IEEE 802.11 Wireless LAN

In this case study we consider model-checking of some properties for IEEE 802.11 Wireless LAN [1]. Since stations in wireless LANs are not able to listen to their own transmission they cannot employ Carrier Sense Multiple Access with Collision Detection (CSMA/CD) as already investigated in one of our other case studies. Hence, other techniques need to be used to handle collisions. The IEEE 802.11 standard describes Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) which is of our main interest in this case study. CSMA/CA also makes use of exponential backoff, but, unlike in CSMA/CD, a randomised exponential backoff rule is used to minimise the likelihood of transmission collisions instead of a “fixed” one. The according PRISM case study [3] uses a manually generated model with digital clocks semantics. We reimplement it in Modest [2] in order to compare the performance of the PRISM code generated by mcpta and that of the hand-written model. The initial setting is rather easy. There are two stations each of which tries to send a message to the other leading to a collision. To obtain the Modest model, we apply a cannonical transformation to the probabilistic timed automaton (PTA) of the PRISM case study. States of the PTA are transformed to Modest processes, every non-deterministic choice in a state is translated to an alt, state invariants are preserved in an invariant construct and every outgoing edge (l, g, a, μ) is transformed to when(g) a followed by a probabilistic choice palt over the destinations. Clock resets are performed according to μ. The exponential backoff is encoded as an assignment backoff := DiscreteUniform(0, 2bc+4 − 1). Mcpta generates the according PRISM code, 150 lines even for this assignment, fully automatic. We check certain probabilistic reachability, expected-time reachability and probabilistic time-bounded reachability properties for the aforementioned model: