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B8ZS is an abbreviation for bipolar with eight-zero substitution . A T-carrier line code in which bipolar violations are deliberately inserted if user data contains a string of 8 or more consecutive zeros.

Original signal:
0 0 0 0 0 0 0 0
B8ZS encoded signal (V=bipolar violation)
0 0 0 V 1 0 V 1

B8ZS 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.

B8ZS is used in the North American hierarchy at the T1 rate. When European E1 was developed much later than T1, it was then common knowledge that forcing 'ones' into a DS0 would corrupt data. E1 uses another method called High Density Bipolar Three (HDB3) code.