Effect of Echo Canceler on Common-Channel Interoffice Signaling Continuity Check

Effect of Echo Canceler on Common-Channel Interoffice Signaling Continuity Check By G. S. FANG (Manuscript received December 12, 1980) Annoying echoes generated by impedance mismatches can occur in long-distance telephone networks. An echo canceler controls echo by synthesizing an echo replica and subtracting it from the actual echo on the return path. For the echo canceler to work properly, it must be able to distinguish between desired signals and echoes. One instance where this can be difficult is during the common-channel interoffice signaling (ccis) continuity check between four-wire switching offices. On ccis trunks the voice and the signaling are routed separately. To ensure a satisfactory transmission path, a voice path continuity check is conducted before call setup. The check is performed on a loop basis by sending a check tone and looping it back at the distant office. Since the echo canceler adaptive filter memorizes information from the last previous call, it may partially cancel the looped-back tone. In this paper we study the effect of the echo canceler adaptive memory on the ccis continuity check. The analytical and experimental results indicate that the occurrence of continuity check failure caused by the presence of an active echo canceler is so infrequent and insignificant compared to the existing statistics of all other pertinent failure mechanisms. Thus, disabling the echo canceler during the ccis continuity check is unnecessary. I. INTRODUCTION An echo may be produced in a transmission system whenever there is an impedance mismatch.