B3ZS

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B3ZS is an abbreviation for bipolar with three-zero substitution, a T-carrier line code in which bipolar violations are deliberately inserted if the stream of user data contains a string of 3 or more consecutive zeros.

B3ZS is used to ensure a sufficient number of transitions to maintain system synchronization when the user data stream contains an insufficient number of "ones" to do so.

Ones in a data stream are used by receiving equipment to synchronize with the sending equipment. If there is an insufficient number of ones in the data stream, then the receiver may not stay synchronized to the sender, which can cause data errors in BOTH directions. To avoid this problem, ones are sent with alternating polarity - a positive pulse is followed by a negative pulse, which is then followed by another positive pulse, etc.

Long streams of zeroes, therefore, can cause complications. Without enough ones in the data stream, there is a risk of losing synchronization. To overcome this, strings of zeroes are replaced with strategically placed one-bits. Sometimes these one bits do NOT alternate polarity as they normally do, but instead are of the same polarity (2 positive pulses in a row, or 2 negative pulses in a row). Consecutive one bits of the same polarity is known as a BPV, or Bi-Polar Violation.

The rules of B3ZS state that a string of 3 zeroes must be replaced with either:

1. 00V, where V is a BPV, that is, a bit of the same polarity as the last one-bit, OR...

2. B0V, where B is a normal one-bit, and V is a BPV

For example, consider the data stream: 10010001000001000000 , where 3 consecutive zeroes are shown in bold-face

Wherever there are 3 consecutive 0's, one of the two BPV codes discussed above will be substituted. Therefore, when this example data stream is sent down the line, it will be sent as

+00-00-+00+00-00-+0+

BPVs are again shown in bold. According to the rules of B3ZS, no 2 BPVs can be of the same polarity. To avoide this, the B0V sequence is used, which is also shown in italics above.

B3ZS is generally used on T3 lines in the North American hierarchy.