Now if you think that for a T1 if you multiply 24 x DS0 (64,000) you do NOT get 1.544 Mbit/s instead you get 24 * 64,000 = 1.536 Mbit/s. The extra bits are lost between 'frames' where a frame consists of one 8 bit sample for each of the 24 channels (remember the DS0 basics). So every 192 bits (24 x 8 = 192) we add a 'frame separator' bit to give 193 bits per frame. The final arithmetic is 193 bits x 8K samples = 1.544 Mbit/s. Easy really.
If you do the same arithmetic for DS1C, T2 etc. the above will not give the right answer. In short, above T1 things get really nasty with M-Frames and M-subframes. Its mind numbing stuff and if you really need this information get hold of ANSI T1.107-2002 and lots of coffee or other mind-altering substances.
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