N-DSL, to Filter or Not to Filter?

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coffeebaron

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# 17812 12-Dec-2007 21:58

A question for those who know the more technical & scientific side of ADSL. With N-DSL coming on stream, what are the effects of still using a filter, i.e. a full central splitter type. This would typically be on a marginal line, far from the exchange.

Does a filter enhance line attenuation, help with noise margin etc, when only an ADSL router is plugged into a N-DSL line?

Or, since the line is ADSL only, would this actually hinder the performance?

Cheers

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Bung

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# 99692 12-Dec-2007 22:14

The only advantage would be isolating a rat's nest of other wiring but if there's only the modem connected disconnecting the unused jacks would be simpler.

kfella2000

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# 99716 13-Dec-2007 06:09

coffeebaron: A question for those who know the more technical & scientific side of ADSL. With N-DSL coming on stream, what are the effects of still using a filter, i.e. a full central splitter type. This would typically be on a marginal line, far from the exchange.

Does a filter enhance line attenuation, help with noise margin etc, when only an ADSL router is plugged into a N-DSL line?

Or, since the line is ADSL only, would this actually hinder the performance?

Cheers

I think you will find the filter was to filter out the DSL from the pstn phone service. I know that when I didnt have a filter on my line the caller display didnt work by memory but as soon as I put the filter in it did. I dont think the filter does anything on the DSL side so if you do go naked DSL it shouldnt make no difference whether the filter is there or not.

cyril7

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# 99721 13-Dec-2007 06:49

I have made comment on the exact reason for the filter previously but will restate it. The DSL modem does not require a filter, unless POTs (Plain Old Telephone) devices are present. The filter provides two primary functions, firstly and most importantly to lift the impedance that POTS devices present to the line, and secondly to remove any DSL energy from reaching the POTS device.

An adsl filter is only present in the leg that goes to the POTS devices, the DSL side of a filter is infact simply the input line.

POTS devices typically present quite a low impedance across the line above the voice band (ie in the DSL band), if a filter was not used to lift this impedance then the DSL signal would be heavily attenuated. Further during ringing and hook transits some POTS device offer an even lower impedance to the line and some HF energy , so once again the filter lifts this attenuating effect and removes the HF energy.

So if you have no POTS devices then you dont reqiure a filter, however I would recommend removing any wiring that is not needed, so that a direct line goes from the demark to the DSL modem, with no Y or stub circuits to cause problems. That said a filter will not cause any problems but offer no advantages

Hope this helps
Cyril

coffeebaron

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# 99851 13-Dec-2007 22:22

cyril7: I have made comment on the exact reason for the filter previously but will restate it. The DSL modem does not require a filter, unless POTs (Plain Old Telephone) devices are present. The filter provides two primary functions, firstly and most importantly to lift the impedance that POTS devices present to the line, and secondly to remove any DSL energy from reaching the POTS device.

An adsl filter is only present in the leg that goes to the POTS devices, the DSL side of a filter is infact simply the input line.

POTS devices typically present quite a low impedance across the line above the voice band (ie in the DSL band), if a filter was not used to lift this impedance then the DSL signal would be heavily attenuated. Further during ringing and hook transits some POTS device offer an even lower impedance to the line and some HF energy , so once again the filter lifts this attenuating effect and removes the HF energy.

So if you have no POTS devices then you dont reqiure a filter, however I would recommend removing any wiring that is not needed, so that a direct line goes from the demark to the DSL modem, with no Y or stub circuits to cause problems. That said a filter will not cause any problems but offer no advantages

Hope this helps
Cyril

Hi Cyril,
I do remember your post from somewhere, and it mentioned improving the line attenuation, but I guess that is in relation to the POTS pulling it down. So that's why I'd thought I'd ask if there was any positive effect where no POTS is present.

I guess the other question would be; what effect would a "straight through" connection have. I.E. rather than go through a normal jackpoint that has capacitors etc, if you wired the line directly to the ADSL router?

Cheers
Fraser

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Bung

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# 99852 13-Dec-2007 22:34

A normal 2 wire jack has 1 capacitor that connects from 1 line wire to the telephone ringer circuit if applicable. The capacitor has no connection or effect on a modem.

So few telephones use the capacitors that Telecom was talking about removing them from new jacks. Not having bought any for years I don't know if that ever happened.

grant_k

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# 99857 13-Dec-2007 23:03

Bung: ...Telecom was talking about removing them from new jacks. Not having bought any for years I don't know if that ever happened.

I bought a new 2-wire jack a couple of years ago. It still had the Bell-Ringing Cap, Resistor and Surge Suppressor. It does seem rather pointless still having the Cap, I agree.

Bung

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# 99871 14-Dec-2007 06:34

grant_k:
Bung: ...Telecom was talking about removing them from new jacks. Not having bought any for years I don't know if that ever happened.

I bought a new 2-wire jack a couple of years ago. It still had the Bell-Ringing Cap, Resistor and Surge Suppressor. It does seem rather pointless still having the Cap, I agree.

2-wire just had a capacitor, by that stage the test termination if still used had become a separate item. Any jack with resistor and surge suppressor is an old 3-wire master jack.

cyril7

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# 99873 14-Dec-2007 06:55

Two wire jacks still come with a capacitor to support legacy phones, however is not needed for anything else. There has been evidence in UK where homes (which are still wired 3wire) have been experincing issues with the bell wire acting as an antenna and coupling MF radio energy into the line and effecting DSL performance. So we did ourselves a favour by departing once from BTs apron strings and dumping the 3wire setup.

Cyril

coffeebaron

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# 102196 29-Dec-2007 16:08

Thanks for the info guys; rewired my phones today. CAT6 from demarcation point to RJ45 socket. RJ45-RJ12 cable to router. A temporary filter box to link into the other jack-points (pre-N-DSL). Once changed over to N-DSL, it will just be a straight in connection (via CAT6, RJ45, RJ12) to the router. Rest of the house wiring is now "unplugged".
Works well, even got close to 10% more speed out of my connection :)

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inquisitor

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# 102835 4-Jan-2008 08:38

Having run NDSL without filter for more than a year here in Germany, I can confirm it works fine and even improved attenuation by 1 dB after dumping the unnecessary filter.

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