ADSL Robustness Verified By Running Over Wet String

A core part of the hacker mentality is the desire to test limits: trying out ideas to see if something interesting, informative, and/or entertaining comes out of it. Some employees of Andrews & Arnold (a UK network provider) applied this mentality towards connecting their ADSL test equipment to some unlikely materials. The verdict of experiment: yes, ADSL works over wet string.

ADSL itself is something of an ingenious hack, carrying data over decades-old telephone wires designed only for voice. ADSL accomplished this in part through robust error correction measures keeping the bytes flowing through lines that were not originally designed for ADSL frequencies. The flow of bytes may slow over bad lines, but they will keep moving.

How bad? In this case, a pair of strings dampened with salty water. But there are limits: the same type of string dampened with just plain water was not enough to carry ADSL.

The pictures of the test setup also spoke volumes. They ran the wet string across a space that looked much like every hacker workspace, salt water dripping on the industrial carpet. Experimenting and learning right where you are, using what you have on hand, are hallmarks of hacker resourcefulness. Fancy laboratory not required.

Thanks to [chris] and [Spencer] for the tips.

44 thoughts on “ADSL Robustness Verified By Running Over Wet String

  1. ADSL will carry over a single wire if the wind is in the right direction. It will also carry over lines so bad you can’t hold a voice conversation.

    Be interested to see someone hack up a point-to-point ADSL line as I’m not sure it can be done with consumer gear and it’d be handy as a cheap way of getting data down more than 100yds of Cat5 – especially with 4 pairs to play with.

      1. I used to work in a small rural town where the telco still had some customers in the back country with phone service lines that were a single wire strung along a fence, and a honking big ground spike at the house, and another by where the one wire hooked into the POTS service X miles away. Back in the ancient times when phone service was installed out there, wire was expensive and a way to save 50% on wire was happily embraced by the phone companies.

        Mostly dialup on those was barely good enough for getting and sending email without attachments, or *very patient* web browsing. ISTR that one of them did get up to 14.4K connection, but I dunno if it ever got anywhere near that transfer speed.

    1. I once had a copper voice line with a very bad 60Hz hum. ATT would never admit a problem (“Your line checks out”, “But there’s a horrible hum”, “I’m sorry sir, I can’t hear you because of the hum, can you speak up”)

      I ordered DSL, partly to force them to fix the line. DSL worked fine without any repair!

    2. I can confirm that one wire is enough to have it connecting and slowly working, although I have no idea how it can still be fine (the ground path is a few km long with a terrible impedance).
      I also tried to use all the 4 pairs in a terrible telephone cable (ADSL + VoIP + 2 half duplex Ethernet across a house).
      Funnily, the DSL line gets disconnected when the VoIP phone rings, so the phone stops ringing. At least negative feedback is good at stabilizing systems.

    1. That few 100 metres could have been in the ground for 50 years though, the old copper (and partly aluminium) UK network is very crusty.

      That said, Openreach culture is “just bodge it up quick and run away”, most of the engineers hate it as they’d far rather do it right, replace crusty old cables etc. than run round fire-fighting.

  2. I still have to service fax units, thankfully, they aren’t stand alone, but integrated into the entire copier/printer/scanner ecosystem. But, VoIp faxing still has challenges that I wish would GO AWAY. It was a mess trying to get 33k faxing on copper, but on VoIp, unless the ATA box is located close to the phone jack, has T.38 installed is a MESS! I’d rather deal with copper sometimes. Pushing data down a 20,000 pair cable with low S/N crosstalk can be a challenge.
    We even had one trunk line in town that after ever storm, would get wet because it had a leak and the pressurization would be zero. They kept a liquid nitrogen bottle on the line FOR YEARS. Recently, I noticed the bottle was gone and about a month later, we had an at&t tech in our office and I asked him, and he said “oh, they finally got around to fixing the leak and pressurizing the line again”.

  3. Circa 1998 I worked in a town where out in the countryside there were still a few farms with phone lines that were a single bare wire strung along a fence on porcelain insulators. The other half was connected to a metal stake driven about 5 feet into the dirt. Connection speed for dialup varied depending on the weather, perhaps 14.4K when it was really dry but not too dry or the ground side would get worse. IIRC power for the customer equipment had to be provided with a transformer at their end, so if there was a power outage they had no phone. The lines dated back to the days of magneto charged batteries in the phones, and party lines.

  4. It is not so much the resistance in the wire, but loose connections and cross talk. Try a loose connection with a lot of static. It will still work, but it will have to work harder.

    I may be showing my age here but I remember using telebit trailblazer modems and you could talk on the phone over them and they would just slow down. In fact we got a ROM upgrade for them because the first generation of them did not have a way to agree to hang up and the remote modem would sometimes hang on to a disconnected line for minutes before giving up.

  5. ADSL works, even on a disconnected phone line that a phone doesn’t work.

    Three years ago, my telecom finally came to install fiber lines for the whole apartment. I think they forgot to plug a copper wire back to the distribution board when they’ve done their work, I only found out that a day later when I picked up my phone and heard nothing. I measured the phone line with my DMM.

    It was 0 (zero) volt, absolutely. Somehow the Internet was still working. The modem was struggling to transmit and error-correct the data, and the bandwidth severely degraded to ~256kbps with high latency, but it was still enough for web browsing and chatting on IRC.

    How could my ADSL work on a disconnected phone line? My best guess is because of the fact that ADSL uses AC signal, if a single wire in a twisted-pair was disconnected, the AC signal can still partially transmitted to the other end and be decoded, provided the modems on both ends shared a common power ground, the earth.

    But only my phone line was disconnected accidentally, I think the disconnected wire was not ground but the “hot” wire. How could a signal travel through solely a ground wire is beyond my understanding of the telephone system. The polarity doesn’t matter for modern phone lines?

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