1312 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 



to tho time duration of each signal. In general, variations in signal time 

 should be within ±5 per cent. All effects of the trunk signal medium 

 should be confined within the trunk terminals or be of such character as 

 to have no adverse reaction in connected circuits. This is necessary for 

 proper operation of switched connections. 



BASIC PLAN 



The in-band single-fret juency signaling system, although fairly com- 

 plex in detail, is very simple in principle. Normally, i.e., when the circuit 

 is idle, steady tone is transmitted over the line and holds relays operated 

 at the receiving end. Signaling is accomplished by removing and re- 

 applying this tone, which in turn releases and reoperates the distant 

 relays. Independent operation is obtained in each direction with one 

 signal frecjuency on four-wire lines, which have separate one-way trans- 

 mission pathes from terminal to terminal, and with two signal fre- 

 quencies, one for each direction of transmission, on two-wire lines. 



The signaling system is provided as a separate entity. It is connected 

 in series with the transmitting and recei\dng branches of the line circuit 

 at each end of a trunk and to the terminal relay circuit (trunk circuit) by 

 two one-way signaling leads. A typical arrangement for a four-wire line 

 terminating in a two-wire switching office at the West terminal and a 

 four- wire switching office at the East terminal is shown in Fig. 1 . 



-^ 



TRUNK 

 CIRCUIT 



-13 DB LEVEL 



BALANCING 

 NETWORK 



+4 DB LEVEL 



+4 DB LEVEL 



SF 

 SIGNAL 

 CIRCUIT 



4-WIRE 

 TOLL 

 LINE 



X 



R '*" 



SF 

 SIGNAL 

 CIRCUIT 



TRUNK 

 CIRCUIT 



-13 DB LEVEL 



> ^ 

 o 



CONTROL 

 LEADS 



Fig. 1 — Application of single-frequencj^ signaling to trunks with four-wire lines. 



