If it is being done intentionally, there are two possibilities. One is passive blocking and the other is active jamming.
The practice of active jamming is doable technically, but I believe it is still illegal in the US and many other countries. Currently, the FCC prohibits "intentionally causing interference", even in the 'unregulated' bands like 802.11. But because of the growth of cell phone networks and use by citizens many countries are reviewing their laws to possibly make room for active jamming. It is a relatively simple matter of radiating a signal that is 1/2 wavelength out of phase with the carrier wave of the cell phone network. It is the same principle as noise cancellation used in aviation headsets or high-end headphones.
The passive route is easy to do and not illegal at all. In fact, I know it is done in gov't buildings where wireless contact from the inside is forbidden. One way is to line the walls of a building with a copper mesh, creating what is called a "Faraday Cage". The conducting enclosure acts as an electro-magnetic shield, isolating the inside of the cage from the outside. The size of the holes in the mesh 'tune' the wavelengths that the cage blocks.
I recently attended a wireless training class and heard a good example of this. Someone told of living in an old house, one with walls made of wood slats covered with chicken-wire and plaster. They couldn't get an 802.11B connection through the house from a router, but had no problem with an 802.11G connection. It was speculated that it was because of the difference in the wavelengths between B and G and the size of the holes in the chicken-wire in the walls filtering the B signals in a kind of Faraday cage.
However, I can't think of a way that the cell phones can report a 'full strength signal' but still have no access to the network. Anything that blocks the cell phone frequencies should result in a total loss of signal at the phone.
Josh
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