AT&T DS1/DMi/ISDN-PRI Differences Between ZCS and B8ZS, Introduction Uncoded BIT Stream Pulse

INTRODUCTION 1-19

UNCODED BIT STREAM

PULSE

STREAM

010011

0 + 0 0 - +

00000000

0 0 0 + 0 - +

111

- + -

00000000

0 0 0 - + 0 + -

00000000

0 0 0 - + 0 + -

01

0 +

VIOLATIONS BASED ON POLARITY

OF LAST 1 TRANSMITTED

Figure 1-8.Example of B8ZS Line Coding

Applications requiring B8ZS line coding are currently in the minority, but it is expected that in the long term they will be in the majority. The B8ZS provides no substantial advantages for voice and voice-grade data signals over ZCS. However, if the data communications protocol does not already maintain proper 1s-density, then B8ZS is essential for transmitting unrestricted digital data. Even if the AT&T network contains unrestricted facilities, the access facilities through the local exchange may not, which means that you would still be required to use the ZCS option.

Differences Between ZCS and B8ZS

Differences between ZCS and B8ZS include:

1.ZCS requires that user data be presented via a data communications protocol that does not generate the all-0s octet, while B8ZS has no such restrictions

2.ZCS monitors each B-channel (not including the framing bits), while B8ZS monitors the entire DS1 facility (including framing bits)

3.ZCS maintains 1s-density at the expense of altering the data, while B8ZS maintains 1s-density without altering the data

4.When detecting the all-0s octet with ZCS, the transmit side inserts a 1 in the second LSB, which will not be corrected from by the receiving side. When detecting eight consecutive 0S with B8ZS, switch in the special B8ZS code word. The receive end monitors the DS1 bit stream and will switch in eight 0s when detecting B8ZS code words.