Power over Ethernet
Encyclopedia
Category 5 cable
Category 5 cable is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. The cable is commonly connected using punch down blocks and modular connectors...
or higher for high power levels, but can operate with category 3 cable
Category 3 cable
Category 3 cable, commonly known as Cat 3 or station wire, is an unshielded twisted pair cable designed to reliably carry data up to 10 Mbit/s, with a possible bandwidth of 16 MHz...
for low power levels. Power is supplied in common mode
Common-mode signal
Common-mode signal is the component of an analog signal which is present with one sign on all considered conclusions. In electronics where the signal is transferred with differential voltage use, the common-mode signal is called a half-sum of voltages:...
over two or more of the differential pairs
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
of wires found in the Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
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.
The original IEEE 802.3af-2003 PoE standard provides up to of DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
power (minimum and ) to each device. Only is assured to be available at the powered device as some power is dissipated in the cable.
The updated IEEE 802.3at-2009 PoE standard also known as PoE+ or PoE plus, provides up to of power. The 2009 standard prohibits a powered device from using all four pairs for power. Some vendors have announced products that claim to be compatible with the 802.3at standard and offer up to of power over a single cable by utilizing all four pairs in the Cat.5 cable
Category 5 cable
Category 5 cable is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. The cable is commonly connected using punch down blocks and modular connectors...
.
Numerous non-standard schemes had been used prior to PoE standardization to provide power over Ethernet cabling. Some are still in active use.
Advantages over other integrated data and power standards
This technology is especially useful for powering IP telephoneVoice over IP
Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...
s, wireless LAN
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
access point
Wireless access point
In computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards...
s, cameras
Video camera
A video camera is a camera used for electronic motion picture acquisition, initially developed by the television industry but now common in other applications as well. The earliest video cameras were those of John Logie Baird, based on the electromechanical Nipkow disk and used by the BBC in...
with pan tilt and zoom
PTZ camera
A Pan tilt zoom camera is a closed-circuit television camera with remote directional and zoom control.In television production PTZ are used with professional video cameras in television studios and referred to as camera robotics. These systems can be remotely controlled by automation systems...
(PTZ), remote Ethernet switches
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
, embedded computers, thin clients and LCD
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
s.
All these require more power than USB
Universal Serial Bus
USB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....
offers and very often must be powered over longer runs of cable than USB permits. Field-spliced outdoor category 5 Ethernet cable can power radios and other low-power devices, for instance, through over 100 m of cable, an order of magnitude further than USB's theoretical maximums.
In addition, PoE uses only one type of connector, an 8P8C modular connector (often called RJ45), whereas there are numerous types of USB connectors and each new USB standard has added more.
PoE is presently deployed in applications where USB is unsuitable and where AC power would be inconvenient, expensiveMains wiring must often be done by qualified and/or licensed electricians for legal or insurance reasons. or infeasible to supply.
However, even where USB or AC power could be used, PoE has several advantages over either, including:
- Cheaper cabling — even high quality outdoor category 5 cableCategory 5 cableCategory 5 cable is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. The cable is commonly connected using punch down blocks and modular connectors...
is much cheaper than USB repeaters or AC wire. The task of meeting building code requirements to run AC power cable safely is eliminated. - A true gigabit connection to every device is possible, which exceeds USB 2.0 (400 Mbps) and current AC powerline networking capabilities, and can be teamed for 2-gigabit or 4-gigabit speeds comparable to USB 3.0 throughput. A 10 Gigabit Power over Ethernet standard is also being created.
- Global organizations can deploy PoE everywhere without concern for any local variance in AC power standards, outlets, plugs, or reliability. This makes a single standard office configuration much easier to maintain, monitor and update based on one standard plan.
- Direct injection from standard batteryBattery (electricity)An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
power arrays; this enables critical infrastructure to run more easily in outages, and make power rationing decisions centrally for all the PoE devices. The priority for power-supply via PoE can be configured via the switches. - Cheap reliable switching. While USB devices require a true computer or router to control the bus, and still require switching or routing to make VPN or Internet connections, powered Ethernet devices require only a switch, which can be unmanaged, which can do both jobs. Further, the experience of ISPs, Carrier EthernetCarrier EthernetCarrier Ethernet is a marketing term for extensions to Ethernet to enable telecommunications network providers to provide Ethernet services to customers and to utilize Ethernet technology in their networks....
and a number of other quality of serviceQuality of serviceThe quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
standards from the Ethernet world are directly applicable to the power-over-Ethernet world, whereas there is no comparable experience base for USB networking. - Symmetric distribution is possible. Unlike USB and AC outlets, power can be supplied at either end of the cable or outlet. This means the location of the power source can be determined after cables and outlets are installed.
