T1 Circuit actual throughput 1290Kbps

In some cases, if you can't get B8ZS line coding, and have to use AMI,
you might have to drop to 24x56=1344kbps (and use one bit on each DS0
to maintin 1's density), but even then, if your routers or DSUs can
invert the data, you can still go 24x64=1536kbps. (HDLC guarantees
zero-density ... so if you invert it, you get guaranteed 1's density.)

Yep. I've done that since 1988 on old circuits that were delivered
before B8ZS was available. It does work. However you still can not
get 1.536 over a T1 in general, close but not quite.

Cisco framing as well as PPP framing uses HDLC encoding. The rules for
HDLC encoding say that any time there are 5 one bits in a row within
the data portion of a frame, a zero bit is inserted by the transmitter
and removed by the receiver. This adds to the overhead of the T1
link. It is of course possible to send data that never has 5 one bits
in a row, but real data will have this. The 5 ones don't have to be
only within a byte. The contiguous one bits can cross byte boundries
to incur this ones stuffing algorithm overhead.