Advanced Guide to ADSL2+

Introduction

Following on from 'About High-speed Broadband' this guide explains ADSL2+ in more detail and includes some useful tips for troubleshooting problems.

1. What is ADSL2+?

ADSL2+ is the latest generation of products from BT Wholesale, used to provide broadband speeds of 'up to' 20Mbps.

As standard, ADSL2+ provides an upload speed of 'up to' 448Kbps, though upload speeds of 'up to' 1Mbps are available. Higher upload speeds can impact the speed of your service and reduce the overall stability of your service.

2. Be prepared

ADSL2+ is the fastest service currently available over standard telephone lines. For most of these lines, ADSL2+ will sync at the fastest speed possible.

Up to 20Mb speeds are received using 'rate-adaptive' technology. This is where your phone line is checked frequently, with the data-transfer speeds changed according to conditions on the line. This means that you can expect to see your broadband speeds go up and down, reflecting the current conditions. Occasionally the connection will sync too fast for your line 'noise' and the connection can drop.

ADSL2+, like Max DSL, is much more at risk to poor quality microfilters, long phone extension cables and electrical interference, where older fixed speed broadband technology might have been more tolerant. To help prevent problems you should try to make sure that anything within your control - your broadband hardware, internal wiring, filters, etc. is set up properly. Many of the issues raised to our fault team are due to these issues, which are outside of our control.

3. The first 10 days

During the first 10 days of ADSL2+ your phone line will be automatically measured to establish what speed it can support, and in particular at what speed the line will remain stable. The test will look to find out the lowest broadband speed you've received during the last 10 days. You'll see your sync speeds vary during this time, and you're likely to get disconnected from time-to-time. Whilst the testing is being carried out you should regularly make new connections to the Internet. It's a good idea to power off/on your modem or router when doing this, as this will force your equipment to reconnect.

ADSL2+ Terminology

One of the most confusing things about ADSL2+ is not only that the speed varies, but that there are a load of technical terms for the different speeds and measurements that are made. This section should help you to make sense of the terms you might come across.

4. Maximum Stable Rate (MSR)

During the first 10 days your connection will be given a value known as the Maximum Stable Rate (MSR). The MSR is the lowest sync speed (line rate) achieved in the last 10 days, not as the name suggests, the maximum sync at which the line was stable.

Here's an example. My new ADSL2+ connection syncs at variable rates between 8 and 10Mbps. It's stable at 8Mbps but occasionally drops the connection at 10Mbps. If conditions change, (perhaps I plug in a phone extension or change the microfilters), I now see the modem synching unreliably at 256kbps and dropping every 5 minutes. I then replace all my microfilters and the line returns to the previously seen pattern. In this case my MSR would be 256kbps but I'd probably see a stable 8Mbps download rate and synch at 9Mbps after the line had stabilised.

The MSR value is just used to work out when a drop in sync speed would be considered a fault*. It's important to understand it is not a guarantee of the 'up to 20Mb' speeds that you should expect for your connection and that ADSL2+ is rate-adaptive. This means your speeds will vary throughout the lifetime of your connection. We don't publish your MSR value as part of your ADSL2+ service...think of it as for administrative purposes only. However, if you'd like to see what data transfer speed you are receiving on your connection please see your BRAS Profile.

*BT will accept a fault report if the line rate drops by 20% or more from your established MSR. This is said to be below the Fault Threshold Rate. e.g. if the MSR during the first 10 days is determined as 5000kbps, a fault can be reported when your sync speed drops to 4000kbps or less.

5. Sync Speed (Line Rate)

Also known as Line Rate, the synchronisation (sync) speed is the speed at which your broadband modem or router communicates with the BT Exchange. This is always higher than the speed at which data will actually be downloaded over the connection. The sync rate is worked out by your phone line's characteristics, such as line length and quality. The shorter the line and the better the quality the faster the line will sync.

6. Throughput Speed (Data Rate)

The throughput speed is the maximum speed that data will transfer across the connection. The maximum throughput speed is always lower than the sync speed. This is because the sync speed includes a certain amount of 'overhead data'. Essentially this is data that makes the connection work, rather than the data that gets added on to the web pages and files you download.

This table shows the maximum possible throughput speeds for the different sync speeds you might see.

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7. Fluctuating Throughput Speeds

It's perfectly normal to see throughput speeds that change from day-to-day, hour-to-hour, even minute-to-minute. It's likely that a ADSL2+ connection in the late afternoon would see speeds vary due to exchange-side contention. The same download at 4am, when the network is very quiet, might give a very steady download speed.

