In this paper a new mathematical model, describing the behavior of simplified Constrained Priority Countdown Freezing (CPCF) protocol, is introduced. The CPCF protocol sets constrains on DCF countdown mechanism and thus requires different mathematical model from those found in the literature. For this reason, the two-dimensional Markov chain analytical model is developed in order to accurately quantify the freezing constrains introduced by CPCF protocol. The results of mathematical model are compared to the simulation results, showing that the proposed scheme models the simplified CPCF medium access method with expected accuracy.