Power over Ethernet or PoE describes any of several standard or ad-hoc systems which pass electric power along with data on twisted pair Ethernet cabling. This allows a single cable to provide both data connection and electric power to devices such as wireless access points, IP cameras, and VoIP phones.
There are several common techniques for transmitting power over Ethernet cabling. Two of them have been standardized by IEEE 802.3 since 2003. These standards are known as Alternative A and Alternative B. For 10BASE-T and 100BASE-TX, only two of the four signal pairs in typical Cat 5 cable are used. Alternative B separates the data and the power conductors, making troubleshooting easier. It also makes full use of all four twisted pairs in a typical Cat 5 cable. The positive voltage runs along pins 4 and 5, and the negative along pins 7 and 8.
Alternative A transports power on the same wires as data for 10 and 100Â 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, all four pairs are used for data transmission, so both Alternatives A and B transport power on wire pairs also used for data.
In addition to standardizing existing practice for spare-pair (Alternative B) and common-mode data pair power (Alternative A) 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.
Standards development
The IEEE standards for PoE require category 5 cable or better for high power levels but allow using category 3 cable if less power is required. Power is supplied in common mode over two or more of the differential pairs of wires found in the Ethernet cables and comes from a power supply within a PoE-enabled networking device such as an Ethernet switch or can be injected into a cable run with a midspan power supply. A midspan power supply, also known as a PoE power injector, is an additional PoE power source that can be used in combination with a non-PoE switch.
The original IEEE 802.3af-2003 PoE standard provides up to 15.4 W of DC power (minimum 44 V DC and 350 mA) on each port. Only 12.95 W is assured to be available at the powered device as some power dissipates in the cable.
The updated IEEE 802.3at-2009 PoE standard also known as PoE+ or PoE plus, provides up to 25.5 W of power for "Type 2" devices. The 2009 standard prohibits a powered device from using all four pairs for power.
Both of these amendments have since been incorporated into the IEEE 802.3-2012 publication.
Standards body IEEE is currently looking at ways of increasing the amount of power transmitted. The upcoming IEEE 802.3bt aka 4PPoE (4 Pair Power over Ethernet) standard slated for early 2018 will introduce two additional power types: up to 55Â W (Type 3)and up to 90-100Â W (Type 4). Each pair of twisted pairs will need to handle a current of up to 600Â mA (Type 3) or 960Â mA (Type 4). Additionally, support for 2.5GBASE-T, 5GBASE-T and 10GBASE-T is planned. This development opens the door to new applications and expands the use of applications such as operating high-performance wireless access points and surveillance cameras or even Ethernet-powered laptop docking stations.
The IEEE 802.3bu amendment introduced PoDL single-pair power delivery for the single-pair Ethernet standard 100BASE-T1 intended for automotive and industrial applications. On the two-pair or four-pair standards power is transmitted only between pairs, so that within each pair there is no other voltage present than that representing the transmitted data. With single-pair Ethernet, power is be transmitted in parallel to the data. PoDL defines 10 power classes, ranging from .5 to 50Â W (at PD).
Uses
Some types of devices powered by PoE include:
- VoIP phones
- IP cameras including panâ€"tiltâ€"zoom cameras
- Wireless access points
- IPTV decoders
- Network routers
- A mini network switch installed in distant rooms, to support a small cluster of ports from one uplink cable. (Mini-switches do not usually provide PoE on their output ports.) (Many modern VoIP phones include a two-port mini-switch that can provide a network connection for a nearby computer.)
- Network Intercom / Paging / Public address systems and hallway speaker amplifiers
- Wall clocks in rooms and hallways, with time set using Network Time Protocol
- Outdoor roof mounted radios with integrated antennas, 802.11 or 802.16 based wireless CPEs (customer premises equipment) used by wireless ISPs.
- Industrial devices (sensors, controllers, meters etc.)
- Access control and Help-points (intercoms, entry cards, keyless entry, etc.)
