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Author Topic: VDSL RFI notching questions / ham radio knocking out router  (Read 420 times)

atomic cafe

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VDSL RFI notching questions / ham radio knocking out router
« on: February 25, 2019, 07:13:53 PM »

Evening everyone - I'm new here and new to telecomms stuff like routers etc so please bear with me.

I'm a licensed Ham Radio operator and I'm finding that every time I transmit on certain frequency bands I kill the link to my router.  I know that my transmissions are clean.  This is with a FTTC "fast fiber" BT/Plusnet service that is pretty quick (60Mbps) with overhead copper to the house so I am ASSUMING that it's VDSL. 

At the moment (see rsgb.org) there are a lot of problems with interference to/from Ham Radio from VDSL.  Ofcom and BT deny that there is a problem, but then Ofcom lied about their own report into interference from powerline ethernet adapters a few years ago until they were forced to unbury their report so there's nothing new there.

The problem is that VDSL uses the amateur bands at 1.8, 3.5, 7, 10 and 14MHz.  For some reason BT cannot understand the laws of physics (ya canne change the laws of physics, Cap'n ...etc) which are pretty clear:  put a radio frequency signal into an unshielded and unbalanced wire and becomes an a-n-t-e-n-n-a.

VDSL standards allow for - and in fact specify - that amateur bands should be notched out of VDSL, but BT don't do it because it decreases line speed and Ofcom are simply shirking their jobs.

So....

I see that I can buy a Huawei Openreach-branded modem (HG612 3B) and put original Huawei firmware on it - as people have kindly shared on this site.  I also see that there are Huawei commands to apply notching:
http://support.huawei.com/hedex/pages/EDOC100015172931189794/01/EDOC100015172931189794/01/resources/cmd/xdsl_rfi-global-config.html

I understand (though correct me if I'm wrong) that if the BT cabinet sees that the device in the home is not responding to a particular frequency then it will avoid it.  I believe that someone out there has made a quarter length open stub of transmission line at the correct frequency (which acts as a band-stop filter) and has successfully notched out certain frequencies.

So here's the question and the plaintive cry for help:

Does anyone know how to apply RFI notching on this modem? 
Will it work?  Can you do it, or are these commands for a different device?
Where do I change config files or type in commands?

Any help would be gratefully received!

Thanks

Dave


« Last Edit: February 25, 2019, 07:43:31 PM by atomic cafe »
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j0hn

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #1 on: February 25, 2019, 07:52:31 PM »

This post may interest you.

No idea if you can disable certain tones. Just because the modem can doesn't mean the DSLAM will allow it.
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Plusnet FTTC 80/20 -  ECI now Huawei cab
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Zyxel VMG1312-B10A bridged with 1508 MTU + Asus RT-AC68U running Asuswrt-Merlin

atomic cafe

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #2 on: February 25, 2019, 08:13:11 PM »

This post may interest you.

No idea if you can disable certain tones. Just because the modem can doesn't mean the DSLAM will allow it.

Thanks, John.  I'd found that post before and had of course tried a number of different ferrite mixes including 31 and 43 and had stuck a 470pF capacitor across the line - all to very little effect.  I also fitted an up to date BT VDSL filter socket. 

The original poster had a line that was notched as it is supposed to be - i.e. to avoid interference to the licensed, primary users of the 1.8-2MHz band as agreed by international convention.  BT are ignoring that convention in order for them to squeeze a bit more bandwidth out of their crappy copper wires.  I believe that if the DSL was configured not to use 1.8-2, 3.5-3.8, 7-7.2, 10.1-10.15 and 14-14.3MHz then the problem of interference to and from Ham Radio would not happen.  That's what I want to experiment with.

Like I said before, someone created a bandstop filter from transmission line (a well known technique but leaves 10m of cable to coil up for 7MHz) and that had the effect of stopping the DSLAM from using that frequency.  Presumably there is some sort of error check and reconciliation like good old X25 used to do - and if responses on a particular frequency fail to check out then the DSL avoids that frequency.  This would make things more error-proof in a world where shortwave broadcast stations get picked up on telephone wires at night and disrupt data flow.

I'd like to try and get a modem to say **** off, I'm not talking to you on XYZ notches and see if the DSLAM plays ball by putting traffic elsewhere.

Anyway, thanks for getting back to me, it's appreciated

cheers

Dave
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ejs

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #3 on: February 25, 2019, 08:50:58 PM »

Those Huawei commands you found for rfi notching appear to be for the DSLAM, not your modem.

