Delivering ATM Video on Demand across ADSL

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					Delivering ATM-based Video-on-Demand across Asymmetrical
                 Digital Subscriber Lines
                     Emile Swanson                                                           Neco Ventura
          Technology Integration, Telkom SA Ltd                         Dept of Electrical Engineering, University of Cape Town
             Private Bag X74, Pretoria, 0001                                        Private Bag, Rondebosch, 7701
                      South Africa                                                            South Africa

ABSTRACT - ADSL technology provides a means for                     discussed in Sections III and IV respectively. Section III
delivering high bit-rate services to residential subscribers.       discusses the need for reliable resource management,
Current service offerings are predominantly IP-based                especially in the access network. Two models for ensuring this
services such as high-speed Internet access. Such services          are presented and evaluated in terms of their suitability for
do not fully exploit the potential offered by the ADSL              various situations. Section IV focuses on shaping the video
technology. This paper investigates the implications of             stream to ensure efficient use of the available network
using the ATM technology that forms part of the ADSL                resources. MPEG-1[6] compression is used as the video format,
standard, for delivering high-quality Video-on-Demand.              as it is a widely used, open standard. Three lossless shaping
Various problems pertaining to delivering such a service            techniques are presented and evaluated in terms of
across ADSL are identified, and solutions are proposed.             computational complexity and network efficiency. Section V
Evaluations of these solutions show improvements in both            describes the implementation of a test-bed combining one of
reliability and bandwidth efficiency. Proposals are also            the models from Section III with the shaping techniques from
made for future research to extend these findings towards           Section IV, and test results are presented for various
practical / commercial implementations.                             situations. Section VI draws conclusions based on the findings
                                                                    presented in this paper, and Section VII presents directions for
Keywords - ATM, Multimedia, Video-on-Demand, xDSL                   future research.

