In telecommunications, identification, friend or foe (IFF) is a cryptographic identification system designed for command and control. It is a system that enables military, and national (civilian-located ATC) interrogation systems to distinguish friendly aircraft, vehicles, or forces, and to determine their bearing and range from the interrogator.
IFF was first developed during World War II. The term is a bit of a misnomer, as IFF can only positively identify friendly targets but not hostile ones. If an IFF interrogation receives no reply, the object can only be treated as suspicious but not as a positively identified foe.
There are many reasons for not replying to IFF by friendly aircraft: battle damage, loss of encryption keys, wrong encryption keys, or equipment failure. Aircraft hugging terrain are very often poor candidates for microwave line-of-sight systems such as the IFF system. Microwaves can't penetrate mountains, and very often atmosphere effects (referred to as anomalous propagation) cause timing, range, and azimuth issues.
more reading http://en.wikipedia.org/wiki/Identification_Friend_or_Foe
photo: An IFF Test Set used for testing transponders on aircraft
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