IS-136 System Description

IS-54 and IS-136 represent the most direct evolution from 1G systems. In
fact, IS-54 and IS-136 were designed to allow significant compatibility with
analog AMPS so that dual-mode handsets could be developed at a reasonable
cost. Since IS-54, and then IS-136, initially began as islands of in a sea
of AMPS coverage, it was important to have dual-mode phones so that subscribers
could still obtain AMPS coverage when roaming outside of IS-54 or
IS-136 coverage.

IS-54 represents the first step in moving from analog AMPS to digital
technology and is often known as Digital AMPS (D-AMPS). IS-54 could be
called a generation 2.5 technology because it is not completely digital. Only
the voice channels are digital—the control channel is still analog. The introduction
of the digital control channel came about with the introduction of
IS-136. Nevertheless, IS-54 was an important step forward as it provided a
number of significant advantages over AMPS, including increased system
capacity and security through support for authentication. Support for
authentication within analog AMPS had already been designed, but since it
involved changes to the air interface, it required support within the handsets.
Unfortunately, millions of handsets were already in the field and these
did not support authentication. IS-54, however, required new handsets and
these new phones incorporated authentication from the start.