支援雙模行動裝置語音通訊之無縫垂直換手機制設計與實作

Abstract

The objective of the thesis is to design and implement algorithms for seamless vertical handoff between GSM and VoWLAN on dual-mode mobile devices. Due to the low cost of WLAN chips, in the future everyone is expected to own one or more dual-mode handsets equipped with the GSM and WLAN modules. Although the users can access GSM and WLAN simultaneously, they still won't feel convenient without the interoperability between GSM and WLAN. For example, people wish to use VoIP through WLAN access to save money but use GSM audio service to maintain session continuity. Currently, there are several research endeavors working toward solving the vertical handoff problems between GSM and VoWLAN access, including proprietary and standardized solutions. However, current proprietary solutions are designed for specific dual-mode clients and the users cannot enjoy the vertical handoff services without the specific handsets. Standardized solutions are designed for GSM telephony operators and hence vertical handoff services will fail if the users are not within the coverage of corresponding WLAN service. Therefore, we need to design a flexible vertical handoff solution to satisfy all situations, including all kinds of dual-mode clients, different WLAN operators, and different GSM operators. To design our intelligent vertical handoff algorithm, we conduct several experiments to figure out potential problems during the vertical handoff procedure. By way of the experimental results we understand that the problems of deciding handoff timing and synchronizing two different audio streams are the most critical issues. To perform seamless vertical handoff between VoWLAN and GSM accesses, we design the handoff decision mechanism to ensure that the GSM and VoWLAN coverage will overlap. By utilizing the audio time-scaling method, we design an audio mixing mechanism to ensure seamless audio switching between GSM and VoWLAN. To implement the algorithm on a dual-mode client, we make several modifications on related entities. At the remote client side, we let it respond to the handoff querying by using winsock programming. At the dual-node client side, we do the handoff triggering and modify the SIP softphone to perform audio waveform tapping for controlling and buffering the instant audio streams of GSM and VoWLAN. The results show that without audio mixing algorithm, there is audio block for more than three hundred milliseconds. With audio mixing mechanism, on the other hand, there is no audio gap during the vertical handoff procedure and the audio streams can be seamlessly migrated. Hence, the proposed vertical handoff algorithms with audio mixing can help the dual-mode client seamlessly switch the audio path and achieve seamless vertical handoffs between GSM and VoWLAN.