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Author Topic: About my possible bridge tap issue  (Read 18712 times)

gazaai

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Re: About my possible bridge tap issue
« Reply #15 on: December 02, 2015, 08:06:15 PM »

It sounds like it could be burakkucat ;), as there defiantly no extra cables in my house, it comes from the telephone to the side of the house then through the atic into the master socket.

Here is my Hlog Data from DSLStats:
http://pastebin.com/RLJ7tri1
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burakkucat

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Re: About my possible bridge tap issue
« Reply #16 on: December 02, 2015, 10:11:10 PM »

The raw data of the Hlog plot was downloaded and saved as Hlog-gazaai.txt (attached below).

Using one of the scripts (saved from the very early days of our circuit statistics analysis) the Hlog plot was created locally as Hlog-gazaai.png (attached below).

Using the latter as a reference guide into the former, the following table was then drawn up --

Code: [Select]
Index Tones at Minima Use as Midpoint Frequency of Midpoint (kHz)
----- --------------- --------------- ---------------------------

  1    160 -  167       163.5   705.09375
  2    496 -  503       499.5 2154.09375
  3    832 -  839       835.5 3603.09375
  4   1144 - 1151      1147.5 4948.59375
  5   1512 - 1519      1515.5 6535.59375
  6   1848 - 1855      1851.5 7984.59375
  7   2192 - 2207      2199.5 9485.34375
  8   2528 - 2535      2531.5 10917.09375
  9   2864 - 2887      2873.5 12391.96875
 10   3208 - 3215      3211.5 13849.59375
 11   3552 - 3567      3559.5 15350.34375
 12   3936 - 3943      3939.5 16989.09375

Reworking the above table to show the delta of the midpoints gives --

Code: [Select]
Index Tones at Minima Use as Midpoint Delta of Midpoints
----- --------------- --------------- ------------------

  1    160 -  167       163.5
336
  2    496 -  503       499.5
336
  3    832 -  839       835.5
312
  4   1144 - 1151      1147.5
368
  5   1512 - 1519      1515.5
336
  6   1848 - 1855      1851.5
348
  7   2192 - 2207      2199.5
332
  8   2528 - 2535      2531.5
342
  9   2864 - 2887      2873.5
338
 10   3208 - 3215      3211.5
348
 11   3552 - 3567      3559.5
380
 12   3938 - 3943      3939.5

The average of the above eleven deltas is 343.

Looking at Table 2 (in the PDF document, referenced in my initial post) we see that there are entries for deltas of 300 and 350 --

Code: [Select]
Tone delta Sub-dips
0   1   2   3

300 77.3 154.6 231.9 309.2
350 66.3 132.5 198.8 265.0

By simple algebra, we can inset a line for a tone delta of 343 --

Code: [Select]
Tone delta Sub-dips
0   1   2   3

300 77.3 154.6 231.9 309.2
343 67.6 135.2 202.8 270.4
350 66.3 132.5 198.8 265.0

The line for the tone delta of 343 now needs to be expanded horizontally to accommodate ten sub-dips.

The entries on the line for the tone delta of 343 may be derived by simple linear programming. The general formula for a straight line is --

y = mx + c          (1)

By inspection of the first four values on the line for the tone delta of 343 it can be seen that --

c = m                 (2)

and c = 67.6

By substituting equation (2) in to equation (1) we obtain --

y = m(x + 1)        (3)

As there are twelve minima seen in gazaai's Hlog plot, we are interested in solving equation (3) for the case of m = 67.6 and x = 10

Substituting those values into equation (3) gives --

y = 67.6 (10 + 1)
  = 743.6 metres

We now have a tentative length for the bridging tap -- approximately 744 metres.

Realistically, where could a bridging tap with a length of 744 metres be located? Where could such a tap of that length start?  :-\

It would be very interesting to know the length of the E-side for gazaai's circuit. If the network records were to show that the E-side length is between 700 and 750 metres, then I would know exactly what to check . . . and where --

The relevant low pass filter for gazaai's circuit in the "fibre cabinet".

b*cat looks towards WWWombat and asks that he please review the above caterwauling.  ;)

[Edited to include the last dip (the twelfth) that had been missed from the original calculations. Look closely at the Hlog plot, attached below.]
« Last Edit: December 05, 2015, 04:50:53 PM by burakkucat »
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gazaai

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Re: About my possible bridge tap issue
« Reply #17 on: December 03, 2015, 02:39:29 AM »

Wow very interesting, also what do you mean by e-side and do you really think there is a 600m bridge tap that seems a bit doubtful lol?

