250 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



6. Basic Circuits 261 



a. Diode Gates 261 



b. Transistor Bistable Circuit 262 



c. Transistor Pulse Amplifier 263 



d. Transistor Ring Counter 264 



e. Crosspoint Operating Circuit 266 



f . Crosspoint Relay Circuit 267 



g. Pulse Signalling Circuit 268 



h. Power Supply 269 



7. Concentrator Operation 270 



a.Line Scanning 270 



b. Line Selection 272 



c. Crosspoint Operation and Check 273 



8. Central Office Circuits 274 



a. Scanner Pulse Generator 279 



b. Originating Call Detection and Line Number Registration 280 



c. Line Selection 282 



d. Trunk Selection and Identification 284 



9. Field Trials 286 



10. Miscellaneous Features of Trial Equipment 287 



a. Traffic Recorder, b. Line Condition Tester 288 



c. Simulator, d. Service Observing 290 



e. Service Denial, f . Pulse Display Circuit 291 



1. INTRODUCTION 



The equipment which provides for the switching of telephone connec- 

 tions has ahvays been located in what have been commonly called "cen- 

 tral offices". These offices provide a means for the accumulation of all 

 switching equipment required to handle the telephone needs of a com- 

 munity or a section of the community. The telephone building in which 

 one or more central offices are located is sometimes referred to as the 

 "wire center" because, like the spokes of a wheel, the wires which serve 

 local telephones radiate in all directions to the telephones of the 

 community. 



A new development, made possible largely by the application of de- 

 vices and techniques new to the telephone switching field, has recently 

 been tried out in the telephone plant and promises to change much of . 

 the present conception of "central" offices and "wire" centers. It is 

 known as a "line concentrator" and provides a means for reducing the 

 amount of outside plant cables, poles, etc., serving a telephone central 

 office by dispersing the switching equipment in the outside plant. It is 

 not a new concept to reduce outside plant by bringing the switching 

 equipment closer to the telephone customer but the technical difficulties 

 of maintaining complex switching equipment and the cost of controlling" 

 such equipment at a distance have in the past been formidable obstacles 

 to the development of line concentrators. With the invention of low 

 power, small-sized, long-life devices such as transistors, gas tubes, and 

 sealed relays, and their application to line concentrators, and with the 

 development of new local switching systems with greater flcxibilit}', it 

 has been possible to make the progress described herein. 



