Design and Evaluation of a Platform for Mobile Packet Telephony

Advances in packet switching technology have made it feasible to support integrated transport of voice, video and data on a common packet switched network infrastructure. This trend has stimulated research in exploring how integrated transport may be supported over wireless networks as well, to provide mobile users access to the same set of services that are available in their desktop environment. Due to spectrum limitations, specially for wide area wireless coverage, it may be practical to initially support only a limited form of integration, for example to enable the transmission of voice and best-effort data on a shared physical channel. However, even this capability makes it possible to design interesting new services. Consider for example, a mobile user who wishes to retrieve personalized directions or traffic updates while driving down a freeway. If the wireless channel supported integrated transport, it becomes feasible to design a service interface in which the user interacts with the appropriate network server using an interactive voice driven interface to specify the information required. This information might then be presented to the user in a variety of styles as audio, text, images etc. Another example is a repairman dispatched to work at a remote location. Integrated transport might provide him the option of consulting with another repairman at headquarters, while perusing product manuals and transmitting realtime sensor feedback describing the product' s current state. Services that result from voice-data integration may be particularly valuable for cellular/PCS telephony users who constitute the fastest growing segment of the wireless industry. A limited form of wireless integrated access is already possible with so-called smart phones like the Nokia communicator [26] and the AT&T PocketNet phone [25] in which the smart phone can function either as a phone or as a (IP) data terminal offering access to e-mail and personalized information on stock quotes, weather reports, etc. These devices use separate wireless access protocols when functioning as a phone or as a data terminal, prohibiting the simultaneous transport of both voice and data. Voicedata integration is underway in the Internet as well with the development of applications centered around Internet telephony, for example click-to-dial and simultaneous voicedata transmission over dial-up (PPP) links. There are several interesting problems that need to be solved to make it possible to support integrated voice-data access over wireless links [20]. First, it is necessary to design wireless channel scheduling mechanisms that can efficiently satisfy the service quality requirements of both voice and data. Second, network protocols need to be extended to support handoffs as mobile terminals change their point of connectivity to the backbone network. Third, forward error correction/packet retransmission mechanisms used over the wireless link need to be be tuned to match the varied loss resilience levels of voice and data. Fourth, integrated voice-data mobile terminals need to be developed that can act as both a phone and information-retrieval device. Finally, there are several interesting issues in providing enhanced voice-data services for mobile-users, for example how to support voice-driven information retrieval and adapt services and data delivery to match the limitations of the terminal and wireless link. In this paper, we describe an experimental testbed that we have designed to support integrated wireless voice-data access. It consists of off-the-shelf network adapters, PCs as basestations, and handheld mobile terminals. This approach of using commodity hardware allowed us to assemble the testbed relatively quickly, providing us with a platform for prototyping algorithms and applications, and examining performance in a realistic but controlled environment. It also provides a clean upgrade path as new technology becomes available. Our goals were to investigate the network mechanisms required to support integrated voice/data transport to mobile terminals, and explore the design of interesting services/applications for this network environment. The initial focus has been on the design and implementation of a Medium-Access-Control (MAC) and handoff protocols that are suitable for integrated voice/data transport. The MAC protocol provides a simple token-based mechanism to support bandwidth reservation an essential element for supporting time-sensitive voice traffic. The handoff protocols allow a wireless terminal to move between geographical