The dial-up telephone network is based upon your analog telephone's signal being converted to a 64kbps digital stream by a codec. One of the limitations of the original design of these systems was that no more than 7 consecutive zeros be transmitted on a T1 carrier. The older AMI changes an 8 zero-bit sample to 00000010. When the T1 signal is received at the next switching or destination point, a time-slot processor generates the 24 DS0 channels.
Software settings determine the line coding used by each processor.
However, if there is a line coding mismatch in the call path, the result can be highly troublesome for 56k modems. If there is a line coding error somewhere in a 56k-modem's call path, the result can be a high bit error rate with a serious hit in throughput. The difference in the sent and received values for any particular 8-bit PCM sample can be extremely large in magnitude. Also, notice that time-slot 2 s data was changed from 00000011 to 01100011. If this were a binary representation of a decimal number, instead of receiving a decimal 60 on time-slot 1, we would have received a decimal 63.