Hello Arul,
because of the AP3915i is nearly a perfect omnidirectional radiator (horizontal plane) you can mount the AP in any direction.
If you use the screw holes on the back you have to mount the AP with the Ethernat jackets downwards. In this case the Extreme Logo is upside down. But this is only an optical disadvantage.
Therefore you need no brackets.
Brackets has however have two man advantages.
The installation is easier and the mounting is more stable.
Best regards
Stephan
Regards
Stephan