The previous section described the various flags that are combined with the forward channel data. These flags or indicators are used by the mobile devices to manage their medium access. This and the following sections detail the procedure followed by the mobile device when it has data to transmit. The state machine for the M-ES is shown in Figure 5.7. Initially, the M-ES is in the Idle state.
When a mobile device has data to deliver, it must transmit Reed-Solomon encoded blocks on the reverse channel. Before the mobile attempts transmission, it must first "listen" to the control flag transmitted by the system on the forward channel. If the reverse channel is busy, the mobile enters the Defer state, "backs off" for a random time interval and then tries again (i.e., "listens" to the reverse channel status again). The random backoff action is the "non-persistent" part of the MAC protocol; the entire listen-before-transmission procedure comprises the "collision-avoidance" aspect of the protocol.
If the reverse channel is idle, the mobile enters the Transmit Blocks state and transmits on a block by block basis. A continuation field is used by the mobile to inform the network that it intends to continue transmission (see Figure 5.8); this allows greater synchrony between the state of the reverse channel and the broadcast reverse channel control flag.5.11 As each block is received by the system, it is decoded and the results of the decode activity are transmitted on the forward channel in the Decode Status portion of the control flag.
While the M-ES is transmitting, it examines the Decode Status flag associated with each block transmitted. If a decode failure is encountered, the mobile stops its transmission and enters the Backoff state. While in the Backoff state, the M-ES waits a random amount of time and then assesses the channel in an attempt to restart its transmission. If the channel is found to be idle, it re-enters the Transmit Blocks state and restarts transmission. If the channel is found to be busy, the mobile remains in the Backoff state and waits another random period before assess the channel status again.
Once the last block has been transmitted, the mobile cannot return to the Idle state immediately. It must enter the Decode Wait state and wait for the Decode Status flag associated with the final block. If the flag indicates a success, the mobile can then return to the Idle state. If the final block was not received successfully, the mobile proceeds to the Backoff state and waits a random delay period before again assessing the channel status.
The forward and reverse channel relationship is displayed in Figure 5.9. The airlink MAC sublayer is defined in Parts 400 and 402 of the CDPD System Specification [CDPD95], and remained essentially unchanged in CDPD Release 1.1.
