max inofrmation about adsl

This page is here to help everyone understand what ADSL Max is and how it works. By the end of the page, you should be able to clearly understand all you need to know about ADSL Max. The guide gets quite techy, but perservere and it will hopefully become clear.

Make a cuppa, take a deep breath, and begin...

There are several new terms that ADSL Max has brought in. These are briefly outlined below along with a few other terms for your reference...
Maximum Stable Rate The MSR is the lowest SYNC rate that your modem has connected at during the first 10 days of your line being connected to the ADSL Max service.
Fault Threshold Rate The FTR is the speed below which a fault can be reported to BT Wholesale via your ISP. It is set to 30% lower than the MSR.
Dynamic Line Management DLM is the system that analyses the performance of your line, such as errors or disconnections - and makes changes to your target SNR margin or interleaving status if required.
Signal-to-Noise ratio The SNR is the amount of signal your line can see as opposed to how much noise it can hear.
SYNC SYNC is short for synchronise, and is the word used when your modem is connecting or connected to the exchange. SYNC rate is the actual speed your modem is connected to the exchange at.
Interleaving Interleaving is a feature that can be used to help stabilise your line.
Data Rate The data rate is the maximum speed at which you can download at.
BRAS The BRAS is a system in the exchange that handles your line profile. This profile contains details such as your data rate.
DSLAM The DSLAM is the main piece of equipment at the exchange. It talks to your modem to initiate and maintain a connection and passes information about your line on to other systems for analysis.

A more advanced description of these, including diagrams where possible, are below...



Maximum Stable Rate (MSR)

As mentioned above, the MSR is initially set during the first 10 days of your line being connected to the ADSL Max service. Each time your modem SYNC's to the DSLAM at the exchange, the modem sends an 'event' to the DSLAM, and between them, they decide at what rate your line can SYNC at, based on your current line stats.

Once the modem and the DSLAM have agreed on the SYNC rate, the DSLAM sends information to the DLM. The DLM analyses your line stats and if required, updates your target SNR margin and/or interleaving status.

A diagram of the first 10 day period is shown below.



The green line indicates the rate at which you are SYNC'd at. At the end of this 10 day period, the lowest SYNC rate that you have had becomes your MSR.



Fault Threshold Rate (FTR)

The FTR is calculated from your MSR (see above), by subtracting 30%. This is then stored in your BRAS profile. The FTR comes into play when you experience a fault on the line. If your modem is SYNCing below your FTR, then you should log a fault via your ISP and in turn they should contact BT Wholesale who should check the line for faults.



Dynamic Line Management

This is the system at the exchange that constantly (not just the first 10 days) monitors your line. Each time your modem SYNC's, the DLM analyses your line stats, and sets your target SNR margin and interleaving status. For example, if your modem continues to SYNC at a rate that causes your line to keep losing SYNC, because of a low SNR margin or high error counts, the DLM may:

If you have a low SNR margin: By default, the target SNR margin is 6 dB. It may be decreased to 3 (rare) or be increased to 9, 12 or 15 dB. If it increass the target SNR margin, it should improve the stability of your line, but by sacrificing some speed.

If you have high error rates: Intereaving may be enabled to cut down the number of errors on your line.

Similarily, if your line improves and the low SNR margin or high error counts disappear, the DLM can change it all back to the default settings. If you think you are having SYNC problems and you can't use your connection, you could call your ISP and ask them to increase the target SNR margin and/or turn on Interleaving. If they are a competent ISP they should pass your request on to BT Wholesale who are able to manually update your profile.



Signal-to-Noise Ratio

The SNR is the amount of signal your line can see as opposed to how much noise it can hear. For example, if you have a short line to the exchange, your SNR is expected to be higher than a line that is a long way from the exchange, as the signal decreases, and the noise increases over distance.

The more SNR your line has, the faster you can SYNC at. The less SNR your line has, the more unstable it can be.

Note that many people confuse the term SNR with SNR margin. The SNR is as stated above, while the SNR margin is the amount of SNR you have left on your line, not the SNR itself.

For a line to be stable, it is widely viewed that a minimum SNR margin of 6 dBis to be observed at all times. If the SNR margin drops below 6 dB, you may begin to have problems with your connection, such as loss of SYNC or lots of errors. This is where the DLM (described above) can kick in.

If your line begins to lose SYNC frequently because of a low SNR margin, the DLM should pick this up and increase the target SNR margin. This means the stability of the line should be increased. Note that a higher target SNR margin means a lower SNYC rate.



