A theorem is proved that makes it possible to calculate the conditional air-time of a node, given another node, as a function of the conditional air-times of the links of both nodes.
The conditional air-time is needed to model the loss probability due to collisions for an accurate evaluation of the (saturation) throughput of a node in a IEEE802.11 network with RTS/CTS enabled access.
The basic access can not be evaluated because the model is invalid if the protocol introduces a bias in the length statistics of packets rescheduled because of collisions as stated in [4].