Power over Ethernet Overview

Power over Ethernet – or “PoE”– is innovative technology that provides a new, more cost-effective method for powering network devices.  As described in the IEEE 802.3af standard, PoE enables delivery of power over existing CAT-5 LAN cabling, eliminating the need for running separate – and expensive power cables.  The benefits of this technology are substantial, and the applications numerous, ranging from powering IP Phones, LAN access points, and WiFi  switches to RFID readers and network security cameras.  And, as the first truly international power distribution standard, manufacturers need not develop separate products for different power standards.

Since the ratification of the IEEE standard in June 2003, PoE has become a “must have” technology for switch and device vendors.  Industry leaders such as Cisco, Avaya, 3Com, HP and D-Link have incorporated PoE into their product lines in order to capitalize on the demand for PoE.   A Gartner Group study found that PoE shipments tripled in 2004, to 18% of wiring closet switch ports.  In the near future, industry analysts expect that 90% of enterprise switches will be PoE-enabled, spawning an explosion of devices that can be powered via standard Ethernet cable.

The 802.3af standard

The two key designations for PoE are 1)  Power Sourcing Equipment (PSE), which provide power, such as a switch and 2) Powered Devices (PD), such as IP phones, access points, or network cameras, which receive power.   The voltage is supplied at 48v DC, providing approximately 13 watts of power to the device. 

An industry initiative known as PoE Plus is working to increase available power to 40W or more per port.  This will enable the deployment of new categories of PoE devices, including multi-band access points, video phones, motorized cameras and possibly even laptops.

The IEEE standard defines how PSEs and PDs work together, including methods for power delivery and detection using CAT-5 Ethernet cable.  The standard cable has four twisted pairs but only two of these are used for 10BASE-T and 100BASE-T data transmission (pins 1,2 and 3,6). Under 802.3af, power transmission can occur on either these "data pairs" or the unused pairs ("spare pairs"), but not both.  PDs are required to accept power from either alternative.  The figure below illustrates the most common configuration of an Ethernet switch delivering power over the data pairs.

Detection

In order to avoid damage to non-PoE devices, a key component of the 802.3af PoE standard is detection.  PSEs must be able to detect the presence of a PoE-enabled device before sending power, and must be able to detect when the powered device is no longer present and remove power.   To achieve accurate detection, a low-level detection current is sent from the PSE through the cable and the response is analyzed to determine if a compliant device is attached.  This concept was originally described in Network-1's '930 patent. .  

IEEE 802.3af-compliant PDs contain a detection circuit providing a defined range of resistance and capacitance (known as the PD "signature") indicating that the device can accept power.  If the PSE detects the appropriate signature it knows a compliant PD is attached and the full 48V is delivered to the DC/DC power supply within the PD.  This circuitry is illustrated in the figure at left.

The detection process also supports optional device classification - enabling the device to specify its maximum power requirements to the PSE.  This can be particularly useful if a PD requires less than 13W, because it permits more devices to be provisioned from a single PSE. 

Deployment Models

There are two types of PoE implementions specified by the standard:    

1) “Endspans” :  Power is applied directly by the switch to the devices.   Under the specification, the endspan PSE can use either the spare pairs or the data pairs, and can be used with gigabit Ethernet transmissions.   This method requires the deployment of a PoE-enabled switch, and therefore is most useful in new installations where new switch purchases are planned. 

1)“Midspans”:  Power is supplied by an intermediary powered patch panel, or injector,  placed between an existing switch and devices.   The Midspan is typically located near the switch, allowing cable to be run to remote devices.   The Midspan is now considered the PSE.   The specification allows midspans to use only the spare pair, and it cannot be used to send power over a gigabit Ethernet connection.  With this method, network investment in legacy switches is preserved.