- Lighting controllers and LED Lighting fixtures
- Stage and Theatrical devices, such as networked audio breakout and routing boxes
- Remote Point of Sale (POS) kiosks
- Physical Security devices and controllers
- Inline Ethernet extenders
Terminology
Power sourcing equipment
Power sourcing equipment (PSE) is a device such as a network Switch that provides (or sources) power on the Ethernet cable. The maximum continuous output power an endpoint device can sink per cable in IEEE 802.3af is 12.95Â W. A later specification, IEEE 802.3at, increases this to 25.50Â W. These require the source to supply 15.4Â W and 30Â W, respectively, with the rest lost in the cable.
When the device is a switch, it is commonly called an endspan (although IEEE 802.3af refers to it as endpoint). Otherwise, if it's an intermediary device between a non PoE capable switch and a PoE device, it's called a midspan. An external PoE injector is a midspan device.
Powered device
A powered device (PD) is a device powered by a PSE and thus consumes energy. Examples include wireless access points, IP Phones, and IP Cameras.
Many powered devices have an auxiliary power connector for an optional, external, power supply. Depending on the PD design, some, none, or all power can be supplied from the auxiliary port, with the auxiliary port sometimes acting as backup power in case of PoE supplied power failure.
Power management features and integration
Advocates of PoE like the Ethernet Alliance expect PoE to become a global long term DC power cabling standard and replace a multiplicity of individual AC adapters, which cannot be easily centrally managed.
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. A typical 48-port Ethernet switch has a 50Â W to 80Â W power supply allocated for the Ethernet switch electronics. Over and above this, it requires typically a 740Â W (for 802.3af) to 1480Â W (for 802.3at) power supply allocation for PoE ports, permitting a maximum draw on each. However, where this central supply replaces several dedicated AC circuits, transformers and inverters, the power loss of long thin DC cable can be justifiable. In many case, power can be introduced to the device directly where AC power is available.
Switch power features
The switches themselves often contain "active", "smart", or "managed" power management features to reduce AC draw of all devices involved.
Multi-protocol teaming standards (G.9960, G.hn, and IEEE P1905) and handoff standards (IEEE 802.21) generally rely on simulating Ethernet features in other media.
By late 2011, some of the energy management features are proprietary. Advertising for power-over-Ethernet devices usually cites its "green" features including less packaging and improvements over previous models.
Integrating EEE and PoE
After integration with the IEEE 802.3az Energy-Efficient Ethernet (EEE) standard, the energy management capabilities of the combined standard are expected to be good. 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 3Â W per link, extremely significant across the tens of millions of new links shipped each year. These losses are especially significant as higher power devices come online. Marvell claims that:
- "With the evolution of PoE from a fairly low power source (up to 12.95Â W per port) to one with devices of up to 25.5Â W, the direct current (DC) power losses over Ethernet cables increased exponentially. Approximately 4.5Â W/port of power is wasted on a CAT5, CAT5e, CAT6 or CAT6A cable...after 100 m... EEE typically saves no more than 1Â W per link, so addressing the 4.5Â W per link loss from PoE transmission inefficiency would provide much more incremental savings. New energy-efficient PoE (EEPoE) technology can increase efficiency to 94% while transmitting over the same 25 ohm cable, powering IEEE 802.3at-compliant devices in synchronous 4-pairs. When utilizing synchronous 4-pairs, powered devices are fed using all the available wires. For example, on a 24-port IEEE 802.3at-2009 Type 2 system (delivering 25.5Â W per port), more than 50Â W are saved."
