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Author Topic: in-house wiring topology, bridge taps, ADSL filters  (Read 11127 times)

asbokid

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in-house wiring topology, bridge taps, ADSL filters
« on: February 16, 2012, 04:07:55 AM »

Hi,

I have no particular knowledge here, but would be interested to hear opinions.

The poor quality of in-house wiring is often blamed for low xDSL sync speeds.  Consumers are advised by providers to install ADSL filters or "splitters" on every piece of voiceband equipment, and to site the xDSL modem as close as possible to the NTE5 demarcation point.

But maybe that tells only part of the story. There could be other things that the customer can do to improve his sync speed, such as changing the overall topology of his in-house wiring.

The paper below is from Westell. [1] It was published in 2003 when consumer ADSL was in relative infancy. Today, the same technologies are still in use, albeit in newer forms, and the problems with those technologies persist, too.

The Westell paper addresses the effect of bridge taps in consumer wiring. That's what specifically interests here.

The bridge tap, also known as a "branch" or a "lateral" is a well documented problem for telcos.   It is where there is a non-terminated cable pair that is 'tapped' onto a subscriber line. The tap introduces impedance mismatches at the splice point of the tap, and at the end point of the tap. These mismatches cause signal reflections. These reflections can impair the performance of the subscriber line. For a line used to provide an xDSL service, the bridge tap is likely to cause notches in the spectral response. These notches reduce the number of 'bins' available in the tone map. This will inevitably affect the overall line sync speed.

Manufacturers of test equipment to detect bridge taps provide some useful background to the problem. [2] [3] [4]

However, bridge taps are not just a problem for telcos, they can also exist in home wiring.  In particular, the tap is known to cause reflections of the strong upstream signal from the CPE xDSL modem in the home.  These signal reflections from the tap are picked up as noise by the sensitive receiver in the modem. This impairs the receiver's ability to recover the much weaker downstream signals from the DSLAM.

It is claimed that the topology of the consumer wiring can directly contribute to the problem of bridge taps.

Intuitively, we assume that it is always better to site the xDSL modem as close as possible to the NTE5, to limit the effect of induction noise and insertion loss in the consumer wiring.  A longer cable run, we are told, will result in greater signal attenuation, additional noise and a lower data throughput. Yet there may be other factors to consider.

The topology of the in-house wiring may also have a bearing.  If the consumer wiring is in a star configuration, where the modem is attached to one of the star 'points', then the other points to the star could have a bridge tap effect on the xDSL signal. Each point of the star introduces its own reflection-loss.

It is said that installing an ADSL splitter at the end of each of those star points should dampen those reflections.  Others however state that the splitter is designed only to perform a low-pass filtering.  Any damping effect it has on signal reflections is entirely perfunctory.  Those reflected signals will still be present to some degree in the home wiring.

Westell's paper is really a sales pitch for the company's 'new' power spectral density shaping algorithm. The algorithm, Westell says, can mitigate the worst effects of a bridge tap. As such, the claims in the paper come with the usual caveat for any commercial vested interest.  In particular, the figures showing attenuation (aka insertion loss) measurements due to bridge taps look distinctly exaggerated.

Westell's measurements vary quite significantly from those published by researchers with a purely academic interest. In his book, Home Networking Basis, Walter Chen publishes his own graphs of measured insertion losses from artificially introduced bridge taps of some 15000 ft. [5]  Chen points out that taps of that length would never be found in consumer wiring. Chen measures the reflection-losses at some 2dB-3dB per tap.

Whatever the scale of the bridge tap effect, the topology of consumer wiring evidently can introduce a bridge tap into the home network.

Further, it's not just the star wiring formation that can pose a problem with bridge taps.  The daisy-chain topology is implicated too. In this wiring layout, the cable from the NTE5 is run in a single line, and where each phone socket is picked up along the cable run.

We might assume in the daisy-chain formation that the best place to site the xDSL modem is at the very beginning of the cable run, right next to the NTE5.  Yet it is possible that the cable running on from the modem to the last socket in the run will itself serve as a bridge tap. And that could introduce the same problems of signal reflection.   Ironically, to overcome the bridge tap problem in a daisychain topology, it might actually be better to site the modem at the far end of the cable run!

cheers, a

[1] http://www.at2.com/downloads/documents/westell/westell_2200white.pdf
[2] http://documents.exfo.com/appnotes/anote233-ang.pdf
[3] http://www.syrus.ru/files/pdf/ssmtt/APP-xDSL-4B.pdf
[4] http://www.jdsu.com/ProductLiterature/sctpsbridgedtap_an_tfs_tm_ae.pdf
[5] http://flylib.com/books/en/2.92.1.16/1/
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roseway

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Re: in-house wiring topology, bridge taps, ADSL filters
« Reply #1 on: February 16, 2012, 06:57:20 AM »

I'll have to give some serious thought to those papers, but in the meantime my initial thought is that this is another strong argument in favour of using a filtered faceplate, so as to isolate the extension wiring from the DSL side.
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waltergmw

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Re: in-house wiring topology, bridge taps, ADSL filters
« Reply #2 on: February 16, 2012, 01:01:54 PM »

@ Eric,

1.  Presumably also try to disconnect all extension wiring by using a DECT phone in the master socket sited well away from the modem and its cabling ?

2.  Perhaps also users should be made aware that using the test socket can, in some circumstances, have significant effects on the modem's performance even on perfect domestic wiring.
(I.e. the user's wiring may not be faulty in its usual sense, but it is deemed so by BT Openreach.)

