COMMON CONTROL SWITCHING SYSTEMS ilUl 



from the callinji; line and llicii ic-cstablislics a coiincclion l)ack to the 

 calHnjj; line. 



Coniinoii coutiols ha\(' Ix'cii cinploycd by llic I^cll Syslcin in a nuinhci' 

 of systems in adchtion to those ali'eady mentioned. These inchuh' ])anel 

 sender tandem, ci'osshar tandem, and Xo. 4, A4A and 4 A toll erossbur. 



COMPARISOX OF COMMON' COXTKOl, SVSTlsMS AND DlltKCT DIAL CONTROL 

 SYSTEMS 



Botii direct dial contrt)l and conunon conliol systems liave been de- 

 A'eloped to meet a wide range of situations for both large and small 

 exchanges but, as previously noted, direct dial control systems ha\-e 

 found their greatest field of use in the smaller exchanges and common 

 control systems in the larger ones. The reasons for this can be brought 

 out by a discussion of some of the features which have an important 

 bearing on costs. These include the features affecting numbering plans, 

 tiunking arrangements, flexibility, quality of service, maintenance and 

 engineering. A discussion of all the factors affecting costs Avill not be 

 attemptetl. However, some of the more important ones will be covered. 



RELATION BETWEEN TY'PE OF SY'STEM AND NUMBERING PLANS 



The requirements of a good numbering plan are well known. A good 

 plan must be universal, i.e., must use the same number for reaching a 

 called line regardless of the point of origin of the call in the area covered 

 by the numbering plan, must permit dialing with acceptable accuracy, 

 must permit directory listings that are readily understood by both dial 

 and manual customers, and should use a minimum number of digits to 

 reduce the labor of diahng. In small networks a satisfactory plan can be 

 set up with almost an}^ kind of system. However, especially in large 

 networks, modern common control systems have outstanding advantages 

 with respect to numbering. 



These advantages of common controls are derived from the mow, 

 flexible method of operation. Direct dial control systems use up tlu^ 

 digits in the various stages of the switching operations whereas common 

 control systems momentarily store them and can retransmit them. The 

 result is that where direct dial control systems are used the numbering 

 plan and the switching and trunking plans must conform whereas with 

 conunon controls numbering, switching and trunking are not directly 

 dependent on each other because the digits can be stored and translated. 

 The effects of these differences on permissible latitude in numbering 

 arrangements can be brought out by some examples. 



