4 ms·

Electrical Characteristics of Telephone Lines (2023)

53 points by Lammy 4y ago

bbanyc 4y ago
Landlines carrying electricity meant that telephones still worked even when the power was out. That's maybe the one thing I'll miss about landlines.
- don-code 4y ago
  During our last extended (3 day) power outage, I was surprised to learn that even though I have all of my gear on a very large UPS, the cable network also ceases to function after a few hours. Local nodes apparently only have backup power for a short time. To their credit, I saw Comcast deploying small gas generators to nearby street poles the next day. The cell network, on the other hand, continued working without a hiccup. This might have been because the nearest cell tower still had power, but even if it didn't, I'd assume that I'd just be able to connect to the nearest in-range tower that _did_ have power.
  - myself248 4y ago
    All of this, plus the lower noise floor during an outage means you can probably get usable signal from towers that would ordinarily be useless. The cable network _should_ have many hours of backup, but they don't take it as seriously as the telephone network. Was built for entertainment, after all, so it doesn't have reliability in its core engineering.
  - doubleg72 4y ago
    We had the same thing for a couple days recently as well. I actually went and topped off the generator in the morning as the cable company had let it run out of gas. I believe it is a regulation in being they provide phone service over their coax internet, and there are 911 requirements.
  - Lammy 4y ago
    > deploying small gas generators to nearby street poles It looks just as amusing as it sounds: https://old.reddit.com/r/Lineman/comments/13c07dl/honda_generator_on_a_cable_line/ https://old.reddit.com/r/Lineman/comments/13c07dl/honda_gene...
    - doubleg72 4y ago
      In our area they are chained to the pole on the ground. I find it hard to believe standard practice anywhere is hanging them off the line..
  - SoftTalker 4y ago
    If you look at almost any cell phone tower site, you'll see that there is a diesel (or maybe propane or natural gas) generator for power outages.
qingcharles 4y ago
The author makes note to discourage the licking of loose phone wiring, and I would just like to double-down on his wise words, from experience. When wiring a phone extension for your grandma and you can't find your wire strippers, please take pains to make sure you are only stripping "dry" wires with your teeth, not the "wet" ones, lest you find out that those in Hollywood movies who fly across the room after touching a live electrical cord actually has some basis in reality.
- anonymousiam 4y ago
  POTS phone wires typically have 45 volts DC across them when the phone is on-hook. When off-hook, the voltage goes down to just a few volts. When the phone is ringing, you'll get about 90 volts AC, so if you have high skin resistance and you were holding the wires, you'll get a jolt. Where the article first refers to "off-hook" voltage, it should be "on-hook" voltage. When the phone is on-hook, it presents (approximately) infinity ohms of resistance to the line. The reactive load will obviously be a lower impedance because of the capacitor-coupled ring circuit. On some of the pre-ESS (GTE) switches I phreaked on as a kid, the polarity of the DC voltage going to the calling phone would reverse when the phone being called was answered. You could put a diode in the right direction on the calling phone and it could only ever receive calls thereafter. If an outgoing call were placed, it would dial and ring, but would disconnect (hang-up) when the remote side answered. You could also put an LED across the same diode (oppositely polarized) and it would light up to show you when the remote phone was answered.
  - gumby 4y ago
    > POTS phone wires typically have 45 volts DC across them when the phone is on-hook. Don’t you mean -48 for the US phone system?
    - anonymousiam 4y ago
      48VDC is specified, but in my experience, it varies. The bottom of the article says the same: On-hook voltage: 40-50 V DC
1827163 4y ago
The modern replacement, as mentioned in the article, is PON. It's a shared network where they typically run a single fiber from the exchange or local node to roughly 32 homes. The signal is optically split between the homes, very much like cable TV, but using light instead of RF signals. So it's somewhat harder to tell where the signal is coming from, you would need to investigate all 32 separate homes. There might be a way, without any form of fraud, to get a more anonymous Internet connection. Which could be useful in combination with a VPN, Tor or another anonymization tool. The Google Fiber GPON ONT adapters are readily available on Ebay, and they have completely open source drivers, down to the actual GPON MAC itself. There is no binary blob firmware or microcode. Here's a link to the driver source, including all the hardware registers: https://gfiber.googlesource.com/kernel/prism/+/master/arch/arm/plat-feroceon/mv_hal/pon/ https://gfiber.googlesource.com/kernel/prism/+/master/arch/a... For testing purposes, you can but an relatively inexpensive OLT on Aliexpress, this is the headend equipment you need for the ONT to function: https://www.aliexpress.com/item/1005005391884685.html https://www.aliexpress.com/item/1005005391884685.html

