AT&T DS1/DMi/ISDN-PRI manual 1s-Density Requirement

INTRODUCTION 1-17

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Figure 1-7.Alternate Mark Inversion

1s-Density Requirement

On the receive side, a DS1 uses the received bipolar pulses of the DS1 signal to recover the 1.544M- bps clock signal that transmitted the bit stream. To do this, the bipolar signal must contain enough pulses (1s) to allow the clock recover circuit to remain synchronized with the bipolar signal. This is known as the 1s-density requirement.

If there are not enough 1s, the clock frequency drifts causing the bits to be received at a different rate than they were transmitted. If this continues, a surplus or deficiency of bits will accumulate at the receiving end. Eventually this surplus or deficiency will equal an entire frame’s worth of bits (192). Then, an entire DS1 frame is either repeated or deleted to compensate for the differences in transmitting and receiving clock frequencies. This is called a slip. (Slips can also be caused by incorrect switch synchronization as discussed in the Synchronization of Digital Facilities chapter.)

The 1s-density requirement specifies that a minimum 1s-density average of 12.5% be maintained and that a maximum of 15 consecutive 0s can occur in the bit stream. If this requirement is not met, it is assumed that an error has occurred and that the network equipment will insert a series of ones into the bit stream to compensate.

Zero Code Suppression and Bipolar with 8 Zero Substitution

To guarantee that data transmitted over a DS1 facility contains enough 1s, a DS1 uses one of two coding options used with AMI line coding. The first option is zero code suppression (ZCS) and the second is bipolar with 8 zero substitution (B8ZS). The option chosen is made through DS1 administration, with ZCS being the default. When ZCS is used, DS1 provides restricted channels. When B8ZS is used, DS1 provides unrestricted or clear channels.