Loop Plant Electronics: The Loop Switching System

Abstract

A new system for achieving more efficient use of cable pairs, the Loop Switching System (LSS), has been developed which uses a microcom puter to control the call processing, traffic measurement and display, system maintenance, and manual testing. The LSS, which works with any central office, switches 96 subscriber lines onto 32 voice frequency trunk pairs by means of a graded multiple space division switching network at the remote terminal (RT) and expands the trunks back into the individual line appearances at the central office terminal (COT) located within a central office. An additional 96 lines and 32 trunks can be added to the LSS, either at the same RT or a second RT on a different cable route, using the same COT common control equipment. Control of the RT is via two standard voice-frequency pairs used for a fullduplex data link. This four-wire data link, operating at 1250 bits per second, is used to transmit concentrator connection orders from the COT to the RT, service request information from the RT to the COT, and other information pertaining to LSS maintenance and alarms. This paper describes the system features, operational characteristics, and circuit designs used in the LSS system. I. THE HISTORY OF LINE CONCENTRATORSThe concept of using subscriber line concentrators to reduce the cost of subscribers loops has been considered for many years. As early as 1908, development engineers proposed the use of remote line concentrators. 1 Line concentrator design effort 2 -8 by the Bell System and other com panies throughout the world has continued ever since on both electro mechanical and, more recently, all electronic versions. Designs for use with specific central offices avoid the expansion stage of switching in the central office terminal needed by universal line concentrators which work LOOP SWITCHING SYSTEM 1157with any central office. They, therefore, can achieve lower costs in a more limited market area. However, past field experience has been with concentration of the universal type.Early universal line concentrators suffered from the technical dif ficulties of maintaining complex switching equipment remote from the central office. The harsh outside plant environment also created a number of electrical and physical reliability problems. The traffic ca pacity was inadequate to handle the wide variations in traffic generated by small groups of subscribers, resulting in an excessive number of blocked calls. Consequently, the use of line concentrators by the tele phone companies fell into disfavor.A new breed of electronic concentrators has solved these basic prob lems and began to reverse these attitudes. Introduced in the early 1970's was the Subscriber Loop Multiplex (SLM m ) system, 9 designed by Bell Laboratories and manufactured by Western Electric. The SLM system concentrates 80 subscriber lines on 24 T l carrier derived channels. T o date, 190 SLM systems have been installed.The SLM system demonstrates that electronic concentrator systems for use in the outside plant are reliable and can be mai...