Power over Ethernet (PoE) describes any of several standards or ad hoc systems that pass electric power along with data on twisted-pair Ethernet cabling. This allows a single cable to provide both a data connection and enough electricity to power networked devices such as wireless access points (WAPs), IP cameras and VoIP phones.
Techniques
There are several common techniques for transmitting power over Ethernet cabling, defined within the broader Institute of Electrical and Electronics Engineers (IEEE) 802.3 standard since 2003. The three techniques are:
Alternative A or Mode A uses the same two of the four signal pairs that 10BASE-T and 100BASE-TX use for data in typical Cat 5 cabling, i.e. pairs 2 and 3. Alternative B or Mode B separates the data and the power conductors for 10BASE-T/100BASE-TX, making troubleshooting easier, i.e. pairs 1 and 4. 4PPoE or 4-pair mode uses all four twisted pairs in parallel, increasing the achievable power. Alternative A transmits power on the same wires as data for common 10 and 100 Mbit/s Ethernet variants. This is similar to the phantom power technique commonly used for powering condenser microphones. Power is transmitted on the data conductors by applying a common voltage to each pair. Because twisted-pair Ethernet uses differential signaling, this does not interfere with data transmission. The common-mode voltage is easily extracted using the center tap of the standard Ethernet pulse transformer. For gigabit Ethernet and faster, both alternatives A and B transmit power on wire pairs also used for data since all four pairs are used for data transmission at these speeds. 4PPoE provides power using all four pairs of the connectors used for twisted-pair Ethernet. This enables higher power for applications like pan–tilt–zoom cameras (PTZ), high-performance wireless access points (WAPs), or even charging laptop batteries. In addition to standardizing existing practice for common-mode data pair (Alternative A), spare-pair (Alternative B), and four-pair (4PPoE) transmission, the IEEE PoE standards provide for signaling between the power sourcing equipment (PSE) and powered device (PD). This signaling allows the presence of a conformant device to be detected by the power source and allows the device and source to negotiate the amount of power required or available while avoiding damage to non-compatible devices.
Standards development
Two- and four-pair Ethernet The original PoE standard, IEEE 802.3af-2003, now known as Type 1, provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA) on each port. Only 12.95 W is guaranteed to be available at the powered device, as some power dissipates in the cable. The first update to PoE, IEEE 802.3at-2009, introduced Type 2, also known as PoE+ or PoE plus. It provides up to 25.5 W and prohibits the use of four pairs simultaneously for power. Both of these standards, 802.3af and 802.3at, were later incorporated into the IEEE 802.3-2012 publication. Later Type 3 and Type 4 were introduced in IEEE 802.3bt-2018, respectively allowing up to 51 W and up to 71.3 W delivered power, optionally by using all four pairs for power. Each pair needs to handle a current of up to 600 mA (Type 3) or 960 mA (Type 4). Additionally, power capabilities are defined for 2.5GBASE-T, 5GBASE-T and 10GBASE-T. This development opens the door to new applications and expands the use of applications such as high-performance wireless access points and surveillance cameras. IEEE 802.3bt was incorporated into 802.3 in the 2022 revision.
Single-pair Ethernet The IEEE 802.3bu-2016 amendment introduced single-pair Power over Data Lines (PoDL) for the single-pair Ethernet standards 100BASE-T1 and 1000BASE-T1 intended for automotive and industrial applications. On the two-pair and four-pair standards, the power voltage is applied between one conductor of each of two pairs, so that within each pair there is no differential voltage other than that representing the transmitted data. With single-pair Ethernet, power is transmitted in parallel to the data. PoDL initially defined ten power classes, ranging from 0.5 to 50 W (at PD). Subsequently, PoDL was added to the single-pair variants 10BASE-T1, 2.5GBASE-T1, 5GBASE-T1, and 10GBASE-T1, and as of 2021 it includes a total of 15 power classes with additional intermediate voltage and power levels.
Uses
Examples of devices powered by PoE include:
VoIP phones IP cameras, including PTZs WAPs IPTV decoders Network routers A small network switch, providing a small number of Ethernet ports from one uplink cable. Such a switch may, in turn, pass PoE to downstream devices (termed PoE pass-through). Intercom and public address systems Wall clocks, with time set using Network Time Protocol (NTP) Roof-mounted radios with integrated antennas, 4G/LTE-, 802.11- or 802.16-based systems used by wireless ISPs Outdoor point-to-point microwave and millimeter-wave radios and some free-space optics units, usually using non-standard, proprietary PoE Industrial control system components, including sensors, controllers, meters, etc. Access control components, including help points, intercoms, keyless entry, etc. Lighting controllers and light-emitting diode (LED) lighting fixtures Stage and theatrical devices, such as networked audio breakout and routing boxes Remote point-of-sale (POS) kiosks Ethernet repeaters, or extenders, which may also pass PoE through to downstream devices PoE splitters that output the power in a different form (e.g. USB Power Delivery), to power a remote device or charge a mobile phone
Terminology
Power sourcing equipment 802.3 refers to Power Sourcing Equipment (PSE), which provides power on the Ethernet cable. This device may be a network switch, in the standard Endpoint PSE (commonly called an endspan device) or a PoE injector, Midspan PSE in the standard, an intermediary device between a switch that does not provide PoE (or one that cannot provide sufficient power) and a PoE-powered device.
Powered device 802.3 refers to any PoE-powered piece of equipment as a Powered Device (PD). Examples include wireless access points, VoIP phones, and IP cameras. Many powered devices have an auxiliary power connector for an optional external power supply. Depending on the design, some, none, or all of the device's power can be supplied from the auxiliary port, with the auxiliary port also sometimes providing backup power in case PoE-supplied power fails.
Power management features and integration
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