Alternate Mark Inversion And Bipolar 8bit Zero Substitution

In 1988, clear channel transmission became an option for T-1 service from AT&T. Users could now transmit DS-0 or DS-1 signals without restricting the consecutive zeros.

T-1 channels are encoded using Alternate Mark Inversion (AMI). AMI encodes each "1" bit as a positive then a negative voltage. The receiving device on a T-1 channel detects dropped bits because it receives two positive or two negative voltage signals back to back when it should always receive a positive then a negative then a positive signal. With digital encoding, there tends to be more "0's" transmitted than "1's" transmitted. The ratio is something like 53% "0's" to 47% "1's." Thus, with AMI, there can be long strings of "0's," which means no positive to negative signal changes. This makes AMI more vulnerable to transmission errors. To overcome this potential problem,

Ami Alternate Mark Inversion
Figure 7-16 Digital Access and Cross-connect System (DACS) and fractional T-carrier channels.

Bipolar 8-bit Zero Substitution (B8ZS) is used to augment T-channel AMI signal encoding.

Technically, T-1 bit streams should not contain more than 15 "0's" in a row. B8ZS solves this problem. Further, B8ZS can provide a full 192-Kbps bandwidth on a T-1 channel. B8ZS augments the normal T-1 AMI encoding by violating AMI bipolar encoding rules when strings of eight consecutive "0's" are found. When eight consecutive "0's" are encountered, a unique coding violation pattern replaces them (see Figure 7-17). This B8ZS coding violation provides sufficient timing pulses to maintain synchronization across the T-1 channel.

B8ZS does not create errors, but rather its coding violations guarantee the minimum pulse state changes needed for synchronization without adding bits into the data stream. B8ZS sidesteps the 15 consecutive "0's" restric-

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  • monika
    How bipolar 8 zero substitution resolves problem of bipolar?
    2 years ago

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