Jaewon Kang

Location-Based Flooding Techniques for Vehicular Emergency Messaging

This paper analyzes the scalability of message flooding protocols in networks with various node densities, which can be expected in vehicular scenarios. Vehicle safety applications require reliable delivery of warning messages to nearby and approaching vehicles. Due to potentially large distances and shadowing, the delivery protocol must forward messages over multiple hops, thereby increasing network congestion and packet collisions. In addition to application-layer backoff delay and duplicate message suppression mechanisms, location-based backoff techniques have been proposed for vehicular networks. We propose a new hybrid method of location-based and counter-based method, and study several variants through simulations. Our preliminary results in the various density scenarios indicate that the proposed hybrid methods outperform conventional backoff delay techniques and adaptively operate in extremely congested network condition

Accurate and energy-efficient congestion level measurement in ad hoc networks

Congestion in ad hoc networks not only degrades throughput, but also wastes scarce energy due to a large number of retransmissions and packet drops. For efficient congestion control, an accurate and timely estimation of resource demands by measuring the network congestion level is necessary. Congestion level measurement in ad hoc networks is more difficult than in wired networks due to time-variant channel capacity, contention among neighboring nodes, and non-deterministic node scheduling. We propose a new congestion detection mechanism that quantifies the congestion level accurately and energy-efficiently at both a node-level (implemented at the MAC layer) and a flow-level (implemented at the routing layer) in ad hoc networks. For accurate congestion measurement, a set of metrics that decouple the measurement from various MAC protocol characteristics is defined. For energy-efficient congestion measurement, an asynchronous channel loading measurement scheme, called lazy measurement, which emulates synchronous measurement by using virtual channel sampling, is incorporated into the proposed scheme. Simulation results show that the proposed mechanism significantly cuts down the energy needed to measure congestion accurately, while maintaining the high level of accuracy needed for timely congestion control.

Analysis of Resource Increase and Decrease Algorithm in Wireless Sensor Networks

In this paper, we first attempt to formally define the resource control framework that adjusts the resource provisioning at the hotspot during congestion. In an effort to find the optimal resource control under the fidelity and energy constraints, we present a resource increase and decrease algorithm called Early Increase/Early Decrease (EIED) that tries to adjust the effective channel capacity quickly to the incoming traffic volume in an energy-efficient manner, thereby increasing the fidelity (or accuracy) level observed by the application during congestion. Under the framework of energy-constrained optimization, we prove this algorithm incurs the lowest overhead of energy consumption for the given fidelity level that is required by the application.

An integrated soft handoff approach to IP fast reroute in wireless mobile networks

This paper presents an integrated approach to IP fast reroute (IPFRR) of both unicast and multicast paths in wireless mobile networks. A distinct feature of the proposed approach is that, instead of modifying existing routing protocols, it employs a soft handoff technique, i.e., temporarily installs pre-computed Loop Free Alternative Paths (LFAPs) until the co-existing routing protocol converges to new routes. The proposed approach builds on our previously proposed IPFRR technology and uses the concept of pre-computed LFAPs not only for local but also for remote link failures within a certain neighborhood to achieve full alternative path coverage for a single link failure. This papers contributions include: i) bandwidth efficient fast failure detection by integrating two novel mechanisms, namely probing and link quality prediction, ii) a novel method for calculating LFAPs, iii) a framework for switching seamlessly between LFAPs and OSPF paths, iv) a multicast fast reroute mechanism, and v) implementation in eXtensible Open Router Platform (XORP). We also present a generic framework for handling multiple simultaneous failures in the integrated IPFRR. The performance evaluation has been performed in both indoor and outdoor environments with real 802.11 radio links. The results confirm that our IPFRR technology consistently provides significant convergence time improvement during a single link failure event.

Resource-controlled MAC-layer congestion control scheme in cellular packet network

In this paper, unlike the current end-to-end congestion control mechanism such as TCP that only reduces its traffic in the presence of congestion, we propose a new wireless congestion control framework, in which a mobile stations MAC-layer tries to increase its available wireless bandwidth in the presence of incipient congestion when available bandwidth can he increased without affecting other flows in a cell. In our framework, wireless congestion is classified into channel congestion, queue congestion, and global congestion. In order to control congestion in a timely manner, the mobile stations MAC-layer decides whether available bandwidth should be increased or incoming traffic should be reduced based on the congestion type. In addition, thanks to the MAC-layers ability to better observe the wireless channel condition, the wireless congestion, which is considered more important than the wired congestion due to its limited bandwidth, can be better handled when the path of a mobile stations connection spans wired network.

