A recent study by In-Stat says that fixed-configuration 24- and 48-port switches can vary by more than 600% in terms of how much switching capacity they support in relation to how much electric power they draw.

The range measured in gigabits per second per Watt ranges from a low of less than .5Gbps/Watt to a high of more than 3Gbps/Watt, the study says. (Compare Ethernet access switches.)

For 24-port switches, In-Stat found the best performing in order were made by 3Com, SMC, Netgear, H3C and D-Link. For 48-port switches, the top five in order were made by 3 com, Force10, Netgear, Extreme Networks and SMC.

Foundry Networks and Cisco placed as the bottom two among 24-port switches and Cisco and HP ProCurve placed as the bottom two among 48-port switches. The most efficient was the 24-port 3Com switch with more than 3Gbps per Watt, and the least efficient was the 24-port Foundry switch at less than .5Gbps per Watt.

The 13 vendors rated were: 3Com, Allied Telesis, Cisco, D-Link, Enterasys, Extreme, Foundry, Force10, H3C, HP ProCurve, NetGear, Nortel and SMC.

In-Stat finds that in general, energy efficiency drops as the number of switch ports increases. Energy efficiency does not vary much between low- and high-end models of switches made by the same manufacturer, the study says.

More important is who makes the switch. "In-Stat determined that even among similarly equipped switches capable of performing identical tasks, there are significant vendor-specific differences in energy efficiency," the study says.

Efforts to improve power efficiencies are underway in the IEEE, but may not bear fruit for some time yet, says David Law, the chairman of the IEEE's 802.3 committee that oversees Ethernet issues.

A working group on energy-efficient Ethernet is considering standards for determining when a link is idle and automatically dropping it down to lower power -- just enough to keep the link alive, then throttling power back up when traffic demands it, says Law, who also works for 3Com.

The key is getting standards so devices made by different vendors can participate in varying power in response to demand, he says. The power reductions would apply to end devices such as workstations and servers.

"This is an active idle, a low-power idle -- in fact off, if possible -- with occasional keep-alive bursts," Law says.