Hence, consistent with the DCF specifications (see Clause 9.1.1 of the standard), a random backoff interval shall
always be selected for the first packet transmission attempt.
Performance Study of IEEE 802.11 DCF and IEEE 802.11e EDCA 77
transmitted frame collides, we say that the station enters backoff stage 1. The next backoff
value will be drawn from a second probability distribution B1, and so forth. In general, a
station entering backoff stage i will extract a backoff value from a distribution Bi.
In the particular case of the DCF Binary Exponential Backoff, B0 is a uniform
distribution in the range [0,CWmin], B1 is a uniform distribution in the range [0, 2(CWmin
+1)-1] and in general, Bi is a uniform distribution in the range [0,CWi] where
CWi=2i(CWmin+1)-1. In addition, the IEEE 802.11 DCF specifies
1. the maximum Contention Window value as CWmax = 2m(CWmin+1)-1 where m is a
parameter that depends on the physical layer considered, and
2. a finite number of retries R, meaning that a frame whose first transmission has failed,
will be retransmitted for at most R times, and then it will be dropped from the
transmission queue.
In what follows, we will show that the performance do not depend on the probability
distributions Bi, but only on their mean values ??
i = E[Bi].
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