| With the development of VOIP technology and the popularity of embedded equipment, VoWLAN - a new researching field has been established to meet the demand for inexpensive,mobile,good quality communication. VoWLAN sends audio packets over wireless LAN, making up for the weak points of traditional VOIP, which has a huge market potential in health care,retailing,manufacturing and hotel as well.With the studies of VoWLAN's key technology, this thesis designs and implements an embedded SIP audio terminal based on PXA 270 development platform in WLAN. Audio communication is implemented between terminals and PC by 802.11 b wireless access,RTP/RTCP real-time transmission and QT user interface. The software design is specifically demonstrated after the analysis of relative protocols and technologies.Firstly, this thesis reviews the development course and trend of VoWLAN, as well as its structure, application areas and current status. Secondly, the functional traits,entity components,message structure and call flow of SIP are deeply analyzed. Then the entire design including hard wares,drivers,soft wares and user interface are shown respectively in this thesis.On the basis of structure design, this thesis introduces he methods to implement SIP and media modules. UAC and UAS are implemented based on oSIP2/eXosip2 open source libraries. Capture and play,encoder and decoder,transmission of audio packets are implemented based on the establishment of RTP media stacks as well as the proposal of circle buffer,jitter elimination and loss resumption.Finally, SIP signaling and QoS of audio are tested with packet capturing tool and VOIP monitoring software. Through the comprehensive performance comparisons and demo display, the result shows that this terminal performs well in SIP interaction and audio communication. |
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