Browsing by Author "Chang, HP"

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A seamless vertical handoff scheme
(2005)
Kang, RJ
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Chang, HP
;
Chang, RC
;
資訊工程學系
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Department of Computer Science
In the last decade, many new wireless communication systems have been proposed and developed These communication systems have different extends of network coverage and form a hierarchical overlay network. A vertical handoff is thus occurred when moving between these different communication systems. In this paper, we propose a new vertical handoff scheme: MOST (Vertical Hand-Off through Seamless TCP-migration). In VHOST, we first design the mechanism to detect an ongoing handoff. Then, on the basis of TCP-migration, we show how a connection is migrated from the old communication system to the new one. Finally, MOST includes a handoff-aware TCP adaptation mechanism for link adaptation to accommodate with the different network characteristics. To verify the feasibility of our approach, we implement a prototype system on Linux. The experimental results show that the vertical handoff time is short and acceptable for most applications. Besides, TCP performance is improved by our proposed handoff-aware adaptation scheme. For example, the TCP throughput by proposed adaptation scheme is 1.8 times the original TCP without adaptation.
Cache-aware real-time disk scheduling
(2004-09-01)
Chang, HP
;
Chang, RI
;
Shih, WK
;
Chang, RC
;
資訊工程學系
Previous real-time disk scheduling algorithms assume that each disk request incurs a physical disk mechanical operation and only consider how to move the disk head under real-time constraints. However, with the increased capacity of on-disk cache, modern disk drives read-ahead data aggressively. Thus, the on-disk cache may service many disk requests without incurring physical disk access. By exploring the design methodology of on-disk cache, in this paper, we propose cache-aware real-time disk scheduling algorithms that take the on-disk cache into consideration during scheduling. Therefore, the scheduling algorithm can help the cache replacement scheme to minimize the cache miss ratio. Besides, the service timing estimation is more accurate in schedulability analysis since the cache effect is considered during scheduling. A simulation-based evaluation shows the proposed scheduling algorithms to be highly successful as compared with the classical real-time disk scheduling algorithms. For example, under sequential workload with 10 sequential streams, the data throughput of our scheme is 1.1 times that of DM-SCAN.
Enlarged-maximum-scannable-groups for real-time disk scheduling in a multimedia system
(2000)
Chang, HP
;
Chang, RI
;
Shih, WK
;
Chang, RC
;
資訊工程學系
In a multimedia system, disk I/O subsystem is the most important component due to its relatively limited throughput and large delay. Previously, by applying SCAN to reschedule tasks having the same deadline, SCAN-EDF tries to improve disk throughput while realtime constraints call be satisfied. In DM-SCAN, groups of tasks that can be successfully rescheduled by SCAN under specified real-time requirements are identified. They are called MSGs (maximum-scannable-groups). In this paper; an enlarged-MSG (E-MSG) is proposed to further expand the MSG concept and thus to obtain more improvement in disk throughput. By removing some excess constraints on MSG, E-MSG merges several MSGs as a new scannable-group. Experiment results show that the E-MSG scheme is better than both SCAN-EDF and MSG in the disk throughput obtained.
Performances and characteristics of a prototype symmetric hybrid adjustable phase undulator
(1997-11-11)
Hwang, CS
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Chang, CH
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Fan, TC
;
Lin, FY
;
Wang, C
In this work, we construct and measure a prototype symmetric Hybrid Adjustable Phase Undulator (HAPU) with nine poles to compare phase change and gap change in terms of their features and performances. HAPU can also provide the different characteristics between the pure and hybrid adjustable phase undulator. The HAPU has been installed in the Synchrotron Radiation Research Center (SRRC) storage ring to test the performance of the beam dynamics and photon flux behavior. A three-orthogonal Hall probe and a long-loop-coil measurement system are also implemented to take the field measurements. The multipole field and electron trajectory correction are made by shimming, the magic finger, and the active end pole coil corrector. After the field measurements, the vertical integral strength of HAPU is markedly smaller than that of the Hybrid Adjustable Gap Undulator (HAGU) and confirm that no obvious distinction arises between HAPU and HAGU in terms of the spectrum flux. The electron beam dynamic performance in the storage ring indicates that the vertical tune shift of HAPU is three times smaller than that of HAGU. Meanwhile, the energy dynamic tuning with the digital global feedback turned on, the photon position and flux in the bending chamber beam line is invariant. In addition, the injection efficiency remains unaffected at any phase position and the electron beam lifetime does not change when HAPU is in the dynamic tuning.
