Harris Intraplex ACS-160 Series - Figure 2-18. AMI Line Coding; Figure 2-19. Original Payload with Eight Consecutive Zeroes

2 – Functional Description ACS-160 Series & STL-160 Series T1 Multiplexer Installation & Operation
Version 2.11, August 2011
Harris Corporation 2-25
Intraplex Products
AMI is a simple bipolar coding scheme. Each “1” bit in the data stream is given an opposite polarity
from the one before it, while each “0” bit is neutral (Figure 2-18). If two 1s in a row have the same
polarity, it is a bipolar violation and indicates a transmission error; the BPV indicator on the CM-5RB
module flashes (flickers erratically) whenever a bipolar violation is detected.
AMI is an older system with a significant limitation: when the payload being transmitted contains too
many 0s in a row (a condition called “insufficient ones density”), the T1 circuit may lose frame
synchronization.
Figure 2-18. AMI Line Coding
To help prevent insufficient ones density on AMI networks, many Intraplex channel modules offer
methods of preventing long strings of 0s from occurring. These methods include
● Placing the payload data into alternate time slots (any unused time slots are automatically filled
with all ones).
● Using only 56 kbps instead of 64 kbps per time slot and filling every eighth bit with a 1.
● Incorporating a data scrambler that applies its own form of zero substitution to the data for that
channel.
B8ZS is a newer and more robust form of bipolar line coding that eliminates the problem of insufficient
ones density. With B8ZS, any time a string of eight consecutive zeroes appears in the payload
(Figures 2-19 and 2-20), it is replaced by a zero substitution code (a special sequence of ones and
zeroes) before transmission. At the receive end, this sequence is converted back to eight zeroes to
maintain data integrity. Intentional bipolar violations are applied to signal the presence of a zero
substitution code; whenever a bipolar violation is detected on a B8ZS circuit, the multiplexer checks
the surrounding bit sequence to determine whether it is part of a zero substitution code or a true
transmission error.
Figure 2-19. Original Payload with Eight Consecutive Zeroes