HIGH POWER OVER ETHERNET
Not surprisingly, there's a renewed clamor among those in the industry concerning how much power a PSE can supply. In November, there will be a "Call for Interest" before the 802.3 committee as a whole. (A Call for Interest is the official way of launching a new Working Group). Assuming the Call for Interest is successful, a new working group (designated 802.3a-something) will be formed.
The new 802.3 subcommittee will hear two proposals for "high power over Ethernet." Not only are there pan/tilt/zoom security cameras that cry out for more power, but conference-room speaker-phones—part of the VoIP phone concept that spawned PoE—could use additional power for extra volume in large venues.
A proposal from PowerDsine, already implemented in its 8000-series midspans, involves powering all four pairs simultaneously. Spare pair pins 4 and 5 and data pins 3 and 6 would carry the positive side of the PoE supply; data pins 1 and 2 and spare pins 7 and 8 would carry the negative side. The spare pairs and the data pairs would each source and sink the same 350 mA as today's PoE. This would permit PSEs to provide up to 39.5 W per LAN cable. On the PD end, 25.9 W would theoretically be available.
An alternate proposal involves simply increasing the allowable maximum current per pair in the existing scheme. Although the gauge of the wire in CAT5 cable pairs is only 24 AWG (0.51 mm dia.), Linear Technology's David Dwelley says the major concern is the current capacity of an RJ45 connector.
On paper, he says, the standard Ethernet connector has a higher maximum current spec than CAT5 cable, but in tests-to-failure in Linear's lab, the connectors always fail first. However, those same tests show that cable and connectors together have a capacity for much more current than the present PoE limit. Empirically, those power-over-LAN security cameras said to run an amp-and-a-half tell their own story. (For a more conservative story, check the Handbook of Electronic Tables and Formulas, which recommends a maximum of 577 mA per AWG 24 wire, or 1.15 A for a pair.)
At first glance, the PowerDsine approach would seem like a winner. Why not use both pairs? But there are two drawbacks to the idea. One is the inevitable doubling of bill-of-materials cost for PoE components on both ends of a cable. The other is incompatibility with legacy installations that have open spare pairs. Today, the PoE standard is backward-compatible with any installed Ethernet plant. The four-pair approach would not be.
On the other hand, PowerDsine, clearly a leader in PoE, is already shipping hardware based on the proposal. The only required change for the standard would be to make it possible to power both sets of pairs at present current levels.
Yet following through on the proposal for increasing current levels would require additional research and experimentation, not to mention coordination with the various authorities that set fire and building codes. No one can predict which alternative the new committee will choose.