ADSL2 speed drop, missing tones!!

Connection issues, drop outs or speed related faults for ADSL and ADSL2+ services

ADSL2 speed drop, missing tones!!

Postby amx on Wed Oct 12, 2011 6:00 pm

I have been having issues with my ADSL2+ speeds. Typically I used to get on average around 10mbps with a noise margin of around 12dB, which is fine. Howevery, I have been getting resyncs, which with the SNR I can get a maximum of 3mbps.
This roughly started around last summer.
I first blamed my Netcomm NB6plus4w rev1 for this, and thought over heating was the cause. I opened the case and looked for bulging capacitors, but none were there. I kind of persisted with the problem for many months, because it was happening about once everyday, and sometimes having good connection for a couple of days, so not too distressing. Also if I were happy with 3mbps, it can stay fine for a long time (I have had for about a month before I really needed more speed).

A couple of weeks ago, that I started needing a more stable connection, I opened the modem again and I saw a slightly bulging cap this time.
I replaced it, but the behaviour was the same. I still blamed the modem. So I pulled the plug and bought a TPlink w8960. The problem is still the same!!

I have checked every hardware connection, and I am 100% sure it is not in the house. Basically the phone line comes direct to the house, after five meters goes to a sockt, then goes to the modem through a 3m extension cable ( I have tried a 20cm piece as well, so no it is not the cable). I have swapped every component in the system, and have soldered were I had doubts about copper being encrusted with some oxide. Still the same.

I have tried to see if there is any sync between any of my wireless devices feeding off the modem and the speed drop. From what I have seen over the many months, there is no relationship.

I used DMT to have a closer look at all the modem parameters, what I have noticed is that although the reported noise margin and most other parameters are very close for both my low/high (i.e. 3/10mbps), in the low sync speed mode I can see that a lot of tones above 1MHz are missing, and below that it seems that the line has been filtered by a comb filter.

I have tried three different filters (theoretically you wouldn't need a filter for a naked plan, but I just tried), and no filter, and all the same.

I was actually on ADSL2+ naked plan, and as I mentioned there is just a single direct connection to curb line. As of a couple of days ago I changed to the OT-100 plan, but the quality hasn't changed. I had a good connection for three days at 9mbps and then it resynced to 2.9mbps again.

The problem is that once it goes to the low speed mode, the only way to get back to a high speed is to reboot, but for that I have to wait a while (I haven't measured exactly how long, but I have tried within 5/10/15 minutes from a speed drop and it still syncs at low speed), but after an hour or so it is ok again.

My guess is that there is something that is either having a severe interference with the line, or an extra load (maybe the result of a drenched pit!!), or maybe a fault in the switch box, or a setting at the switch or server that cuts off higher tones.

Before I raise a service request, has anyone else seen this behavior? Is there anything else that I can test?

-------------------------------------
Edit1:

I have tried many different modem settings such as enable and disabling different adsl profiles, enabling/disabling SRA, doing factory resets to erase all existing profiles, etc, to no avail.
Attachments
adsl_low.jpg
This is the DMT screen capture for the low speed. Notice the sparse tones and the missing tones above 1MHz.
adsl_low.jpg (211.87 KiB) Viewed 1001 times
adsl_high.jpg
This is the DMT screen capture for when the modem has synced at a high speed.
adsl_high.jpg (167.14 KiB) Viewed 1001 times
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Re: ADSL2 speed drop, missing tones!!

Postby ShaminG on Wed Oct 12, 2011 6:46 pm

Dear amx,

I have conducted few tests on your connection and could see few issues. Hence, I have created a support ticket with the Ticket ID: 4495130 :) .
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Re: ADSL2 speed drop, missing tones!!

Postby Dazzled on Wed Oct 12, 2011 6:58 pm

That app saves some work. I use an expect script on the telnet interface to grab that stuff. 2-15 bits per bin are needed, so the bits from the missing bins are got from the better bins (bit allocation), but the effect is a reduced line speed. If there is insufficient SNR in a channel it is marked as unusable. There are a couple of unusable parts anyway (eg guards).

+1 for ShaminG's post.
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Re: ADSL2 speed drop, missing tones!!

Postby ShaminG on Wed Oct 12, 2011 7:03 pm

I have created a support ticket with the Ticket ID: 4495130 :) .


As discussed, I have conducted the test at 8 pm. You can plug in the devices back. It does shows the errors even without the equipment.

I have escalated the issue with our wholesale supplier for further investigations.
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Re: ADSL2 speed drop, missing tones!!