Providing mobile LAN access capability for bluetooth devices
(2002)
Chiu, SY
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Chang, HP
;
Chang, RC
;
資訊工程學系
;
Department of Computer Science
In this paper, we design and implement a Bluetooth system with mobile LAN access capability. First, by adding a virtual network interface card (NIC) driver between IP and Bluetooth protocol stack, Bluetooth users can use their Bluetooth devices to access the LAN resources via the Bluetooth access points. Besides, to accommodate with the mobility characteristics in Bluetooth network, we also propose a new handoff scheme. Instead of passively waiting for a handoff indication by the expiration of link supervision timer, in this paper, we detect a handoff actively and promptly by measuring the RSSI (Receive Signal Strength Indication) value. In addition, our proposed handoff scheme eliminates the inquiry procedure and only requires paging procedure during a handoff. We have implemented a prototype of Bluetooth system with mobile LAN access capability by our proposed scheme. Experimented results show that the handoff duration is significant reduced than the normal scheme.
Real-time disk scheduling with on-disk cache conscious
(2003)
Chang, HP
;
Chang, RI
;
Shih, WK
;
Chang, RC
;
資訊工程學系
Previous real-time disk scheduling algorithms assume that each disk request incurs a disk mechanical operation and only consider how to move the disk head under real-time constraints. However, with the increased capacity of on-disk cache, modern disk drives read-ahead data aggressively. Thus, the on-disk cache may service lots of requests without incurring physical disk access. By exploring the design methodology of on-disk cache, in this paper, we propose CARDS: a cache-aware real-time disk scheduling algorithm that takes the on-disk cache into consideration during scheduling. Therefore, the scheduling algorithm can help to minimize the cache miss ratio. Besides, the service timing estimation is more accurate in schedulability analysis since the cache effect is considered. A simulation-based evaluation shows CARDS to be highly successful as compared to the classical real-time disk scheduling algorithms. For example, under sequential workload with 10 sequential streams, the data throughput of CARDS is 1.1 times of DM-SCAN.
Reschedulable-Group-SCAN scheme for mixed real-time/non-real-time disk scheduling in a multimedia system
(2001-11-15)
Chang, HP
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Chang, RI
;
Shih, WK
;
Chang, RC
;
資訊工程學系
Real-time disk scheduling is important for time-critical multimedia applications. Previous approaches, such as SCAN-earliest deadline first (EDF) or DM-SCAN, applied the SCAN scheme to reschedule service sequence of input tasks to reduce tasks' service time. However, they required the input to be in EDF order. In DM-SCAN, a deadline modification scheme was employed to obtain a pseudo-EDF sequence from non-EDF ordered input tasks. Because the modified deadlines were smaller than the original ones, number of tasks that could be rescheduled is decreased and thus data throughput is reduced. In this paper, we propose Reschedulable-Group-SCAN (RG-SCAN), a new real-time disk scheduling algorithm using the concept of Reschedulable-Group (R-Group). Differing from previous approaches, RG-SCAN has no limitation on the input task's sequence. In addition, by exploiting the service time's reduction after rescheduling in each R-Group, RG-SCAN has been extended to serve mixed real-time and non-real-time workloads. As shown in experimental results. our approach can support more tasks than DM-SCAN, both real-time and non-real-time. Additionally, our approach can provide larger data throughput and offer better response time to non-real-time tasks. For example, given 30 random-generated real-time tasks, the number of non-real-time tasks that can be supported by RG-SCAN is 1.3 times that supported by DM-SCAN. In addition, our data throughput is 1.1 times DM-SCAN's. (C) 2001 Elsevier Science Inc. All rights reserved.