Bipolar encoding
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In telecommunication, bipolar encoding is a type of line code (a method of encoding digital information to make it resistant to certain forms of signal loss during transmission).
A binary 0 is encoded as zero volts as in unipolar encoding. A binary 1 is encoded alternately as a positive voltage and a negative voltage. This prevents a significant build-up of DC, as the positive and negative pulses average to zero volts. Little or no DC-component is considered an advantage because the cable may then be used for longer distances and to carry power for intermediate equipment such as line repeaters. The DC-component can be easily and cheaply removed before the signal reaches the decoding circuitry.
Bipolar encoding is preferable to non-return-to-zero where signal transitions are required to maintain synchronization between the transmitter and receiver. Other systems must synchronize using some form of out-of-band communication, or add frame synchronization sequences that don't carry data to the signal. These alternative approaches require either an additional transmission medium for the clock signal or a loss of performance due to overhead, respectively. A bipolar encoding is an often good compromise: runs of ones will not cause a lack of transitions, however long sequences of zeroes are still an issue. Since the signal doesn't change for as long as the data to send is a zero, they will result in no transitions and a loss of synchronization. Where frequent transitions are a requirement, a self-clocking encoding such as the manchester code may be more appropriate, though they introduce significant overhead.
When used on a T-carrier, the code is known as Alternate Mark Inversion because in this context, a binary 1 is referred to as a "mark", while a binary 0 is called a "space". The coding was used extensively in first-generation PCM networks, and is still commonly seen on older multiplexing equipment today, but successful transmission relies on no long runs of zeroes being present. For voice traffic the least significant bit of each frame (every eighth bit) is always set to '1', generating enough transitions on the line. This is acceptable for voice, but not for data.
Other T1 encoding schemes ensure regular transitions regardless of the data being carried. B8ZS is a newer format for North America, where HDB3 is the original line coding type used in Europe and Japan.
A very similar encoding scheme, with the logical positions reversed, is also used and is often referred to as pseudoternary encoding. This encoding is otherwise identical.
