Does Public Switched Telephone Network (PSTN) use two voice channels for a single voice call - one for each direction?
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1\$\begingroup\$ How many wires are there between your PSTN phone and the wall? \$\endgroup\$– dimCommented Oct 9, 2019 at 8:14
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\$\begingroup\$ @dim There are four wires. However, I am confused because of 2-wire to 4-wire between customer premises and local exchange, and vice versa. \$\endgroup\$– QuirikCommented Oct 9, 2019 at 8:18
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1\$\begingroup\$ for the local loop, only 2 wires are used. That makes a single pair. When using the old analog telephone system, both call directions are therefore merged in the same channel. The other 2 wires you have are likely unused (but maybe this is country dependent - I know that in France, there is only a single analog channel). \$\endgroup\$– dimCommented Oct 9, 2019 at 8:27
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1\$\begingroup\$ @dim What about core network? Does it use one voice channel for each direction? \$\endgroup\$– QuirikCommented Oct 9, 2019 at 8:31
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\$\begingroup\$ @Marcus Müller I am aware of that, but I am doing a little research and want to understand how it worked. If it uses only one voice channel why then four wires were used in the core network (one pair for each direction)? \$\endgroup\$– QuirikCommented Oct 9, 2019 at 8:35
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1 Answer
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Regular analogue phones require two wires to operate and by operate I mean transfer voice signals in both directions simultaneously without the earpiece you are using getting bombarded with your own amplified voice from the handset microphone.
Basically the transmit and receive amplifiers are wired as a kind of wheatstone bridge and this cancels local sounds from being locally received: -
Picture from here.
The cancellation of local transmitted voice in the earpiece is called "sidetone balancing" and relies on the transmitted signal to line producing exactly the same voltage on both inputs of the differential receiver for the earpiece. For this to work effectively, the line's characteristic impedance has to be controlled throughout the telephone network.
If you understand wheatstone bridges then you should understand this.
Does Public Switched Telephone Network (PSTN) use two voice channels for a single voice call - one for each direction?
The network may convert the forward and return voice signals into two separate signals so that line amplifiers can be added when line lengths are particularly long: -
Picture from here (ELC-523).
The above circuit is one half of what is known as a telephone hybrid and here's the bigger picture that shows 2 to 4 to 2 wire amplifier: -
Picture from Elliot sound.
In detail, the hybrid can be realized with two of these although solid state versions are used in the modern world: -
Picture from Elliot sound.
The two wires entering your home are also used to transmit the ringing signal (circa 90 volts AC).
See also this answer for other details.
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1\$\begingroup\$ Could you please provide links or citations to the original images that you copied into your answer? \$\endgroup\$ Commented Oct 9, 2019 at 17:57