I. INTRODUCTION                                                     II. ADSL NETWORK ARCHITECTURE
Asymmetrical Digital Subscriber Line (ADSL)[1,2] technology         In order to identify the factors affecting the performance of
is seen as a cost-effective means of delivering high bit-rate       ATM-based VoD over an ADSL link, a basic network
content to residential subscribers. ADSL is an overlay network      architecture is used, comprised of only those elements
that leverages existing local-loop copper infrastructure, and       necessary for delivering the video stream. This architecture is
employs advanced digital signal processing to allow high bit-       shown in Figure 1.
rate 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
                                                                     ATM / ADSL                                       Video-on-
originally designed for streaming video [5], as indicated by the     End-Systems
                                                                                    DSLAM          ATM Network
                                                                                                                    Demand Server
error-reduction techniques employed, low latencies offered,
and relatively high downstream data rates offered. In addition,              Figure 1: Simplified ATM/ADSL Network
the use of ATM as the Layer 2 technology for ADSL further
suggests that this technology is highly suited for an application   From Figure 1 it can be seen that the ADSL network closely
with strict network resource requirements such as Video-on-         resembles a conventional ATM network, with the addition of
Demand (VoD).                                                       an ADSL link in the access leg. This link serves as a physical
                                                                    medium for transporting the ATM traffic to the end-user.
This paper explores the factors affecting the delivery of video     However, it should be noted that ADSL was designed as a
content across ADSL links, and proposes and evaluates               transparent technology, and that the ADSL link transparently
solutions for optimizing such a content delivery system, in         bridges the information between the end-user's 25.6Mbps
terms of network efficiency as well as received video quality.      ATM link, via the ADSL line, and the backhaul link from the
Section II of this paper presents a simplified ADSL network         DSLAM, which may be an STM-1 connection. This means,
architecture containing only those elements necessary for           amongst other things, that the ATM network does not know
delivering video content. From this model the factors affecting     the ADSL resources, and that the ADSL equipment does not
streaming video delivery are identified. These factors may be       participate in connection admission control (CAC). Figure 2
addressed either at the network- or application-level, as           illustrates the Q.2931[7] signaling for a virtual circuit being
created from the end-system, through the ADSL network, to                                         line through a direct connection to the ATU-R. The end-
the video server. It can be seen that the ADSL equipment does                                     system would then be able to integrate this information
not participate at any point, and that the ADSL link's resources                                  into its ATM stack for more reliable resource
are not considered at any point in the CAC process. ATM                                           management. This option may not always be available, as
Quality-of-Service (QoS) can therefore not be guaranteed                                          it is heavily dependent on whether the ATU-R supports
when operating across an ADSL network. Section III of this                                        such access to its management information. Current ATU-
paper presents methods of addressing this shortcoming at the                                      Rs also have various ways of providing this access, so the
network level, to ensure ATM QoS in an ADSL network.                                              implementation would be tied to a specific set of ATU-
 ADSL End-        ADSL            ATM                       ATM                      VoD
  System         Access
                                 Switch A                  Switch B                 Server
                                                                                             2.   Querying the DSLAM via in-band management.
                                                                                                  The end-system may be able to access the management
                                      SETUP                                                       information stored on the DSLAM, and use this
                                                                  SETUP                           information to update its own ATM stack. Disadvantages
                                                              CALL                                of this approach are that it compromises security by
                                                                                                  allowing access to the ADSL network management
                                                                                                  system, and that the end-system needs detailed
                                                                      CONNECT ACK
                                                                                                  information about which line's details to query, e.g. shelf,
                                                                                                  slot and port number.
                                             CONNECT ACK                                     3.   Querying the DSLAM via out-of-band management.
                                                                                                  This method is similar to that described above, with the
         CONNECT ACK                                                                              main difference being the use of out-of-band
                                                                                                  management. This method uses IP-based SNMP to query
     Figure 2: Q.2931 Signalling for successful SVC setup                                         the DSLAM. In addition to the disadvantages for the in-
                                                                                                  band method, this method also needs the IP networking
