Hands-On Powerline Networking: How Well (Or Not) Are Latest-Generation Devices Working?by Brian Dipert on September 1, 2011 12:41 AM EST
If you made it through the italicized technical protocol descriptions of the prior page, you encountered numerous references to bit errors and their mitigation; forward error correction approaches, retransmit schemes, hundreds or thousands of discrete transmission channels with independently configured modulation densities, etc. Such workarounds exist because the power grid was never intended for networking purposes and, as such, is a quite unfriendly environment for reliable high-speed data transfers.
Consider, first and foremost, that every AC-fed device creates a momentary dip or surge (however slight) when it first powers up or off. Such situations are usually occasional, and as such can be dealt with packet retransmission (in the case of TCP) or brief loss (with UDP). More egregious, on the other hand, are devices that inject a constant stream of high frequency noise onto the power grid, such as:
- Switching power supplies (including AC-to-DC converters used in cellphone chargers and the like)
- Motors in devices such as fans, hair dryers, vacuum cleaners, washers and dryers, furnaces and air conditioners, and refrigerator compressors
- Illuminated CFLs (compact fluorescent lamps)
Such devices' noise patterns can destructively interfere with one or multiple channels' worth of powerline networking data. And at this point, I should also point out that the active powerline network can itself be a destructive interference source, specifically for shortwave radios, by virtue of the fact that current passing through a wire creates a magnetic field surrounding that wire, thereby turning it into an antenna. Powerline technologies are a longstanding sworn enemy to many 'ham' radio operators, although LAN-based powerline approaches are far less egregious in this regard than are WAN BPL (broadband over powerline) approaches spanning a large region. And powerline adapters are also intentionally designed with notch filters that, when activated, create channels (at the tradeoff of reduced peak bandwidth) that might interfere with other transmitters and receivers in a particular geography.
Next is the issue of networking signal attenuation, which is first and foremost caused by old or otherwise low-quality electrical wiring. Other potential problems include narrow-gauge wiring, with excessively high impedance; poor intra-span connections and variable gauge wiring across the span both result in unwanted reflections. Powerline packet 'jumps' across circuit breakers are performance-problematic; even more so are source-to-destination paths that involve a transition from one 110V (U.S.) phase of the incoming 220V source to the other phase. Even within a particular circuit breaker wiring spur, the presence of GFCI (Ground Fault Circuit Interrupter) outlets can cause problems, even if a powerline adapter isn't directly connected to them.
Don't try to connect a powerline adapter to a surge protector, which will filter out the high frequency data modulated on the 50 or 60 Hz carrier, unless the adapter is three-prong and implements Sigma Designs' ClearPath approach. ClearPath, according to Sigma Designs, alternately routes packets over the earth ground connection, which is normally not filtered. (Atheros also eventually plans to implement a similar approach, called Smart Link.) Keep in mind that surge protection circuitry is increasingly not just included in standalone power strips but also embedded within wall outlets. And a UPS (uninterruptable power supply) also acts as an effective deterrent to powerline packet propagation.
Speaking of circuit breakers, now's as good a time as any to discuss security. Don't worry about your next-door neighbor accessing your LAN if a transformer is in-between your respective street-side power connections. On the other hand, there's a tangible possibility that multiple powerline networking users sharing a common transformer feed (such as, for example, in the same multi-apartment building) could tap into each others' equipment. That's where encryption comes in. HomePlug 1.0 and 1.0 Turbo harness 56-bit DES encryption, while HomePlug AV leverages even more robust 128-bit AES. And altering an adapter's password requires access to a 16-digit unique password stamped on the unit. Just change your equipments' passwords from the 'HomePlug' or 'HomePlugAV' default, and other folks on the same transformer feed won't subsequently have access to them.