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Re: T1 Circuit actual throughput 1290Kbps

  • From: Ron Buchalski
  • Date: Thu Jul 09 14:55:02 1998

>Date: Thu, 9 Jul 1998 10:18:51 -0400 (EDT)
>From: "Robert E. Seastrom" <rs@bifrost.seastrom.com>
>To: dorian@blackrose.org
>CC: ltd@interlink.com.au, tonyh@noc.cbn.net.id, nanog@merit.edu
>Subject: Re: T1 Circuit actual throughput 1290Kbps
>
>
>   From: Dorian Kim <dorian@blackrose.org>
>
>   > A T1 is capable of achieving 1536 kbps maximum (24 x 64 kbps).
>
>   While this doesn't seem to apply to Tony's case, I wouldn't make a 
blanket
>   statements like that. If the T1 is provisioned ESF, yes you can get 
>   1536 kbps, but there are places where you still can only get SF/D4 
framing.
>
>Precisely.  The most likely explanation is that the T1 is actually D4
>framed, which gives it a throughput of (24 * 56kbps) == 1344k.  Or the
>T1 may be properly provisioned but the CSU/DSUs incorrectly configured
>for the old-fashioned framing (I think I saw this working at one
>point, can't remember for sure).  The PPP is hardly eating anything at
>all in the grand scheme of things.
>
>                                        ---Rob

It isn't D4 or ESF framing that causes a loss in usable bandwidth on a 
T1, but AMI line coding.  As stated above, when using AMI line coding, 
the T1 will not be provisioned for clear channel (64k) DS0s, but will be 
provisioned for 56k DS0s, thus providing an aggregate bandwidth of 1344 
kbps.  The least significant zero of each DS0 is forced to a 'one', thus 
insuring correct ones density on the line.  The better solution is B8ZS 
coding, which allows full use of each DS0's bandwidth.

Inverting the data on the router won't give you back the "lost" 192 kbps 
of bandwidth, since you're still limited to using 56kb/s of each DS0.

-rb


Ron Buchalski
SE, Cisco Systems


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