Sajal K. Das and Kalyan Basu
The success of wireless voice services has created opportunities for new information and entertainment services, leading to a ubiquitous information environment for the future. The potential for wireless data services has been demonstrated recently through the tremendous success of i-mode services in Japan. More and more people are getting accustomed to the concept of wireless appliances that can provide many attractive services by integrating content, voice, and text communications. Along with this development, core global information networks are becoming a reality that converge multimedia (audio, video, and text) services on a single, seamless network infrastructure. Such convergence will not only reduce network complexity and infrastructure equipment, but also will be easy to maintain. The switching and routing technologies of core networks are evolving toward packet-based transmission such as IP (Internet Protocol), ATM (asynchronous transfer mode), frame relay, and optical routing. Thus, the migration of the core network toward packet-based transmission has created the opportunity for integrating voice and data traffic on the same wireless and IP network infrastructure.
Although the evolution of the core network to IP is enabling the migration of traditional circuit-switched voice- and call-signaling message traffic over the Internet using VoIP (Voice over IP) technology, there are many technical issues and challenges that need to be resolved for its successful commercial deployment. Before proceeding further, we analyze the benefits offered by such a unified end-to-end IP network:
Cost reduction: The convergence of voice and data traffic can reduce operations costs and improve network efficiency.
Simplification: An integrated infrastructure that supports all forms of communication allows more standardization and reduces network complexity.
Consolidation: Because personnel are among the most-significant expense elements in a network, any opportunity to combine operations would eliminate points of failure and reduce expense.
Advanced applications: Although telephony is the basic application for voice over all-IP networks, the long-term benefits are expected to be derived from multimedia and multiservice applications. For example, E-commerce solutions can combine World Wide Web access to information with a voice call button that allows immediate access to a call center agent from a personal computer.
Voice traffic transfer through the packet network involves the following important components:
Coding of voice signals for packet mode transfer, and the resulting impact of the code on subjective voice quality
Network impediments that are acceptable for voice packets and their allocation to different parts of the network
Voice connection signaling mechanisms
Core network quality of service (QoS) management for voice communication
There are three general areas of research that will influence the successful migration of wireless voice traffic to integrated voice, data, and text on the IP network:
Migration of traditional circuit-switched voice-call session signaling to the Internet-based signaling scheme that meets voice signaling performance requirements
Selection of appropriate voice coding and decoding methods, and the mechanism for assembling and dissembling wireless packet frames to transfer VoIP frames through the wireless link, to meet network performance requirements
Developing a successful micro-mobility management scheme to hand over VoIP frames from one base station to another without impacting voice quality
At present these issues are not fully resolved to provide a wireless VoIP service that can be compared with the voice quality offered on current wireless voice services. Many efforts are being made by the different standards bodies and research laboratories to address these challenging issues. In this chapter, we identify and explore some of these issues and describe the current state of the art in VoIP services in wireless networks. The focus of this discussion is mostly on the system issues of this problem, and hence all design- and architecture-related issues might not be covered.
This chapter presents a summary of our original research in VoIP services in wireless data networks. Section 7.2 deals with the basics of wireless networks, including GPRS (General Packet Radio Service) and the challenges in providing VoIP services. Section 7.3 briefly summarizes the principles of voice coding, while Section 7.4 analyzes the network quality requirements for VoIP implementation. Sections 7.5 and 7.6 give an overview of the H.323 Protocol and SIP (Session Initiation Protocol) for multimedia services, respectively, and Section 7.7 discusses the Radio Link Protocol (RLP) standard in wireless networks. Section 7.8 details the architecture implementation of H.323 using wireless links, presents the delay analysis for control messages and call set-up message with and without RLP, as well as the experimental verification using Microsoft NetMeeting . Media packets blocking and VoIP traffic blocking analysis in GPRS are discussed in Section 7.9. Section 7.10 concludes the chapter.