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Re: T1 Circuit actual throughput 1290Kbps

  • From: Patrick W. Gilmore
  • Date: Thu Jul 09 00:28:17 1998

At 10:44 AM 7/9/98 +0700, Tony S. Hariman wrote:
>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 ?
>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 

That is.... well, not correct.  (Please insert favorite euphemism for "not
very smart" in regards to this upstream.)  First of all, PPP overhead is
not even close to 200 Kbps on a T1.  Second of all, I believe MRTG includes
the overhead in it's graphs.  Could someone please correct me on that if
I'm wrong?  And lastly, there are no "cells" in PPP.  If you are doing ATM
over that T1, you would have cells.  But you say you are doing PPP, not
ATM.  Besides, I think you'd get less than 1.3 Mbps - probably more like
1.1 or 1.0.

According to my MRTG graphs, the most I've ever gotten on a PPP
encapsulated link is 1521.4 kb/s.  I think that's a bit higher than your
upstream told you is possible. :)  This is a Cisco talking to a Bay router
(hence the PPP encap as opposed to HDLC).  So tell your upstream he's full
of it.

>Tony S. Hariman


Patrick W. Gilmore                      voice: +1-650-482-2840
Director of Operations, CCIE #2983        fax: +1-650-482-2844
              "Tomorrow's Performance.... Today"