Seongkwan Kim

A high-throughput MAC strategy for next-generation WLANs

Wireless LAN technology has been shown to he a revolutionary development during the last decade. Recently popularized IEEE 802.11a/g-based products can support up to 54 Mb/s physical layer rate and provide wireless access to the Internet. However, in order to deal robustly with the unreliable wireless nature, the 802.11 medium access control protocol has a relatively large overhead and hence, the throughput performance is much worse than the underlying physical layer rate. Moreover, along with many emerging applications and services over WLANs, such as voice over WLAN and audio/video streaming, the demand lor faster and higher- capacity WLANs has been growing recently. In this article, we propose a new medium access control protocol for the next-generation high-speed WLANs. The proposed medium access control, called multi-user polling controlled channel access, is composed of two components: multi-layer frame aggregation, which performs aggregation at both the medium access control and the physical layers; and multi-user polling, used to reduce the contention overhead and in turn, achieve higher network utilization. Multi-user polling controlled channel access is compared with the 802.11e-enhanced distributed channel access medium access control. Highly enhanced medium access control efficiency can be achieved by applying multi-user polling controlled channel access. We show the improved medium access control performance in terms of the aggregate throughput of non-QoS Hows with relevant QoS requirements.

Available bandwidth-based association in IEEE 802.11 Wireless LANs

The performance of an IEEE 802.11 station heavily depends on the selection of an AP (Access Point) that the station is associated with to access the Internet. The conventional approach to the AP selection is based on the received signal strength called RSSI (Received Signal Strength Indication) from APs within the transmission range. This approach however, might yield unbalanced traffic load among APs as the station chooses an AP only based on the signal strength, instead of considering the AP load and the level of contention on medium access. Accordingly, the station that is associated with the highest-RSSI AP might suffer from poor network performance. In this paper, we propose a new association metric, EVA (Estimated aVailable bAndwidth) with which a station can find the AP that provides the maximum achievable throughput among scanned APs. EVA is designed to estimate the available bandwidth on a channel with respect to a station that is to join a WLAN (Wireless Local Area Network). A station equipped with EVA observes a channel state in a per-slot basis, and yet does not request any external information from nearby APs or neighbor stations. Our estimation mechanism is non-intrusive, fully distributed, and independent of the infrastructure. Through simulation study, we evaluate the accuracy of the estimation and show that EVA-based association yields enhanced throughput performance compared with the legacy scheme.

Comparative analysis of link quality metrics and routing protocols for optimal route construction in wireless mesh networks

We provide a comparative analysis of various routing strategies that affect the end-to-end performance in wireless mesh networks. We first improve well-known link quality metrics and routing algorithms to enhance performance in wireless mesh environments. We then investigate the route optimality, i.e., whether the best end-to-end route with respect to a given link quality metric is established, and its impact on the network performance. Network topologies, number of concurrent flows, and interference types are varied in our evaluation and we find that a non-optimal route is often established because of the routing protocols misbehavior, inaccurate link metric design, interflow interference, and their interplay. Through extensive simulation analysis, we present insights on how to design wireless link metrics and routing algorithms to enhance the network capacity and provide reliable connectivity.

Reliable, Low Overhead Link Quality Estimation for 802.11 Wireless Mesh Networks

We propose QUEST (QUality ESTimation), a new method that accurately estimates IEEE 802.11 wireless link quality with no in-band signaling overhead. Existing link quality estimation methods either are based on hello exchanges by fixing or varying transmission rates or rely on the history (e.g., delivery ratio) of previously sent data packets in a per-rate/-neighbor manner. QUEST on the other hand, is based on a delivery ratio vs. SNR (Signal to Noise Ratio) relation, called profile, that is managed offline. QUEST estimates the target link quality in terms of delivery ratio by performing profile lookup for any incoming messages including broadcast hello, beacon, data packets, etc. Therefore, it does not depend on a designated protocol to obtain the delivery ratio. Instead, in QUEST, the per-rate/-neighbor management of link quality is achieved by profile lookup. We perform testbed experiments to achieve the profile and also unravel two major bugs in MadWifi driver, widely employed by many researchers to build an 802.11-based system. Utilizing the large database of transmitter and receiver traces with an indigenously developed tool, we study the impact of altering the averaging time period on the profile for different transmission rates.

Collision-Aware Rate Adaptation in multi-rate WLANs: Design and implementation

Many rate adaptation algorithms have been proposed for IEEE 802.11 Wireless LAN devices and most of them operate in an open-loop manner, i.e., the transmitter adapts its transmission rate without using the feedback from the receiver. A key problem with such transmitter-based rate adaptation schemes is that they do not consider the collision effect. Accordingly, they often result in severe throughput degradation when many transmission failures are due to frame collisions. In this paper, we present a transmitter-based rate adaptation scheme, called CARA (Collision-Aware Rate Adaptation), and its MadWifi-based implementation. The key idea of CARA is that the transmitter combines adaptively the RTS/CTS (Request-to-Send/Clear-to-Send) exchange with the CCA (Clear Channel Assessment) functionality in order to differentiate frame collisions from transmission failures due to channel errors. The effectiveness of CARA schemes is evaluated via extensive ns-2 simulations and testbed experimentations.

