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

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

Introduction 3 Evolution of different wireless technologies

Challenges and Requirements for Wireless Transmission of Multimedia 4 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

Need for Cross-Layer Optimization 5 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.

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

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

Functionalities Provided by 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)

Distributed Coordination Function (DCF) 9 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 DIFS SIFS Contention window Next frame Defer access Wait for reattempt time Time Busy medium

Enhanced Distributed Channel Access 10 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

Point Coordination Function (PCF) 11 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) DIFS PIFS SIFS Contention window Next frame Defer access Wait for reattempt time Time Busy medium

Example of Cross-Layer Impact on Throughput, Efficiency, and Delay for Video Streaming 12 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

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

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

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

Throughput Efficiency 16 (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:

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

Summary 18 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.

References 19 Schaar and Chou (editors), Multimedia over IP and Wireless Networks: Compression, Networking, and Systems, Elsevier, Solomon_error_correction Solomon_error_correction us/library/cc757419%28WS.10%29.aspx us/library/cc757419%28WS.10%29.aspx