OpenWrt Modem VDSL2 Profile 35b which can change DSL max speedrate

Hi all,

i've a German VDSL2 250Mbit/s Down 40Mbit/s Up Profile 35b internet connection. The connection isn't stable and i have to decrease the down speed manually at my side. Is there any (OpenWRT) modem which has this feature? I've bought a FritzBox 7530 but can not find something about that feature on the internet.

Only this topic with no solution: Changing max vdsl sync speed

Thank you!

I think you can use option ds_snr_offset '0' in /etc/config/network* to modify the SNR margin the modem enforces (or probably what it reports to the DSLAM). And you can install recent snapshots (starting from either yesterday or a few days ago) on fritzbox 7520/7530 (ATTENTION some firmware files eed to be copied manually after first boot). So this might actually help, but I am not sure how well this option works on profile 35b and vrx518 modems....

In my experience though this is unlikely to fully help you and the first step needs to be to figure out why your link is so unstable in the first place, and then try to remedy the root causes if possible.
I note that for unstable link's Deutsche Telekom's DLM system should by itself intervene and step by step reduce the allowed maximal sync speed until the link is stable again. Do you know why this has not happened or why if it did that did not help?

To start out in diagnosing the issues, maybe post a screen shot of what dsl-gui reports for your Firtzbox (still running FritzOS) that might indicate what kind of issues your link suffers from.

*) I think this field is also exposed in the luci GUI under networking in s dsl specific tab, but not sure about the details.

Hi @moeller0,

thank you for your detailed answer. The technician was able to measure the error (95 errors per minute, maximum is 1200) already in the distribution box and can therefore exclude that there is a problem inside the building. So I have no way to fix the cause. Since the error rate is so low, the Telekom does not want to do anything further, except a downgrade. Currently I have a DrayTek Vigor 167 modem with a Zyxel USG Flex 200 router. A Telekom technician recommended a FritzBox (7590) because it allows me to throttle the line manually. I chose the 7530 because it supports OpenWRT and I only need a simple modem. The router Zyxel USG Flex 200 should stay for now, in the long run I probably want to switch to OPNsense. The FritzBox 7530 will arrive tonight. I hope I manage to run the FritzBox with the original firmware in modem mode, then I'll post a screenshot. I have no experience with a FritzBox and OpenWRT yet. But I already have OpenWRT running on other devices, so this would be a good option for me.

Telekom's system for setting the data rate does not handle my borderline case well. The technicians do not have a solution for this :frowning:

P.S. I can still change the modem if there is a modem that is more suitable for this case

Here is something to try:
simply disconnect your modem from the TAE socket multiple times with little time in between an repeat that for a few minutes spread over say 30-60 minutes. That should get alarming retrain counters into the DLM system to which it should respond with the only thing it knows reducing your "sync corridor". You might have to repeat that exercise every week/fortnight if the DLM upgrades you again.

I honestly can not answer that, lacking both a Fritzbox modem (last I used one was 2008) or a profile 35b link. To try OpenWrt on a semi-modern modem I think 7530/7520/7590? are currently the best and only bet. Given that vrx518 modems are rumored to have better analog part quality I am pondering replacing my xrx200 OpenWrt bridged modem with a 7520, conditional on being able to get one cheaply enough or my xrx200 dying ;).

Nice idea :smiley: But no solution for me, the conection must be always stable

Oh i didnt see the 7520. Is it nearly the same hardware? I think it is also sufficient for me

Well, the thing is that DLM is quite conservative in upgrading a link after reducing the syncrate, so such a game might only be required every fortnight, but sure doing that routinely would be a pain in the ass (I would probably think about getting a WiFi/network controlled powerstrip and imitate the retrains from cron automatically).