Standard implementation
Standards-based Power over Ethernet is implemented following the specifications in IEEE 802.3af-2003 (which was later incorporated as clause 33 into IEEE 802.3-2005) or the 2009 update, IEEE 802.3at. A phantom power technique is used to allow the powered pairs to also carry data. This permits its use not only with 10BASE-T and 100BASE-TX, which use only two of the four pairs in the cable, but also with 1000BASE-T (gigabit Ethernet), which uses all four pairs for data transmission. This is possible because all versions of Ethernet over twisted pair cable specify differential data transmission over each pair with transformer coupling; the DC supply and load connections can be made to the transformer center-taps at each end. Each pair thus operates in common mode as one side of the DC supply, so two pairs are required to complete the circuit. The polarity of the DC supply may be inverted by crossover cables; the powered device must operate with either pair: spare pairs 4â€"5 and 7â€"8 or data pairs 1â€"2 and 3â€"6. Polarity is required on data pairs, and ambiguously implemented for spare pairs, with the use of a diode bridge.
Notes:
Powering devices
Three modes, A, B, and 4-pair are available. Mode A delivers power on the data pairs of 100BASE-TX or 10BASE-T. Mode B delivers power on the spare pairs. 4-pair delivers power on all four pairs. PoE can also be used on 1000BASE-T Ethernet, in which case there are no spare pairs and all power is delivered using the phantom technique.
Mode A has two alternate configurations (MDI and MDI-X), using the same pairs but with different polarities. In mode A, pins 1 and 2 (pair #2 in T568B wiring) form one side of the 48Â V DC, and pins 3 and 6 (pair #3 in T568B) form the other side. These are the same two pairs used for data transmission in 10BASE-T and 100BASE-TX, allowing the provision of both power and data over only two pairs in such networks. The free polarity allows PoE to accommodate for crossover cables, patch cables and Auto MDI-X.
In mode B, pins 4â€"5 (pair #1 in both T568A and T568B) form one side of the DC supply and pins 7â€"8 (pair #4 in both T568A and T568B) provide the return; these are the "spare" pairs in 10BASE-T and 100BASE-TX. Mode B, therefore, requires a 4-pair cable.
The PSE (power sourcing equipment), not the PD (powered device), decides whether power mode A or B shall be used. PDs that implement only Mode A or Mode B are disallowed by the standard.
The PSE can implement mode A or B or both. A PD indicates that it is standards-compliant by placing a 25 kΩ resistor between the powered pairs. If the PSE detects a resistance that is too high or too low (including a short circuit), no power is applied. This protects devices that do not support PoE. An optional "power class" feature allows the PD to indicate its power requirements by changing the sense resistance at higher voltages. To stay powered, the PD must continuously use 5â€"10 mA for at least 60 ms with no more than 400 ms since last use or else it will be unpowered by the PSE.
There are two types of PSEs: endspans and midspans. Endspans (commonly called PoE switches) are Ethernet switches that include the power over Ethernet transmission circuitry. Midspans are power injectors that stand between a regular Ethernet switch and the powered device, injecting power without affecting the data.
Endspans are normally used on new installations or when the switch has to be replaced for other reasons (such as moving from 10/100Â Mbit/s to 1Â Gbit/s or adding security protocols), which makes it convenient to add the PoE capability. Midspans are used when there is no desire to replace and configure a new Ethernet switch, and only PoE needs to be added to the network.
IEEE 802.3at capable devices are also referred to as "type 2". An 802.3at PSE may also use layer2 communication to signal 802.3at capability.
Class 4 can only be used by IEEE 802.3at (type 2) devices, requiring valid Class 2 and Mark 2 currents for the power up stages. An 802.3af device presenting a class 4 current is considered non-compliant and, instead, will be treated as a Class 0 device.
Configuration via Ethernet layer 2 LLDP
The setup phases are as follows:
- PSE (provider) tests PD (consumer) physically using 802.3af phase class 3.
- PSE powers up PD.
- PD sends to PSE: I'm a PD, max power = X, max power requested = X.
- PSE sends to PD: I'm a PSE, max power allowed = X.
- PD may now use the amount of power as specified by the PSE.