@ Asbokid & Eric

3. Are the higher frequency VDSL services are more susceptible to bridge tap effects also ?

Kind regards,
Walter
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silversurfer44

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Re: in-house wiring topology, bridge taps, ADSL filters
« Reply #3 on: February 16, 2012, 01:26:08 PM »

>> (I.e. the user's wiring may not be faulty in its usual sense, but it is deemed so by BT Openreach.) <<
Quite correct Walter.
I had a visit for line fault and the OR engineer tried to blame my hard wired telephone. He even suggested that I may be charged for the call out even though he repaired the fault in the cabinet. ???
I never received the bill, which is a good job otherwise I would have kicked up right stiink.
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Bald_Eagle1

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Re: in-house wiring topology, bridge taps, ADSL filters
« Reply #4 on: February 16, 2012, 06:43:29 PM »

Hi walter,


3. Are the higher frequency VDSL services are more susceptible to bridge tap effects also ?


FWIW, please see my attached graphs, showing in particlular my Hlog data & sync speed before an engineer messed up my connection when he was installing a service for another user & the data after he had finished messing about.

I noticed a re-sync while he was working, but of course he denied any responsinility for my connection re-syncing unexpectedly.

The dips in the Hlog (attenuation over frequency) graph at around Tone 1725 apparently signify a bridged tap, as per the JDSU document linked to by asbokid & an alternative JDSU document I have downloaded.

The graph timestamped at 08:31 2nd February (Sync speed 29441 Kbps) was before his involvement & the graph timestamped 16:39 also 2nd February (Sync speed down to 22668 Kbps) was immediately after the engineer left.

Overall attenuation values after the engineer's intervention had also increased slightly.

I still occasionally see "valleys" in my Hlog graphs, suggesting that an intermittent fault has been introduced by the engineer.


Paul.

[attachment deleted by admin]
« Last Edit: February 16, 2012, 06:58:47 PM by Bald_Eagle1 »
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burakkucat

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Re: in-house wiring topology, bridge taps, ADSL filters
« Reply #5 on: February 16, 2012, 07:22:08 PM »

Quote
Ironically, to overcome the bridge tap problem in a daisychain topology, it might actually be better to site the modem at the far end of the cable run!

That certainly would be the case if none of the extension sockets were used by a telephone and could still be the case, when one considers the impedance mismatch introduced by plugging a telephone into a socket early in the daisy-chain.  :-X
« Last Edit: February 16, 2012, 07:28:45 PM by burakkucat »
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asbokid

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Re: in-house wiring topology, bridge taps, ADSL filters
« Reply #6 on: February 16, 2012, 10:52:46 PM »

@ Asbokid & Eric

3. Are the higher frequency VDSL services are more susceptible to bridge tap effects also ?
Presumably so.. higher frequency tones are the usually first to fall victim to signal attenuation.
Interestingly, according to Chen, it is the length of the bridge tap that determines the frequency of the notch.

Chen writes:
Quote
Figure 2.14 shows insertion loss of a 26-gauge 150-ft twisted pair cable as well as insertion losses of those with an in-house wiring branch of 10, 20, and 30 ft.


Figure 2.14. A 150-ft 26-Gauge Flat Wire with 15-ft Branches

Compared with the insertion loss without any branch, a 30-ft cable branch causes the lowest frequency notch at around 4.8 MHz and the second frequency notch at around 14.5 MHz. On the other hand, the first frequency notch of a 10-ft in-house wiring branch is also at around 14.5 MHz. The frequency notch is caused by reflections from a branch with a 180-degree phase shift. In other words, the first frequency notch is the result of the reflection from the branch with a half-wave delay, the second frequency notch is the result of a one-and-a-half wave delay, and so on.

The location of a notch frequency can be estimated based on branch length according to the following expression:
 


 where fi is the ith frequency notch, l is the branch length, and c is the speed of transmission over the in-house wiring. We have c ≈ 5.8 x 10^8 feet per second (ft/sec).
 
 With that transmission speed, we can estimate that the first frequency notch in [Figure 2.14] corresponding to a 20-ft branch is at around 7.25 MHz.

Edit:

Looking at Baldie's second graph.... we can see two significant notches.. These occur around DMT #1400 (≈6.0MHz) and around DMT #1725. (≈7.4MHz).

There are also artifacts immediately above and below those two notches, and a meaningless clatter of spikes and notches at the higher end of the VDSL2 spectrum. 

We are not necessarily looking at bridge taps though. Reflection-loss notches could occur from other causes of impedance mismatch -  maybe from splicing an aluminium pair to a copper pair or from splicing two copper cables of differing gauges..


« Last Edit: February 17, 2012, 03:31:14 AM by asbokid »
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burakkucat

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Re: in-house wiring topology, bridge taps, ADSL filters
« Reply #7 on: February 17, 2012, 12:19:48 AM »

Quote
We are not necessarily looking at bridge taps though.

Can you be absolutely sure that GCHQ have not commissioned a tap on Baldy_bird's line, so as to monitor his activities?  :-X
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waltergmw

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Re: in-house wiring topology, bridge taps, ADSL filters
« Reply #8 on: February 17, 2012, 08:15:08 AM »

Gentlefolk,

Paraphrasing a well know "pome":-

In days of old,
When the GPO was bold...
They had a brilliant idea to equip all overhead lines with Line Protection Units.
These were either pole mounted as shown here in November 2010 or in small ground-mounted pedestals often incased in asbestos clad "sausages".

Probably al have now been decommissioned by removing the metrosil "fuses" which connected each pair leg to earth.
Some in this area have been disconnected properly; although to my certain knowledge the bridge tap lines were NOT always disconnected in the Tee joints at that time. The coiled aerial (Earth wire) and one pair was removed at my suggestion by the BT Openreach engineer.
(Sadly in this case there was so much other noise it had no noticeable effect)

Kind regards,
Walter




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