Time Synchronization for VoIP Quality of Service

Time synchronization provides receivers with precise information about end-to-end delays, enabling effective playout of time-sensitive voice data over the Internet. A lthough packet-based and unman-aged networks like the Internet are ideal for delivering time-insensitive data such as e-mail or static Web traffic, the growing demand for Web-based multimedia data exposes the limitations of core Internet protocols. With no end-to-end delay bounds, the public Internet's best-effort service is unsuitable for delivering time-sensitive data and for interactive applications such as voice-over-IP. Normal speech consists of talkspurts, which typically last a few hundred milliseconds , and silence periods, which occur both within a spoken word and between words. In a packet-based network , voice packets are generated periodically at the sender and transmitted across the network. To cope with the packet interarrival variance inherent in best-effort service, current VoIP implementations generally implement fixed buffer schemes. Fixed buffer schemes make no attempt to match receiver operation to current network performance. An alternative approach is to use a more complex adaptive buffer scheme. This continuously matches playout delay to network conditions , at the expense of some packet loss due to late arrivals and some distortion of intertalkspurt silence periods. Various approaches seek to optimize the quality of service of VoIP applications (see the sidebar, " Quality of Service for VoIP, " next page, for a categorization of QoS approaches). We propose a system that uses synchronized time to combine the useful characteristics of both fixed and adaptive buffer strategies, thereby improving VoIP quality of service. Using a combination of global positioning system (GPS) technologies and the network time protocol (NTP), hosts can learn the precise end-to-end delay for each packet. This information can benefit both domestic and business Internet telephony users. In this article, we outline our proposed system and discuss issues arising from the use of synchronized time.