One of the most important developments in introducing decentralized wireless networks and devices over the last five years is Power over Ethernet, or PoE, which enables a single cable to provide both data and electrical power to networked pieces of equipment such as switches, wireless access points, IP video cameras and even licensed microwave bridges.
PoE works across standard network cabling (i.e. CAT5) to supply power directly from the data ports to which networked devices are connected; but did you know that not all PoE is created equal? There are two standards for PoE set by IEEE, each containing different power level classifications. There are also non-standards-based PoE offerings.
We’ve come to expect that when solutions are labeled “standard” that everything should play together nicely. The fact is that not all standards are created equal; thus, when planning or installing a network to support remote devices using Power over Ethernet, homework, research and planning are necessary.
IEEE PoE Standards
In order to create some industry standardization on the physical connections that are made between nodes and/or infrastructure devices, the IEEE standards body currently supports two standards. The standards ensure that all devices that use PoE are compatible and will interoperate with each other.
In 2003, the IEEE introduced the 802.3af-2003 standard. This original PoE standard provides up to 15.4 W of DC power to each device using 48V.In most cases, only 12.95 W is assured to be available at the powered device, as some power is dissipated in the cable. Examples of devices that only require this lower wattage power include standard box and dome IP cameras, most WLAN access points and IP phones.
As remote devices became more complex and more computing was required at the device, power requirements at each device consequently increased. Some of these products include outdoor IP PTZ cameras, microwave bridges and some 802.11n access points. In 2009, the IEEE updated the standard to the 802.3at-2009 PoE standard, also known as PoE+ or PoE plus, which provides up to 25.5 W of power.
The 2009 standard prohibits a powered device from using all four pairs for power; however, some vendors have announced products that claim to be compatible with the standard but offer up to 51 W of power over a single cable by using all four pairs in the Category 5 cable.
Not all 802.3af and .3at conformant products pull the same amount of power. According to IEEE 802.3-2008 section 2, clause 33.3.4, there are multiple levels of power that each device can draw (illustrated in the graphic below). This becomes critical when selecting a switch or midspans for network design.
Standards-conformant Power over Ethernet products contain two individual active pieces, the injector and the splitter. Each active piece includes an electrical circuit which ensures the product functions according to the standard-based active POE distance limitation of 100 m (328 ft).
Non-Compliant PoE Variations
To make matters even more complex, there are many variants of PoE that are not IEEE compliant, including passive and high wattage solutions.
In devices where cost is a major concern, such as lower-cost wireless bridges, access points and IP cameras, manufacturers may use Passive Power over Ethernet (Passive PoE) for lower cost. Passive PoE uses no active electronic components for transmission; thus, it is a simple way of connecting the cables in order to transfer the data and power supply along the same Ethernet cable at the same time. Ethernet cable (CAT5 and above) contains 8 copper wires (pins) — 4 pins (1,2,3,6) are used for data transmission and the rest (4,5,7,8) are used for supplying power. Positive power is received on pins 4 and 5 of the Ethernet cable, with negative returns on pins 7 and 8. The use of Passive PoE is only optimal when the distance of the cable does not exceed 30-40 meters (100-130 ft), but can be a less-expensive alternative at these distances.