A9. AMI, B8ZS, and HDB3 are different types of line coding used in T1 and E1 communications systems. AMI (alternate mark inversion) is used in both T1 and T1 systems. B8ZS (bipolar with 8 zeros substitution) is used in T1 systems. HDB3 (high-density bipolar 3) is used in E1 systems. AMI is the most basic encoding scheme, in which ones are represented by voltage pulses and zeros are represented by the lack of a voltage pulse. In AMI, the polarity of each pulse is the opposite of the previous pulse. The problem with using AMI is that if not enough voltage pulses are sent down the line, the receiver will not correctly decode the data stream. To solve this problem, two different encoding schemes were developed to replace a sequence of zeros with a special code word of voltage pulses. The B8ZS encoding scheme replaces each sequence of 8 zeros with the code of 000VB0VB, where the V pulse (bipolar violation) has the same polarity as the previously pulse and the B pulse (correct bipolar pulse)
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