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Power over Ethernet (POE) analysis of the key technologies

 Updated by ACORID R&D May.2008

Power over Ethernet (POE) Overview

       POE (Power Over Ethernet) refers to the existing Ethernet Cat.5 cabling infrastructure, not to make any alteration of the case, for some IP-based terminals (such as IP telephones, wireless LAN access point AP , network cameras, etc.) transmit data signals at the same time, but also to provide DC power for such equipment technology. POE technology to ensure the safety of the existing structured cabling while ensuring the normal operation of the existing network, minimizing costs.

      POE (Power Over Ethernet) refers to the existing Ethernet Cat.5 cabling infrastructure, not to make any alteration of the case, for some IP-based terminals (such as IP telephones, wireless LAN access point AP , network cameras, etc.) transmit data signals at the same time, but also to provide DC power for such equipment technology. POE technology to ensure the safety of the existing structured cabling while ensuring the normal operation of the existing network, minimizing costs.

      IEEE in 1999, started to develop the standard, the first vendors to participate with 3Com, Intel, PowerDsine, Nortel, Mitel, and National Semiconductor. However, the shortcomings of the standard has been constrained the expansion of the market. Until June 2003, IEEE approved the 802. 3af standard, it specifies a remote system in the power detection and control matters, as well as routers, switches and hubs via Ethernet cable to IP phones, security systems as well as wireless LAN access points and other equipment the way power has been stipulated. IEEE 802.3af development includes the efforts of experts in many companies, which also makes the standard can be tested in all respects.

     A typical Ethernet power supply system shown in Figure 1. In the wiring cabinet to retain the Ethernet switch equipment, with a hub with a power supply (Midspan HUB) twisted-pair to provide power to the LAN. In the twisted pair at the end, the power supply used to drive phones, wireless access points, cameras and other equipment. In order to avoid power outages, you can use a UPS.

 Figure 1 A typical Ethernet Power Supply System

 1. POE composition and power system parameters

       POE system consists of a complete power supply terminal equipment (PSE, Power Sourcing Equipment), and by the Powered Device (PD, Power Device) in two parts. PSE device is a client device for Ethernet-powered device, but also the whole process of POE Power over Ethernet manager. The PD device is powered PSE accept the load, that is, POE system, customer premises equipment, such as IP telephony, network security cameras, AP, and Pocket PC (PDA) or mobile phone chargers and many other Ethernet devices (in fact, Any power not exceeding 13W devices are available from the RJ45 socket to obtain the corresponding electricity). Both establish standards based on the IEEE 802.3af powered device PD, client connectivity, equipment type, power level, etc. information links and used as a basis PSE to the PD power supply through Ethernet.

POE standard power supply system's main power supply characteristic parameters as follows:

    1. Voltage between 44 ~ 57V, the typical value of 48V.
    2. To allow the maximum current of 550mA, the maximum inrush current of 500mA.
    3. Typical operating current of 10 ~ 350mA, overload detection current 350 ~ 500mA.
    4. In the no-load conditions, the maximum required current of 5mA.
    5. For the PD devices 3.84 ~ 12.95W five levels of electric power request, the maximum does not exceed 13W.

 POE-powered work process

 When a PSE power supply arrangement of the network-side equipment, POE Power over Ethernet working process as follows.

    1. Detection: a beginning, PSE equipment in the port output low voltage cable terminal until it detects a connection to a support IEEE 802.3af-standard powered end equipment.
    2. PD premise equipment Category: When detected by the PD electrical equipment after the end, PSE devices may be classified as a PD device and to assess the PD of equipment for power loss.
    3. Began to power: in a configurable time (usually less than 15μs) the launch period, PSE equipment began from a low voltage power to PD device up to provide 48V DC power supply.
    4. Power Supply: Equipment for the PD to provide a stable and reliable 48V DC to meet the PD device does not cross the 15.4W power consumption.
    5. Power: If the PD device, when disconnected from the network, PSE will be fast (normally within 300 ~ 400ms) to stop power supply equipment for the PD, and repeat the testing process to test the cable terminal is connected PD device .