more complicated line code may be more appropriate, though they introduce significant overhead. 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. There are two popular ways to ensure that no more than 15 consecutive zeros are ever sent: robbed-bit signaling and bit stuffing. T-carrier uses robbed-bit signaling: the least-significant bit of the byte is simply forced to a "1" when necessary. 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. Of course, this lowers the effective data throughput to 56 kbit/s per channel. 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, recreating the correct data. Using this method the data sent between the coder and the decoder is longer than the original data by less than 1% on average. Error detection Another benefit of bipolar encoding compared to unipolar is error detection. In the T-carrier example, the bipolar signals are regenerated at regular intervals so that signals diminished by distance are not just amplified, but detected and recreated anew. Weakened signals corrupted by noise could cause errors, a mark interpreted as zero, or zero as positive or negative mark. Every single-bit error results in a violation of the bipolar rule. Each such bipolar violation (BPV) is an indication of a transmission error. (The location of BPV is not necessarily the location of the original error). Other T1 encoding schemes For data channels, in order to avoid the need of always setting bit 8 to 1, as described above, other T1 encoding schemes (Modified AMI codes) ensure regular transitions regardless of the data being carried. In this way, data throughput of 64 kbit/s per channel is achieved. B8ZS is a newer format for North America, where HDB3 is the original