ADSL is a multi-tone technology that measures the available tone spectrum from close to zero to very high inaudible frequencies on your Copper pair phone line. A small lower frequency portion is reserved in case you also have a voice line service over the same DSL line. And a Low bandpass "signal splitter" can further isolate that from harmonic interference both above and below the demarc established for voice or data traffic. This can improve voice clarity, and data transmission reliabilty.
But this takes place from the CO to the CPE (Central Office to Customer Premise Equipment) over a copper pair that is assumed dedicated and trouble free to the NID (Network Interface Device) or the little gray box on the outside of your house. Often the NID is supplied by the local telco and conforms to an industry standard like the Seicor Corning CAC 7600 style NID box.
The NID box has a cable entering generally from below on one side through a Subscriber Grommet to keep moisture out and is terminated on a terminal bus then bridged to the customer side of the NID box through a [test jack] which is a conventional RJ11 plug and socket. The RJ11 is then terminated at two screw terminals where a customer can connect "premise" wiring which travel down the opposing side of the NID box from the telco side, through another Subscriber grommet to keep moisture out and back into the house.
All of the telephone wiring and Copper pair to and from the NID is subject to the outdoors. UV and Weatherproof or "burialproof" cable should be use to make sure the Copper pair is not compromised by the elements when it runs from the NID to either the CO or into the customers house.
Once the Copper pair enters a house it often gets split off into multiple pairs or "tapped" in several places to run Copper pair to different rooms. Each bridge or "tap" introduces signal losses and reflections like in a fun house echo chamber into the line. Which a phone or DSL modem must struggle to nullify and reduce or ignore. If a DSL modem is not successful it can end up passing along some of this "noise" into the DSP (digital signal processing) chips used by the modem to read and write ATM packets to and from the Copper pair.
And each connection jack exposes the Copper pair direct to the air where moisture and corrosion can introduce short circuits, random snap crackles and pops and unstable or unpredictable noise. The additional runs also act like mini-antenna into the house which can pickup additional noise from light switch current surges, CFL/LED light bulbs, blenders, microwave ovens, WiFi routers, computers, cell phones and all manner of electrical noise.
Our homes are very noisy electrical dens and can make trouble for a DSL modem trying to sift through all of the noise to find a "true" DSL signal.
The best DSL connections are a "Home Run" or a straight line from the DSL modem direct to the NID and to either a splitter or the actual Copper pair assigned from the CO at the NID. But this requires quite a bit of preparation, and the use of specific outdoor grade telephone cable, and possibly an enhanced grade of telco cable like CAT5 or above to eleminate any crosstalk between the wires and dampen any external signals that might be picked up when running from the DSL modem to the NID.
Further, however it is wired to the NID or the DSL modem, its best to coat the terminals or jacks with an Anti-Corrosion resistant Moisture proof or resistant gel which will allow electrical contact without a short circuit and prevent the connection from degrading over time.. essentially a kind of di-electric gel that does not dry out. This can be hard to find.
Finally it goes without saying, any line length added by bringing the Copper pair into the house to service additional RJ11 jacks for other phones, also increases the distance from the CO to the CPE, sometimes significantly because builders often want to leave "spare" cable length. This is so they do not have to "pull cable" later if they want to add additional connection drops.. these "loop lengtheners" can add up fast into the 100's of 1000's of feet. It does not matter if the DSL modem is the "first drop" or last drop along this extra cable.. it all counts to the total length due to reflections and additonal noise level introduce simply by the extra resistance of the total length of cable.. it raises the ground floor of the noise and makes the DSL modem behave as if it were at the very end of the cable.