Myth and magic of Max

[Realist]

Myth and magic of Max

There is a lot of confusion about Max services, and how it works. Even within BT there is confusion over the different aspects. People are confused as to what the 10 day period is when you first get Max, and what the BRAS cap is... Hopefully this page can clear up some of these issues.
Sync

Max uses fully rate adaptive ADSL. This means that your modem or router will sync at the best speed it can. However, it is not as simple as that! The modem measures the noise that it sees on the line, and works out what frequencies to use, and how much of each frequency. This ends up giving a specific speed. The fact that multiple frequencies are used over a wide frequency band is why it is called broadband in the first place.

The sync process has some parameters. The main one is the signal margin. This is how much room the modem gives itself to work correctly. In a perfect world one could say you don't need a margin, but in practice, the noise on your line can vary so a margin is needed. Some lines change over the day due to external interference, and so need more margin to allow continued operation throughout the day. Because each line is different, the correct margin is not something you can accurately predict. We'll see how the DLM is used to control this later.

Another aspect is interleaving. This is a technique that allows some of noise to be tolerated more easily. It is combined with forward error correction (FEC). An interleaved line cannot achieve quite the same level of max sync speed as a non interleaved line (presumably because of the overhead of the FEC). Interleaving does increase the latency on the line (the time for packets to be sent/received). There are different levels of interleaving and correction which can be used, but BT only use two settings, on and off. Non interleaved mode is called fast mode.

It is important to realise that these settings affect the way the router will sync, and do not directly affect the speed it syncs to. The same parameters will get a higher speed on a less noisy (or cleaner) line. The make of modem/router and the internal wiring can have an effect on the speed you get for a specific margin setting.
DLM: Dynamic Line Management

Because the parameters needed to get the most from a line depend on the line itself, they need a little fine tuning. BT have a range of ADSL profiles they can apply, as described above. These allow interleaving on or off, along with a margin of 3dB, 6dB, 9dB, 12dB, or 15dB of noise margin. The default is 6dB without interleaving

If the line syncs at a level where it is trying to work too fast for the line conditions, the modem will give up and re-sync (trying again). Every time the line syncs at a different rate BT notice. If the line syncs a lot then Bt will change the profile.

The process can be time consuming. One of the rules is number of re-syncs inan hour, with a very fast change in profile, but others are number of resyncs in a day, and a new profile applied the next day. It can take several days for the line to eventually settle at the right rate.
BT will normally apply interleaving if there are errors on the line, but we can over-ride this forcing interleaving on or off, or leaving it to BT. To change this takes us one working day though.

Eventually the line should reach a level where the margin is sufficient for a reliable service. This may mean the line syncs at a higher speed initially, but is unreliable, and after a few days it syncs at a lower rate and is stable.
Because the profile set is not a specific speed, but a margin, fixing any problems with internal wiring, or getting a better modem, can mean the modem syncs at a higher speed right away. BT say that they can also spot a line is good quality now, and adjust the profile back to a lower margin, but this takes many days and the exact algorithm is not clear.

BT are also planning to monitor the corrected errors when in interleave mode. The way this works is that errors can be detected and corrected, hence making the service work in the face of some interference. However this also provides stats of how many errors were corrected, and so allows BT to see the line is now clean and interleaving is no longer necessary. They plan to have a system to remove interleaving automatically in such cases.

Important: The DLM process is continuous. It is not just for the first 10 days, or some such - it applies to the line all the time. It is also important to realise that whilst we can control interleaving, we cannot set a specific DSLAM profile via BT.
BRAS rate

The DSLAM is the equipment at the other end of the phone line and with which the modem negotiates the sync/speed when it connects. However, further within BT's network is a BRAS (broadband remote access server). BT insist that they need to rate limit the traffic from the BRAS to the DSLAM. They have not said why, but they insist that the rate must be the same or lower than the sync rate of the line. The BRAS has a distinct number of steps which can be used.

BT have an ongoing process to notice the rate the line syncs at and update the BRAS profile. The system should react to any decrease in sync speed within 75 minutes, and react to any increase in sync speed within 3 days. However, at present, it is generally reacting to any change within 15 minutes or so, up or down. Also the 3 day rule does not apply when a line is first converted to Max - it should have the new rate set within 75 minutes, before which it is set to a default 2Mb/s rate. BT then notify us of the BRAS rate (most of the time), and we update our management pages and systems.

There are problems which BT are working on. Sometimes the rate is not set for days, or at all (and we have to work through BT faults to fix this). However, for most customers, this process does work.

Why do we need to know the rate? We set a rate limit at our end to match the BRAS rate. This is not to stop you getting the full performance of the line. We default it to the full 7.15Mb/s rate for a full speed line. We use the rate to balance traffic when customers have multiple lines bonded. We also use it to limit traffic in the event of a denial of service attack. If you need the rate adjusting at our end (e.g. BRAS rate should have increased but we have not been told by BT), ask support.

Important: The BRAS rate setting process is continuous. It does not just apply for the first 10 days.
MSR: Maximum Stable Rate

BT reckon that the DLM process will always achieve a stable line rate after 10 days. With ideal lines there are no adjustments, and with even quite bad lines the line is usually stable within a couple of days. In extreme cases it could take near to 10 days to finally become stable.

Once a new line is installed, or a regrade completed, BT wait 10 days for the DLM to stablise the line, and then take a note of the BRAS rate. They use this rate as a reference for speed related faults in the future. All this means is that if the line characteristics change and you no longer achieve the same sync speed on the line, BT will only consider it a fault if below around 30% below the MSR rate that was set after the initial 10 days. They are giving themselves some margin to allow for lines that just gradually change characteristics. Interestingly, they also limit the scope of the DLM adjustments if it would try and adjust a line such that it may sync below this fault threshold rate.

So, the 10 day period you may have heard of is just a reference used to set a fault threshold for speed related faults in future. The DLM and BRAS rate setting processes are continuous.

Hope this helps,

Laters