Network load and voice quality are usually the first issues examined when planning a Voice over IP (VoIP) implementation.
VoIP phones are powered through the network using Power over Ethernet (PoE) technology. Power is injected onto the network cable by the switch nearest the phone or by a midspan. Switches and midspans are usually located in a wiring closet. Switches supporting PoE consume more power than non-PoE switches. Much of the additional power is transmitted through the network to phones and other powered devices, but some is consumed in the wiring closet. Midspans also consume power.
More power consumed by a switch or midspan means more heat in the wiring closet. Make sure that AC power supplied to outlets in the closet is adequate for the additional load. Failure to calculate the amount of additional heat and offset it with additional cooling will cause shortened equipment life or even a sudden failure of both the data and voice networks.
Your plan must provide adequate power, but it must do so efficiently. Each unnecessary watt adds cost twice: first, when consumed by a switch or midspan; second, when consumed by additional air conditioning.
Planning your implementation requires five key decisions:
1. Determine how much power is required.
The current PoE standard, IEEE 803.3af, specifies a maximum of 15.4 W per port, but most VoIP phones require much less power. Find out from your phone vendor how much power the phones need. Phones require more power when off-hook than on-hook. If your installation is a call center where phones are off-hook nearly all the time, your power requirement will be higher than in an office area where only a few phones are off-hook at any one time.
Factor in PoE-powered devices other than phones. Wireless access points and surveillance cameras are typically powered via PoE and usually require more power than a phone.
2. Decide between PoE-capable switches and midspans.
If your switches are otherwise adequate and do not need to be replaced, midspans can be more cost effective than replacing switches just to gain PoE capability.
If you intend to replace switches, determine the number of powered devices and the amount of power they will consume. If all or most of the switch ports will support PoE devices, PoE switches can be the most efficient approach. If only a fraction of the ports require PoE support, a midspan can be less expensive to buy and can use less power than a switch with PoE available on all ports (but unused on most).
Midspans also require rack space. If space in the closet is at a premium, midspans may not be an option.
3. Choose switches or midspans that are adequate but as efficient as possible.
Some switches and midspans can supply the full 15.4 W on all ports simultaneously. Others are limited to a lower total amount of power but provide sufficient power because most powered devices do not require 15.4 W.
Consider two switches driving an identical set of devices: one capable of 15.4 W on all ports simultaneously and the other capable of less total power. The switch with higher power capacity will operate its power supply at a lower fraction of capacity. Power supplies are less efficient at lower fractions of capacity, so the higher-capacity switch will consume more power and radiate more heat into the closet.
Switches and midspans that cannot provide full power on each port must measure the power needed by each powered device. When the total required reaches the power supply maximum, no further devices will be powered.
Some switches and midspans use a method specified in the standard to measure the power required. When a powered device starts up, the switch or midspan exchanges signals with it to determine its power requirement.
The device can request one of the three levels defined in the standard. The standard defines levels of 4.0 W, 7.0 W and the full 15.4 W. A device that requires, for example, 7.1 W exceeds the 7.0 W level and must request 15.4 W. The device may need 7.1 W at certain times only, but since the requirement is sensed only on startup, the device must be allocated 15.4 W at all times.
Other switches and midspans use "managed power" to continually sense device power requirements and allocate to each the exact amount of power needed at that time. "Managed power eliminates the need to allocate maximum power to each PoE port. Managed power utilizes the power supply in a more efficient way by running a lower-capacity supply at higher levels of efficiency," said Sharon Dagan, product manager at PowerDsine, a manufacturer of midspans and PoE integrated circuits used by leading switch manufacturers.
4. Protect your network against power failures.
If you already have an uninterruptible power supply (UPS) to maintain your data network through a power failure, you will need to increase its capacity. If you don't currently provide uninterruptible power, you will need to add a UPS. Your users will expect phones to continue to work through a power failure.
Remember to factor in the power required and heat generated by either a newly added UPS or a higher-capacity unit.
5. Plan for the future.
Devices such as video phones and full-motion video cameras require more power than 802.3af-capable switches or midspans can provide. The IEEE Power over Ethernet Plus committee is currently working on 802.3at with a goal of increasing the amount of power available. If these or other power-hungry devices may be in your future, discuss with your switch or midspan vendor how to plan now for a transition to support for the new standard.
David B. Jacobs has more than 20 years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software startups.
This was first published in October 2006