Probabilistic Model Checking of the IEEE 802.11 Wireless Local Area Network Protocol

The international standard IEEE 802.11 was developed recently in recognition of the increased demand for wireless local area networks. Its medium access control mechanism is described according to a variant of the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme. Although collisions cannot always be prevented, randomized exponential backoff rules are used in the retransmission scheme to minimize the likelihood of repeated collisions. More precisely, the backoff procedure involves a uniform probabilistic choice of an integer-valued delay from an interval, where the size of the interval grows exponentially with regard to the number of retransmissions of the current data packet. We model the two-way handshake mechanism of the IEEE 802.11 standard with a fixed network topology using probabilistic timed automata, a formal description mechanism in which both nondeterministic choice and probabilistic choice can be represented. From our probabilistic timed automaton model, we obtain a finite-state Markov decision process via a property-preserving discrete-time semantics. The Markov decision process is then verified using PRISM, a probabilistic model checking tool, against probabilistic, timed properties such as "at most 5,000 microseconds pass before a station sends its packet correctly."