Experiments Involving the Transmission of Layered Video over a Local ATM Network

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1 Experiments Involving the Transmission of Layered Video over a Local ATM Network Larry Baptiste Electrical and Computer Engineering University of Toronto Toronto ON, Canada M5S 3G4 larry@comm.utoronto.ca Anindo Banerjea Information Sciences Institute University of Southern California Marina del Rey, CA 9292, USA banerjea@isi.edu. Abstract In this paper we report our experience with the transmission of layered video over a local ATM network. The layered video was obtained through the use of a subband video codec and an MPEG2 codec. The video was transmitted as a constant bit rate (CBR) base layer and an unspecified bit rate (UBR) enhancement layer. We then introduced background traffic (both CBR and UBR) in order to determine their effect on the layered video in terms of delay, jitter and loss. The results obtained were quite promising, with competing cross traffic in the network having very little effect on base layer jitter or loss. We also found that specifying the cell delay variation tolerance (CDVT) was a factor in preventing base layer loss. Losses in the enhancement layer occurred only when total utilization exceeded 1%; also maintaining synchronization between the two layers did not prove to be problematic even at very high network utilization. Losses in the base layer had a disastrous effect on the quality of the received video, while losses in the enhancement layer allowed graceful degradation in the quality of received video. Keywords: Layered Video, Subband Coding, ATM Networks, ATM Analyzer, Experimental Systems, PSNR, 1- Point CDV, MPEG-2 Video. 1. Introduction Asynchronous transfer mode (ATM) is the technology chosen by the International Telecommunications Union (ITU) for Broadband Integrated Services Digital Networks (B-ISDNs). In addition to supporting traditional data and voice services, B-ISDNs will support the transport of images, video, audio, and large amount of interactive computer data. The provisioning of video services is one of the driving forces behind the implementation of B-ISDNs, and hence the efficient transport of video over ATM networks is an important issue. Video can be constant bit rate (CBR) or variable bit rate (VBR). CBR video has the advantage of being easy to manage in the network. However, it suffers from variable picture quality, since when the scene being transmitted has a lot of information content (motion, details, etc.), more aggressive quantization must be used to keep the bit-rate constant. Alternatively, the CBR reservation in the network can be made for the highest bit-rate required, resulting in bandwidth wastage. Conversely, VBR video is difficult to manage in the network, since statistical multiplexing of VBR sources can cause network congestion, leading to cell losses and delay jitter. However, VBR video has the potential advantage of providing consistent quality, while at the same time saving on bandwidth through the use of statistical multiplexing. Layered source coding can be used to overcome this drawback and maintain the advantage of VBR coding. In the layered video concept an encoder produces video made up of a number of layers. Typically two layers are produced: a base layer and an enhancement layer. 1 / 13

2 The following list describes some of the motivating factors behind layered video: The use of layered video allows the transmission of VBR video, while simultaneously allowing CBR reservations to be made in the network. This is done by keeping the base layer at a constant bit-rate, while allowing the sum of the base and enhancement layers to be variable to achieve a constant quality. The base layer can then be sent using CBR service in the network, while the enhancement layer can be sent without any reservations. Digital networks are made up of a heterogeneous mix of subnetworks, which may support varying bandwidths and qualities of service. Encoding video into a range of layers allow subnetworks to subscribe to the appropriate amount of layers that they can support. Different endpoints may simply want to subscribe to a smaller number of layers because that is all they are willing to pay for. The objective of our work is to examine the practical issues involved in implementing such a layered video system in an ATM network. The kind of questions that we try to answer include: Is there sufficient QoS difference between the various service classes of ATM network to warrant a layered video scheme at the end-system? How much of the quality difference is due to the end-system effects and how much can be attributed to the network? What are the synchronization issues that arise due to the different delay experienced by the different layers? What is the effect of the QoS seen by the different streams on the resulting video? How do these results differ for different encoding schemes? While there exists previous experimental work on layered video [1, 2], they did not include systematic measurements, at the frame and cell level, as well as at the level of the decoded video, of the effect of network level impairments on video quality. Other previous work on ATM cell/frame measurements [3, 4] have not had the applications focus on layered video that this paper brings. This work is the first to use detailed experimental measurement to understand the problems that arise when transmitting layered video over ATM networks. In the following sections we report on results obtained from the transmission of layered video over a local ATM network. The layered video was obtained using both a subband encoder, and an MPEG-2 encoder. It should be noted that the results obtained pertain to the particular experimental setup described and particular ATM switch used, and should by no means be taken as a sweeping generalization for all ATM switches. 2. ATM Cell Level Measurements The basic setup for cell level measurements is shown in Figure 1 for two loopback hops, where the interwatch 95 real-time ATM traffic analyzer [5] captured and timestamped cells at the output port of the ATM network interface card (NIC) of the source workstation Saturn. The cells were then forwarded by the analyzer to the ATM switch (Fore ASX-2WG [6]), where they were processed and sent out over loopback connections. A loopback connection is where an output port of the ATM switch is connected via OC3 fiber optic cable to one of the input ports of the ATM switch. The use of loopback connections facilitated the introduction of competing background traffic at the output port of the loopback, and also allowed the switch to emulate a network with multiple hops. The cells of the foreground traffic were then forwarded to another port of the ATM analyzer, where they were again captured and timestamped. Finally they were forwarded to the input port of the destination workstation Jupiter. One hundred thousand cells were captured at both the source and destination workstations by the ATM analyzer. The analyzer was also used to introduce competing background traffic over the loopback connection as shown in the diagram. The analyzer has built-in traffic generation modules based on various models. We set it to generate traffic to emulate multiple multiplexed MPEG-2 streams. All our connections were set up 2 / 13

