On Wed, 2005-11-09 at 01:16 -0800, trixter aka Bret McDanel wrote: > Bandwidth is a tricky issue. You have your IP + UDP + RTP + whatever > headers (iax2 combines stuff so potentially that skews this a bit) but > something most often forgotten is link layer framing.
I should have added to this that for home users using DSL with PPPoE (which seems increasingly popular) the framing issues can be compounded because PPPoE has 6 bytes of headers and PPP has 2 bytes, this is added to the total frame sent over a DSL + PPPoE link. But if you are running max connections on VoIP over DSL you are either doing special purpose stuff or a very bad provider and have other issues. DSL, especially with PPPoE is not a 'carrier class' link :) Addiitonally ATM has a 8 byte SAR trailer, and I forgot to subtract that from the 16 bytes of padding so that is really only 8 bytes of padding (not nearly as bad). To clarify, if you have a straight ATM link, then you have 5 bytes per cell padding and 8 bytes at the end of all the cells that make the packet. So 40 bytes then add in increments of 48 bytes of usable data, to avoid waste. A little isnt that big of a deal (you shouldnt be trying to run your links at 100% anyway), but if you transmit 41 bytes of packet data for example you have 47 bytes of padding, which puts you about 50% efficiency, which is really bad. -- Trixter http://www.0xdecafbad.com Bret McDanel UK +44 870 340 4605 Germany +49 801 777 555 3402 US +1 360 207 0479 or +1 516 687 5200 FreeWorldDialup: 635378
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