A BT guy climbed my pole, and I lost 9Mbps

[#1]

Good luck getting Openreach's official technical documentation then.

Have you got the documentation, can you supply a link or place a copy somewhere?

I did some experimentation by adjusting the SNR on my draytek modem and observed the changes in speed.

https://community.plus.net/t5/Fibre-Broadband/SNRM-target/td-p/1654577

I don't know if the modem sends the target SNR to the DSLAM which then makes the changes, but the point is by altering the SNR on the modem the speed changed.

This is what it looks like, note this is just a snapshot taken today, with no adjustments, I don't intend to disrupt an operational  system to prove a point.

[#2]

@Gandalf @LordFox @Enverex 

The question you need to consider is how the DSLAM 'knows' the state of the line (disregard line breaks, thats too obvious as nothing will come back).

Bear in mind the frequencies used differ between the uplink and downlink, so therefore SNR will not be constant across each  link.

It would be logical to expect the modem to pass the information back to the DSLAM, and visa versa, so each end may alter the transmit power accordingly. Also errors could be used to determine line status (but not too conclusive when Wendy the Welder starts repairing her car!! )

Cunningly if the modem does send SNR back to the DSLAM, this can be tweaked by the modem user (tsk tsk) to gain a bit more throughput.

So is there a modem-dslam protocol? (wireshark will not help here, you would need to analyse the RF (radio frequencies) to determine this.).

A tip, FTTC is basically radio based with a bit of fibre thrown in, a really rubbish solution, implemented due to government interference in things they are not capable of understanding.

I look forward to your thoughts.

[#3]

You CAN tweak the downstream SNRm target on a Lantiq chipset modem (if the UI supports it) and if you are on an ECI cabinet.

You cannot (or more accurately SHOULD NOT, as DLM can already tweak it right down to 3dB on a stable line) do so if you are on an Huawei cabinet or from any other chipset modem that I'm aware of.

Another caveat, I did this and after a month of a perfectly stable line, DLM suddenly decided to band my line so I couldn't do it any more.

As for the OPs initial problem, its likely they climbed the pole to fix another users line and when their broadband started working properly the crosstalk caused that 9Mbit loss.  Many people have seen around 10Mbit loss from just a single new user coming online, myself included.

[#4]

@alexatkin 

Hi

AFAIK Draytek use Lantiq chipset.

I did this when I went from ADSL 17Mbs to VDSL 20Mbs, DSLAM is Huawei, and got a small improvement.

Unfortunately my line is 1500m, over the golden 800m (Huawei actually state that the VDSL line should not exceed 1000m in their DSLAM document).

Still waiting for @Enverex to come up with the Openreach document.

[#5]

Why would I have the Openreach documentation? I'm not an Openreach engineer and I'm pretty sure they're not supposed to distribute them, hence my comment "Good luck getting Openreach's official technical documentation then".

[#6]

@Enverex 

FYI Openreach publish quite a lot of technical & pricing documentation on their website.

It would not be a surprise if you had managed to find the documentation regarding SNR and DSLAM on a VDSL circuit, and my suggestion was you backed up your statement with evidence.

[#7]

Interesting points folks, thank you all. I get the bit about 'how does the DSLAM know what SNRm to use", also about the RF (being a Class A Full Amateur Radio licence holder).

It seems my Zyxel modem can't do anything to my Huawei DSLAM. It has the commands (via telnet) to apply a delta to the VDSL SNRm, or set the max attainable speeds, but they have no effect other than to cause a resync at exactly the same stats.

The idea that BT might have installed/fixed another FTTC, and that has caused my rate drop, is plausible. Such is life. One day they might get around to bringing FTTP to my area, although I might be pushing up daises by then since my backwater isn't even in the 'on plan' stage yet.

[#8]

@LordFox 

Re Full Amateur license holder.

As one you don't need me to tell you about interference (REIN)  on phone lines, especially above ground lines, great big antenna's!!.

REIN affected my road, lost 4Mbs, so maybe you have a REIN issue, there is lots of info on the kitz pages.

Time to get the headphones on!!

73's