數位化的行動通信技術逐漸使人群之間的交流更為頻繁，在過去Open Mobile Alliance(OMA)發展出即按即說(Push-to-talk)的行動數據服務，即按即說的使用方式類似傳統AM/FM類比式無線電對講機，不過更方便且成本更低，並能將語音同時清晰的廣播給許多人聽到。雖然目前付費語音數據服務已經普及，但是近幾年使用資料網路來傳送即時語音資料的技術VoIP(Voice over IP)，已經應用在人們的生活中，讓更多人期待能用低成本的設備與技術來解決遠距離語音通信的問題，來滿足一對一或一對多人的通話需求。新一代即按即說服務使用VoIP技術，結合不同性質的網路，將不再局限於一個網路之間互通，可將語音通話等多媒體服務擴展到更大的區域，依特性來延伸各種多元化服務，並可與公眾交換電話網路PSTN(Public Switched Telephone Network)及公眾陸地行動網路PLMN(Public Land Mobile Network)內的裝置相互溝通廣播。本文用開放原始碼軟體Linphone在Linux系統下實現語音即按說廣播平台，並且針對語音廣播服務在異質網路做探討與分析，討論如何在相同的軟硬體及網路條件下，有效的運用系統去控制發送的排程及調整接收裝置上緩衝區，對語音傳輸服務做優化，並得到更清晰的語音品質。

The evolving mobile technologies have made communications between people easier. Such as Push-to-talk over Cellular(PoC) developed by Open Mobile Alliance in 1990s, similar to analog AM/FM handheld walkie-talkie, but more convenient and cheaper, and can converse over half-duplex communication lines. And besides the wide spread cellular voice services, there are technologies like Voice over Internet Protocol (VoIP) using data network to transmit voice for long distance communication at lower cost and fulfilling the increasing demands, either one-on-one of one-to-many. The next generation of Push-to-talk technologies using VoIP, can provide voice communication and multimedia services over heterogeneous networks, such as WiFi, 3G and LAN, and can connect with devices from Public Switched Telephone Network(PSTN) and Public Land Mobile Network(PLMN). In this study, we use open sources software Linphone with Linux system to implement a Push-to-talk platform in order to increase the voice quality and decrease transition delay by optimizing OS scheduling and device buffer, and discuss its performance within homogenous and heterogeneous networks.

[1] F. Belqasmi, F. Chunyan, M. Alrubaye, and R. Glitho, "Design and implementation of advanced multimedia conferencing applications in the 3GPP IP multimedia subsystem," IEEE Communications Magazine, vol. 47, pp. 156-163, 2009.
[2] L. Cai, Y. Xiao, X. Shen, and J. W. Mark, "VoIP over WLAN: voice capacity, admission control, QoS, and MAC: Research Articles," Int. J. Commun. Syst., vol. 19, pp. 491-508, 2006.
[3] C. Walravens and B. Gaidioz, "Breaking up gigabit ethernet's VoIP bottlenecks," IEEE Potentials, vol. 27, pp. 12-17, 2008.
[4] K. Yonghyuck, K. Younghan, and K. Namhi, "Multimedia push-to-talk service in home networks," IEEE Transactions on Consumer Electronics, vol. 56, pp. 1480-1486, 2010.
[5] S. Coulombe, "Real-Time Multi-user Transcoding for Push to Talk over Cellular," in Next Generation Mobile Applications, Services and Technologies, The Second International Conference on NGMAST '08, pp. 351-356, 2008.
[6] S. Sangho and H. Schulzrinne, "Measurement and Analysis of the VoIP Capacity in IEEE 802.11 WLAN," IEEE Transactions on Mobile Computing, vol. 8, pp. 1265-1279, 2009.
[7] ITU-T, “Pulse Code Modulation (PCM) of Voice Frequencies,” Rec. G.711, 1988.
[8] H. Schulzrinne and J. Rosenberg, ”A Comparison of SIP and H.323 for Internet telephony,” in Proc. OSSDAV, Cambridge, U.K., July 1998.
[9] Internet Engineering Task Force,“RTP:A transport protocol for real-time applications,”July 2003.
[10] Open Mobile Alliance,”Push using SIP Architectur Approved Version 1.0”, Aug 2011.
[11] D. P. Bovet and M. Cesati, Understanding the Linux Kernel, O’Relly, November 2005.
[12] Open-Source Voip Software, “Linphone” , Retrieved July 25,2012, from the World Wide Web: http://www.linphone.org/
[13] S. B. Moon, P. Skelly, and D. Towsley, "Estimation and removal of clock skew from network delay measurements," Eighteenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings, vol.1, pp. 227-234 1999.
[14] A. W. Rix, J. G. Beerends, M. P. Hollier, and A. P. Hekstra, "Perceptual evaluation of speech quality (PESQ)-a new method for speech quality assessment of telephone networks and codecs," IEEE International Conference on, Acoustics, Speech, and Signal Processing, vol. 2, pp. 749-752, 2001.