什么是以太网供电.doc

什么是以太网供电(PoE)？以太网供电(PoE)是一种将功率从中央交换机传输到以太网连接设备（VoIP电话、WLAN发射机、安防摄像头）的供电方法。利用现有的CAT-5电缆，这些设备可以不需要AC供电（同时节省配线成本）。交换机也可以对分配给用电设备(PD)的功率进行控制，这样允许对某些重要系统进行精确不间断的功率管理。工作方式基本上，一个PoE负载或用电设备（PD）为了实现与发送端PSE（功率发送设备）的连接，要完成三个步骤，包括检测、分级及欠压锁存。检测阶段当具有PoE功能的以太网电缆插入到一个PD设备时，PSE会询问该PD设备以确定其是否为具有PoE功能的设备。这段时间定为检测阶段。在检测阶段，PSE会给PD施加一个斜坡电压，用来寻找负载端是否有25 k的阻抗特性。如果没有检测到该阻抗，PSE会假定负载不是具有PoE功能的负载，从而关断PoE发送端。此时系统以标准的以太网连接方式工作。如果检测阶段检测到识别阻抗，PSE会继续进行到分级阶段。阻抗识别电压是介于2.5 V和10 V之间的斜坡电压。在检测阶段，一个24.9 k的电阻可以提供恰当的识别阻抗（参见图1）。图1. 检测阻抗分级阶段PSE给PD提供的斜坡电压继续上升。在15 V到20 V之间，开始进行分级。在此电压过渡期间，PD设备必须拉出规定的电流以确定其设备级别（参见图2）。使用24.9 k识别电阻可以确定其为最简单的级别（0级）。分级电流用来确定该PD设备在正常工作期间所消耗的总功率大小。此电流信息由PSE反馈到控制器，由系统来确定功率预算的要求。表1列出了分级电流的大小及相应PD工作时的功率要求。级别PMINPMAXICLASS（最小值）ICLASS（最大值）RCLASS00.44 W12.95 W0 mA4 mA开路10.44 W3.84 W9 mA12 mA150 23.84 W6.49 W17 mA20 mA82.5 36.49 W12.95 W26 mA30 mA53.6 4保留保留36 mA44 mA38.3 表1.分级功率级别。 启动阶段在分级阶段之后，PSE继续将输入斜坡电压上升至30 V，此时欠压锁存(UVLO)电路解除，PD设备开始有供电。要求有软启动电路以限制从PSE拉出的电流。图3为典型的欠压锁存电路。图3. 使用MOSFET导通开关的0级PoE接口电路通过这样的过程，PSE与PD协同工作，确定了负载特性并仅向具有PoE功能的设备供电。在中心位置的系统控制器可以确定负载要求，在停电期间根据UPS电源预算及各级工作需要对功率加以分配。有关驱动PoE兼容负载设备和实现1级到3级分级的电路的其他信息，请参见设计参考DI-70和DI-88。IEEE P802.3at DTE Power EnhancementsTask ForceThe work of the P802.3at DTE Power Enhancements Task Force is now complete with the approval of IEEE Std 802.3at-2009 and IEEE Std 802.3bc-2009 at the September 2009 IEEE Standards Board meeting. IEEE Std 802.3at-2009 and IEEE Std 802.3bc-2009 are available from the IEEE. Approved IEEE P802.3at Project Authorization Request 30-Jan-09. Approved IEEE P802.3at 5 Criteria 14-Nov-08. Approved IEEE P802.3at Objectives 14-Nov-08. IEEE P802.3at search. IEEE P802.3at public area. IEEE P802.3at e-mail reflector archive. IEEE P802.3at comments received during balloting. IEEE P802.3at Big ticket items list May-07. IEEE P802.3af-2003 (available at no charge through the Get IEEE 802 program). Subscribing and unsubscribing to the IEEE P802.3at e-mail reflector. IEEE 802.3au (IEEE P802.3-2005/Cor 1) DTE Power Isolation Corrigendum. IEEE 802.3 Power over Ethernet plus Study Group public area The next meeting of this Task Force will be during the IEEE 802 March plenary meeting. Return to IEEE 802.3 Home Page Last Update: 02 October 2009Commonly Used Terms for PoEClassA protocol for describing Powered Device (PD) load requirements (Class 0 is 0 watts to 12.95 watts, Class 1 is 0.44 watts to 3.84 watts, Class 2 is 3.85 watts to 6.49 watts and Class 3 is 6.50 watts to 12.95 watts. Class 4 is reserved for future use). See DI-70 and DI-88 for implementing PD classification circuitry.ClassificationThe PSE applies a voltage of 14.5 VDC to 20.5 VDC to the PD. The PD draws a constant current during this voltage range to indicate its class. This current is monitored by the PSE, which reports power level and sets sending end power limits accordingly. See DI-70 and DI-88 for details.Data pairsCat-5 cable pairs that carry data and may also be used for power in a Power over Ethernet (PoE) application The data pairs are either pairs 1,2 or 3,6. For PoE both wires in a data pair must be used.DetectionThis is the method (sometimes referred to as “Signature”) by which a PD indicates to the PSE that it is capable of receiving power via PoE. The PD exhibits a specific impedance (typically 24.9 k) when the PSE first applies a detection