In this paper a mathematical model of Constrained Priority Countdown Freezing (CPCF) protocol is introduced. In the new model presented, the existing model of simplified CPCF is expanded in order to include the binary exponential backoff (BEB) algorithm. Thus, the behavior of a single CPCF station is modeled using four-dimensional Markov chain. In addition, the probability of the contention loss after a busy timeslot (TS) is differentiated based on the value of the station's freezing counter. The results of mathematical model are compared to the simulation results, showing that the proposed expanded scheme models the CPCF medium access method with expected accuracy.