3 Foreground Traffic ATM Analyzer Port ATM Analyzer Port Foreground fgggg Traffic ATM Switch Foreground Traffic Background Saturn Traffic Jupiter ATM Analyzer Figure 1 : Experimental setup - 2 hops. using permanent virtual circuits (PVCs). We multicast the background traffic from the analyzer to each loopback output port as shown in the diagram, so that the foreground traffic would have contention at every output port. Each port supported a maximum bandwidth of 15 Mbits/s. The packet size used for the foreground traffic was 1324 bytes and the bandwidth for the CBR and UBR foreground traffic was 45 kbits/s and Mbits/s respectively. Figure 2 shows the CTD mean and standard deviation for CBR foreground traffic, 1 and 3 hops, with increasing CBR background traffic load level. Here, we see that CTD mean is in the range 3 to 45 µs for 1 hop, and 65 to 1 µs for 3 hops. The mean and standard deviation both gradually increase with load, till the background traffic is around 9%, after which they increase very rapidly. Figure 3 shows the CTD mean and standard deviation for UBR foreground traffic for 1 and 3 hops with increasing CBR background traffic. The mean varies in this case from 1 s of microseconds to as much as 25 ms, and both the mean and standard deviation increase rapidly with increasing load level. Increasing the number of hops also has no apparent effect on the mean and standard deviation. A possible reason for this is the fact that the load levels on the three hops were the same. Thus, the service rates received by the reference connection are also about the same at the three hops. Hence, the burst of traffic arriving at the first hop would cause significant delay to cells at the back of the burst. However, at the second hop (and subsequent hops) the arrival rate and the service rate are roughly the same, so the queuing delay at these hops are much smaller than at the first hop. 3 / 13

4 CTD mean and standard deviation for CBR foreground traffic vs CBR background traffic (1,3 hops) Mean (1 hop) Standard deviation (1 hop) Mean (3 hops) Standard deviation (3 hops) 7 6 us % CBR Background Traffic Load Level Figure 2 : CTD Mean and Standard Deviation for CBR foreground traffic vs CBR background utilization (1,3 hops). 3 UBR foreground Mean and Standard deviation vs CBR background load level (1,3 hops) 25 2 Mean (1 hop) Standard deviation (1hop) Mean (3 hops) Standard deviation (3 hops) Time (us) % CBR background traffic load level Figure 3 : CTD Mean and Standard Deviation for UBR foreground vs CBR background utilization (1,3 hops). We conclude from this that the QoS difference between UBR and CBR service is sufficient to explore the layered video paradigm. However, a potential exists for resynchronization problems between the base and enhancement layers at the receiver, because of the marked difference in delay. 3. Results for Subband Coded Layered Video For these experiments we used subband coded layered video, obtained using a codec developed at the University of California at Berkeley [7, 8]. The video consisted of 25 sublayers, numbered to 24, with lower numbered sublayers containing basic information, and higher numbered sublayers containing 4 / 13

