Delivering ATM-based Video-on-Demand across Asymmetrical Digital Subscriber Lines

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1 Delivering ATM-based Video-on-Demand across Asymmetrical Digital Subscriber Lines Emile Swanson Technology Integration, Telkom SA Ltd Private Bag X74, Pretoria, 0001 South Africa Neco Ventura Dept of Electrical Engineering, University of Cape Town Private Bag, Rondebosch, 7701 South Africa ABSTRACT - ADSL technology provides a means for delivering high bit-rate services to residential subscribers. Current service offerings are predominantly IP-based services such as high-speed Internet access. Such services do not fully exploit the potential offered by the ADSL technology. This paper investigates the implications of using the ATM technology that forms part of the ADSL standard, for delivering high-quality Video-on-Demand. Various problems pertaining to delivering such a service across ADSL are identified, and solutions are proposed. Evaluations of these solutions show improvements in both reliability and bandwidth efficiency. Proposals are also made for future research to extend these findings towards practical / commercial implementations. Keywords - ATM, Multimedia, Video-on-Demand, xdsl discussed in Sections III and IV respectively. Section III discusses the need for reliable resource management, especially in the access network. Two models for ensuring this are presented and evaluated in terms of their suitability for various situations. Section IV focuses on shaping the video stream to ensure efficient use of the available network resources. MPEG-1 [6] compression is used as the video format, as it is a widely used, open standard. Three lossless shaping techniques are presented and evaluated in terms of computational complexity and network efficiency. Section V describes the implementation of a test-bed combining one of the models from Section III with the shaping techniques from Section IV, and test results are presented for various situations. Section VI draws conclusions based on the findings presented in this paper, and Section VII presents directions for future research. I. INTRODUCTION Asymmetrical Digital Subscriber Line (ADSL) [1,2] technology is seen as a cost-effective means of delivering high bit-rate content to residential subscribers. ADSL is an overlay network that leverages existing local-loop copper infrastructure, and employs advanced digital signal processing to allow high bitrate operation across this infrastructure. Research has been done into the effect of using ADSL as a "last mile" technology, focussing mainly on the performance of TCP/IP over ADSL [3,4]. While these findings benefit TCP/IP-based applications such as high-speed Internet access, which are currently the main applications of ADSL, they do not fully explore the possibilities offered by ADSL. ADSL was originally designed for streaming video [5], as indicated by the error-reduction techniques employed, low latencies offered, and relatively high downstream data rates offered. In addition, the use of ATM as the Layer 2 technology for ADSL further suggests that this technology is highly suited for an application with strict network resource requirements such as Video-on- Demand (VoD). This paper explores the factors affecting the delivery of video content across ADSL links, and proposes and evaluates solutions for optimizing such a content delivery system, in terms of network efficiency as well as received video quality. Section II of this paper presents a simplified ADSL network architecture containing only those elements necessary for delivering video content. From this model the factors affecting streaming video delivery are identified. These factors may be addressed either at the network- or application-level, as II. ADSL NETWORK ARCHITECTURE In order to identify the factors affecting the performance of ATM-based VoD over an ADSL link, a basic network architecture is used, comprised of only those elements necessary for delivering the video stream. This architecture is shown in Figure 1. ATM / ADSL End-Systems DSLAM ATM Network Video-on- Demand Server Figure 1: Simplified ATM/ADSL Network From Figure 1 it can be seen that the ADSL network closely resembles a conventional ATM network, with the addition of an ADSL link in the access leg. This link serves as a physical medium for transporting the ATM traffic to the end-user. However, it should be noted that ADSL was designed as a transparent technology, and that the ADSL link transparently bridges the information between the end-user's 25.6Mbps ATM link, via the ADSL line, and the backhaul link from the DSLAM, which may be an STM-1 connection. This means, amongst other things, that the ATM network does not know the ADSL resources, and that the ADSL equipment does not participate in connection admission control (CAC). Figure 2 illustrates the Q.2931 [7] signaling for a virtual circuit being

