ADVANCED MOBIILE PHONE SERVIICE

ADVANCED MOBIILE PHONE SERVIICE
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
ADVANCED MOBILE PHONE SERVICE 7
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.
Summary
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.