(From Wikipedia, the free encyclopedia)
Power over Ethernet or PoE describes any of several standardized or ad-hoc systems which pass electrical power along with data on twisted pair Ethernet cabling. This allows a single cable to provide both data connection and electrical power to devices such as wireless access points or IP cameras. Unlike standards such as Universal Serial Bus which also power devices over the data cables, PoE allows long cable lengths. Power may be carried on the same conductors as the data, or it may be carried on dedicated conductors in the same cable.
There are several common techniques for transmitting power over Ethernet cabling. Two of them have been standardized by IEEE 802.3 since 2003.
Only two of the total four twisted pairs in an Ethernet cable are used for data in 10BASE-T or 100BASE-TX Ethernet; thus, power can be transmitted on the unused two pairs. In IEEE standards, this is referred to as Alternative B. It is a common PoE technique because it separates data and power conductors, making troubleshooting easier.
Power may also be transmitted on the data conductors by applying a common-mode voltage to each pair. Because twisted-pair Ethernet uses differential signalling, this does not interfere with data transmission. The common mode voltage is easily extracted using the center tap of the standard Ethernet pulse transformer. This is similar to the phantom power technique commonly used for powering audio microphones. In the IEEE standards, this is referred to as Alternative A.
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 signalling 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. Up to 25.5 watts is available for a device.
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) to each device. 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. 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 over Ethernet cabling. The upcoming 4PPoE (4 Pair Power over Ethernet) standard will introduce two new levels of power: 55 W (Level 3) and 90-100 W (Level 4). Each twisted pair can handle a current of up to one ampere. This development opens the door to new applications such as operating high-performance WLAN antennas and surveillance cameras.
However, the increased current brings fresh challenges with it. The greater the currents, the greater the heat produced due to resistance. And warm cables slow data transmission rates more than was previously the case. As a result, the signal reaching the receiver could be too weak, causing data transmission to fail. This effect must be taken into account when planning new PoE-compatible LAN cabling. One way is to adapt the maximum transmission distance to temperature conditions and shorten it as necessary.
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. This is because the metal in the shield helps to transport heat from inside the bundle to the outside.
Comparison with other integrated data and power standards
PoE provides both data and power connections in one cable, so equipment doesn’t require a separate cable for each need. For equipment that does not already have a power or data connection, PoE can be attractive when the power demand is modest. For example, PoE is useful for IP telephones, wireless access points, cameras with pan tilt and zoom (PTZ), and remote Ethernet switches. PoE can provide long cable runs e.g. 100 m (330 ft) and deliver 12 W of galvanically isolated power. PoE-plus provides even more power.
Universal Serial Bus (USB) and IEEE 1394 (FireWire) both provide data and power over limited distances. USB and FireWire are good choices for connecting peripherals to a PC.
If a device already has power available but no data link, then PoE may not be attractive. A wireless data connection such as IEEE 802.11 may be more economical than running a data cable for the device. Alternatively, there are power line communication technologies that can use power cables for transmitting data. Using some power line modems may be more economical than running a cable.
When data rate and power requirements are both low, other approaches may be viable. Mobile phones, for example, use batteries for power and antennas for communication. Remote weather sensors use very low data rates, so batteries (sometimes supplemented with solar power) and custom wireless data links are used.
Depending on the application, some of the advantages with PoE over other technologies may be:
Inexpensive cabling carries both data and power
Power to equipment can be remotely cycled
Fast data rate
Power sourcing equipment
Power sourcing equipment (PSE) is a device such as a switch that provides (or sources) power on the Ethernet cable. The maximum allowed continuous output power per cable in IEEE 802.3af is 15.40 W. A later specification, IEEE 802.3at, offers 25.50 W.
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.
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
Most advocates[who?] expect PoE to become a global longterm DC power cabling standard and replace “wall wart” converters, which cannot be easily centrally managed, waste energy, are often poorly designed, and are easily vulnerable to damage from surges and brownouts.
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 50 W to 80 W power supply allocated for the traditional Ethernet switch and transceiver IC. Over and above this it requires typically a 740 W (for 802.3af) to 1480 W (for 802.3at) power supply allocated solely for PoE ports, permitting a maximum draw on each. This can be quite inefficient to supply through long cables. However, where this central supply replaces several dedicated AC circuits, transformers and inverters, and prevents expensive human interventions (AC installations) the power loss of long thin DC cable is easily justifiable. Power can always be introduced on the device end of the Ethernet cable (radically improving efficiency) 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 3 watts 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 12.95W per port) to one with devices of up to 25.5W, the direct current (DC) power losses over Ethernet cables increased exponentially. Approximately 4.5W/port of power is wasted on a CAT5, CAT5e, CAT6 or CAT6A cable…after 100m… EEE typically saves no more than 1W per link, so addressing the 4.5W per link loss from PoE transmission inefficiency would provide much more incremental savings. New energy-efficient PoE (EEPoE) technology can change increase efficiency to 94% while transmitting over the same 25ohm 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 25.5W per port), more than 50W are saved.”
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 4–5 and 7–8 or data pairs 1–2 and 3–6. Polarity is required on data pairs, and ambiguously implemented for spare pairs, with the use of a diode bridge.
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Two modes, A and B, are available. Mode A delivers power on the data pairs of 100BASE-TX or 10BASE-T. Mode B delivers power on the spare 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 48 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-MDIX.
In mode B, pins 4–5 (pair #1 in both T568A and T568B) form one side of the DC supply and pins 7–8 (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 5–10 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 10/100 Mbit/s to 1 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.
Power capacity limits
Category 5 cable uses 24 AWG conductors, which can safely carry 360 mA at 50 V according to the latest TIA ruling. The cable has eight conductors (only half of which are used for power) and therefore the absolute maximum power transmitted using direct current is 50 V × 0.360 A × 2 = 36 W. Considering the voltage drop after 100 m (330 ft), a PD would be able to receive 31.6 W. The additional heat generated in the wires by PoE at this current level (4.4 watts per 100 meter cable) limits the total number of cables in a bundle to be 100 cables at 45 °C (113 °F), according to the TIA. This can be somewhat alleviated by the use of Category 6 cable which uses 23 AWG conductors.
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