the signal itself to determine where bits begin and end. Because the duration of a mark is known, synchronization can be achieved simply receiving marks. When AMI is used, a long progression of spaces will result in a loss of synchronization. With no marks in the signal, the receiving end will eventually lose track of where bits begin and end. The risk of an all zeros signal exists, SO AMI sets every eighth bit to a 1, regardless of its original value. This ensures that there are enough signal transitions in the line (i.e., that the ones density of the signal stream is sufficiently high to ensure synchronization). As few as 16 zeros in a row can cause the remote end to lose synchronization. Voice signals can easily absorb having every eighth bit set to 1. The human ear can't hear the difference if a single bit in the stream is changed. Data signals, though, cannot tolerate having any bits changed. If one bit is different in a TCP packet, the Cyclic Redundancy Check (CRC) will fail, and the packet will be resent (CRCs are not per- formed for UDP packets). Because of this limitation, AMI is not an acceptable encoding technique for use on data T1s. B8ZS encoding was introduced to resolve the shortcomings of AMI. The idea behind B8ZS is that if eight zeros in a row are detected in a signal, those eight zeros are converted Chapter 21: T1 to a pattern including intentional BPVs. When the remote side sees this well-known pattern, it converts it back to all zeros. Figure 21-3 shows how long strings of zeros are converted on the wire. The top signal consists of a one, followed by nine zeros, and then three ones. B8ZS takes the first eight zeros and converts them to a pattern including two BPVs. You would not see this pattern on a normal, healthy circuit. When the remote side receives the pattern, it converts it back into eight zeros. This technique allows data streams to contain as many consec- utive zeros as necessary while maintaining ones density. Nine spaces (zeros) Original signal B8ZS converted signal Bipolar violat