—The paper presents an analytical modular model of a DiffServ network supporting Assured Forwarding PHB. Individual DiffServ network elements such as meter/markers and droppers are modeled separately. Additional equations make it possible to describe behavior of a complex DiffServ network in mathematical terms. The modular model makes it possible to calculate several network performance parameters under various configurations and traffic volume. The model can be used for designing networks, is helpful in initial setting of configuration parameters and enables quick evaluation of network performance.