5 enhancement information. If a sublayer was missing, all higher numbered sublayers were useless. The video sequence consisted of a head and shoulders sequence called Mother, at a resolution of 352 x 24 pixels, 25 frames per second. Four frames for each sublayer were combined into a single 1324 byte packet. With the header and AAL overhead, each sublayer came to a bit-rate of 75 kbits per second. We constructed a base layer by combining 6 sublayers (45 kbits/s) which was sent using the CBR traffic class - and an enhancement layer of 19 sublayers (1.4 Mbits/s) sent using the UBR traffic class. The experimental setup was similar to the one described for cell level measurements, except in this case we recorded packet numbers and timestamps at the destination workstation Jupiter. For every run of the experiment we transmitted 1 groups (of 4 frames) by continually retransmitting the same 75 unique groups, to improve the statistics significance of the results. The retransmission of the same video scene does cause long range dependencies beyond 12 seconds to be lost. However, the reference stream is a small fraction of the overall traffic load, hence its effect on the delay, jitter and loss statistics, which are the focus of the experiments, are negligible Base Layer Jitter We measured base jitter at the receiver by using the inter-arrival times of the last sublayer packet in each group. We subtracted 16 ms (the ideal group inter-arrival time at 25 frames/s, since each group was made up of 4 frames and the frame inter-arrival time at 25 frames/s is 4 ms) from this inter-arrival time to gauge the base layer jitter. Figure 4 shows the base layer jitter with increasing CBR background traffic load level for a single loopback hop. The distributions are almost identical for the different load levels, which shows that the background traffic had little effect on the base layer jitter. We saw from Figure 2 that the mean and standard deviation of the CTD, even at the highest load levels, was on the order of tens of microseconds. The range of jitter shown is on the order of several milliseconds, hence, the main source of jitter was not due load levels within the network. The major contribution to jitter was at the source and destination workstations. This consisted mainly of the variation in time needed to copy packets between buffers, to carry out interrupt calls, and to segment and reassemble packets Enhancement Layer Packet Inter-arrival times Figure 5 shows the average enhancement sublayer packet inter-arrival time. The reference stream was kept fixed, while the load level and number of hops were varied. Here we see that there was very little change as number of hops was increased, this agrees with the results we got for cell level UBR traffic. The significance of enhancement layer average inter-arrival times lies in the fact that we would like all 19 sublayers packets belonging to the same group, to arrive within the 16 ms group time period. Hence, we would like the average inter-arrival time to be considerably less than about 8 ms (16/19). From the graph we see that even at high load levels, the average times were less than 4 milliseconds. This implied there would be no problem synchronizing the two streams at the receiver. In fact, we found that even at the highest load levels all the enhancement layer sublayer packets within a particular group arrived before any of the corresponding base layer packets Base and Enhancement Layer Loss Figure 6 shows the percentage of base layer packets lost with increasing CBR background traffic load level. Up to 95% background traffic there is no loss. However, at very high load levels (97.5 % background traffic) loss was observed from the base layer. This loss also increased with number of hops. 5 / 13

6 .12.1 Jitter Distribution for varying CBR Background Traffic Utilization (1 hop) No background traffic 7% Utilization 95% Utilization 97.5% Utilization.8.6 Probability Jitter (ms) Figure 4 : Base Layer Jitter Average enhancement layer packet inter-arrival time vs CBR background load (1-3 hops) 4 1 hop 2 hops 35 3 hops Average packet inter-arrival time (us) CBR background load level Figure 5: Enhancement Layer Packet Inter-arrival Times with increasing CBR Background Traffic 25 2 Base Layer Loss with Increasing CBR Background Utilization 1 hop 2 hops 3 hops 15 % Loss % Background Utilization Figure 6 : Base Layer Loss vs CBR Background Traffic Load Level. 6 / 13

