Real-time Communication in Vehicular Ad Hoc Networks (VANETs)

Figure 2.1 Time Relationship
There are two carrier sense functions, physical and virtual carrier-sense
function, used to determine the state of the medium. Either of the two
functions can determine the busy status of medium.
In the BEB mechanism, a random Backoff Time value is chosen from a
discrete uniform distribution with values between 0 and CW (Contention
Window) by each node. The value is decreased by one for each idle time slot.
During of DIFS and EIFS interval, the decrement stops. Once the backoff
timer reaches zero, transmission will commence. In contrast, when there are
collisions during the transmission or when the transmission fails, the value of
CW will be doubled until it reaches the maximum CW value. Then, the
backoff process will be restarted and the data packet will be retransmitted
when the backoff process is complete. If the failure limit of transmission is
reached, the retransmission process will stop, CW is reset to the initial value
and the original data packet is discarded. But in broadcast models, there will
not be any retransmissions and CW value. Figure 2.2 illustrates backoff
process.
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