[Gllug] odd hardware

[rich at annexia.org]

On Thu, Jan 16, 2003 at 12:40:08PM +0000, Sean Burlington wrote:
> Ok - what's ATM ?

A marvellously neat idea (label switching rather than IP routing),
thoroughly corrupted by a terrible, over-engineered, baroque
implementation.

OK, cynicism aside, ATM (Asynchronous Transfer Mode, not that this
means much) was an attempt by telephone companies to define a connection-
oriented service which was well suited to real-time applications.
Things like streaming video-on-demand, telephone calls, video
conferencing etc. where (supposedly) it's vital that you can
guarantee that there will be less than a certain amount of latency
and more than a certain amount of bandwidth available at all times.

The implementation was horribly sucky, because, being telephone
companies they use weird telephone signalling protocols like Q.2931
(?) which are baroque in the extreme. Although ATM defines lots
of 'layers' (basically: protocols), only one is really used, which
is called 'AAL5' which roughly corresponds in capability to IP.

Key features of ATM: fixed packet ('cell') size of 48 bytes. The
fixed cell size is useful for real-time. The choice of 48 bytes is 
unfortunate. Because of various characteristics of TCP ACK packets,
this is about the worst possible size of cell you could have chosen
to carry TCP. On a nominally 155 Mbps ATM circuit, we achieved
75 Mbps throughput with TCP, the rest lost to protocol overhead.

Apocryphally the size of cell was chosen as a compromise between
the US (which wanted 64 bytes which is good for data) and Europe
(which wanted 32 bytes which is good for phone calls).

Don't get me started on the addressing scheme ... It's based, naturally,
on telephone numbers (!)

These days the only serious use for ATM is in very large backbone
switches, and even there it's being replaced rapidly by gig ethernet
and other technologies. Oh, and also for historical reasons it's
the core protocol used by ADSL. Hence the need to know VPI/VCI
numbers when setting up ADSL connections (which is really an
ATM PVC).

Some more ATM stuff here:

http://vulcan.ee.iastate.edu/~dougj/ATM/introduction.html

Rich 'I used to work with ATM' Jones.

-- 
Richard Jones, Red Hat Inc. (London, UK) http://www.redhat.com/software/ccm
http://www.annexia.org/ Freshmeat projects: http://freshmeat.net/users/rwmj

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