2002 - cesnet

low with respect to the capacity of backbone lines of existing networks, reachingseveral Gbps. However, multimedia transmissions are also used betweenpoints connected at a lower capacity lines with higher loss rate. As an example,we may use a transmission of a lecture from a hall, connected to the Internetwith a temporary wireless device. We were therefore interested in the influenceof QoS network characteristics on the quality of multimedia transmissions inthe MPEG format.For the configuration of the experiment, see Figure 9.7. The sending PC wasequipped with an Optibase MPEG MovieMaker 200 card, the receiving PC withan Optibase Videoplex Xpress card. Both cards used MPEG1 and MPEG2, formatsSIF, QSIF, Full-D1 and Half-D1. The MovieMaker 200 card is capable ofsending real-time encoded data in MPEG1, from the S-video port, or it can usepreviously encoded data in MPEG1 or MPEG2, saved in a file on the disk. Weused different combinations of the formats mentioned above, with differentspeeds of the sent data flows. The observations described below were almostidentical under all circumstances.Optibase MPEGMovieMaker 200100BASE-TXLinux + NIST Net100BASE-TXOptibase Videoplex XpressFigure 9.7: Configuration of experimental transmission of MPEGThe sending and receiving PCs were linked through a router running the LinuxOS. We installed NIST Net on this router, for the emulation of QoS network characteristics.The program makes it possible to set up the required throughput,loss rate and delay, including the distribution.As we had expected, the packet loss rate proved to be a critical parameter.MPEG transmission without an error correction code (FEC) does not tolerateany loss of packets at all. For instance, with the data flow of 10 Mbps, the lossrate of 0.02 % for 1,500-Byte long packets means that one packet gets lost every6 seconds. Such a single lost packet was visible as a pixelization. We used relativelydynamic scenes from a demo video presented by Optibase. It is likelythat the effect would not be as visible in less dynamic scenes. For the effect observedat the loss rate of 0.02 % and 0.1 %, see Figure 9.8 and 9.9, respectively.On the other hand, MPEG transmission proved to be resistant to delay and jitter.Delays of up to 1 second, fairly common in real networks, were without anyproblem.See Figure 9.10 for the development of loss rate on the Prague–Poděbrady wirelessline, measured during a period of five days. The loss rate reached approx.1.7 %. Unfortunately, we did not have the chance to try multimedia transmissionthrough this line, even though it is very likely that such a transmission wouldHigh-speed National Research Network and its New Applications 2002127