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
On Fri, 19 Jan 1996, Shikhar Bajaj wrote:
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.
Thanks for the correction. I knew that number didn't quite look right. -dorian ______________________________________________________________________________ Dorian Kim Email: dorian@cic.net 2901 Hubbard Drive Network Engineer Phone: (313)998-6976 Ann Arbor MI 48105 CICNet Network Systems Fax: (313)998-6105 http://www.cic.net/~dorian
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