COMMON CONTROL SWITCHING SYSTEMS 111') 



occur. In most marker systems a lockout arrangement permits onl}' one 

 marker at a time to test trunks in a given group. There are cases where 

 trunks arc common to two offices and two markers are allowed to test 

 trunks simultaneously. In these cases special circuit arrangements are 

 provided at nominal expense to avoid double connections. Modern com- 

 mon control S3^stems with markers are, therefore, free of double con- 

 nections resulting from weaknesses of the sj^stcm and they can occur 

 only as a consequence of defects in circuits or apparatus. 



THEORETICAL OFFICES 



It is sometimes desirable to assign more than one office designation to 

 customers in a single central office unit. A new unit may be planned for 

 sometime in the future and if growth on the existing unit can be taken 

 with a new office designation, then when this new office is placed in 

 service it can be done without directory changes by transferring a block 

 of lines from the old unit. Another occasion for assigning more than one 

 designation to a single unit arises when customers served by the unit 

 are in two rate zones, and service to lines in one of the rate zones must 

 be restricted or extra charges collected. The lines served by an additional 

 designation are called a theoretical office. Common control systems 

 handle theoretical offices with little difficulty. In the first case mentioned 

 the translating equipment in the originating offices recognizes that the 

 physical office and theoretical office designations require identical treat- 

 ment until the new unit is cut into service at which time translator cross- 

 connection changes take care of the new routings. AVhere different rate 

 treatments are involved, records for billing purposes depending on both 

 the origin and destination of the call can be made by methods previously 

 mentioned. In some cases where the billing data is determined at a 

 tandem office and different treatments for the same destinations must 

 be given to customers calling from one office, split trunk groups must be 

 provided to tandem, one for each treatment. 



In the step-by-step system, theoretical offices can be opened up by 

 multipling two selector levels together. For example, if the physical 

 office is designated 25 and it is desired to open a theoretical office, say 26, 

 the 5 and G levels on the proper second selectors in the network can be 

 strapped until the 26 office is changed to a physical office. At that time 

 the levels are split and trunks to the new office are connected to the 

 6 levels of the second selectors. Restrictions in reaching blocks of numbers 

 can be applied by spHtting selector multiples and intercepting calls to 

 restricted blocks from one of the splits. 



