Moonsoo Kang

Investigation of handoffs for IEEE 802.11 networks in vehicular environment

Users in vehicles who are interested in accessing Internet services can be achieved by connecting to IEEE 802.11 WLAN APs along the roadside. However, due to the small coverage range of 802.11 APs and high mobility of vehicles, handoffs frequently occur as vehicles migrate across a series of adjacent access points which can be a major source of interruption to seamless connections and throughput reduction. In this paper, we investigate the well-known handoff schemes that have worked to reduce the 802.11 handoff latency and address the possible issues that arise when applied to vehicular scenarios. Our main objective is to provide groundwork for future research on the enhancement of fast handoffs for vehicular scenarios and emphasize that smart and optimized fast handoffs are a critical requirement for seamless connectivity. To meet these requirements, we also suggest that trajectory information (on-board navigation systems) along with neighbor AP information (802.11k) have the potential to provide smart and fast handoffs to support seamless communication in a vehicular scenario.

COMIC: Intelligent Contention Window Control for Distributed Medium Access

In this letter, a scheme for Collision Mitigation with Intelligent Contention Window Control (COMIC) is proposed for backoff based collision resolution algorithm. COMIC intelligently mitigates collisions by probabilistically maximizing the selection likelihood of relatively less collision-probable contention slots over the backoffed contention window. A unified Markovian model for the Distribution Coordination Function (DCF) that incorporates COMIC, DCFcomic, is formulated for the performance analysis. The performance results show that DCFcomic outperforms the conventional DCF in both throughput and average packet delay due to the significant reduction in packet collisions.

Isomorphic strategy for processor allocation in k-ary n-cube systems

Due to its topological generality and flexibility, the k-ary n-cube architecture has been actively researched for various applications. However, the processor allocation problem has not been adequately addressed for the k-ary n-cube architecture, even though it has been studied extensively for hypercubes and meshes. The earlier k-ary n-cube allocation schemes based on conventional slice partitioning suffer from internal fragmentation of processors. In contrast, algorithms based on job-based partitioning alleviate the fragmentation problem but require higher time complexity. This paper proposes a new allocation scheme based on isomorphic partitioning, where the processor space is partitioned into higher dimensional isomorphic subcubes. The proposed scheme minimizes the fragmentation problem and is general in the sense that any size request can be supported and the host architecture need not be isomorphic. Extensive simulation study reveals that the proposed scheme significantly outperforms earlier schemes in terms of mean response time for practical size k-ary and n-cube architectures. The simulation results also show that reduction of external fragmentation is more substantial than internal fragmentation with the proposed scheme.

A cross-layer approach for TCP optimization over wireless and mobile networks

We propose a path recovery notification (TCP-PRN) mechanism to prevent performance degradation during a handoff. Even though Freeze-TCP achieves a performance increase during handoff, detecting accurate handoff time and the vulnerability of high variation in the round trip time (RTT) become obstacles for deploying Freeze-TCP in a real environment. The proposed protocol, TCP-PRN, quickly recovers lost packets by restoring the congestion window, preventing the congestion window to decrease, or immediately initiating the slow start algorithm. The simulation results of our study show better performance in a more realistic environment where the handoff prediction is difficult and the variation of RTT is higher.

A concurrent access MAC protocol for cognitive radio ad hoc networks without common control channel

Cognitive radio ad hoc networks (CRAHNs) consist of autonomous nodes that operate in ad hoc mode and aim at efficient utilization of spectrum resources. Usually, the cognitive nodes in a CRAHN exploit a number of available channels, but these channels are not necessarily common to all nodes. Such a network environment poses the problem of establishing a common control channel (CCC) as there might be no channel common to all the network members at all. In designing protocols, therefore, it is highly desirable to consider the network environment with no CCC. In this article, we propose a MAC protocol called concurrent access MAC (CA-MAC) that operates in the network environment with no CCC. The two devices in a communication pair can communicate with each other even if they have only one common channel available. Therefore, the problems with CCC (such as channel saturation and denial of service attacks) can also be resolved. In CA-MAC, channel accesses are distributed over communication pairs, resulting in increased network connectivity. In addition, CA-MAC allows different communication pairs to access multiple channels concurrently. According to our performance study, CA-MAC provides higher network connectivity with shorter channel access delay compared to SYN-MAC, which is the conventional key MAC protocol for the network environment with no CCC, resulting in better network throughput.

