[Gllug] ADSL modem -- WTF?

[John Hearns]

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.

>  The data that interests us is
> sent over the ATM system, now modified to work better when the MTU is 1500.
> There are other users sending different data at much higher rates, so I
> guess the system may now handle even larger or perhaps variable size frames
> without problems.
That's what the adaptation layers mentioned earlier do.
As I understand it they 'saw up' the various types of traffic into 48
byte (plus header) chunks.
There's audio, video, IP type data and circuit emulation.
An variable length IP packet will be encapsulated in 48 byte chunks.

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