TANDEM OPERATION IN THE BELL SYSTEM 393 



tors' positions equipped with key sets having a row of keys for each of 

 the last four digits and the party line letters in subscribers' numbers, 

 and, in addition, there are coordinate routing keys for selecting trunk 

 groups to the various ofifices either inside or outside the New York 

 City numbering plan area. In the more recent type of panel sender 

 tandem system, trunk groups to offices outside the Tumibcring plan area 



Fig. 7 — Panel sender tandem operators' positions — Suburban Tandem — 

 New York City. 



are given 3-digit codes which do not conflict with office codes within the 

 numbering plan area, and trunk selection is made by setting up these 

 codes on the key set. The trunks to the operators' positions of the 

 semi-mechanical tandem are on a straightforward key-listening basis 

 and do not have the call distributing feature. 



Frequently, it is found desirable in cities served by panel central 

 office equipment to consolidate the traffic, originating in one central 

 office building and destined to a number of central offices in one or more 

 distant buildings, over a single group of trunks to a distant office which 

 serves as a distributing point. This is done by placing panel office 

 selectors at the distant point. Such equipment constitutes an office 

 selector tandem and is arranged to complete connections incoming and 

 outgoing over panel dial trunks only. In all cases the connections are 

 set up by dialing on the part of a subscriber, or by dial or key set opera- 

 tion on the part of an operator. The operation of the office selector 

 tandem equipment, in establishing a connection, differs from that of 

 the full selector sender tandem in that it is controlled by the senders 



