Tutorial TUT-4: Cross-layer Optimized Wireless Multimedia Transmission: Principles, Standards, Solutions and New Paradigms

Instructors

M. van der Schaar; University of California, Davis
S. Shankar; Philips

Time & Location

Friday, March 18, 14:00 - 17:00, Location: CC: Room 113-B

Abstract

Tutorial level: Basic to Advanced

Wireless Local Area Networks (LANs) are poised to enable a variety of delay-sensitive multimedia applications, such as videoconferencing, emergency services, surveillance, telemedicine, remote teaching and training, augmented reality, and entertainment because of their flexibility and low cost infrastructure. However, existing wireless networks provide only limited, time-varying Quality of Service (QoS) for the delay-sensitive, bandwidth-intense and loss-tolerant multimedia applications. To achieve a high level of acceptability and proliferation of wireless multimedia, in particular wireless video, several key requirements need to be satisfied:

1. Easy adaptability to wireless bandwidth fluctuations due to co-channel interference, multipath fading, mobility, handoff, competing traffic, etc.;
2. Robustness to partial data losses caused by the packetization of video frames and high packet error rates; and
3. Support for heterogeneous wireless clients with regard to their access bandwidths, computing capabilities, buffer availabilities, display resolutions and, importantly, power limitations.

The variability of wireless resources has considerable consequences for multimedia applications and often leads to unsatisfactory user experience due to their following characteristics:

• High bandwidths – many consumer applications, e.g. High Definition TV (HDTV), require transmission bit-rates of several Mbps;
• Very stringent delay constraints – delays of less than 200 milliseconds are required for interactive applications, such as videoconferencing, surveillance etc., while for multimedia streaming applications delays of 1-5s are tolerable. Packets that arrive after their display time are discarded at the receiver side or, at best, can be used for concealing subsequently received multimedia packets.

Fortunately, multimedia applications can cope with a certain amount of packet losses depending on the sequence characteristics and error concealment strategies available at the receiver (e.g. packet losses up to 5% or more can be tolerated at times). Consequently, unlike file transfers, real-time multimedia applications do not require a complete insulation from packet losses, but rather require the application layer to cooperate with the lower layers to select the optimal wireless transmission strategy that maximizes the multimedia performance.

In recent years, the research focus has been to adapt existing algorithms and protocols for multimedia compression and transmission to the rapidly varying and often-scarce resources of wireless networks. However, these solutions often do not provide adequate support for multimedia applications in crowded wireless networks, when the interference is high or when the stations are mobile. This is because the resource management, adaptation and protection strategies available in the lower layers of the OSI stack - Physical (PHY) layer, Medium Access Control (MAC) layer, and Network/Transport layers - are optimized without explicitly considering the specific characteristics of the multimedia applications, and conversely, multimedia compression and streaming algorithms do not consider the mechanisms provided by the lower layers for error protection, scheduling, resource management etc. This “layered” optimization leads to a simple, independent implementation, but results in suboptimal multimedia (objective and/or perceptual quality) performance. Alternatively, under adverse conditions, wireless stations need to optimally adapt their multimedia compression and transmission strategies jointly across the protocol stack in order to guarantee a predetermined quality at the receiver.

In this tutorial, we present a cross-layer framework for jointly analyzing, selecting and adapting the different strategies available at the various OSI layers in terms of multimedia quality, consumed power, and spectrum utilization. Developing such an integrated cross-layer framework is of fundamental importance, since it not only leads to improved multimedia performance over existing wireless networks, but it also provides valuable insights into the design of next generation algorithms and protocols for wireless multimedia systems. The discussed cross-layer approach does not necessarily require a re-design of existing protocols, and can be performed by selecting and jointly optimizing the application layer and the strategies available at the lower layers, such as admission control, resource management, scheduling, error protection, power control etc.

We then proceed to discuss the challenges in solving the cross-layer optimization problem, identify various classes of solutions, and illustrate how the cross-layer optimization can be performed using several examples. Different methods for cross-layer design are discussed in order to gain further insights into the principles that guide such designs and to compare the various solutions. In particular, we discuss:

• Top-down approach – The higher layers protocols optimize their parameters and the strategies at the next lower layer. This cross-layer solution has been deployed in most existing systems, wherein the APP dictates the MAC parameters and strategies, while the MAC selects the optimal PHY layer modulation scheme.
• Bottom-up approach – The lower layers try to insulate the higher layers from losses and bandwidth variations. This cross-layer solution is not optimal for multimedia transmission, due the incurred delays and unnecessary throughput reductions.
• Application-centric approach – The APP layer optimizes the lower layer parameters one at a time in a bottom-up (starting from the PHY) or top-down manner, based on its requirements. However, this approach is not always efficient, as the APP operates at slower time scales and coarser data granularities (multimedia flows or group of packets) than the lower layers (that operate on bits or packets), and hence it is not able to instantaneously adapt their performance to achieve an optimal performance.
• MAC-centric approach – In this approach the APP layer passes its traffic information and requirements to the MAC, which decides which APP layer packets/flows should be transmitted and at what QoS level. The MAC also decides the PHY layer parameters based on the available channel information. The disadvantage of this approach resides in the inability of the MAC layer to perform adaptive source-channel coding tradeoffs given the time-varying channel conditions and multimedia requirements.
• Integrated approach – In this approach, strategies are determined jointly. Unfortunately, exhaustively trying all the possible strategies and their parameters in order to choose the composite strategy leading to the best quality performance is impractical due to the associated complexity. Several new solutions for solving this problem will be discussed.

