| SIP (Session Initiation Protocol) is an IP telephony signaling protocol proposed by the IETF (Internet Engineering Task Force). It follows the principle of the Internet such as simplicity, openness, compatibility and extensibility. Therefore, SIP is suitable for the development of new, Internet-related voice applications. With the development of Softswitch technology, SIP has become a most widely used signaling control protocol in the field of IP, and it has also become a core protocol of the next generation network.Concurrently, with the continuous development of the embedded technology, embedded systems have rapidly expanded from the single application mode in industrial control to the field of consumer electronics products. Increasing attention to the combination of embedded technology and network communication technology, embedded VoIP (Voice over Internet Protocol)will be one of the major applications in embedded systems with broad prospects for development.Due to the requirement of voice security in the embedded Environment, this paper has implemented a secure IP telephone system for mobile communications. The system is running in the embedded Environment based on Windows Mobile 2003 and adopts SIP. All the work bases on the project-Mobile Communication Security System for Internal Members of National Security Bureau of Yunnan Province.First, this paper introduces the related work of SIP. Second, the SIP stack is analyzed. Then an embedded SIP stack is implemented with reference from some open source system. Finally, some tests and practical applications are verified that this system is to meet the basic need of applications of SIP. Through the development of a prototype, a VOIP system prototype is implemented and is in practical use.The main jobs of this study are as follows:1. Analysis the SIP architecture in depth, including the principle of protocol stack, message mechanism, signaling path, and other key techniques of the SIP, such as the registration process. Additionally, the structure of SIP and the principle of SIP stack implementation are discussed in detail. 2. Designs an embedded SIP stack with the ideas of PJSIP,Sofia SIP and other open source systems. A simple embedded SIP stack has been designed with technologies such as buffer pool and zero-copy. These technologies have effectively improved the response speed in embedded SIP stack. And then a SIP stack prototype applied in embedded devices is implemented.3. Develop a softphone based on the embedded SIP stack running on the Windows Mobile 5.0. After conducting response speed and voice quality tests, the results show that the SIP stack prototype has practicality to a certain degree.4. Adopt OpenVPN to solve the NAT Traversal to the problem generated by the firewall in SIP applications. |
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