SuKyoung Lee

Vertical Handoff Decision Algorithms for Providing Optimized Performance in Heterogeneous Wireless Networks

There are currently a large variety of wireless access networks, including the emerging vehicular ad hoc networks (VANETs). A large variety of applications utilizing these networks will demand features such as real-time, high-availability, and even instantaneous high-bandwidth in some cases. Therefore, it is imperative for network service providers to make the best possible use of the combined resources of available heterogeneous networks (wireless area networks (WLANs), Universal Mobile Telecommunications Systems, VANETs, Worldwide Interoperability for Microwave Access (WiMAX), etc.) for connection support. When connections need to migrate between heterogeneous networks for performance and high-availability reasons, seamless vertical handoff (VHO) is a necessary first step. In the near future, vehicular and other mobile applications will be expected to have seamless VHO between heterogeneous access networks. With regard to VHO performance, there is a critical need to develop algorithms for connection management and optimal resource allocation for seamless mobility. In this paper, we develop a VHO decision algorithm that enables a wireless access network to not only balance the overall load among all attachment points (e.g., base stations and access points) but also maximize the collective battery lifetime of mobile nodes (MNs). In addition, when ad hoc mode is applied to 3/4G wireless data networks, VANETs, and IEEE 802.11 WLANs for a more seamless integration of heterogeneous wireless networks, we devise a route-selection algorithm for forwarding data packets to the most appropriate attachment point to maximize collective battery lifetime and maintain load balancing. Results based on a detailed performance evaluation study are also presented here to demonstrate the efficacy of the proposed algorithms.

Vertical Handoff Decision Algorithm Providing Optimized Performance in Heterogeneous Wireless Networks

There are currently heterogeneous wireless access networks. A large variety of applications utilizing these networks will demand features such as real-time, high-availability and even instantaneous high-bandwidth in some cases. Therefore, it is imperative for network service providers to make the best possible use of the combined resources of available heterogeneous networks for connection support. Thus, with regard to vertical handoff performance, there is a critical need for developing algorithms for connection management and optimal resource allocation for seamless mobility. In this paper, we develop a vertical handoff decision algorithm that enables a wireless access network to not only balance the overall load among all attachment points but also to maximize the collective battery lifetime of mobile nodes (MNs). Simulation results are also presented to demonstrate the efficacy of the proposed algorithms.

Channel Condition Based Contention Window Adaptation in IEEE 802.11 WLANs

In IEEE 802.11 standard, the backoff parameters of its collision avoidance mechanism can be very inefficient and hence, the network becomes far from its optimal behavior. There have been several mechanisms to tune the Contention Window (CW) with the aim to achieve the optimal throughput in the IEEE 802.11 WLAN, however, the mechanisms do not specifically address a proper setting of the backoff parameters under non-saturated conditions. Noting that typical 802.11 networks are usually non-saturated, in this paper, we analytically derive the CW sizes that maximize the WLAN system throughput under both saturated and non-saturated conditions. Then, using the CW sizes derived, we propose a distributed algorithm that enables each station to dynamically adapt its CW according to the channel congestion status. The performance of the proposed algorithm is investigated through simulation. Simulation results indicate that our proposed backoff algorithm provides a remarkable performance improvement in terms of the delay experienced by a packet in the MAC layer, while maintaining an optimal throughput close to the theoretical throughput limit of the IEEE 802.11 Distributed Coordination Function (DCF) access scheme.

Energy-efficient multi-hop transmission in Body Area Networks

Compared to Wireless Sensor Network (WSN), Body Area Network (BAN) has its own unique requirements and further, it is more difficult to equip the medical sensors with replaceable batteries as this reduces the comport of the person wearing them. In this respect, a lot of efforts are being made by 802.15.6 WG to standardize a more efficient Medium Access Control (MAC) for BAN. Thus, in this paper, we propose an energy-efficient MAC scheme designed for BAN. The main idea is to allow for body sensors to transmit their data to the coordinator using multi-hop transmission in the BAN with the aim to maximize the lifetime of BAN. The performance of the proposed MAC scheme is evaluated by computer simulations in terms of lifetime and resource utilization.

A probabilistic call admission control algorithm for WLAN in heterogeneous wireless environment

In an integrated WLAN and cellular network, if all mobile users whose connections originate in the cellular network migrate to the WLAN whenever they enter the double coverage area, the WLAN will be severely congested and its users will suffer from performance degradation. Therefore, we propose a Call Admission Control (CAC) algorithm that allows the WLAN to limit downward Vertical Handovers (VHOs) from the cellular network to reduce unnecessary VHO processing. Numerical and simulation results demonstrate that our CAC scheme reduces the unnecessary VHO processing while keeping the DVHO blocking rate within acceptable limits and maintaining reasonable throughput in the WLAN.

An efficient power-saving mechanism for integration of WLAN and cellular networks

In this letter, we propose an efficient power-saving mechanism using paging of cellular networks for WLAN in heterogeneous wireless networks, where WLAN interface is turned off during idle state without any periodic wake-up in order to save power consumption while at the same time, the existing paging of cellular network is utilized in place of beacons in WLAN. For the proposed mechanism, the mean power consumption is investigated via analytical and simulation results.

