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Line Coding
The DS1 signal consists of a continuous stream of ones and zeros, encoded into bipolar pulses for transmission. Only the ones create a pulse; the zeros represent the absence of a pulse. Pulses alternate between positive and negative. This type of line coding is called bipolar or alternate mark inversion (AMI). The line-coding formats guarantee that the ones-density requirement is met to achieve clock recovery.
To meet the ones-density requirement, either zero code suppression (ZCS) or bipolar 8 zero substitution (B8ZS) line coding can be chosen, but the selected line coding must match the line coding at the far end. Use this coding for tandem trunks.
ZCS line coding monitors each DS0 channel and prevents strings of eight or more zeros. Upon detecting eight consecutive zeros in a channel octet, ZCS line coding forcibly changes the seventh zero (the second least significant bit) to a one. The factory-set line coding is ZCS.
B8ZS line coding matches the ones-density requirement by using a special sequence with a bipolar violation in bit positions 4 and 7. Normally, for bipolar transmission, ones are encoded alternately as a positive then negative, or negative then positive, pulse. If two positive or two negative pulses are received in succession, a bipolar violation occurs. Bipolar violations are normally caused by noise hits to the signal; however, B8ZS uses a specific binary sequence with bipolar violations as a code for an all-zero channel octet.
B8ZS line coding is preferred over ZCS because it provides no possibility of corrupting data transmissions.
B8ZS violations are passed by the ACCULINK 3150 and 3160/3164, and ESF T1 channel service units (CSUs) but not by other CSUs.
