The conventional goal of a context-aware computer system is to improve the users interaction with the system. But it is useful also to employ a users context information to optimise the operation of the system itself. In particular the context can be used to improve the use of network resources within the system. Furthermore, in achieving this goal, it might be useful to allow the system to influence the users context in a controlled and restricted way. In this paper we report on our investigation regarding the impact of system-initiated context modifications on the usage of network resources in a context-aware system. A model is presented that allows this impact to be succinctly described and quantified. The model is then evaluated by analysing its behaviour for a specific class of applications - context-aware guides for a museums and art galleries.