COMMON CONTROL SWITCHING SYSTEMS 1099 



codes. Changeable translation of office codes removes tiie limitation 

 that the trunks for a given office designation must be located in a definite 

 position on the switches which is the necessary result of fixed translation. 

 Increased flexibility of numbering is now possible because office designa- 

 tion changes no longer require rearrangements of switch multiple. More 

 economical arrays of switches are also possible because the switching plan 

 can conform to traffic requirements without regard to numbering. Other 

 ad\'antages of translation— and as a practical matter, flexible transla- 

 tion — -include the ability to operate with tandems, to operate with more 

 than one type of outpulsing, and to operate with varying numbers of 

 digits. The originating equipment of the panel system is an example of a 

 system using changeable translation. This type of translation is also used 

 for called line numbers as well as office codes in No. 1 and No. 5 crossbar 

 thereby permitting these systems to shift lines for load balancing pur- 

 poses without requiring numbering changes. 



Finally, there is the most flexible but also the most complicated plan 

 of all in which the selection of paths and trunks or lines is divorced from 

 the selectors and placed in markers. In this plan the size of group is not 

 limited by the number of terminals that a switch can hunt over in one 

 sweep. No. 1 crossbar is an example of a system using the marker method 

 of operation. In this system a switch generally has access to only ten 

 trunks but on any one call a marker can test 160 trunks distributed over 

 a number of switches. 



Typical common control arrangements for systems using translation 

 are shown in Fig. 3 for the panel system and in Fig. 4 for No. 1 crossbar. 



The advantages noted are, in each case, the fundamental ones. Many 

 others are inherent in common control and some will be brought out in 

 further discussion. 



A number of common control systems embodying the principles dis- 

 cussed have been designed. Rotary, panel and coordinate have been 

 previously mentioned. Although the coordinate system never reached the 

 commercial stage as a complete system, some of its features were adopted 

 in the panel system starting in 1927 with the introduction of the decoder 

 to replace the original three digit panel translator which used special 

 panel selectors and pulse generating drums to do the translating job. 

 This translator was limited in the digit combinations and number of 

 three digit codes it could handle and also demanded a great deal of atten- 

 tion by the maintenance force. In place of the panel translators a small 

 group of all-relay decoders, ranging from three to six, depending on 

 traffic, was provided for each office. Senders were connected to decoders 

 for about one-third of a second per call to obtain the information derived 



