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Publication Date: 2010-Mar-17
Document File: 2 page(s) / 27K

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Only a single detection circuitry is provided. Detection is performed in a round robin method, with the ports queued in the reverse of their priority. Thus, lowest priority ports are first detected, followed by higher priority ports. The highest priority port, if detected and optionally classified, is then enabled within the above mentioned time window. In an embodiment in which a plurality of ports may be enabled, and time allows, a plurality of high priority ports may be enabled. In the next detection cycle lower priority ports will be enabled, again while ensuring that the highest priority unpowered ports are enabled before lower priority ports.

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Background:  Power over Ethernet (PoE) provides power to a DTE over the same cabling as the data.  Power may be provided either from an endspan power sourcing equipment (PSE) or a midspan PSE.  PoE is usually provided in accordance with the IEEE 802.3af standard, or with the IEEE 802.3af standard.

Each of the above standards sets a predetermined time window from the end of detection until power is to be supplied from the PSE to the PD, and a maximum amount of time is provided for detection.  The time window is denoted Tpon, and amount of time allowed for detection is denoted Tdet.  If power is not applied as specified, a new detection cycle is initiated. 

A PSE may be power limited in that connected PDs may request power in excess of the power available to the PSE.  In such a case, power management is implemented to prevent the connection of PDs which would draw in excess of the available power.  Thus, in the event that a PD is detected, as described above, it is possible that insufficient power is available for the detected PD.

Since power may be limited, preferably power is only supplied in order of priority.  Thus, a PSE port which has been tagged as high, or critical priority, should be enabled, i.e. a PD attached thereto should be powered, even when additional PDs connected to low priority PSE ports are detected and not powered.  In the above mentioned IEEE-802.3at standard three levels of priority are defined: critical, high and low.  In order to avoid confusion and ensure consistent results, typically all available PSE ports are assigned a particular sub-priority level.  For example, if 8 high priority ports are detected, the high priority ports are further organized in order of port number.