The 802.11 standard and the FHSS has redundant data built into its methodology that allows for radio packet loss without the loss of the data being sent and the need to retransmit that data. The 2.4 Ghz frequency range is affected by some lighting devices, microwave ovens and other radio equipment that uses the 2.4 Ghz range. These radios put out a very low power signal and as such are not likely to interfere with other devices. Some hospital equipment can be affected by these signals if, and only if, the unit is used in very close proximity to some devices. The ability for the antenna units to see each other with out obstruction is clear line of site.
Many factors influence and affect the radio signal.
Concrete can be penetrated in close proximity to the antennas. Wooden buildings can be penetrated. The fresnel zone is a conical shaped area extending out from an antenna in the direction the signal is traveling. Think of it as a megaphone. The fresnel zone for antennas one mile apart, has a radius of 27' in the middle where the two cones are as large as they get.