Bipolar Encoding - Alternate Mark Inversion

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. No more than 15 consecutive zeros should ever be sent to ensure synchronization. The modification of bit 7 causes a change to voice that is undetectable by the human ear, but it is an unacceptable corruption of a data stream. Data channels are required to use some other form of pulse-stuffing, such as always setting bit 8 to '1', in order to maintain a sufficient density of ones. If the characteristics of the input data do not follow the pattern that every eighth bit is '1', the coder using alternate mark inversion adds a '1' after seven consecutive zeros to maintain synchronisation. On the decoder side, this extra '1' added by the coder is removed, resulting that the correct data arrives for the receiver. Due to this, the data sent between the coder and the decoder is longer than the original data by less than 1% on average. Of course, this lowers the effective data throughput to 56 kbit/s per channel.