The impact of IEEE 802.11 MAC strategies on multi-hop wireless mesh networks

This paper analyzes the impact of different MAC (medium access control) and transmission rate adaptation schemes on wireless mesh networks. The considered protocols include three different MAC protocols specified in IEEE 802.11 standards, i.e., 802.11, 802.11e, and 802.11n, and three rate adaptation schemes, i.e., ARF (automatic rate fallback), RBAR (receiver-based auto rate), and 802.11n rate adaptation. We also study the interactions of these MAC strategies with the state-of-the-art routing metric ETT (expected transmission time). Through comparative simulation evaluations, we investigate the effectiveness of these protocols when they coexist on both single-hop and multi-hop wireless mesh network environments. As these MAC strategies are designed for single-hop WLANs, we observed their limitations on multi-hop wireless mesh networks. We analyze their performances and suggest solutions for improvements. Based on our simulation results, we also argue for the need of a new routing metric that takes advantage of the new emerging MAC features.

MCCA: A high-throughput MAC strategy for next-generation WLANs

Today, IEEE 802.11 wireless LAN (WLAN) has emerged as a prevailing technology for broadband wireless networking. Along with many emerging applications and services, the demands for faster and higher-capacity WLANs have been growing fast. We propose a new medium access control (MAC) scheme for the next-generation high-speed WLANs, such as IEEE 802.11n. The proposed MAC, called multiuser polling controlled channel access (MCCA), is composed of two sub-schemes. The first is multiuser polling, in order to achieve higher network utilization. The second is a frame aggregation scheme, which performs aggregations at both MAC and physical (PHY) layers, and can achieve an even higher throughput gain as a result. From simulations, we confirm that the MCCA scheme enhances the aggregate throughput of non-quality-of-service (non-QoS) traffic by an order of magnitude from 17.4 Mbps to 129.9 Mbps, while the aggregate throughput and QoS requirements continue to be satisfied.

An Empirical Measurements-based Analysis of Public WLAN Handoff Operations

Recently, IEEE 802.11 wireless LAN (WLAN) has emerged as a widely accepted technology for broadband wireless networking. A WLAN handoff operation, which occurs when a wireless station (STA) changes its association from one access point (AP) to another, has not been well understood so far since the mobility has not been a major concern for typical WLAN environments. However, the mobility within WLAN is becoming more and more important as the portable WLAN devices, e.g., voice over WLAN (VoWLAN) devices, emerge today. In this paper, we introduce an empirical approach to evaluate the 802.11 handoff process in a public WLAN service network, i.e., KT (Korea Telecom) NESPOT. We analyze the handoff delay by characterizing individual delay components in terms of protocol-oriented delays and network configuration-dependent ones. From the analysis, we identify that the user authentication-related procedure, especially, with delay terms not rooted in the employed protocols, could become a major delay in a public service WLAN, and hence should be reduced in order to achieve a faster handoff

MAC-aware routing metric for 802.11 wireless mesh networks

We develop a new wireless link quality metric, ECOT (Estimated Channel Occupancy Time) that enables a high throughput route setup in wireless mesh networks. The key feature of ECOT is being applicable to diverse mesh network environments where IEEE 802.11 MAC (Medium Access Control) variants are used. We take into account the detailed operational features of various 802.11 MAC protocols, such as 802.11 DCF (Distributed Coordination Function), 802.11e EDCA (Enhanced Distributed Channel Access) with BACK (Block Acknowledgment), and 802.11n A-MPDU (Aggregate MAC Protocol Data Unit), and derive an integrated link metric that enables finding maximum throughput end-to-end routes. Through simulations in randomized topological environments, we evaluate the performance of the proposed link metric and routing strategy to demonstrate that our proposed schemes can achieve up to 354.4% throughput gain over existing ones.

Enhanced rate adaptation schemes with collision awareness

While many existing rate adaptation schemes in IEEE 802.11 Wireless LANs result in severe throughput degradation since they do not consider the collision effect when selecting the transmission rate, CARA (Collision-Aware Rate Adaptation) [1] shows improved system performance thanks to its collision-awareness capability. In this paper, we propose two enhancements to the original CARA scheme to further improve the system performance. The first one is called CARA-RI, which extends CARAs collision-awareness capability in making rate increase decisions, while the second one, called CARA-HD, incorporates a hidden station detection mechanism. Simulation results show that the proposed schemes outperform the original CARA significantly under various randomly-generated network topologies.