ers, or other appropriate media. In Bellcore (now Telcordia) terminology, the central office termination (COT) is the digital terminal colocated with the local serving switch. The RT is the remote terminal. The RT must provide all of the features to a subscriber loop that the local serving switch normally does, such as supervision, ringing, address signaling, both dial pulse and touch tone, and SO on (Ref. 7). 6.10.1 New Versions of DSL ADSL, or asymmetric digital subscriber line, as described by Bellcore (Telcordia), pro- vides 1.544 Mbps service "downstream," meaning from the local serving switch to the subscriber, out to 18,000 ft (5500 m). In the upstream direction 16-kbps service is fur- nished. Such service has taken on new life in providing a higher bit rate for Inter- net customers. There is an ANSI version of ADSL that can provide 6-Mbps downstream service using a complex digital waveform and devices called automatic equalizers to improve bandwidth characteristics, particularly amplitude and phase distortion. The upstream bit rate can be as high as 640 kbps. Some manufacturers purport to be able to extend this service out to 12,000 kft (3700 m). 6.11 DIGITAL SWITCHING 6.11.1 Advantages and Issues of Digital Switching There are both economic and technical advantages to digital switching; in this context we refer to PCM switching. The economic advantages of time-division PCM switching include the following: 6.11 DIGITAL SWITCHING There are notably fewer equivalent cross-points for a given number of lines and trunks than in a space-division switch. A PCM switch is of considerably smaller size. It has more common circuitry (i.e., common modules). It is easier to achieve full availability within economic constraints. The technical advantages include the following: It is regenerative (i.e., the switch does not distort the signal; in fact, the output signal is "cleaner" than the input). It is noise-resistant. It is computer-based and thus incorporates all the advantages of SPC. The binary message format is comp