  The IEEE 802.11ad standard extends WiFi operation to the millimeter wave
frequencies, and introduces novel features concerning both the physical (PHY)
and Medium Access Control (MAC) layers. However, while there are extensive
research efforts to develop mechanisms for establishing and maintaining
directional links for mmWave communications, fewer works deal with transmission
scheduling and the hybrid MAC introduced by the standard. The hybrid MAC layer
provides for two different kinds of resource allocations: Contention Based
Access Periods (CBAPs) and contention free Service Periods (SPs). In this
paper, we propose a Markov Chain model to represent CBAPs, which takes into
account operation interruptions due to scheduled SPs and the deafness and
hidden node problems that directional communication exacerbates. We also
propose a mathematical analysis to assess interference among stations. We
derive analytical expressions to assess the impact of various transmission
parameters and of the Data Transmission Interval configuration on some key
performance metrics such as throughput, delay and packet dropping rate. This
information may be used to efficiently design a transmission scheduler that
allocates contention-based and contention-free periods based on the application
requirements.