The above cross-layer approaches exhibit different advantages and drawbacks for wireless multimedia transmission, and the best solution depends on the application requirements, used protocols and algorithms at the various layers, complexity and power limitations etc. We will give several simple illustrative examples on how to perform the cross-layer optimization and highlight the improvements in multimedia quality and power consumption.

Presenter Information

Mihaela van der Schaar received her PhD degrees in electrical engineering from Eindhoven University of Technology, the Netherlands. She is currently an Assistant Professor in the Electrical and Computer Engineering Department at University of California, Davis. Between 1996 and June 2003, she was a senior member research staff at Philips Research in the Netherlands and USA, where she led a team of researchers working on scalable video coding, networking, and streaming algorithms and multimedia architectures. From January to September 2003, she was also an Adjunct Assistant Professor at Columbia University. She is also an active participant in the ISO MPEG video standardization for which she received an ISO recognition award. She is currently chairing the ad-hoc group aimed at standardizing the next-generation video compression technology entitled MPEG-21 Scalable Video Coding as well as the ad-hoc group on Interframe Wavelet Video Coding exploration activity. In the past, she was also co-chairing the ad-hoc group on Multimedia Test-bed. She has co-authored more than 100 book chapters, conference and journal papers in this field and holds 15 patents and several more pending. She has also chaired and organized numerous conference sessions in this area, was part of the technical program committee of numerous IEEE conferences, including ICIP, ICASSP, MMSP, ICME, ICC etc. She is also the General Chair of the Picture Coding Symposium 2004. She was also a guest editor of the EURASIP Special issue on multimedia over IP and wireless networks in January 2004. She gave numerous tutorials on scalable video coding and wireless multimedia transmission at numerous IEEE conferences, including IEEE ICIP, IEEE Globecom etc. She is also currently co-editing, with Dr. Phil Chou (Microsoft Research), a Multimedia Communications and Networking book. She was also elected as a Member of the Technical Committee on Multimedia Signal Processing of the IEEE Signal Processing Society and is an Associate Editor of IEEE Transactions on Multimedia and of SPIE Electronic Imaging Journal. Her current research interests include user-centric video compression, scalable video compression, cross-layer optimized wireless video transmission, multimedia transmission over peer-to-peer networks, multimedia transmission over spectrum agile wireless networks, power and architecture driven multimedia compression and streaming etc.

Sai Shankar N received his PhD degree from the department of Electrical Communication Engineering from Indian Institute of Science, Bangalore, India in the area of ATM networks. In 1998, He was awarded the German Fellowship, DAAD, in the department of mathematics, University of Kaiserslautern, Gernany to work on queueing approaches in manufacturing. In 1999, he joined Philips Research, Eindhoven, the Netherlands, where he served as Research Scientist in the department of New Media Systems and Applications. He worked on various problems involving Hybrid, Fiber, Co-axial Cable (IEEE 802.14) Networks and IP protocols and provided efficient algorithms to improve protocol efficiency. In the year 2001 he joined Philips Research USA, Briarcliff Manor, NY and is working in the area of Wireless LANs. He is an active contributor of the wireless LAN and MBOA standards and has submitted more than 20 proposals in shaping QoS related issues in the IEEE 802.11e. He was the leading contributor on proposals related to traffic specification in the 802.11e. He is also an active participant in the Ultra Wide Band (UWB) working group of IEEE 802.15.3 Multi-Band OFDM Alliance (MBOA) and is the prime inventor and author of the new MBOA MAC standard that will roll out next year for UWB. Also Sai Shankar is a participant in IEEE 802.11n (High Throughput Study Group), a new standard that will define the wireless MAC that provides high throughput. He was reviewer for ICICS 2001, Globecom 2001-2004, WCNC 2002-2004, ICME 2003, Infocom 2003-2004, VTC 2003-2004, special issue on WLANs in ACM Mobile Networks journal, IEEE Transactions on Multimedia and ACM Wireless Networks. He is the Chair pof poster session at IEEE INFOCOM, publicity coordinator of MC2R journal, Sponsorship Chair of IEEE/ACM Broadnets 2005 and Industry Liaison Chair of IEEE WICON 2005. Apart from that he is a TPC member of IEEE Globecom 2005, IEEE Globecom 2004 organizing a workshop on mobile applications and also TPC member of workshop on Wireless LAN hotspots organized in conjunction with ACM Mobicom 2004. He has recently edited a book on “Recent Trends in WLANs” that was published by Wiley Interscience in Dec. 2004. He was the session chair of Networks performance evaluation session in IEEE ICICS 2001, Wireless Networks session, Chair and TPC member of 2nd New York Metro Area Workshop and the TPC member of 3rd New York Metro Area workshop. He also organized a workshop on Broadband wireless Applications and services (Broadwise) in California in conjunction with Broadnets 2004. Besides these activities he is IEEE Standards Association Member as well as Senior Member of IEEE. He has authored more than 35 conference and journal papers, 20 accepted standard contributions and holds more than 30 patents.