In the AMI signaling format, the binary value of 1 is represented by the square wave (i.e., pulse); the bnary value of 0 is represented by a straight line (i.e., the absence of a pulse). Note that each pulse alternates between positive and negative polarity, making the signal bipolar in format.

The primary advantage of the bipolar format is that it allows the DS-1 signal to travel twice as afr on a pair of copper wires. Another advantage of the bipolar format is its ability to offer a built-in method of error detection. When consecutive pulses of the same polarity are detected, it constitutes a bipolar violation (BPV). BPVs indicate that signal input has been disrupted due to defective equipment or poor environmental conditions (e.g. storms).

To correctly identify DS-1 input, the regenerative repeater must know when to sample the bipolar signal to determine whether a 0 or 1 is being transmitted at any given time. To ensure proper sampling, the repeater relies on a timing method that uses binary pulses (ie: ones) to maintain synchronization with the network equipmentthat is transmitting the DS-1 signal.

Since pulses are critical to maintaining proper signal timing, all DS-1 signals are required to meet specific ones density standards. These standards require at least one pulse within any eight bit sequence (ie:12.5% ones density). Further, since long strings of consecutive zeros between digital values can also hinder signal timing, ones density standards prohibit the transmission of more than 15 zeros in succession.

Success in meeting ones density requirements can vary based on application. For example, since the size and content of the bit patterns that represent human speech are consistent, acceptable ones density in voice applications is a virtual certainty. But since computer data is highly variable in size and content, conformance to ones density standards cannot always be guaranteed. This technical problem is why a coding technique known as bipolar with 8-zero substitution (B8ZS) has gained in popularity.

B8ZS uses intentional BPVs to break up long strings of zeros, allowing their transmission through the T1 link without violating the ones density standard.

With B8ZS, network equipment replaces any string of eight consecutive zeros with two intentional BPVs before the DS-1 signal is transmitted over the T1 link: the first BPV replaces the fourth zero; the second replaces the fifth and seventh zeros. Additionally, the eighth zero bit, which normally would be coded as a zero, is assigned a plus value.

Using this format, the DS-1 signal can pass through the repeaters on the T1 link with an acceptable level of pulse density. When the signal arrives at the receiving network equipment, the pattern is recognized as the B8ZS substitute for eight consecutive zeros; the equipment then replaces the intentional BPVs with their zero values.