252 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



4. Each concentrator would serve up to 50 lines with the central office 

 control circuits common to a number of concentrators. (Experimental 

 equipment described herein was designed for 60 lines to provide addi- 

 tional facilities for field trial purposes.) No extensive change would be 

 made in central office equipment not associated with the line switches 

 nor should concentrator design decrease call carrying capacities of exist- 

 ing central office equipment. 



5. To provide data to evaluate service performance, automatic traffic 

 recording facilities to be integrated with the design. 



6. Remote equipment designed for pole or wall mounting as an addi- 

 tion to existing outside plant. Therefore, terminal distribution facilities 

 would not be provided in the same cabinet. 



7. Power to be supplied from the central office to insure continuity 

 of telephone service in the event of a local power failure. 



8. Concentrators to operate over existing types of exchange area fa- 

 cilities without change and with no decrease in station to central office 

 service range. 



9. Maintenance effort to be facilitated by plug-in unit design using 

 the most reliable devices obtainable. 



3. NEW DEVICES EMPLOYED 



»! 



I 



Numerous products of research and development were available for 

 this new approach. Only those chosen will be described. 



For the switching or "crosspoint" element itself, the sealed reed switch 

 was chosen, primarily because of its imperviousness to dirt.* A short coil 

 magnet with magnetic shield for increasing sensitivity of the reed 

 switches were used to form a relay per crosspoint (see Fig. 1). 



A number of switching applications^ '^^ for crosspoint control using 

 small gas diodes have been proposed by E. Bruce of our Switching Re- 

 search Department. They are particularly advantageous when used in 

 an "end marking" arrangement with reed relay crosspoints. Also, these 

 diodes have long life and are low in cost. One gas diode is employed for 

 operating each crosspoint (see Fig. 6). Its breakdown voltage is 125v ± 

 lOv, A different tube is used in the concentrator for detecting marking 

 potentials when termination occurs. Its breakdown potential is lOOv ± 

 lOv. One of these tubes is used on each connection. 



Signaling between the remote concentrator and the central office con- 

 trol circuits is performed on a sequential basis with pulses indicative of 

 the various line conditions being transmitted at a 500 cycle rate. This 

 frequency encounters relatively low attenuation on existing exchange 

 area wire facilities and j^et is high enough to transmit and receive in- 

 formation at a rate which will not decrease call carrjdng capacitj^ of the 



