[Terry_Roll]

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.

[leoedin]

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.

[Eelongate]

Couldn't you send an amplitude modulated signal down a DC line by modulating the voltage but keeping that voltage always above zero? If the current never actually reverses, it wouldn't be AC as I understand it. It would be PDC, pulsed direct current.

[goodpoint]

> modulating the voltage but keeping that voltage always above zero

In signal transmission theory people almost always think in terms of frequency ranges.

Any DC component is simply a frequency set to 0 - that can be ignored.

Similar to the earth magnetic field when talking about radio transmission in air.

[Eelongate]

Sure, but it's still not AC. It's VDC or PDC, not AC. It's not AC if the Current isn't Alternating.

[hereforphone]

48VDC but yes. Sounds like we're agreed that the information is being transmitted via AC or another alternating signal.

[Terry_Roll]

I wasnt taught that radio waves were to be considered identical to AC power down a cable but when thinking about it, they probably are identical with waveform properties and the difference being the medium they are travelling through.

[jcrawfordor]

The exact details become confusing and get into some physics details, but in practice radio frequency on a cable is basically an AC voltage that needs special care to not be lost to inductance and other effects. This is of course part of the general trend that DC through gamma radiation are in many ways the same thing.

It's common parlance to say "DC component" to refer to any offset from zero in the AC waveform. So, for example, a typical analog telephone line when in use could be described as having a DC component of around 5 volts (referred to as the battery voltage for historic reasons), and then an AC component of around a few volts (varying by signal amplitude) is superimposed. Someone else mentioned the case of an AC signal with its center point not at zero actually being a pulsed DC signal... but both are correct in their own ways. An AC signal with a DC component will have its "neutral" voltage wherever the DC component puts it. This isn't usually referred to as pulsed DC because the AC signal usually starts out that way---as AC, and the DC component gets added. To receive the signal, the DC component is essentially removed. A lot of real systems end up this way either intentionally (in the case of phones) or unintentionally. Much of the time people talk about a DC component its in the sense of an undesirable one induced for some reason. Many people who use SDRs are familiar with this as common direct-conversion SDRs virtually always pick up a spurious DC voltage in the down-converter used to bring the selected frequency band into the range of the ADC. This results in the so-called "DC spike" in the middle of the tuned band.

Now, others have said, and elsewhere you have probably read, that telephone battery voltage is 48-ish volts (varies somewhat by central office equipment and line loss, phones are expected to tolerate a wide range). That's true, but when a phone is taken off hook it closes the loop (while presenting some resistance) and the voltage drops much lower. One of the odd things about DSL from a telephony perspective is that, unlike normal telephone applications, it is designed to function whether the phone is on or off hook. As a result, DSL devices do not make assumptions about the battery voltage, which during DSL operation can vary from off-hook of a few volts to ringing of around 100 volts.

Another odd detail of telephone circuits is that typical local loops use two wires, one pair, for audio both directions. The telephone, though, needs an "in" and an "out" to connect to the microphone and speaker. Similarly, the telephone network itself predominantly operates using pairs of two separate signal circuits, one for each direction, as this greatly simplified analog telephone systems and is required for digitization for digital ones. This is achieved by the use of a hybrid on each end of the phone line, which historically was a type of matching transformer that used some clever electrical tricks to provide three taps. One has signals both directions, the other two have one of each signal cancelled out based on matching or mismatching the impedance of the telephone line. It's a bit hard to wrap your head around and rather clever. Unfortunately hybrids, being analog devices, are never perfect and introduce some oddities on the line. DSL devices must use DSP methods to contend with phase shifts and other issues caused by hybrids. Today, it is increasingly common for not just telco equipment but also consumer phones to also use DSP instead of a hybrid to isolate the directions, since the DSP can self-tune to achieve a more perfect result. Amusingly, so-called "sidetone" in telephones (being able to hear yourself in the speaker) is an undesired result of imperfect performance of the hybrid but turns out to be an important comfort to humans, so DSP-based systems usually intentionally mix the outbound audio into the inbound at a low level.

All of this adds up to DSL being surprisingly robust. Unfortunately, there is a downside to the fact that DSL relies on frequencies beyond what telephone circuits were originally designed to convey: line loss of DSL signals is very high, which results in a rather short practical range for DSL, typically only a few miles even with a local loop in good condition. DOCSIS is able to achieve tens of miles, even at the very high speeds it supports, because coaxial cable and the fittings and amplifiers used are designed to carry high frequencies with minimal loss. Even so, the push to greater-than-gigabit speeds has required outside plant upgrades for cable networks, just as the push to expand DSL coverage (and less so, but in some markets, speed) has lead to outside plant improvements to the telephone network, such as heavy use of remote DSLAMs that "convert" most of the subscriber loop to a longer-range medium like fiber.