7 This was an unexpected result, since reservations were made for the base layer. We hypothesized the following reason for this loss. At very high CBR background traffic load levels, the jitter in the base layer stream was large enough to cause the cell delay variation tolerance (CDVT) component of the switch s single leaky bucket policing mechanism to be exceeded, causing the switch to drop the offending cells. Figure 7 shows the probability distribution of the 1-point cell delay variation (CDV), y k (which mirrors the way the switch implements its CDVT policing mechanism), for base layer cells. As load level was increased the distribution shifts to the right, indicating greater values of the y k. Positive values for y k correspond to cell clumping, and when the value exceeds the CDVT tolerance at the switch, the switch would drop the offending cells. Increasing the CDVT value at the switch to a large value prevented any loss at the switch as shown in Figure 8. No loss was observed in the enhancement layer for CBR background traffic load level of up to 97.5 %. Hence, we decided to use UBR background traffic, which allowed us to exceed 1% background traffic load levels. The results obtained are shown in Figure 9. The graph indicates that there was a precipitous onset of loss in going from just under 1% background utilization, to 1 % background utilization. As expected, no losses were observed in the base layer when using UBR background traffic Effect of Loss on Quality of received video Figure 1 shows the quality of the first 3 frames of received video, in terms of Peak-Signal-to-Noise ratio (PSNR), when losses occurred in the base layer. Also shown is the PSNR when only the base layer was received without loss, as well as when both base and enhancement layers were received without loss. The graph shows that there are wide fluctuations in the quality with base layer loss, which would be very disturbing to the human visual system. Hence, base layer loss is to be avoided at all cost. The lesson to be learnt here is the importance of specifying the CDVT value appropriately when setting up an ATM connection, if loss is to be prevented. Figure 11 shows the quality of the received video when there were losses in the enhancement layer. As background traffic is introduced (around frame 2), the PSNR of the combined signal drops from the noloss level of around 4 db, to the base layer level of around 34 db. However, it never drops below that level, showing that we can guarantee the minimum quality of the received video through the use of the base layer. The fluctuations in the quality are also much smaller than in Figure 1, implying graceful quality degradation in this case. 4. Results for Scalable MPEG-2 Video In this section we look at results obtained results using MPEG-2 video. The MPEG-2 standard [9] describes several methods for producing layered video. In our experiments we used the signal to noise ratio (SNR) scalable MPEG-2 option, whereby two layers are produced: a base layer which can be decoded to give a course quality video output, and an enhancement layer which can be decoded to give a high(er) quality video output. We produced the video using an encoder toolkit, MPEG2Tool [1], developed at the University of Pennsylvania. The sequence used, Flower, consisted of a camera panning past a row of multicolored flowers with a row of houses in the background. The raw video was in CCIR-61 format, at a resolution of 72 x 486. One hundred and forty eight frames were encoded. Table 1 gives the size statistics for the I, P, and B pictures for both the base and enhancement layers. 7 / 13

8 point CDV distribution with increasing CBR background traffic No background traffic 5% Background traffic 7% Background trafic 9% Background traffic 95% Background traffic 97.5% Background traffic Probability yk (us) Figure 7: 1-Point CDV Distribution. 1 9 Percent base packets lost vs CDVT - 1 hop (97.5% CBR background traffic) "cvdtloss.txt" 8 % Base layer packets lost CDVT (us) Figure 8 : Base Layer loss vs CDVT value. % Enhancement layer packet loss Enhancement layer packet loss vs UBR background traffic 1 hop 2 hops 3 hops % UBR background traffic load level Figure 9 :Enhancement Layer Packet Loss vs UBR Background Traffic Load Level. 8 / 13

9 PSNR (db) PSNR comparison - No loss vs loss with CBR background traffic PSNR - 25 layers (base + enhancement layers) PSNR - 6 layers (base layer) PSNR - 2 hops (97.5% CBR background) Frame Number Figure 1 : PSNR Comparison - No loss vs Base Layer Loss (2 hops 97.5 % CBR Background Load Level). PSNR comparison - No loss vs loss with UBR background traffic 44 PSNR - 25 layers (base + enhancement layers) 42 PSNR - 6 layers (base layer) PSNR - 2 hops (1% UBR background) 4 PSNR (db) Frame Number Figure 11 : PSNR Comparison - No Loss vs Enhancement Layer Loss (2 hops 1 % UBR Background Traffic Load Level). Due to the fact that I, P, and B pictures can be compressed to differing degrees, if we made CBR reservations based on the size of the largest frame, most of the other frames would be significantly smaller and a lot of bandwidth would be wasted. The group of pictures (GOPs) format was IBBPBBPBBPBB. Therefore we had a larger I or P frame, followed by two smaller B frames. We chose the reservation level based upon the largest group of IBB or PBB frames in the entire sequence. What this implied was that there would be some overlap of the time to send the I or P frame into the time period to send the following B frames. For example using our frame rate of 25 frames/s, there are 4 ms between frames. The effect of choosing our reservation levels was to allow the larger P or I frame to take more than 4 ms to be sent, but still ensure that the largest set of three frames took at most 12 ms to send. The reservation level obtained for the base layer using this scheme was 5.6 Mbits/s. The advantage of our reservation scheme was that the peaks caused by the I and P frames were smoothed to some extent. The disadvantage of our scheme was that we needed to buffer at least three frames before playback at the decoder. Because we expected little or no losses in the base layer, and the size of the largest base picture was smaller than the maximum AAL5 packet size of bytes, we used the base layer 9 / 13

