The first mobile phones were simple radios. Users paid high prices for the priviledge of sharing a handful of frequncies. Calls were placed through a mobile operator.

Then AMPS was born - Advanced Mobile Phone Service. A large set of frequencies was made available and calls were directed by computers instead of operators. Making reaching a mobile phone as easy as dialing the assigned number.

The big breakthrough in AMPS was the cellular nature of the service. The frequency set was divided and each set assigned to a small area - with towers up to 15 miles apart. (Wider spacing usually left holes in the service area.) The frequency sets could be reused in non-neighboring cells, and if a cell was being used beyond its capacity a new tower could be constructed and the frequencies resorted to provide additional connections.

The "A" companies joined together in roaming agreements (since they generally did not compete against each other in the same areas) and the "B" companies joined in their own agreements. This made it possible to leave your local cellular company's area and still use your phone.

The "A"/"B" division made it easier to select a carrier and compare coverage. This site once contained simple maps intended to help with that comparison - however recent changes in wireless phone systems, ownership of systems, and cooperative agreements have rendered such simple comparisons nearly useless.

Mega-systems are now emerging that provide 'local' coverage across several states - with roaming coverage that doesn't nessisarily follow the "A" roams on "A" - "B" roams on "B" trend of the not so distant past.