The IEEE 802.11 standard is one of the most used wireless local area network technology. Even if it is not a deterministic protocol, it is possible to bound its performances, and thus to provide some quality of service. Its evolution 802.11n integrates an aggregation system which improves its efficiency under specific conditions. Particularly, such a system could be interesting in a wireless sensor and actuator network, where data are gathered before being forwarded to a sink. We propose to model this aggregation system since it significantly impacts the frames delay. The network calculus theory is used to compute worst-case performances. Rather than considering the aggregates in a macroscopic way and providing global performances indicators, this paper aims at defining the service offered to each incoming flow by evaluating backlogs and end-to-end delays.