Coexistence of ZigBee-Based WBAN and WiFi for Health Telemonitoring Systems

The development of telemonitoring via wireless body area networks (WBANs) is an evolving direction in personalized medicine and home-based mobile health. A WBAN consists of small, intelligent medical sensors which collect physiological parameters such as electrocardiogram, electroencephalography, and blood pressure. The recorded physiological signals are sent to a coordinator via wireless technologies, and are then transmitted to a healthcare monitoring center. One of the most widely used wireless technologies in WBANs is ZigBee because it is targeted at applications that require a low data rate and long battery life. However, ZigBee-based WBANs face severe interference problems in the presence of WiFi networks. This problem is caused by the fact that most ZigBee channels overlap with WiFi channels, severely affecting the ability of healthcare monitoring systems to guarantee reliable delivery of physiological signals. To solve this problem, we have developed an algorithm that controls the load in WiFi networks to guarantee the delay requirement for physiological signals, especially for emergency messages, in environments with coexistence of ZigBee-based WBAN and WiFi. Since WiFi applications generate traffic with different delay requirements, we focus only on WiFi traffic that does not have stringent timing requirements. In this paper, therefore, we propose an adaptive load control algorithm for ZigBee-based WBAN/WiFi coexistence environments, with the aim of guaranteeing that the delay experienced by ZigBee sensors does not exceed a maximally tolerable period of time. Simulation results show that our proposed algorithm guarantees the delay performance of ZigBee-based WBANs by mitigating the effects of WiFi interference in various scenarios.

Enhanced fast handover for proxy mobile IPv6 in vehicular networks

To reduce the handover latency in PMIPv6, Fast Handover for PMIPv6 (PFMIPv6) is being standardized in the IETF. On the other hand, vehicle-roadside data access has been envisioned to be useful in many commercial Internet services; however, integrating the current Internet into Vehicular Networks (VNs) presents a new set of challenges. In particular, to provide rapid IP handover in the VNs, simply applying PFMIPv6 to VNs may not improve handover performance since PFMIPv6 handover restricts the previous Mobile Access Gateway (MAG) from forwarding the packets until it receives an HAck/HI from the next MAG, even though the vehicle may have already arrived at the next MAG. We also note that PFMIPv6 does not consider the impact of geographic restriction on vehicular mobility. Therefore, in this paper, we propose an enhanced PFMIPv6 (ePFMIPv6) for VNs in which the serving MAG pre-establishes a tunnel with candidate next MAGs for next MAG so that the packets can be immediately forwarded to the next MAG once the serving MAG is indicated the vehicle’s handover by the serving road side unit. To evaluate the performance of the proposed protocol, we derive analytical expressions for packet loss, latency and signaling overhead caused by ePFMIPv6 and PFMIPv6 handovers. Our analytical study is verified by simulation results.

Performance analysis of fast handover for proxy Mobile IPv6

In Proxy Mobile IPv6 (PMIPv6), any involvement by the Mobile Node (MN) is not required, so that any tunneling overhead can be removed from over-the-air. However, during the PMIPv6 handover process, there still exists a period when the MN is unable to send or receive packets because of PMIPv6 protocol operations, suffering from handover latency and data loss. Thus, to reduce the handover latency and data loss in PMIPv6, Fast Handover for PMIPv6 (PFMIPv6) is being standardized in the IETF. Nevertheless, PFMIPv6 has a few weaknesses: (1) handover initiation can be false, resulting in the PFMIPv6 handover process done so far becoming unnecessary. (2) Extra signaling is introduced in setting up an IP-in-IP tunnel between the serving and the new Mobile Access Gateways (MAGs). Therefore, in this paper, we present our study on the protocol overhead and performance aspects of PFMIPv6 in comparison with PMIPv6. We quantify the signaling overhead and the enhanced handover latency and data loss by conducting a thorough analysis of the performance aspects. The analysis is very meaningful to obtain important insights on how PFMIPv6 improves the handover performance over PMIPv6, especially in a highway vehicular traffic scenario where Base Stations (BSs)/Access Points (APs) can be placed in one dimensional space and MNs movements are quasi one-dimensional, so that the degree of certainty for an anticipated handover is increased. Further, our analytical study is verified by simulation results.

Fast Handover Latency Analysis in Proxy Mobile IPv6

In Proxy Mobile IPv6 (PMIPv6), any involvement by the Mobile Node (MN) is not required so that any tunneling overhead could be removed from over-the-air. However, during the PMIPv6 handover process, there still exists a service interruption period during which the MN is unable to send or receive data packets because of PMIPv6 protocol operations. To reduce the handover latency, Fast Handover for PMIPv6 (PFMIPv6) is being standardized in the IETF MIPSHOP working group. In PFMIPv6, however, handover initiation can be false, resulting in the PFMIPv6 handover process done so far becoming unnecessary. Therefore, in this paper, we provide a thorough analysis of the handover latency in PFMIPv6, considering the false handover initiation case. The analysis is very meaningful to obtain important insights on how PFMIPv6 improves the handover latency. Further, our analytical study is verified by simulation results.