The outside BPL has its issues with interference and cost. The inside BPL using HomePlug has none of those problems. It's affordable for individuals and small businesses and has been designed and tested to avoid interference to Ham Radio receivers. So how does HomePlug work and how can you use it?
HomePlug is the product of the HomePlug Powerline Alliance, an industry association that formed 5 years ago. The goal was to be able to use the normal power wiring within a house to function as an Ethernet-type data network. That goal seems more ambitious once you know some of the ugly things that are hanging around power lines just looking to cause trouble. You know those filtered surge protectors you buy to protect your valuable electronic equipment? HomePlug has to work in that environment without the filtering.
The allure of home power line networking is that its already in place. All houses, even those creaky hundred year old structures, are wired for 120 VAC power. Every room has at least one outlet and usually several. Why reinvent the wheel or, in this case, wiring when it's sitting there free for your use? With HomePlug, you simply plug an adaptor for Ethernet or USB into the nearest power outlet and connect your networked device. That could be a PC, printer, wireless access point and so on. The adaptors talk among themselves over the power wiring and act just like they were on their own network wiring.
HomePlug creates an Ethernet-like MAC (Media Access Control) layer with a throughput of 8.2 Mbps, which compares favorably with a 10 Mbps Ethernet speed of 9.8 Mbps and a 802.11b WiFi speed of 7.48 Mbps at the MAC layer. The physical layer for HomePlug has a throughput of 13.78 Mbps. No, these aren't 100 Mbps Fast Ethernet speeds, but they are likely more than adequate for most home office and small business networking.
HomePlug transmits on a frequency band of 4.5 to 21 MHz, which actually works much better than lower radio frequencies at crossing circuit breakers and bridging the two phases of typical residential 60 Hz power. This wide spectrum is generated using Orthogonal Frequency Division Multiplexing (OFDM) with up to 84 separate carriers. In actual use, some of the carriers have to be turned off due to interference from fluorescent lights, sparking motor brushes, switching power supplies in electronic equipment and light dimmers. Carriers that would interfere with Amateur Radio frequencies are also not used.
How about network security? You might think that it isn't an issue since the wires are in your walls. But there's another wire that leaves the house and connects at a power transformer outside, along with maybe 6 other houses. Yes, the power drop for your electrical service makes a convenient node for interconnecting multiple dwellings. Your HomePlug signals may well be sneaking into your neighbor's wiring. The solution is the same as for WiFi that radiates beyond your property. All devices on your network must share a common encryption key for 56 bit DES data encryption.
