8 ms·

ADSL, being essentially a form of alternating current, will pass through an air gap. I was enlightened on this by an AT&T tech who pointed out that one of my AD

ADSL, being essentially a form of alternating current, will pass through an air gap. I was enlightened on this by an AT&T tech who pointed out that one of my ADSL lines had several breaks and could not carry DC but the ADSL signal still came through. The other line was dead so I was running essentially on one wire with an earth ground. Here is an interesting discussion: "Phone line with 1 broken wire still gets ADSL2" https://forums.whirlpool.net.au/archive/9yzp5wr3 https://forums.whirlpool.net.au/archive/9yzp5wr3 Anyway, my hat is off to the people who designed and implemented ADSL.

deleted 5y ago
[deleted]
geofft 5y ago
In some sense, that's "just" radio. An antenna is a wire... it's sending a signal and picking it up from a very nearby antenna. Radio transmissions are all AC. (Arguably, the real insight here is that the very existence of radio is impressive / unintuitive.)
- y04nn 5y ago
  The gap on the line acts more as a capacitor than an antenna, AC pass through capacitors, but DC does not.
- Johnythree 5y ago
  It's a differential signal using a center-tapped transformer to drive the signal. It's actually using three wires if you count the ground. This means it's symmetrical about ground and thus cancels it's own radiated field (eg doesn't interfere with adjacent lines). But with one side open there is still a path for AC, which is through the other side and using the ground return.
DaiPlusPlus 5y ago
> Anyway, my hat is off to the people who designed and implemented ADSL. It's a blessing-and-a-curse: too many incumbent ISPs in highly-developed nations used ADSL's ability to run on anything as an excuse to put-off FTTH deployments (looking at you, BT).
- unixhero 5y ago
  Telenor They also did the same, stalling and delaying, insisting on introducing ISDN 64/128kbit instead of going straight to DSL like Holland did.
- samwillis 5y ago
  I think we (in Stamford Lincolnshire, UK) must be pretty unique right now with TWO competing FTTP startups both rolling out on the same streets. One day you will see Lightspeed Broadband pulling their fibre through the BT ducts and putting boxes on the top of telegraph polls. The next day Upp Brordband will be on the same street. So bizarre they they are doing the same town! Apparently we are due to have BT put in their own fibre in the next few months too. Really don’t understand why they are doubling up the infrastructure. Hoping for a local price war!
  - aphrax 5y ago
    How strange to see my hometown mentioned! Still not too far away and yes hoping for some local competition
  - rconti 5y ago
    Very strange! Mine was not quite as happy a story, but here in Silicon Valley (1mi from Facebook) I got a local ISP announcing fiber, and the very next week AT&T was running their own lines. This was back in 2019, and it felt like "everyone" had fiber before we did. 2 years later I guess it feels like old hat. And the local ISP has not yet run the fiber they promised. But one fast ISP is better than zero! Good luck with your deployment. I'm still so happy about it.
    - bcrl 5y ago
      Your incumbent killed the local ISP's project. Fibre customers generally won't change ISPs in the first 2-3 years of having service, so being undercut by the incumbent makes it worthless to continue with the cost of construction when you'll only get a handful of customers. I know because the incumbent has done exactly the same thing to me. A project that planned on having 140 customers can't be supported by 5.
tiernano 5y ago
this explains a problem i had in Ireland... ADSL worked, but phone line had to dial tone... engineer had no idea how it was working... mind you, ISP had sent him out to get better internet... i was on 8mb and was promised closer to 24... after he "fixed" it, it went to 6... so they were really not happy with the "engineer" when i canceled 2 days later...
andoma 5y ago
Same thing happened once at my parents house. Quite confusing for some :)
Terry_Roll 5y ago
Its DC. If you had to imagine it, think of it as a range of radio stations being broadcast down a wire instead of over the air and your router can tune in to all the radio stations at the same time and then piece together the different bits of data being broadcast. Long Wave would be ADSL, ADSL2, Medium Wave ADSL2+, and FM would be higher forms of ADSL to give you your 40Mb, 80Mb, 100Mb download speeds. With this broadcasting of signals down a wire now known, it becomes somewhat unsurprising anything capable of transmitting an electrical voltage & current would be capable of transmitting ADSL. Not knocking their effort though, Arnold and Arnold have always like to demonstrate their knowledge. One of them has a personal blog which can be quite interesting. I wonder if they have considered trying to adapt an SDR dongle to become an ADSL transmitter?
- hereforphone 5y ago
  What's DC? "Radio stations" communicate via alternating quantities (EM waves)
  - Terry_Roll 5y ago
    The telephone cable coming into the house is 50volts DC is not AC like a powerline. I know radio waves are just that, waves, its how things like noise cancelling headphones work. Different frequency's give you different ranges or distances for 1 watt, which is why you can bounce Long wave around the planet.
    - hereforphone 5y ago
      48VDC but yes. Sounds like we're agreed that the information is being transmitted via AC or another alternating signal.
    - leoedin 5y ago
      Radio waves down a wire are by definition AC. Otherwise they don't work. The voltage must change, thus the current must change, thus it is AC. Just because it's not 50/60Hz AC like a powerline doesn't change that. There might be a DC component, but ultimately all the ADSL modem cares about is the AC part.

