This paper presents the problem of optimally dimensioning a new geographically distributed computer system that handles all communications between aircraft and airline operation centers. Since computers are distributed on three continents, the service demand per site (as measured by messages to be treated) can vary greatly with the time of day. The model optimizes costs while taking advantage of the variation of demand over time and the sharing of load among the sites. The model is a mixed integer program with non-linear constraints. An algorithm is proposed to solve the problem to optimality. Test data provided by SITA were used to evaluate the quality of the solution.