Interleaving

Interleaving is a 'feature' that can be turned on to help stabilise your line. It works by chopping up the packets that are being transferred into smaller chunks that are put back together at the other end. Please refer to the following diagram:



If you take a look at the above diagram, you can see the difference in how data is transferred over a line that has the normal 'fast' mode compared to 'interleaved' mode. In fast mode, the data is sent in a continuous stream and no re-assembling is required at the other end. But with interleaving, the data is split up into chunks and then re-assembled at the other end.

Interleaving is only beneficial if there are errors on the line and the data being transferred becomes corrupt. See the following diagram:



In this diagram we can see that some of the data has become corrupt. In fast mode, this produces a bigger problem as a whole packet of data is lost, but with interleaving, only a minor section of each packet is lost, and with the present technology of ADSL, it can re-assemble each packet by sending a request back down the line for the bits that are corrupt which are then received, and re-assembled. This is also why interleaving increases latency (ping) times.

Interleaving also reduces the possibility of losing SYNC due to the way it reduces errors on the line. BT Wholesale say that interleaving significantly improves error correction and the stability of long and/or marginal lines.

The downside to iInterleaving is that it can slightly slow down your connection. Latency (ping times) may increase by around 20ms - so watch out gamers. I would imagine most people would rather have a slower, yet stable connection over a faster, unstable connection.

There is a lot more to the way interleaving can reduce errors, but the above description is the easiest way I can think of showing it.



Data Rate

The data rate is set in your BRAS profile and is simply the maximum speed at which you can download at. For example, if you SYNC at a rate of
8128 Kbps, you would normally expect to be able to achieve a maximum download speed of 8128, but it's not as simple as that, unfortunately.

The data rate is set in increments of 500 Kbps bar a few exceptions, starting from 250 Kbps to 7150 Kbps.

To work out your expected data rate, refer to the table below:
SYNC rate Data rate
288 - 575 250 Kbps
576 - 1151 500 Kbps
1152 - 1727 1.0 Mbps
1728 - 2271 1.5 Mbps
2272 - 2847 2 Mbps
2848 - 3423 2.5 Mbps
3424 - 3999 3.0 Mbps
4000 - 4543 3.5 Mbps
4544 - 5119 4.0 Mbps
5120 - 5695 4.5 Mbps
5696 - 6239 5.0 Mbps
6240 - 6815 5.5 Mbps
6816 - 7391 6.0 Mbps
7392 - 7967 6.5 Mbps
7968 - 8127 7.0 Mbps
8128 7.15 Mbps

This means that if you SYNC at a rate of 3800 for example, you will only ever get a maximum download speed of 3.0 Mbps. So to get any higher, you would have to SYNC at a rate of 4000, to step up to the next level. Then your maximum would rise to 3.5 Mbps. It seems a bit strange doesn't it? Yes... but that is how BT Wholesale have set it up, so unfortunately, we have to live with it.

It is very important to note that your data rate does not increase straight away...

If you are currently SYNC'd at 3800, to get up to the next level(s) as described above, you would need to re-SYNC at a rate of 4000 (or higher) - for three solid days on the run, with no re-SYNC's at a lower SYNC rate. It doesn't work that way if you SYNC at a lower rate though. If your modem was to SYNC at a rate lower than 3424 twice in a three day period, you would instantly be down to the 2.5 Mbps data rate.

Not fair? I agree. Again, thats how BT Wholesale want to play it.

NOTE: Some lines that have just been enabled for the ADSL Max service have what's known as a stuck BRAS profile. This is where the data rate is stuck at 2.0 Mbps no matter what. You can usually identify this by taking several speed tests over a few days. If they all come back at around 1.5 - 2.0 Mbps - then its possible your line has a stuck BRAS profile too. If after the first 10 days of your line being enabled for ADSL Max you are still not able to download at speeds above 2.0 Mbps, you will need to contact your ISP and ask them to reset your BRAS profile. (That is, presuming your line is SYNC'd at above 2848 - the rate at which above 2.0 Mbps is possible)



Conclusion

That is all pretty much all you need to know about the way ADSL Max works. There are a few other bits which I could have included, but to keep it as simple as possible, I opted to leave them out, as they aren't that important.

If you have any queries regarding this guide, or would like to correct any of the topics covered, please feel free to post on our Message board and I will endeavour to make any corrections.