MaxDSL

2007-03-24T09:38:00.000-07:00

L8NC plot

an example....

2006-11-23T14:22:00.000-08:00

What not to buy....

We had an ADSL line running at ~600 kbits/s downstream speed instead of the expected 3M. The christmas tree of extensions, phones and faxes didn't help but replacing the T-shaped Excelsus Z-200UK microfilter with a better one (ie any other type of filter) allowed the modem to achieve 3.6 Mbits/s downstream - result !

Our ISP replaced the filter, we took it to bits after reading up at ADSLnation and discovered that the Z-200UK is a poor design where the input wires on pins 3 and 4 of the BT socket are passed out to the same pins on the output socket, neatly bypassing the filter circuitry. If you study the photo below you can see the offending red and green wires, with the black and yellow signal pair (2 & 5) going through the filter. The testmeter showed a low resistance DC path from the centre two input to the output pins, the other (better) filters we tested were open circuit between these points.

BT's filter spec SIN346 specifically requires that this bypassing is avoided "The bell wire must either be filtered by the filter or left open at the Line Port of the filter and re-created at the Telephony Port of the filter" so the Z-200UK is a non-compliant non-effective item.

So don't buy (or be given) an Excelsus Z-200UK, ask for their Z-420UK-A or Z-420UKP2J instead, where all the wires go through the filtering in the approved manner.

For the anoraks - the problem caused by this is that RF interference collected in the extension wiring or from the phones etc on the line will pass along the bell wire, through the filter into the master socket and then through the ring capacitor into the signal wire, greatly reducing the SNR of the ADSL signal.

2006-08-29T02:26:00.000-07:00

Speeds in practice - DSLzoneUK speed test results as a % of reported modem sync speed, plotted against time of day for four ISPs in the second half of August 2006 :-

With apologies for the pink background - no idea what happened there !

2006-08-24T05:02:00.000-07:00

Routers....

the challenges of MaxDSL and longer lines are handled with varying degrees of success by different models of ADSL router. This is an attempt to identify routers with pertinent features. Some folks have found that modems from the same chipset as the exchange DSLAM work best, so I'll try to include that as well. The abbreviations used for Vendor ID are a bit confusing at times.

The features identified include :-

MaxDownRate - the ability to limit downstream modem sync speed. This gives a higher SNR margin and hence more reliability, or the capacity to accept a night-time noise increase without a resync.

SNR Margin tweaks - commands that allow the user to adjust the target SNR margin so that the modem syncs at a lower speed (higher margin) or higher speed (lower margin) than it would if left to its own (and the exchange's) standard settings.

DMT - compatibility with the German DMT software tool which provides a GUI access to router configuration and tweaking commands plud monitoring information.

SNMP - SNMP functions to allow graphing of throughput, sync speed, SNR margin etc using tools like PRTG.

Make	Model	Chipset	MaxDownRate	SNR tweak	DMT	SNMP
BT Voyager	205	Globespan	Yes	No	No	pass
Speedtouch	5x6 family	Broadcom	No	Yes	Yes	pass
Netgear	DG834(G)	TI AR7	No	No	No	pass
Belkin	7633	Broadcom	No	Yes	No	pass
Belkin	7632v1000	Conexant	Yes	No	No	pass
USR	9105	Broadcom	No ?	Yes	No	Yes
Draytek	2800	??	Yes	No	No	Yes
Westell	6100	TI AR7	No ?	Yes	No	Yes
Dlink	DSL-300T	TI AR7	Yes	No	No	Yes
Westell	6301	Broadcom	No ?	Yes	No	Yes

Note that some features may be available in individual versions of firmware and not in other versions, or may be confined to certain versions or batches of the stated model. It is common for manufacturers to maintain a model number through a range of alternative hardware & firmware configurations and the user may not be able to check this before purchasing.

The TI AR7 chipset has a good reputation on long lines, especially if combined with a TI DSLAM. MaxDownRate limits are helpful to sure instability, and SNR tweaks to get more speed out of a line with a high target SNR margin. SNR tweaking can also reduce speed and increase margin, when I tested a USR9105 it appeared to only tweak the margin up and the speed down.

When using these "semi supported" features like maximum down rate or SNR tweaking you may find the setting disappears on a reboot. An automated telnet program can be used to quicky restore favourite settings.

2006-07-25T16:18:00.000-07:00

Fixed !

The BT change had not been implemented until this morning. Now it's in, it works and the correct priority has been restored. The Max Premium line now runs at over 4M regardless of what the slower Standard line is doing, but the standard line drops noticeably whenever the Premium line is on stream.

Note how the upper (Standard) trace drops at 16:20 when the lower one rises, and when the lower one drops off at 16:30 the upper one rises to its full 1.5M (temporary go-slow after thunderstorm). Bringing the Premium line on at 16:50 knocks the Standard line down a fair bit, but taking the Standard line off doesn't result in an increase to the Premium line. Order has been restored.