It should also be noted that an ADSL network imposes stricter                                     stack to be initialized before the ATM resource
bandwidth constraints than other fixed-line ATM networks,                                         management can be fully initialized. Fortunately, IP is a
particularly for bursty traffic such as compressed video.                                         best-effort service, and therefore typically uses a UBR
Research has addressed the requirements of delivering VoD in                                      class-of-service across an ADSL link. This means that the
ATM networks[8,9,10,11,12,13,14]. However, much of this research                                  end-system's ATM resource management stack may be
focuses on achieving statistical multiplexing gains. This                                         passed an updated set of line parameters without
research is not applicable to ADSL, as the limited bandwidth                                      negatively affecting the performance of the existing IP
available on an ADSL link (up to 8Mbps downstream) does                                           connectivity.
not allow for multiplexing video streams. Instead, the traffic
stream should be shaped to minimize the peak bandwidth                                       The primary advantage of using a distributed solution is that it
required. Ideally this shaping should be done at the application                             may be implemented in software, by extending the ATM
level, as this will allow more intelligent shaping based on the                              networking stack at the end-system. This type of solution is
characteristics and requirements of the traffic. Section IV                                  suited to existing networks, as it does not require costly
addresses this problem by presenting and evaluating lossless                                 hardware replacement or upgrades. Software implementations
shaping techniques that impose minimal additional                                            may also be developed and deployed more quickly than
computational load on both the server and the client systems.                                hardware-based solutions.

III. NETWORK-LEVEL RESOURCE MANAGEMENT                                                       B. Centralized Solution
As discussed above, current ADSL equipment does not                                          In this solution, the DSLAM is extended to contain more
participate in resource management activities such as CAC,                                   complete ATM switching functionality than current systems,
making it impossible to guarantee ATM QoS. This problem                                      effectively evolving into an ATM switch. The DSLAM now
may be overcome by including the ADSL resources in the                                       uses the information about the specific ADSL line, in
CAC already being performed at one of the participating ATM                                  conjunction with information about the backhaul ATM link,
nodes, using either a distributed or a centralized solution.                                 when creating new virtual connections. The DSLAM therefore
                                                                                             acts as a proxy for each end-system, assisting with the CAC
A. Distributed Solution                                                                      process. An additional advantage is that, as an ATM node, the
In this solution, the ATM resources of the ADSL line are                                     DSLAM would also be able to form an active part of an ATM
managed by the end-system's ATM stack. The end-system                                        multicast tree, improving the efficiency of multicasting to
determines the available resources on the line in one of three                               ADSL end-users. While this solution offers the advantage of
ways, listed below in decreasing order of preference:                                        resource management at a single, central point, it comes at the
                                                                                             cost of increased hardware complexity in the DSLAM. This
1.   Querying the ATU-R.                                                                     solution would therefore be more suited to new networks than
     Ideally the end-system would be able to access                                          existing installations.
     information about the available resources on the ADSL
In this research, the solution presented in Section III.A.3 has     A. Short-Term Shaping
been implemented and evaluated. Details about this are              When studying an MPEG-1 stream, two things are noticed.
provided in Section V.                                              Firstly, the I-frames are typically largest and therefore
                                                                    contribute most to the peak bandwidth requirement of the
IV. APPLICATION-LEVEL TRAFFIC SHAPING                               traffic stream. Secondly, the I-frames are always preceded by
Applications operating across limited capacity media such as        B-frames, which are smallest. It should therefore be possible
ADSL should minimize their network resource requirements,           to reduce the peak requirement of the stream by buffering and
specifically their peak bandwidth requirement. This should          transmitting the average of an I-frame and its preceding B-
ideally be done at the application level, as the application        frame during their original transmission periods. The sequence
possesses detailed information about the traffic stream, and        now becomes …B(BI)'(BI)'BBPBBPBBPBBPB(BI)'(BI)'B…,
can therefore perform more intelligent traffic shaping than the     where (BI)' refers to the averaging of the I- and preceding B-
network. MPEG-1 compressed video, for example, has a                frame. Figure 4 shows the effect of applying this technique on
repetitive structure, as shown in Figure 3.                         the sequence in Figure 3. The figure shows a noticeable
                                                                    reduction in the peak bandwidth requirement, from 25610
25000                                                               bytes per frame (in the GoP in Figure 3) to 16090 bytes per
                                                                    frame (in the smoothed GoP in Figure 4). Specific details on
20000                                                               improvements obtained with this technique are provided in
                                                                    Section V.


