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Re: MCI [ATM overhead]

  • From: Tim Salo
  • Date: Thu Mar 21 00:55:40 1996

> From: Wolfgang Henke <wolfgang@whnet.com>
> Subject: Re: MCI [ATM overhead]
> To: johnc@msc.edu
> Date: Wed, 20 Mar 1996 08:38:08 -0800 (PST)
> Cc: nanog@merit.edu
> a	[...]
> SONET (Synchronous Optical Network) speeds given in Mbps
> 
>                 nominal   w/o Sonet  ATM   TCP/IP
>                           overhead
> 
> OC-3   STS-3c   155.520     149      122     137   future net backbone
> 	[...]

I think your 122 Mbps "ATM" number could be a bit confusing, even knowing
the assumptions you described in earlier mail.  (Also, more bandwidth seems
to be available to "TCP/IP" than appears to be available from ATM...)

If it helps, the following numbers are from John Cavanaugh's paper:

	Line Rate		155.520 Mbps

	Available to ATM	149.760
	  (SONET payload)
	Available to AAL	135.632
	  (ATM payload)

John then computes the overhead for three MTUs, and yields rates 
available to IP and TCP:

                                                 MTU
				576		9180		65527

	Available to IP		125.198		135.102		135.547

	Available to TCP	116.504		134.513		135.464

These are the maximum available rates, namely they assume MTU-sized
packets.

The reader can apply their favorite packet size distributions to these
numbers.


Having said all that, I am not sure where that leaves us.

One could theoretically remove the SONET overhead, but then one looses
the ability to manage the SONET link.

One could remove the ATM overhead, but then one has a point-to-point
link, rather than a link over which data from many sources can be
multiplexed.

-tjs





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