I really want to get an openreach engineer out here and see what his testing equipment shows. :)
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burakkucat

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Re: About my possible bridge tap issue
« Reply #18 on: December 03, 2015, 03:51:59 PM »

First, just consider just the classic telephony circuit.

There is a pair which leaves the exchange building and, as part of a cable bundle, proceeds to the PCP. That is the E-side for your circuit. Let us call the physical length of your E-side LE.

In the PCP, the E-side is connected to the D-side of your circuit. Let us call the physical length of your D-side LD.

The total circuit length, from telephony equipment within the exchange building to the NTE5/A in your home, will therefore be the sum of the E-side length plus the sum of the D-side length plus the sum of the final drop (or service feed) into your home plus any internal wiring (on the Openreach side of the NTE5/A). Thus --

LTotal = LE + LD + LOther

where LOther is the sum of the final drop (or service feed) plus any internal wiring to the NTE5/A.

A Broadband Internet (VDSL2) service is now added to the above.

Within your home, let us assume that a centralised filter is fitted to the NTE5/A. The low frequencies (300 - 3400 Hz) now only reach the telephony socket and the higher frequencies (~26 kHz and upwards) only reach the modem (or modem/router). (See xDSL Frequency Plan for Annex A.)

In the PCP, at the junction of the D- & E-side cables, the direct connection of those two parts of the circuit are severed. They are each connected to a tie cable pair to link the "fibre cabinet's" DSLAM into the circuit. Within the "fibre cabinet", those two pairs are connected to the same point -- the line card port of the DSLAM. In the case of the D-side pair, they are connected directly. In the case of the E-side pair, they are connected via a low pass filter.

The length of the metallic pathway for the Broadband Internet service is --

LOther + LD + LD-tie          (1)

where LD-tie is the length of the D-side tie cable.

The length of the metallic pathway for the telephony service is thus lengthened to --

LOther + LD + LD-tie + LE-tie + LE          (2)

where LE-tie is the length of the E-side tie cable.

In terms of the location, the low pass filter should "appear" between LD-tie + LE-tie, thus --

LOther + LD + LD-tie + Low Pass Filter + LE-tie + LE          (3)

where I now make use of the formula that defines the telephony total circuit length as indicators of the wires of the circuit pair.

Consider the telephony circuit defined in equation (2); all is good the telephone operates satisfactorily.

Consider the telephony circuit defined in equation (3); all is good the telephone operates satisfactorily.

Equation (3) is the normal situation when a VDSL2 service is provisioned to share the pair (D-side + Other) to your home.

If the situation depicted by equation (2) becomes true (the low pass filter becomes faulty, falls out or has never been correctly inserted) then there is no effect on the telephony service.

However, for the VDSL2 service, which is normally depicted by equation (1) the situation becomes sub-optimal (or even disastrous). The VDSL2 service now "sees" the situation as depicted by equation (2). The E-side of the pair, which just brings the telephony service to the cabinet, has become a bridging tap of length LE-tie + LE.

It is my current speculation that LE-tie + LE is of the order of approximately 744 metres. The only cause for this defect would be a problem with the low pass filter within the "fibre cabinet".

[Edited to correct the approximate length, as a result of reworking my earlier calculations to include the last dip (the twelfth) that had been missed from the original calculations.]
« Last Edit: December 05, 2015, 04:54:37 PM by burakkucat »
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licquorice

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Re: About my possible bridge tap issue
« Reply #19 on: December 03, 2015, 04:02:43 PM »

Or to summarise, if your exchange is approximately 682 metres from your PCP, the hypothesis looks a good one.
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gazaai

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Re: About my possible bridge tap issue
« Reply #20 on: December 03, 2015, 04:22:30 PM »

The exchange may well be 600m away from the cabinet, here is the layout of my line here:

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burakkucat

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Re: About my possible bridge tap issue
« Reply #21 on: December 03, 2015, 04:55:19 PM »

Having "brought up" Carstairs [1] on Google Maps, it was asked to provide the distance between the PCP/Fibre Twin and the exchange building (image attached, below).

It came up with approximately 0.6 mile which, according to my arithmetic [2], is 960 metres.



