North American Network Operators Group

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Re: T1 Circuit actual throughput 1290Kbps

  • From: Lincoln Dale
  • Date: Thu Jul 09 00:48:15 1998

In message <[email protected]>, Tony S. Hariman writes:
>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 

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

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.