REMOTE CONTROLLED LINE CONCENTRATOR 251 



2. OBJECTIVES 



Within the telephone offices the first switching equipment through 



which dial lines originate calls concentrates the traffic to the remaining 



equipment which is engineered to handle the peak busy hour load with 



the appropriate grade of service.^ This concentration stage is different for 



different switching systems. In the step-by-step system^ it is the line 



' finder, and in the crossbar systems it is the primary line switch.^ Pro- 



1 posals for the application of remote line concentrators in the step-by- 



i step system date back over 50 years/ Continuing studies over the years 



have not indicated that any appreciable savings could be realized when 



such equipment is used within the local area served by a switching center. 



When telephone customers move from one location to another within 



a local service area, it is desirable to retain the same telephone numbers. 



The step-by-step switching system in general is a unilateral arrangement 



where each line has two appearances in the switching equipment, one 



for originating call concentration (the line finder) and one for selection 



of the line on terminating calls (the connector) . The connector fixes the 



line number and telephone numbers cannot be readily reassigned when 



moving these switching stages to out-of-office locations. 



Common-control systems^ have been designed with flexibility so that 

 the line number assignments on the switching equipment are independ- 

 ent of the telephone numbers. Furthermore, the first switching stage 

 in the office is bilateral, handling both originating and terminating calls 

 through the same facilities. The most recent common-control switching 

 system in use in the Bell System, the No. 5 crossbar,^ has the further 

 advantage of universal control circuitry for handling originating and 

 terminating calls through the line switches. For these reasons, the No. 

 5 crossbar system was chosen for the first attempt to employ new tech- 

 niques of achieving an economical remote line concentrator. 



A number of assumptions were made in setting the design require- 

 ments. Some of these are influenced by the characteristics of the No. 5 

 crossbar system. These assumptions are as follows: 



1. No change in customer station apparatus. Standard dial telephones 

 to be used with present impedance levels, transmission characteristics, 

 dial pulsing, party identification, superimposed ac-dc ringing,^ and sig- 

 naling and talking ranges. 



2. Individual and two-party (full or semi-selective ringing) stations 

 to be served but not coin or PBX lines. 



3. Low cost could best be obtained by minimizing the per line 

 equipment in the central office. AMA^ charging facilities could be used 

 but to avoid per station equipment in the central office no message reg- 

 ister operation would be provided. 



