[AUUG-Talk]: Documenting how ADSL modems work?

Chris Maltby chris at sw.oz.au
Fri Sep 9 11:08:17 EST 2005

On Thu, Sep 08, 2005 at 05:05:03PM +0930, Andrew Rutherford wrote:
> Another thing not many people realise is from the ATM heritage. Your 
> 1.5M service isn't really 1.5M usable - it's the old hard drive space 
> con all over again.
> 53 byte cells are on the wire, of which 48 is the payload and 5 is 
> the headers, means your 256k back-channel is really 231k. With a 
> payload size of 48 bytes set - smaller items are padded - if you want 
> to send a TCP ACK over PPPoE you need 14 bytes PPP, 6 bytes PPPoE, 20 
> bytes IP header, and 20 bytes TCP header, that's 60 bytes - or two 
> cells, so 106 bytes on the wire, nearly doubling the required 
> bandwidth - so your 256k backchannel can only send 99k of actual 
> TCP/IP traffic. With some TCP implementations, this can mean your 
> download speed is controlled by the rate at which you can send ACK's. 
> :-(

I don't suppose anyone's considered re-enabling the old TCP header
compression stuff from dialup days. After all, the bloat applies
even worse for the archetypal 1 byte "keystroke" packet... counting
the echo plus headers and acks that's 424 octets per key...


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