2 created from the end-system, through the ADSL network, to the video server. It can be seen that the ADSL equipment does not participate at any point, and that the ADSL link's resources are not considered at any point in the CAC process. ATM Quality-of-Service (QoS) can therefore not be guaranteed when operating across an ADSL network. Section III of this paper presents methods of addressing this shortcoming at the network level, to ensure ATM QoS in an ADSL network. ADSL End- System SETUP ADSL Access Network CALL PROCEEDING CONNECT ACK CONNECT ATM Switch A SETUP CALL PROCEEDING CONNECT CONNECT ACK ATM Switch B SETUP CALL PROCEEDING CONNECT CONNECT ACK VoD Server (ATM) Figure 2: Q.2931 Signalling for successful SVC setup It should also be noted that an ADSL network imposes stricter bandwidth constraints than other fixed-line ATM networks, particularly for bursty traffic such as compressed video. Research has addressed the requirements of delivering VoD in ATM networks [8,9,10,11,12,13,14]. However, much of this research focuses on achieving statistical multiplexing gains. This research is not applicable to ADSL, as the limited bandwidth available on an ADSL link (up to 8Mbps downstream) does not allow for multiplexing video streams. Instead, the traffic stream should be shaped to minimize the peak bandwidth required. Ideally this shaping should be done at the application level, as this will allow more intelligent shaping based on the characteristics and requirements of the traffic. Section IV addresses this problem by presenting and evaluating lossless shaping techniques that impose minimal additional computational load on both the server and the client systems. III. NETWORK-LEVEL RESOURCE MANAGEMENT As discussed above, current ADSL equipment does not participate in resource management activities such as CAC, making it impossible to guarantee ATM QoS. This problem may be overcome by including the ADSL resources in the CAC already being performed at one of the participating ATM nodes, using either a distributed or a centralized solution. A. Distributed Solution In this solution, the ATM resources of the ADSL line are managed by the end-system's ATM stack. The end-system determines the available resources on the line in one of three ways, listed below in decreasing order of preference: 1. Querying the ATU-R. Ideally the end-system would be able to access information about the available resources on the ADSL line through a direct connection to the ATU-R. The endsystem would then be able to integrate this information into its ATM stack for more reliable resource management. This option may not always be available, as it is heavily dependent on whether the ATU-R supports such access to its management information. Current ATU- Rs also have various ways of providing this access, so the implementation would be tied to a specific set of ATU- Rs. 2. Querying the DSLAM via in-band management. The end-system may be able to access the management information stored on the DSLAM, and use this information to update its own ATM stack. Disadvantages of this approach are that it compromises security by allowing access to the ADSL network management system, and that the end-system needs detailed information about which line's details to query, e.g. shelf, slot and port number. 3. Querying the DSLAM via out-of-band management. This method is similar to that described above, with the main difference being the use of out-of-band management. This method uses IP-based SNMP to query the DSLAM. In addition to the disadvantages for the inband method, this method also needs the IP networking stack to be initialized before the ATM resource management can be fully initialized. Fortunately, IP is a best-effort service, and therefore typically uses a UBR class-of-service across an ADSL link. This means that the end-system's ATM resource management stack may be passed an updated set of line parameters without negatively affecting the performance of the existing IP connectivity. The primary advantage of using a distributed solution is that it may be implemented in software, by extending the ATM networking stack at the end-system. This type of solution is suited to existing networks, as it does not require costly hardware replacement or upgrades. Software implementations may also be developed and deployed more quickly than hardware-based solutions. B. Centralized Solution In this solution, the DSLAM is extended to contain more complete ATM switching functionality than current systems, effectively evolving into an ATM switch. The DSLAM now uses the information about the specific ADSL line, in conjunction with information about the backhaul ATM link, when creating new virtual connections. The DSLAM therefore acts as a proxy for each end-system, assisting with the CAC process. An additional advantage is that, as an ATM node, the DSLAM would also be able to form an active part of an ATM multicast tree, improving the efficiency of multicasting to ADSL end-users. While this solution offers the advantage of resource management at a single, central point, it comes at the cost of increased hardware complexity in the DSLAM. This solution would therefore be more suited to new networks than existing installations.