An optimal resource control scheme under fidelity and energy constraints in sensor networks

To control congestion, either the traffic from sources should be “reduced” (traffic controlling) or the available resources should be “increased” (resource controlling). Compared to the wired and other wireless counterparts, wireless sensor networks usually have elastic resource availability, and the applications require a certain level of throughput called fidelity. As a result, resource control strategies cannot only alleviate congestion but also ensure the required fidelity level during congestion by accommodating higher incoming traffic. In this paper, we first attempt to formally define the resource control framework that adjusts the resource provisioning at the hotspot nodes during congestion. In an effort to find the optimal resource control under the fidelity and energy constraints, we present a resource increase and decrease algorithm called Early Increase/Early Decrease (EIED) that tries to adjust the effective channel capacity quickly to suit the incoming traffic volume in an energy-efficient manner, thereby increasing the fidelity level observed by the application. Under the energy-constrained optimization, we prove this algorithm incurs the lowest overhead of energy consumption for the given fidelity level that is required by the application. We also prove that the EIED algorithm performed in a distributed manner also lowers the energy consumption per packet at an end-to-end level. The effectiveness of the EIED algorithm is verified by simulations based on realistic sensor network configurations.

Robust PIM-SM Multicasting Using Anycast RP in Wireless Ad Hoc Networks

Due to its bandwidth efficiency, multicast makes a group-centric communication more viable in wireless ad hoc networks with limited radio resources. PIM-SM, a de facto standard multicast protocol known for its high scalability, is a good fit for a large-scale ad hoc network. However, it does not provide a robust multicast communication under RP outage and host mobility. In this paper, we propose a robust way of configuring PIM-SM using Anycast RP in wireless ad hoc networks. We analyze the impact of cardinality and locations of anycast RPs on the network performance under node mobility. Based on these observations, we find metrics for near-optimal cardinality of anycast RPs and propose a novel RP selection scheme. The proposed scheme is proven to make PIM-SM robust against mobility while satisfying QoS requirements and maintaining the scalability of PIM-SM.

Adaptive QoS control by toggling voice traffic between circuit and packet cellular networks

As cellular packet data services become widely deployed by the rollout of the networks such as general packet radio service (GPRS) and 3G cellular networks, packet-switched voice service such as voice over IP (VoIP) can soon be expected to be offered as an alternative to circuit-switched voice service. While circuit-switched voice offers better quality, packet-switched voice offers better resource utilization due to its multiplexing and compression techniques that can be used in packet radio principle. Therefore, the capability of having voice service over either circuit-switched network or packet-switched network provides a tradeoff between voice quality and bandwidth utilization. This paper proposes three schemes that allow users to toggle between circuit-switched voice and packet-switched voice based on the desired tradeoff. The basic idea is to allow ongoing voice traffic to alternate between circuit-switched network and packet- switched network using a 3-way calling mechanism in the SIP-enabled GSM/GPRS cellular network. We also show how these toggle schemes can be used depending on the cell load. Detailed performances of the proposed schemes are evaluated using simulation with realistic parameters of a cellular environment. The result shows our schemes can alleviate cell overloading as well as provide the smooth transition of an ongoing call between circuit-switched network and packet-switched network. Our schemes can be easily deployed in any cellular mobile network where circuit-switched network and packet-switched network coexist.

ALE: Adaptive link establishment in OSPF wireless ad-hoc networks

Link state routing protocols such as OSPF synchronize the topology databases by flooding link state update messages periodically or whenever there is a connectivity change. When deployed in wireless networks with mobility and noisy channels, OSPF incurs frequent link creation and deletion and subsequent route changes. To cope with this problem, we propose an Adaptive Link Establishment (ALE) scheme that creates a link based on its stability in terms of connectivity and channel condition. The key characteristic of the ALE scheme is that it dynamically controls the latency of a new link creation by either accepting or dropping incoming Hello messages without generating additional control packets on the wireless channel. In addition, the ALE scheme is fully compatible with the legacy OSPF routing protocol. The ALE scheme has been implemented in the real OSPF routing protocol. We measure the performance of the ALE scheme in terms of flooding overhead, convergence time, packet drop rate and throughput. Our experiment results show that the ALE scheme effectively suppresses the unnecessary link establishment caused by intermittent connectivity while having no impact on the convergence time when the network topology is stable. They also show that the end-to-end application throughput is improved by up to 66%.

A study on switching voice traffic seamlessly between GSM and GPRS cellular networks

As cellular carriers start deploying packet data services such as General Packet Radio Service (GPRS) and 3G cellular networks, packet-switched voice services such as VoIP will be available over the cellular network in the near future. When the existing circuit-switched radio interface and the newly deployed packet-switched radio interface coexist in a cell, the mobile station user may have an option between the circuit-switched call and the VoIP call. While the circuit-switched voice offers a better call quality, the VoIP connection offers a better radio resource utilization. In this paper, we introduce dynamic QoS control schemes for voice traffic, where Class-A GPRS mobile station users can switch their ongoing GSM calls to VoIP calls and vice versa seamlessly in the SIP-enabled GSM/GPRS cellular network based on their desired quality of service. Due to the VoIPs less radio resource demand, these schemes can also be used by the base station controller to handle the dynamically changing load in a cell.