The soon-to-be-released IEEE 802.3bt standard specifies increased power over previous Power-over-Ethernet standards,...
along with increased efficiency and cost savings. Devices such as point-of-sale terminals, building lighting systems and nurse call systems designed to use earlier standards could not be powered via the Ethernet cable connecting them to the switch. Using the new standard, however, these devices can be designed to operate without a power cord and a nearby AC outlet.
The rise of PoE
Power over Ethernet (PoE) was originally developed to support devices such as Wi-Fi access points (APs), which are typically installed on the ceiling, where adding an AC outlet would increase expenses. The original IEEE 802.3af PoE standard, released in 2003, provided a maximum of 15.4 watts at the power sourcing equipment (PSE). Losses in the cable reduce PSE power to 12.95 watts at the powered device (PD) when connected by 100 meters of cable. Less power is lost with shorter cables, and CAT6 cables reduce power loss compared to CAT5e.
Equipment built to the 2003 standard delivered enough power for APs, but devices such as pan-and-tilt video surveillance cameras -- also typically installed on a ceiling or outside a building -- require more power.
The Institute of Electrical and Electronics Engineers (IEEE) responded to the need for more power by releasing IEEE 802.3at in 2009. The standard, also known as PoE+, specifies approximately 25.5 watts at the PD, which is sufficient for pan-and-tilt cameras and a number of other devices. But soon after the standard was released, some equipment vendors realized other equipment that would benefit from PoE required even more power than IEEE 802.3at specified.
Rather than wait for the IEEE to develop a higher power standard, vendors developed a number of proprietary implementations, including Universal PoE, High PoE and PoE++. Equipment built to these standards delivers more power than 802.3at by supplying power through all four wire pairs in a Cat5 cable. IEEE 802.3af and 802.3at both supply power over only the two wire pairs that carry the signal.
Equipment built to any of the proprietary standards will interoperate with a device built to 802.3af or 802.3at when supplying power levels defined by those standards. Despite the fact that all of the proprietary standards use four pairs, they are not compatible at higher power levels. A PSE built to one proprietary standard will support a PD built to the same standard, but it generally will not support a PD built to another standard. The need to match PSE and PD to the same proprietary standard has inhibited vendors from developing new devices that can be installed on any network without concern that it will be incompatible with existing equipment.
Features of 802.3bt standard
The new IEEE 802.3bt standard replaces the need for proprietary standards. A PD built to the standard will be compatible with any PSE built to it. Like the proprietary standards, IEEE 802.3bt specifies use of all four wire pairs in order to supply enough power to support LED lighting, kiosks, terminals and a variety of other devices.
Devices designed to the IEEE 802.3bt standard will not be compatible with devices designed to any of the proprietary standards, but it is anticipated that vendors using the proprietary standards will upgrade device software to allow interoperability with devices built to the new standard; hardware limitations may prevent this in some cases, however.
Each of the standards define PoE Types, with each type able to support multiple power classes. IEEE 802.3af only defines Type 1, which specifies any of four power classes ranging from 3.84 watts to 12.95 watts at the PD. IEEE 802.3at supports Type 1 devices and its associated power classes, and it adds Type 2 and an additional class that specifies 25.5 watts at the PD. IEEE 802.3bt adds Types 3 and 4 and four additional power classes. The new types and classes define power levels ranging from 45 watts at the PSE and 40 watts at the PD up to 100 watts at the PSE and 71 watts at the PD.
Reducing costs, managing power
The new 802.3bt standard also defines a way to supply two different power levels simultaneously. A dual-signature device uses two of the four pairs of wires in the cable to operate as one device type, supplying power at one class level, and the other two pairs to operate as another type supplying power at a different class level. Dual signature is useful for applications such as workstations where the signal and power go to the processor, and the monitor requires a different power level, but does not need access to the signal.
Powered devices request the required power level via an initialization sequence that begins when the PSE places a low voltage on the cable to detect whether the device on the other end of the cable is a PoE device. If the current flow indicates there's a 25,000 ohm resister in the device, it is a PoE device. Both devices then operate as if the PD is a Type 1 device, while the PSE and PD carry on an initialization sequence.
There are two ways the PD can specify its power requirement: It can exchange a series of pulses with the PSE or use link level discovery protocol (IEEE802.1ab), which defines parameters that the two devices exchange to determine type and required power level. If power requirements change at any time, the PD can send another LLDP packet to inform the PSE of the change.
Another benefit of the IEEE 802.3bt standard is devices built to it conserve power, compared to devices designed to the earlier standards. When a PD is shut off, it must continue to draw power periodically to inform the PSE it is still connected. While earlier standards require that a PD draw 10 ma 20% of the time, IEEE802.3bt requires that a PD draw power only 1.875% of the time -- a significant difference for applications such as LED lighting, with a large number of devices that are turned off at night and on weekends.
The new standard also reduces costs in other ways. Powering lights via PoE eliminates the need to run AC power to the ceiling and build an AC power control system separate from network management. Lights can be turned off and on by sending Simple Network Management Protocol commands to the PSE.
The fact that proprietary four-wire standards were developed so soon after 802.3at indicates many applications demand more power than the standard could supply. Now that an industry standard is in place, PoE usage will undoubtedly grow, and even more applications will be developed.