Power over Ethernet
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.
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 transmits power on the same wires as data for common 10 and 100 Mbit/s Ethernet variants.
The common-mode voltage is easily extracted using the center tap of the standard Ethernet pulse transformer.
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).
[24] Critics of this approach argue that PoE is inherently less efficient than AC power due to the lower voltage, and this is made worse by the thin conductors of Ethernet.
[25] In any case, where the central PoE supply replaces several dedicated AC circuits, transformers and inverters, the power loss in cabling can be justifiable.
The integration of PoE with the IEEE 802.3az Energy-Efficient Ethernet (EEE) standard potentially produces additional energy savings.
Pre-standard integrations of EEE and PoE (such as Marvell's EEPoE outlined in a May 2011 white paper) claim to achieve a savings upwards of 3 W per link.
If the PSE detects a resistance that is too high or too low (including a short circuit), no power is applied.
If the PD goes more than 400 ms without meeting this requirement, the PSE will consider the device disconnected and, for safety reasons, remove power.
Endpoint devices (commonly PoE switches) are Ethernet networking equipment that includes the power-over-Ethernet transmission circuitry.
Endpoint devices are normally used in new installations or where the switch has to be replaced for other reasons (such as moving from 10/100 Mbit/s to 1 Gbit/s), which makes it convenient to add the PoE capability.
Midspan PSE can be used e.g. to power a single piece of equipment added to a network that does not provide PoE.
PoE standards define TLV structures used by PSE and PDs to signal and negotiate available power.
Under Cisco's pre-standard scheme, the PSE (switch) will send a fast link pulse (FLP) on the transmit pair.
The PSE will provide a common mode current between pairs 1 and 2, resulting in 48 V DC[51] and 6.3 W[52] default of allocated power.
[53] In 2014, Cisco created another non-standard PoE implementation called Universal Power over Ethernet (UPOE).
UPOE can use all four pairs, after negotiation, to supply up to 60 W.[54] A proprietary high-power development called LTPoE++, using a single Cat 5e Ethernet cable, is capable of supplying varying levels at 38.7, 52.7, 70, and 90 W.[55] PowerDsine, acquired by Microsemi in 2007, which was then acquired by Microchip in 2018, has been selling power injectors since 1999.
Several companies such as Polycom, 3Com, Lucent and Nortel used PowerDsine's older Power over LAN PoE implementation.
Earlier versions of passive PoE 24 VDC power sources shipped with 802.11a, 802.11g and 802.11n-based radios are commonly 100 Mbit/s only.
[58] The ISO/IEC TR 29125 and Cenelec EN 50174-99-1 draft standards outline the cable bundle temperature rise that can be expected from the use of 4PPoE.
