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This paper addresses the issue of cost-optimal voice over IP (VoIP) network design. In the applied model, the whole VoIP network is divided into two logical components: the access network and the transport network. The access network consists of VoIP end-points that connect to the transport network through edge routers serving as gateways. Since multiple edge routers may be available for any given VoIP node, one task of the design process is to assign a particular edge router to every VoIP node. The edge routers have to be connected in a way that security and availability can be assured for the VoIP traffic. One obvious approach to fulfilling these requirements, which is assumed throughout the paper, is to define a virtual private network (VPN). Supposing a large volume of VoIP traffic, the cost of the VPN can be significant; thus, the other task of VoIP network design is to specify the transport VPN in the most economical way. These two tasks of VoIP network design can be solved separately using existing methods; nevertheless, the specification of VoIP regions influences the cost of the final solution to a great extent. Therefore, in this paper a novel approach is proposed in which the edge router assignment process takes the objective function of VPN specification into consideration as well. In order to realize the new approach, multiple methods are introduced which are based on the paradigms of genetic algorithms and simulated annealing. These methods perform a sophisticated optimization of the gateway assignments using various cost calculation methods. To evaluate the new algorithms, a method based on a well-known greedy solution to the problem is used as reference. Moreover, a VPN specification algorithm is presented which utilizes the stepwise nature of the cost functions. The performance of the presented methods is evaluated with the help of simulations. It is shown that the proposed methods outperform the reference algorithm significantly in the simulation scenarios investigated. |
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