Before the age of digital services, the predominant technology for analog cellular phone services in North America adhered to a set of standards for Advanced Mobile Phone Service (AMPS). Originally, AMPS operated in the 800-MHz frequency band using 30-kHz-wide channels. Avariant of AMPS, known as Narrowband AMPS (NAMPS), uses 10-kHz-wide channels and consequently has triple the capacity of AMPS. Although AMPS or a variation of AMPS is still around— chances are that your cellular phone allows you to switch between analog and digital mode—its use is rapidly declining in the face of more sophisticated digital cellular standards.
The mobile telephone service that preceded AMPS was known as Improved Mobile Telephone Service (IMTS), which operated in several frequency ranges: 35 to 44 MHz, 152 to 158 MHz, and 454 to 512 MHz. But IMTS suffered from call setup delay, poor transmission, and limited frequency reuse. AMPS overcame the limitations of IMTS and set the stage for the explosive growth of cellular service, which continues today worldwide. Interestingly, Pacific Bell finally dropped IMTS in 1995.
Proposed by AT&T in 1971, AMPS is still the standard for analog cellular networks. It was tested in 1978, and in the early 1980s cellular systems based on the standards were installed throughout North America. Although AMPS was not the first system for wireless telephony, the existence of a single set of standards enabled the United States to dominate analog cellular throughout the 1980s. Today, Europe dominates cellular primarily because it is a lower-cost alternative to conventional telephone service.
Analog cellular is delivered from a system of cellular hubs and base stations with associated radio towers. Amobile telecommunications switching office (MTSO) authenticates wireless customers before they make calls, switches calls between cells as mobile phone users travel across cell boundaries, and places calls from land-based telephones to wireless customers.
AMPS uses a technique called “frequency reuse” to greatly increase the number of customers that can be served at the same time. Low-powered mobile phones and radio equipment at each cell site permit the same radio frequencies to be reused in different cells, multiplying calling capacity without creating interference. This spectrum-efficient method contrasts sharply with earlier mobile systems that used a highpowered, centrally located transmitter to communicate with high-powered mobile equipment installed in vehicles over a small number of frequencies. Once a channel was occupied with a call, its frequency could not reused over a wide area.
Despite the success of AMPS, this method of transmission has its limitations. Analog signals can be intercepted easily and suffer signal degradation from numerous sources, such as terrain, weather, and traffic volume. Analog systems also could not handle the transmission of data very well. Adigital version of AMPS—referred to as DAMPS—solves many of these problems while providing increased capacity and a greater range of services.
Both AMPS and DAMPS operate in the 800-MHz band and can coexist with each other. DAMPS is implemented with Time Division Multiple Access (TDMA) as the underlying technology, which provides 10 to 15 times more channel capacity than AMPS and allows the introduction of new feature-rich services such as data communications, voice mail, call waiting, call diversion, voice encryption, and calling-line identification.
Adigital control channel available with DAMPS supports such advanced features as a sleep mode, which increases battery life on newer cellular phones by as much as 10 times over the current battery capabilities of analog phones. DAMPS also can be implemented with Code Division Multiple Access (CDMA) technology to increase channel capacity by as much as 20 times and provide a comparable range of services and features. Unlike TDMA, which can be added onto existing AMPS infrastructure, CDMA requires an entirely new network infrastructure.
DAMPS also allows operators to build overlay networks using small micro- and picocells, boosting network capacity still further in high-traffic areas and providing residential and business in-building coverage. Advanced software in the networks’ exchanges continuously monitors call quality and makes adjustments, such as handing calls over to different cells or radio channels, when necessary.
The network management system provides an early warning to the network operator if the quality of service is deteriorating so that steps can be taken to head off serious problems. Graphical displays of network configuration and performance statistics help ensure maximum service quality for subscribers.
In 1983, AMPS was approved by the Federal Communications Commission (FCC) and first used in Chicago. In order to encourage competition and keep prices low, the U.S. government required the presence of two carriers in every market, known as Aand B carriers. One of the carriers was normally the Local Exchange Carrier (LEC); in other words, the local phone company.