53 points by Lammy 4y ago

Landlines carrying electricity meant that telephones still worked even when the power was out. That's maybe the one thing I'll miss about landlines.

don-code 4y ago
During our last extended (3 day) power outage, I was surprised to learn that even though I have all of my gear on a very large UPS, the cable network also ceases to function after a few hours. Local nodes apparently only have backup power for a short time. To their credit, I saw Comcast deploying small gas generators to nearby street poles the next day. The cell network, on the other hand, continued working without a hiccup. This might have been because the nearest cell tower still had power, but even if it didn't, I'd assume that I'd just be able to connect to the nearest in-range tower that _did_ have power.
- myself248 4y ago
  All of this, plus the lower noise floor during an outage means you can probably get usable signal from towers that would ordinarily be useless. The cable network _should_ have many hours of backup, but they don't take it as seriously as the telephone network. Was built for entertainment, after all, so it doesn't have reliability in its core engineering.
- doubleg72 4y ago
  We had the same thing for a couple days recently as well. I actually went and topped off the generator in the morning as the cable company had let it run out of gas. I believe it is a regulation in being they provide phone service over their coax internet, and there are 911 requirements.
- Lammy 4y ago
  > deploying small gas generators to nearby street poles It looks just as amusing as it sounds: https://old.reddit.com/r/Lineman/comments/13c07dl/honda_generator_on_a_cable_line/ https://old.reddit.com/r/Lineman/comments/13c07dl/honda_gene...
  - doubleg72 4y ago
    In our area they are chained to the pole on the ground. I find it hard to believe standard practice anywhere is hanging them off the line..
- SoftTalker 4y ago
  If you look at almost any cell phone tower site, you'll see that there is a diesel (or maybe propane or natural gas) generator for power outages.

qingcharles 4y ago

The author makes note to discourage the licking of loose phone wiring, and I would just like to double-down on his wise words, from experience. When wiring a phone extension for your grandma and you can't find your wire strippers, please take pains to make sure you are only stripping "dry" wires with your teeth, not the "wet" ones, lest you find out that those in Hollywood movies who fly across the room after touching a live electrical cord actually has some basis in reality.

anonymousiam 4y ago
POTS phone wires typically have 45 volts DC across them when the phone is on-hook. When off-hook, the voltage goes down to just a few volts. When the phone is ringing, you'll get about 90 volts AC, so if you have high skin resistance and you were holding the wires, you'll get a jolt. Where the article first refers to "off-hook" voltage, it should be "on-hook" voltage. When the phone is on-hook, it presents (approximately) infinity ohms of resistance to the line. The reactive load will obviously be a lower impedance because of the capacitor-coupled ring circuit. On some of the pre-ESS (GTE) switches I phreaked on as a kid, the polarity of the DC voltage going to the calling phone would reverse when the phone being called was answered. You could put a diode in the right direction on the calling phone and it could only ever receive calls thereafter. If an outgoing call were placed, it would dial and ring, but would disconnect (hang-up) when the remote side answered. You could also put an LED across the same diode (oppositely polarized) and it would light up to show you when the remote phone was answered.
- gumby 4y ago
  > POTS phone wires typically have 45 volts DC across them when the phone is on-hook. Don’t you mean -48 for the US phone system?
  - anonymousiam 4y ago
    48VDC is specified, but in my experience, it varies. The bottom of the article says the same: On-hook voltage: 40-50 V DC

1827163 4y ago

The modern replacement, as mentioned in the article, is PON. It's a shared network where they typically run a single fiber from the exchange or local node to roughly 32 homes. The signal is optically split between the homes, very much like cable TV, but using light instead of RF signals. So it's somewhat harder to tell where the signal is coming from, you would need to investigate all 32 separate homes. There might be a way, without any form of fraud, to get a more anonymous Internet connection. Which could be useful in combination with a VPN, Tor or another anonymization tool. The Google Fiber GPON ONT adapters are readily available on Ebay, and they have completely open source drivers, down to the actual GPON MAC itself. There is no binary blob firmware or microcode. Here's a link to the driver source, including all the hardware registers: https://gfiber.googlesource.com/kernel/prism/+/master/arch/arm/plat-feroceon/mv_hal/pon/ https://gfiber.googlesource.com/kernel/prism/+/master/arch/a... For testing purposes, you can but an relatively inexpensive OLT on Aliexpress, this is the headend equipment you need for the ONT to function: https://www.aliexpress.com/item/1005005391884685.html https://www.aliexpress.com/item/1005005391884685.html