2006-07-24T03:17:00.000-07:00

No fix yet.

Either it didn't happen last night or it didn't work. I had sort of convinced myself it was running better around 07:00 but testing after 09:00 showed the familiar pattern - a standard line always runs at 2.5M irrespective of the Premium line and while doing so it will hold down a Max Premium line at around 2M, with the Premium line stepping up to over 4M when the Standard line is done.

Download rate always around 263 kbytes/s on Standard, 200 - 620 on Premium.

2006-07-19T04:27:00.000-07:00

BT have discovered a problem....

the BRAS server which serves the affected circuits has a configuration problem which leads it to ignore the priority of Premium circuits. This is planned to be fixed Sunday evening so we'll see what Monday brings :-)

On another note, when Entanet were asked if their proposed traffic management would give priority to Max Premium or 20:1 business circuits over residential / 50:1 they said "that is an option on the list". Makes you wonder how much priority we're paying for - still, at least there's the extra upload.

2006-07-18T06:16:00.000-07:00

and then there were three....

We started up a new line with a new ISP on a MaxDSL standard circuit today, and discovered that it operates independently of the other Max Premium and Standard lines - no interaction whatever ! It is as if it is on a different exchange, which suggests it may be on a different virtual path from the exchange.

The new line is the 3rd trace below, the Premium line the centre and the standard reisdential MaxDSL is the top :-
Once again the "standard" Max residential line does its own thing and gets its full line rate, the only exception being the small dip at 2pm when a second similar line was added in.

The combination of 2 standard lines downloading at 2M at 14:00 was enough to flatten the "Premium" line to well below 2M. It recovered in steps after 14:00 as the residential lines finished their downloads.

The new line (bottom trace) didn't impact on the top trace, nor did it bother the Premium (middle) line much at 13:30. The subsequent download around 13:35 on the top line certainly caused a dip on the Premium line.

2006-07-16T13:56:00.000-07:00

Update on priority.

I thought about this some more, and did a test where I started 100M downloads on the standard line twice, and half way through each I started the same on the Max Premium line. Then I reversed the plan and started a download on the Premium line and part way through commenced the same on the Standard line.

The standard line on the lower trace clearly gets its way every time, whether it starts first or joins the Premium line part way through. It always gets priority!

The premium line, on the other hand, starts slowly especially if the standard line is downloading, and picks up some speed when the standard line finishes its download.

Whichever way you look at it, the Max Premium line does not deliver a Premium service.

2006-07-16T03:42:00.000-07:00

When is a priority not a priority ?

The "business" service MaxDSL Premium has a higher upload speed and is "given higher priority at times of congestion". Some ISPs, for example Griffin, interpret the product spec as Max Premium giving double the minimum throughput of standard Max. Max Premium is effectively the "up to 20:1 contention" product in the old language.

We have been experiencing poor performance on the Max Premium lines that feed part of the SWBB Wireless network and for some weeks now had observed that the Premium lines actually performed worse than the Standard lines on the same exchange from the same wholesale ISP (Entanet).

Our observation is that a Max Standard line will not drop its throughput if a Max Premium line comes online with a heavy download, but that the opposite will occur. Two standard lines affect each other to a small extent, but one or two standard lines can have a big impact on a premium line. So premium price gives discount performance and the priorities appear in practice to be reversed.

The top trace is the throughput of the Premium line. Just before 9am (Sunday test) it downloaded a 10M file when the bottom (standard line) was downloading a 100M file. The main observation here is that the standard line showed no effect whatsoever.

After 9am the premium line was set off on a series of downloads of a 100M file while the other lines were used periodically. Between 09:00 and 09:10 the bottom line makes a series of three downloads of the 10M file and the impact on the premium line can be seen by the three dips in its speed trace. The "office" line slows down when the "home" user makes a download.

Around 09:15 the two standard lines made a total of 3 downloads of the 10M file and "blipped" the throughput of the premium line.

At 09:20 the middle line started on a download of the 100M file, with the bottom line joining it after a couple of minutes. The combination of two competing downloads proved fatal for the premium line which dropped in speed to below 1000 kbits/s from over 4000. Both standard lines continued at over 2000 kbits/s - well over double the premium line despite having about half the maximum line speed.

When the bottom line finishes just before 09:30 the middle line gains some speed and the premium line also increases, finally restoring its original speed when the middle line is done.

So clearly the priority system is working contrary to expectations, reducing throughput of MaxDSL Premium lines on the same exchange in order to meet the bandwidth demand of MaxDSL standard lines.

[The 100M file source can supply well over 10 Mbits/s as witnessed by pulling 5M from a different exchange on a Standard line without impacitng the above line's throughput. The 10M file is taken from Microsoft.com and the 100M from a UK hosting centre.]

2006-06-01T06:11:00.000-07:00

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.

2006-06-01T03:25:00.000-07:00

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.