Centrex

A local telephone company service that leases a portion of the central office switch. Centrex is similar to PBX systems.

C.O. line

A phone line coming from the Central Office directly to the end user.

Digital Subscriber Line (or Loop) (DSL)

DSL uses modulation algorithms to send more data over copper wire. Sometimes referred to as "last-mile" because its only used for connections between a telephone switching station and a home or office, not between stations. DSL and ISDN both work over copper telephone lines (POTS) and have the additional requirement of the end user being located near a central telephone office. Depending on the type of DSL, the range is .2 to 15 miles. There are several types of DSL, mainly ADSL (Asymmetric) and SDSL (Symmetrical). Also gaining in popularity are High bit-rate DSL (HDSL) and Very high bit-rate DSL (VDSL).

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