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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Midspan or Endspan?
Once PoE is incorporated into a switch/hub, the switch/hub becomes PoE-capable, and as per IEEE terminology, it is then an endpoint PSE (or colloquially, an Endspan), because of its location at one end of the cable.
One question that arose during the creation of the IEEE PoE standard was would the standard force people to throw out all their old switches, hubs, and so on, and buy brand-new Endspans? That could prove very expensive. Wasn’t there some way to simply upgrade existing installations if desired? The answer to that was the PoE Midspan (or injector). As its name indicates, it is a unit inserted somewhere between the switch/hub and DTE (PD). It contains a PSE by which it can inject PoE onto the cable. We therefore ...

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