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 'About 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. 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 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. With Max this means that you can expect to see your broadband speeds go up and down, where previously you'd have got one set speed. Occasionally the connection will sync too fast for your line 'noise' and the connection can drop.

Max is much more at risk to poor quality microfilters, long phone extension cables and electrical interference, where a lower broadband speed 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. More than a third of all faults raised to our technical team about Max are issues about these issues and not within our control.

The first 7 days
During the first 7 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. The test will look to find out the lowest broadband speed you've received during the last 7 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.

Max Terminology
One of the most confusing things about Max 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.

Maximum Stable Rate (MSR)
During the first 7 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 7 days, not as the name 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 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 3Mbps download rate and synch at 3.5Mbps 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 8Mb' speeds that you should expect for your connection and that Max 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 Max 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 7 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 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.

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.

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 Max 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.

As a general rule so long as your throughput speeds fit within the following bands it's 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 phone 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 it's not already active. Note: 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 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.

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.

Summary
Broadband Max 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 7 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 7 days. The line will be constantly monitored as long as Max 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 8Mbps broadband and what this means. Up to 8Mbps doesn't mean that every line will see 8Mbps sync speeds, many lines will see sync speeds a lot lower than this, due to length or quality. Up to 8Mbps 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.

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