390 BELL SYSTEM TECHNICAL JOURNAL 



phia. These straii;htforward toll lines are arranged for one-way 

 operation and terminate on single -ended cords at automatic-listening 

 tandem positions in the Long Lines offices in New York and Phila- 

 delphia. Regular toll switching trunks are multipled at the tandem 

 positions for reaching the various local offices on incoming calls; idle 

 trunks are found by the tandem operators by tip test, no visual busy 

 signals being provided. The order for connection to the called station 

 is given to the " B " operator at the local office by the originating opera- 

 tor. Ringing on the toll switching trunks is controlled by equipment 

 in the toll line circuits. Switchhook supervision from both called and 

 calling station is received by the originating operator as in local tandem 

 systems. The disconnect signals at the tandem board are controlled 

 by the originating operator. The trunks from the tandem board to 

 the local offices are of the straightforward type. In the case of dial 

 central offices, these trunks terminate on selectors, but an operator at 

 an associated "B" position sets up on a key set the connection to the 

 called subscriber's line. 



Toll Switching Trunk Tandems 

 In a number of the larger toll offices, equipment limitations prevent 

 the multipling of toll switching trunks to all of the local offices at all of 

 the outward positions. Each operator has direct access in the multiple 

 at her position to trunks to the offices from which she normally receives 

 calls. Occasionally, however, it is necessary for operators to reach 

 subscribers connected to other local offices, and to permit this a toll 

 switching trunk tandem is provided. The trunks incoming to the 

 tandem positions are of the cord-ended, key-listening, straightforward 

 type. The originating operator passes to the tandem operator the 

 name of the local office desired. 



Dial Tandems 



Dial tandems receive calls from operators and, in panel areas, from 

 subscribers also, and are of several types. 



Where there is a considerable concentration of short-haul toll traffic 

 within an area served by two or more dial tandem systems which indi- 

 vidually serve limited areas, it is sometimes desirable to interconnect 

 such systems and to route calls through one or more of these tandem 

 centers, as required. An example of this is shown in Fig. 9. 



Panel Tandems 

 Panel tandem systems employ panel selectors and are of two general 

 types; one known as the panel sender tandem, and the other as the 



