To satisfy QoS requirements for multimedia flows transmitted over WLANs, deterministic channel access mechanisms can be used. A number of them use channel reservation, i.e. they allow the stations to reserve channel times in advance for future packets. To minimize protocol overhead and simplify the description of the reserved channel times, a reservation is done in a periodic manner — the reserved time intervals are periodic and of the same duration. Choosing the parameters of a reservation needs to be done in such a way that channel resource consumption is minimal provided that QoS requirements are met. The case when a reservation is established for a particular flow is widely studied in the literature. At the same time, multiplexing several flows and using a common reservation for their transmission instead of individual reservations may be more efficient. This paper fills the gap and presents an analytical model, which allows choosing optimal parameters of such a common reservation used for multiplexed QoS-sensitive streaming.