3 In this research, the solution presented in Section III.A.3 has been implemented and evaluated. Details about this are provided in Section V. IV. APPLICATION-LEVEL TRAFFIC SHAPING Applications operating across limited capacity media such as ADSL should minimize their network resource requirements, specifically their peak bandwidth requirement. This should ideally be done at the application level, as the application possesses detailed information about the traffic stream, and can therefore perform more intelligent traffic shaping than the network. MPEG-1 compressed video, for example, has a repetitive structure, as shown in Figure A. Short-Term Shaping When studying an MPEG-1 stream, two things are noticed. Firstly, the I-frames are typically largest and therefore contribute most to the peak bandwidth requirement of the traffic stream. Secondly, the I-frames are always preceded by B-frames, which are smallest. It should therefore be possible to reduce the peak requirement of the stream by buffering and transmitting the average of an I-frame and its preceding B- frame during their original transmission periods. The sequence now becomes B(BI)'(BI)'BBPBBPBBPBBPB(BI)'(BI)'B, where (BI)' refers to the averaging of the I- and preceding B- frame. Figure 4 shows the effect of applying this technique on the sequence in Figure 3. The figure shows a noticeable reduction in the peak bandwidth requirement, from bytes per frame (in the GoP in Figure 3) to bytes per frame (in the smoothed GoP in Figure 4). Specific details on improvements obtained with this technique are provided in Section V , ,000 12, ,000 0 I B B P B B P B B P B B P B B Figure 3: Relative frame sizes and sequence for MPEG-1 As can be seen, the stream consists of three frame types (I-, P-, and B-frames), with a regularly repeating sequence, arranged to form Groups-of-Pictures (GoPs). This sequence is described in terms of M and N parameters, referring to the spacing between consecutive I-frames, and I- and P-frames respectively, e.g. the sequence PBBIBBPBBPBBPBBPBBIBBP etc would be described as M=15, N=3. These frames can also be characterized relative to each other in terms of size. I-frames are typically largest, followed by P-frames, with the B-frames being smallest. Example ratios for I:P:B frame-sizes are 6:2:1 (for low-motion streams) or 4:4:1 (for higher-motion streams). The average bitrate for an MPEG-1 compressed stream is also known. Together, these parameters may be used to describe an MPEG-1 stream. A video server may use the information provided by these parameters to shape the traffic stream As stated previously, an advantage of performing traffic shaping at the application level is that the server can use a priori information (e.g. frame-type sequence, relative frame sizes) to perform intelligent shaping. While other researchers use lossy shaping techniques [15], in this research we have elected to use only lossless shaping techniques, thereby ensuring no degradation of video quality due to the presence of the limited-bandwidth ADSL link. Based on the structure of an MPEG-1 compressed stream (described above), the following three types of lossless shaping techniques may be used, in increasing order of the expected reduction of peak requirements: 8,000 6,000 4,000 2,000 - (BI)' B B P B B P B B P B B P B (BI)' Figure 4: Reduced peak bandwidth after short-term smoothing B. Medium-Term Shaping Further investigation of the GoP used in Figures 3 and 4 suggest that further reductions in peak requirements may be obtained by using the remaining B-frames to reduce the peaks due to the large I-frames. In addition, the peaks due to the medium-sized P-frames may be reduced in the same manner. This method effectively buffers the entire GoP and transmits at the much lower average rate of the GoP. Figure 5 illustrates the reduced peak requirement of the smoothed GoP. Note that Figure 5 spans 3 GoPs, to show the smoothing per GoP. Note also that "GoP A" in the figure refers to the GoP used in Figures 3 and 4. A potential disadvantage of using this form of traffic shaping is that delays are required for the buffering of the stream prior to transmission. During the implementation and evaluation of this technique, however, it was found that it is possible to employ the existing de-jittering buffers in standard MPEG-1 servers and clients without imposing additional delays or computational load. Details of the specific improvements obtained with this technique are provided in Section V.

