The aim of our work is to provide a quantified means of helping in the definition of a new architecture for CAUTRA, a subset of the French Air Traffic Control system. In this paper, we define a set of alternative architectures, give some elements for constructing their dependability models, and compare their availability. Modeling is carried out following a modular and systematic approach, based on the derivation of block models at a high level of abstraction. In a second step, the blocks are replaced by their equivalent Generalized Stochastic Petri Nets to build up the detailed model of the architecture. The evaluations performed permit identification of a subset of architectures whose availability meets the dependability requirements and also identification of the best architecture among this subset.