Zero code suppression (ZCS) is a technique that used to be implemented on T1 systems to ensure ones density.Ones density is a requirement imposed by the used of
AMI as the line code on a T1. In this line code, a long series of the zeroes (i.e., more than 15) will lead to loss of clock and loss of synchronization. This was not a problem for T1 used in the
PSTN because the PAM/PCM voice digitizing scheme never generated the all-zero code (i.e., 00000000). When these same facilities were used to send data, however, a long series of zeroes became possible.
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