394 BELL SYSTEM TECHNICAL JOURNAL 



associaU'd with the local central office equipment at the originating 

 office, or by senders associated with the panel tandem equipment in 

 the case of traffic first routed through the latter. 



While panel sender tandem systems ordinarily include both operator 

 tandem and full selector tandem equipment, Knickerbocker Tandem 

 in New York City is entirely of the full selector type, being designed for 

 traffic incoming over dial trunks only. 



Step-by-Step Tandems 



Step-by-step tandem systems employ step-by-step selectors, and, 

 ordinarily, are of the full selector type without tandem operators' 

 positions. Under these conditions, no senders are required at the 

 tandem office and the pulses which select the central office to which 

 connection is to be made are received over the tandem trunk. Con- 

 nections may be made through a step-by-step tandem system to both 

 dial and manual offices; to the latter by the use of straightforward, call 

 indicator, or automatic-signaling ringdow^n tandem completing trunks. 

 Call announcer trunks are not used. Release of the tandem trunk at 

 the originating office automatically releases the tandem selectors and 

 at the same time releases the selectors, or (except in the case of ring- 

 down trunks) gives a disconnect signal, at the called office. 



Figures 9 and 10 show, schematically, the step-by-step tandem sys- 

 tem in Connecticut, over which most of the toll traffic within the state 

 is handled. Similar systems are in use in Southern California,^ and on 

 a smaller scale, in other places. 



In certain cases the increased trunk efficiency of step-by-step tandem 

 operation is obtained, without the necessity of providing a tandem 

 switching equipment, by locating some of the second selectors of local 

 step-by-step central offices in a distant building serving two or more 

 central offices to which calls are to be distributed. These "distant 

 second selectors" combine all of the traffic to the terminating office over 

 a single group of trunks. In other cases, increased trunk efficiency is 

 obtained through the use of certain levels on the regular step-by-step 

 selectors in a distant dial central office on which to terminate trunks to 

 other central offices. 



As stated, step-by-step tandems generally receive the controlling dial 

 pulses over the incoming tandem trunks. When it is desired to com- 

 plete connections through a step-by-step tandem system from manual 

 offices, this may readily be done if the manual positions are equipped 

 with dials. Where the volume of traffic on which dials could be used is 



' For a detailed description of the design of the Los Angeles tandem system, see 

 paper by F. D. Wheelock and E. Jacobsen, Transactions A. I. E. E., Vol. 47. 