10000                                                               14,000



   0                                                                8,000

        I   B   B   P   B   B   P   B   B   P   B   B   P   B   B   6,000

  Figure 3: Relative frame sizes and sequence for MPEG-1            4,000


As can be seen, the stream consists of three frame types (I-, P-,     -
and B-frames), with a regularly repeating sequence, arranged
                                                                             (BI)' B   B   P   B   B   P   B   B   P   B   B   P   B (BI)'
to form Groups-of-Pictures (GoPs). This sequence is described
in terms of M and N parameters, referring to the spacing                     Figure 4: Reduced peak bandwidth after short-term
between consecutive I-frames, and I- and P-frames                                               smoothing
respectively,            e.g.           the            sequence
…PBBIBBPBBPBBPBBPBBIBBP… etc would be described
                                                                    B. Medium-Term Shaping
as M=15, N=3. These frames can also be characterized relative
to each other in terms of size. I-frames are typically largest,     Further investigation of the GoP used in Figures 3 and 4
followed by P-frames, with the B-frames being smallest.             suggest that further reductions in peak requirements may be
Example ratios for I:P:B frame-sizes are 6:2:1 (for low-motion      obtained by using the remaining B-frames to reduce the peaks
streams) or 4:4:1 (for higher-motion streams). The average bit-     due to the large I-frames. In addition, the peaks due to the
rate for an MPEG-1 compressed stream is also known.                 medium-sized P-frames may be reduced in the same manner.
Together, these parameters may be used to describe an               This method effectively buffers the entire GoP and transmits
MPEG-1 stream. A video server may use the information               at the much lower average rate of the GoP. Figure 5 illustrates
provided by these parameters to shape the traffic stream            the reduced peak requirement of the smoothed GoP. Note that
                                                                    Figure 5 spans 3 GoPs, to show the smoothing per GoP. Note
As stated previously, an advantage of performing traffic            also that "GoP A" in the figure refers to the GoP used in
shaping at the application level is that the server can use a       Figures 3 and 4.
priori information (e.g. frame-type sequence, relative frame
sizes) to perform intelligent shaping. While other researchers      A potential disadvantage of using this form of traffic shaping
use lossy shaping techniques[15], in this research we have          is that delays are required for the buffering of the stream prior
elected to use only lossless shaping techniques, thereby            to transmission. During the implementation and evaluation of
ensuring no degradation of video quality due to the presence        this technique, however, it was found that it is possible to
of the limited-bandwidth ADSL link.                                 employ the existing de-jittering buffers in standard MPEG-1
                                                                    servers and clients without imposing additional delays or
Based on the structure of an MPEG-1 compressed stream               computational load. Details of the specific improvements
(described above), the following three types of lossless            obtained with this technique are provided in Section V.
shaping techniques may be used, in increasing order of the
expected reduction of peak requirements:
                             GoP B: Peak =7414   GoP C: Peak =7452   The test-platform was built around a Siemens XpressLink D
                                   bytes               bytes
8000                                                                 v1.0 DSLAM, connecting two ATM/ADSL clients to an
         GoP A: Peak =6491
                                                                     ATM-based MPEG-1 video server via an ATM network. A
                                                                     development version of the Linux operating system (kernel
                                                                     version 2.3.41) 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
3000                                                                 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
1000                                                                 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
       Figure 5: Reduced peak variations after medium-term           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
C. Long-Term Shaping
                                                                     link. Before enabling the improved resource management, all
Long-term shaping of compressed video traffic is discussed in        SVCs were accepted by the ATM network. Data transfers
[14], with the emphasis on renegotiating the allocated network       across these links were possible, but data losses and reduced
resources to match the requirements of the smoothed traffic.         throughput due the associated retransmissions were observed.
This method allows the reserved bandwidth to be dynamically          With the resource management mechanisms enabled, requests
tailored to match the traffic stream, in order to obtain high        for resources in excess of those available on the link were
bandwidth efficiency through the network for compressed              rejected by the network, ensuring reliable operation of
streams in general. In this research however, we focus               connections across the link.
specifically on MPEG-1 compressed video, and can therefore
simplify our long-term smoothing by using our knowledge of           Having confirmed correct operation of the underlying
the stream's structure.                                              network, the second stage of testing focussed on evaluating
                                                                     the performance of the application-level shaping mechanisms.
MPEG-1 compression works by reducing spatial and temporal
redundancy at a frame- and GoP-level. However, each GoP is           The shaping techniques described in Section VI were
effectively encoded independent of the rest of the stream, for       implemented on the video server and clients. Two source
error-resilience amongst other reasons. An advantage of this         streams were used (SeatSpin.mpg and AlienSong.mpg), as
compression method is that the average bandwidth                     samples of high-motion and low-motion streams respectively.
requirement of each GoP does not deviate from that of the            "SeatSpin.mpg" was also specifically chosen to represent a file
overall stream as much as the individual frames do. It should        having high instantaneous requirements, to determine how
therefore be possible to transmit the entire stream at the           effectively the smoothing techniques would handle large
overall average rate without imposing heavy buffering                peaks. More detailed information on these streams are
requirements as suggested in [14]. This method effectively           provided in Tables 1 and 2.
converts the original (highly bursty) variable-bit-rate (VBR)
video stream to a constant- bit-rate (CBR) for transporting
                                                                                  MPEG Movie Info (SeatSpin.mpg)
across the ADSL link, thereby minimizing the peak
                                                                                  Audio Bitrate (average)      176400
requirements of the stream. The evaluation in Section V shows                     Picture Size               352 x 288
that this technique is practically applicable, both for low- and                  Video Bitrate (Average)     1152000
high-motion streams.                                                              Frame rate                        25
                                                                                  File Size                   3897998
V. IMPLEMENTATION AND PERFORMANCE EVALUATION                                      I:P:B ratio                    4:4:1