Postby amx on Wed Oct 12, 2011 8:09 pm

Thanks ShaminG,

I will post here about the outcome of the tests and the resolution.
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Re: ADSL2 speed drop, missing tones!!

Postby felix on Wed Oct 12, 2011 9:17 pm

Sophos won't let me use the tool for my Netcomm. Reports spyware and the website reports as high risk. Anyone else got this problem?

http://dmt.mhilfe.de/

shame cause it looks like a good tool.
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Re: ADSL2 speed drop, missing tones!!

Postby Dazzled on Wed Oct 12, 2011 9:48 pm

Felix, it has to make a telnet-type connection, presumably on port 23, to request the data. Sophos and perhaps other anti-malware scans might think that resembles a zombie server.

You can get a similar result, but much less pretty, after ordinary telnet login (eg on a Dynalink - adsl show, followed by the --arg for the item you want), and sending the results to a spreadsheet. Commands vary between brands - most modems will list the available commands.
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Re: ADSL2 speed drop, missing tones!!

Postby amx on Fri Oct 14, 2011 4:07 pm

Today the subcontractors came. They spent more than four hours going from pit to pit and to the pillar.
I was with them most of the time, as couldn't work anyway (working from home).
The problem was with a short (not a complete short more like a leakage) through the line. It happened to be from the cables in one of the pits.
The cable had water in it, and despite all effort, apparently the cable enclosure had been breached. Cleaning it up and repatching didn't help, and they were saying that this could be due to the gels in the cable, which overtime can harden and cause trouble.

The line could not be repaired without cutting up the cable, and they said that they are not allowed to do that. So they had to find a good spare cable and swap the line pit by pit, and at each point ask optus to run the tests and pass it.
At a couple of points the line resistance was 800 Ohm and optus did not pass. Aparently it has to about 1000.

The credit should go to the contractors with persisting with it and not taking shortcuts.

The guys that worked on this issue should really be commended.

The line is back up now. I will monitor and report back in a couple of weeks as of the stability.
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Re: ADSL2 speed drop, missing tones!!

Postby KavindaS on Fri Oct 14, 2011 4:19 pm

amx wrote:Today the subcontractors came. They spent more than four hours going from pit to pit and to the pillar.
I was with them most of the time, as couldn't work anyway (working from home).
The problem was with a short (not a complete short more like a leakage) through the line. It happened to be from the cables in one of the pits.
The cable had water in it, and despite all effort, apparently the cable enclosure had been breached. Cleaning it up and repatching didn't help, and they were saying that this could be due to the gels in the cable, which overtime can harden and cause trouble.

The line could not be repaired without cutting up the cable, and they said that they are not allowed to do that. So they had to find a good spare cable and swap the line pit by pit, and at each point ask optus to run the tests and pass it.
At a couple of points the line resistance was 800 Ohm and optus did not pass. Aparently it has to about 1000.

The credit should go to the contractors with persisting with it and not taking shortcuts.

The guys that worked on this issue should really be commended.

The line is back up now. I will monitor and report back in a couple of weeks as of the stability.


Thank you for your valuable feedback. As you mentioned supplier tech has done some changes and we have sent you a text accordingly. You may monitor the service and let us know if you require further assist. :)
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Re: ADSL2 speed drop, missing tones!!

Postby amx on Fri Oct 14, 2011 7:16 pm

I just ran DMT again to see how it looks like. Compared to what it was before when I was getting high speed the response has less notches in the 200-300 tones, but a few small ones around 390.

Interestingly the downstream tx pwr is reduced from 19 dB to 0 dB, maybe an indication of the short telling that the switch was trying to put a lot of power out to get the required data rate.

Also the "Channel characteristics" (the bottom plot) looks different, similar to the lower sync speed before. I am not sure how to interpret this one.

As I mentioned I will still monitor the line, but there is some signs of improvement.

It would be good if exetel or other ISPs come up with some similar tools that they can give customers to monitor the connection, and then upload the data to the ISP for diagnostics.
Attachments
adsl_fixed_dmt20111014_2053.jpg
DMT response after the line fix.
adsl_fixed_dmt20111014_2053.jpg (210.29 KiB) Viewed 918 times
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Re: ADSL2 speed drop, missing tones!!

Postby Dazzled on Fri Oct 14, 2011 9:10 pm

I'm glad they fixed the line, as that moisture penetration problem is very common. The design has since been changed.

There is a readable description of what happens in the modem (without the maths) at http://www.kitz.co.uk/adsl/adsl_technology.htm and another at http://en.wikipedia.org/wiki/G.992.1.

