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Author Topic: BitLoading  (Read 6843 times)

ryant704

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BitLoading
« on: April 18, 2013, 12:36:34 PM »

Hey guys, not really a problem... maybe.

I've had packet loss issues for the last 2 months which has cleared by itself but I had a REIN engineer here a couple of weeks ago who reported they found a very low bit/tone with a weird result. He didn't mention what this weird result was but said he is going to look into, then it was cleared. Though I'm just looking for how a BitLoading Graph is actually meant to look as mine has a dip in SNR which looks to be wrong from my opinion though I don't have a clue how the graph is meant to look.

My graph

http://tinypic.com/view.php?pic=2nsq47&s=6

Just before the 300 tone you can see it take a massive DIP into the BitLoading is this normal or?

Cheers :)
« Last Edit: April 18, 2013, 12:50:24 PM by ryant704 »
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asbokid

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Re: BitLoading
« Reply #1 on: April 18, 2013, 01:21:39 PM »

The two graphs (Bit Loading and SNR) are linked but should be viewed independently.

The drop in SNR around DMT#300 will be due to line noise - hence the drop in the Signal to Noise Ratio.   DMT#300 is slap-bang in the middle of the frequency band used for medium-wave broadcast radio.

So it is possible, perhaps even likely, that the big trough is due to radio frequency ingress from a broadcast radio station.  If so, there's not much you can do about it.    Unless the radio transmitter is poorly tuned - e.g. a pirate station - it tends not to be a major problem.  Rarely more than three tones are knocked out from the interference.

I haven't recently used Eric's excellent programme but can you zoom in on that graph to check the 'damage' to the Bit Loading from the RFI (if that is what it is) ?

cheers, a
« Last Edit: April 18, 2013, 02:08:26 PM by asbokid »
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ryant704

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Re: BitLoading
« Reply #2 on: April 18, 2013, 01:29:34 PM »

My line SNR is fine, 5 - 6 daytime, though it can get as low as 3 in the morning though is normally around 4 minimum.

I can post a few uncompressed pictures, I will post some tonight.
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Bald_Eagle1

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Re: BitLoading
« Reply #3 on: April 18, 2013, 04:39:35 PM »

As your graph is displaying 'Shared/Other' bitloading with no gaps (coloured blue), I presume you are still using the original unlocked firmware with the original BLOB.

The gaps in the red DS & green US bitloading are of concern though.

I have only seen such gaps on connections with physical 'issues' such as bridged taps etc.

Your Hlog graph may identify issues at the same tones, which block bitloading at those frequencies.

However, it is possibly a REIN issue of which I have seen very little evidence & have little first-hand knowledge.

I have attached a montage of the relevant graphs from my own low speed connection, using data obtained via the the original firmware.

You can see that my connection is quite noisy from the SNR (not SNRM) graph & QLN graph, but doesn't really display any physical issues as the Hlog graph is fairly smooth (in the D1 & D2 band frequencies that my connection can actually use).

I suspect the 'noise' is due to increased crosstalk as more users are being connected.

I live in a very open area on very high ground so if anyone would see radio transmitter interference I imagine it would be me.

I live nowhere near Wales, but I can receive very strong Welsh TV signals & may years ago when using CB radio, I saw very strong signals when talking with people based in wales.

In the evenings, I get very strong foreign radio stations, but they don't seem to have too much effect on my VDSL2 connection.

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ryant704

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Re: BitLoading
« Reply #4 on: April 18, 2013, 05:52:05 PM »

I've just put the unlocked modem back on and updated the firmware, I will post some graphs later when I've got a some data. I've never used QLN or Hlog because I have no idea what they are though have enabled them.

The REIN engineer looked for crosstalk and said there was basically none. I already have a RF filter on my line, this was installed over a month ago this was 2/3 weeks before the REIN engineer came.

What's the best way to try and detect RF? there is a small home aerial station about 50/60 ft away from the cabling along the road where my cable is laid.