10 packet size equal to the picture size. The enhancement layer was sent best effort, and because some losses could be expected here, we used 1 packets for each picture so that the loss of one cell would not cause the entire frame to be lost. The average bit rate of the base layer was 3.84 Mbits/s, while the average bit rate of the enhancement layer was Mbits/s. At the beginning of each 4 ms time period the base packet was sent followed immediately by the enhancement layer packets. Average size (bytes) Standard deviation Largest (bytes) I pictures P pictures B pictures All pictures Flower Base Layer Picture Sizes. Average size (bytes) Standard deviation Largest (bytes) I pictures P pictures B pictures All pictures Flower Enhancement Layer Picture Sizes. Table 1 : Picture size comparison for Flower sequence Base Layer Inter-arrival Distribution Figure 12 shows the results obtained for the percentage of base layer frames that arrive T milliseconds after the reference frame inter-arrival time of 4 ms. Twenty thousand frames were used in each run of the experiment by repeatedly transmitting the same 148 frames. The setup was similar to the one described in the previous section for subband video. The results show that an increase in load had little effect on the inter-arrival time distribution of the packets. In this case because the frames are of different sizes, then the theoretical arrivals times vary with each frame; the percentage of frames that arrive after multiples of the inverse of the frame rate, 4 ms (i.e. at 4, 8 ms etc.) is a useful parameter. It helps in determining how much buffer space is needed, and also how much time to wait initially before starting to decode the video. Figure 12 shows that frames arrive at most 12 ms after their reference arrival time, hence it would be reasonable to buffer 3 frames before initiating decoding and playback of the base layer. This ensures that preceding frames would always be available for playback while waiting for a particular frame to arrive. The results shown are for 1 hop no load; and also for 93% CBR background load over 3 hops. The two graphs are indistinguishable, illustrating that the high load level and increased number of hops had little effect on base layer packet jitter Synchronization of Base and Enhancement Layers In order to provide a measure of the effect of load on the relationship between the two streams, we defined distance as the difference between the packet number of the base layer packet, and the packet number of the first enhancement layer packet that directly followed it. A negative value for this distance implied that a majority of time the base layer frames lagged behind the enhancement layer frames with the same packet number, while a positive value implied the opposite. 1 / 13

11 .35.3 Graph showing cumulative probability that frames arrive > T ms after reference arrival time 1 hop (No load) 3 hops (93% CBR background load.25 Probability T (ms). Figure 12: Base layer inter-arrival time relationship. Distance statistics for CBR background load 2 Distance hop (mean) 1 hop (standard deviation) 2 hops (mean) 2 hops (standard deviation) 3 hops (mean) 3 hops (standard deviation) % CBR background load Figure 13: Distance Mean and Standard Deviation vs CBR Background Traffic Load Level (1-3 hops). Figure 13 shows the distance statistics at increasing CBR background traffic load levels. At low load levels the values for the mean are small (less than one in magnitude) and negative, indicating that the majority of time the base layer packet arrived after the enhancement layer packets. While at higher load levels the distance is positive and increases with load and number of hops. At a background load level of around 73% the difference changes from a negative value to a positive value, and at a load level of 93% can be as high as 2. What this means is that although enhancement packets are received at higher load levels, they may be useless due to the delay they experience in the network Enhancement Layer Loss Figure 14 shows the percentage of enhancement layer packets lost, with increasing load over 1 to 3 hops. The total bandwidth of the reference MPEG-2 video was just less than 12% of the available 15 Mbits/s bandwidth, and as shown in the figure, losses generally began occurring at about 88% background traffic where the total load becomes 1%. This indicated that once the total utilization is less than 1% very little or no loss occurs; this is similar to the results obtained in the previous section for subband video. In 11 / 13

