<P> The problem became so large that some carriers required the use of a PIN before making calls . Eventually, the cellular companies initiated a system called RF Fingerprinting, whereby it could determine subtle differences in the signal of one phone from another and shut down some cloned phones . Some legitimate customers had problems with this though if they made certain changes to their own phone, such as replacing the battery and / or antenna . The Oki 900, the ultimate tool of cell - phone hackers, could listen in to AMPS phone - calls right out - of - the - box with no hardware modifications . </P> <P> AMPS was originally standardized by American National Standards Institute (ANSI) as EIA / TIA / IS - 3 . EIA / TIA / IS - 3 was superseded by EIA / TIA - 553 and TIA interim standard with digital technologies, the cost of wireless service is so low that the problem of cloning has virtually disappeared . </P> <P> AMPS cellular service operated in the 850 MHz Cellular band . For each market area, the United States Federal Communications Commission (FCC) allowed two licensees (networks) known as "A" and "B" carriers . Each carrier within a market used a specified "block" of frequencies consisting of 21 control channels and 395 voice channels . Originally, the B (wireline) side license was usually owned by the local phone company, and the A (non-wireline) license was given to wireless telephone providers . </P> <P> At the inception of cellular in 1983, the FCC had granted each carrier within a market 333 channel pairs (666 channels total). By the late 1980s, the cellular industry's subscriber base had grown into the millions across America and it became necessary to add channels for additional capacity . In 1989, the FCC granted carriers an expansion from the previous 666 channels to the final 832 (416 pairs per carrier). The additional frequencies were from the band held in reserve for future (inevitable) expansion . These frequencies were immediately adjacent to the existing cellular band . These bands had previously been allocated to UHF TV channels 70--83 . </P>

In cellular mobile communications the forward channel frequency band of amps system is