Binary 8-zero substitution. The line coding standard for T1 data transmission which allows full 64kbps per second per channel.
Since sending eight zeros in a row could cause T1 equipment to lose synchronization with the sending equipment, it is important that that pattern is not sent. Using the old standard, the eighth bit is modified, so that only seven bits could be reliably transmitted per channel.
In order to eliminate a string of eight zeros without modifying the data being transported, the equipment replaces the pattern with a pair of bipolar violations or BPVs. This means that the voltage, instead of the correct inversion for each one-bit, allows the voltage to be positive twice in a row and negative twice in a row. This pattern looks like this:
0 0 + - 0 - + 0
-- OR --
0 0 - + 0 + - 0
depending on the polarity of the last 1-bit sent. The first voltage sent in the B8ZS pattern will be a bipolar violation, and will be follwed by another in the pattern, seperated by a zero.
Using B8ZS line coding, 64kbps is available per channel, for a total of 1.536mbps.