Such an intuitive statement can be formally reworded by means of the two key
assumptions:
1. Regardless of the history of the head-of-line (HOL) frame in terms of the number of
retransmissions and accumulated backoff stage, we assume that each frame transmission
suffers from a constant and independent collision probability;
2. If p is the collision probability and N is the number of competing stations, we assume
that p is computed as the contribution of N-1 remaining stations, each independently
accessing a channel slot with a constant permission probability ?„.
As shown in what follows, these assumptions enable a very simple, though accurate,
analytical modelling of the DCF.
For the sake of generality, it is useful to develop the model considering more general
backoff rules than the exponential backoff specified in the DCF standard. To this end, let
us define the term ???Backoff Stage??? as the number of retransmissions suffered by a HOL
frame. A station in backoff stage 0, i.e., willing to transmit a new MPDU, will select7 an
integer random backoff value drawn from a general probability distribution B0. If the
7 Saturation conditions imply that a packet in backoff stage 0 immediately follows a previously transmitted one.
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