>> So how do Sky restrict their LLU customers to 8mbps, and O2 to their various speed levels (8 and 16 or is it 20?). Or is that just standard ADSL variants?
In the way you mean its a line parameter.
Some ISPs will also restrict on their network such as AOL/tiscali may have users on a 2Mb account but if they looked at their router it would still sync at 8128 so Im ignoring ISP side capping as it doesnt apply for this discussion.
Maximum sync speed is one of the parameters set for the initialisation stage and before authentication. Its one of the very first steps that occurs - even max dsl which is a "rate adaptive product" has several different parameters that can be assigned to your line.
With Maxdsl, BTw doesnt have that many profiles for the downstream its just:
576 fixed,
1152 fixed.
2272 fixed or
8128 fully rate adaptive.
Tiscali LLU downstream its something like:
574 fixed, 168 Adaptive
1148 fixed, 1148 Adaptive
2296 fixed, 2296 Adaptive
4096 fixed, 4096 Adaptive
6144 fixed, 8160 Adaptive
I dont know Skys :/
This isnt the only parameter set in the profile the other parameters included are
~ Interleaving / Depth of Interleaving,
~ Target SNR
~ adsl standard (eg G.992.1 / G.992.5 and Annex type A/M).
~ Upstream speed.
All of these line characteristics are set at the intialisation handshake... which then goes on to the training and analysis part. If a line is fixed rate the dslam needs to know before doing the channel analysis and way before sync is actually achieved.
I may be wrong but I dont see anyway how capping by frequency could be stored in the profile. It would make things way too complicated.. and now having thought this through a bit further - power cutback wouldnt be stored in the profile either. That would occur at a later stage of intialisation. The dslam would/should recognise that the SNR is too high at this stage and cut back power to make sure neighbouring lines aren't being drowned out. The reason Im not sure at which stage is because power cut back can also occur after sync and seems to be consistantly monitored by the dslam. I think therefore there is 2 types of power cutback 1.) Spectral mask type (stored on the dslam) and 2.) Continually monitored by the dslam if theres later a large increase in SNR causing a surplus of too strong a signal which could cause NExT.
DMT is designed to work on the full 256/512 subchannels as long as the SNR is good enough.. and its the adsl standard in use (g.992.1/g.992.3) which defines the subchannels and therefore frequencies available for use.
The stages of initialisation are listed
here. Any profiles set by the ISP would have to be passed during stage 1 and before the end of stage 2.
Theres also a difference between fixed rate lines and adaptive lines in that fixed rate will either sync at 'x'speed or it wont. Theres no Target SNR stuff and "oh it wont sync so lets try again at a lower speed".
I dont know where the LLU providers store their line profiles - BTw do it at the RAS (see
NCAS in the diagram).
A simplified version and the way I would imagine it to work would be something like..
Tel no 01234567890 attempts to connect to the dslam.
Dslam quickly looks up line profile for the phone no.
Dslam says hello... knowing at this stage what profiles to use.
Router then says hi I want to connect (and at this stage can pass any of its own parameters that it wishes to over-ride such as the target SNR).
All this would be the 'Handshake'These are my ramblings on the subject - as you know I will sometimes theorise on the forums rather than the main site which provides the hard facts. Therefore if Im wrong about anything then Im happy to be corrected.