Cross-Layer Wireless Multimedia

1. Cross-Layer Wireless Multimedia Presented by Scott Kristjanson CMPT-820 Multimedia Systems Instructor: Dr. Mohamed Hefeeda

2. Outline • Challenges and requirements for wireless transmission of multimedia • Need for cross-layer optimization • Short summary of802.11 wireless LAN standard and impact on wireless multimedia • Example of cross-layer impact on throughput efficiency

3. Introduction • Evolution of different wireless technologies

4. Challenges and Requirements for Wireless Transmission of Multimedia • Wireless Networks Exhibit a large Variation in Channel Conditions • Noise, Mobility, Multipath fading, Cochannel interference, Handoff, … • variability of wireless resources leads to unsatisfactory user experience • High bandwidths • transmission bit rates of several Mbps. • High-definition TV • Very stringent delay constraints: • delays of less than 200 ms are for interactive applications • delays of 1–5 s for multimedia streaming applications • Quality of Service (QoS) issues becomes essential

5. Need for Cross-Layer Optimization • Normal Design • Multimedia compression and streaming algorithms do not consider the mechanisms provided by the lower layers • Resource management, adaptation, and protection strategies available in the lower layers of the OSI optimized without explicitly considering the specific characteristics of the multimedia applications • Simpler implementation, but local optimization of all layers may not lead to global optimization.

6. 802.11 wireless LAN standard • Wireless version of Ethernet • Specifications for the physical layer and the media access control (MAC) layer

7. Functionalities Provided by 802.11 • PHY layer: • Several modulation and coding schemes • to adapt to changing channel conditions, varying code rates can be employed • 802.11a PHY provides eight different PHY modes with different modulation schemes and code rates

8. Functionalities Provided by 802.11 • MAC layer: • Control of access to the shared wireless medium • Access to the shared wireless medium is paramount For transmitting delay-sensitive multimedia • Mechanisms • Distributed coordination function (DCF) • Point coordination function (PCF) • Enhanced Distributed Channel Access (EDCA)

9. Contention window DIFS DIFS SIFS Busy medium Next frame Time Wait for reattempt time Defer access Distributed Coordination Function (DCF) • DCF provides basic access service • Best-effort data transfer • All stations contend for access to medium and relinquishe control after transmitting a single packet • CSMA-CA • Ready stations wait for completion of transmission • All stations must wait Interframe Space (IFS) • Distributed, fair access to the wireless medium • Not appropriate when dealing with real-time multimedia applications that exhibit different delay deadlines and bandwidth requirements

10. Enhanced Distributed Channel Access • Four levels of priorities or access categories (AC) • Higher priority → shorter maximum back-off time → higher priority wins access to the medium more frequently than the lower priority • Provides (DiffServ) QoS • Nondeterministic nature → not possible to guarantee parameters such as bandwidth, jitter, and latency → not suitable for multimedia streaming

11. DIFS Contention window PIFS DIFS SIFS Busy medium Next frame Time Wait for reattempt time Defer access Point Coordination Function (PCF) • Designed to support delay-sensitive applications • Contention free access to the wireless medium - controlled by a point coordinator (PC) • based on a poll-and-response protocol: all stations are polled for a certain amount of time during a service interval → provides real-time applications a guaranteed transmission time (opportunity, no actual guarantee)

12. Example of Cross-Layer Impact on Throughput, Efficiency, and Delay for Video Streaming • Assumptions: • Polling based mode of MAC standard (PCF) • Adaptive retransmission at MAC layer • Reed-Solomon (RS) codes at application layer • Video packets size: La bytes • Packets are not fragmented in any of the lower layers • Overhead of higher layers: O bytes

13. Average Packet Transmission Duration • : The average transmission duration for a packet with an L-byte payload, given that the transmission is successful with the retransmission limit of R • : The average transmission duration for a packet with an L-byte payload, given that the transmission is successful with the retransmission limit of R

14. Average Packet Transmission Duration • Good Cycle: successful packet transmission • Tgood: average transmission duration for a good cycle • Bad Cycle: retransmission due to packet or ACK error • Tbad: average transmission duration for a good cycle • Probability of a successful transmission

15. Average Packet Transmission Duration • Average successful transmission duration: • the probability that the packet with L-byte data payload is transmitted successfully after the ith retransmission • the probability that the packet with L-byte data payload is transmitted successfully after the ith retransmission • Average unsuccessful transmission duration

16. Throughput Efficiency • (N,K)RS, decoder can correct up to N-k packet erasure. • Probability of error after RS decoding: • Where the error probability of data packet after R retransmission is: • Throughput efficiency:

17. Impact of Cross-Layer Optimization on Video Quality • Optimal packet size to maximize video quality • Given RS code, retransmission limit

18. Summary • QoS is essential for multimedia transmission over wireless networks • Local optimization of all layers may not lead to global optimization. Even poor performance when wireless resources are limited. • Cross layer design has impact on performance such as throughput efficiency.

19. References • Schaar and Chou (editors), Multimedia over IP and Wireless Networks: Compression, Networking, and Systems, Elsevier, 2007 • http://en.wikipedia.org/wiki/Reed-Solomon_error_correction • http://technet.microsoft.com/en-us/library/cc757419%28WS.10%29.aspx