[Asterisk-Users] Beyond T1

[Kevin P. Fleming]

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...