COMMON CONTROL SWITCIIIXG SYSTEMS 1117 



with a ixM'maneiit signal by perforating a liouhlc ircoidci- card has 

 cliniinated the need for tracing permanents. 



A number of schemes are employed to detect tr()ul)les in markers and 

 (l(>co(lers and in circuits which connect to them. These include detectoi's 

 for wrong sequences of operations, wrong combinations of i'e]a3\s, exces- 

 si\-e current, false potential and lack of contiiuiity. These are generally 

 iiiti-odu(;ed at small cost since the circuits to which they are applied ai'c 

 small multipliers. Howe\'er, some of them do a majoi- job of testing since 

 they reach out and test the numerous elements of the switcliing system 

 to which markers have access. In this category are the tests of the cross- 

 bar linkages for opens, false grounds and double connections, tests of the 

 switch crosspoints for continuity, tests of lines for false grounds, and for 

 receivers off the hook on coin first coin lines. 



To obtain clear trouble records, markers are designed with interlocked 

 progress signals. This has made trouble analysis easier and has tended 

 to improve design by eliminating relay races. 



Starting with the panel system tests have also been intioduced in 

 senders for detecting open and reversed trunks. These tests have been of 

 considerable help in maintaining outside plant and in detecting condi- 

 tions that could lead to false charges. 



DIJ^ADVANTAGES OF COMMON CONTROLS 



Up to this point the stress has been mainly on the ad\'antages of com- 

 mon controls. There are also some disadvantages. One of the major ones 

 is the substantial getting started cost due to the necessity of providing a 

 minimvmi amount of common equipment. This minimum is provided to 

 maintain operation in case of trouble and during intervals when, for 

 example, cross-connections require change because of changed or added 

 routes. The minimum requirements establish economic })arriers which 

 tend to prohibit the economical use of common controls for small iso- 

 lated systems. 



Another disadvantage is the performance of common control systems 

 under severe and protracted overloads. Experience with these systems 

 indicates that although they compare quite favorably to direct dial con- 

 trol systems with respect to capability of handling moderate overloads, 

 they are not able to handle severe overloads as well. In part this is a 

 consequence of the fact that elements in common control systems are 

 used at high efficiency and hence there is relatively less free equipment 

 at full load for soaking up an overload than there is in systems that 

 operate with smaller and less efficient groupings. AMienever the number 



