How did old dial phones figure out where the call is going without having a computer inside? What is the mechanism behind the dial?



How did old dial phones figure out where the call is going without having a computer inside? What is the mechanism behind the dial?

In: Technology

The phone doesn’t do the routing, the phone line provider does. Old days they had operators and now they have computers that connect your call. There is computers just not in your phone

They sent analog signals through the line, different frequencies for each number and then the network can use analog electronics to route the signal to the correct destination. (By identifying the frequencies with filters and then using relay switches)

Before this method existed every phonecall you made reached some operator who would then connect your phone physically to the destination you told them by connecting your ports with a wire. (My grandma worked that job)

there are two legacy way for older thelephone to compose a number: pulse signaling and tone dial.

pulse signaling, more often found in phones with rotary dial, flip the hang button as many time as the number you put: if you want to call, let’s say the number 123, it will create the following electrical pulse on the “hang” wire: – — —.

Tone signals instead produce a sound, a note for every number you digit directly on the voice wire: it’s the “beep” you hear when you’re composing a number.

today provider sides calls are all routed on various VoIP networks but for compatibility there is a device between your old phone and their network that take the duty of converting those electrical signals into digital signals that can be managed by them.

The old, rotary dial telephones were just a small part of a much more complicated machine – the local telephone switch (local being local to a small town or several large neighborhoods.)

As part of the greater machine (local switch), and in order to minimize the number of individual wires connecting the handset (rotary telephone) to the larger switch, a signalling mechanism was used involving completing and interrupting a circuit with multiple voltages, both AC and DC.

When a call was received AND the phone was “on hook” (such as the receiver being in the cradle), the local switch would send a high voltage AC signal to activate the bell[2] , at around 20Hz.

Once the receiver was picked up (the phone was “off hook”), the local switch would detect that and stop the ringer signalling. The mouthpiece in the receiver (the part you spoke into) was a “carbon microphone”[3] that would change the electrical resistance with the pressure waves of the sound impacting on it. This change in resistance would produce the analogue electrical signal representing the voice of the speaker.

Conversely, the sound from the other end was impressed on the pair of wires at a higher frequency (than the ringer). A system of passive electronic filters made of inductors, capacitors, and resistors separated the mouthpiece signal from the earpiece signal.

In order to make a call, the receiver would be lifted off the cradle, changing the phone from “on hook” to “off hook” state, completing the DC circuit. The local switch would detect this (and would usually provide feedback as a low frequency buzz.) The local switch would connect the phone circuit to a “stepper” switch.

Operating the dial would (quickly) switch the DC circuit at a fixed rate (frequency.) This frequency was low enough that it was possible for some agile people to actually make phone calls by tapping the cradle “hook” button fast enough, with the right timing. (A parlor trick for nerdy teenagers.)

This fixed rate of on-off switching is the “pulses” of pulse dialing[4] . In the original design, it caused the stepper switch[5] to step through it’s various positions and then (usually) proceed to connect to another stepper switch.

These stepper switches were electro-mechanical and hideously complex (for the time – and even for now.) The sheer physical size and number of these stepper switches needed for each subscriber line, the number of cross connects needed, etc. meant that most of these switches didn’t support more than 5-digit dialing.

The phones themselves had no idea what they were doing.

They were just a speaker, a microphone, a bell and a way to create signals/make some noises by pressing buttons or turning a dial.

The clicks when the dial was turned and the beeps when buttons were pressed were send down the wire the same as the words you spoke into the speaker.

It was up to the switchboard to turn these signals into instructions of how to connect your call.

These machines did not need to especially smart either. We could make them before we had real computers.

The numbers you pressed/dialed were the instructions on how to connect your call.

Phones don’t decide where calls are going. The only thing any phone does is send and receive signals to its network. Even when you make a cell phone call, all it’s doing is sending a phone number to its tower, and it’s the machinery behind the tower that does the work of connecting a call.

Originally, you didn’t even dial numbers. When you picked up the phone, you waited for an operator to notice (you might have a way to ring a bell to draw their attention). Then you’d tell them who you wanted to connect to, and they’d manually make the connection. They sat at a machine called a “switchboard”, and they were a “switchboard operator”, which is why we called them “an operator” for short.

Phones still work that way, but we’ve mostly replaced operators with machines.

The phone is connected to machines that can receive those signals. When you use a touch-tone phone, the machine is listening for those specific sounds and it uses the series of numbers to talk with other machines to connect the call, just like the operator did. When you use a rotary phone, the spinning part of the phone generates electrical pulses at a specific speed. Dialing 9 makes 9 pulses, dialing 1 makes 1 pulse, etc. The machines at the phone company count the pulses, figure out the number, then do the same thing they’d do for a touch-tone call.

In a way, it’s a very simple version of how we used to get internet over plain old phone lines! A modem is just a device that can send signals over a phone wire. Some people say it plays sounds over a phone, but sound is just an electrical signal on the wire to the phone. So it would “pick up” the line, send the signals for the number to an ISP, then start sending signals to the ISP’s machines that, while we could hear the noise if we picked up the phone, these signals were meant to efficiently transmit data about internet traffic, not sound pleasant!