DCE—Connections to Telephone Lines

Data circuit-terminating equipment (DCE), also referred to as analog and digital modems, sit between the telephone line and computers or multiplexers. The multiplexers, computers and PBXs, data terminating equipment (DTE) (see Figure 7.1) is connected to the modem. The increase in the computing power of chips and functionality of digital signal processors (DSPs) has resulted in the shrinking size of DCE devices. The following are sample configurations:

• Modems for PCs are either cards located in an internal slot of PCs or external devices that plug into the serial port of the computer. In either case, they are plugged directly into a telephone company jack. Standalone modems need their own power source. Integrated modems do not.

• NT1s and CSUs plug directly into a telephone company T-1 or ISDN two-pair data jack.

• A BRI ISDN NT1 device can be an extension of a PBX or key system. In these configurations, the ISDN adapter plugs into the same type of PBX jack as the telephones. The PBX/key system is then connected to a PRI ISDN trunk.

• Modems and CSU/DSUs can be cards within multiplexers.

• CSU/DSUs also are supplied as cards within PBXs, which have internal T-1 multiplexer cards. A cable from the CSU/DSU card connects the PBX to the telephone company jack.