuit-Switching Services." See "Telecommunications and Telephone Systems" for more details.

  
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