PoE is defined by IEEE standard 802.3af, which specifies voltage, current and power levels. Previous to completion of the standard in 2003, many network equipment vendors developed proprietary methods for supplying power through the network cable. In the years since standardization, all of the major network equipment vendors have adopted the standard, making it possible to mix components from different vendors.
The standard classifies compliant devices as either Power Sourcing Equipment (PSE) or Powered Equipment (PD). Switches from Cisco, Extreme, Foundry and other vendors all adhere to the standards for PSEs. But just because the specifications for a switch say that it is 802.3af compliant doesn't mean that it can supply full power on all of its ports simultaneously. The standard specifies three different power levels: 15.4W, 7.0W, or 4.0W. Some switches, especially those with a large number of ports, can supply 4 or 7W on all ports, but cannot supply 15.4W on all ports at the same time.
PoE doesn't require buying new switches. Vendors such as PowerDsine and ADC offer devices called midspans or injector hubs. Midspans are typically located adjacent to switches. The midspan supplies power to the powered device through the network cable while transparently passing data between the switch and the device.
The cable from an output port of the switch is connected to an input port of the midspan. Then the cable to the powered device is connected to an output port of the midspan. Midspans are available ranging from single port models, often called injectors, to units supporting 48 or more ports.
A picker or tap provides a way to power non-PoE devices. The picker is located near the device to be powered. It removes power from the network cable, converts it to the proper DC voltage level for the device, and then supplies it to the device through the device DC power connector.
Cost and other considerations
PoE reduces network implementation costs by eliminating the need to provide individual power supplies for each VoIP phone and by making it unnecessary to install AC outlets near ceiling-mounted access points. These savings may be offset by the cost of new switches or midspans. If only a few PoE devices are being added, adding a midspan will probably be less expensive than replacing a switch.
Power, heat and space in the data closet must also be considered when planning a PoE installation. Switches supporting PoE draw more power and dissipate more heat than switches without the feature. Midspans consume rack space and also draw power and dissipate heat. Be sure to review the specifications for each new device and add power and cooling as necessary.
Moving the power supply from individual devices to a central location can result in increased reliability. It isn't practical to supply uninterruptible power to every VoIP phone or wireless access point spread throughout a building. It is practical to add a UPS or upgrade an existing UPS in the data closet. Users are accustomed to having phone service even during a power failure. Maintaining this capability can be vital for a successful VoIP implementation.
PoE also adds management flexibility. The network management capabilities in the 802.3af standard make it possible for a network manager at a central location to monitor the power use of an individual device and shut off power to the device if necessary.
PoE was originally developed to reduce the cost of supplying power to VoIP phones and other low-power network devices. It has met that goal while also enabling network architects to increase reliability and improve monitoring and management of remote devices.
About the author:
David B. Jacobs has more than twenty years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software start-ups.