Though I doubt I can get this looked into anymore as BT have cleared my local loop and the wide area network. Though last time when I had speed problems I had to point it out to them, then after 5 months they listened and it was fixed within a day.

I live in a very rural part of England, Norfolk to be specific.

The packet loss has been clear for just over a week, compared to how it use 2 be anyway...

You can see a graph here, I have these bursts of ping spikes. Do you have any idea what they are?

http://www.thinkbroadband.com/ping/share/94fd9595af0544134bbd247a1f513aa3-18-04-2013.html

I'm pinging a SamKnows probe through DMZ

Here is a graph when I'm pinging my PC (wirelessly)

http://www.thinkbroadband.com/ping/share/1953c4f31e2f113d632e3f9bb28eece1-15-04-2013.html

When the ping rises that is when I'm streaming...
« Last Edit: April 18, 2013, 05:56:40 PM by ryant704 »
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asbokid

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Re: BitLoading
« Reply #5 on: April 18, 2013, 07:29:29 PM »

I've just put the unlocked modem back on and updated the firmware, I will post some graphs later when I've got a some data. I've never used QLN or Hlog because I have no idea what they are though have enabled them.

QLN and HLog are an integral requirement of G.993.2 - the ITU-T specifications for VDSL2. 

QLN is a measure of Quiet Line Noise - the background noise from RFI and other forms of external interference on your line.
Hlog is a measure of the attenuation present across the frequency bands.

Quote
The REIN engineer looked for crosstalk and said there was basically none.

Unless you are the only person served by your cabinet, there must be some crosstalk.  It will increase over time as more people subscribe to VDSL2 from your cabinet.

Quote
I already have a RF filter on my line, this was installed over a month ago this was 2/3 weeks before the REIN engineer came.

What sort of RF filter is it?  It's conceivable that it's actually degrading the xDSL signal, since VDSL uses the same frequencies as AM radio (LW, MW and frequencies up into the amateur radio bands)

Quote
What's the best way to try and detect RF?

By plotting the QLN graph for each subcarrier.

Quote
The packet loss has been clear for just over a week, compared to how it use 2 be anyway...

In the network protocol stack, 'packets' are a layer 3 entity.  The quality of the DSL connection from the CPE to the DSLAM has no bearing on packet loss.  Packet loss and packet delay are due to upstream switching issues - undesirable capacity problems such as buffer overflows at routers, and intentional limits set through traffic management and traffic policing policies.

Quote
You can see a graph here, I have these bursts of ping spikes. Do you have any idea what they are?

http://www.thinkbroadband.com/ping/share/94fd9595af0544134bbd247a1f513aa3-18-04-2013.html

Capacity issues, traffic management, traffic policing.  That sort of thing.  All layer 3 issues.

cheers, a
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ryant704

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Re: BitLoading
« Reply #6 on: April 18, 2013, 07:46:59 PM »

I do have rather a small uptake at the minute, about a month ago there was around 20 connected since the cab had been installed in September.

Since the install I have noticed lower SNRM at night. Normally goes down to 4.3/4 but it started hitting 3 some nights now since this install but could just be a coincidence...



Various images attached now, including the RF filter.

If you want any more just say, it has now changed shape in the expanded view. I've edited with the bold line being how it use 2 be, it was like this for months maybe the REIN engineer did something...



I also have various dips on the BitLoading where that tone/bit isn't being used, there is probably about 15 max. About 7/8 on DS and the same on US, none on the other (blue).
« Last Edit: April 18, 2013, 08:06:06 PM by ryant704 »
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asbokid

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Re: BitLoading
« Reply #7 on: April 18, 2013, 08:21:50 PM »

I do have rather a small uptake at the minute, about a month ago there was around 20 connected since the cab had been installed in September.