In order to verify the operation of the resource management                Table 1: Stream information for SeatSpin.mpg
solutions described in Section III, the technique described in
Section III.A.3 was implemented and evaluated on an ADSL                          MPEG Movie Info (AlienSong.mpg)
test-bed in the Communications Research Group at the                              Audio Bitrate (average)       88200
                                                                                  Picture Size               320 x 240
University of Cape Town. This technique was chosen for                            Video Bitrate (Average)      480000
implementation due to limited time, limitations on the                            Frame rate                        24
available equipment and the lack of access to the hardware                        File Size                   3177818
specifications required for implementing the DSLAM-based                          I:P:B ratio                     6:2:1
solution. The implementation and evaluation of the remaining
                                                                          Table 2: Stream information for AlienSong.mpg
solutions are left for future research projects.
Statistical analysis of the streams used showed that the peak    two categories, namely network-level and application-level,
buffer requirements for each of the smoothing techniques         and then addressed as such. This paper ensures reliable
never exceeded the size of one GoP (for the two streams          network operation (in terms of QoS guarantees) by proposing
used). However, due to the small sample set used, it was         and implementing the network-level solutions, before
decided to dimension the buffers to accommodate up to four       addressing the application-level problems. By integrating both
(4) GoPs, with the operating target being 50% buffer             classes of solution into a system, we have shown that it is
utilization. Table 3 provides information on the buffer sizes    indeed possible to reliably deliver high quality ATM-based
(in bytes) and associated delays introduced (in seconds).        streaming video across ADSL links, without imposing
                                                                 significant computational or storage requirements on either the
                            Buffer size    Latency               server or the client systems.
                             (bytes)        (sec)
         SeatSpin.mpg           398520         1.20              VII. DIRECTIONS FOR FUTURE RESEARCH
         AlienSong.mpg          177563         1.25              The remaining models proposed in Section III should be
                                                                 implemented and evaluated relative to the model used in
    Table 3: Buffer requirements and latencies / delays          Section V. All the models should also be evaluated to
              introduced for two test streams                    determine applicability to various scenarios. Factors to be
                                                                 taken into account include network size and structure, number
The results of tests under various ADSL line conditions were     of users, traffic volumes, redundancy and reliability, and cost
as expected. SVC setup requests were correctly accepted or       implications.
rejected based on the availability of reserve capacity on the
ADSL link, with no cell losses due to congestion on the ADSL     A more detailed study of the statistical properties of MPEG-1
link. The smoothing techniques also performed as expected,       streams is also required. The implementation described in this
with the long-term shaping providing the greatest reduction in   paper makes assumptions based on descriptive statistics of the
peak requirements, down to 18% of the original peak              stream and achieves acceptable results. However, the accuracy
requirements for the "AlienSong.mpg" stream. Error detection     of the buffer size calculation becomes more critical as the size
was enabled at the video client applications, but no stream      of the network being served by the video server grows. It
errors were detected. It was originally intended to conduct a    would therefore be desirable to have an empirical formula to
subjective assessment of the delivered video stream as well,     accurately calculate the minimum required buffer sizes for an
but this was deemed unnecessary since the objective measures     arbitrary stream.
were reporting error-free operation. It was therefore accepted
that the implementation was performing as intended,              VIII.    REFERENCES
combining reliable resource management and intelligent           [1] International     Telecommunication      Union,   "ITU-T
traffic shaping to deliver high-quality video across an ADSL         Recommendation G.992.1 - Asymmetrical Digital
link. Tables 4 and 5 list the results obtained when using the        Subscriber Line (ADSL) Transceivers"
"SeatSpin.mpg" and "AlienSong.mpg" respectively.                 [2] American National Standards Institute, “ANSI T1.413
                                                                     Issue 2”
      Smoothing         PCR Relative Reduction                   [3] Hari Balakrishnan, Venkata N. Padmanabhan, "How
     None              13339    -          -                         Network Asymmetry Affects TCP", IEEE Communications
     Short-Term         9790    73%       27%                        Magazine, April 2001, pp60-67.
     Medium term        4247    32%       68%                    [4] Ryoichi Kawahara, Hiroshi Saito, "Performance of
     Long-Term          3665    27%       73%                        TCP/IP over ATM over an ADSL", IEICE Transactions on
                                                                     Communication, Volume E83-B, No.2, February 2000,
   Table 4: Reduced peak bandwidth requirements (in                  pp140-154.
         cells/sec) after smoothing SeatSpin.mpg                 [5] ADSL Forum, "ADSL Forum: Technical FAQs",
                                                                     available at
      Smoothing         PCR Relative Reduction                   [6] International Organisation for Standardisation, "MPEG-1
     None               8625    -          -                         Coding of moving pictures and associated audio for
     Short-Term         4668    54%       46%                        digital storage media at up to about 1,5 Mbit/s"
     Medium term        1739    20%       80%                    [7] International Telecommunication Union, "ITU-T
     Long-Term          1564    18%       82%                        Recommendation Q.2931 - Digital Subscriber Signalling
                                                                     System No. 2 - User-Network Interface (UNI) layer 3
   Table 5: Reduced peak bandwidth requirements (in                  specification for basic call/connection control"
        cells/sec) after smoothing AlienSong.mpg                 [8] Marwan Krunz, "Bandwidth Allocation Strategies for
                                                                     Transporting VBR Video Traffic", IEEE Communications
VI. CONCLUSIONS                                                      Magazine, January 1999, pp40-46.
                                                                 [9] Pedro Cuenca, Antonio Garrido, Fransisco Quiles, Luis
Although a number of impairments still prevent the proper            Orozco-Barbosa, “Error-Resilient Video Transmission
operation of ATM over ADSL links, it is possible to address          over ATM Networks”, IEEE Communications Magazine,
these problems. The problems may be broadly classified into          December 1999, pp106-111.
[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, pp67-74.
[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, pp38-47.
[12] Marcel Graf, "VBR Video over ATM: Reducing Network
     Resource Requirements through Endsystem Traffic
     Shaping", IEEE Infocom'97 Proceedings Volume 1, pp48-
[13] Patrick Vine, Neco Ventura, "Maximising the
     simultaneous users of a Video on Demand Server",
     SATNAC98 Proceedings, pp329-336.
[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, pp58-66.
[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-0065.000

Author's Biography: Emile Swanson
Emile Swanson completed his B.Sc(Eng) degree at the
University of Cape Town in December 1998. 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.

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