IS-54B and IS-136

 IS-136 came about through a two-stage evolution
from analog AMPS. As described in more detail later, AMPS is a frequency
division multiple access (FDMA) system, with each channel
occupying 30 KHz. Some of the channels, known as control channels, are
dedicated to control signaling and some, known as voice channels, are dedicated
to carrying the actual voice conversation.
The first step in digitizing this system was the introduction of digital
voice channels. This step involved the application of time division multiplexing
(TDM) to the voice channels such that each voice channel wdivided into time slots, enabling up to three simultaneous conversations on
the same RF channel. This stage in the evolution was known as IS-54 B
(also known as Digital AMPS or D-AMPS) and it obviously gives a significant
capacity boost compared to analog AMPS. IS-54 B was introduced in
1990.
Note that IS-54 B involves digital voice channels only, and still uses analog
control channels. Thus, although it may offer increased capacity and
some other advantages, the fact that the control channel is analog does
limit the number of services that can be offered. For that reason, among
others, the next obvious step was to make the control channels also digital.
That step took place in 1994 with the development of IS-136, a system that
includes digital control channels and digital voice channels.
Today AMPS, IS-54B, and IS-136 are all in service. AMPS and IS-54
operate only in the 800-MHz band, whereas IS-136 can be found both in the
800-MHz band and in the 1900-MHz band, at least in North America. The
1900-MHz band in North America is allocated to Personal Communications
Service (PCS), which can be described as a family of second-generation
mobile communications services.as