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.

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.

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.

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.

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.

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.]