846 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



Fig. 12 — Toll switchboard position with kej- set used lur lull dialing. 



vantages. The}^ are iiiherentl}^ of the "four-wire" type which minimizes 

 the number of possible singing and echo paths on a circuit. Also, the 

 speeds of propagation over carrier systems are generally higher than o\'er 

 voice frequency systems thereby further minimizing the echo problem. 

 These features are of great advantage in reducing limitations on circuit 

 design and layouts of the general toll switching plan. 



Signaling Systems 



In addition to the ability to carry messages, intertoll trunks must be 

 provided with suitable signaling facilities. ' These must provide a means 

 of: first, attracting the attention of the distant point, either an operator 

 or automatic equipment, to the fact that a connection is to be established ; 

 and second, in the case of dial operation, transmitting coded information 

 in the form of pulses for establishing the connection; and third, trans- 

 mitting a general class of super\'isory signals including connect and 

 disconnect signals, on and ofT switch hook signals, recall signals and 