deleted 5y ago

[deleted]

geofft 5y ago

In some sense, that's "just" radio. An antenna is a wire... it's sending a signal and picking it up from a very nearby antenna. Radio transmissions are all AC. (Arguably, the real insight here is that the very existence of radio is impressive / unintuitive.)

y04nn 5y ago
The gap on the line acts more as a capacitor than an antenna, AC pass through capacitors, but DC does not.
Johnythree 5y ago
It's a differential signal using a center-tapped transformer to drive the signal. It's actually using three wires if you count the ground. This means it's symmetrical about ground and thus cancels it's own radiated field (eg doesn't interfere with adjacent lines). But with one side open there is still a path for AC, which is through the other side and using the ground return.

DaiPlusPlus 5y ago

> Anyway, my hat is off to the people who designed and implemented ADSL. It's a blessing-and-a-curse: too many incumbent ISPs in highly-developed nations used ADSL's ability to run on anything as an excuse to put-off FTTH deployments (looking at you, BT).

unixhero 5y ago
Telenor They also did the same, stalling and delaying, insisting on introducing ISDN 64/128kbit instead of going straight to DSL like Holland did.
samwillis 5y ago
I think we (in Stamford Lincolnshire, UK) must be pretty unique right now with TWO competing FTTP startups both rolling out on the same streets. One day you will see Lightspeed Broadband pulling their fibre through the BT ducts and putting boxes on the top of telegraph polls. The next day Upp Brordband will be on the same street. So bizarre they they are doing the same town! Apparently we are due to have BT put in their own fibre in the next few months too. Really don’t understand why they are doubling up the infrastructure. Hoping for a local price war!
- aphrax 5y ago
  How strange to see my hometown mentioned! Still not too far away and yes hoping for some local competition
- rconti 5y ago
  Very strange! Mine was not quite as happy a story, but here in Silicon Valley (1mi from Facebook) I got a local ISP announcing fiber, and the very next week AT&T was running their own lines. This was back in 2019, and it felt like "everyone" had fiber before we did. 2 years later I guess it feels like old hat. And the local ISP has not yet run the fiber they promised. But one fast ISP is better than zero! Good luck with your deployment. I'm still so happy about it.
  - bcrl 5y ago
    Your incumbent killed the local ISP's project. Fibre customers generally won't change ISPs in the first 2-3 years of having service, so being undercut by the incumbent makes it worthless to continue with the cost of construction when you'll only get a handful of customers. I know because the incumbent has done exactly the same thing to me. A project that planned on having 140 customers can't be supported by 5.

tiernano 5y ago

this explains a problem i had in Ireland... ADSL worked, but phone line had to dial tone... engineer had no idea how it was working... mind you, ISP had sent him out to get better internet... i was on 8mb and was promised closer to 24... after he "fixed" it, it went to 6... so they were really not happy with the "engineer" when i canceled 2 days later...

andoma 5y ago

Same thing happened once at my parents house. Quite confusing for some :)

Terry_Roll 5y ago

Its DC. If you had to imagine it, think of it as a range of radio stations being broadcast down a wire instead of over the air and your router can tune in to all the radio stations at the same time and then piece together the different bits of data being broadcast. Long Wave would be ADSL, ADSL2, Medium Wave ADSL2+, and FM would be higher forms of ADSL to give you your 40Mb, 80Mb, 100Mb download speeds. With this broadcasting of signals down a wire now known, it becomes somewhat unsurprising anything capable of transmitting an electrical voltage & current would be capable of transmitting ADSL. Not knocking their effort though, Arnold and Arnold have always like to demonstrate their knowledge. One of them has a personal blog which can be quite interesting. I wonder if they have considered trying to adapt an SDR dongle to become an ADSL transmitter?

hereforphone 5y ago
What's DC? "Radio stations" communicate via alternating quantities (EM waves)
- Terry_Roll 5y ago
  The telephone cable coming into the house is 50volts DC is not AC like a powerline. I know radio waves are just that, waves, its how things like noise cancelling headphones work. Different frequency's give you different ranges or distances for 1 watt, which is why you can bounce Long wave around the planet.
  - hereforphone 5y ago
    48VDC but yes. Sounds like we're agreed that the information is being transmitted via AC or another alternating signal.
  - leoedin 5y ago
    Radio waves down a wire are by definition AC. Otherwise they don't work. The voltage must change, thus the current must change, thus it is AC. Just because it's not 50/60Hz AC like a powerline doesn't change that. There might be a DC component, but ultimately all the ADSL modem cares about is the AC part.