SUPPORTING REAL-TIME AND MULTIMEDIA APPLICATIONS ON THE MERCURI TESTBED

This paper describes the distributed system, network and software arch itecture, the application development environment, the performance, an d the early lessons learned on the ATM LAN testbed Mercuri established at the Honeywell Technology Center, to develop distributed multimedia technologies for realtime control applications. We have developed a c lient-server-based software architecture on Sun Sparcstation-2s connec ted by a Fore Systems' ASX-100 ATM switch, with video processing handl ed by Parallax's XVideo cards, The architecture enables network-transp arent applications and provides simple primitives for multimedia captu re, display, transmission, storage, and retrieval, A real-time multime dia-in-the-loop control application was developed as the vehicle for t esting the capabilities and performance of the network, Our test measu rements focus on the end-user-level performance metrics such as messag e throughput and round-trip delay as well as video-frame jitter under no-load and load conditions, Our results show that the maximum burst t hroughput that can be supported at the user level is 48 Mb/s using AAL 5, while round-trip delays for 4-kbyte messages are about 3 ms, Our e xperience reveals a number of performance bottlenecks and open issues in using commercial ATM switches for practical applications, Our concl usions are: 1) For end-to-end performance, the primary bottlenecks are in the protocol processing at layers above ATM (as currently implemen ted) and the host operating-system's performance for burst data transf ers; 2) the current video-processing hardware and its integration with the host operating system are also severe limiting factors; and 3) be sides performance issues, other issues that limit ATM for practical ap plication and experimentation are the lack of analysis tools and the s upport for deadline-driven real-time traffic.