Broadband pricingMonthly Inc VAT. See VAT Exc.
| Up to: | 8Mb/s | 8Mb/s | 24Mb/s* |
|---|---|---|---|
| Tariff | Std | Prem | ADSL2+ |
| A | £18.99 | £31.00 | £17.95 |
| C | £26.99 | £39.00 | £26.95 |
| D | £34.99 | £47.00 | £32.95 |
| E | £46.99 | £59.00 | £38.95 |
Setup £59.99 Full tariff list The package Max *ADSL2+ trial Extra charges
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There is a lot of confusion about Max services, and how it works. Even within BT there is confusion over the different aspects. The way it works is constantly being improved and fine tuned which makes matters even more confusing.
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 (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. 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.
Good internal wiring is often a key factor in getting the best performance out of your broadband line.
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.
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. Changing this can take 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 hours or even days it syncs at a lower rate and is stable. When a line is initially installed or regraded to Max the line characteristics are measured and used to set the initial settings, which usually means a line will just work without these initial problems.
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.
Important: The DLM process is continuous. It is not just for the first few 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.
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 - see our Max/ATM pages for details.
BT have an ongoing process to notice the rate the line syncs at and update the BRAS profile. The idea is that the system reacts to changes in sync speed and adjusts the BRAS rate. Sadly this is not done on the fly as the connection is made, but via a configuration change in BTs systems. This means that BT have to notice the change and apply the change to the BRAS.
The system scheduled the change to the BRAS automatically, but because of the volume of changes they will queue these based on priority. The shortest being up to 4 hours and the longest being up to 120 hours (5 days). The priority is based on the percentage change of speed of your line. So if your line changes from 1M to 2M sync, then it will get a fast BRAS update. If it changed from 8M to 8.1M (a BRAS change from 7M to 7.15M) it could take days.
When the rate changes BT then advise us within 15 minutes, and we will put a note of the change on our system which you can see in the logs. We will also at that point drop the PPP link on your line making an outage of a few seconds so that we can apply the rate change to our systems as well.
Why do we need to know the rate? We set a rate limit at our end to match the BRAS rate. 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. This is not used as a traffic shaping to manage customer usage. 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, but this should never be necessary.
BT reckon that the DLM process will always achieve a stable line rate after the first 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.
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 20% 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.
One feature BT have is a temporary sync capping in the DSLAM profile. This is specifically where there are lines syncing hundreds of times a day. Basically - lines that never actually get a reliable sync and constantly keep trying.
Such lines generate a vast number of sync events which cause problems for the DLM systems. As such, when identified, a cap is put on the speed the line will sync. This then allows the line to sync reliably at this capped speed.
This is only done as a temporary measure. We are notified when this is done so that we can get whatever fault is causing the problem to be fixed. Typically this is some makes of router on particular line characteristics, and the solution may be sorting wiring, replacing splitters or even the router.
If this happens, we will contact you to resolve it.
