Mei-Hsuan Lu

Design, implementation and evaluation of an efficient opportunistic retransmission protocol

This paper presents an efficient opportunistic retransmission protocol (PRO, Protocol for Retransmitting Opportunistically) to improve the performance of IEEE 802.11 WLANs. PRO is a link-layer protocol that allows overhearing nodes to function as relays that retransmit on behalf of the source after they learn about a failed transmission. Relays with stronger connectivity to the destination have a higher chance of delivering the packet than the source does, thereby resulting in a more efficient use of the channel. PRO has four main features. First, channel reciprocity coupled with a run-time calibration process is used to estimate the instantaneous link quality to the destination. Second, a local qualification process filters out poor relays early. Third, a distributed relay selection algorithm chooses the best set of eligible relays among all qualified relays and prioritizes them. Finally, 802.11e Enhanced Distributed Channel Access (EDCA) is leveraged to make sure high priority relays transmit with high probability. PRO is designed to coexist with legacy 802.11 stations. We have implemented PRO in the driver of a commodity wireless card. Our extensive evaluation on both a controlled testbed and in the real world shows that PRO boosts throughput in diverse wireless environments, and especially in when there is significant contention for the channels, under fading, and with user mobility.

Video Streaming Over 802.11 WLAN with Content-Aware Adaptive Retry

Robust video streaming over error-prone wireless LANs (WLANs) poses many challenges. In this paper, we propose a timestamp-based content-aware adaptive retry (CAR) mechanism for MPEG video streaming over 802.11 WLANs, where the MAC dynamically determines whether to send or discard a packet based on its retransmission deadline. The retransmission deadline is assigned to each packet according to its temporal relationship and error propagation characteristics with respect to other video packets in the same GOP. The proposed scheme avoids late packets by eliminating the impact of random backoff deference and co-channel interference with proper initial delay introduced at the receiver. Simulation results show CAR significantly improves video quality and saves channel bandwidth

A time‐based adaptive retry strategy for video streaming in 802.11 WLANs

Video streaming over time-varying, error-prone wireless LANs (WLANs) poses many challenges. One problem is that WLANs are designed without awareness of the characteristics of application data, which causes performance degradation in a noisy or congested environment. In this paper, we propose a time-based adaptive retry (TAR) mechanism for MPEG-like video streaming over 802.11 wireless networks. TAR dynamically determines whether to send or discard a packet based on its retransmission deadline instead of adopting a static retry limit uniformly over all the packets. Our approach can adapt the retry limit for each individual packet, thus providing indirect unequal error protection over different types of video frames. Analytical and simulation results show that TAR significantly improves video quality and saves channel bandwidth. We also describe a preliminary software-based implementation of TAR and use it to demonstrate the practicality of the proposed approach. Copyright

Video transmission over wireless multihop networks using opportunistic routing

Wireless multihop networks comprise of mobile or stationary stations interconnected via an ad hoc multihop path. The dynamically self-organized and selfconfigured nature of such network provides a flexible alternative in a variety of contexts where the deployment of a fixed infrastructure may be impractical. While this type of network provides new opportunities in multimedia streaming, the high rate requirement coupled with stringent time constraints poses a certain range of challenges in deploying high quality video services. This paper presents an analytical model to study the performance of multihop video streaming using opportunistic routing, an emerging technique that achieves high throughput in the face of volatile wireless links. Specifically, we use a discrete-time Markov chain to model the expected number of transmissions needed for a successful delivery in a multihop network. This model produces a closed-form expression that is later validated by the Monte-Carlo simulation. Using the proposed model, we compare opportunistic routing with the traditional routing method which considers the best predetermined multihop path. We demonstrate performance gains in throughput, latency, as well as decoded video quality from adopting opportunistic routing. The problem of out-of-sequence delivery inherently resulted from opportunistic communication is also discussed.

Time-Aware Opportunistic Relay for Video Streaming Over WLANs

Robust video streaming over time-varying, error-prone wireless LANs (WLANs) poses many challenges. In this paper, we present a time-based opportunistic relay (TOR) scheme for high-quality video streaming over WLANs. The proposed scheme exploits path diversity in relaying packets with awareness of the time constraints of video data. Specifically, relay nodes continuously overhear and opportunistically forward packets to enhance end-to-end delivery rates. To relay a packet in a time-aware manner, a relay deadline is computed for each packet and used by relay nodes to determine whether they should relay or discard the packet. Simulation results show that TOR improves video quality by up to 9.76 dB.