Maybe something might be possible with "xdslcmd snrclamp ..." but I have no idea what the exact parameters for that command are.

I can't really see how the DSLAM could disallow it, particularly for the downstream. If the modem says that the received SNR for a particular tone is off-scale low making it unusable, I don't see how the DSLAM could know if that's true or not.
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tubaman

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #4 on: February 26, 2019, 09:53:29 AM »

Have you tried using a different brand of router, as it may just be that your's is susceptible to your broadcasts whereas another may not be so?
 :)
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atomic cafe

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #5 on: February 26, 2019, 12:49:19 PM »

Hi Tubaman - I tried a Netgear D7800 but it was just the same. 
Itís been suggested to me by another amateur they I should ask Openwretch to balance the line as an unbalanced line will cause more problems.  In theory twisted pair is balanced even if itís unscreened.  I always use s/ftp and properly screened rj45  plugs around the house rather than utp. The copper drop wire from the pole is over head.

The problem is that BT have a crappy snr (3db) and have not notched out the amateur bands like they have everywhere else in Europe.  If the two devices use the same frequency then even a dolt would know to expect mutual interference.  Do you know of any more rf immune modems?

Thanks for the idea, itís appreciated
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j0hn

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #6 on: February 26, 2019, 12:57:52 PM »

If the SNRM only being 3dB is what you believe is part of the issue you can always try increase this back to the default 6dB (or higher) and see if the connection holds when using the HAM radio.

With a Broadcom modem compatible with DslStats you could monitor/graph the effects of using the HAM radio and see what effect different frequencies have on the line.

Maybe something might be possible with "xdslcmd snrclamp ..." but I have no idea what the exact parameters for that command are.

I can't really see how the DSLAM could disallow it, particularly for the downstream. If the modem says that the received SNR for a particular tone is off-scale low making it unusable, I don't see how the DSLAM could know if that's true or not.

Happy to test this if you can provide the necessary command in full.
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retx low @ 3dB target SNRM
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johnson

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #7 on: February 26, 2019, 01:20:03 PM »

I have a faint memory of someone using an asus router/modem and being able to select tone ranges. First google hit is this:

https://forum.kitz.co.uk/index.php/topic,14399.msg274583.html#msg274583

So I think its possible to have some control over tones used, but it may be just setting the lowest/highest, which is probably not useful your your situation.
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j0hn

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #8 on: February 26, 2019, 01:31:24 PM »

Disgusted by the suggestion of using a Mediatek chipset!  :P

If on a Huawei cabinet that would be the end of G.INP, but that's insignificant compared to the rest of the problems with the chipset.
Just read that that and see why Ixel was trying to disable tones. The chipset is pretty horrendous.
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Plusnet FTTC 80/20 -  ECI now Huawei cab
retx low @ 3dB target SNRM
Zyxel VMG1312-B10A bridged with 1508 MTU + Asus RT-AC68U running Asuswrt-Merlin

krypton

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #9 on: February 26, 2019, 02:41:29 PM »

With the xdslctl connection --tones command it should be possible to select the used tones. But sadly it has no effect on my vdsl connection. It seems to work only on adsl.

The problem is that BT have a crappy snr (3db)...

With the broadcom chipset you can limit the data rate to increase the SNRM. Probably this would help to get your connection stable. Normally bitswap should automatically swap out disturbed frequencies to other unaffected ones but I don't know if there are enough unaffected tones with only 3db SNRM.
Do you have root access to your router so you can take a look with dslstats what happens?
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johnson

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #10 on: February 26, 2019, 03:07:10 PM »

Agree j0hn, shouldnt spread its use.  :-[

Krypton has the right idea, if you lower your sync rate and the interference is as localised to specific frequencies as you say the bits should swap out to other tones rapidly without causing a drop of sync.

My bitloading graph looks like swiss cheese when certain neighbours turn on devices, the system is very resilient if you dont mess with it too much.
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atomic cafe

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Re: VDSL RFI notching questions / ham radio knocking out router
« Reply #11 on: February 26, 2019, 05:44:14 PM »

Well at the moment I just have the bog standard Plusnet router like the BT Hub 5.  I havenít done anything with that to get root access and wouldnít know where to start.  Can you recommend and particular modem or router thatís worth experimenting with?
Cheers
Dave
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