In cognitive radio networks (CRN), the secondary network opportunistically access the wireless channels that are free from primary user (PU). Recently, the usage of quiet period (QP) scheduling mechanism to detect transmission opportunities for 802.11 secondary network has gained popularity.The QP-CSMA-CA is a 802.11 based CRN MAC protocol that uses DIFS-based QP scheduling mechanism. In this paper, we propose an analytical model for DIFS-based QP scheduling mechanism. We study the impact of number of secondary nodes on QP scheduling and on their throughput and delay. The access point of 802.11 secondary network can use our analytical model to map the optimal QP scheduling interval TQ P (in time units) to the number of DIFS occurrences ND I F S which is required for QP-CSMA-CA MAC protocol to detect the reappearance of PU. We validate our mathematical model in finding the optimal ND I F S through extensive MATLAB simulation. © 2016, Springer Science+Business Media New York.