AT&T Bell Laboratories has been involved in testing equipment for conformance with network and product interface specifications for more than five years. X.25 conformance testing has been conducted for over four years. The ISDN Basic Rate Interface protocol as specified for the 5ESS switch followed shortly thereafter. Testing of the ISDN Primary Rate Interface protocol, the ISDN interface to long-distance network service has been in process for about one year. Experience gained in protocol testing with almost 100 units tested for conformance with these interfaces is summarized.>

The time compression multiplexing (TCM) technique offers a solution to the problem of achieving digital penetration of the two-wire loop plant. TCM is planned to provide the digital loop transmission in the new circuit switched digital capability of the Bell System, so that a full-duplex 56 khit/s digital connection is obtained between subscriber's premises and the serving central office. Herein, the concept of TCM and its parameters are described. Tradeoffs leading to determination of system parameters such as burst frequency, burst length, guard time, and delay as they affect the range of the system are discussed. Also, the performance of the digital loops in terms of eye diagrams, system range limitations due to crosstalk and impulse noise, and percentage of loops covered are summarized. In addition, the circuit implementation using both analog and digital technologies is described. Experiences with various digital architectures to implement TCM and tradeoffs leading to a low-power digital architecture suitable for VLSI are discussed.