  Real-time applications (RTA) tend to play a crucial role in people's everyday
life. Such applications are among the key use cases for the next generations of
wireless technologies. RTA applications are characterized by strict guaranteed
delay requirements (in the order of a few milliseconds). One of the pillars of
enabling RTA in next-generation Wi-Fi standards is Restricted Target Wake Time
(R-TWT), which provides Wi-Fi stations exclusive channel access within
negotiated service periods (SPs). If each RTA data flow uses dedicated SPs for
data transmission, they are completely isolated from each other and do not
experience any contention. To ensure the satisfaction of RTA QoS requirements
while minimizing the channel airtime consumption, it is important to properly
select the R-TWT parameters, namely the duration of SPs and the period between
SPs. In this paper, we develop a mathematical model that estimates the delay
probability distribution and packet loss probability for a given set of
network, traffic and R-TWT parameters. Using this model, the access point can
select the optimal R-TWT parameters for the given QoS requirements. The high
accuracy of the model is proven by means of simulation.
