[Gllug] ADSL modem -- WTF?

[Richard Jones]

On Tue, Jul 20, 2004 at 10:13:37AM +0100, John Hearns wrote:
> On Tue, 2004-07-20 at 09:53, Chris Bell wrote:
> 
> > > 
> >    ATM is the system designed to carry low data rate (speech) with minimal
> > delay, so has (had?) a very small packet size (something like 48 bytes plus
> > header), it was adapted and adopted by BT.
> Yes, that's right.
> 48 bytes chosen as a compromise between the States, who wanted 64 bytes
> and the Europeans, who wanted 32 bytes.
> 
> The fixed cell size means that you can rapidly switch the cells
> (packets) in hardware. At the time, the state of the art in IP routers
> was very much behind what it is now.

Indeed.  48 bytes is absolutely the worst choice for cell size too.  A
lot of TCP traffic is 64 byte ACKs.  These get split across two cells.
Each cell is 53 bytes, which means your 64 byte ACK just got turned
into 53 * 2 = 106 bytes on the wire.  This is a 65% overhead.

Actually it's worse than that.  Underneath ATM is another layer called
SDH which adds even more cruft.  I have some 155 Mbps ATM cards around
here which I've never managed to get to transmit TCP/IP data at a
higher rate than 77 Mbps.  Basically around half the advertised speed.

Rich.

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