AMPS is a first-generation cellular technology that uses separate frequencies, or "channels", for each conversation (see FDMA). It therefore requires considerable bandwidth for a large number of users. In general terms, AMPS is very similar to the older "0G" Improved Mobile Telephone Service, but uses considerably more computing power in order to select frequencies, hand off conversations to PSTN lines, and handle billing and call setup.
What really separates AMPS from older systems is the "back end" call setup functionality. In AMPS, the cell centers can flexibly assign channels to handsets based on signal strength, allowing the same frequency to be re-used in various locations without interference. This allowed a larger number of phones to be supported over a geographical area. AMPS pioneers fathered the term "cellular" because of its use of small hexagonal "cells" within a system.
It suffered from some weaknesses when compared to today's digital technologies. Since it is an analog standard, it is very susceptible to static and noise and has no protection from eavesdropping using a scanner. In the 1990s, "cloning" was an epidemic that cost the industry millions of dollars. An unscrupulous eavesdropper with specialized equipment can intercept a handset's ESN (Electronic Serial Number) and MIN (Mobile Identification Number, aka the telephone number). An Electronic Serial Number is a packet of data which is sent by the handset to the cellular system for billing purposes, effectively identifying that phone on the network. The system then allows or disallows calls and or features based on its customer file. If an ESN/MIN Pair is intercepted, it could then be cloned onto a different phone and used in other areas for making calls without paying.
Cell phone cloning became trivial in the 90's with the use of three key elements. A radio receiver that could tune into the Reverse Channel (the frequency that the phones transmit data to the tower on), such as the famed Icom PCR-1000, a software program called Banpaia, and an easily clonable phone such as the Oki 900. By tuning the radio to the proper frequency, Banpaia would decrypt the ESN/MIN pair, and display it on the screen. The person could then input that data into the Oki 900, reboot it, and the phone network could not distinguish the Oki from the original.
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, where 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 modifications to their own phone, such as replacing the battery and/or antenna. The Oki 900 was the ultimate tool of cell phone hackers because it could listen in to AMPS phone calls right out of the box with no hardware modifications.
AMPS was originally standardized by ANSI as EIA/TIA/IS-3. This was later superseded by EIA/TIA-553 and TIA interim standard IS-91. AMPS has been replaced by newer digital standards, such as Digital AMPS, GSM, and CDMA2000 which brought improved security as well as increased capacity. Though cloning is still possible even with digital technologies, the cost of wireless service is so low that the problem has virtually disappeared.
AMPS cellular service operates in the 800 MHz Cellular FM band. For each market area, the United States Federal Communications Commission (FCC) allowed two licensee (networks) known as "A" and "B" carriers. Each carrier within a market uses 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.
At the inception of cellular in 1983, the FCC had granted each carrier within a market 333 channels (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 current 666 channels to the now 832 (416 per carrier). The additional frequency was available in the upper 800 MHz band which also was home to UHF channels 70–83. This meant that these UHF channels could no longer be used for UHF TV transmission as these frequencies were to be used for AMPS transmission.
The anatomy of each channel is composed of 2 frequencies. 416 of these are in the 824–849 MHz range for transmissions from mobile stations to the base stations, paired with 416 frequencies in the 869–894 MHz range for transmissions from base stations to the mobile stations. Each cell site will use a subset of these channels, and must use a different set than neighboring cells to avoid interference. This significantly reduces the number of channels available at each site in real-world systems. Each AMPS channel is 30 kHz wide.
Later, many AMPS networks were partially converted to D-AMPS, often referred to as TDMA (though TDMA is a generic term that applies to many cellular systems). D-AMPS is a digital, 2G standard used mainly by AT&T Mobility and U.S. Cellular in the United States, Rogers Wireless in Canada, Telcel in Mexico, Vivo S.A. and Telecom Italia Mobile (TIM) in Brazil, VimpelCom in Russia, Movilnet in Venezuela. In Latin America, AMPS is no longer offered and has been replaced by GSM and new UMTS networks.
AMPS and D-AMPS are now being phased out in favor of either CDMA2000 or GSM which allow for higher capacity data transfers for services such as WAP, Multimedia Messaging System (MMS), and wireless Internet access. There are some phones capable of supporting AMPS, D-AMPS and GSM all in one phone (using the GAIT standard).
In 2002, the FCC decided to no longer require A and B carriers to support AMPS service as of February 18, 2008. Since the AMPS standard is analog technology, it suffers from an inherently inefficient use of the frequency spectrum. All AMPS carriers have converted most of their consumer base to a digital standard such as CDMA2000 or GSM and continue to do so at a rapid pace. Digital technologies such as GSM and CDMA2000 support multiple voice calls on the same channel and offer enhanced features such as two-way text messaging and data services.
Unlike in the United States, the Canadian Radio-television and Telecommunications Commission (CRTC) and Industry Canada have not set any requirement for maintaining AMPS service in Canada. Rogers Wireless has dismantled their AMPS (along with IS-136) network, the networks were shut down May 31, 2007. Bell Mobility and Telus Mobility, who operated AMPS networks in Canada, announced that they would observe the same timetable as outlined by the FCC in the United States, and as a result would not begin to dismantle their AMPS networks until after February 2008.
OnStar relied heavily on North American AMPS service for its subscribers because, when the system was developed, AMPS offered the most comprehensive wireless coverage in the US. In 2006, ADT asked the FCC to extend the AMPS deadline due to many of their alarm systems still using analog technology to communicate with the control centers. Cellular companies who own an A or B license (such as Verizon and Alltel) were required to provide analog service until February 18, 2008. After that point, however, most cellular companies were eager to shut down AMPS and use the remaining channels for digital services. OnStar transitioned to digital service with the help of data transport technology developed by Airbiquity, but warned customers who could not be upgraded to digital that their service would permanently expire on January 1, 2008.