TANDEM OPERATION IN THE BELL SYSTEM 391 



office selector tandem. Both are designed for use in cities employing 

 panel type central offices, and therefore these tandems are used to 

 serve only the larger cities and their environs. 



Panel sender tandems use senders associated with the tandem equip- 

 ment to control the electrical operation of the system. The completing 

 trunks may be of the dial, call indicator, or call announcer types. The 

 sender is a device which receives the impulses from the incoming trunk 

 or tandem operators' positions, determines the routing for the call, 

 and sets up the necessary electrical conditions for operating the panel 

 type equipment in the tandem office and the associated equipment in 

 the completing trunks. 



These systems ordinarily include both operator tandem and full 

 selector tandem equipment, the latter for traffic routed directly through 

 the selectors from dial system "A" boards equipped with key sets. 

 Local calls dialed directly by subscribers are also routed through full 

 selector tandems when the volume of such calls is so small as not to 

 warrant direct trunks between the originating and terminating offices. 



While the equipment arrangements of the full selector tandem are 

 such as to permit operators at dialing type dial system "A" boards 

 (as distinguished from boards equipped with key sets) to dial numbers 

 at the distant offices direct, it usually is more economical to route calls 

 from these boards through the operator tandem positions. Manual 

 "A" boards in panel areas usually are not equipped wath either dials 

 or key sets. 



All of the trunks to a panel sender tandem office terminate on selec- 

 tors, but on incoming calls, other than those from operators at switch- 

 board positions equipped with key sets, or dialed by subscribers, the 

 trunk automatically is connected to an idle tandem operator who sets 

 up the called number on a key set provided with a row of keys for 

 each letter and digit in the number. The tandem operator's position 

 then is released automatically from the connection. On connections 

 completed through either the operator tandem or the full selector 

 tandem, disconnection, both at the tandem office and at the called 

 office, is under control of the originating operator, or calling subscriber 

 on calls dialed direct. 



The use of call announcer trunks, which at present are designed for 

 panel tandem systems only, permits tandem connections to be com- 

 pleted by key set or dialing operation to small outlying manual offices 

 where, for equipment or other reasons, call indicators are not pro- 

 vided. At the called office call announcer trunks are similar to straight- 

 forward trunks, but are so arranged that the connection of the "B" 

 operator's telephone circuit to the trunk causes the call announcer 