The rules for this power negotiation are:
- PD shall never request more power than physical 802.3af class
- PD shall never draw more than max power advertised by PSE
- PSE may deny any PD drawing more power than max allowed by PSE
- PSE shall not reduce power allocated to PD that is in use
- PSE may request reduced power, via conservation mode
Non-standard implementations
Cisco
Some Cisco manufactured WLAN access points and IP phones supported a proprietary form of PoE many years before there was an IEEE standard for delivering PoE. Cisco's original PoE implementation is not software upgradeable to the IEEE 802.3af standard. Cisco's original PoE equipment was capable of delivering up to 10 W per port. The amount of power to be delivered is negotiated between the endpoint and the Cisco switch based on a power value that was added to the Cisco proprietary Cisco Discovery Protocol (CDP). CDP is also responsible for dynamically communicating the Voice VLAN value from the Cisco switch to the Cisco IP Phone.
Under Cisco's pre-standard scheme, the PSE (switch) will send a Fast Link Pulse (FLP) on the transmit pair. The PD (device) connects the transmit line to the receive line via a low pass filter. And thus the PSE gets the FLP in return. And a common mode current between pair 1 and 2 will be provided resulting in 48 V DC and 6.3 W default of allocated power. The PD has then to provide Ethernet link within 5 seconds to the auto-negotiation mode switch port. A later CDP message with a type-length-value tells the PSE its final power requirement. A discontinued link pulses shuts down power.
In 2014, Cisco created another non-standard PoE implementation called Universal Power over Ethernet (UPOE). UPOE can use all 4 pairs, after negotiation, to supply up to 60Â W.
Linear Technology
A proprietary high-power development called LTPoE++ using a single CAT-5e Ethernet cable is capable of supplying varying levels at 38.7Â W, 52.7Â W, 70Â W, and 90Â W in addition to being backwards compatible with IEEE 802.at.
Microsemi
PowerDsine, acquired by Microsemi in 2007, has been selling midspan power injectors since 1999 with its proprietary Power over LAN solution. Several companies such as Polycom, 3Com, Lucent and Nortel utilize PowerDsine's Power over LAN.
Passive
The common 100 Mbit/s passive applications use the pinout of 802.3af mode B - with DC plus on pins 4 and 5 and DC minus on 7 and 8 (see chart below) and data on 1-2 and 3-6. Gigabit passive injectors use a transformer on the data pins to allow power and data to share the cable and is typically compatible with 802.3af Mode A. In the common "passive" PoE system, the injector does not communicate with the powered device to negotiate its voltage or wattage requirements, but merely supplies power at all times. Passive midspan injectors up to 12 ports simplify installations. Devices needing 5 Volts cannot typically use PoE at 5 V on Ethernet cable beyond short distances (about 15 feet (4.6 m)) as the voltage drop of the cable becomes too significant, so a 24 V or 48 V to 5 V DC-DC converter is required at the remote end. Passive DC-to-DC injectors also exist which convert a 9 V to 36 V DC input power source to a stabilized 24 V 1 A or 48 V 0.5 A PoE feed with '+' on pins 4 & 5 and 'âˆ'' on pins 7 & 8. These DC-to-DC PoE injectors are used in various telecom applications.
Power capacity limits
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. A distinction is made between two scenarios: 1.) bundles heating up from the inside to the outside, and 2.) bundles heating up from the outside to match the ambient temperature. The second scenario largely depends on the way that the cable bundle has been installed, whereas the first is solely influenced by the physical make-up of the cable. In a standard U/UTP cable, the PoE-related temperature rise increases by a factor of 5. In a shielded cable, this value drops to between 2.5 and 3, depending on the design. Put another way, the temperature increases by twice as much in a U/UTP cable bundle than in a comparable bundle of S/FTP cables.
Pinouts
References
External links
- ieee802.org: Download the IEEE 802.3 standards
- ieee802.org: IEEE 802.3af Task Force
- ieee802.org: IEEE 802.3at Task Force
- POE pinouts of various manufacturers