Equal-Size Clustering for Irregularly Deployed Wireless Sensor Networks

This study examines the problem that sensors are irregularly deployed in a wireless sensor network (WSN). Such irregularity makes clustering protocols less efficient. This paper proposes a new clustering algorithm, called balanced clustering algorithm (BCA), for irregularly deployed WSNs. In BCA, each node determines the probability that the node itself becomes the cluster head (CH) by considering the sensing population, which is defined as the number of nodes within the sensing range of a node. As a result, the coverage area of each cluster is distributed almost equally and unused redundant nodes are turned into sleep mode. Therefore, the large deviation of the coverage areas of clusters in a network can be decreased and the unnecessary duplication of sensing and transmission can also be decreased. In addition, the inefficient energy consumption is reduced significantly because the sleeping nodes do not send duplicated information over high populated areas. According to the simulation, the proposed BCA reduces energy consumption, increases the network lifetime and distributes the detection area of each cluster evenly, compared to the conventional schemes.

Time-Slotted Scheduling Schemes for Multi-hop Concurrent Transmission in WPANs with Directional Antenna

To achieve high-speed (Giga-bit) connectivity for shortrange wireless multimedia applications, the mmWave wireless personal area networks with directional antennas are gaining increased interest. Due to the use of directional antennas and mmWave communications, the probability of non-interfering transmissions increases in a localized region. The network throughput can immensely increase by the concurrent time allocation of non-interfering transmissions. The problem of finding optimum time allocation for concurrent transmissions is an NP-hard problem. In the literature, few “sub optimum concurrent time slot allocation” schemes have been proposed. In this paper, we propose two enhanced versions of previously proposed Multihop Concurrent Transmission (MHCT) scheme. To increase the network capacity, proposed schemes efficiently use the free holes in the time allocation map of MHCT scheme and make it more compact.

A new loss recovery architecture for wireless TCP

The loss recovery architecture of TCP under wireless environment is considered. We propose sent-time ordered two lists architecture as an alternative to sequence number ordered single list architecture. By keeping the sent-time order, the recovery mechanism can be more efficient and-simpler and transmission decision is decoupled from loss recovery using the second list. Simulation results show the superiority of our mechanism.

Job-based queue delay modeling in a space-shared hypercube

Processor allocation problem in a space-shared hypercube parallel computer is considered. Since jobs are dynamically allocated and deallocated, existence of more free processors than required does not guarantee the immediate allocation due to the fragmentation problem. If the immediate allocation fails, the processor allocator should decide whether to wait or to assign a smaller subcube to the job. Here, the decision is made depending on the wait or queue delay time. There have been several attempts to predict the queue delay analytically based on the number of busy processors, not the busy subcubes. Main disadvantages of the processor-based prediction is that the number of busy processors does not accurately explain the queue delay. A subcube with the minimum number of running jobs is probably the best candidate for the future allocation even though it has less free processors. We propose a job-based queue delay prediction. We view the allocation moment when all the existing jobs, not the busy processors, in one of the possible subcubes complete their execution. We performed an extensive simulation study and it reveals that the predicted queue delay matches very well with the simulation results. Using the predicted queue delay time, the processor allocator is able to make a right decision and thus, enhances the system performance.

Interpolation using wavelet transform and discrete cosine transform for high resolution display

We propose a new interpolation method based on the hybrid technique combining the discrete wavelet transform (DWT) and discrete cosine transform (DCT). The high frequency wavelet coefficients are interpolated using the zero pad method in the DCT domain. The upscaled image is generated using inverse DWT of interpolated coefficients and original image.