As a quick of a post as I can, because its very late... long days and also having to do lots of backend site stuff when I do get home:(... so Im just going to blurb.
@boost - Theres an explanation and more information on such things as MSILs, APs etc on this page
http://www.kitz.co.uk/adsl/wbc_wbmc.htmThe main Core network is highly unlikely to be congested..ever. Would take a real catatrophic problem to see issues there. Its highly resiliant and the way its meshed means that all major routes could be re-routed.
It's unlikely to see congestion on any of the core nodes (think of this as entry point to the core), although this also could become a problem in a major outage (Say for eg when there was the Manchester fire near to / at the RAS many years ago).
Usual points of congestion are either the VP (for 20CN) or SVLAN (for FTTC) OR the ISP Host links (WBMC/IPsC).
OR Another point of congestion could be the MSILs/APs, but since PN is supposedly WBMC - Shared, then its BTw's responsibililty to maintain sufficient bandwidth on the WBC Interconnects. Ive no idea how BTw monitor these and what their proceedure is for upgrading them.
Re the SVLANs - Im not 100% certain on these.
CVLAN is the customer tag (inner), whilst SVLAN is the Service tag (outer). I think the SVLAN is basically kind of the same as the old VPs and the amount of bandwidth available on that particular tunnel's 'backhaul', ie between the DSLAM and the RAS.
The new MSE bRAS may change things somewhat, because now many of the bRAS are closer to the dslam than they used to be.
It is possible to change SVLANs with much persausion (ie like BT would change you to a different VP if you moaned enough). AFAIK you cant change your CVLAN, but you can (and will be) be grouped with other CVLANs in to an SVLAN. There's 2 distinct different types of SVLANs 1)Copper-21CN 2)Fibre- FTTC. Whatever way, I'm pretty sure that the SVLAN is the backhaul to the bRAS.
Im unsure on FTTC if its from the DSLAM to RAS, or perhaps because of the fact DSLAM is BTor, then its possible it could be from the OLT to the bRAS. Thinking about it further probably most likely to be OLT because this is the point where GEA is handed over... and in which case we also have a possible BT Openreach congestion point between the dslam and the OLT.
Only your ISP or BTw has access to which SVLAN you are on. The SVLANS are constantly monitored to make sure they dont run hot and are supposed to be upgraded whenever they reach a certain threshold. The SVLAN status reports are updated twice weekly, but you need to know which SVLAN youre on to be able to check the reports.