Uses
Some types of devices with PoE include:- IP Security Cameras
- Network routers
- A mini network switchNetwork switchA network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
installed in distant rooms, to support a small cluster of ports from one uplinkUplinkA telecommunications link is generally one of several types of information transmission paths such as those provided by communication satellites to connect two points on earth.-Uplink:...
cable. (These ports on the mini-switch do not themselves provide PoE.) (In most modern VoIP phones a two-port switch is embedded to which a local workstation can be installed using a different VLANVirtual LANA virtual local area network, virtual LAN or VLAN, is a group of hosts with a common set of requirements that communicate as if they were attached to the same broadcast domain, regardless of their physical location...
from the voice-VLAN used by the phone itself) - Network webcamWebcamA webcam is a video camera that feeds its images in real time to a computer or computer network, often via USB, ethernet, or Wi-Fi.Their most popular use is the establishment of video links, permitting computers to act as videophones or videoconference stations. This common use as a video camera...
s - Network IntercomIntercomAn intercom , talkback or doorphone is a stand-alone voice communications system for use within a building or small collection of buildings, functioning independently of the public telephone network. Intercoms are generally mounted permanently in buildings and vehicles...
/ PagingPagingIn computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called...
/ Public addressPublic addressA public address system is an electronic amplification system with a mixer, amplifier and loudspeakers, used to reinforce a sound source, e.g., a person giving a speech, a DJ playing prerecorded music, and distributing the sound throughout a venue or building.Simple PA systems are often used in...
systems and hallway speaker amplifiers - VoIP phones
- Wall clocks in rooms and hallways, with time set using Network Time ProtocolNetwork Time ProtocolThe Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...
- Wireless access pointWireless access pointIn computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards...
s - 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
- Remote Point of Sale (POS) kiosks
Power sourcing equipment
Power sourcing equipment (PSE) is a device such as a switchNetwork switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
that provides ("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's called an endspan. 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 pointWireless access point
In computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards...
s, 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
Wall wart
The AC adapter, AC/DC adapter or AC/DC converter is a type of external power supply, often enclosed in what looks like an over-sized AC plug. Other names include plug pack, plug-in adapter, adapter block, domestic mains adapter, line power adapter, or power adapter...
" converters, which cannot be easily centrally managed, waste energy, are often poorly designed, and are easily vulnerable to damage from surges
Voltage spike
In electrical engineering, spikes are fast, short duration electrical transients in voltage , current , or transferred energy in an electrical circuit....
and brownouts
Power outage
A power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...
.
Critics of this approach argue that DC power is inherently less efficient than AC power to push through long cables, 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 370 W (for 802.3af) to 740 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
Beyond the inherent advantages of an optimized inject-anywhere AC-and-DC cabling infrastructure, the switches themselves often contain "active", "smart", or "managed" power management features to further reduce AC draw of all devices combined.Multi-protocol teaming standards (G.9960, G.hn
G.hn
G.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
, and IEEE P1905) and handoff standards (IEEE 802.21
IEEE 802.21
802.21 is an IEEE standard published in 2008. The standard supports algorithms enabling seamless handover between networks of the same type as well as handover between different network types also called Media independent handover or vertical handover...
) generally rely on simulating Ethernet features in other media. These standards enable more optimal energy and bandwidth management solutions than would otherwise be possible. For instance, networking on existing AC power lines to an outlet where a PoE router is plugged, making it capable of moving a gigabit per second to every device, with minimal wiring, participating fully in both AC and DC device power demand management. Or, letting a session migrate from a high-power Ethernet switch to a low-power power-over-ethernet wireless routing when the need for bandwidth is low and there is no need for power on the Ethernet cable to be supplied to the device.
By late 2011, managed switches with both powered and unpowered Ethernet ports, from Netgear
Netgear
Netgear is a U.S. manufacturer of computer networking equipment and other computer hardware....
and others, featured many significant energy management features, some of which are proprietary but are likely to migrate into the eventual standard. For instance, "auto power-down and cable-length detection" allowing lower signal strength to be used. Given that such features were available in switches selling for under US$250, power savings alone could justify some users switching to a security, VoIP or wireless AP infrastructure based on power over Ethernet, as they would pay for it very quickly.
Power advantages are a major sales appeal of powered over unpowered Ethernet or unpowered alternatives (such as strictly wireless sensor networks, which must in practice rely on batteries if they draw more than their own solar capacity). 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 formidable. Pre-standard integrations of EEE and PoE (such as MarvellMarvell
-People:* Andrew Marvell, 17th-century English metaphysical poet* Marvell Wynne , American soccer player* Marvell Wynne , American baseball player-Businesses:* Marvell Technology Group, U.S. semiconductor company...
