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In buildings, it is common for the same cable (coax) to be used for pay-TV and the Internet.

It makes sense that I can transmit several TV channels on the same cable, since it can be modulated by the frequency and sends them. However, in Internet communication, data must be sent and received. There is no cable for the transmission and another for reception. So how is Internet communication on a coaxial cable?

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    \$\begingroup\$ Have you tried looking this subject up? The idea of stack exchange is for questions which can't be answered by simple web searches. \$\endgroup\$ – Chris Stratton Mar 2 at 15:47
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    \$\begingroup\$ It's not entirely simple, but the answer is en.m.wikipedia.org/wiki/DOCSIS \$\endgroup\$ – pjc50 Mar 2 at 15:52
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    \$\begingroup\$ Lucas, the same as for cable can be said for wireless communication... \$\endgroup\$ – Marcus Müller Mar 2 at 15:55
  • \$\begingroup\$ @Chris, I do not consider this question trivial because I asked two professors, one PhD in engineering, and another master. one could not answer me and another said that there is a multiplexing by time (ie, the current would stop and wait to be able to change the direction.) what for this new information or it is wrong, or who answered me. \$\endgroup\$ – Lucas Mar 2 at 19:50
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    \$\begingroup\$ @Lucas, I did not say the question was trivial, but that it could be answered a simple web search. It is the search which is simple, not the search results. And part if the point is that what you can find there is more comprehensive than would fit in a response here. \$\endgroup\$ – Chris Stratton Mar 2 at 20:05
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Energy moves through the cable in both directions simultaneously. Just as different video signals are modulated on different channel frequencies, incoming and outgoing data streams are modulated on different carrier frequencies, and pass each other without interference.

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    \$\begingroup\$ @Lucas depending on the standard, both USB(USB 2 and lower) and ethernet (1000 base-T) use a bidirectional interface. In USB 2's case, it's half duplex so only the root or device can transmit at one time. In Ethernet's case some special processing is used to subtract the transmitted signal from the data so the other signal can be received. \$\endgroup\$ – C_Elegans Mar 2 at 20:01
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    \$\begingroup\$ Think about what happens when two people try to talk at the same time. Do their voices actually collide? To make it even clearer, say it is a young child and their father, so we might say different frequency bands are being used. \$\endgroup\$ – Chris Stratton Mar 2 at 20:02
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    \$\begingroup\$ @Lucas the reason many interfaces use separate rx and tx lines is because of cost. Adding the extra processing and circuitry to be able to separate the two signals on one wire makes the hardware cost more. Granted, using separate lanes makes the cable cost more, but that tends to be less of an issue for how long the cables are (ethernet is pretty long, but USB, SATA and PCIe are not). Compare this to how much cable the cable company needs to lay, and it's a bit more reasonable that they would favor expensive hardware and cheap(er) cable \$\endgroup\$ – C_Elegans Mar 2 at 20:04
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    \$\begingroup\$ It is very common. The telephone was invented in 1876. \$\endgroup\$ – AnalogKid Mar 2 at 20:09
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    \$\begingroup\$ @Lucas as with most electronics questions, ignore the electrons - they will confuse you. The real signal carrier is the electromagnetic fields. It's much easier to understand it as a radio signal that has been contained in a tube. \$\endgroup\$ – pjc50 Mar 2 at 20:32
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Internet over CATV is called DOCSIS (https://en.wikipedia.org/wiki/DOCSIS).

It uses several channels separated by frequency for downstream and upstream. Think about FM radio. How can you have several channels on radio? They just use different frequencies.

This is called "Frequency-division multiplexing" (https://en.wikipedia.org/wiki/Frequency-division_multiplexing)

Here is an article that covers it: https://volpefirm.com/docsis101_rf-fundamentals/

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Radio frequency communications operate the same when travelling through a coaxial cable as they do through open-air. They are just shielded from outside interference (called ingress) and leakage (called egress). As such, signals of differing frequencies can co-exist, with each travelling in different directions.

Amplification, however, is a different story. Since amplifiers work in only one direction, the incoming and outgoing signals need to be separated when amplification is necessary. This is performed by a device called a diplex filter, which is sort of like a splitter/combiner that splits/combines based upon the frequency of the signal. In legacy CATV systems, downstream signals were generally about 50 MHz (around the bottom of analog channel 2) and up, while upstream signals were from around 5 MHz to 40 MHz.

An amplifier assembly would (basically) consist of a diplex filter on one end separating the two frequency ranges, followed by an amplifier for each frequency range oriented in opposite directions, and then a second diplex filter to merge the two frequency ranges to its original full spectrum signal.

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  • \$\begingroup\$ Thanks for editing for readability and adding the link, Peter! \$\endgroup\$ – Hitek Mar 3 at 19:18

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