COMMON CONTROL SWITCHING SYSTEMS 1109 



provide the digit storage and digit spilling features in the oflice that 

 does the alternate routing so that it can spill forward to the alternate 

 route point the digits the latter requires. 



Common controls have other ad\-antages with respect to trunking 

 which have already been covered in part. They also simplify the problems 

 of assignment and load balancing as groups change in size or as new 

 groups are added. An example of the difference in the methods of han- 

 dling trunk groAvth in step-by step and crossbar is of interest. In step- 

 bv-step when groups grow beyond 10 trunks a grade must be introduced 

 in the switch wiring, or trunks must be sub-grouped or rotary out trunk 

 switches used. If further growth occurs, regrades must be made or re- 

 arrangements may be required in the sub-grouping or in the rotary out- 

 trunk switches. In a crossbar system, howe\xr, in most cases added 

 trunks are merely assigned to spare switch terminals which are left 

 \-acant for this purpose. 



ROUTINGS FOR IRREGULAR CONDITIONS 



Common controls are adapted to the efficient recognition and handling 

 of irregular conditions such as permanent signals, vacant codes, and dis- 

 continued or temporarily intercepted lines. 



Registers or senders detect line troubles which cause permanent signals 

 or receivers off the hook by a timing circuit which waits for a short time 

 for dialing to start. If the dialing does not start ^^dthin the interval al- 

 lowed the line is directed to a common group of permanent signal trunks 

 which may appear before operators or at a test board. In No. 5 crossbar 

 a trouble recorder card can be produced on which the location of the 

 line in trouble is indicated. The step-by-step system indicates permanent 

 signals by alarms to the maintenance force on a line group basis, and 

 lines in trouble must be traced. 



Vacant codes are detected by the translators, decoders and markers 

 of common control systems and the calls are routed to a common tiunk 

 group which appears before operators or which returns "no such numl^er 

 tone." The corresponding arrangement in step-by-step requires con- 

 nections from the switch multiple to operator or tone trunks. 



In systems like No. 1 crossbar and No. 5 crossbar which have common 

 controls in the terminating equipment, lines on which service has been 

 discontiinied or temporarily intercepted can be recognized by the mark- 

 ers and the calls rerouted to a common group of intercepting trunks. 

 For example, temporary discontinuation of service is indicated by lifting 

 a single cross-connection at the numboi- group frame. In the step-by-step 



