COMMON' CONTROL SWITCHING SYSTEMS 1119 



systems do a good job of haiidliiiji; moderate overloads and, by provision 

 of load control features, viui operate satisfactorily even with severe 

 o\'erloads. 



From a maintenance standpoint, a disadvantage of common controls 

 is the relative complexity of the circuits. While this has introduced a 

 training problem, maintenance forces have had no difficulty in acquiring 

 the knowledge^ nec(h'd to do a conipe^tcnt niaint(>nanc(^ jol). 



CONCLUSION 



The full fledged common control systems exemplified by the crossbar 

 local and toll s^'stems have a number of important advantages over 

 systems where the switches are dii\(Mi directly by the customer's dial. 

 The advantages arise largely from the ability to store digits, to translate 

 them, use them fiexibl}' for switching within the office, and transmit as 

 many of them as desired to distant points for subsequent switching 

 operations. The digits can be converted to others of different value 

 whenever it is adA'antageous to do so. The inherent flexibility of common 

 control equipment makes it possible to adopt any kind of numbering 

 plan for a local area or a nationwide network that is best suited for the 

 purpose without regard to the manner in which calls will be trunked fi-om 

 one point to another. Codes can be assigned at will to represent destina- 

 tions and the best route for the call can always be taken. The best route 

 may in some cases involve tandem operation or even a half-dozen 

 switches in tandem. It may be the I'oute selected as an alternate after 

 previous trial of one or more other routes. A connection may be set up 

 between offices of different types and over trunk groups requiring differ- 

 ent forms of pulsing. These conditions may be met b}" common control 

 equipment and the ability to meet such conditions makes it possible to 

 provide cheap step-by-step eciuipment in places for which it is best 

 suited, compensating for some of its deficiencies with common control 

 eciuipment in other places. 



With marker type common controls, ti-unk groups out of an office 

 can be of any desired size regardless of the switch design. The individual 

 crossbar switch, for example, gives access to only ten or twenty outlets 

 as normally wired but full access single trunk groups of hundreds of 

 trunks can be employed in some crossbar systems. 



Schemes for recording billing data, aside from the relatively simple 

 ones where metering equipment is associated with the customer's line 

 and operated once per call, make use of common control equipment. 

 This seems to be necessary where detail records must be made on indi- 

 vidual culls for charging purposes. 



