Asterisk and POTS / PSTN Fundamentals

I’m working on a pro bono project for a non-profit public-service organization. The PBX is installed and running using SIP phones at two locations and several people’s homes, and all seems to be working well.

The next step is to implement a Radio-over-IP system where an incident commander can pick up a phone, dial a 3-digit extension, and talk to an aircraft or a ground search team via radio.

I would like to implement the radio interface as a standard analog telephone lookalike that plugs into a standard PBX FXS port. To do that I need to learn all about how the PSTN connections work, i.e. what voltages and signals are generated at the FXS port to drive a telephone ringer, what happens at the FXS port (voltage levels, etc.) when the phone goes off-hook, and how the audio gets superimposed over the voltage levels so that audio, ring signaling, and off-hook/on-hook signaling can all happen on one pair of wires.

Can any of you recommend a good book or (preferably) website I can use to learn about all of this? Yes, I can (and did) google it but have little way of separating the good resources from the bad other than the experiences of people such as yourselves.


Eric /

BT (the orginal UK PTT) publish their interface specifications on

It is possible that some of this assumes standard practice that is too well known to document.

I imagine the datasheets for line interface chips will also include the information.

I imagine radio interface are normally four wire. You will need some way of operating PTT on the radio. There are specific radio interfaces for Asterisk, designed from an amateur radio point of view.

In commercial terms, the buzz word for this is “phone patch”.

PSTN lines typically have nominal 48 volts DC on them (from lead acid batteries, for an exchange line) and ringing is 60 volts AC, superimposed on this. These are exchange end voltages, and may well be less for PABX line cards.

Any DMM will give you the DC voltage. A peak reading meter would be needed to measure the AC component. Subtract the DC component and divdie by root 2, to get the RMS value. If you are going to construct custom interfaces, such tools should be available or easily made.

Good afternoon, David.

I have several commercial phone patches around here, mostly hooked up to radios. I was considering probably pressing one of them into service to do the actual phone line interfacing. But, their function is to extract/insert audio from/to the phone lines and that’s all. They don’t support any kind of controlling or signaling, at least not without modification.

If at all possible I prefer to handle ALL audio and control over the two-wire POTS line. It must be possible… taking a phone off-hook does something to the two-wire line to alert the CO that the phone has been taken off hook. And there is something called “answer supervision” or something close to that, that alerts the calling device that the call has been answered at the other end. I’m told that is done by reversing the polarity of the DC component on the two-wire line. Thus there must be at least one way to signal control or status information in each direction, plus audio, plus ringing, all on two wires. If I can figure out what those ways are, I can certainly design a circuit that uses them to operate the PTT on the radio.

If you find the right supplier information note (the single line one) from the site I gave you, it will tell you how it is done in the UK.

Seize is a lowering of the DC loop resistance.

If answer supervision is provided (not necessarily true, and probably not true on an FXS port on a PABX), the most common way is battery reversal. However, I believe that dahdi can do reversals on FXS ports, as I think it can forward caller-ID, which is normally preceded by a line reversal. That doesn’t mean it can send answer supervision, though. (Analogue modes relied on detecting carrier, or the echo suppressor disable tone, to detect answer, not any answer supervision.)

Traditionally the audio was fed using a hybrid transformer, assuming a particular AC load (600 ohm??). Nowadays, it is done electronically. If you present the wrong load, you are likely to generate far end echo for the other side.

Disconnect supervision can be a temporary removal of battery voltage, a long tone, or not supported. Again, I wouldn’t expect a PABX FXS port to generate disconnect supervision, but maybe Asterisk can.

have a google for apt_rpt which is the asterisk radio repeater project