Steve Underwood wrote:
Even with the robbed bit thing you get 62666.7 bits/s, since it only steals the LSB every 6 samples. :-)
Yeah, 56Kb/s per channel comes from the days when T-1 circuits were installed using AMI (alternate mark inversion) instead of B8ZS (bipolar 8th-zero substitution) for the actual line coding (layer 1). In those days, the channels weren't "clear"; you couldn't use all 8 bits of each channel for data, because every 8th bit (at least) had to be a 1 to maintain the clocking between the two ends.
Nobody installs T-1s like that anymore, because no new equipment needs it, and it's wasteful. I can remember when ordering B8ZS was an extra-cost/extra-build-time option for a T-1 span from USWest, though, and it wasn't all that long ago...
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