4 GoP A: Peak =6491 bytes GoP B: Peak =7414 bytes GoP C: Peak =7452 bytes Figure 5: Reduced peak variations after medium-term smoothing C. Long-Term Shaping Long-term shaping of compressed video traffic is discussed in [14], with the emphasis on renegotiating the allocated network resources to match the requirements of the smoothed traffic. This method allows the reserved bandwidth to be dynamically tailored to match the traffic stream, in order to obtain high bandwidth efficiency through the network for compressed streams in general. In this research however, we focus specifically on MPEG-1 compressed video, and can therefore simplify our long-term smoothing by using our knowledge of the stream's structure. MPEG-1 compression works by reducing spatial and temporal redundancy at a frame- and GoP-level. However, each GoP is effectively encoded independent of the rest of the stream, for error-resilience amongst other reasons. An advantage of this compression method is that the average bandwidth requirement of each GoP does not deviate from that of the overall stream as much as the individual frames do. It should therefore be possible to transmit the entire stream at the overall average rate without imposing heavy buffering requirements as suggested in [14]. This method effectively converts the original (highly bursty) variable-bit-rate (VBR) video stream to a constant- bit-rate (CBR) for transporting across the ADSL link, thereby minimizing the peak requirements of the stream. The evaluation in Section V shows that this technique is practically applicable, both for low- and high-motion streams. V. IMPLEMENTATION AND PERFORMANCE EVALUATION In order to verify the operation of the resource management solutions described in Section III, the technique described in Section III.A.3 was implemented and evaluated on an ADSL test-bed in the Communications Research Group at the University of Cape Town. This technique was chosen for implementation due to limited time, limitations on the available equipment and the lack of access to the hardware specifications required for implementing the DSLAM-based solution. The implementation and evaluation of the remaining solutions are left for future research projects. The test-platform was built around a Siemens XpressLink D v1.0 DSLAM, connecting two ATM/ADSL clients to an ATM-based MPEG-1 video server via an ATM network. A development version of the Linux operating system (kernel version ) with ATM extensions (version 0.66) was used on the client machines. The resource management extensions were integrated into the device driver software installed at the ATM/ADSL end-systems. All three smoothing techniques were implemented in a custom-developed MPEG-1 video server-client system, developed with the SDL multimedia development libraries. In order to create switched virtual circuits (SVCs) from either of the end-systems, paths were mapped through the DSLAM to the edge ATM switch, and ILMI [16] was activated on the additional paths. SVCs were used to confirm the acceptance or rejection of call-setup requests based on available resources. In the first stage of testing, the end-systems attempted to create SVCs exceeding the available resources on the ADSL link. Before enabling the improved resource management, all SVCs were accepted by the ATM network. Data transfers across these links were possible, but data losses and reduced throughput due the associated retransmissions were observed. With the resource management mechanisms enabled, requests for resources in excess of those available on the link were rejected by the network, ensuring reliable operation of connections across the link. Having confirmed correct operation of the underlying network, the second stage of testing focussed on evaluating the performance of the application-level shaping mechanisms. The shaping techniques described in Section VI were implemented on the video server and clients. Two source streams were used (SeatSpin.mpg and AlienSong.mpg), as samples of high-motion and low-motion streams respectively. "SeatSpin.mpg" was also specifically chosen to represent a file having high instantaneous requirements, to determine how effectively the smoothing techniques would handle large peaks. More detailed information on these streams are provided in Tables 1 and 2. MPEG Movie Info (SeatSpin.mpg) Audio Bitrate (average) Picture Size 352 x 288 Video Bitrate (Average) Frame rate 25 File Size I:P:B ratio 4:4:1 Table 1: Stream information for SeatSpin.mpg MPEG Movie Info (AlienSong.mpg) Audio Bitrate (average) Picture Size 320 x 240 Video Bitrate (Average) Frame rate 24 File Size I:P:B ratio 6:2:1 Table 2: Stream information for AlienSong.mpg

5 Statistical analysis of the streams used showed that the peak buffer requirements for each of the smoothing techniques never exceeded the size of one GoP (for the two streams used). However, due to the small sample set used, it was decided to dimension the buffers to accommodate up to four (4) GoPs, with the operating target being 50% buffer utilization. Table 3 provides information on the buffer sizes (in bytes) and associated delays introduced (in seconds). Buffer size (bytes) Latency (sec) SeatSpin.mpg AlienSong.mpg Table 3: Buffer requirements and latencies / delays introduced for two test streams The results of tests under various ADSL line conditions were as expected. SVC setup requests were correctly accepted or rejected based on the availability of reserve capacity on the ADSL link, with no cell losses due to congestion on the