[Gllug] odd hardware

[John Hearns]

On Thu, 2003-01-16 at 12:40, Sean Burlington wrote:
> Rev Simon Rumble wrote:
> > On Thu 16 Jan, Sean Burlington bloviated thus:
> > 
> > 
> >>I've tried just connecting to my LAN hoping it would request an IP 
> >>address via dhcp - but no joy.
> > 
> > 
> > That isn't ethernet.  It's ATM.
> 
> Ok - what's ATM ?

ATM = Asynchronous Transfer Mode.

ATM is a networking technology.
Ironically enough, it was first engineered for the purpose of delivering
video to the home via telephone lines.
The major telecomms and datacomms companies developed it further, and a
few years ago it was supposed to be the telecomms technology which would
cover everything - IP data transfer, telephony, video transfer.

You might suss out that I'm a bit of a fan of ATM :-)

ATM technology is based on transferring digital data in fixed size 48
byte packets, with a five byte header.
Why fixed size and 48 bytes you ask?
Well, back in the days when IP routers were software-only, this meant
that fast switches could be constructed, to switch the traffic in
hardware. (These days of course there are now big fast IP routers)
Oh, and a I should mention that ATM is connection-oriented, so a 
'virtual circuit' is established between the two endpoints, right
through the network. Unlike IP, where routing is applied at every hop.

48 bytes is a compromise in size between 32 and 64 bytes.
Such small datagrams mean that voice can be carried on an ATM circuit
too.


ATM held out the promise of an end-to-end network, all based on the same
networking technology - so ATM circuits could come to the home, over
ADSL, and continue through the big core networks.
'Twas not to be.






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