DSL on Fax/Modem lines.

[Shockeye]

I was wondering if anyone knew whether DSL can interefere with modem/fax communications if they are on the same line.

Any help would be appreciated.

[DarkDryad]

Shouldn't. Phone/fax/modem use only 2 of the 4 wires in a box DSL uses an entirely differnt channel as these devices so if you can use your phone you should be able to use fax/modem on the same line.

[Miasma]

As long as the fax/modem line has a functioning dsl filter on it I don't think there should be interference.

[Murgos]

Shouldn't. Phone/fax/modem use only 2 of the 4 wires in a box DSL uses an entirely differnt channel as these devices so if you can use your phone you should be able to use fax/modem on the same line.

Its a bit more complicated than that.  The simple answer is you should be fine.  The complex answer is that the dsl signal is a much higher frequency signal and SHOULD be completely seperate from the wave sent by the modem or fax machine (see freshman trig: the sine function).  The reality is that sometimes you need to use a band-pass filter to give the signals a better seperation, your DSL provider will usually give you 4 - 6 of these for your phone lines free of charge if you ask politely.

Edited to add:  Though as DD said often you are only using one line in your house and the other can then be a dedicated DSL line, no need to worry about interferance then.  But if you are using a FAX and MODEM and telephone lines then chances are you are all ready using all the lines available to you.

[Viin]

I've always recommended that you get a different line for DSL, no matter what. Piggybacking DSL on an existing phone line is asking for trouble.

Usually, if you go through a third party DSL provider (covad, etc) they can get you a new line with your installation. Only the phone companies force you to piggyback.

Or has this changed since '99?

[Alkiera]

My DSL 'piggybacks' on the phone line.  I've never had any problem with the phone signal, no crossover or anything.  Admittedly, we only have 1 phone jack in the house, and I ran the wire from the network box on the back of the house to that jack myself (after the old one had shorted out due to rubbing on a corner of the house).  We have a DSL filter on that jack, phone and DSL modem both Y'd into it.

And I love my 3mbit down/384kbit up DSL.  It's crazy fast when downloading from good servers.

Alkiera

[DarkDryad]

Sorry I was assuming they were using the filter supplied with the kit.

[Lanei]

In nearly all cases, running a DSL signal on the same pair of wires as a phone signal causes no problem at all.  Modems and Faxes (faxes being just a subset of modems) use the same signal bandwidth on the wire as the voice signal. Typical frequency range is 200Hz to 4KHz.  Yes this is MUCH less than human hearing range, which is why phones don't reproduce voices very well (and truly mangle music).  The ADSL signal runs from around 25kHz to 1MHz.   Notice even the low end of that is above the top end of the range of human hearing.

All a DSL filter is, is a low-pass filter. Basically an inductor and a capacitor in a series-parallel arrangement.  Everything below a cutoff frequency gets off with just a little reduction in signal, everything above that frequency gets hit with a whole fisking LOT of reduction in signal.  Slap the cutoff point in about 10kHz or so, and the DSL doesn't get thru the filter.

The key thing for sharing the copper between the signals is that EVERY SINGLE THING that connects to the phone line that doesn't need to see the DSL signal, doesn't GET to see the DSL signal.  IOW: filter everything but the DSL modem, and its all good.

Regarding Viin's comments; yes it has changed since '99.  A lot.  Firstly because Covad went insolvent a few years ago and is gone.  Secondly because technology - especially in the DSLAM boxes, has improved a LOT since then.  I work (indirectly) for $Three_Letter_Phone_Company (I actually work for $company_what_makes_DSL_modems_with_routers_builtin).  They will sell you an additional line for your DSL if you ask for one, but IMO it's an unnecessary expense.