For the past century, people have let phone companies and cable television providers run new cables throughout their homes, and new construction will often include coax and phone lines by default. However, you rarely encounter homes, even new ones, with runs of CAT5 or CAT6 cable, and un-keyed 8P8C recievers in the walls.
Natrually, the reason for this is that Wi-Fi has become the preffered standard for connecting devices in a home network. This makes sense for cell phones and laptops, but many people still have dedicated workstations, or smart TVs in fixed positions in their home, and relying on Wi-Fi for those devices is often an unnecessary downgrade.
I think most people would expect devices that don't move to have the best connectivity, especially since those devices are likely to make up the bulk of the network demand. If your TV happens to be in an optimal position relative to your Wi-Fi router, perhaps you get good download speed, but I imagine many people are stuck with a TV that buffers all the time, because it's too far from the router, and there's no better place to put the thing.
This really shouldn't be an issue, but a proper fix would mean potentially thousands of dollars in installation costs for multiple runs of some twisted pair cable, and putting in proper network switch. For some, the cost and inconvenience might be worth it, but it certainly isn't for most people.
Various electronics manufacturers have identified this problem, and have come out with a great variety of communications standards and specifications that aim to leverage the existing cabling in a home; like coaxial, phone lines, and power lines.
Here I cover some of these technologies, where they may be useful for a given application, and what considerations to make if you intend on using any of them.
I imagine that powerline ethernet adapters are the most well known out of this class of devices.
The principle they operate on is simple. Powerlines normally carry only a 60Hz sine wave, with 120v potential to ground. A powerline ethernet adapter will impose an additional signal at a much higher frequency (4.5MHz to 21MHz), on those copper lines, and listen for signals of that frequency range coming from other devices. These signals are at such a low power level compared to AC power, that they have no impact on other devices plugged into other outlets.
There is a catch, however.
Powerlines are typically unshielded. They don't really need to be; no transmitted signal is going to be powerful enough to significantly affect the AC waveform. This is a problem for us, however, because signals picked up by the lines in our frequency range could very well interfere with our signal, slowing down transmission rates, or even stomp it out completely.
If you're lucky enough to live in a home where the wiring must be run through metal conduit, like those in Chicago, Illinois, your powerlines are ideal for this application, as metal conduit may largely shield the lines from electromagnetic interference.
Other homes with thin interior and exterior walls, with only unshielded Romex running through the walls, may not fare nearly as well.
The primary standard for ethernet over coaxial cables is MoCA, for "Multimedia over Coax Alliance".
Originally developed for distributing IP Television, MoCA is now a general-purpose ethernet link
You're probably already familiar with broadband service delievered over the coaxial cables originally installed to serve cable television. This is the same cable, and the same standard, that MoCA uses. The only difference is that MoCA uses a different frequency range than the one used by cable television, so that the two signals can coexist on the same cable.
Because all coaxial cables are connected at a common point, it can be used to create a wired LAN in a home without running new cables.
The modern MoCA standard is capable of speeds up to 2.5Gbps, so while not as fast as a dedicated CAT6 run, it is still more than sufficient for most home applications.
By and large, using coaxial cables is preferred to using powerlines, as coaxial cables are shielded, and will not suffer from the same electromagnetic interference that powerlines do. While some powerline adapters are technically capable of speeds up to 2.5Gbps, they intereference they are subject to will often result in much lower speeds, closer to 500Mbps or lower.
Fewer and fewer people are holding onto their landlines, but many homes still have phone lines running through them. These lines are typically unshielded twisted pair (UTP) cables, and are capable of carrying ethernet signals. However, the maximum speed you can expect from these lines is 100Mbps, which is the same as Fast Ethernet.
This is because the phone lines are designed to carry voice signals, which are much lower frequency than the signals used by modern ethernet standards. The maximum frequency that can be used over these lines is 31.25MHz, which is the same as Fast Ethernet (100BASE-TX). This is why the lines are unshielded, as the low frequency, analog signals, can suffer much more interference before it becomes unintelligible.
There are two options for using phone lines for ethernet: modified phone lines, or G.hn. The existing phone lines use 6P4C connectors, which are not compatible with standard 8P8C connectors used by ethernet. You can either modify the phone lines to use 8P8C connectors, or use a G.hn adapter, which can use the existing phone lines without modification. If you choose to modify the phone lines, you will need to ensure that the lines are not connected to any active phone service, and take steps to prevent issues like signal reflections and crosstalk.
While Wi-Fi is the most convenient way to connect devices in a home network, it is not always the best option. Powerline, coaxial, and phone lines can all be used to create a wired LAN in a home without running new cables, but each has its own limitations and considerations.
There are other technologies that can be used to improve home networking, like Wi-Fi over Coaxial, which helps propagate existing Wi-Fi signals by effectively turning coaxial cables into Wi-Fi antennas, or plastic optical fiber, which uses cables like TOSLINK to create a high-speed, low-latency network over short distances. The G.hn set of standards covers pretty much all conductors you might find in a home, so you're unlikely to find a cable that can't be used for some form of networking.