Hopefully not, but maybe a case of enjoy it while you can :-)

Quote
Since the install I have noticed lower SNRM at night. Normally goes down to 4.3/4 but it started hitting 3 some nights now since this install but could just be a coincidence...

Lower SNR at night is normally due to greater interference from electrical lighting circuits, especially 'noisy' street lighting.

Quote
Various images attached now, including the RF filter.

Is that the RF filter they removed?  Or the one that's still in situ? If so, that's an RF2 dubbed the "ADSL Killer" !

I can see from the interference profile, that you're near the Postwick MW transmitter.  For example, the interference on DMT#198 is due to AM Radio Norfolk station on 855kHz  (198 * 4312.5 Hz) pounding out 1500 watts :-)

That RFI is evident in the QLN graph (check the spike at DMT #198).   That noise lowers the SNR for that tone and surrounding tones, and consequently, the bit loading goes down for those affected tones.

Recall each subcarrier (or "tone") has its own SNR.   These SNRs are then averaged across the upstream and across the downstream bands to give us a single SNR figure for up, and a single SNR figure for down.  Those average SNR figures tend to mask frequency-specific interference.

Quote
I also have various dips on the BitLoading where that tone/bit isn't being used, there is probably about 15 max. About 7/8 on DS and the same on US, none on the other (blue).

At a guess from your proximity to Postwick, many of those dips are due to RFI from that transmitter.

That transmitter also belches out Radio Five Live on 693kHz (10kW), TalkSport on 1053 (18kW), Virgin Radio on 1215kHz (1.2kW)

See: http://www.mediumwaveradio.com/uk.php

The spikes and troughs look ugly but they're no biggie.  Each tone carries at most 15 bits, so if you lose at maximum three tones per AM radio station, you're only losing a few tens of bits in throughput.   Besides what can you do about it, short of cutting the guy ropes to the antenna and lopping the wretched thing :-)

From: http://tx.mb21.co.uk/gallery/postwick.php



- - - - - - - - - - - - - - - - - Cut here (and run!) - - - - - - - - - - - - - - - - -

cheers, a
« Last Edit: April 20, 2013, 01:33:30 PM by asbokid »
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ryant704

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Re: BitLoading
« Reply #8 on: April 18, 2013, 08:28:57 PM »

Yes, the RF filter is still in-place. Though I haven't noticed any decrease in service except in upload from 5.2 throughput to 4.5. Though when I had Infinity first Installed the upload would never go above 4.5Mbps then after about 20 powercuts it went above 5... also turning QoS off gave me no better. :(

I'm a considerable amount of miles from postwick but not that far (Lingwood).

Thanks for your help, guess everything is as it should be. :)
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ryant704

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Re: BitLoading
« Reply #9 on: April 19, 2013, 11:36:22 AM »

While I have an topic open topic I thought I would get some words of advice about my SNRM.

See Attachment :P
« Last Edit: April 19, 2013, 11:42:05 AM by ryant704 »
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roseway

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Re: BitLoading
« Reply #10 on: April 19, 2013, 11:47:53 AM »

It looks as though some source of moderate interference was switched on at 10:40 and switched off at 11:30. Unless you can relate that to something happening in your own home, it won't be easy to trace, and possibly not worth bothering about.
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  Eric

ryant704

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Re: BitLoading
« Reply #11 on: April 19, 2013, 11:54:51 AM »

I did think as much and I do think like that but... I just found out what it was and it was so obvious. I have a laptop which is broken and I guess the Electrical Interference is coming off one of the circuit boards that is partially damaged.

Agreed I would't give it too much effort I was looking for more recommendations of what it could of been but it was the obvious (too obvious)... though at night the BitLoading can struggle as some very rare nights the SNRM can drop to 1. :)
« Last Edit: April 19, 2013, 11:58:52 AM by ryant704 »
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ryant704

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Re: BitLoading
« Reply #12 on: May 15, 2013, 07:39:53 PM »

Should I be concerned about these gaps?

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