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According to this answer by @asndre,

It is very bad idea to use Cat5e-based wiring/cabling to establish a xDSL connection.

However, my telephone/internet provider technician recommended -- and gave me 100ft of -- Cat5e cable to carry the telephone signal to my DSL modem so that I could move the modem. Others have had the same experience.

Of course phone/internet provider technicians can be wrong. (Mine was wrong in saying that the WiFi modem he installed would reach the other end of the house with no problems.) But they could be right. Hopefully they have tried what they're recommending and found it to work.

Why could Cat5e for a DSL connection be a bad idea?

Cat5e wiring is not designed for so low frequencies and therefore does not feature an appropriate characteristic impedance (i.e. Z(f)) at the frequencies employed by xDSL which works in telephone line bandwidth (<20 kHz).

Using Cat5e for xDSL also results in dramatical signal attenuation, about 3~5 times worser comparing with an appropriate cable/wire.

To be short, POTN is about 600-Ohm techniques while Cat5e if for 100-Ohm ones.

That sounds to me like the answerer knew what he was talking about. (He also said he had done testing that supported his conclusions.) However the original question was not about the suitability of Cat5e cable for carrying a telephone signal to a DSL modem, so that issue didn't get discussed much.

On the other hand, maybe the results of the impedance mismatch are such that it wouldn't affect actual DSL performance in a typical household? I understand that the effect of impedance mismatch could be signal reflection (causing distortion) and reduced power transfer (= signal attenuation?). This Wikipedia article section suggests that impedance matching to telephone lines in modern devices is no longer very important due to active amplification and filtering.

Does anyone have experience or insight into whether using Cat5e wiring to carry a POTS signal to a DSL modem causes noticeable problems?

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    \$\begingroup\$ I fail to see where cat5 cable is worse than the normal telephone wiring in your house, or the 50 year wires used to connect the house to the DSLAM. ADSL is made to operate over cheap, plain old wire. Cat5 can't be any worse than that. \$\endgroup\$ – JRE Dec 29 '18 at 13:26
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    \$\begingroup\$ And, I've used Cat5 and satellite TV cable to connect a DSL modem. No troubles. \$\endgroup\$ – JRE Dec 29 '18 at 13:28
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    \$\begingroup\$ 100 ft = about 30 m. Typically, xDSL spans km-long distances. It has enough power to overpass (i.e. waste on) an additional (let's take 1-km example) 30/1000=0.03=3% of the "renewed" full path. Try to look on your situation so your technician did have no cable other than Cat. 5e TP :-)). Yes, Cat. 5e is so universal while you are in the house/office (in-door), i.e. the link distances are (typically) no longer than about 100 m, but in the "out-door world's" distances it is not so. \$\endgroup\$ – asndre Dec 29 '18 at 13:40
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    \$\begingroup\$ Yes, a tire swing would definitely need Cat6. \$\endgroup\$ – Transistor Dec 29 '18 at 15:08
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    \$\begingroup\$ If you wish to test whether this matters for you, set up a computer beside your phone terminal block (where the DSL signal enters your house) and connect your modem with the full length of Cat5e(spread it on the floor, don't leave it in a coil), run a few connection speed and ping tests, then replace the cat5e with a short length of regular phone cable. Chances are you will have your full connection speed and near identical ping. If the Cat5e does show a measurable difference, shorten it to the length it would actually be installed and check again. \$\endgroup\$ – K H Dec 29 '18 at 18:07
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During my conversation with Jack Creasey, I “refreshed” in memory some normative material on ADSL and think that the next could be interesting:

excerpt form Annex E/G.996.1

Source: ITU-T G.992.1 (06/1999), ADSL.

Along with small guidelines and useful limitations, it could be considered “indirect evidence” that CAT5 cable is at least assumed as a possible in-home wiring by the then-days researchers.

Moreover, ITU-T G.996.1 (02/2001), Test procedures for DSL transceivers, considers CAT3 (predecessor?) cable appropriate among in-home wiring models (see subsection 6.2).

Maybe, this will give you additional confidence around your cat.5e cabling.

