Wan-Seon Lim

Implementation and performance study of IEEE 802.21 in integrated IEEE 802.11/802.16e networks

In this paper, we propose a framework for the implementation of the IEEE 802.21 Media Independent Handover (MIH) standard and evaluate its performance through experiments in integrated 802.11/802.16e networks. The IEEE 802.21 standard defines three types of MIH services (event, command, and information) that facilitate the mobility management and handover process in heterogeneous networks. To support MIH services, we develop a MIH-capable mobile node and the MIH information service server. Then, we introduce the Connection Manager (CM) which utilizes MIH services. Two main roles of CM are supporting seamless vertical handovers and efficient access point (AP) discoveries. From the experimental results in the real test-bed, we can know that the MIH services can be used to reduce packet losses during a vertical handover and to reduce the AP discovery time and energy consumption of mobile nodes.

Design of Efficient Multicast Protocol for IEEE 802.11n WLANs and Cross-Layer Optimization for Scalable Video Streaming

The legacy multicasting over IEEE 802.11-based WLANs has two well-known problems-poor reliability and low-rate transmission. In the literature, various WLAN multicast protocols have been proposed in order to overcome these problems. Existing multicast protocols, however, are not so efficient when they are used combining with the frame aggregation scheme of IEEE 802.11n. In this paper, we propose a novel MAC-level multicast protocol for IEEE 802.11n, named Reliable and Efficient Multicast Protocol (REMP). To enhance the reliability and efficiency of multicast services in IEEE 802.11n WLANs, REMP enables selective retransmissions for erroneous multicast frames and efficient adjustments of the modulation and coding scheme (MCS). In addition, we propose an extension of REMP, named scalable REMP (S-REMP), for efficient delivery of scalable video over IEEE 802.11n WLANs. In S-REMP, different MCSs are assigned to different layers of scalable video to guarantee the minimal video quality to all users while providing a higher video quality to users exhibiting better channel conditions. Our simulation results show that REMP outperforms existing multicast protocols for normal multicast traffic and S-REMP offers improved performance for scalable video streaming.

PR-MAC: A practical approach for exploiting relay transmissions in multi-rate WLANs

In this paper, we propose a new MAC protocol, called proxy relay-enabled MAC (PR-MAC), to improve the capacity of multi-rate WLANs. PR-MAC introduces a new entity called proxy relay point (PRP), which serves as a relay between the AP and stations. The cooperation of the AP with PRP replaces direct transmissions for low-rate stations with fast two-hop transmissions while the stations think that they communicate directly with the AP. Our experiment and simulation results showed that PR-MAC can significantly improve the throughput of legacy stations without any modification on the MAC operation of the stations.

Achieving Fairness Between Uplink and Downlink Flows in Error-Prone WLANs

This letter considers an unfairness problem between uplink and downlink in 802.11 WLANs. One of existing solutions for this problem is giving a larger transmission opportunity (TXOP) limit to an AP than stations. This TXOP differentiation scheme, however, does not work well in error-prone environments since a packet bursting during a TXOP duration is terminated when a data transmission fails due to channel errors. To overcome this, we propose a new scheme in which an AP controls its minimum contention window size and TXOP limit dynamically according to the packet error rate and the number of stations. Our simulation results show that the proposed scheme provides fair channel accesses between uplink and downlink in error-prone 802.11 WLANs.

A Robust and Cooperative MAC Protocol for IEEE 802.11a Wireless Networks

In wireless networks, it is well known that the interference of hidden nodes can interrupt frame receptions. Although several solutions have been proposed to alleviate the problem of DATA corruptions at receivers, control frame corruptions at transmitters have not been considered yet. In this paper, we propose an enhanced MAC protocol, called Robust and Cooperative Medium Access Control (RCMAC), to improve the network throughput and fairness by reducing control frame losses at transmitters. RCMAC uses a relay mechanism to allow transmitters of long distance links to receive control frames more robustly by relaying control frames via relay nodes. Furthermore, RCMAC improves the network throughput through fast two-hop DATA transmissions via relay nodes. Our extensive simulation results show that RCMAC has better performance than existing well-known MAC protocols.

