[1455] in North American Network Operators' Group
Re: value of co-location
daemon@ATHENA.MIT.EDU (Shikhar Bajaj)
Fri Jan 19 17:53:26 1996
Date: Fri, 19 Jan 1996 17:21:07 -0500
From: bajaj@faline.bellcore.com (Shikhar Bajaj)
To: dorian@cic.net
Cc: nanog@merit.edu
> > What's the average packet size now? I think the overhead on DS-3 ATM/SMDS
> > is worse than on OC-3, if I recall correctly, because the DS-3 PLCP is so
> > wasteful as well. Someone should redo these calculations more accurately.
>
> With PLCP, maximum bit rate available for ATM cells =
>
> 53 cells * 12 rows * 8bits / 125us = 40.704 mpbs.
>
> So, 40.704 * 48/53 = 36.864 mpbs = data rate.
>
> Now if you use AAL5, then it's another 2 bytes for SAR, and 2 bytes for
> CS, so, 40.704 * (48-4)/53 = 33.792 mpbs.
You are correct up til you get to the efficiency at the AAL level. AAL-5 does
not have any SAR overhead but it does have 8 bytes of CS overhead. Now, this
overhead is on a per PDU basis and NOT a per cell basis. Thus, you maximum
efficiency will be
36.864 * [(x-8) / x] where x = PDU size in bytes.
Thus, if you just send 64-byte packets, your min throughput will
be 36.8 * (56 / 64) = 32.3 Mbps.
Of course, larger PDUs are more efficient.
Shikhar
