Yeonchul Shin

Wi-Fi could be much more

Wi-Fi has become an essential wireless technology in our daily lives, although the original intention of its introduction was to replace Ethernet cable. In this article, we outline the most remarkable features introduced during its ongoing technological evolution in terms of three major directions: throughput enhancement, longrange extension, and greater ease of use. By stitching these advanced features together, we also envision a promising future that Wi-Fi technology will bring us in terms of spectrum heterogeneity, seamless service provisioning, and possible relations with cellular networks.

Video multicast over WLANs: Power saving and reliability perspectives

Video streaming service via multicast is one promising technology for multimedia services over wireless local area networks. Given that many portable devices are battery-powered while video quality relies on the reliable delivery of video traffic, video multicast frames need to be reliably delivered to power-saving stations. In this article, we study video multicast from the power saving and reliability perspectives. The IEEE 802.11 standard allows multicast receivers to enable power management while receiving multicast frames. However, from measurements with several commercial WLAN devices, we observe that many devices are not standards- compliant, thus severely degrading video multicast performance when multicast receivers are in the power saving mode. We identify various standards-non-compliant malfunctions of both access points and stations, and then categorize these malfunctions. Meanwhile, the legacy 802.11 standard is known to support only unreliable multicast service due to the lack of retransmissions. We present various reliable multicast schemes including application layer forward error correction and reliable multicast protocols defined in the emerging IEEE 802.11v and 802.11aa, and comparatively evaluate them via simulations.

Empirical analysis of video multicast over WiFi

Video multicast service is becoming one of the most important applications over WiFi, due to the increasing popularity of WiFi for multimedia communication. On the one hand, energy efficient operation is required to WiFi technology due to the limited battery power of most WiFi-equipped devices. In this paper, we empirically study the video multicast operation, especially, along with power management operation in two aspects: (1) whether commercial WiFi devices correctly operate as defined in IEEE 802.11 standard and (2) what problem the standard-compliant operation can induce. From our experimental results, we first figure out that some of commercial WiFi devices do not follow the standard with respect to the power saving operation, and this noncompliance worsens interoperability. We also find that the standard-compliant operation may cause significant delay of voice over IP (VoIP) traffic, when video multicast coexists with VoIP. Through this experimental study, we provide the guidelines for energy-efficient video multicast service.

Toward realistic WiFi simulation with smartphone “Physics”

Various packet-based simulation tools (e.g., NS-3) have been employed for design, validation, and evaluation of new protocols for WiFi networks since they offer cost efficiency, scalability, and reproducibility. These benefits come, however, at the expense of lack of realism compared to live testbed experiments. This is attributed in a major part to the difficulty of capturing detailed characteristics of channel dynamics, bit-level protocol specification (PHY layer), and application/user behaviors in a high-fidelity manner. The performance gap predicted by simulation and live testbed becomes even more pronounced when one considers a wide diversity of device characteristics and the way each device is used by end users. For example, smartphones generally show worse WiFi performance than other WiFi devices (e.g., laptops and tablets) because smartphones suffer from additional signal loss due to hand-grips and the low antenna gains of their embedded antennas. The goal of this study is to significantly close the gap by incorporating survey- and measurement-based smartphone WiFi characteristics and realistic hand-grip models into traditional WiFi network simulators (NS-3 in this study). The enhanced WiFi simulation tools performance prediction capability is validated through an comparative study between testbed experiments and simulations.

Scalable video multicast using flexible multicast service over IEEE 802.11 WLAN

In video multicast, it is a challenging task to deliver video at a satisfactory level to multiple receivers with different channel quality and different battery capacities. In order to resolve such receiver heterogeneity, we propose a video multicast system that integrates scalable video coding (SVC) and flexible multicast service (FMS). The system is designed to transmit the base layer (BL) and the enhancement layer (EL) of an SVC-encoded video at different delivery traffic indication maps (DTIMs) such that each receiver can freely select either energy-intensive high quality video or energy-saving baseline quality video. Moreover, we enhance reliability by adding a functionality of intra-flow network coding (NC). As a measure of user satisfaction, we develop a multi-attribute utility model, and then, based on the utility model, we develop a Stakelberg game-based optimization framework to determine modulation and coding scheme (MCS) and the number of NC-generated packets for each layer. Through simulations, we confirm that the proposed scheme achieves higher user satisfaction by exhibiting performance gain of 16.4% to 47% over the legacy 802.11 scheme.

A novel metric for Dynamic Channel Assignment in enterprise wireless networks

In enterprise wireless networks where Access Points (APs) are densely deployed, the importance of Dynamic Channel Assignment (DCA) is more emphasized since the network is under severe inter-cell interference. In this paper, we propose Contention Factor (CF) as a metric for DCA in enterprise wireless networks. The metric is designed to minimize the performance degradation by separating collision domains of APs. Simulation results demonstrate that the proposed metric precisely predicts the network performance so that DCA utilizing the proposed metric improves the overall network throughput.

InFRA: Interference-Aware PHY/FEC Rate Adaptation for Video Multicast over WLAN

Multi-rate forward erasure correction (FEC)-applied wireless multicast enables reliable and efficient video multicast with intelligent selection of physical (PHY) layer data rate and FEC rate. The optimal PHY/FEC rates depend on the cause of the packet losses. However, previous approaches select the PHY/FEC rates by considering only channel errors even when interference is also a major source of packet losses. We propose InFRA, an interference-aware PHY/FEC rate adaptation framework that (i) infers the cause of the packet losses based on received signal strength indicator (RSSI) and cyclic redundancy check (CRC) error notifications, and (ii) determines the PHY/FEC rates based on the cause of packet losses. Our prototype implementation with off-the-shelf chipsets demonstrates that InFRA enhances the multicast delivery under various network scenarios. InFRA enables 2.3x and 1.8x more nodes to achieve a target video packet loss rate with a contention interferer and a hidden interferer, respectively, compared with the state-of- the-art PHY/FEC rate adaptation scheme. To our best knowledge, InFRA is the first work to take the impact of interference into account for the PHY/FEC rate adaptation.

ACT-AP: ACTivator access point for multicast over WLAN

Multicast is a major solution in supporting the explosive growth of the wireless video traffic demand. Also, power saving is a key technology to extend battery life of mobile devices. To meet both purposes, IEEE 802.11 wireless local area network (WLAN) supports power save mode (PSM) for station (STA) while receiving multicast packets. According to recent studies, off-the-shelf chipsets configured to use PSM show un-desired functions, thus resulting in many multicast packet losses. From our extensive measurement, we also verify degradation of multicast performance with widely-used off-the-shelf chipsets using PSM, and present previously-unknown undesired functions. Without modification of the chipsets, STA in PSM cannot enjoy reliable multicast service. Given this, we develop a practical and readily-applicable AP-side solution, called ACT-AP, which avoids multicast packet losses by preventing STA from operating in PSM. Our prototype implementation with off-the-shelf chipsets demonstrates that ACT-AP improves packet delivery ratio by up to 216% with little additional protocol overhead. To our best knowledge, ACT-AP is the first practical effort to support multicast to real devices with undesired functions in PSM.