Distributed Architecture Of VoIP For Firewall/NAT Traversing Based On SIP

Voice over IP technology develops with the rapid development of network applications. However, NAT(Network Address Translation) and firewall technologies, which are brought forward for the insufficiency of IP address and the network security, conduce to issue between end to end communications. Consequently, how to solve the problems mentioned above effectively will greatly affect the promotion and application of VoIP. Developed from a new technology named SBC(Session Border Controller), this paper proposes distributed architecture to resolve firewall/NAT problem.Compare to other Firewall/NAT traversing solution, SBC technology become popular because it is suitable to be applied in all firewall/NAT environment nowadays and it needs no modification on existing network However, classic SBC system faces serious problems as follows: SBC is the bottle-neck: Being in charge of media and signaling forwarding makes SBC the bottle-neck of the system. Handling packets inspection for security and QoS(Quality of Service) purpose increases SBC's load. Furthermore, SBC system is hard to extend.Based on the research on the SBC technology, this paper proposes a distributed architecture to resolve firewall/NAT traversing: SBC is split into two servers: SAG (Signaling Access Gateway) and MCC (Media Channel Controller) to let signaling and media flow pass through separately. The relationship between the SAGs and the MCCs is determined by the NMC(Network Manage Center). The media subsystem which is made up of MCCs and the signaling subsystem which is made up of SAGs can achieve great capacity enhancement that a single SBC can never reach. The greater significance is that: in a fully distributed architecture, every subsystem can be easily extended and dynamically controlled without any modification on the existing system.This paper also gives a prototype of the architecture based on SIP(Session initial protocol). It contains the main structure and key technology of SAG, MCC and NMC. The test result shows that: the SAG's ability of call construction is nearly 100 times greater than the SBC; if there are enough MCCs providing media channels, the firewall/NAT traversing system under a distributed architecture can achieve great capacity enhancement than the formal SBC system.