Analog Telephony (Part 1)

Welcome to Analog telephony, as part of the QSC Quantum Training,

an advanced service and troubleshooting curriculum.

My name is Patrick Heyn and today we’re going to be talking about

the most technologically advanced element of the meeting room, just kidding we’re just talking about POTS... I'm sorry!

Obviously, analog telephony is a very old technology.

In fact, it was first demonstrated at the World's Fair in 1876 over a distance of about 10 miles.

In the beginning, there was only very basic signaling and audio, but the centuries progressed,

and we started superimposing additional features on those same two wires.

Analog telephony is often referred to as “POTS”, or Plain Old Telephone Service,

which is probably the saddest but most descriptive name you can think of.

Analog telephone service is hosted on what is called the ‘Central office’, or ‘CO’.

The central office provides the voltage, signal and switching to connect one phone to another during a call.

A phone is wired to a ‘wall jack’ (typically a female connector on a wall plate).

This is called the ‘Foreign Exchange Subscriber’ (or FXS) port.

The port on the phone (or Q-SYS in this case) is called the ‘Foreign Exchange Office’ port, or FXO.

Central offices connect phones to each other using a ‘Public Switched Telephone Network’, or PSTN.

The links from the CO to the PSTN are typically ‘trunks’,

this is where many calls are ‘bundled’ to the global telephony network.

These days, trunks are no longer bundles of analog cabling.

Instead, networks link all these in some sort of Voice over IP technology.

To put all these together, we have FXO devices (phones or Q-SYS)

connected to FXS ports provided by the Central Office.

We make a call through the CO, to the public switched telephony network,

which can theoretically connect us to any other phone in the world.

Many terms used in analog telephony today were named after their analog predecessors so many years ago.

For example, we often talk about a phone being on or off ‘hook’.

That’s because an old phone used to have a literal hook to hold the handset.

When it was off hook, it meant that you were speaking with somebody.

This term somehow survived today, even without the hook!

Sounds like laziness, I know.

There are also a number of terms used in analog telephony to describe

the signals required to receive or make calls.

The most basic signal is known as the ‘talk battery’.

This simple DC voltage provides a modest amount of power to the telephone electronics

and allows the central office to detect when the phone is on or off hook.

The CO detects the phone has gone off hook by detecting the current on the line.

This is known as the ‘loop current’. A phone line using this method is called a ‘loop start’ line.

It takes a relatively large AC voltage riding on top of the talk battery voltage

to activate the ring circuitry in a given phone.

This is called the ‘ring voltage’.

When I took my phone off hook, I should hear a ‘dial tone’.

That tells me that the central office knows that I’m off hook and is ready to accept my input.

When the phone is ringing, the call’s originator needs to know what’s happening.

The ‘ringback tone’ tells the original caller that the destination is ringing.

Call progress tones include dial tones,

ringback tone and any other tones used to help a caller understand what’s taking place.

The reorder tone is another good example of call progress tones.

A hook flash is a short burst on the hook switch that typically involves special features on a line,

like connecting a second call.

DTMF denotes the type of tones used for touchtone dialing, which replaced pulse dialing in the 1960s.

Using this model, each key on the telephone dial pad plays two tones simultaneously.

You know…these are the tones that you used to play ‘mary had a little lamb’ on your telephone!

The tones can be ‘heard’ by the receiving device and interpreted as needed.

If you have ever tried to route yourself through a technical support line,

chances are you've used an Interactive Voice Response, or IVR system,

which uses give us voice prompts and collects your input through a DTMF

or keyword to negotiate the menus and accomplish tasks.

Caller ID is modulated data technology that communicates the callers information.

A Private Branch Exchange (or PBX) is a localized phone system that can be placed in an office building

or structure to handle local extensions.

Many of these systems make use of multi-pair cabling or proprietary features,

so you should be careful when these come up in integration with Q-SYS POTS.

External calls are made through a PBX by way of an analog or digital trunk.

Many PBX systems make use of Ground Start lines rather than loop start lines.

This means one side of the line is temporarily grounded to signal the off condition rather than the current loop.

An analog telephone adapter (or ATA) is a piece of hardware

that converts a VoIP subscriber line to analog telephony.

This may be used in situations where Q-SYS softphones can't subscribe directly to an on-premise VoIP system.

Let’s have a more detailed look at an FXO device.

You can see the ‘tip’ and ‘ring’ ; literally just two wires that go back to the service provider.

Inside the FXO device, you can see the following:

This is a ring detector or a simple ring circuit. The AV ring voltage will excite this circuit if somebody is calling

There's the hook switch. When the hook switch is closed, it connects another component: the current loop.

To the right of these components, you can see other important parts.

There’s an audio ‘hybrid’, which is the system of transformers

used to add or remove audio signal from the line.

And finally, there’s the DTMF generator that allows for user input.

Let’s take a closer look inside the central office circuitry.

The talk battery is a -48VDC power supply. Note that telephony devices are generally NOT polarity sensitive.

It will still work if the tip/ring pair is reversed.

The hook (or current) detector signals the ‘smart’ part of the CO that the subscriber has gone off hook.

Like the telephone, the CO has an audio hybrid for each subscriber.

Then there’s a LOT of switching and control circuits to make the right thing happen at the right time.

Let’s image that I want to call the phone on the right. I pick up my phone and dial the appropriate number.

The CO understands this and connects the AC ring voltage to the subscriber line to the destination.

The DC battery is never disconnected though, the ring voltage simply rides on top.

In the US standard ring voltage is 90VAC at 20Hz. This is going to vary from country to country, as does the ring cadence.

The ‘cadence’ defines how long the bursts of ring voltage occurs and how often.

In this case, the central office would also send me a ringback tone so I know my destination is ringing.

When someone answers the phone, the hook switch closes, and causes current on the line.

When the central office detects that current,

it knows that the call has been answered and can stop ringing the circuit.

The typical loop current is between 25 and 75 milliamps.

Now that sounds like something you probably want to write down… like … for a quiz …

that might be happening later. Write it down. "Between 25 and 75 milliamps."

If the circuit was not ringing and somebody picked up the receiver to make a call,

the CO would detect the loop current and send a dialtone.

Loop current would be the same in both cases.

Alright, that’s a good place to stop! We’ll get into how this gets set up in Q-SYS when we get back.