[1] In the early 1970s, I lived in Carstairs Road, Liverpool.  :)
[2] 0.6 x 8 x 1000 / 5 = 960
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gazaai

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Re: About my possible bridge tap issue
« Reply #22 on: December 03, 2015, 05:10:32 PM »

So what do you reckon then, do you think it could still be the cable from the exchange acting as a bridge tap?
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Black Sheep

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Re: About my possible bridge tap issue
« Reply #23 on: December 03, 2015, 05:33:56 PM »

If I may, on the back of all the great posts from the regulars ....... my educated guess would be that the 'Tap' is on the D-side. These kind of 'Taps will only occur (on both E and D sides) if the Cab has Quante or Krone connection strips.
The chances of accidentally connecting a 2nd D-side pair to the one the engineer is already working on, in a 'Crimp-only' Cab, is incredibly extreme.

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burakkucat

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Re: About my possible bridge tap issue
« Reply #24 on: December 03, 2015, 06:43:54 PM »

Something odd is occurring with the circuit.  :(

I have no definitive answer. All my calculations are "best effort", using approximations.  :-\

Based on my calculations, if I had "the ways and means", I would inspect the terminations in the fibre cabinet and also give a critical examination to the low pass filter for the circuit.  :-X

For Mr B*Sheep -- I use the terminology "bridging tap" in the correct sense of the phrase but, in the scenario described above, there is no abnormal wiring proposed as connected to the circuit. It is the speculated absence of the low pass filter or a speculated defective low pass filter for the circuit in question that makes the telephony E-side appear as a bridging tap across the VDSL2 circuit.  ;)

[Couldn't you experiment on your own VDSL2 circuit? Nip down to the fibre cabinet, identify the relevant low pass filter, remove the same and then, back in the sheep-fold, check your circuit's behaviour with your JDSU HST-3000c?  :D  ]
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Black Sheep

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Re: About my possible bridge tap issue
« Reply #25 on: December 03, 2015, 07:10:24 PM »

Something odd is occurring with the circuit.  :(

I have no definitive answer. All my calculations are "best effort", using approximations.  :-\

Based on my calculations, if I had "the ways and means", I would inspect the terminations in the fibre cabinet and also give a critical examination to the low pass filter for the circuit.  :-X

For Mr B*Sheep -- I use the terminology "bridging tap" in the correct sense of the phrase but, in the scenario described above, there is no abnormal wiring proposed as connected to the circuit. It is the speculated absence of the low pass filter or a speculated defective low pass filter for the circuit in question that makes the telephony E-side appear as a bridging tap across the VDSL2 circuit.  ;)

[Couldn't you experiment on your own VDSL2 circuit? Nip down to the fibre cabinet, identify the relevant low pass filter, remove the same and then, back in the sheep-fold, check your circuit's behaviour with your JDSU HST-3000c?  :D  ]

Ha ha, alas ...... Openreach engineers have no access keys for the DSLAM cabs.

My apologies, I thought that there was an inference that there was some kind of 'Wired tap' ??. I know from personal experience that D-side 'Taps' can occur, especially when the Cab is a 'rats nest' and more often than not the terminating strips will be at low-level, making it difficult to notice if there is indeed another pair connected to the EU's circuit.

I'll bow out now as I seem to be muddying the waters.  ;) :) :)

 
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burakkucat

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Re: About my possible bridge tap issue
« Reply #26 on: December 03, 2015, 08:30:14 PM »

I'll bow out now as I seem to be muddying the waters.  ;) :) :)

I will dispute that! You have made valid contributions, based on real life experiences.  :thumbs:
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NewtronStar

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Re: About my possible bridge tap issue
« Reply #27 on: December 03, 2015, 08:57:31 PM »

Can see why BS calls it a rats nest

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gazaai

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Re: About my possible bridge tap issue
« Reply #28 on: December 03, 2015, 11:02:27 PM »

A lot of information in this topic I don't have much of a clue on haha

I am getting a call back from BT tomorrow to hopefully arrange an engineer visit, I think I might have to pay for it tho and its 130  :o

Although I really want to get his opinion, do you guys have any advice for me. And is it true that he doesn't have an access key to the cabinet. As that could be an issue would you not agree? as he probably wont bother to escalate it further as my line is performing fine.
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Weaver

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Re: About my possible bridge tap issue
« Reply #29 on: December 03, 2015, 11:31:24 PM »

> I'll bow out now as I seem to be muddying the waters.  ;) :) :)

Even if that were true, you're still massively in credit anyway BlackSheep !

Your contributions are always essential reading.
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