<P> Initially, T1 used Alternate Mark Inversion (AMI) to reduce frequency bandwidth and eliminate the DC component of the signal . Later B8ZS became common practice . For AMI, each mark pulse had the opposite polarity of the previous one and each space was at a level of zero, resulting in a three level signal which however only carried binary data . Similar British 23 channel systems at 1.536 megabaud in the 1970s were equipped with ternary signal repeaters, in anticipation of using a 3B2T or 4B3T code to increase the number of voice channels in future, but in the 1980s the systems were merely replaced with European standard ones . American T - carriers could only work in AMI or B8ZS mode . </P> <P> The AMI or B8ZS signal allowed a simple error rate measurement . The D bank in the central office could detect a bit with the wrong polarity, or "bipolarity violation" and sound an alarm . Later systems could count the number of violations and reframes and otherwise measure signal quality and allow a more sophisticated alarm indication signal system . </P> <P> The decision to use a 193 - bit frame was made in 1958 . To allow for the identification of information bits within a frame, two alternatives were considered . Assign (a) just one extra bit, or (b) additional eight bits per frame . The 8 - bit choice is cleaner, resulting in a 200 - bit frame, twenty - five 8 - bit channels, of which 24 are traffic and one 8 - bit channel available for operations, administration, and maintenance (OA&M). AT&T chose the single bit per frame not to reduce the required bit rate (1.544 vs 1.6 Mbit / s), but because AT&T Marketing worried that "if 8 bits were chosen for OA&M function, someone would then try to sell this as a voice channel and you wind up with nothing ." </P> <P> Soon after commercial success of T1 in 1962, the T1 engineering team realized the mistake of having only one bit to serve the increasing demand for housekeeping functions . They petitioned AT&T management to change to 8 - bit framing . This was flatly turned down because it would make installed systems obsolete . </P>

When did t1 and e1 stop being used for data transmission