MaxDSL lines have a number of speeds or "rates" associated with them, which can be confusing.

Sync speed

First there is the modem sync speed which you can see on your equipment, a 2M line is 2272 kbits/s downstream and a Max line will be between 160 and 8128 kbits/s and anywhere in between in steps of 32k. This speed is set by negotiation between your modem and the exchange equipment (DSLAM) every time it connects or re-connects. This speed variation goes on for ever and does not stop after a period of 10 days.

Data rate (BRAS profile)

BT's systems set a data rate profile for each circuit, this lives in a Broadband Remote Access Server and its job is to ensure that the speed data is sent to you is less than the maximum the modem can handle. The BRAS profile has 0.5M steps and will always be less than or equal to your modem sync speed. At 2272k sync you have a 2M profile, at 2240k the data rate drops to 1.5M. These step sizes look a bit big on the slow speed products, which may deny the user any useful throughput increase if (for example) a 512k line was upgraded to Max and ran at 1120 kbits/s.

The BRAS data rate "follows" your modem sync speed, if the modem speed falls the profile should drop down quickly (minutes). If your modem starts to sync at higher speeds the BRAS profile should eventually (3 days) increase to follow it. This process continues through the life of the service, and does not stop after 10 days.

Initial BRAS data rate profiles when switching to Max are those of the previous service, for example 2M. They should change to match the modem after ~75 minutes however it is fairly common for this not to happen, especially if you get the full 8128 sync speed. Switching the modem/ router off for 30 minutes each day may wake the system up but if it doesn't change in 10 days talk to your ISP.

If the BRAS rate is stuck at 2M you will see speed tests of 1800-1900 kbits/s even with the modem sync speed at 6 or 8M. The table below shows the BRAS data rate appropriate to different sync speeds :-

ATM Line sync speed	IP Data Rate
from 288Kbps	0.25Mbps
from 576Kbps	0.5Mbps
from 1152Kbps	1Mbps
from 1728Kbps	1.5Mbps
from 2272Kbps	2Mbps
from 2848Kbps	2.5Mbps
from 3424Kbps	3Mbps
from 4000Kbps	3.5Mbps
from 4544Kbps	4Mbps
from 5120Kbps	4.5Mbps
from 5696Kbps	5Mbps
from 6240Kbps	5.5Mbps
from 6816Kbps	6Mbps
7392Kbps	6.5Mbps
7968Kbps	7Mbps
8128Kbps	7.15Mbps

(Table from ADSLguide). Note that the fastest TCP/IP data rate of an "8M" line is in fact 7.15 Mbits/s.

Maximum Stable Rate

During the initial 10 day period on MaxDSL the BT systems monitor the line and note the slowest speed it operates at. This becomes the Maximum Stable Rate (MSR). In other words the slowest it is seen to work at becomes the fastest it is deemed capable of !

The MSR is effectively a number chalked on the wall, it does not limit your speed, fix it or anything else. It is just a recording of the line's capability in the initial period.

The BRAS rate profile and the modem sync speed can be above or below the MSR if the line conditions change.

Fault Threshold Rate

This is 70% of the maximum stable rate, so if your MSR is 2272 your fault threshold or FTR will be 70% of 2272 or 1590 kbits/s. This becomes the modem sync speed below which BT will accept there is a fault, it has no other role.

Some ISPs believe that the FTR is 70% below the MSR, which is not correct. It is 30% below the MSR or 70% of the MSR. Accept no substitutes.

FTR does not affect sync speed, BRAS rate or anything else. It is simply to benchmark the line for future fault reporting.

Upstream

The upstream sync speed on a Max line rate adapts in the same way as the downstream, in the range 64-448 kbits/s for Home products and 64-832 kbits/s for Max Premium. In practice most lines will see the full upload speed on the Home product, but lines achieving less than 6M downstream may not manage the full 832 of the Premium product. Steps of 32k are used upstream.

The ATM rates of 448 and 832 kbits/s upstream correspond to TCP/IP rates of 398 and 750 kbits/s, compared to the old 256 kbits/s of the fixed speed products.

There is no BRAS profile for the upstream, your equipment is required to limit the data rate to match the modem sync speed.

BT have introduced "MaxDSL" services which give "up to 8M" depending on line conditions. These services are "rate adaptive" which means the speed achieved by the modem depends on the line condition - a good short line will achieve 8128 kbit/s downstream and the longest poorest line will either drop to 160 kbits/s or fail to connect. In general this means people are getting faster speeds with Max than before, as the previous limits were conservative.

Not all ISPs are offerring MaxDSL services as yet. Those that are have experienced some ups and downs both with individual customers lines and with their systems as a whole - giving the users 3 times the bandwidth has caused some bottlenecks at exchanges and ISPs. Individual users that have in the past had rock solid lines are now finding that the target SNR margin of 6 dB gives them lots of speed but perhaps lots of errors and disconnections as well.

More explanations of the Max service coming soon. Meanwhile you can read the results of an early-adopter survey which shows most people getting high modem speeds but somewhat disappointing data throughputs.