's EEPoE outlined in a May 2011 white paper http://www.marvell.com/switching/assets/Marvell-PoE-An-Energy-Efficient-Alternative.pdf) 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." http://www.marvell.com/switching/assets/Marvell-PoE-An-Energy-Efficient-Alternative.pdf
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 powerPhantom power
Phantom power, in the context of professional audio equipment, is a method for transmitting DC electric power through microphone cables to operate microphones that contain active electronic circuitry....
technique is used to allow the powered pairs to also carry data. This permits its use not only with 10BASE-T
10BASE-T
Ethernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. Other Ethernet cable standards employ coaxial cable or optical fiber. Early versions developed in the 1980s included StarLAN followed by 10BASE-T. By the 1990s, fast, inexpensive...
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
Common-mode signal
Common-mode signal is the component of an analog signal which is present with one sign on all considered conclusions. In electronics where the signal is transferred with differential voltage use, the common-mode signal is called a half-sum of voltages:...
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
Ethernet crossover cable
An Ethernet crossover cable is a type of Ethernet cable used to connect computing devices together directly. Normal straight through or patch cables were used to connect from a host network interface controller to a network switch, hub or router.A cable with connections that "cross over" was used...
; 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
Diode bridge
A diode bridge is an arrangement of four diodes in a bridge circuit configuration that provides the same polarity of output for either polarity of input. When used in its most common application, for conversion of an alternating current input into direct current a output, it is known as a...
.
| Property | ||
|---|---|---|
| Power available at PDMost switched power supplies Switched-mode power supply A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power... within the powered device will lose another 10 to 25% of the available power. |
||
| Maximum power delivered by PSE | ||
| Voltage range (at PSE) | ||
| Voltage range (at PD) | ||
| Maximum current | per mode | |
| Maximum cable resistance | (Category 3 Category 3 cable Category 3 cable, commonly known as Cat 3 or station wire, is an unshielded twisted pair cable designed to reliably carry data up to 10 Mbit/s, with a possible bandwidth of 16 MHz... ) |
(Category 5 Category 5 cable Category 5 cable is a twisted pair cable for carrying signals. This type of cable is used in structured cabling for computer networks such as Ethernet. It is also used to carry other signals such as telephony and video. The cable is commonly connected using punch down blocks and modular connectors... ) |
| Power management | Three power class levels negotiated at initial connection | Four power class levels negotiated at initial connection or steps negotiated continuously |
| Derating of maximum cable ambient operating temperature | None | with one mode (two pairs) active |
| Supported cabling | Category 3 and Category 5 | Category 5More stringent cable specification allows assumption of more current carrying capacity and lower resistance (20.0 Ohms for Category 3 versus 12.5 Ohms for Category 5). |
| Supported modes | Mode A (endspan), Mode B (midspan) | Mode A, Mode B |
Powering devices
Two modes, A and B, are available. Mode A delivers phantom 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, not the powered device (PD), 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 are Ethernet switches that include the power over Ethernet transmission circuitry. Endspans are commonly called PoE switches. 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.
| Stage | Action | Volts specified [V] |
|
|---|---|---|---|
| 802.3af | 802.3at | ||
| Detection | PSE detects if the PD has the correct signature resistance of | ||
| Classification | PSE detects resistor indicating power range (see below) | ||
| Signals PSE is 802.3at capable. PD presents a load. | — | ||
| PSE outputs classification voltage again to indicate 802.3at capability | — | ||
| Signals PSE is 802.3at capable. PD presents a load. | — | ||
| Startup | Startup voltage | ||
| Normal operation | Supply power to device | ||
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 | Usage | Classification current [mA] | Power range [Watt] | Class description |
|---|---|---|---|---|
| 0 | Default | 0–4 | 0.44–12.94 | Classification unimplemented |
| 1 | Optional | 9–12 | 0.44–3.84 | Very Low power |
| 2 | Optional | 17–20 | 3.84–6.49 | Low power |
| 3 | Optional | 26–30 | 6.49–12.95 | Mid power |
| 4 | Valid for 802.3at (Type 2) devices, not allowed for 802.3af devices |
36–44 | 12.95–25.50 | High power |
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
| TLV Type-length-value Within data communication protocols, optional information may be encoded as a type-length-value or TLV element inside of the protocol. TLV is also known as tag-length value.... Header |
MED Header | Extended power via MDI Medium dependent interface A medium dependent interface describes the interface in a computer network from a physical layer implementation to the physical medium used to carry the transmission.... |
|||||
|---|---|---|---|---|---|---|---|
| Type |
Length |
TIA Telecommunications Industry Association The Telecommunications Industry Association is accredited by the American National Standards Institute to develop voluntary, consensus-based industry standards for a wide variety of ICT products, and currently represents nearly 400 companies... OUI Organizationally Unique Identifier An Organizationally Unique Identifier is a 24-bit number that is purchased from the Institute of Electrical and Electronics Engineers, Incorporated Registration Authority... |
|||||
| 127 | 7 | 00-12-BB | 4 | PSE or PD | Normal or Backup conservation Emergency power system Emergency power systems are a type of system, which may include lighting, generators, fuel cells and other apparatus, to provide backup power resources in a crisis or when regular systems fail. They find uses in a wide variety of settings from residential homes to hospitals, scientific... |
Critical, High, Low |
in steps |
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
Cisco
Cisco manufactured WLAN access points and IP phoneIP Phone
A VoIP phone uses voice over IP technologies allowing telephone calls to be made over an IP network such as the Internet instead of the ordinary PSTN system. Calls can traverse the Internet, or a private IP network such as that of a company...
