Wei-Kuan Shih

TCP Throughput Enhancement over Wireless Mesh Networks

TCP is the predominant technology used on the Internet to support upper layer applications with reliable data transfer and congestion control services. Furthermore, it is expected that traditional TCP applications (e.g., Internet access) will continue to constitute the major traffic component during the initial deployment of wireless mesh networks. However, TCP is known for its poor throughput performance in wireless multihop transmission environments. For this article, we conducted simulations to examine the impact of two channel interference problems, the hidden terminal and exposed terminal, on TCP transmissions over wireless mesh networks. We also propose a multichannel assignment algorithm for constructing a wireless mesh network that satisfies the spatial channel reuse property and eliminates the hidden terminal problem. The simulation results demonstrate the effectiveness of the proposed approach in improving the performance of TCP in wireless multihop networks.

Deadline-modification-SCAN with maximum-scannable-groups for multimedia real-time disk scheduling

Real-time disk scheduling is important to multimedia systems support for digital audio and video. In these years, various approaches are presented to use the seek-optimizing scheme to improve the disk throughput of a real-time guaranteed schedule. However, as these conventional approaches apply SCAN only to the requests with the same deadline or within the same constant-sized group, their improvements are limited. In this paper, we introduce the DM-SCAN (deadline-modification-SCAN) algorithm with an idea of MSG (maximum-scannable-group). The proposed DM-SCAN method can apply SCAN to MSG iteratively by modifying request deadlines. We have implemented the DM-SCAN algorithm on UnixWare 2.01. Experiments show that DM-SCAN is significantly better than that of the best-known SCAN-EDF method in both the obtained disk throughput and the number of supported disk requests.

Current Results on EDZL Scheduling for Multiprocessor Real-Time Systems

Many optimal uniprocessor schedulers, such as earliest deadline first (EDF) and rate monotonic (RM), do not have a good schedulability bound on multiprocessor systems. In this paper, we study an on-line algorithm earliest deadline first until Zero laxity (EDZL) for multiprocessor systems. A set of tasks scheduled by EDZL is scheduled using EDF until a job experiences a zero laxity. To avoid the job from missing its deadline, the priority of the job is immediately promoted to the highest priority. We derive the schedulability bound of 3/2+\umax-1/2\ for two-processor systems, where umax is the maximum utilization of an individual task in the given task set. We also discuss the best known upper bound and lower bound on EDZL schedulability conditions.

Exploiting GSM short message service for ubiquitous accessing

The Short Message Service (SMS) in the GSM system is designed to provide a vehicle for delivering length-limited data messages between mobile users. This paper describes a prototype system that is capable of providing a Short Message Command Interface (SMCI) for mobile users employing GSM-enabled terminal devices to access remote PCs or workstations by executing DOS or UNIX commands. In addition, based on our successful testing of the implemented SMCI prototype system, we further devise a framework of the Home Network Service Centre by expanding this prototype system for mobile users to access emerging information appliances connected by Home Networks in a reliable and cost-efficient way. The SMCI prototype system is composed of three parts: the SMCI Clients, the SMCI Servers, and the SMCI Gateway. The SMCI Clients are mobile users who can send commands to the SMCI Servers and receive results from the SMCI Servers both through the SMCI Gateway. By taking advantage of this prototype system and the proposed framework, mobile users can login remote machines and can access value-added services as well as Home Network services provided on those machines or information appliances.

An Advanced ECC ID-Based Remote Mutual Authentication Scheme for Mobile Devices

Recently, Elliptic Curve Cryptosystem (ECC) based remote authentication scheme has been used for mobile devices. For instance, Yang and Change proposed an ID-based remote mutual authentication with key agreement scheme for mobile devices on Elliptic Curve Cryptosystem in 2009. However, in this paper, we found that their scheme still is vulnerable to insider attack and impersonation attack. Therefore, we proposed an advanced ECC ID-Based remote mutual authentication scheme for mobile devices to solve the issues. Furthermore, we analyzed our scheme to show that our scheme is more secured to authenticate users and remote servers for mobile devices.

