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GUEST EDITORIAL
ADVANCES IN WIRELESS VOIP
Apostolis K. Salkintzis Fotini-Niovi Pavlidou
T
Qian Zhang
he unprecedented growth of the Internet, along with growing exigencies for flexible network access, spurred the development of innovative applications. In this context, Voice over IP (VoIP), also referred to as Internet or IP Telephony, constitutes one of the most flourishing applications. Beginning as a frolic among computer enthusiasts, VoIP has managed to set off a feeding frenzy in both the industrial and scientific communities and it has the potential to radically change telephone communications. The trend toward voice communications over the Internet is mainly fuelled by the salient advantages Internet telephony offers. VoIP holds considerable appeal both from the users’ and service providers’ viewpoint, the most important one being the edge in cost savings over the Public Switched Telephone Network (PSTN). The interest in VoIP dates back to the end of the previous decade and the first Special Issue was published in 2000 in the IEEE Communications Magazine. Since then, huge strides have been made in all aspects related to voice communications over the Internet and Internet Telephony now enjoys the fruits of labors during the past few years as an alternative to traditional telephony in homes and enterprises. Nonetheless, the literature lacks a recent Special Issue that would bring together state-of-the-art research activities in the field of VoIP communications. Users and service providers are lured by the voices that praise countless benefits and new business opportunities. On the one hand, VoIP opens up exciting possibilities for users, such as flexibility and monetary savings. On the other hand, VoIP promises new revenue sources to service providers, giving them also an easy and cost-efficient way to compete with incumbent operators. Notwithstanding, when discussing the merits of VoIP, one question springs in mind. Can VoIP compete with traditional telephony systems? Albeit huge strides have been made since its inception, there still exist numerous daunting challenges that must be met in order for speech quality in VoIP networks to be on a par with the one in PSTN. The aim of this special issue is to cover a wide spectrum of topics related to VoIP communications, ranging from QoS mechanisms to the design and performance evaluation of different VoIP components with the purpose of informing both industry and academia about this important research area. We received about 20 papers for this call. All of them were reviewed by experts in the field and six of them have been selected for publication. The first article “Voice Capacity Analysis of WLANs with Channel Access Prioritizing Mechanisms” covers issues of
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MAC techniques for VoIP applications. As a delay-sensitive realtime application, a VoIP flow is usually given higher priority in accessing the shared wireless channel, compared to delay-insensitive non-realtime flows. In contention based wireless networks, two widely used prioritizing medium access control (MAC) mechanisms are: 1) class dependent arbitration interframe space (AIFS); and 2) class dependent contention window (CW). The authors propose an analytical model to evaluate the effect of the two mechanisms in the voice capacity (the maximum number of two-way voice flow pairs supportable) of ad hoc mode and infrastructure mode wireless LANs (WLANs). They show that the AIFS mechanism has a relatively strong effect on the voice capacity in the ad hoc mode WLAN, but not in the infrastructure mode WLAN; further, the CW mechanism, when properly configured, has a mild effect on the voice capacity of WLANs in both modes. The second article “Efficient Silence Suppression and Call Admission Control through Contention-Free Medium Access for VoIP in WiFi Networks” underlines the tremendous demand on the wireless technologies to support VoIP, specially on the WiFi technologies that have already been matured commercially. The authors are addressing two core QoS issues of the on-going research work, i.e. efficient silence suppression, and call admission control. They present a QoS-aware wireless MAC protocol, called Hybrid Contention-Free Access (H-CFA) protocol, and a VoIP call admission control technique, called the Traffic Stream Admission Control (TSAC) algorithm. The H-CFA protocol is based on a novel idea that combines two contention-free wireless medium access approaches, i.e. round-robin polling and time-division multiple access (TDMA)-like time slot assignment, and it provides substantial multiplexing capacity gain through silence suppression of voice calls. The TS-AC algorithm ensures efficient admission control for consistent delay-bound guarantees and further maximizes the capacity through exploiting the voice characteristic that it can tolerate some level of non-consecutive packet loss. The third article “QoS-Driven Adaptive Congestion Control for Voice over IP in Multi-Service Wireless Cellular Networks” establishes a QoS-driven adaptive Congestion Control framework that provides QoS guarantees to VoIP service flows in mixed traffic scenarios for wireless cellular networks. The framework is composed of three Radio Resource Management algorithms: Admission Control, Packet Scheduling and Load Control. The proposed framework is scalable to several services and can be applied in any current or future
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GUEST EDITORIAL packet switched wireless system. By means of dynamic systemlevel simulations carried out in a specific case-study where VoIP and WWW service flows compete for shared access in an HSDPA wireless network, the proposed framework is able to increase the overall system capacity twofold depending on the traffic mix, while keeping the system operating optimally in its target QoS profile. The fourth article “An Enhanced SIP Proxy Server For Wireless VoIP In Wireless Mesh Networks” discusses VoIP applications on wireless mesh networks and specifically the provision of high quality multimedia service in a flexible and intelligent manner. The authors study the session initiation protocol (SIP) for wireless voice over IP (VoIP) applications. They investigate the technical challenges in WMN VoIP systems, and propose to design an enhanced SIP proxy server to overcome them. The analysis of signaling process and study of simulation results have shown the advantages of our proposed approach. The fifth article “VoIP Support using Group Resource Allocation Based on the UMB System” refers to the latest evolution of the wireless communication system, such as the UMB(tm) (Ultra Mobile Broadband) system developed in 3GPP2. A detailed end-to-end scheme for supporting VoIP in the air interface, designated for the UMB system, is described. Full duplex simulations are performed to evaluate the proposed scheme. The results show that the UMB system has significantly improved upon the VoIP capacity. A 5 MHz UMB system can support 320 VoIP users while simultaneously supporting a forward link data throughput of over 2.26 Mb/s. Although more of the delay budget was expanded on the reverse link, the system is still reverse link limited. The last article “Voice Communications over ZigBee Networks” provides an overview of ZigBee enabled wireless networks, and discusses the feasibility of supporting voice communications over ZigBee networks. It begins by providing an overview of the ZigBee technology followed by an evaluation of voice quality and performance over such an impoverished wireless channel. Two types of voice communications namely full-duplex Voice over IP (VoIP) and half-duplex Push-To-Talk (PTT) are considered. Voice quality of VoIP is measured using R-factor [1] (a well known objective speech quality metric). The quality of PTT however, is evaluated based on packet loss rate, delay and jitter. The simulation results demonstrate that a low power low rate wireless sensor network with minimal capabilities can support a limited range of voice services.