signal. This identifies the PD as a PoE enabled device. See DI-70 and DI-88 for details.Detection resistorThe detection function in a PD can be implemented using a single 24.9 k resistor. This resistor will also draw the correct classification current from the PSE for Class 0 classification.EndpointEndpoint (or Endpoint PSE) refers to an Ethernet switch or hub which is equipped to inject PoE power onto the Ethernet cables using either of the data pairs.Endpoint PSESee Endpoint.EndspanAlthough IEEE802.3af describes only “Endpoint” or “Endpoint PSE”, the term “Endspan” is in common use. See “Endpoint” for the definition of this term.IEEE802.3atStandard in development for next generation PoE delivering higher power over multiple wire pairs. This standard is the implementation of PoEP.InjectorA device for putting PoE onto a non PoE enabled Cat-5 cableMidspanAlso referred to as a “Midspan PSE” or “Power Injector”. The terms describe a PoE system using redundant pairs 4,5 and 7,8 to transmit power. A midspan is most often added to legacy Ethernet equipment as a separate module to create a PoE enabled system.Midspan PSESee Midspan.PDPD (Powered Device) is a device that consumes power from a PoE enabled Ethernet port. PDs can consume up to a maximum of 12.95 watts. PDs are classified (presently Class 0,1,2 and 3) to indicate to the PSE the power range over which the PD will operate.PickerA device for extracting the power and data from an endspan solution.PoEPower over Ethernet is the method for transmitting up to 12.95 watts (load end) over Ethernet cable as described by IEEE standard 802.3af.PoEPPower over Ethernet Plus is a standard being developed by the IEEE. Designated as IEEE802.3at the new standard will permit higher power transmission than is allowed by the current standard (IEEE802.3af). This will be accomplished by using multiple cable pairs for power transmission.PPPatch panel. A distribution panel between the switch and load equipment.PPPPower patch panel. As PP but also adds PoE power to the Cable.PSEPSE (Power Sending Equipment) is the device that provides power to the PoE network. The PSE can be either and endpoint PSE (as part of the Ethernet switch/hub), or a midspan PSE (where power is injected separately onto the Ethernet network after the switch/hub).Redundant PairsUsed to refer to Ethernet cable pairs 4,5 and 7,8 that do not carry data.SignatureThe method by which a PD indicates to a PSE that it is capable of receiving power via PoE (see “discovery”).VoIPVoice over Internet Protocol.WAPWireless Application Protocol connectivity algorithm that joins a wireless device to a wired transmitter/receiver module (WLAN)WAP Wireless Access PointWAPWAP Wireless Access PointWLANWireless Local Area Network.PoE Interoperability TestingThe University of New Hampshire in cooperation with key players in implementation of the IEEE802.3af power over Ethernet (PoE) standard has established an interoperability test laboratory. It is necessary to demonstrate adherence to the IEEE standard and to show interoperability with the different types of Power Sending Equipment (PSE) available today. Full details are available on the UNH-IOL Power over Ethernet Consortium page.DI-88 Class 2 PoE Power Supply Power Integrations has submitted Powered Device Power supplies (PDs)