Last week, I moderated a Power-Over-Ethernet (POE) session at the Ethernet Technology Summit held in San Jose. The idea of supplying high-speed communications and power over one standard cable that works anywhere in the world is certainly compelling because there are many difficulties in developing a power-delivery scheme that works worldwide thanks to differing wall-socket connectors and line voltages but that RJ45 Ethernet plug works anywhere. Consequently, POE makes a lot of sense for a large number of diverse, high-volume products including VOIP phones, thin clients, wireless access points, security cameras, and digital signage.
In an interesting twist on the concept, the amount of power delivered over a POE cable fairly well defines the power envelope of the end product. The initial 802.3af POE specification can deliver a maximum of 13W to the end product, after power losses in the cable are accounted for. The new 802.3at specification nearly doubles the available power to 25.5W, but that’s not even enough to run the laptop I’m using to write this blog post, so there’s clearly a limitation there. (However, as Microsemi’s Dan Feldman explained in his presentation, there’s also a way to get nearly 50W from a POE cable by using all four cable pairs to deliver power. Two of the pairs must then serve double duty by carrying both power and communications.)
In addition to the lack of a universal wall power plug, most of the devices that are candidates for POE adaptation are currently powered by wall warts, the ubiquitous black boxes that most of us plug as best we can into inexpensive power strips that are not designed to accommodate these “fat plugs.” In addition to their inconvenient form factor and the extra wiring they create—which often ends up creating a rat’s nest of wiring under your desk—wall warts bleed energy because they are not efficient. Because they’re generally inexpensive, not much engineering goes into the design of a wall wart. In fact, said Matthew Tyler of ON Semiconductor, the best you can get from a 50W wall wart is 84% efficiency. Wall warts delivering less power are even less efficient.
Consequently, one of POE’s advantages is the ability to develop high-quality, highly efficient power supplies for the network switches that deliver power to POE devices. (The POE realm refers to these switches as PSEs, power sourcing equipment, and calls the end devices PDs—powered devices.) Using POE topologies, says Tyler, you can drive efficiency up to 90%. In addition, says Akros Silicon’s Amit Gattani, the PSE becomes an intelligent controller that can manage the power supplied to the network’s PDs. For example, in a digital-signage application, the PSE can automatically power down the signs 15 minutes after a store closes and can power them back up 15 minutes before a store opens.
Reduced energy use saves companies money, so that’s one economic advantage enjoyed by POE. However there’s an even bigger economic advantage—one you might not expect. You need licensed electricians and permits to run ac power cables while you generally do not need either to run Category 5 Ethernet cable. Consequently, a system employing POE has lower installation costs compared to a system that employs a combination of ac power and wired Ethernet or even one that uses ac power and wireless Ethernet. There are real installation saving to be had for applications such as VOIP telephony, thin clients, and digital signage.
There is far more subtlety in the design of POE equipment than I’ve described here. All three of the companies listed above have developed expertise in the design of POE equipment and are willing to share that expertise with you.
