T1 Circuit actual throughput 1290Kbps

You have a 1.536Mbps port. However, there is the overhead

    > from PPP and the translation overhead which takes place in
    > all circuits.

Further comments... translation overhead affects *latency*, not
*throughput*. Overhead in the circuit may delay the leading edge of
the bits, but it does not affect the bit rate. PPP framing overhead
is close to negligible, and certainly does not account for the missing
246 Kb/s.