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ADSL2 Rate Limits - comment

Community Veteran
Posts: 38,460
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Registered: ‎15-06-2007

ADSL2 Rate Limits - comment

From AAISP blog
Tuesday, September 15, 2009
[sor] 11107 Disable 21CN BRAS rate limit
BT has BRAS (Broadband Remote Access Servers) which are the other end of the L2TP (layer 2 Tunneling Protocol) between us and BT.
One of the key features of 20CN BRASs was that they had a rate limit, and BT have a complex system to set this if the actual line sync rate changes. They can take hours to reflect a change and sometime it can go wrong or get stuck. It is a source of faults in itself.
The logic is, as we understand it, that it is not efficient to rate limit at the line because it handles ATM cells and not IP packets. ATM cells are small chunks of data and many are needed to make an IP packet. This logic is that dropping ATM cells would kill the whole packet but means sending the rest of the cells anyway. If you dropped 1 in 10 cells you would probably manage to break all IP packets, not 1 in 10. So the rate limit is set at an IP level on the BRAS. The BRAS has finite settings so has descrete steps, like 250K, 500K, 750K, 1M, 1.5M 2M, etc... This is always lower than the line rate so means losing some capacity.
The argument is, of course, flawed. The main reason a link fills is TCP which is designed to fill a link! That is what it does. Dropping at the ATM link will be inefficient, but TCP will back off. It will push up speed until it drops packets again. The TCP stack will adapt so that it is hardly dropping any packets. This means that it does not matter whether they are dropped at ATM or IP level anyway - the effect is the same.
However, even if it was a valid argument it does not apply to 21CN. This is because, untile 20CN which used ATM from BRAS to DSLAM, on 21CN it is IP from BRAS to MSAN. So the MSAN can drop at IP level.
BT have not managed to explain why the 21CN has BRAS rate limiting other than "it was there on 20CN".
We'd like them to just turn it off, but someone somewhere must have wasted a lot of money on this, so to let them save face we are suggesting now (at the suggestion of someone in BT as it happens) that they give us the option to turn off on selected lines. That way the people that wasted lots of money save face!
Of course, if that happens, we'll turn off on all lines. We can manage rate control our end to match the line rate anyway (as we can do it better) but not have the delay or large steps in the rate that BT have. i.e. on connection we can set an IP rate limit to exactly match the sync speed at that instant. We can then give users finer control as we do now to either squeeze every packet out of the link or have us limit just below the line rate to allow VoIP to work better.
Its all about choice!
Tuesday, September 15, 2009
[sor] 11098 Control of MSAN profiles
Basically, we are asking BT to let us turn off their DLM (Dynamic Line Management) on any lines and give us direct control over the MSAN profile in use on a line.
Importantly, we need the control to be instant, or as close to instant as possible. It may go in a queue for BT to action, but we want it to happen promptly and not take a working day to change as now.
This would give us the same features as the competition we use, BE, as they let us change profiles whenever we like will immediate effect and we pass that control on to customers. We will do the same if BT offer this feature.
At the end of the day we expect most customers to be happy with the DLM. We know it can be a pain, but for most people it is probably fine. Its the exceptions we want to manage - where the DLM cannot cope, and where our customers want control themselves.
Rising Star
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Registered: ‎05-04-2008

Re: ADSL2 Rate Limits - comment

The Andrews & Arnold blog is a great source of information, isn't it?
And now there's also the personal blog by one of their top people (maybe the big boss himself - I'm not quite sure). Take a look at:
Few punches pulled! Not everything's about their relationship with BT, but quite a lot of it is. At least we know Plusnet isn't the only ISP that has problems with "a major telco" as it's sometimes called.