A radio channel consists of two frequencies within the radio-frequency (RF) spectrum that are separated by a fixed distance. These two frequencies allow a cell site and wireless phone to transmit and receive signals simultaneously. Cell sites communicate with wireless phones using two different radio channels: a voice channel and a control channel.

In TDMA systems, each digital-radio channel can carry up to three voice calls by time-multiplexing voice traffic into time slots. A DCCH is introduced into the TDMA system by reprogramming one of those traffic channels, called DTCs, to become the DCCH on a frequency that contains the existing DTCs.

Figure 2 depicts the DTC slot pair (1, 4) used for a DCCH, and shows each cell divided into sectors (A, B, C). Only one slot pair is required for a DCCH in each cell sector regardless of the number of digital radios in the sector.

Figure 2. IS–136 DCCH Operation

Operating Principle

Information carried on the DCCH flows in two directions over the air interface: from the system to the phone (downlink), and from the phone to the system (uplink). In Figure 2, the base station represents the system.

DCCH–capable and PCS phones monitor (camp on) a digital-control channel in each sector of a wireless system that supports IS–136 services. A PCS phone will scan for this channel, gain synchronization, and begin to decode the information provided over a broadcast-control channel on the DCCH. The DCCH serves as the phone's control channel until the phone finds another cell that is more appropriate.

PCS phones receive pages, send originations, and communicate with the system on the DCCH. After receiving a page or performing a call origination, a traffic channel is then designated for the call, and the phone will hand off from cell to cell as it moves around the system. At call completion, the phone returns to the DCCH to await further interaction.