By mean of external aerial antennas (that replace the standard access points antennas) lets possible to extend the access point working area.
An external antenna is characterized by some parameters, where the most important one is the gain. The gain is defined as ratio between the antenna efficiency in a predefined direction, compared to the efficiency associated to an ideal antenna (that means no one can't build it, also called as "isotropic") performing the same on all directions. The gain is measured on dBi, a logaritmic unit. Strictly speaking, if an antenna gains +3dBi means that it's able to concentrate their radiation in one specific direction two times better than what an "isotropic" antenna could do in the same direction.
