[NTYNUT (OP)]

Hello
what is power line communication modem i have seen it but dont understand its working

[evan_au]

It is possible to transmit data over the 50Hz power lines. There are 3 main applications, but they all seem to be declining in popularity:
1. A power company can transmit data over its long-distance high-voltage transmission lines between substations, for frequency control, fault handling, etc. This is now mostly replaced by optical fiber which is faster, and can carry lots of other data too.
2. A power company can transmit data to and from electronic meters. However, there is a lot of electrical noise on power lines in homes and factories which causes interference. The 50Hz power grid does not have a controlled impedance, which also makes communication difficult. Many power companies are now trying to use a wireless cellular network; the more adventurous are trying to use an ad-hoc wireless network or even the home internet connection...
3. A home owner can install Homeplug devices to communicate between computers within their home, so they don't have to drill holes in the walls. As well as the electrical noise noted above, homes with 3-phase power have real problems communicating between different phases. Most home owners are now using WiFi.

As you can imagine, all 3 of these can cause interference to each other and they tend to radiate interference that can interfere with other services. The 50Hz power grid was not designed to carry signals in the frequency range of 100kHz to 100MHz.

See: https://en.wikipedia.org/wiki/HomePlug

[chris]

They are pretty rubbish, to be honest. I've got a cluster of them that I was using for transmission of smart-tv streaming media from router to TV on the other side of the house where the wifi struggles. The bandwidth was tragically poor...

I've since abandoned it and moved the TV instead!

[Colin2B]

My experience is the same.
Wifi extenders seem to work well, Ive got one to cover dead spots and it will stream tv ok.