In this paper, we compare the availability and performance of a wireless TDMA system with and without automatic protection switching. Stochastic reward net models are constructed and solved by SPNP (Stochastic Petri Net Package). Hierarchical decomposition is adopted to simplify the analysis. The optimization of the number of guard channels reserved for the handoff calls is studied. Numerical results prove the accuracy of the decomposition and show the significant improvement of the availability and the performance indexed as the new call blocking probability and handoff call dropping probability, brought by automatic protection switching.