The DMT tool is only for Windows, and it essentially downloads the various status figures from the telnet interface of a range of modem-routers. Unfortunately, while pretty well all modems run Linux, they each have it arranged differently, and the various utilities are maker specific. Very few makers and their firmware programmers (eg Montavista) publish this stuff, not even a telnet CLI manual, being anal about "intellectual property" even when dealing with open-source software. You have to poke about in telnet. Eg, this script fragment works on a Dynalink, and shows the sort of thing DMT is doing out of sight:
#!/usr/bin/expect
spawn telnet 192.168.1.1
expect "Login name:"
send -- "admin\r"
expect "Password:"
send -- "admin\r"
expect ">"
send -- "adsl info --show\r"
expect ">"
send -- "logout\r"
exit

Standard Linux/Unix commands like cat /proc/net/dev (list the kernel transmission stats) usually work well. One developer imitator has started to pretty up this at http://www.spida.net/projects/software/ ... ex.en.html but he only has one modem family available at present.

There are replacement open source firmware projects, eg DD-WRT and Tomato (Polarcloud), and the users of these get full access to all information.
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Re: ADSL2 speed drop, missing tones!!

Postby amx on Sat Oct 15, 2011 10:58 pm

Unfortunately, it happened again. Today when I came back home, I noticed the speed had dropped to below 3mbps, with all high speed tones missing. I tried several reboots to finally get a 10mbps speed.

I am really running out of things to test here. What I think is happening is this:

For some reason the ATU-C goes into a low power mode, or maybe as a result of an event (a shock, high voltage interference, some solar storm or ET phenomenon) decides to change the profile to a low speed one, and holds onto this profile.
If this is a power mode issue, the first thing to ask is "does exetel support power mode switching?".
If no, then is there a log that indicates something disruptive has happened, or any reason why a new lower speed profile has been used?
If there is no log, can this be set up to monitor the line behavior over some period?
If exetel uses power state switching, then can this be the cause?

Could this be a fault at the exchange?
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Re: ADSL2 speed drop, missing tones!!

Postby chathurad on Mon Oct 17, 2011 10:46 am

amx wrote:Unfortunately, it happened again. Today when I came back home, I noticed the speed had dropped to below 3mbps, with all high speed tones missing. I tried several reboots to finally get a 10mbps speed.

I am really running out of things to test here. What I think is happening is this:

For some reason the ATU-C goes into a low power mode, or maybe as a result of an event (a shock, high voltage interference, some solar storm or ET phenomenon) decides to change the profile to a low speed one, and holds onto this profile.
If this is a power mode issue, the first thing to ask is "does exetel support power mode switching?".
If no, then is there a log that indicates something disruptive has happened, or any reason why a new lower speed profile has been used?
If there is no log, can this be set up to monitor the line behavior over some period?
If exetel uses power state switching, then can this be the cause?

Could this be a fault at the exchange?


Dear amx,

I have checked your connection and could not find any issue. Please find the answers to your questions bellow:

For some reason the ATU-C goes into a low power mode, or maybe as a result of an event (a shock, high voltage interference, some solar storm or ET phenomenon) decides to change the profile to a low speed one, and holds onto this profile.
-- The Profile has not been Changed

If this is a power mode issue, the first thing to ask is "does exetel support power mode switching?".
--Exetel does not support power mode switching

If no, then is there a log that indicates something disruptive has happened, or any reason why a new lower speed profile has been used?
--We have no logs on the speed changes on your connection

If there is no log, can this be set up to monitor the line behavior over some period?
--The speed changes can only be monitored at the customers end. You may run speed tests to monitor this.

If exetel uses power state switching, then can this be the cause?
-- Exetel does not support power mode switching

Could this be a fault at the exchange?
--According to the line test and the field technician attended few days ago, the exchange port confirmed to be in full working order.

Regards
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Re: ADSL2 speed drop, missing tones!!

Postby amx on Mon Oct 17, 2011 2:57 pm

Thanks Chathurad,

It looks like that I will have to dig deeper on my side and see what is triggering this.
I will setup a full log server on the modem, and will try to write a perl
script to grab the data off the modem, and see if I can find the root of the problem.

Thanks
AMX
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Re: ADSL2 speed drop, missing tones!!

Postby Dazzled on Mon Oct 17, 2011 3:33 pm

Old NB6 firmware supports SNMP, which makes it a bit easier. For just the line rates telnet is easiest.
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