Video Transcript
Analog Telephony (Part 1)
8m 16s
00:07
Welcome to Analog telephony, as part of the QSC Quantum Training,
00:11
an advanced service and troubleshooting curriculum.
00:14
My name is Patrick Heyn and today we’re going to be talking about
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the most technologically advanced element of the meeting room, just kidding we’re just talking about POTS... I'm sorry!
00:25
Obviously, analog telephony is a very old technology.
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In fact, it was first demonstrated at the World's Fair in 1876 over a distance of about 10 miles.
00:36
In the beginning, there was only very basic signaling and audio, but the centuries progressed,
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and we started superimposing additional features on those same two wires.
00:46
Analog telephony is often referred to as “POTS”, or Plain Old Telephone Service,
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which is probably the saddest but most descriptive name you can think of.
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Analog telephone service is hosted on what is called the ‘Central office’, or ‘CO’.
01:03
The central office provides the voltage, signal and switching to connect one phone to another during a call.
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A phone is wired to a ‘wall jack’ (typically a female connector on a wall plate).
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This is called the ‘Foreign Exchange Subscriber’ (or FXS) port.
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The port on the phone (or Q-SYS in this case) is called the ‘Foreign Exchange Office’ port, or FXO.
01:28
Central offices connect phones to each other using a ‘Public Switched Telephone Network’, or PSTN.
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The links from the CO to the PSTN are typically ‘trunks’,
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this is where many calls are ‘bundled’ to the global telephony network.
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These days, trunks are no longer bundles of analog cabling.
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Instead, networks link all these in some sort of Voice over IP technology.
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To put all these together, we have FXO devices (phones or Q-SYS)
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connected to FXS ports provided by the Central Office.
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We make a call through the CO, to the public switched telephony network,
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which can theoretically connect us to any other phone in the world.
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Many terms used in analog telephony today were named after their analog predecessors so many years ago.
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For example, we often talk about a phone being on or off ‘hook’.
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That’s because an old phone used to have a literal hook to hold the handset.
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When it was off hook, it meant that you were speaking with somebody.
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This term somehow survived today, even without the hook!
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Sounds like laziness, I know.
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There are also a number of terms used in analog telephony to describe
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the signals required to receive or make calls.
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The most basic signal is known as the ‘talk battery’.
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This simple DC voltage provides a modest amount of power to the telephone electronics
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and allows the central office to detect when the phone is on or off hook.
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The CO detects the phone has gone off hook by detecting the current on the line.
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This is known as the ‘loop current’. A phone line using this method is called a ‘loop start’ line.
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It takes a relatively large AC voltage riding on top of the talk battery voltage
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to activate the ring circuitry in a given phone.
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This is called the ‘ring voltage’.
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When I took my phone off hook, I should hear a ‘dial tone’.
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That tells me that the central office knows that I’m off hook and is ready to accept my input.
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When the phone is ringing, the call’s originator needs to know what’s happening.
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The ‘ringback tone’ tells the original caller that the destination is ringing.
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Call progress tones include dial tones,
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ringback tone and any other tones used to help a caller understand what’s taking place.
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The reorder tone is another good example of call progress tones.
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A hook flash is a short burst on the hook switch that typically involves special features on a line,
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like connecting a second call.
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DTMF denotes the type of tones used for touchtone dialing, which replaced pulse dialing in the 1960s.
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Using this model, each key on the telephone dial pad plays two tones simultaneously.
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You know…these are the tones that you used to play ‘mary had a little lamb’ on your telephone!
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The tones can be ‘heard’ by the receiving device and interpreted as needed.
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If you have ever tried to route yourself through a technical support line,
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chances are you've used an Interactive Voice Response, or IVR system,
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which uses give us voice prompts and collects your input through a DTMF
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or keyword to negotiate the menus and accomplish tasks.
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Caller ID is modulated data technology that communicates the callers information.
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A Private Branch Exchange (or PBX) is a localized phone system that can be placed in an office building
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or structure to handle local extensions.
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Many of these systems make use of multi-pair cabling or proprietary features,
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so you should be careful when these come up in integration with Q-SYS POTS.
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External calls are made through a PBX by way of an analog or digital trunk.
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Many PBX systems make use of Ground Start lines rather than loop start lines.
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This means one side of the line is temporarily grounded to signal the off condition rather than the current loop.
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An analog telephone adapter (or ATA) is a piece of hardware
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that converts a VoIP subscriber line to analog telephony.
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This may be used in situations where Q-SYS softphones can't subscribe directly to an on-premise VoIP system.
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Let’s have a more detailed look at an FXO device.
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You can see the ‘tip’ and ‘ring’ ; literally just two wires that go back to the service provider.
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Inside the FXO device, you can see the following:
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This is a ring detector or a simple ring circuit. The AV ring voltage will excite this circuit if somebody is calling
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There's the hook switch. When the hook switch is closed, it connects another component: the current loop.
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To the right of these components, you can see other important parts.
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There’s an audio ‘hybrid’, which is the system of transformers
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used to add or remove audio signal from the line.
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And finally, there’s the DTMF generator that allows for user input.
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Let’s take a closer look inside the central office circuitry.
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The talk battery is a -48VDC power supply. Note that telephony devices are generally NOT polarity sensitive.
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It will still work if the tip/ring pair is reversed.
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The hook (or current) detector signals the ‘smart’ part of the CO that the subscriber has gone off hook.
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Like the telephone, the CO has an audio hybrid for each subscriber.
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Then there’s a LOT of switching and control circuits to make the right thing happen at the right time.
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Let’s image that I want to call the phone on the right. I pick up my phone and dial the appropriate number.
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The CO understands this and connects the AC ring voltage to the subscriber line to the destination.
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The DC battery is never disconnected though, the ring voltage simply rides on top.
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In the US standard ring voltage is 90VAC at 20Hz. This is going to vary from country to country, as does the ring cadence.
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The ‘cadence’ defines how long the bursts of ring voltage occurs and how often.
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In this case, the central office would also send me a ringback tone so I know my destination is ringing.
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When someone answers the phone, the hook switch closes, and causes current on the line.
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When the central office detects that current,
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it knows that the call has been answered and can stop ringing the circuit.
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The typical loop current is between 25 and 75 milliamps.
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Now that sounds like something you probably want to write down… like … for a quiz …
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that might be happening later. Write it down. "Between 25 and 75 milliamps."
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If the circuit was not ringing and somebody picked up the receiver to make a call,
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the CO would detect the loop current and send a dialtone.
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Loop current would be the same in both cases.
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Alright, that’s a good place to stop! We’ll get into how this gets set up in Q-SYS when we get back.
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Analog Telephony (Part 1)
8m 16s