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[Retro~Burn]

I have an unusual issue. When I connect with three routers individually, I see a different connection rate with each one.

D-link DSL-G624T:





DrayTek Vigor 2830n:






Netgear DGN 1000:




I looked at the ADSL stats for the Vigor and DGN routers to confirm the test results. I'm curious, how would connecting with different vary the connection rate? Where my oldest (and slightly intermittently working) router is the one that provides the maximum connection rate with 1.73 being the zenith.

My guess is the settings are different or it's down to the individual firmware.

Any suggestions as to what it could be?

[c6em]

Different routers have different chipsets in them.
Some chipset are better on some line characteristics, others are better on different lines (short, long, noisy, quiet,variable etc).

Sometimes its even a case of what chip is in the router AND what chip is in the exchange DSLAM.  Some work better together....and some worse(!)

It all depends on how the chip manufacturer has decided to implement the firmware as you suggested.

I've certainly heard it stated on forums that to get the best out of your line particularly in marginal conditions you need to experiment with routers to see which is 'best'.
Note however that best is not necessarily the highest sync' speed.  Too higher a sync speeds for what the line really can handle often leads to higher errors on the line, which in turn will lead to re-sync events if excessive and too many re-sync events in a short time over an extended period will get the exchange equipment 'upset'.

[asbokid]

Lots of excellent info from c6em :-)

There is a very good paper on xDSL. It was authored by Matthew Ernelli, an employee at Telenor, Norway's incumbent telco.    [1]

In chapter 3 of his report, Ernelli performs lab tests using two different CPE modems (Broadcom & Texas chipsets) and two different DSLAMs (Conexant & Broadcom chipsets). 

In the lab set up, he forms a 0-loop (zero length loop) between the Central Office kit (the DSLAM) and the Remote Equipment (the CPE modem).  The 0-loop was kept as short as possible, so as to minimise the effect the cable might have on the measurements.

The measurements obtained from each of the modems (QLN and Hlin/Hlog) all had to be normalised before comparisons could be performed.

Ernelli offers no specific conclusions but it's clear that he wasn't impressed with the Texas chipset.

This would make an excellent project.  Probably the DSLAM kit is just too expensive to do it for anything other than a commercial project. No doubt big telcos like British Telecom have performed their own extensive in-house testing of all major chipsets.

I'm no authority at all, but would guess that the most critical component is the digital signal processor and the competence of the DSP codecs at recovering the symbols from the QAM scatterplot.  Different codecs will perform this better in different conditions, e.g. under varying levels of noise. There must be many other issues though.  The sensitivity of the receiver is doubtless important. Maybe it is complicated by non-linearity of the signal amps in the modem.  Internal noise seemed to be a problem for the (unidentified) chipset from Texas.

cheers, a

[1] https://eeweb01.ee.kth.se/upload/publications/reports/2008/XR-EE-KT_2008_003.pdf

[c6em]

That is an interesting report.
I see I'm going to have to get out my electrical engineering university notes from the early 1980's and remind myself of such things as modeling of transmission lines.

As my line sync's at the 20CN max limit of 8128 I've only had one opportunity for some sync level testing when as a result of a line fault the target SNR margin was raised.  Following the fault being mended and before I asked Plusnet to do a SNR reset I did experiment with 2 broadcom and 1 connexant based routers.
Obviously I could not test them at the same time for a true comparative test in identical conditions but rotated them round every 24 hours on the line for a week.  The two broadcoms were indisguishable from each other.
The connexant chipped one always sync'd at a lower rate but it always also recorded fewer errors on the line: so it was more "conservative" in its negotiations with the DSLAM.  What was interesting though was that as evening drew in and the SNR margin dropped as expected, the drop registered by the Connexant router was always far higher than that registered by the Broadcom ones: - and it dropped at a steeper rate as well. (This is a rural line 2.5Km long, all underground bar the last 50 yards via pole & dropwire).