Slow Motion Over Plain Old Cellular
In
1969, engineers at Bell Labs developed the cellular telephone
technology known as Advanced Mobile Phone System (AMPS). This system
uses the 800-MHz frequency band and has been widely deployed in North
and South America for mobile voice communications. Although the AMPS
cellular network was designed primarily for voice transmission,
techniques have been developed to send data over the network. In order
for the mobile worker to do a database query or check e-mail over a
dial-up circuit-switched connection, it was necessary to dial in;
establish the communication channel through the cellular network,
server, mainframe, and database; and stay connected while the
application is launched and information is retrieved. The process was
slow and cumbersome, and oftentimes sending a fax was faster and
easier. In that sense, the AMPS, in much the same way as the "plain old
telephone system" used to access the Internet with a trusty 28.8-kbps
analog modem, was prone to data loss and high and variable propagation
delay, impeding reliability and reducing effective throughput.
AMPS wireless data service was similar to a standard
cellular phone call, using the same channels and the same frequency as
the cellular voice call, but with specialized protocols used by the
modems on each end for circuit switched cellular data. The mobile
device required a modem, such as SpeedPaq 336 offered by Compaq, which
connected to a cellular phone and supported the necessary cellular
protocols. To send a data signal using AMPS over-the-air protocols, a
temporary dedicated path was established for the duration of the
communication session. All signals flow continuously over the same
path, and billing for AMPS data service was generally a function of
airtime used, typically in 1 minute increments, with charges based on
the user's selected rate plan. And, just as with a normal phone call,
all applicable long distance charges, roaming charges, and taxes also
were billed.
In 1991, U.S. cellular operators initiated an
activity to see if they could offer a digital data service for uses
like email and telemetry. The analog cellular worked, but the operators
did not see the expected return from the subscribers and the technology
cost was too high. The data throughput offered over the AMPS networks
was very slow, ranging from 2.4 to 14.4 kbps, affected by interference,
noise, fading, and overall channel degradation, common RF-related
affects, and varied from one location
to another. Dropped calls also were common. Security was another issue.
There was a limited amount of the content available for the wireless
Internet users. Mainly the AMPS wireless networks were used to connect
to the custom build corporate gateways that were designed to serve data
often in proprietary format. Browsers also were primarily proprietary,
and limited standards existed for the Mobile Data communications.
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