[Terry_Roll]

> Amusingly, so-called "sidetone" in telephones (being able to hear yourself in the speaker) is an undesired result of imperfect performance of the hybrid but turns out to be an important comfort to humans, so DSP-based systems usually intentionally mix the outbound audio into the inbound at a low level.

I wonder if the circuit lag from sidetone is something that affects hearing aid users, considering they have another circuit in which audio has to go through before the user gets to hear, but I wonder if the sidetone passes through their skull or jaw bone instead, totally bypassing their hearing aid. I guess the speed of electrical circuits, the speed of electricity passing through a circuit, is a bonus for our slower nervous system and brain which is only running at speeds of up to the old 286's/386's/486's iirc.

>DOCSIS >coaxial cable and the fittings and amplifiers used are designed to carry high frequencies with minimal loss So the old Token Ring network never died out it just morphed into Cable networks?

>heavy use of remote DSLAMs that "convert" most of the subscriber loop to a longer-range medium like fiber. And then there is Deep Packet Analysis (DPA) which can not only be used as a firewall like solution, but also used for compression/decompression to increase the bandwith of fibre by virtue of using compression algo's that give high rates of compression for different types of packet streams based on the type of data and also the way its delivered.

May be Youtube's delivery of packets from a variety of servers in a co-ordinated manner could negatively affect the compression algos, that could then be used to deliver over fibre and other networks, because we cant assume all network traffic for a service comes from one server. At the same time, this method of delivery can also be used to work out where compression of packet data is taking place on a "hop" or number of hops on the network to the end user, by virtue of packets arriving out of order. I think that method could be used to work out physical layouts of telephone networks between their servers and the end user. I then wonder are the spooks/telco providers in some country's using crypto currency like "tumblers" to make the telephone network change so no route is ever the same.

Its interesting but frustrating when looking back at some of the ways things were explained when at school, education can be used for so much more intelligence gathering than meets the eye.

[Johnythree]

The reason for your confusion is that the telephone exchange sends a constant Current, not a constant Voltage. There is 48V, but it's behind a 300+300Ohm relay which limits the current.

[jcrawfordor]

More the reason that I said it varies is because it depends on the equipment. In the US it is nominally derived from 24x lead-acid cells yielding 48v, but in older switches the maintenance charger is connected when the switch has external power and increases battery voltage to around 50v, sometimes a bit higher, like 54v. Basically due to the charger, other countries often specify 50v as the nominal voltage (this is of course similar to how we call automotive systems 12v when in practice they're around 13.7v most of the time).

All this results in 40-50v being considered a normal on-hook line voltage, but it can vary more in the real world. Of course modern SAIs or RLCs or what have you tend to use solid-state regulators that keep a very tight 48v, so I'm sure the variation is much lower in like modern suburban neighborhoods than it is in cities or with older exchanges.

When off-hook, current starts flowing and the line relays and local loop come into play. The line relays are not tightly standardized and range from say 400-700 ohm, but unless you're pretty close to the exchange the line resistance of the local loop is greater, which can be 1kohm or more. Then the actual telephone instrument has a resistance due to the current it uses for operation, 200 ohm is perhaps average but it varies plenty, I think the WECo phones were usually 180 ohm nominal. Both line resistance and telephone resistance vary widely. A total loop resistance of 2400 ohm could be called a maximum because it allows the phone the 20 mA that's considered a minimum for reliable telephone operation, but lots of equipment will work out of that range, especially since so many newer phones have an independent power supply and digital voice circuit. On older switches, where "older" includes plenty that are still in wide service like 5ESS, the line cards have a couple of jumper options to adjust the relay resistance in order to bring loop current up or down depending on length of the loop. That's mostly because high loop currents due to a short loop can shorten the service life of equipment.

Nothing is really regulated (on older equipment and per specs, newer equipment tends to have voltage regulation as a result of using more advanced transistorized power supplies), neither voltage nor current, and so it can all vary within a fairly wide range. This is surprising from the modern perspective but not so much when you consider that the "standards" here are a hundred years old. Newer switches, RLCs, etc often measure the current on lines and raise trouble alarms when it's too high or low, which does impose certain tighter bounds.

[Johnythree]

Phone lines use an AC signal which is superimposed on a DC current. The DC is used to power the phone.