B8ZS

B8ZS is an abbreviation for Bipolar with 8 Zeros Substitution, which is a method of line coding used in the T-carrier system which allows full 64 kbit/s per second per channel, though it does not allow for Clear Channel Capability (CCC - 64 Kbit/s)in and of itself. The service would still have to be point-to-point(PTP)and not switched through a digital switching network. The standard is to use B8ZS as the line encoding option when providing PTP circuits and services. The older AMI scheme was implemented in channel cards that always robbed a bit for signaling regardless if the service on that channel was switched or P2P. B8ZS cards can be optioned not to rob that bit.

On a T1, ones are sent by applying voltage to the wire, where a zero is sent by having no voltage on the wire. Sending excessive zeros in a row could cause receiving equipment to lose synchronization with sending equipment, so it is important that such a pattern not be sent.

The original standard of line coding, Alternate Mark Inversion, specifies that there are three states of the line, no voltage is a zero, positive voltage is a one (or mark), and negative voltage is also a one (or mark). Because of the inversion of the voltage for each "mark," or one, sent, the receiving equipment can easily determine the data rate of the line and not lose synchronization.

B8ZS builds upon this, by using violations of this rule to replace a pattern of eight zeros in a row.

Original signal:
0	0	0	0	0	0	0	0
B8ZS encoded signal (V=bipolar violation)
0	0	0	V	1	0	V	1
Signal Polarity (assuming that the previous mark was negative)
0	0	0	-	+	0	+	-

B8ZS is used in the North American hierarchy at the T1 rate. E1 uses another method called High Density Bipolar Three (HDB3) code.