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Can any body tell me in simple words why we are required to use Power injector and whats the purpose of it.
You would only need to use a Power Injector if the SwitchPort that the AP connects to does not supply PoE (Power) and you don't want to connect the AP using the A/C power.
Hope this helps!
AP can be powered up through PoE facility to decrease the cost and complexity of installation.
If the PoE is not supported by network switches that the AP connected through, Power injector must be used to inject the power through unused pair.