Broadband Terminology

Following on from About High-speed Broadband this guide explains the some of the terminology used in more detail.

The terms covered can apply to both Broadband Max and ADSL2+ services.

• ADSL2+ is the latest generation of broadband, used to provide 'up to' 20Mbps download speeds and 'up to' 1Mbs upload speed
• Broadband Max (a.k.a. Max DSL or DSL Max) is used to provide 'up to' 8Mbps download speeds and 'up to' 448kbps or 832kbps

1. Maximum Stable Rate (MSR)
2. Sync Speed (Line Rate)
3. Throughput Speed (Data Rate)
4. Fluctuating Throughput Speeds
5. Broadband Remote Access Server Profile (BRAS)
6. Dynamic Line Management (DLM)
7. Blip Logic
8. Interleaving

1. 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 speeds that you should expect for your connection and that ADSL2+ and Max are 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 broadband 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.

2. 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.

3. 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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4. 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 broadband 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.

5. Broadband Remote Access Server Profile (BRAS)

The BRAS profile is responsible for regulating the maximum throughput (data rate) you will receive on your broadband 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 your first connection. 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.

6. Dynamic Line Management (DLM)

DLM is a feature that is run on all Max 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.

Generally, the performance of a broadband 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.

DLM for ADSL2+ is able to use 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+:

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7. Blip Logic

Some customers have very stable lines, but for one reason or another can occasionally see a one-off sync rate at a lower speed. In the first few months of Max such a one-off event would cause the BRAS profile to be decreased and it would take at least 3 days before it went back up again.

BT Wholesale have a 'Blip Logic' function on the system. This means if a stable line, (one that hasn't seen a BRAS speed profile decrease in the last 14 days), experiences a short drop in sync rate and then goes straight back up again, it won't result in a 3 day wait for the BRAS profile to catch up. A second decrease in sync speed would lower the BRAS profile however.

8. 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.

This page last updated 9th February 2010