Observations of errors and error rates on T1 digital repeatered lines

Measurements of errors on T1 repeatered lines were made at five Bell System offices during 1973 and 1974. They included an informal survey of error rates and error-free seconds on lines in service, and detailed recordings, normally of 24 hours duration on selected lines. The detailed recordings show the existence of at least two distinct error mechanisms, differing significantly in diurnal variation of error rate, distribution of intervals between errors, and dependence on the transmitted bit pattern. It was found that certain T1 lines made errors when in service, driven by D1 channel banks, but not when driven by a pseudorandom test signal.