Odds & ends.

A few odds and ends remaining to report. Several users posted about problems with SNR dropping at night and causing retrains. A speed that during the day gave an SNR margin of 6 dB would not be sustainable in the evening and a resync to a lower speed would occur, possibly several such retrains if the noise got progressively worse.

In answer to the question "Does your SNR / noise margin and/or modem sync speed vary a lot, for example between night and day." the response was :-

Don''t know / haven''t noticed 27%
Variations are small 54%
Slower speeds at night 16%
Faster speeds at night 2%

so for 81% of respondents the variability isn't a big problem. 16% see the behaviour described above and 2% see the reverse.

On the upstream front there are far fewer issues than downstream. The 448k sync speed of the Home version of MaxDSL is not especially ambitious and it is therefore not a suprise that 88% of respondents have no upstream issues at all. 9% reported upstream speed problems, and 2% had issues with errors or stability.

Since the survey was done BT have temporarily withdrawn their bulk upgrade tool to fix some issues with stuck BRAS profiles etc. Another glitch to be acknowledged was that the DLM process would not raise the target SNR margin to a level where the downstream rate fell below the fault reporting rate (FTR) which is itself set at 70% of the MSR recorded during the 10 day training period. This prevented the DLM stabilising a line that had fallen more than 30% in speed from its original stable rate. The latter problem is reportedly fixed and the bulk upgrade process is due to restart on 06/06/06.

Zen Internet's Technical Support Manager recently posted that "Despite all of the above the actual increase in fault rate we've seen hasn't been significant, particularly given the volume of regrades and new activations we've seen."

My office line went Max today, full 8132/832 sync speed. I had a 2M rate limit until I left the router off for 30 minutes while I was out for lunch, on return it was running with download rates around 2.3 times that of the 2M fixed speed service.

Overall satisfaction

With 640 responses in the bag, 46% of them say they are pleased with MaxDSL and 54% are disappointed.

54% say that given a free choice in the next month they would stick with MaxDSL, 9% would go back to fixed speed and 36% would stay but only if it improves.

The five largest responses were from the following ISP's users :-
BT / BT Yahoo 23%
Plusnet / Metronet 17%
Eclipse 14%
Nildram 10%
Zen 7%

80% of Zen users said they were pleased with MaxDSL but the other four in the list above all swung to the disappointed side - only 39% of BT/BT Yahoo users were pleased.

AOL registered a small majority of pleased users, Andrews & Arnold users followed the Zen trend and of the smaller ISPs captured in the "others" category 54% of users were pleased with MaxDSL.

If we were to assume that the ISPs have a random scattering of line speeds etc there looks to be something to be said for the way a product is handled by the ISP influencing the user satisfaction level. One third of the users wanting to go back to fixed speed service were with BT/BT Yahoo.

Data throughput

Having a high modem sync speed is one thing, but what of the data rate experienced by users ? This is a complex area, as the throughput can be limited by the ADSL line, the data rate profile setting, the connection from the exchange to the rest of the BT network, the ISP link to BT, the ISPs onward transit, the internet in general or the capacity of the target server.

The BT STIN defining the service in its trial phase set out the expectation that the throughput of MaxDSL would be no better than the equivalent fixed speed service when the network was congested, so a line sync over 2M may on that basis deliver the same throughput as a 2M fixed speed service at busy times.

The survey asked about data rates measured by speed test off-peak (06:00) and peak time (20:00).

The off-peak results were :-
<450 kbits/s 5%
>450k but <1Mbits/s 4%
1M or higher but <2M 15%
2M or higher but <4M 33%
4M or higher but <6M 26%
6M or higher 18%

so at the top end we only have 18% of users experiencing speeds over 6M despite 58% of users having a sync speed of 6M or higher. On the positive side there are 77% of users seeing an increase in off-peak data rate over the previous 2M service.

This is probably the root cause of most dissatisfaction with MaxDSL, the discrepancy between modem sync speed and actual throughput. The largest group of users is the 2M-4M download rate and in this group only 35% say they are pleased with the service. 82% of those getting >6M data rate say they are pleased, as do 63% of those getting 4M-6M.

Below 2M download rate the "disappointed" respondents are very much the majority.

So the user response to the service is based very much on download rate experienced and you have to look at 4M or higher to find the happy users. There is a bit of a clash between the product design (may be the same as 2M) and the user expectation. Neither BT nor ISPs have invested in a doubling or trebling of network capacity to match the increase in end user modem speeds, so this situation was inevitable.

Moving onto the peak time performance, 22% of respondents see a peak time download rate that is 25% or less than the off-peak rate. 82% of this group say that overall they are disappointed with MaxDSL which is perhaps understandable !

Nearly 30% see peak time rates similar to their off-peak rate, but only 55% of this group are pleased with MaxDSL overall. This could be because speeds are bad 24/7, or other factors not yet identified.

19% report peak rates at ~75% of off-peak and 29% see the rate slow to half at peak times. 2/3 of the 75% group are pleased overall, but 55% of the 50% group are disappointed overall.

The other question that addressed this topic was 7. Have you found your download rate to be well below the indicated modem speed ?

No, it seems pretty close 15%
Yes, 75-100% of modem speed 29%
50-74% of modem speed 31%
25-49% of modem speed 13%
Its worse than 25% 12%

In terms of overall satisfaction, the 50-74% of modem speed group has 36.5% pleased with the service, those getting lower rates are largely disappointed and of those getting 75-100% of modem speed more than 70% are pleased.

