Two Major Styles
Powerline networking devices come in two major flavors, exemplified by the Siemens SpeedStream Powerline series and Phonex Broadband Corp.'s NeverWire family. Both can work with almost any Ethernet-enabled device. Both offer 56-bit DES encryption, plug-and-play setup, intrinsic maximum speeds of about 14 Mbps, and can bridge to any normal 10/100-Mbps Ethernet network. (Full specs are available via the above links.) But beyond these basics, the units differ:

The Speedstream units are "zero footprint" devices (no desk or shelf space needed) roughly the size and shape of a power brick: about 4" high by 2.5" wide by 1.5" thick (approximately 10 centimeters by 6.5 centimeters by 4 centimeters). Setup is extremely simple: Plug one Speedstream unit into a plain-vanilla wall socket anywhere, and connect it to a PC's network card or to a network hub using a standard RJ45 network cable. (Siemens also offers units that connect to a PC via a USB cable.) Repeat the process at a second location with a second PC or hub. That's it; you're done -- a basic network now exists between the PCs or hubs, using the intervening electrical wiring to carry the signals. No special software or drivers are needed, and no special configuration. To go beyond the basics -- say, if you want enhanced security (a good idea) -- you then load special software onto the PCs that connect to the powerline devices, and use that to set unique passwords and permissions for the powerline network devices. The same software also helps troubleshoot any problems that may occur; and the setup CD contains still more tools and information to help get the powerline network going with just about any PC configuration. (But I needed none of this in my tests: The Siemens units worked right out of the box, with no special setup needed for basic networking.) The Siemens units retail for about $90; you need at least two units to set up a powerline network.

The NeverWire units are larger than Speedstream's; or about the size of a standard freestanding external cable modem: roughly 6.5" high x 1.5" wide x 5.5" deep (call it 16.5 centimeters by 4 centimeters by 5.5 centimeters). As such, they're less portable than the Siemens units. On the other hand, they are even easier to set up because all the important features -- including troubleshooting and enhanced security -- are available via push-button controls built into the units. These extra controls mean you don't need extra software or a PC to set up even the NeverWire's advanced features. This is particularly useful if any portion of your powerline network will connect to a hub instead of to a PC; you can fully set up and configure the unit using only the built-in control panel. Beyond this key difference, the NeverWire units are much the same as the SpeedStream; they even cost roughly the same; with most retail outlets selling them for around $100 per unit (you need at least two to create a network), but with some retailers offering them for $80 each.

To me, the small SpeedStream-style units seem ideal in situations where portability or ad hoc networking is key. For example, if you have a notebook that you want to carry from room to room, or if your powerline networking setup is temporary, the small size, zero footprint, and easy transport of the SpeedStream units may make them the better choice.

On the other hand, the NeverWire-style units seem better suited for permanent or semi-permanent installation. In these cases, the NeverWire's self-contained, software-less setup and operation is a plus, and the slightly larger size is less important.

Note that in addition to creating new networks from scratch, both types of units can be used to extend an existing network: For example, if building geometry or local interference prevents easy coverage of an area via a wireless node, a powerline network can send the network data to any location served by the building's electrical system -- including deep basements, the building core, or other areas where a radio signal just won't go. At the remote location, you can even switch back to wireless via a simple hub and standard wireless access point. Siemens even offers a combo powerline/wireless access point that connects wireless devices to the powerline network in one step. Either way, the powerline network acts as a transparent link between two otherwise disconnected wireless bubbles.