The signal-bearing medium of claim 20 whereby a plurality of service delivery models are available for retrieval, wherein each service delivery model is applicable to a range of values the at least one service variable may assume during a service delivery event, and wherein retrieving a service delivery model in dependence on the value that the service variable will assume further comprises:

choosing a service delivery model specifying a range of possible values of the at least one service variable to which the service delivery model applies within which the value that the at least one service variable will assume during the service delivery event falls.