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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: MCI [ATM overhead]
Jim Forster <[email protected]> writes: He's talking about the overhead due to carrying variable length IP packets in fixed length ATM cells. Consequently the last cell of an AAL5 frame will contain 0 - 39(?) bytes of padding, which is wasted bandwidth. Assuming random length distributions (which they're not), the average waste is about 20 bytes per packet. My rough estimate, based on an average packet size of 200 bytes (used to be correct, not sure anymore), is that the waste due to cell padding is about 10%. Due to the highly skewed packet size distribution the actual overhead might vary substantially. Note that this 10% is on top of the ~10% overhead due to the 5 byte ATM cell headers (5/53 ~= 10%), and various other overheads (some of which are also present in frame over Sonet schemes). There's beginning to be some expectation that there will be a transmission capacity crunch in the carrier's Sonet nets, and this ~25% ATM cell tax may be looked at carefully as packet over Sonet solutions emerge. Great. Let's estimate the ATM bandwidth available on an OC-3. With STS-3c SONET at 155.520 Mbps it gets reduced to 149.760 Mbps due to section, line and path SONET overhead. Next we reduce 10% due to 20 bytes lost on average 200 byte long packets due to incomplete ATM cell fills resulting in 134.784 Mbps. Then we need to subtract 9.43% due to ATM headers of 5 bytes in 53 byte packets. Thus we end up with a 122.069 Mbps true information rate which is about 78.5% of the nominal OC-3 capacity. And we still have not raised the issues of wasted reserved bandwidth and routing ease which were recently addressed very eloquently by Vadim Antonov when comparing connection based with packet based transport. Wolfgang Wolfgang Henke [email protected] . http://www.whnet.com/wolfgang/ WH Networks ... ftp.whnet.com /pub/wolfgang 2672 Bayshore Parkway Suite 503 ....... +1 415 390 9316 Mountain View CA 94043 ................... fax +1 415 390 9317
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