Stability

Stability can be a bit subjective, and in general MaxDSL will be less stable than the fixed speed products with their conservative margins. G.dmt ADSL can only adapt to changes in noise or line condition by retraining, which the user sees as a temporary disconnection. ADSL2 and ADSL2+ have features like bit swapping and seamless rate adaption that can compensate for changes without requiring a retrain.

The response to question 6. "How stable is your connection, in terms of speed and disconnections. " was :-

Very stable. 53%
Occasional disconnections 30%
Frequent disconnections 13%
Totally unstable. 5%

so 83% are either very stable or have occasional disconnections.

48.5% of the "very stable" respondents knew they were on Fast path, and 16% knew they were interleaved. As 22% of the total knew they were interleaved this suggests that the interleaved lines were less well represented in the "very stable" category. 28% of the "occasional" and "frequent" disconnectors were interleaved, and the same in the "totally unstable" category, so the DLM had spotted them but the interleaving itself had not solved the problem.

For the speed ranges of 2M and over the "totally unstable" response covered about 4% of the lines. For lower speeds the % rose but the numbers reporting were small at 1 or 2 in each speed category.

5.8% of the fastest lines had "frequent disconnections", compared to 13% overall. 24% of the 4-6M, 18% of the 2-4M and 24% of the 1-2M sync speed lines also fell into the "frequent disconnection" category.

Looking at the influence of SNR margin, 70% of those with margins of 11 dB and higher said their connections were "very stable". In the 6-10 dB range this fell to 47.5% and in the 0-5 dB range to 41%.

There appears to be no substitute for having a good margin in terms of stability, which is why some are turning to tweakable routers to achieve higher SNR margins, lower speeds and greater reliability.

SNR Margin & Interleaving

BT's MaxDSL sets out with a 6 dB target SNR margin and the modem takes the speed as high as it can within that constraint. If the line is detected as being unstable the DLM (Dynamic Line Management) can turn on interleaving and ratchet up the SNR margin target to 9, 12 and 15 dB.

In the survey, 22% of respondents had interleaving on their line, 40% were still on Fast Path and 38% didn't know. If the "don't knows" were split along the same lines as those that do know then perhaps 35% of lines have been unstable enough to warrant application of interleaving.

45% of respondents had an SNR margin in the 6-10 dB range, 11% were below that and 16% didn't know.

24% fell into the 11-15 dB bracket and 5% were over 15 dB. 75% of the >15 dB group had speeds of 6M or higher, suggesting that these are the very short lines iwth margin to spare at high sync speeds.

Line speeds

90% of survey responses had MaxDSL modem downstream sync speeds faster than 2M. 58% reported 6M and higher, with 17% of 4M up to 6M and 15% 2M up to 4M.

73% of users had previously been on the 2M fixed speed service but 10% of them reported a drop in modem sync speed. 70% of them reported 6M or higher.

45% of users previously on 1M moved into the 2M up to 4M bracket, with 24% at 4-6M and 19% at 6M or higher. 7.5 % moved up a little into the "1M up to 2M" bracket. 3% saw a speed reduction into the 512k-1M range.

Of the previous 512k users some 40% went to the 6M and over sync speed. 1 user (2%) fell below 512k and the rest were spread over the rest of the speed range with 40% between 1M and 4M. We don't know if the former 512k users were necessarily on long lines, or on 512k through choice.

Overall, 93.4% of respondents saw their modem line speed increase by changing to Max.

MaxDSL survey - background

This is background information to a survey of user experiences with the MaxDSL broadband service provided by BT in the UK and used by many ISPs to provide ADSL services to their customers.

MaxDSL was introduced at the beginning of April 2006 as an "up to 8M" service, the line speed depending on the quality and length of the phone line and the level of interference.

In the "fine print" of the trial product description it was made clear that the end user of a >2M MaxDSL service may see the same data throughput as a 2M fixed speed user in times of congestion, in other words it is a "best efforts" service to give faster than 2M speeds where possible. AOL describe it as a "2M up to 8M service" which covers this. The maximum IP rate in the spec is 7320 kbits/s which is 3.6 times that of the fixed speed 2M service.

Originally the MaxDSL trial was limited to lines running at 2M fixed speed. On release of the final product this eligibility constraint was removed however the product retains 0.5M steps in the "data rate" profile applied to the line to control the ATM capacity of the circuit. This rather clunky arrangement means a user originally on 512k fixed speed may migrate to MaxDSL and get a line speed of 1120 kbits/s on their modem but their data rate profile will remain at 0.5M. The only benefit they get is the increased upload speed of 400 kbit/s or 750 kbits/s on the "Max Premium" business service.

MaxDSL is the first BT product to use rate adaption rather than fixed speed. This means the lines run at the fastest speed they can, rather than the previously conservative regime which gave high reliability but limited speed to 2M on lines capable of far more. The trade-off for higher speed is reduced SNR/noise margin, more errors and more retrains / disconnections. BT has a process of "Dynamic Line Management" designed to find a compromise between stability and speed by upping the target SNR margin (which reduces the speed) on unstable lines. DLM can also apply interleaving, unless the end user has opted out of this via their ISP. Interleaving reduces errors and increases stability on lines with "the right sort of interference", it adds 10-20 ms to ping times and reduces the maximum speed possible from 8128 to 7616 kbits/s, details in the BT SIN.

The online survey was conducted to assess the user experiences of actual data throughputs, stability, line sync rates etc. It was promoted on Usenet and on the ADSLguide forums and attracted over 600 responses.