cility to your facility. While a T1 may be converted and multi- plexed into a larger circuit like a DS3 or SONET circuit, the last mile is usually copper. Multiplexing Multiplexing is the act of taking multiple signals and sharing them on a single signal. The act of converting twenty-four 64 Kbps channels into a single T1 is an example of multiplexing. PBX is the abbreviation for private branch exchange. A PBX is essentially a phone system as most people know it; it offers a phone network to an entity such as an enterprise. Some of the main features of a PBX are the ability for many phones to share a limited number of public phone lines, and the ability to number individual extensions with, typically, three- or four-digit extension numbers. PBX systems have traditionally been large hardware devices with cryptic control systems and proprietary hardware. VoIP is often controlled by software versions of PBXs. Ex- amples include Cisco's Unified Communications Manager (formerly Call Man- ager) and the open source product Asterisk. Chapter 20: Telecom Nomenclature POTS is short for the clever phrase plain-old telephone service. A POTS line is one into which you can plug in a normal analog phone or fax machine. Most home phone lines are POTS lines. RBOC (pronounced are-bock) is short for Regional Bell Operating Company. In 1875, Alexander Graham Bell (and two others who agreed to finance his inven- tions) started the Bell Telephone Company. Bell Telephone later became known as the Bell System as it acquired controlling interests in other companies such as Western Electric. In 1885, American Telephone and Telegraph (AT&T) was in- corporated to build and operate the US's original long-distance telephone network. In 1899, AT&T acquired Bell. For almost 100 years, AT&T and its Bell System operated as a legally sanctioned, though regulated, monopoly, building what was by all accounts the best telephone system in the world. In 1984, however, a judge-citing antitrust monopoly issues-broke up AT&T's Bell System, known as Ma Bel