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Author Topic: About my possible bridge tap issue  (Read 19109 times)

gazaai

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About my possible bridge tap issue
« on: December 01, 2015, 09:59:11 AM »

Hi all, I thought I would create my own topic on here about the Bridge Tap, 'WWWombat' has found with my stats on MyDSLWebStats. Here is an image that proves my line does not look normal:



I have had a look online about bridge taps and none seem to look like mine, as others seem to have one v dip where as mine seems to continue from the start of the graph to the end. So can anyone help explain why my graph looks like this?

Thanks
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burakkucat

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Re: About my possible bridge tap issue
« Reply #1 on: December 01, 2015, 05:48:55 PM »

It is possible for a bridging tap to cause multiple dips in an Hlog graph.

I will recommend that you download the "Detecting Bridged Tap and Noise Interference in VDSL2 Access Networks using the JDSU SmartClassTM TPS" PDF document and take note of the method described on Page 6, which makes use of Table 2 on Page 7.
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tickmike

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Re: About my possible bridge tap issue
« Reply #2 on: December 01, 2015, 09:33:12 PM »

It is possible for a bridging tap to cause multiple dips in an Hlog graph.

I will recommend that you download the "Detecting Bridged Tap and Noise Interference in VDSL2 Access Networks using the JDSU SmartClassTM TPS" PDF document and take note of the method described on Page 6, which makes use of Table 2 on Page 7.

Interesting read :).
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gazaai

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Re: About my possible bridge tap issue
« Reply #3 on: December 01, 2015, 11:12:49 PM »

It is interesting although I don't understand most of it lol :D
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Weaver

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Re: About my possible bridge tap issue
« Reply #4 on: December 02, 2015, 04:03:03 AM »

@gazaai  it's to do with the reflections back from the far end of the loose wire. These reflections setup an interference pattern when they are combined with the outgoing waves heading towards the far end. The combined interference pattern from these two wave chains produce a resonance, the resonance pattern is determined by the wavelength of the waves travelling up the wire. The resonance is exactly like an organ pipe's resonance, a guitar string is not quite the same as both ends of it are tethered whereas here one end is free to waggle about.

The reason for the dips in the graph is to do with the fact that changing frequency means changing wavelength. That means either a whole number of wavelengths fit into the length of the loose wire or not, and it's the pattern of fitting a varying number of waves of varying wavelength into the fixed length of the wire that produces the repeating pattern you see. Going to a higher frequency means shortening wavelength until n+1 whole waves will fit into the wire's length.

I do hope I made sense of this; it's easier with nice pictures.
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JGO

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Re: About my possible bridge tap issue
« Reply #5 on: December 02, 2015, 08:32:25 AM »

Incidentally the same thing can happen on a TV aerial installation if someone thinks 50Hz techniques apply ! 
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Weaver

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Re: About my possible bridge tap issue
« Reply #6 on: December 02, 2015, 12:35:10 PM »

@JGO  quite so. And with (parallel) SCSI hard disk cables, which have an absorber at the end of the cable to eat up the signal and prevent back reflections.
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burakkucat

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Re: About my possible bridge tap issue
« Reply #7 on: December 02, 2015, 04:54:33 PM »

It is interesting although I don't understand most of it lol :D

If you would be willing to make the raw Hlog data available, I'll see if I can do the relevant calculations.  :)
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WWWombat

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Re: About my possible bridge tap issue
« Reply #8 on: December 02, 2015, 05:23:32 PM »

I have had a look online about bridge taps and none seem to look like mine, as others seem to have one v dip where as mine seems to continue from the start of the graph to the end. So can anyone help explain why my graph looks like this?

Short answer: your dip appears to be caused by a long length of extra wire (I calculated 66m); other examples may well be shorter lengths.

Long answer:
As others mention, the technical explanation is that the dip is caused by reflections from the far end of the extra bit of wire; the frequency the first dip is seen at depends on the length of that wire, and the wavelength of the matching frequency; Table 1 in that linked PDF gives you an idea of the relationship.