P.S. Also, Annex A/G.996.1 describes the characteristics (from 1 Hz to 5 MHz) of 26 AWG, 24 AWG, and 22 AWG cables (and test line models based on them) which are used in (lab) performance testing of G.992.1 ADSLs. Who knows, may be modern Cat. 5e fits the then-expected requirements...

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Short answer: Cat5 cable will work just fine for your DSL AND for your phone line.

The element missed by most answers and the listed one is a great example of how to be completely wrong, is that the characteristic impedance of a cable is frequency sensitive …..IT IS NOT A CONSTANT VALUE.

Plains old POTS telephone cable is NOT 100 Ohms at voice frequencies, it's actually more like 300-700 Ohms at the voice frequencies used over the phone line (about 300 - 4000Hz). The characteristic impedance drops as the frequency gets higher and asymptotically ends up at about 100-110 Ohms beyond 1Mhz.

Here is what the POTS cable looks like with frequency:

enter image description here
Here for more detail.

POTS only requires about 4kHz of bandwidth and the cable impedance is in the 300-700 Ohms range.

Next you need to understand that the bandwidth required for xDSL varies, but the spectrum usage looks like this:

enter image description here

More detail here.

Notice that the frequencies are all between 25kHz - 1MHz so the cable impedance will vary from around 140 Ohms to around 100 Ohms for all the frequencies involved.

Cat5 cable is just the same ….it's about 100 Ohms for the frequencies above 1 Mhz and you don't see specifications for frequencies below 1Mhz, but you can guess it tends toward 300-700 Ohms for voice frequencies.
You may find this informative.

While there will be differences between POTS telephone cable and Cat5 the differences will be small for the frequencies involved.

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  • \$\begingroup\$ While your answer may be correct, I don't see how you can say that the linked answer missed the fact that the characteristic impedance of a cable is frequency sensitive. It says that cat5e wiring is "not designed for so low frequencies" and "does not feature an appropriate characteristic impedance (i.e. Z(f)) at the frequencies employed by xDSL". \$\endgroup\$ – LarsH Dec 29 '18 at 23:18
  • \$\begingroup\$ @LarsH The linked answer says this: "To be short, POTN is about 600-Ohm techniques while Cat5e if for 100-Ohm ones." ....which compares the impedance of phone cable at audio frequencies to Cat5 beyond 1MHz ...which is completely incorrect. Both cables have about the same impedance whether at audio or RF frequencies. One could just as well say that phone cable is not and was never designed for RF frequencies, but is used for xDSL quite successfully. \$\endgroup\$ – Jack Creasey Dec 29 '18 at 23:24
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    \$\begingroup\$ @JackCreasey IMHO, [expression/function] Z(f) explicitly shows that "the characteristic impedance of a cable is frequency sensitive …..IT IS NOT A CONSTANT VALUE". Is not it for you? Why? In the linked answer there is no claim about Z(f)=Zo=const. Instead, the impedance is mentioned as a function of frequency and therefore expressed as Z(f). is not it? \$\endgroup\$ – asndre Dec 30 '18 at 9:22
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    \$\begingroup\$ @JackCreasey I could agree with your short answer ("Cat5 cable will work just fine for your DSL AND for your phone line") if you'll make some important addition to it like this: "at the distances insignificant to the POTN (i.e. from the provider to the household) span". \$\endgroup\$ – asndre Dec 30 '18 at 9:40
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    \$\begingroup\$ @JackCreasey I also want to draw your attention to the second factor in the linked answer: the signal attenuation. Taking in the consideration the same cat. 5e cabling, one can see that while its characteristic impedance in the range of 1 to 100 MHz is specified to be nearly "constant" (100 +/- 15 Ohms), its insertion loss in the same range behaves other, non-"constant" way: 2.2 dB/(90H + 100E)m @ 1 MHz, ..., 24 dB/(90H + 100E)m @ 100 MHz per TIA/EIA-568. And it is not specified below 1 MHz again... \$\endgroup\$ – asndre Dec 30 '18 at 10:09

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