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

Miles Goodhew miles at henrygoodhew.com
Sat Sep 10 23:43:42 EST 2005


Andrew Rutherford wrote:
> At 9:58 PM -0700 7/9/05, Michael Paddon wrote:
>> The ADSL link layer consists of a synchronised stream of fixed
>> length frames. You may remark that this looks a lot like ATM and, indeed,
>> the multiplexing layer of the ITU ADSL standard is built on ATM.
>> Knowing this, a bunch of jargon begins to make sense...
> The reason for this is at the time ADSL standards were being developed, 
> carriers thought ATM was going to take over the world, instead of IP. It 
> seemed logical to use ATM framing to the end device so then no 
> conversion would be required when routing through their networks to the 
> end point.

	I dunno about that take on DSL. Here's mine: ATM is a telco-oriented 
technology and all the DSLs are simply part of ATM as a whole. DSL was 
created partially to allow telcos to extend the same switching 
technology right-out to the customer premises (rather than using bizarro 
things like analogue modems that are just plain wrong and have been 
since they were created - a cool hack that went on waaay too long). The 
other thing DSL did was allow telcos to compete with cablecos and 
deliver video to their customers. Unfortunately this is where the "ATM 
camp's" eyes were bigger than their collective stomach as it's only this 
year that we're seeing standardised MPEG-4 video equipment that can 
squeeze bitrates down to ADSL speeds (well, at least the ADSL speeds 
we've been given in Australia). Currently you need about 6 to 10Mib/s to 
get _decent_ MPEG-2 video (This is what's used in terrestrial digital 
video broadcast - DVB-T).
	The idea of switching Internet access over DSL was a bit of an 
afterthought (like SMS on mobiles) and played second fiddle to the video 

> There were a lot of people who thought people would place calls over ATM 
> (like phone calls) and that multiple people in a house would place 
> multiple ATM calls simultaeneously, so a worker could be connected to 
> the comapny LAN while someone else was connected to a random ISP.
> None of this ever came off - well, not much of it, and certainly not in 
> Australia.

	I currently work for A Canberra-based tripleplay provider (count 'em!) 
whose products include off-the-air and on-demand video delivered to 
customers over an ATM network with a 54Mib/s VDSL last leg).


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