The IS-54 Digital Voice Channel

IS-54 takes the existing 30-kHz AMPS voice channel and, applying Time
Division Multiplexing (TDM), divides the 30-kHz channel into a number of
time slots, as shown in Figure 3-14. Rather than having a full 30-kHz channel
for a conversation, each user is assigned a number of time slots, each
known as a Digital Traffic Channel (DTC). In IS-54, typically three users
are supported on a given RF channel. Having three users per RF channel
implies an obvious increase in capacity over analog AMPS, which supports
just a single user on an RF channel.