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What is POE

Nov 02, 2017

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POE (Power Over Ethernet) refers to the existing Ethernet Cat.5 cabling infrastructure without any changes in the case, for some IP-based terminals (such as IP telephones, wireless LAN access point AP, network camera Etc.) while transmitting data signals, but also for such devices to provide DC power supply technology. POE is also known as a LAN-based power supply system (POL) or active Ethernet (Active Ethernet), and sometimes referred to as Ethernet power supply, which is the use of existing standard Ethernet transmission cable while transmitting data and The latest standard specification for electrical power, and maintains compatibility with existing Ethernet systems and users.




POE technology can ensure the safety of existing structured wiring while ensuring the normal operation of existing networks, to minimize costs. The IEEE 802.3af standard is a new standard based on the POE of the Ethernet power supply system. It adds the standard for direct power supply via the network cable on the basis of IEEE 802.3. It is the extension of the existing Ethernet standard and the first international power distribution standard.



Related standards


PoE early application there is no standard, the use of idle power supply.


1, IEEE 802.3af


IEEE 802.3af (15.4W) became the first PoE power supply standard, which stipulates the Ethernet power supply standard and is the mainstream implementation standard for PoE applications.


IEEE began to develop the standard in 1999, the first to participate in the manufacturers are 3Com, Intel, PowerDsine, Nortel, Mitel and National Semiconductor. However, the shortcomings of the standard has been restricting the expansion of the market. Until June 2003, IEEE approved the 802. 3af standard, which clearly defined the power system in the remote detection and control issues, and routers, switches and hubs through the Ethernet cable to IP phones, security systems and wireless LAN access Point and other equipment to provide the way the provisions of the power supply. The development of IEEE 802.3af contains the efforts of many company specialists, which also allows the standard to be tested in all respects.


A typical Ethernet power supply system. Keep the Ethernet switch in the wiring closet, and use a power supply hub (Midspan HUB) to provide power to the local area network twisted pair. At the end of the twisted pair, the power supply is used to drive the phone, the wireless access point, the camera and other devices. To avoid power outages, you can use a UPS.



2, IEEE 802.3at


IEEE802.3at (25.5W) should be the needs of high-power terminal was born, compatible with 802.3af based on the provision of greater power supply needs to meet the new needs.


In order to comply with the IEEE 802.3af specification, PoE power consumption on powered devices (PDs) is limited to 12.95W, which is sufficient for traditional IP telephony and webcams, but with dual band access, video telephony, PTZ video surveillance system and other high-power applications, 13W power supply is clearly unable to meet the demand, which limits the scope of application of Ethernet cable power supply. In order to overcome the limitations of PoE's power budget and push it to new applications, IEEE has set up a new task force to explore ways to improve the power limits of the international power supply standards. The IEEE802.3 Working Group established PoEPlus's research team in November 2004 to evaluate the possibility of IEEE802.3at implementation technically and economically. And then in July 2005 approved the establishment of IEEE 802.3at Commission of Inquiry program. The new standard, called Power-over-Ethernet Plus (PoEP) IEEE 802.3at, defines a device with a power requirement higher than 12.95W as Class 4, which is described in IEEE 802.3af but is reserved for future use. Extends the power level to 25W or higher.


System Components


POE system structure:


A complete POE system includes two parts: power supply equipment (PSE) and power supply equipment (PD). The PSE device is a device that powers the Ethernet client device and is also the manager of the entire POE power over Ethernet process. The PD device is a PSE load that receives power, such as POE system client devices such as IP phones, network security cameras, APs and handheld computers (PDAs) or mobile phone chargers, and many other Ethernet devices (in fact, any power No more than 13W of equipment can be obtained from the RJ45 socket corresponding power). Both based on the IEEE 802.3af standard to establish the connection of the receiving device PD, equipment type, power level and other aspects of information contact, and as a basis for the PSE through the Ethernet to the PD power supply.


Power supply work process


When the POE power supply terminal is arranged in a network, the POE power supply operation process is as follows.


1. Detection: Initially, the POE device outputs a very small voltage at the port until it detects that the cable terminal is connected to a receiving terminal that supports the IEEE 802.3af standard.


2.PD terminal device classification: When the receiving terminal device PD is detected, the POE device may classify the PD device and evaluate the power loss required for the PD device.


3. Start power supply: During a configurable time (typically less than 15μs), the PSE device begins to supply power from the low voltage to the PD device until a 48V DC power supply is provided.


4. Power supply: PD equipment to provide a stable and reliable 48V DC power to meet the PD device does not exceed 15.4W of power consumption.


5. Power off: If the PD device is disconnected from the network, the PSE will quickly (usually within 300 ~ 400ms) stop the PD power supply, and repeat the detection process to detect whether the cable terminal connected to the PD device.


The principle of power supply


The standard five-wire cable has four pairs of twisted pairs, but only two of them are used in 10M BASE-T and 100M BASE-T. IEEE80 2.3af allows two kinds of usage, the application of idle feet power supply, 4,5 feet connected to the positive, 7,8 feet connected to the negative.


Application of data feet power supply, the DC power supply in the transmission transformer at the midpoint, does not affect the data transmission. In this way the line pair 1, 2 and the pair 3, 6 can be of any polarity.


The standard does not allow the simultaneous application of the above two cases. The power supply device PSE can only provide one usage, but the power application device PD must be able to accommodate both situations at the same time. The standard provides power supply is usually 48V, 13W. PD devices provide 48V to low voltage conversion is easier, but at the same time there should be 1500V insulation safety voltage.


The latest development


PowerDsine, an Ethernet-powered chip maker, will hold an IEEE conference to formally submit a "high-power Ethernet power" standard that will support powering equipment such as laptops. PowerDsine will submit a white paper, it is recommended to 802.3af standard 48v input, 13w available power limit increased by 1 times. In addition to notebook computers, the new standard is also possible for LCD monitors and video phones and other power supply.


Recently, IEEE has a new 802.3AT, which provides the POE can provide higher power, more than 13W, can reach 30W!

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