Slip detection during bit-error-rate measurement

Abstract

A bit-error-rate detector (20) in a test set (10) for a frame-based communications channel employs a pseudo-random-number generator (46) at the channel's output end that generates a sequence the same as that produced by a pseudo-random-number generator (16) at the input end, but typically with a timing offset. A chain of delay circuits (38, 40, 42, and 44) receives the channel output. Each delay circuit imposes a delay equal to a single frame time and produces a respective output. One such output (CENTER) is normally compared in an XOR gate (52) with the output of the output-end pseudo-random-number generator (46). The XOR gate (52) applies signals indicative of any symbol mismatches to a shift register (88), which forwards them, after a delay, to a bit-error-rate counter (90). At the same time, another XOR GATE (70) compares the output of the channel or of one of the other delay circuits (38, 42, and 44) with the pseudo-random-number-generator output, and a decoder (80) generates a slip-indicating output when a counter (76), which counts the number of consecutive matches that the latter XOR GATE (70) detects, indicates that the output of the channel or other delay circuit (38, 42, or 44) has matched the output-side pseudo-random-number-generator output a number of times in a row indicative of the likelihood of a frame slip. In response, a slip counter (92) is incremented and the shift register (88) cleared to avoid counting as ordinary bit errors mismatches that occurred in the CENTER signal during the matching sequence in the other signal.