| Great changes have been processed in the information and traditional industries with the high development of computer networks, and technologies have brought inevitable consequences to peoples' communications. VoIP(Voice over Internet Protocol) techniques prompt the transfer of audio service from circuit switch to package-based switch. Accordingly, legal eavesdropping of VoIP has become focuses from diverse fields in the national departments.Currently, from the perspective of national security, licit VoIP eavesdropping is necessary. Certain characteristics of VoIP including massive audio data and low coding speed barricade efficient monitoring. To solve the problems of VoIP's real-time eavesdropping and lagged eavesdropping, a distributed VoIP eavesdropping system is introduced, the architecture and critical techniques are designed and implemented.The research work mentioned in the thesis is mainly about:1. Describes VoIP techniques and its advantages comparing with traditional audio communications. Introduces current developments of VoIP, presents the importance of audio eavesdropping and problems that are needed to focus on.2. Introduces H.323 recommendation and SIP(Session Initiation Protocol), analyses TCP/IP protocol and codices including G.711 and G723.1 that are widely used in VoIP architectures.3. Presents the architecture of the distributed VoIP eavesdropping system and its destination functions. Designs the hierarchy that contains eavesdropping agent, eavesdropping platform and eavesdropping terminal, introduces responsible abilities of the three parts.4. Realizes audio data capturing, data analyzing, audio decoding and audio playback and related techniques. Gives solutions to efficiency optimizing, payload balancing, UDP-based communicating optimization, extensible audio decoding and audio concurrency.5. Experiments system tests, results demonstrate that on the basis of playback quality assurance, the system could eavesdrop VoIP talks effectively. |
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