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SDSL and microfilters

Robtheplod
Grafter
Posts: 320
Registered: 17-08-2007

SDSL and microfilters

title says it all really, do you need to use a microfilter for SDSL? - the activation pack says yes, but i thought the filter was to split the line for other uses, which you don't do with SDSL?Huh
15 REPLIES
Mand
Grafter
Posts: 5,560
Thanks: 1
Registered: 05-04-2007

Re: SDSL and microfilters

Hi there,
You shouldn't need to use a filter for SDSL, as you will have a dedicated line for the SDSL connection.
imroberts
Grafter
Posts: 52
Registered: 15-06-2007

Re: SDSL and microfilters

You don't actually need to use a filter for your ADSL router Smiley you can actually gain a slightly higher sync speed by removing the filter.
But you would be asked to plug one in if you ever had any problems with your connection Grin
VileReynard
Seasoned Pro
Posts: 10,917
Thanks: 256
Fixes: 10
Registered: 01-09-2007

Re: SDSL and microfilters

Surely if you use a filter on the modem signal you will get nothing?
I've never used one - I save mine for telephones etc

James
Grafter
Posts: 21,036
Registered: 04-04-2007

Re: SDSL and microfilters

Hi,
SDSL should be a managed install with SSFP (Service Specific Faceplate) as far as I'm aware, I'll just double check this though.  SDSL comes with a dedicated phoneline so there should/will be no need for additional filtering.
James
Grafter
Posts: 21,036
Registered: 04-04-2007

Re: SDSL and microfilters

I used to explain it in a similar fashion, although poerhaps a little more easier to understand.  You have your phone line which uses analogue signals for your telephone.  when you have broadband activated, you have a digital signal applied to the phone line so that you can use a high speed internet connection.
the filter is used to separate the two different kind of signals to ensure that both work at an optimal level.  If there is interference between the two you can see poor speed levels or at a more extreme level, intermittent or complete loss of broadband connection and in some cases, oddness around the broadband connection dropping when the telephone receiver is lifted and so on.
VileReynard
Seasoned Pro
Posts: 10,917
Thanks: 256
Fixes: 10
Registered: 01-09-2007

Re: SDSL and microfilters

I would disagree.
In the case of ADSL, I don't think it uses the low frequencies used by telephones. In addition all non-active filters introduce a slight signal loss and AFAIK a some amount of delay.
I get a good stable sync ratel anyway.

oliverb
Grafter
Posts: 606
Registered: 02-08-2007

Re: SDSL and microfilters

I think its worth repeating that all the filters I've seen do nothing to the DSL signal, it just passes straight through. That includes two makes of faceplate filter, and yes I have looked inside, also if you plug a RJ11-to-BT adaptor into the DSL side and plug a phone in then YES YOU WILL GET DIALTONE!!
The main purpose of the filter is to prevent the line impedance changing every time a phone goes on or off hook, otherwise this would force a resync. The secondary purpose is to prevent DSL signals interfering with phone equipment.
itsme
Grafter
Posts: 5,924
Thanks: 1
Registered: 07-04-2007

Re: SDSL and microfilters

Quote from: James
when you have broadband activated, you have a digital signal applied to the phone line so that you can use a high speed internet connection.

My understanding is no digital signals are applied to the phone line but modulated signals on a carrier seperated approximately at 4.3kHz intervals.
Perhaps a better analogy is to think of the filter as a radio tuner. With it tuned in to the voice components of the signal so stopping interference from other stations (DSL part of the signal).
john32
Grafter
Posts: 25
Registered: 18-06-2007

Re: SDSL and microfilters

I hope it is OK to post this link (if not please delete)
http://www.adslnation.com/support/filters.php
This provides details of a number of filters available.
Cool
john32
Grafter
Posts: 25
Registered: 18-06-2007

Re: SDSL and microfilters

Nice one James - could you expand you brick anaolgy to explain harmonics  Wink
itsme
Grafter
Posts: 5,924
Thanks: 1
Registered: 07-04-2007

Re: SDSL and microfilters

1st harmonic will be quarter size bricks so some of these will pass through the half brick size holes Grin
john32
Grafter
Posts: 25
Registered: 18-06-2007

Re: SDSL and microfilters

Quote from: itsme
1st harmonic will be quarter size bricks so some of these will pass through the half brick size holes Grin

and these are created as the whole bricks bump into each other as they are thrown off the scaffold.............
VileReynard
Seasoned Pro
Posts: 10,917
Thanks: 256
Fixes: 10
Registered: 01-09-2007

Re: SDSL and microfilters

Wikipedia http://en.wikipedia.org/wiki/ITU_G.992.1 gives the story in fuller, more gory detail.
Note that the upstream ADSL signal occupies the portion of the spectrum just above telephones - so harmonics of the half bricks are not a problem (because the upstream S/N ratio is usually excellent).

oliverb
Grafter
Posts: 606
Registered: 02-08-2007

Re: SDSL and microfilters

Quote from: James_H
Quote from: oliverb
all the filters I've seen do nothing to the DSL signal, it just passes straight through.

You've clearly not seen them all then!
I'll try to remember to post up some pics later on, but basically you don't *need* to filter the DSL signal, but in many situations its beneficial to do so, as filtering the DSL signal may eradicate interferance coming from just below the wavelengths used by ADSL equipment.

I think the low frequencies are [usually] removed by digital filtering, they just end up in the bottom of the spectrum that the DSL modem is programmed not to use. I think the only exception would be if the signal was enough to saturate the converter, in which case you'd have major voice service problems too.