In layman terms, power line communications (PLC) is the transmission of both data and voice over the same cables that carry electricity to your house. This is done by adding to the power line a high frequency carrier signal that can carry data and voice. This therefore means a power line will have two frequency components namely the power frequency at 50-60 Hertz and the carrier signal at a 20MHz-20Ghz range. PLC can therefore turn every power socket in every house into a data and voice port.
This technology is popularly used in SCADA systems by utility companies to perform such tasks as pre-paid metering, switching on/off of street lights and controlling remote power substations. All these operations require low data rates to perform.
With the advent of the internet and ISPs some operators thought of using PLC as a last mile solution to connect customers to the Internet. several ISPs could lease the power lines and by using various frequencies avail services to all customers that have electric supply to their homes. All the end users need to buy is a device to plug on any socket and start enjoying internet services such as the one pictured. This seemed a very viable alternative to laying separate infrastructure to provide internet services on. PLC was once touted as the panacea to last mile access problems.
However, some technological barriers have prevented PLC from being fully exploited for use as a viable last mile solution for internet access and other forms of communication.
warning: geek speak starts here:
When you pass a signal in a conductor/wire, it distributes itself across the cross-section of the conductor, however, as you increase the frequency of the signal, it starts moving away from the center of a conductor towards the surface so that the center of the conductor ends up not carrying any signal (this explains why most high frequency conductors are hollow at the center). As the frequency becomes high, a point is reached when all the signals have moved so far away from the center of the conductor that the conductor now transmits from the surface and the conductor begins to glow with a pale blueish color. The glowing effect is known as corona discharge while the act of the signal moving towards the surface (or skin) is known as skin effect and is the reason why high voltage power lines glow at night(on the other hand Corona discharge produces Ozone gas that can harm living organisms). Skin effect causes the resistance of the cable to increase. Now, on the other side, to transmit higher data speeds requires a higher frequency carrier. This therefore means that as more data is pumped through a cable, its resistance increases and this affects the electricity flow in the cable.Most power companies therefore started opposing the use of PLC as a last mile solution for ISPs as it started affecting the quality of power they transmitted to their customers.
From basic high school physics, a transformers primary and secondary coils are not electrically connected but are magnetically connected/coupled. However, electric coupling occurs at lower frequencies and the normal electric power frequency of 50-60Hz can jump the transformer gap. Sadly, higher frequencies cannot cross a power transformer and therefore this effectively stop the data signal carrier. This is circumvented by the use of a PLC data coupler that enables this high frequency carrier to do the jump from the primary to secondary coil such as this one here by ABB. At higher frequencies however, the coupling starts to get affected by the requirement of a more stringent impedance (resistance) matching of both sides of the transformer. In layman terms, if you switch on or off any device in your home, you change the impedance on one side of the transformer and at lower frequencies, the impedance mismatch is not an issue, however, at higher frequencies like those needed for data transmission, this change has an effect on the system and can cause undesired effects. This is the second reason why PLC has not picked up as the preferred last mile solution for ISPs.
To overcome the issues highlighted above, power companies have implemented work arounds. They have done this by installing fiber links alongside or within the power transmission lines. majority of long distance power lines actually come with a fiber strand inside the cable that can be used for data transmission. In fact this is the capacity that KPLC was selling to ISPs such as Wananchi, Jamii and Safaricom to connect Nairobi and Mombasa. The other alternative is to lay separate fiber link along the power lines and this is what KPLC is currently doing in the pre-paid meter rollout in the country. The fiber is laid to the nearest transformer and the a coupler is used to inject the data signal to the distribution cables that go to the customer premises. I foresee KPLC leasing out the extra capacity of the links after they are done with the prepaid meter roll out that is currently ongoing.
With every household in the country that has mains electricity on prepaid meter, KPLC is bound to turn the tables on the provision of last mile solutions in the country. The infrastructure it is laying in place for prepaid metering will make it the only company in the country with the most extensive last mile network because every power outlet socket in every house will also be a data point. ISPs that are busy building last mile solutions better take note of this for KPLC will soon be able to offer last mile access to anyone at a very low fee.