Once a DL-MAP message has been detected, the MAC sublayer will listen for downlink and uplink transmission parameter. By listening for UCD messages from the Base Station (BS), the Subscriber Station (SS) is able to determine a usable uplink channel.
UCD messages are broadcast messages, send out periodically, providing pertinent parameters for all available uplink channel, and will attempt to establish communications on suitable channel.
If communications fail on one channel, the SS will move on to next suitable channel until a connection is establish or the list has been exhausted, in which case it will begin the scanning process again.
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