s 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 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
Cisco Discovery Protocol
The Cisco Discovery Protocol is a proprietary Data Link Layer network protocol developed by Cisco Systems. It is used to share information about other directly connected Cisco equipment, such as the operating system version and IP address...
(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
Low-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut 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 and default of allocated power. The PD has then to provide Ethernet link within to the auto-negotiation mode switch port. A later CDP message with a type-length-value
Type-length-value
Within data communication protocols, optional information may be encoded as a type-length-value or TLV element inside of the protocol. TLV is also known as tag-length value....
tells the PSE its final power requirement. A discontinued link pulses shuts down power.
PowerDsine
PowerDsine, now a Microsemi brand, have been selling midspan power injectors since 1999 with its proprietary Power over LAN solution. Several companies such as PolycomPolycom
Polycom is a multinational corporation with approximately 3,200 employees worldwide and an annual revenue of approximately $1.2 billion in 2010. The company manufactures and sells telepresence and voice communications solutions.-Company History 1990:...
, 3Com
3Com
3Com was a pioneering digital electronics manufacturer best known for its computer network infrastructure products. The company was co-founded in 1979 by Robert Metcalfe, Howard Charney, Bruce Borden, and Greg Shaw...
, Lucent and Nortel
Nortel
Nortel Networks Corporation, formerly known as Northern Telecom Limited and sometimes known simply as Nortel, was a multinational telecommunications equipment manufacturer headquartered in Mississauga, Ontario, Canada...
utilize PowerDsine's Power over LAN.
Passive
Numerous devices exist which make use of positive power received on pins 4 and 5 of the Ethernet cable, with negative return on pins 7 and 8. Power over Ethernet injectors exist which convert 110-240 V 50/60 Hz AC power to 9 V, 15 V, 24 V, 48 V or 55 V DC with currents ranging from 0.5 A to 2.0 A. In the common "passive" PoE system the injector does not communicate with the powered device to negotiate its wattage requirements, but merely supplies power. 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 pins +4,5 and -7,8. These DC to DC PoE injectors are used in a variety of different telecom applications.See also
- Network switchNetwork switchA network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
, connects network nodesNode (networking)In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
with independent pipes (efficient) - Power line communicationPower line communicationPower line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...
, data communication over mains electricityMains electricityMains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power... - Switched-mode power supplySwitched-mode power supplyA switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
, efficient electrical power conversion - ITU-TITU-TThe ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
G.hnG.hnG.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
, a home networkHome networkA home network or home area network is a residential local area network . It is used for communication between digital devices typically deployed in the home, usually a small number of personal computers and accessories, such as printers and mobile computing devices...
using existing home wiring (power linesPower line communicationPower line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...
, phone lines and coaxial cablesEthernet over coaxEthernet over Coax is a family of technologies that supports the transmission of Ethernet frames over coaxial cable.- History :The first Ethernet standard, known as 10BASE5 in the family of IEEE 802.3, specified baseband operation over coaxial cable...
) - Phantom powerPhantom powerPhantom power, in the context of professional audio equipment, is a method for transmitting DC electric power through microphone cables to operate microphones that contain active electronic circuitry....
, long established standard technique to power microphoneMicrophoneA microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
s - HomePlug Powerline AllianceHomePlug Powerline AllianceThe HomePlug Powerline Alliance is a trade association of electronics manufacturers, service providers, and retailers that establishes standards for, and tests members' devices for compliance to, the various power line communication technologies known as HomePlug.The alliance developed standards...
, an industry trade groupIndustry trade groupA trade association, also known as an industry trade group, business association or sector association, is an organization founded and funded by businesses that operate in a specific industry...
on datacommunication over mains electricityMains electricityMains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...