B8ZS: 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.

Note 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.

Note 2: 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.

Source: from Federal Standard 1037C