Broadband works as it does, and at the low prices we as customers pay, because it works on a shared network. The available capacity on the network is shared out amongst all the customers that are using the Internet at that time. This means that at busy times, when more people are using the Internet, each person gets a smaller share of the bandwidth and so lower speeds than when the network is quiet. As different people use the Internet in different ways and have different usage patterns. This can mean that the speed you see can be constantly going up and down, especially when it's busy.

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8. Broadband Remote Access Server Profile (BRAS)

The BRAS profile is responsible for regulating the maximum throughput (data rate) you will receive on your ADSL2+ service. Initially this is set to 2Mbps (if the line rate is above 2272kbps), but it will increase if your line can support higher speeds than this. The first data rate increase should take place within 75 minutes of you receiving ADSL2+ service. Your BRAS profile however, changes only when a stable connection has been made for a period of time; this may be up to 3 days in some cases. The BRAS Profile will also drop should the line rate decrease significantly.

9. Dynamic Line Management (DLM)

Dynamic Line Management is a feature that is run on all ADSL and ADSL2+ services that are provided by BT Wholesale, to analyse line performance information on their network. It uses this information to determine whether or not a line is unstable and, if it is, will make some configuration changes to try and stabilise the line at the fastest possible speed.

The DLM processes that run on ADSL2+ are generally very similar to those used for Max DSL. There have, however, been a number of changes made, designed to improve both the speed and efficiency of how it reacts to lines which are experiencing dropped connections and poor speeds.

It is also designed to adapt much faster, should a line's performance improve, such as if the sync speed is improved after a voice line fault is fixed.

Generally, the performance of an ADSL2+ line is reviewed on a daily basis, unless it is what BT refer to as a 'flapping line' or one which drops the connection regularly. These are generally reviewed more frequently. Once a line has been stabilised, the DLM changes become less frequent.

It is only when a line experiences a period of instability where this process is restarted. The reconfiguration of these lines will involve changes in the 'line profile'. Once a change to a line profile has been requested, there is a delay of approximately 2 hours before the change is actually made. When this change occurs, there is a brief drop in connection, typically for about 30 seconds.

Where there is a deterioration in service the speed at which the DLM will react will depend upon the severity of the problem. A connection which drops once a day will see profile changes occur less quickly than if a line was disconnecting far more frequently.

Another addition to DLM for ADSL2+ is that of banded profiles. Initially, all lines are configured to run in a fully rate adaptive mode. That means there isn't really any maximum or minimum capability of the broadband service within technical capabilities.

For lines which drop frequently, a banded profile can be applied, which will restrict a line's connection speed from connecting at a level which it can't maintain. Banded profiles operate with a minimum and maximum connection speed band. For example, one banded profile will control the connection speed from between 3.3 and 6.6Mbps.

The following table shows the various banded profiles that are currently used on ADSL2+:

10. Interleaving

Interleaving is a form of error correction that can help reduce the number of 'errors' on a line. This will be activated by default on new broadband services and where a fault has previously been reported. It helps to stabilise a line that might otherwise suffer frequent disconnections. One drawback of interleaving is that it can increase ping times, which may cause problems for people that play online games. Whilst we'd recommend against it, if it is switched on, interleaving can be turned off if you ask us.

11. Summary

ADSL2+ is very much a 'best efforts' service. This means that it will try and sync at the best speed your line can support and means that it's normal for throughput speed to vary all the time.

During the first 10 days of a service the automated systems will try and establish the ideal sync speed for your phone line. This is in order for your connection to be stable. It may mean that your sync speed will vary and that you might be disconnected from time-to-time.

It's important to know the training period doesn't end after the first 10 days. The line will be constantly monitored as long as ADSL2+ is enabled. Over time the line may get better or worse, however the Dynamic Line Management will attempt to keep the line as stable as it can.

It's also very important to remember the 'up to' in up to 20Mbps broadband and what this means. Up to 20Mbps doesn't mean that every line will see 20Mbps sync speeds, many lines will see sync speeds a lot lower than this, due to length or quality. Up to 20Mbps also doesn't mean that throughput speeds will always be full speed. Local contention will play a big factor and speeds will vary.

Remember that the Maximum Stable Rate (MSR) despite its somewhat misleading name doesn't have anything to do with throughput speeds. The Maximum Stable Rate is just the lowest sync speed reported in the first 10 days. The MSR is only used as a guideline figure in order to decide if a line's sync speed has dropped too low.

12. More Help

Our online tool can be used to measure your current data transfer speed as defined by your sync speed.

This page last updated 20th October 2009