Anyway, more often than not unstable links have clear root-causes for their instability and occasionally understanding that cause can help in finding a way to increase stability again.
E.g. in my case I retired my TAE in the middle of the apartment and replaced it with a simple old RJ45 style ISDN socket almost directly where the POTS wires enter my apartment; getting rid of ~12m of low quality internal wiring, often close to power cables significantly increased the SNR on my link and helped stabilizing my internet connection. (Looking at SNR/bitloading/Hlog/QLN plots confirmed that the quality of the DSL connection increased massively from that change alone). If I could have done so, I would have replaced the wiring all the way to the APL, but this being a rented apartment in a larger house it was not realistically feasible. Not saying this will work on your link as well, just as an illustration that occasionally try to dig deeper can help improving one's link quality.
Ultimately, the real 'solution' is to replace the heroic but tricky "analog" electric signaling over coper wires with much more robust photons through glass fiber, but given the speed of the FTTH roll-out in Germany it still can be a good idea to try to stabilize one's DSL link so that waiting for FTTH is not a painful experience.

My TAE 1. was renewed in 2013 with good cables to APL (~5m). Two cables come into my house. The thechnican has switched to the other cable and my up SNR increased from 7 to 9.4dB but down still ~6dB when it gets unstable. It is around a week at ~7.3dB than my connection will be upgraded by Telekom and SNR down decreases to 6db. After multipe disconnects (around 3-20) it will be stable again for some days.

FTTH will come in 2025 in my area :smiley:





DSL Version5.12.24.0_B_A60901

Power Management Mode--

Vendor IDfe004452 41590000

ATU-C Information

Vendor IDb5004244 434dc27f

Line Statistics

Downstream Upstream
Actual Rate 250073 kbps 42460 kbps
Attainable Rate 299175 Kbps 47288 Kbps
Path Mode Interleaved Fast
Interleave Depth 16 1
Actual PSD 0 dB 0 dB
SNR Margin 7.4 dB 9.3 dB
Near End Far End
Trellis - -
Bitswap ON ON
Attenuation 5.9 dB 0.1 dB
CRC 0 0
FECS - -
ES 51 s 0 s
SES 15 s 0 s
LOSS - -
UAS 3463 s 3463 s
HEC Errors 0 0
RS Corrections - -
LOS Failure 0 0
LOF Failure 4 0
LPR Failure 0 0
NCD Failure - -
LCD Failure 0 0
NFEC - -
RFEC 10 8

That is the standard SNR margin on Telekom links IIRC, this probably also means that you do not reach full sync as you then seen SNR-margin limited.

OK, so that is DLM in action, as I said you might as well cause these reconnects manually yourself thereby removing the stability data that convinced the DLM to increase the maximal sync on your link again as a stop-gap measure. That will not magically increase your links stability but will allow you to schedule the disruptions to time windows where they are less problematic.

In my limited experience TAEs are pretty lousy connectors prone to oxidation, so I would still try to remove it out of the loop.
And if I had my own house I would place the modem directly next to the APL and use the shortest possible connection (use ethernet wiring to connect the modem next to the APL with the router placed for best WiFi coverage).

Excellent, that seems like an acceptable time to wait, why not simply downgrade the link to 100/40 Mbps then? This probably should be stable?

I assume that adding serious SNR to your config like +5dB should show decrease in sync speed
option ds_snr_offset '50'
But You might try even more
I don't think fall back to VDSL2 17a 100/40 will fix your libe but SNR should be much better

I'm on VDSL2 with lantiq xrx200 but this should work on 35b profile as well

Good point, this really depends on where the instability comes from.
Profile35b is pretty sensitive to powerline devices (these typically offer some DSL compatibility modes that essentially are tailored for profile 17b's 17 MHz and tend to interfere severely with the upper half of profile 35b). It would really be good to see SNR and bitloading plots for this link to get some idea where the problems come from...

@baba2k you (or your neighbors) do not happen to use PLC/power-line adapters? Because these are known problematic for DSL/VDSL, as both happen to use overlapping frequency bands and can interfere with each other, but PLC being local typically sees not issues while DSL becomes unhappy pretty fast.

The next smaller packet is 100/25 Mbit/s for the same price.. do not use power-line adpaters. I am living in a detached house, but have no influence at my neighbors :slight_smile: Before the upgrade it was a 50/25 Mbit/s 17a connection and it was very stable.

Telekom says that it is useless to change the line inside the house, because the fault can already be measured in the distribution box. The APL and the TAE 1 socket may only be connected by the provider in Germany. They now want to open a fault ticket again and send a technician out.