GSR: A global seek-optimizing real-time disk-scheduling algorithm

Earliest-deadline-first (EDF) is good for scheduling real-time tasks in order to meet timing constraint. However, it is not good enough for scheduling real-time disk tasks to achieve high disk throughput. In contrast, although SCAN can maximize disk throughput, its schedule results may violate real-time requirements. Thus, during the past few years, various approaches were proposed to combine EDF and SCAN (e.g., SCAN-EDF and RG-SCAN) to resolve the real-time disk-scheduling problem. However, in previous schemes, real-time tasks can only be rescheduled by SCAN within a local group. Such restriction limited the obtained data throughput. In this paper, we proposed a new globally rescheduling scheme for real-time disk scheduling. First, we formulate the relations between the EDF schedule and the SCAN schedule of input tasks as EDF-to-SCAN mapping (ESM). Then, on the basis of ESM, we propose a new real-time disk-scheduling algorithm: globally seek-optimizing rescheduling (GSR) scheme. Different from previous approaches, a task in GSR may be rescheduled to anywhere in the input schedule to optimize data throughput. Owing to such a globally rescheduling characteristic, GSR obtains a higher disk throughput than previous approaches. Furthermore, we also extend the GSR to serve fairly non-real-time tasks. Experiments show that given 15 real-time tasks, our data throughput is 1.1 times that of RG-SCAN. In addition, in a mixed workload, compared with RG-SCAN, our GSR achieves over 7% improvement in data throughput and 33% improvement in average response time.

Real-Time Disk Scheduling for Multimedia Applications withDeadline-Modification-Scan Scheme

Real-timedisk scheduling (RTDS) is important for time-critical multimediaapplications. In conventional approaches of real-time disk scheduling,such as SCAN-EDF, the seek-optimizing SCAN scheme is appliedto reduce the disk service time of the real-time EDF schedule.Since only tasks with the same deadline are seek-optimized, theobtained improvement of SCAN-EDF is limited. In this paper, basedon the maximum-scannable-group (MSG) concept, a deadline-modification-scan (DMS or DM-SCAN) algorithm is proposed.Our algorithm uses MSG to automatically decide the suitable taskgroups for seek-optimizing. Based on the MSG concept, we repeatedlyapply DMS to further increase disk throughput to support moretasks. We have implemented the proposed algorithm on UnixWare2.01. The appropriate problem behaviors and parameter valuesto yield good solutions are investigated. Experiments show thatDMS is better than conventional approaches in both the obtaineddisk throughput and the supported tasks. Moreover, our proposedapproach can schedule task sets that are not schedulable by EDFand SCAN-EDF.

CacheRAID: An Efficient Adaptive Write Cache Policy to Conserve RAID Disk Array Energy

Cloud storage is a hot topic at the moment with Googles Google Storage, Microsofts Sky Drive, iCloud, Drop box, Mozy and dozens of others. Because of these applications, conserving energy of storage systems is becoming a growing concern in current storage technology. The factors of disk power consumption include disk idle time, poor random writing performance and random read in distributed file systems. Hence, we present an adaptive write cache mechanism - Cache RAID. Redundant Arrays of Inexpensive Disk (RAID) is widely used in modern distributed storage systems. Our Cache RAID aims to improve the random access problems that implicitly exist in RAID techniques to create more idle time of hard drives, and conserve RAID disk array energy. The experimental results show that Cache RAID storage system can conserved 50%~70% of the power consumption compared to the conventional software RAID system.

New Schedulability Conditions for Real-Time Multiframe Tasks

The real-time multiframe task model first studied by Mok and Chen assumes that the computation times of a periodic task vary instance by instance. They have derived an utilization bound for verifying the schedulability of multiframe task sets. Their schedulability test has since been improved by other researchers. In this paper we use the information about the relative period ratios between tasks in a system to derive a new schedulability condition. By considering the smallest and the largest period values in a system, we can show that the RM schedulability bound can be improved significantly. This method also can be applied to other test methods studied earlier to improve the schedulability of real-time multiframe systems.

A fast algorithm for scheduling imprecise computations with timing constraints to minimize weighted error

Scheduling tasks with different weights in the imprecise computation model is rather difficult. Each task in the imprecise computation model is logically decomposed into a mandatory subtask and an optional subtask. The mandatory subtask must be completely executed before a deadline to produce an acceptable result; the optional subtask begins after the mandatory subtask to refine the result. The error in the results of a task is measured by the processing time of the unexecuted portion of the optional subtask. This paper proposes a fast algorithm for scheduling imprecise computation with timing constraints on uniprocessor systems. The proposed algorithm can obtain the optimal schedule for different weighted tasks with time complexity O(n log/sup 2/n).