Byte the power: PoE for VoIP is very juice-intensive
A report in Network World magazine casts new light on a Power over Ethernet, which can be used to run the juice for hundreds of IP phones to corporate desktops.
The problem with PoE for IP is in the extra juice required to run network connections via wiring closets.
"It's the issues in the last mile so to speak, that might get us," Network World's Phil Hochmuth quotes Saukville, Wisc.-based Charter Steel's telcom manager Peter Schwei a saying. Charter just upgraded its Avaya Definity phone switch to a network of Avaya S8700 IP PBX's, which are hooked together via IP.
Roen Heldman, vice-president of product management for PoE component supplier PowerDsine, explains the problem to Hochmuth.
A typical switch is 70 to 100 watts, Heldman says. A switch with PoE can add 200 to 700 watts, depending on how it is implemented. Heldman says that if a PoE switch drives out 300 watts of power, and its efficiency is 75%, then: 300/75 = 120. That 120 watts equals 360 BTUs per hour of extra heat that has to dissipate in the communications room or wiring closet.
Add to that the demanding power requirements of IP phones.
"The IEEE 803.3af standard for PoE specifies 15.4 watts of power," Hochmuth writes. "But most switches aren't built to blast out that much electricity on all ports, Heldman says. PowerDsine and other PoE gear makers build power management features into their products that distribute only the power needed to endpoint devices. For example, most IP phones require from eight to 12 watts of power, Heldman says. Also, most typical PoE switch deployments do not have every port powering an end device."
Hochmuth adds Heldman's note that in VoIP deployments,, the extra heat dissipation of PoE switches must be added to the heat dissipation from additional UPSs.
"Heldman says most UPSs are AC, which run hot because they convert their power streams from DC," Hochmuth writes. "But DC UPSs are something that should be looked at down the line, he says, especially if firms must dramatically increase the amount of UPSs installed to support VoIP-enabled switches in wiring closets.
"Today, most DC input is only used in the telecom and service provider markets," Heldman says. "My expectation is that in the future, more devices in communications and in data centers will go to DC," Heldman says. "But the practical issue is that most devices today use only AC. So even though theoretically DC may be the more effective way," it is less-widely used.