Electrical Characteristics of Telephone Lines (2023) | Hacker News Reader

Electrical Characteristics of Telephone Lines (2023)

computer.rip

53 pointsLammy16 comments

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.

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.

> 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?

48VDC is specified, but in my experience, it varies. The bottom of the article says the same:

On-hook voltage: 40-50 V DC

Pretty sure from memory AT&T spec’d negative voltage to reduce corrosion.

Ah. I did not see that you meant to emphasize the negative vs. positive voltage. It's true that it's negative, but given that phones are polarity independent, it doesn't really matter. I was expressing the voltage as an absolute value.

Back in the late 1980s when I was a teenager I found plans and built this little circuit with a LED that would light up if someone else in the house was trying to listen in on my phone calls. More than one phone off the hook... LED lights up.

Good project for a comparator chip! Just set the trigger threshold slightly below the off-hook voltage for your extension and it shows when another local phone is also off-hook. Only works locally though, and will not let you know if your phone is tapped, or if a nosy operator or phone company technician is listening.

A long time ago, I helped a co-worker tap his own phone because he suspected his wife of cheating. I gave him a device that would connect to a phone line, activate a cassette recorder when any phone went off-hook, and record the audio. He ended up getting evidence for a divorce shortly afterward.

The "butt set" phones that telco field techs have will let you connect to a line and monitor a call without affecting the voltage, so the comparitor/LED doesn't ensure privacy beyond your family extensions. I still have an old one (with a dial). It's also useful to find the right pair with a flicker test set. I still have a few of those too.

Sometimes a field tech needs to access the CO and will find an unused line on a nearby demark. Long after my phreaking days had passed, a friend of mine had connected a DTMF decoder to his phone line (AC coupled, so it was undetectable). One day he looked at his logs and found that he had recorded a session between a field tech and the CO. The phone number and access codes were his for the taking! Fortunately for the phone company, my friend is mostly a trustworthy law-abiding guy.

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/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

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.

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.

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.

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.

> 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_gene...

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..

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.

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