Wireless Ad Hoc and Sensor Networks

96 Wireless Ad Hoc and Sensor Networks3.3.3.5.2 VBR SourcesIn high-speed networks such as ATM and Internet, real-time videoapplications will become major traffic sources. The moving pictureexpert group (MPEG) is one of the most promising interframe compressiontechniques for such applications (Jagannathan and Talluri 2002).Therefore, one of the critical issues is to realize the effective MPEGvideo transfer through networks. Our scheme takes advantage of theflexibility of MPEG video encoding and the feedback mechanisms ofABR service.Compressed video also is often rate adaptive, that is, it is possible tomodify the source rate dynamically by adjusting the compression parametersof a video coder (Liu and Yin 1998). The video source rate can bematched to the rate returned in the RM cell by modifying the quantizationlevel used during compression. Therefore, a feedback approach can beemployed even with variable bit rate (VBR) type traffic.The MPEG data set used in the simulations was found at a Bellcore ftpsite (Liu and Douligeris 1997). This data set comes from the movie “StarWars.” The movie length is approximately 2 h and contains a diversemixture of material ranging from low-complexity scenes to scenes withhigh action. The data set has 174,138 patterns, each pattern representingthe number of bits generated in a frame time, F. In this trace, 24 framesare coded per second, so F is equal to 1/24 sec. The peak bit rate of thistrace is 185,267 b/frame, the mean bit rate is 15,611 b/frame and thestandard deviation of the bit rate is about 18,157. We used 4000 framesfrom this trace to run our simulations. The trace used for offline trainingis different from the one used for testing.The sources were driven with MPEG data, which cannot be adequatelymodeled using a conventional traffic model. This source behavior can beaccommodated by the ABR service, because the explicit-rate schemeallows sources to request varying amounts of bandwidth over time. Whenthe bandwidth demand cannot be met, the network provides feedback tomodify the source rate.Figure 3.10 shows the multiplexed MPEG traffic created through multipleVBR sources. The combined PCR for all the sources is given by13,247 cells/sec with the MCR being 3444 cells/sec. Simulations usingmultiple VBR sources were performed with a measurement interval of41.67 msec. Simulation Example 3.3.1 deals with results obtained whenthe sources are of ON/OFF type, whereas the Example 3.3.2 illustratesthe results for VBR type. In Simulation Example 3.3.3, additional delayswere injected in the feedback and the controller performance was compared,whereas Example 3.3.4 presents the performance of the controllerin the presence of cross-traffic and fairness in the end-to end case.Example 3.3.5 deals with multiple bottlenecks with switches connectedin tandem.