The IEEE 802.15.4e amendment provides important functionalities to address timeliness and reliability in timesensitive WSN applications, by extending the IEEE 802.15.4-2011 protocol. Nevertheless, in other to make the appropriate network design choices, it is mandatory to understand the behavior of such networks under worst-case conditions. This paper contributes in that direction by proposing a methodology based on Network Calculus that will, by modeling the fundamental performance limits of such networks, enable in the future a quick and efficient worst-case dimensioning of the networks’ schedule and resources.