Using commodity hardware platform to develop and evaluate CSMA protocols

Developing and evaluating wireless protocols is challenging because it requires flexible network interface hardware, which is not readily available. In this paper, we present FlexMAC, a wireless protocol development and evaluation platform based on commodity hardware. FlexMAC targets CSMA wireless protocols and allows customization of functions such as backoff, retransmission, and packet timing. We describe our implementation of FlexMAC and quantify FlexMACs precision for 802.11b compared with commercial hardware implementations. The results show that FlexMACs performance is very close to that of hardware implementations. We also present two case studies that illustrate FlexMACs flexibility: the use of opportunistic relaying to boost throughput and an investigation of temporal and throughput fairness. We found that FlexMAC is a useful tool for conducting 802.11-style protocol research.

FlexMAC: a wireless protocol development and evaluation platform based on commodity hardware

Developing and evaluating wireless protocols is challenging because it requires flexible network interface hardware, which is not readily available. To reduce the efforts of conducting realistic wireless experiment, we have developed FlexMAC, a wireless protocol development and evaluation platform based on commodity hardware. FlexMAC targets CSMA wireless protocols and allows customization of functions such as backoff, retransmission, and packet timing. In this demo, we will show that FlexMAC is a useful tool for wireless protocol development. We will demonstrate two projects that are developed using FlexMAC, including a software-based 802.11 implementation and an opportunistic relaying protocol in 802.11.

CMUseum: A Location-aware Wireless Video Streaming System

Location-aware computing enables automatic tailoring of information and services based on the current location of a mobile user. We have designed and implemented CMUseum, a system that enables location-based video streaming applications, as well as other add-on services on top of the purposed infrastructure. Our novel design incorporates an 802.15.4 Zigbee sensor network for collecting location information as well as an 802.11 Wi-Fi network for streaming video contents. This framework is largely event-driven in order to support the real-time nature of the video streaming services it handles. We have demonstrated the viability of this system through implementing a working system in a museum tour guide scenario

Opportunistic Retransmission in WLANs

This paper presents an efficient opportunistic retransmission protocol (PRO, Protocol for Retransmitting Opportunistically) to improve the performance of IEEE 802.11 WLANs. PRO is a link-layer protocol that allows overhearing nodes to function as relays that retransmit on behalf of a source after they learn about a failed transmission. Relays with better connectivity to the destination have a higher chance of delivering the packet than the source, thereby resulting in a more efficient use of the channel. PRO has four main features. First, channel reciprocity coupled with a runtime calibration process is used to estimate the instantaneous link quality to the destination. Second, a local qualification process filters out poor relays early. Third, a distributed relay selection algorithm chooses the best set of eligible relays among all qualified relays and prioritizes them. Finally, 802.11e Enhanced Distributed Channel Access (EDCA) is leveraged to make sure high-quality relays transmit with higher probability. PRO is designed to coexist with legacy 802.11 stations. Our extensive evaluation on both a controlled testbed and in the real world shows that PRO can improve throughput in diverse wireless environments. PRO helps the most when there is significant contention for the ether, under fading, and with user mobility.

Robust wireless video streaming using hybrid spatial/temporal retransmission

Bandwidth demands and timing constraints are two major challenges for wireless video streaming applications. In this paper, we present a hybrid spatial/temporal retransmission protocol that tackles both of these challenges. To increase individual throughput as well as overall network capacity, the system uses an opportunistic retransmission protocol (PRO, Protocol for Retransmitting Opportunistically) that relies on overhearing nodes distributed in physical space to function as relays that retransmit failed packets on behalf of the source. Specifically, the best relay out of the set of nodes that currently have the copy of the packet is responsible for retransmitting (relaying) the packet. Relays with stronger connectivity to the destination have a higher chance of delivering packets successfully than the source, thereby resulting in a more efficient use of the channel. To meet timing constraints, a Time-based Adaptive Retransmission strategy (TAR) is applied by both the source and the relays. With TAR, the MAC dynamically determines whether to (re)transmit or discard a packet based on the retransmission deadline of the packet assigned by the video server. This significantly reduces the number of late packet arrivals at the receiver. Our extensive evaluation results both on a testbed and in the real world demonstrate that hybrid temporal/spatial retransmission can boost streaming performance in diverse wireless environments. The benefits are most pronounced for busy networks, under fading conditions, or for mobile users.