AUTOMATIC TESTING OF INTERTOLL TRUNKS 943 



just previously from some other originating office, this call waits on the 

 test line. When the transmission measuring and noise checking circuit 

 is ready to serve this call, it connects to the test line on which this call 

 is waiting and then returns a steady off -hook signal to the originating end. 

 This notifies the originating end that the transmission test may begin. 



The philosophy of a two-way transmission measurement is as follows. 

 The near end sends test power over the trunk and the far end measures 

 the loss as previously described. In this process the loss of the trans- 

 mitting pad at the far end is adjusted to a value equal to the trunk loss 

 in the near to far direction. The far end then returns test power, first 

 directly over the trunk and next, through the transmitting pad. The 

 power levels received at the near end are a measure of first, the trunk 

 loss in the far to near direction and next, the sum of the losses in the two 

 directions. Measurement of these levels provides data for recording the 

 loss in each direction at the near end. 



The two-way transmission measurement takes place in four steps as 

 shown in Fig. 6. These steps are described below. 



Slevl 



The near end sends one milliwatt and the far end adjusts its pads and 

 checks the measurement. After about 3 seconds the near end removes 

 the test power and then pauses for a short interval to wait for a signal 

 denoting whether or not the accuracy tests were successful. 



If they were unsuccessful, the far end will restore itself to the condi- 

 tion prevailing at the start of Step 1 and will also return a short (about 

 3^2 second) on-hook signal to the near end. The near end then reconnects 

 the test power for three seconds for another trial. The test frame at the 

 near end stops and sounds an alarm after a third unsuccessful trial. 



If the far end is successful in any one of the first three trials, an on-hook 

 signal will not be returned to the near end when the test power is re- 

 moved. The near end, after the short pause, then sends a short spurt of 

 test power which reoperates the measuring relay at the far end. This 

 signal at the far end, after a successful Step 1, indicates to the far end 

 that this is a full automatic test. 



Step 2 



For Step 2 the near end connects a far-near amplifier, a set of far-near 

 receiving pads and an amplifier-rectifier. The far end disconnects its 

 receiving eciuipment and returns one milliwatt over the trunk. The far- 

 near receiving pads at the near end are now inserted in the proper com- 



