Does anybody have experience having a T1 circuit with PPP
encapsulation getting only 1290 Kbps maximum throughput looking
at "sh int" result from cisco router or MRTG ?
A T1 is capable of achieving 1536 kbps maximum (24 x 64 kbps).
typically, the output shown both via snmp interface-counters
and from a cisco "show int" includes all of the associated
PPP framing.
bear in mind that the output of a "show int" by default
shows a 5-minute-exponentially-decayed average of the throughput.
this will 'smooth' out instantaneous traffic bursts and troughs.
the figure reported as a 'kbps' figure is most likely bursting
much higher than this.
This is the explanation our upstream provider gave us:
You have a 1.536Mbps port. However, there is the overhead from
PPP and the translation overhead which takes place in all circuits.
Judging by your settings that limit ends up somewhere between
1.3 and 1.4. This overhead would be the non-data portion of cells
or frames for example. For example, you might have 1.3 Mbps of
data which gets framing or cell information appended onto it before
sending taking up additional bandwidth. It is to be expected in all
circuits.
PPP doesn't have much of the overhead of IP-in-ATM-cells
(fixed-cell-size, very-badly-chosen-prime-number-cell-size, ..)
that the discussion given to you by your upstream is talking
about.
what kind of traffic are you sending over the link ?
perhaps there isn't enough traffic / the traffic isn't of the
variety to actually fill the link capacity.
if its mostly TCP traffic, don't expect it to fill the whole
pipe all the time.
cheers,
lincoln.