Downstream Throughput Performance of "MAX" ADSL

It is important to understand that your sync rate on a BT MAX ADSL line is only the rate at which your ADSL modem syncs with the DSLAM at your local BT exchange. Your sync rate actually has limited effect on your overall downstream throughput.

Your downstream throughput is the rate at which IP data packets travel from their originating source to the destination software application running on the PC or device at your premises. If you download data say from a computer located somewhere in Germany, your throughput will be determined by the slowest link along the path taken by the data.

Continuing with this example, the data must first travel via the connection the computer in Germany has to the internet backbone. Then the packets are routed to the Upstream Internet network over the internet backbone. Once they reach our network we route them into BT's UK ADSL backbone. BT then routes them through their network to your local exchange, where they are finally sent down your ADSL link to your premises.

Your ADSL link is only one of several physical links the packets must travel through. Any one of these links are a potential bottleneck. Your exchange is likely to be a bottleneck. Most BT exchanges are heavily contended, with a limited amount of bandwidth being shared by the geographical area it serves.

Below are some quotes from material given to us by BT about their MAX products. We hope it will help our users form realistic expectations of BTs MAX service. In particular please see the two tables at the botton of the page, which BT have given us to shows what IP throughput rates you can expect for particular sync rates. Please note if your throughput falls in the ranges in the table then BT will not accept any speed related faults from us.

Where BT refer to Max "Premium" below this is equivalent to the Upstream Internet Priority Upgrade.

"BT IPstream Max Premium has a higher priority through the shared part of the BT IPstream network than BT IPstream Max. This means that under network congestion conditions an IPstream Max Premium line will receive a higher Downstream throughput than the equivalent BT IPstream Max line."

"IPstream Max is a “best efforts” product, and thus no guarantee is given that increased data throughput will be experienced over that of the IPstream Home/Office fixed rate services (i.e. IPstream Home/Office 2000)."

"Throughput of Max services operating at line rates of 2Mbit/s (2272kbit/s) or above will be similar to that of IPstream Home/Office 2000 fixed rate services at busy (high network usage) times, irrespective of line rate achieved."

"Data throughput associated with the maximum line rate may not be possible on some lines provided with the Service, due to the size of the virtual path (VP) deployed in the network until such time as the average data throughput demands requires a VP to be upgraded in accordance with our (BT) standard planning rules."

Max sync speeds can fall anywhere in the following ranges:
"Rate adaptive 288kbit/– 8Mbit/s (8128Kbit/s) Downstream
Rate adaptive 64kbit/s- 448kbit/s Upstream (64kbit/s- 832kbit/s Upstream with Max Premium)"

"BT IPstream Max and BT IPstream Max Premium products have major differences on how they operate over that of fixed rate IPstream products (i.e. IPstream Home 2000)."

"In general it is likely that Rate Adaptive products provide higher Upstream and Downstream data rates for End Users who are closer to a Broadband enabled exchange, with lower rates for End Users who are further away."

"Rate Adaptive (RA) line rate (speed) is purely the rate which is set up between the DSLAM and the End Users CPE, and should not be confused with the end to end data throughput for the end to end service."

"A feature of the RA service is that an End Users CPE will automatically re-train on occasions (i.e. lose sync, and then regain sync). This is not a fault: it is a function of Rate Adaptation. Automatic (forced) re-trains of End Users’ CPE to the DSLAM occur when the line no longer supports the previously set up line rate (i.e. it is below the noise threshold set for the rate)."

"Re-training of the “End Users” CPE (i.e. modem, router) may cause a service outage of up 2 minutes (average of 1 minute)."

"During the first 10 days of the service, BT will be calculating the stable line rate. The line is designed to continue to work at a line rate of 288kbit/s or above throughout this time. If line rate falls below 288kbit/s, or if the line loses synch, then we will accept this as a fault."

"No guarantee of Upstream rate will be given, and therefore you should expect Upstream line rates (speeds) to fall anywhere within the range."

"BT will not accept as an Upstream Line Rate fault any services that are experiencing Upstream rates of 64kbit/s or above."

"Having initially expressed BT IPstream End User performance on our network in terms of “contention ratios”, BT has progressed to a situation where performance on IPstream Max products will be described in terms of ‘End User experience’ rather than “contention ratios”. We will articulate the expected service performance in terms of data throughput which will provide you with a more accurate indication of End User experience. The following tables shows this with figures based on efficient fill of the underlying transport layers in the IP packets. These are based on sustained download rates over a number of minutes and not an instantaneous measure of throughput."