Control Channel

Even though the IS-54 control channel is analog, and even though IS-54 is
designed to include a certain degree of compatibility with analog AMPS, the
control channel contains a number of significant differences from the analog
control channel. These changes were introduced to overcome known
problems in AMPS and to provide control channel support for digital voice
channels. For example, when assigning a mobile to a given traffic channel,
the downlink control channel must specify the time slots to be used by the
mobile. Obviously, such capability does not exist in the standard AMPS control
channel.

Access to the TDMA system is either achieved through the primary control
channel, utilized for analog communication, or the secondary dedicated
control channel. During the initial acquisition phase, the mobile reads the
overhead control message from the primary control channel and determines
if the system is digital-capable. If the system is digital capable, a
decision will be made whether to utilize the primary or secondary dedicated
control channel. The secondary dedicated control channels are assigned as
FCC channels 696 to 716 for the A band system and channels 717 through
737 for the B band system. The use of the secondary dedicated control channels
enables a variety of enhanced features to be provided by the system
operator to the subscribers.

IS-136 brings to the table the Digital Control Channel (DCCH) and it
enables the delivery of adjunct features that in cellular were not really possible.
The DCCH occupies two of the six time slots and therefore if a physical
radio also has a DCCH assigned to it, only two subscribers can use the
physical radio for communication purposes.
The DCCH’s can be located anywhere in the allocated frequency band;
however, certain combinations of channels are preferred to be used. The
preference is based on the method that the subscriber unit scans the available
spectrum looking for the DCCH.
The preferred channel sets are broken down into 16 relative probability
blocks for each frequency band of operation, both cellular and PCS. The relative
probability block
1 is the first group of channels the subscriber unit
uses to find the DCCH for the system and cell. The subscriber unit will then
scan through the entire frequency band, going through channel sets according
to the relative probability blocks until it finds a DCCH. In the case of
cellular, if no DCCH is found, it reverts to the control channel for a dualmode
phone and then acquires the system either through the control channel
or is directed to a specific channel that has the DCCH.