Whatever the first frequency (or tone number N) the dip first appears on, there will be repeats at 2N, 3N etc. This is a kind of resonance effect - whatever length of wire is an exact match (in terms of whole wavelengths) for frequency F is always *also* going to match 2 wavelengths of half that size (ie 2F frequency), and 3 wavelengths of a third that size (ie 3F frequency).

In figure 2 of the PDF, the example shows a dip at tone 1400. By table 1, that means the tap is 8m long. It also means there will be further dips at tone 2800, 4200 etc ... but the display just doesn't show them.

Your picture has the first dip at about tone 164, which is too low for table 1 to help - suggesting the tap is longer than 46 metres, up to maybe 100m-ish. However, table 2 does apply, even though the text describing how to use it (at the end of page 6, "Occasionally, this is too difficult ...") can be hard to follow.

The way to use table 2 is to see that your first dip is at tone 164, and your third dip is at tone 834. The difference between tone 164 and tone 834 (or tone delta) is 670. Using the column titled "1 sub-dip" (because there is one sub-dip between the first & third), we can see the tap is of a length between 66.3m and 71.3m
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WWWombat

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Re: About my possible bridge tap issue
« Reply #9 on: December 02, 2015, 05:27:25 PM »

If you would be willing to make the raw Hlog data available, I'll see if I can do the relevant calculations.  :)

Sorry - I've had my calculations in a partially-written post all afternoon, waiting for me to finish it off!

Raw data is on MyDslWebStats, but I don't think you use that, do you?
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burakkucat

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Re: About my possible bridge tap issue
« Reply #10 on: December 02, 2015, 05:38:52 PM »

Hmm . . . Assuming that the tone number per minima can be ascertained reliably, then a simple multiplication of the tone number by 4.3125 kHz will give the relevant frequency.

But as you are already "part way there", I'm happy for you to finish the task.  :D

Edited to add -- I am fairly sure that I have read (somewhere) how it is possible to determine the length from the observation point to the start of the tap. And with that figure, it should then be easy to "point a paw" to where in the circuit that the tap begins.
« Last Edit: December 02, 2015, 05:43:49 PM by burakkucat »
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tbailey2

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Re: About my possible bridge tap issue
« Reply #11 on: December 02, 2015, 05:57:19 PM »

I seem to remember that the TDR facility on the HST-3000C will detect bridge taps - but at the Cab?
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gazaai

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Re: About my possible bridge tap issue
« Reply #12 on: December 02, 2015, 06:10:52 PM »

Hi, how can I get my raw Hlog data then. Is there a command? and yes I am on myDSLwebstats. Username: GaZaai
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roseway

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Re: About my possible bridge tap issue
« Reply #13 on: December 02, 2015, 06:32:17 PM »

Hi, how can I get my raw Hlog data then. Is there a command? and yes I am on myDSLwebstats. Username: GaZaai

I see you're using DSLstats. Look at Telnet Data --> HLog. Copy that text to a message here.
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burakkucat

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Re: About my possible bridge tap issue
« Reply #14 on: December 02, 2015, 06:56:52 PM »

I seem to remember that the TDR facility on the HST-3000C will detect bridge taps - but at the Cab?

Indeed. Any time domain reflectometer will be capable, even my Tester 301C.

Thought experiment.

Once upon a time, there was a primary cross connection cabinet. As it was a lonely cabinet, a "fibre twin" was located nearby and those two cabinets were then interlinked with a pair of tie cables. Looking into the newer "fibre" cabinet, we see that the two pairs of tie cables can be designated as D-side and E-side. The only difference between the two separate cable bundles is that the circuit pairs in the designated E-side cable are connected via low pass filters, one per circuit.

Question: What would be the result if one low pass filter was faulty? Or was never correctly fitted?

Answer: The E-side pair of that particular circuit would act as a very definite bridging tap of significant length.

Musing: Could the above be the cause of what is visible in the Hlog plot for gazaai's circuit?  :hmm:
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