DS-3 links support Alternate Mark Inversion (AMI), Bipolar Three Zero Substitution (B3ZS), and High-Density Bipolar Three (HDB3) line coding. AMI is the same as the AMI used on T1s, discussed in Chapter 21. B3ZS is similar to B8ZS, also discussed in Chapter 21, except that it replaces occurrences of three rather than eight consecutive zeros with a well-known bipolar violation. HDB3 is used primarily in Japan and Europe. The default line coding on Cisco DS3 interfaces is B3ZS. When channelized DS3s are used, the line coding may be AMI, depending on how the circuit was ordered. Usually B3ZS is preferred.