Some businesses or people who want a fast internet connection are probably interested in having a T1 connection.  If this describes your situation, it is good to know that at first, the T1 format carried 24 pulse-code modulated, time-division multiplexed speech signals each encoded in 64 kbit/s streams, leaving 8 kbit/s of framing information which facilitates the synchronization and demultiplexing at the receiver.

Remember that a D3 and D4 channel banks had an extended frame format, allowing eight bits per sample, reduced to seven every sixth sample or frame when one bit was "robbed" for signaling the state of the channel. The standard does not allow an all zero sample which would produce a long string of binary zeros and cause the repeaters to lose bit sync. However, when carrying data there could be long strings of zeroes, so one bit per sample is set to "1" leaving 7 bits x 8000 frames per second for data.

The 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago were conducted underground. To accommodate loading coils, cable vault manholes were physically 6600 feet apart, and so the optimum rate was chosen empirically--the capacity was increased until the failure rate was unacceptable, then reduced to leave a margin. Companding allowed acceptable audio performance with only seven bits per PCM sample in this original T1/D1 system.

The highest voice frequency which the telephone system transmits is 4000 Hz, the required digital sampling rate is 8000 Hz. Since each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs then 8000 frames per second to maintain those 24 simultaneous voice channels. T1 used Alternate Mark Inversion (AMI) to reduce bandwidth and eliminate the DC component of the signal.

Then, for AMI, each pulse had the opposite polarity of the previous one, resulting in a three level signal which however only carried binary data. Similar British 23 channel systems at 1.536 Mbaud 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. The AMI or B8ZS signal allowed a simple bit error measurement. The D bank in the central office could detect a bit with the wrong polarity, or "bipolarity violation" and sound an alarm.

Remember that to allow for the identification of information bits within a frame, two alternatives were considered.  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.