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Power over Ethernet basics for network architects
Power over Ethernet provides a way to supply electricity to low-power devices through a standard Ethernet cable. Learn about the different standards and PoE types.
Power over Ethernet, or PoE, has been an immensely valuable tool for network architects. It is intended for devices that require power but are located far from an AC outlet.
PoE was first introduced in 2003 as the IEEE 802.3af standard. Its target applications were ceiling-mounted Wi-Fi access points (APs), surveillance cameras and VoIP phones. Many network architects did not want to add expenses by creating an outlet or running an extension cord from an outlet to power those devices. A preferable alternative was to supply power through the Ethernet cables that connected them.
Current environments now require reduced energy use and tighter security in office areas, resulting in new PoE applications. These applications require network architects to understand the capabilities of PoE as defined by the IEEE standard and the requirements of emerging applications.
Prior to the introduction of the first PoE standard, major equipment vendors had developed proprietary standards, making it impossible to mix equipment from various vendors in the same network. When IEEE published 802.3af, vendors updated their products to support the standard, making it possible for enterprises to choose the best product for each purpose -- even if that meant mixing equipment built to different vendors' standards.
The original IEEE 802.3af standard delivers 15.4 watts (W) at the power source, which is typically a network switch. When a powered device is connected by 100 meters of cable, the power usually degrades to 12.95 W, due to losses in the cable. Shorter cables will not lose as much power, and Cat6 cables reduce power loss compared with Cat5e.
Equipment built to the 2003 standard delivers enough power for APs and rigidly mounted surveillance cameras, but devices such as pan-and-tilt cameras require more power.
IEEE responded to the need for more power by releasing IEEE 802.3at in 2009. The 802.3at standard, also known as PoE+, specifies approximately 25.5 W at the powered device. This power is sufficient for pan-and-tilt cameras and a number of other devices. But, soon after IEEE released the standard, some equipment vendors realized other equipment that would benefit from PoE required even more power than IEEE 802.3at specified.
Rather than wait for IEEE to develop a higher power standard, vendors again developed a number of proprietary implementations. Equipment built to these standards all deliver more power than 802.3at by supplying power through all four wire pairs in a Cat5 cable. In contrast, IEEE 802.3af and 802.3at both supply power over only the two wire pairs that carry the signal. Although all the proprietary versions used four pairs, they were not compatible. All maintained compatibility for the two earlier IEEE standards, however.
Once IEEE again recognized the need for higher power, the organization began work on an updated standard. The result was IEEE 802.3bt in 2018, which also supports use of all four wire pairs and delivers 60 W to 100 W of power, depending on the type. Devices upgraded to IEEE 802.3bt can connect to products from other vendors.
The 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 that require different levels of power. For example, a workstation might require the signal and power to go to the processor, while the monitor requires a different power level but doesn't need access to the signal.
Each of the standards defines PoE types, with each type able to support multiple power classes.
IEEE 802.3af defines Type 1, which specifies any of four power classes ranging from 3.84 W to 12.95 W at the powered device.
IEEE 802.3at supports Type 1 devices and its associated power classes. It also adds Type 2 and an additional class that specifies 25.5 W at the powered device.
IEEE 802.3bt adds Types 3 and 4 and four additional power classes. The new types and classes define power levels ranging from 60 W at the power source and 51 W at the powered device (Type 3) up to 100 W at the power source and 71 W at the powered device (Type 4).
How PoE works
Powered devices request the required power level via an initialization sequence. This sequence begins when the power source 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 the device has a 25,000 ohm resister, the power source and powered device carry on an initialization sequence.
Powered devices specify their power requirements in the two following ways:
- Exchange a series of pulses with the power source.
- Use Link Layer Discovery Protocol (LLDP), or IEEE 802.1ab, which defines parameters that the two devices exchange to determine type and required power level. If power requirements change at any time, the powered device can send another LLDP packet to inform the power source of the change.
When a powered device is shut off, it must continue to draw power periodically to inform the power source it is still connected. While earlier standards require a powered device to draw 10 milliamperes 20% of the time, IEEE 802.3bt requires a powered device to draw power only 1.875% of the time. This reduction is a significant difference for applications such as LED lighting, where a large number of devices are turned off nights and weekends.
PoE 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.
The use of PoE is constantly expanding. The variety of new product areas where it's being used means network architects must keep up to date on these new products and understand their capabilities in order to determine which are most suitable for their networks. With the rapid growth of PoE applications and products, network architects can expect to invest considerable time in evaluating PoE usage and applications.