520 BKLL SYSTEM TECHNICAL JOURNAL 



is similar to that already described, but additional equipment is 

 provided for starting the motor from the switchboard. An attempt 

 is made to complete the call on an attended basis as outlined above 

 and, if the called subscriber does not answer, the operator asks the 

 calling subscriber if he wishes to leave his message. If he does, she 

 presses a key in the cord circuit, which starts the motor at the absent 

 subscriber's teletypewriter. The operator then instructs the calling 

 subscriber to proceed with the communication. 



Long Subscriber Lines 



The subscriber stations just discussed are connected to the central 

 office by two wires, known as a loop, the maximum distance between 

 station and switchboard for loop connections being approximately 

 38 miles. A network is placed in the loops where the mileage makes its 

 use necessary to improve the transmission efficiency. 



It is necessary in some instances to connect subscribers situated at 

 greater distances from the switchboard, perhaps as much as 200 or 

 250 miles. Two methods are available for accomplishing this: the 

 d-c. method using telegraph facilities, and the carrier method using 

 telephone facilities. 



\A'ith the d-c. method a standard telegraph repeater is used at the 

 central office, and a simplified repeater is placed on the subscriber's 

 premises. These repeaters, with suitable signaling apparatus, provide 

 a high grade of transmission and also the same type of supervisory 

 signals as would obtain on the shorter loop connection. 



The carrier method is used to a limited extent in the few instances 

 where telegraph facilities are not available. In this method both the 

 central office and the station are equipped with carrier apparatus and 

 the regular telephone facilities are used. When the subscriber oper- 

 ates the power switch of the station, an answering lamp is lighted 

 before the operator of the local telephone switchboard. The local 

 operator, knowing by the multiple marking that this is a teletype- 

 writer station, immediately connects through to the TWX switchboard 

 over the regular toll telephone facilities. When the TWX switch- 

 board is reached the call is handled in the same manner as a regular 

 d-c. telegraph connection. All the signaling facilities available for 

 the other subscriber stations are also available here. Completion to 

 the subscriber is also made by the TWX operator over the regular 

 toll telephone facilities, and the local telephone operator at the switch- 

 board to which the subscriber is connected rings the subscriber. 



Inter-toll Trunk Supervision 

 To provide inter-toll trunk supervision in the TWX network, it 

 was necessary to select different types of signals than those occurring 



