LOS, also LoS - Line of Sight. Two things are LOS or in LOS if they can see each other (at least theoretically).
Refers to the requirement of having a direct visual path between two wireless devices, usually in a point-to-point arrangement using some type of highly directional antenna. An example would be a wireless link from a home to an office using a yagi antenna at each location. Trees and buildings are probably the most common objects outdoors that interfere with line of sight. 2.4GHz, technically a near-microwave frequency, can shoot through some obstructions at short ranges (less than a mile in dense foliage). Do-able obstructions include houses (wood), walls, trees, bushes, reflections off buildings, etc. Your signal will take a hit through each one, however.
Visual obstructions such as clouds, fog, rain, snow, etc. will not effect 2.4GHz. Even in the heavest precipitation, expect 0.02dB loss per mile.
VHF/UHF ducting during the summer time will effect microwaves as well. Links from a mountain several thousand feet higher than a lower destination point may experience strange results. Under perfect conditions, very long distance non-line-of-sight links can be functional for hundreds of miles. This is diffcult to do, but many hams have done this before -- even at 72GHz!
Keep in mind that obstructions such as concrete/steel buildings and ground/hills/mountains are definite signal killers. Some windows have special anti-UV/radio coatings, which also kill 2.4GHz.
Its a dangerous world out there -- for 2.4GHz that is!
