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T1 can mean any data circuit that runs at the original 1.544 Mbit/s line rate. Originally 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. T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in transmission rates of up to 44.736 Mbit/s.
Supposedly, the 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago were conducted underground, and cable vault manholes were physically 6600 feet apart, and so the optimum rate was chosen empirically--the speed was increased until the failure rate was unacceptable, then reduced.
A more common understanding of how the rate of 1.544 Mbit/s was achieved is as follows. (This explanation grazes over T1 voice communications, and deals mainly with the numbers involved.) Given that the highest frequency at which voice communications occurs is at 4000 Hz, one needs, when converting analog voice to digital data, at least double that frequency for the sample rate. This yields the number 8000. 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. Since each frame of a T1 is 193 bits in length (24 channels X 8 bits per channel + 1 control bit = 193 bits), 8000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s (8000 X 193 = 1544000).
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