Advanced Guide to Broadband Max

Max Terminology
Maximum Stable Rate (MSR)
Sync Speed (Line Rate)
Throughput Speed (Data Rate)
Fluctuating Throughput Speeds
Broadband Remote Access Server Profile (BRAS)
Dynamic Line Management (DLM)
Blip Logic
Interleaving
Summary

Introduction
Following on from 'Get Ready for High-speed Broadband' this guide explains Broadband Max DSL in more detail and includes some useful tips for troubleshooting problems.

What is Broadband Max?
Broadband Max, also known as Max DSL or DSL Max, is a set of products from BT Wholesale that is used to provide fast broadband speeds 'up to' 8Mbps.

The two main products that form part of the Max DSL family are known as IPStream Max and IPStream Max Premium. Max offers downstream line speeds up to 8128kbps and upstream up to 448kbps. Max Premium, used on our Broadband Business product, offers potentially faster upstream speeds of up to 832kbps and at busier times is given a higher priority on BT Wholesale's network.

Be Prepared
BT's Max DSL operates at the very limits of ADSL technology. For most lines DSL Max will sync at the absolute fastest speeds they can. Thus when compared to the old fixed rate 500, 1000 and 2000 services most lines no longer have a 'comfort zone' of what is called noise margin or 'Signal to Noise ratio' (SNR).

Up to 8Mb speeds are obtained using 'rate-adaptive' technology where your line is checked frequently, with the data-transfer speeds changed according to conditions on the line. This means that with Max, you can expect to see your broadband speeds vary where previously you would have one set speed. Occasionally the connection will sync too fast for your line 'noise' and the connection can drop.

Max is much more susceptible to poor quality filters, long phone extension cables and electrical interference, where a previous lower broadband speed might have been more tolerant. To help prevent problems you should try to ensure that anything within your control - your ADSL hardware, internal wiring, filters, etc. is set up correctly. Upwards of a third of faults raised to our technical team about Max are issues relating to these issues and not within our control.

The first 10 days
During the first 10 days of Max 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. You will see your sync speeds vary, and commonly may experience disconnections and some instability of your connection. During the testing period you should frequently make new connections to the Internet so these tests can take place. It's a good idea to power off/on your modem or router when doing this - this will force your equipment to reconnect.

Max Terminology
One of the most confusing things about Max is not only that the speed varies, but that there are a whole load of technical terms for the different speeds and measurements that are made. The following should help make sense of the different terms you might come across.

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 first 10 days, not as the name somewhat confusingly suggests, the maximum sync at which the line was stable.

Here's an example. My new Max connection syncs at variable rates between 3 and 4Mbps. It's stable at 3Mbps but occasionally drops the connection at 4Mbps. If conditions change, perhaps I put in a phone extension or change the filters, I now see the modem synching unreliably at 256kbps and dropping every 5 minutes. I then replace all my filtering and the line returns to the previously seen pattern. In this instance my MSR would be 256kbps but I would probably see a stable 3Mbps download rate and synch at 3.5Mbps after the line had stabilised.

The MSR value is solely used to determine when a drop in sync speed would be considered a fault*. It is important to understand it is not a guarantee of speeds that you should expect for your 'up to 8Mb' connection and that Max is rate-adaptive, meaning your speeds will vary throughout the lifetime of your connection. We do not publish your MSR value as part of your Max service...think of it as for administrative purposes only. If you would 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.

Sync Speed (Line Rate)
Also known as Line Rate, the synchronisation (sync) speed is the speed at which your ADSL 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 determined 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.

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 because the sync speed includes a certain amount of 'overhead data', this is essentially data that makes the connection work rather than the data that gets added on to the web pages and files you download.

The following table represents the maximum possible throughput speeds for the different sync speeds you might see.

Fluctuating Throughput Speeds
It is perfectly normal to see throughput speeds that change from day to day, from hour to hour, even from minute to minute. A Max connection in the late afternoon would likely see variable speeds 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 is busy.

As a general rule so long as your throughput speeds fit within the following bands it is considered to be normal.

Broadband Remote Access Server Profile (BRAS)
The BRAS profile is responsible for regulating the maximum throughput (data rate) you will receive on your Max DSL 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 receiving Max 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. See Blip Logic for more information.

Dynamic Line Management (DLM)
Dynamic Line Management (or DLM) is the collective term for the automated BT systems that log information about your line's performance and processes that are used to stabilise the line. DLM uses the information it gathers about disconnections and reconnections to establish what speeds are likely to be most stable for your particular line. It may also determine that interleaving should be activated on your line if not already present. It should be noted that frequently restarting your router or modem may be interpreted by the DLM as a problem and may subsequently reduce the controlled line rate.

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 recently introduced a 'blip' logic function to the system so that 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.

Interleaving
Interleaving is a form of error correction that can help reduce the number of 'errors' on a line. On new orders and where a fault has previously been reported this will be present by default. 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 would recommend against it, if switched on, interleaving can be turned off on request.

Summary
Broadband Max is very much a "best efforts" service. Best efforts means that it will try and sync at the best speed your line can support and means that it is normal for throughput speed to vary all the time.

During the first 10 days the automated systems will try and establish the ideal sync speed for your line, in order for your connection to be stable. This may mean that your sync speed will vary and may mean some customers will see disconnections.

It is important to realise that the training period doesn't end after the first 10 days. The line will be constantly monitored so long as Max is enabled. Over time the line may get better or the line may get worse and the Dynamic Line Management will attempt to keep the line as stable as it can.

It is also very important to remember the 'up to' in up to 8Mbps broadband and what this means. Up to 8Mbps does not mean that every line will see 8Mbps sync speeds, many lines will see sync speeds a lot lower than this because of the length or quality. Up to 8Mbps also does not 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.

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

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