An adaptive collision resolution scheme for energy efficient communication in IEEE 802.15.4 networks

IEEE 802.15.4 provides a widely accepted solution for low-cost and low-power wireless communications, and it is known to be applicable to many types of WSN application scenarios. To enhance energy efficiency and throughput of sensor nodes, many CSMA/CA schemes based on IEEE 802.15.4 have been proposed. However, they still suffer from unnecessary waste of energy and bandwidth due to inefficient backoff management. In order to overcome these problems, we propose an enhancement to existing schemes, called adaptive collision resolution (ACR). ACR appropriately adapts backoff exponent (BE) to the current network contention level based on physical and link-layer measurements. In addition, it adjusts backoff periods (BP) to avoid meaningless backoffs and additional clear channel assessments (CCAs) by using the estimated time remaining until the channel becomes idle. To verify the feasibility and applicability of our scheme, we studied mathematical analysis and tested it on the Matlab and USRP/GNU Radio testbed. We also performed simulation study using OPNET, and our simulation results indicate that ACR shows improved energy efficiency and throughput performance over existing schemes.

A link contact duration-based routing protocol in delay-tolerant networks

Delay Tolerant Networks (DTNs) provide message delivery services to users via intermittently connected nodes. In DTNs, routing is one of the most challenging issues since end-to-end connectivity between nodes may not be available most of the time. Although many routing protocols for DTNs have been proposed, they do not achieve satisfactory performance, since they exploit only some of the network characteristics. In this paper, we present a new DTN routing protocol, called the Link Contact Duration-based Routing Protocol (LCD). Like existing protocols, LCD uses the disconnect duration of a link between two nodes to find the routing path with the shortest end-to-end delay. In addition, LCD uses the contact duration of a link and the number of buffered messages to deliver as many messages as possible in a short time. Our simulation results show that LCD has better performance than existing DTN routing protocols.

Efficient WLAN Discovery Schemes Based on IEEE 802.21 MIH Services in Heterogeneous Wireless Networks

Discovering currently available radio access networks (RANs) is one of the most challenging issues in the heterogeneous wireless network environment. Especially, discovering WLANs, which support high data rates but have limited service coverage, has a significant effect on the energy consumption of multi-mode terminals (MMTs). In this paper, we propose three WLAN discovery schemes which are based on the upcoming IEEE 802.21 standards. In the proposed schemes, a MMT exploits the information on neighboring WLANs from a MIH information server to discover available WLANs as soon as possible while minimizing energy consumption. Our simulation results show that the proposed schemes can enhance the performance of MMTs in terms of energy consumption and the detection time of WLANs compared to conventional WLAN discovery schemes.

Achieving per-station fairness in IEEE 802.11 wireless LANs

In this paper, we address the issue of per-station fairness in IEEE 802.11-based wireless local area networks (WLANs). Although the IEEE 802.11 Distributed Coordination Function (DCF) has been designed to provide a fair channel access for all competing stations, it does not guarantee fairness between uplink and downlink flows in infrastructure mode WLANs. In the literature, several solutions have been proposed, where they give a higher channel access priority to an access point (AP) in order to avoid the starvation of downlink flows. However, we found that existing solutions cannot guarantee per-station fairness when uplink and downlink flows coexist in a station. In this paper, we propose a new MAC protocol for IEEE 802.11 WLANs, which aims at providing a fair medium access for all stations regardless of the direction of flows. In the proposed protocol, stations are classified into four states according to their traffic patterns, and they access the medium in different ways according to their states. The proposed protocol is fully backward compatible with the IEEE 802.11 standard and it does not include any complex scheduling algorithm. We evaluated the performance of the proposed protocol via extensive simulations and experiments in a real test-bed. Compared to existing work, the proposed protocol shows improved fairness performance in resource sharing among WLAN stations.

Throughput Enhancement for WDS-Based WLANs

A wireless distribution system (WDS)-based wireless local area network (WLAN) is composed of a base access point (AP) which is connected to wired line and one or more linked APs which are connected to the base AP or other linked APs via wireless links. Usually, WLAN stations try to associate with an AP from which the stations receive the strongest signal strength. However, in WDS-based WLANs, the base AP may provide higher throughput than linked APs although the signal strength of base AP is weaker than those of linked APs. In this paper, we propose a new medium access scheme in which each station uses an AP that can provide the highest throughput to improve the capacity of WDS-based WLANs. To implement the proposed scheme, no modification is required to stations, while some modifications are required to APs. Our experiment and simulation results show that the proposed scheme achieves improved throughput performance over existing schemes.