ElI5 How do telephone wires carry multiple messages at once? How do messages not interrupt each other?


ElI5 How do telephone wires carry multiple messages at once? How do messages not interrupt each other?

In: Technology

Communicating use a pattern called multiplexing to carry multiple digital signals–each one gets an allotted window of time to share data within a given period, usually fractions of a second. For analog signals, they can either use different carrier signals to separate multiple channels, or an old fashioned party line simply shares conversations openly. Most phones today are digital, and randomly generated channel hopping (wireless, usually) and multiplexing keeps conversations separate effectively.

There are very, very few “telephone wires” in the US. They exist in other parts of the world, but almost all telephones in the US are digital.

When you have analog copper wire telephones (what’s sometimes called POTS, for plain old telephone service) the wires only carry one call at a time.

In modern systems, the phone calls are just packets, like the Internet, with headers that say what call they are part of. The packets are all poured into a super-fast pipe and sorted out at the other end.

The analogy is to consider listening to music say from a band or orchestra. All the instruments send their sound messages through the air. It is still possible for us to discern between a drum and a guitar and a piano – and very likely hear the different notes they are playing even though they sound at the same time. In a similar way, multiple messages can be encoded on a single wire and with the proper filtering at the receiving end, these messages can be “heard” separately.

This happens all the time with analog radio (for example). All the radio signals from all the stations are present everywhere and the radio is designed so that you can “tune in” to a particular station and ignore the others.

With digital, there is even more ability to interleave messages and use digital processing to de-interleave the received digital data into various streams which is why one internet cable can simultaneously support multiple devices browsing, streaming, gaming etc.

How do highways carry multiple vehicle at one time?

There’s something called channels. That like having multiple lanes on a highway where you cannot change lanes.

There’s something called muxing (multiplexing). That’s like having a lane on the highway that’s being shared by different trucks, some from Walmart, some from Amazon. On highways, trucks are separated by distance, I.e 50 foot separation. On a wire, or a fiber, the packets are separated by a slight (time) gap.

Once upon a time in the Stone Age, there were huge bundled cables packed with wire strands distributed all over the country, and each wire strand carried a single call signal, typically one frequency was used for “upstream” and another frequency for “downstream”. However, there were two wires in your phone jack where the final signal was sent/received by the telephone itself while indoors, and this was handled by the junction box down the street. Switching stations along the call route determined which wire was used (any not already in use) — in the old days, this was done manually, but after WW2 there was increasing interest in mechanical methods of directing call signals, which is why old rotary phone had clicks when dialing a number. Click dialing was eventually rendered obsolete by “touch tone” keypads, and around this time there was a slow increase in digital switching and signalling, and today everything is simply multiplexed.