Wireless LANs have somewhat different properties than wired LANs and require
special MAC sublayer protocols. In some wireless LANs, not all stations
are within range of one another, which leads to some complications.
In order to illustrate the hidden and exposed station problem,
consider a scenario with four wireless stations. The radio range is such
that only direct neighbors are within each other's range and can potentially
interfere with one another. All stations use CSMA (Carrier Sense Multiple
Access) that means before starting a transmission they just listen for
other transmissions. The problem is that a station really wants to know
whether there is activity around the receiver, but CSMA merely tells if
there is activity around the station sensing the medium.