12 % enhancement layer packets lost Percentage of enhancement packets lost vs CBR background traffic load level 1 hop 2 hops 3 hops % CBR background traffic load level Figure 14 : Enhancement layer loss vs CBR background traffic load level Frame number this case however, our enhancement layer had a much higher bandwidth and burst size. The low probability of loss at less than 1% utilization points to the ATM switch having an effective buffering scheme Effect of Loss on PSNR of Decoded Video PSNR db PSNR for first 148 frames Base and enhancement (no loss) Base Base and enhancement (loss) Figure 15: Effect of Loss on PSNR. Finally, Figure 15 shows the PSNR values for the base layer, base and enhancement layers when both are received without loss, and also base and enhancement layers when there was 27% loss in the enhancement layer for the case of 1 hop 9% UBR background load. The PSNR for the first 148 reconstructed frames are shown, these frames were decoded off-line using the MPEG-2 decoder developed by the Software Simulation Group (SSG) [11]. The luminance components from both the original raw video and the reconstructed decoded video were used to calculate the PSNR. Background load is introduced at around frame 1, leading to a clearly visible drop in the PSNR of the combined video. The quality of the decoded video is extremely low when there are losses in the enhancement layer, hence it would be better to monitor if enhancement layer loss occurs at the decoder, and decode only the base layer if consistent loss is observed. The reasons for the low quality of the video were twofold; firstly, when enhancement layer packets were lost, the decoder was unable to combine the base and enhancement frames correctly because headers essential to their proper recombination were missing. Secondly, when using I and P frames as reference for other frames, these reference I and P frames were reconstructed using both the base and enhancement layers. Hence if these frames contained errors, they would propagate to other frames. 12 / 13

13 5. Conclusions In this paper we investigated the feasibility of transmitting layered video over a local ATM network. Two layered video schemes were looked at - subband coded layered video and MPEG 2 SNR scalable layered video. In both cases the video was transmitted as a CBR base layer and a UBR enhancement layer. We then introduced background traffic (both CBR and UBR) in order to determine their effect on the layered video in terms of delay, jitter and loss. Because no reservations were made for the enhancement layer, this allowed the video to be coded to a constant quality without close consideration of what the final bit rate would be, however without reservations, there were no delay or loss guarantees for the enhancement layer. The use of the base layer, which did have quality of service guarantees, maintained a minimum quality for the received video. Also the fact that UBR enhancement layer cells were sent in bursts at line rates helped to compensate for the lack of delay guarantees. This was because ATM cells from the CBR base layer were staggered over the period to send a frame, hence the enhancement layer could potentially arrive before the base layer. The results obtained for the subband layered video were quite promising, with competing cross traffic in the network having very little effect on base layer jitter or loss. We also found that specifying the cell delay variation tolerance (CDVT) was a factor in preventing base layer loss. Losses in the enhancement layer occurred only when total utilization exceeded 1%, also maintaining synchronization between the two layers did not prove to be problematic even at very high network utilization. Losses in the base layer had a disastrous effect on the quality of the received video, while losses in the enhancement layer allowed graceful degradation in the quality of received video. For the SNR scalable MPEG 2 video, the results were similar, except that the higher bit rates and burst sizes involved resulted in significant delay for the enhancement stream at high load levels, causing problems with maintaining synchronization between the two layers. 6. References 1. S. Mccanne, V. Jacobson, and M. Vetterli. Receiver-driven layered multicast. Proc. SIGCOMM '96, pp , Stanford, California, August M. Sudan and N. Shacham. Gateway Based Approach For Managing Multimedia Sessions over Heterogeneous Signaling Domains. INFOCOM' R. Tsang, D. Du, and A. Pavan. Experiments with video transmission over an ATM network. Multimedia Systems, vol.4, August J-R. Louvion, B. Piller. Performance measurements and traffic characteristics on the ATM Pilot network. European Transactions on Telecommunications, vol. 7., Sep/Oct GN Nettest InterWATCH 95 ATM Traffic Generator Manual, October White Paper, ForeThought Bandwidth Management Version 1., Technical report, Fore Systems, A. Banerjea, W. Tan, A. Zahkor. A Layered Compression Scheme for Multicasting Medical Images across Heterogeneous Networks. SPIE Medical Imaging 97, Feb. 1997, Newport Beach, CA, pp W. Tan, E. Chang, and A. Zakhor. Real Time Software Implementation of Scalable Video Codec. Proc. International Conference on Image Processing, Lausanne, Switzerland, Sep ISO/IEC Generic Coding of Moving Pictures and Associated Audio. In Recommendation H.262 Committee Draft, Nov MPEG2Tool Version 1., February ftp://sokaris.ee.upenn.edu/pub/mpeg2tool. 11. MPEG Software Simulation Group, MPEG-2 Encoder/Decoder, Version 1.2, ftp://ftp.mpeg.org/pub/mpeg/mssg/mpeg2decode. 13 / 13