ADSL link. The smoothing techniques also performed as expected, with the long-term shaping providing the greatest reduction in peak requirements, down to 18% of the original peak requirements for the "AlienSong.mpg" stream. Error detection was enabled at the video client applications, but no stream errors were detected. It was originally intended to conduct a subjective assessment of the delivered video stream as well, but this was deemed unnecessary since the objective measures were reporting error-free operation. It was therefore accepted that the implementation was performing as intended, combining reliable resource management and intelligent traffic shaping to deliver high-quality video across an ADSL link. Tables 4 and 5 list the results obtained when using the "SeatSpin.mpg" and "AlienSong.mpg" respectively. Smoothing PCR Relative Reduction None Short-Term % 27% Medium term % 68% Long-Term % 73% Table 4: Reduced peak bandwidth requirements (in cells/sec) after smoothing SeatSpin.mpg Smoothing PCR Relative Reduction None Short-Term % 46% Medium term % 80% Long-Term % 82% Table 5: Reduced peak bandwidth requirements (in cells/sec) after smoothing AlienSong.mpg VI. CONCLUSIONS Although a number of impairments still prevent the proper operation of ATM over ADSL links, it is possible to address these problems. The problems may be broadly classified into two categories, namely network-level and application-level, and then addressed as such. This paper ensures reliable network operation (in terms of QoS guarantees) by proposing and implementing the network-level solutions, before addressing the application-level problems. By integrating both classes of solution into a system, we have shown that it is indeed possible to reliably deliver high quality ATM-based streaming video across ADSL links, without imposing significant computational or storage requirements on either the server or the client systems. VII. DIRECTIONS FOR FUTURE RESEARCH The remaining models proposed in Section III should be implemented and evaluated relative to the model used in Section V. All the models should also be evaluated to determine applicability to various scenarios. Factors to be taken into account include network size and structure, number of users, traffic volumes, redundancy and reliability, and cost implications. A more detailed study of the statistical properties of MPEG-1 streams is also required. The implementation described in this paper makes assumptions based on descriptive statistics of the stream and achieves acceptable results. However, the accuracy of the buffer size calculation becomes more critical as the size of the network being served by the video server grows. It would therefore be desirable to have an empirical formula to accurately calculate the minimum required buffer sizes for an arbitrary stream. VIII. REFERENCES [1] International Telecommunication Union, "ITU-T Recommendation G Asymmetrical Digital Subscriber Line (ADSL) Transceivers" [2] American National Standards Institute, ANSI T1.413 Issue 2 [3] Hari Balakrishnan, Venkata N. Padmanabhan, "How Network Asymmetry Affects TCP", IEEE Communications Magazine, April 2001, pp [4] Ryoichi Kawahara, Hiroshi Saito, "Performance of TCP/IP over ATM over an ADSL", IEICE Transactions on Communication, Volume E83-B, No.2, February 2000, pp [5] ADSL Forum, "ADSL Forum: Technical FAQs", available at [6] International Organisation for Standardisation, "MPEG-1 Coding of moving pictures and associated audio for digital storage media at up to about 1,5 Mbit/s" [7] International Telecommunication Union, "ITU-T Recommendation Q Digital Subscriber Signalling System No. 2 - User-Network Interface (UNI) layer 3 specification for basic call/connection control" [8] Marwan Krunz, "Bandwidth Allocation Strategies for Transporting VBR Video Traffic", IEEE Communications Magazine, January 1999, pp [9] Pedro Cuenca, Antonio Garrido, Fransisco Quiles, Luis Orozco-Barbosa, Error-Resilient Video Transmission over ATM Networks, IEEE Communications Magazine, December 1999, pp

6 [10] Marwan Krunz, Satish K. Tripathi, "Exploiting the Temporal Structure of MPEG Video for the Reduction of Bandwidth Requirements", IEEE Infocom'97 Proceedings Volume 1, pp [11] T. V. Lakshman, Partho P. Mishra, K. K. Ramakrishnan, "Transporting Compressed Video over ATM Networks with Explicit Rate Feedback Control", IEEE Infocom'97 Proceedings Volume 1, pp [12] Marcel Graf, "VBR Video over ATM: Reducing Network Resource Requirements through Endsystem Traffic Shaping", IEEE Infocom'97 Proceedings Volume 1, pp [13] Patrick Vine, Neco Ventura, "Maximising the simultaneous users of a Video on Demand Server", SATNAC98 Proceedings, pp [14] Wu-chi Feng, Jennifer Rexford, "A Comparison of Bandwidth Smoothing Techniques for the Transmission of Prerecorded Compressed Video", IEEE Infocom'97 Proceedings Volume 1, pp [15] Nicholas Yeadon, "Supporting Quality of Service in Multimedia Communications via the use of Filters", BT Labs, Ipswich/ Lancaster University Internal Report [16] ATM Forum, "Integrated Local Management Interface (ILMI) Specification", af-ilmi Author's Biography: Emile Swanson Emile Swanson completed his B.Sc(Eng) degree at the University of Cape Town in December He joined the Communications Research Group in the Department of Electrical Engineering at the University in 1999, and is studying towards an M.Sc(Eng) degree, focussing on ATM and ADSL technologies. He is currently employed as an electrical engineer at Telkom SA, working with ATM integration issues.