BIOGRAPHIES APOSTOLIS K. SALKINTZIS [SM’04] (
[email protected]) received his Diploma (honors) and his Ph.D. degree from the Department of Electrical and Computer Engineering, Democritus University of Thrace, Xanthi, Greece. Since 1999 he has been with Motorola Inc. working on the design and standardization of wireless communication networks, focusing in particular on IMS, GPRS, UMTS, WLANs and TETRA. In 1999 he was a sessional lecturer at the Department of Electrical and Computer Engineering, University of British Columbia, Canada, and from October 1998 to December 1999 he was also
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a post-doctoral fellow in the same department. During 1999 he was also a visiting fellow at the Advanced Systems Institute of British Columbia, Canada; during 2000, he was with the Institute of Space Applications and Remote Sensing (ISARS) of the National Observatory of Athens, Greece. He has many pending and granted patents, has published more than 60 papers in referred journals and conferences and is a co-author and editor of two books in the areas of Mobile Internet and Mobile Multimedia technologies. His primary research activities lie in the areas of wireless communications and mobile networking, and particularly on seamless mobility, IP multimedia over mobile networks, and mobile network architectures and protocols. He is a member of the Technical Chamber of Greece. He is an editor of IEEE Wireless Communications and Journal of Advances in Multimedia and has served as lead guest editor in a number of special issues of IEEE Wireless Communications, IEEE Communications Magazine, etc. He is an active participant and contributor in 3GPP and vice chair of “Quality of Service” Interest Group (QoSIG) of IEEE Multimedia Communications Technical Committee. F OTINI -N IOVI P AVLIDOU (
[email protected]) received the Diploma degree in Mechanical/Electrical Engineering and the PhD degree from the Aristotle University of Thessaloniki (AUTh), Greece, in 1979 and 1988 respectively. She is now a Full professor at the department of Electrical and Computer Engineering (ECE) in AUTh where she is engaged in teaching and research on traffic analysis, design (MAC protocols and routing techniques) of wireless (terrestrial, satellite, ad hoc, sensor) networks, performance evaluation and QoS studies of mobile satellite communications and multimedia applications over the Internet. She is the author of more than 150 papers in international journals and conferences. She has been involved in various editorial activities, she is a reviewer for book publishers, reviewer for research projects funded by various organizations. She is permanently included in the Technical Program Committee of many IEEE conferences and has been the organizer and Technical Program Chair of a number of conferences. She has served as external Advisor for the promotion of many academics worldwide. She is involved in many European and National Projects, she has been the Delegate of Greece in the European COST Program on Telecommunications (1998–2004) and served as Chairperson for the COST262 Action “Spread Spectrum systems and techniques for wired and wireless Systems.” She currently serves as the national Delegate of Greece in the FP7 Cooperation Program in the ICT theme. She is a Senior Member of IEEE (Communications and Vehicular Technology Society), currently chairing the Joint VTS & AES Greece Chapter. QIAN ZHANG (
[email protected]) received the B.S., M.S., and Ph.D. degrees from Wuhan University, China, in 1994, 1996, and 1999, respectively, all in computer science. Dr. Zhang joined Hong Kong University of Science and Technology in Sept. 2005 as an Associate Professor. Before that, she was in Microsoft Research, Asia, Beijing, China, from July 1999, where she was the research manager of the Wireless and Networking Group. She has published about 150 refereed papers in international leading journals and key conferences in the areas of wireless/Internet multimedia networking, wireless communications and networking, and overlay networking. She is the inventor of about 30 pending patents. She also participated many activities in the IETF ROHC (Robust Header Compression) WG group for TCP/IP header compression. She is the Associate Editor for IEEE Transactions on Wireless Communications, IEEE Transactions on Multimedia, IEEE Transactions on Vehicular Technologies, Elsevier Computer Communications, and Elsevier Computer Networks. She served as Guest Editor for special issues in IEEE Journal on Selected Areas in Communications (JSAC), IEEE Wireless Communication Magazine, Elsevier Computer Networks, ACM/Springer Journal of Mobile Networks and Applications (MONET). She has received TR 100 (MIT Technology Review) world’s top young innovator award. She also received the Best Asia Pacific (AP) Young Researcher Award elected by IEEE Communication Society in year 2004. She received the Best Paper Award in Multimedia Technical Committee (MMTC) of IEEE Communication Society in 2005 and Best Paper Award for QShine 2006 and IEEE Globecom 2007. She received the Oversea Young Investigator Award from the National Natural Science Foundation of China (NSFC) in 2006.
