Hsung-Pin Chang

Adaptive TCP congestion control and routing schemes using cross-layer information for mobile ad hoc networks

Compared with traditional networks, ad hoc networks possess many unique characteristics. For example, ad hoc networks can drop a packet due to network events other than buffer overflow. Unfortunately, the current layered network architecture makes it impossible to pass the information specific to one layer to other layers. As a result, if a packet is lost due to reasons other than buffer overflow, TCP adversely invokes its congestion control procedure. Similarly, the routing algorithm may misinterpret that a path is broken and adversely invoke the route recovery procedure. This study addresses the limitations of the current layered network architecture by adopting a cross-layer protocol design for TCP and routing algorithms in ad hoc networks. The objective of this approach is to enable the lower-layered ad hoc network to detect and differentiate all possible network events, including disconnections, channel errors, buffer overflow, and link-layer contention, that may cause packet loss. Using the information exploited by lower layers, the upper layer-3 routing algorithm, and the layer-4 TCP can take various actions according to the types of network events. Simulation results demonstrate that the combination of the cross-layer optimized TCP and routing algorithms can effectively improve the performance of TCP and DSR, regardless of whether it is in a stationary or a mobile ad hoc network.

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.

A seamless vertical handoff scheme

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: VHOST (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, VHOST 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.

A Tracking-Assisted Routing Scheme for Wireless Sensor Networks

Wireless sensor networks (WSNs) offer much promise for target tracking and environmental monitoring. While many WSN routing protocols have been proposed to date, most of these focus on the mobility of observers and assume that targets are fixed. However, in reality, many applications require for sensing data to be propagated from multiple mobile targets to multiple mobile observers. In addition, WSNs often operate under strict energy constraints, and therefore reducing energy dissipation is also an important issue. In this paper, we present a grid-based routing scheme known as TRENS. First, we address the issue of the WSN comprising multiple mobile targets and observers—with TRENS being the first scheme of its kind to use tracking technology to increase the efficiency of routing procedures in the context of dynamic topology. Next, we introduce a shortcutting approach to resolve energy issues by optimizing routing paths and thus decreasing communication costs and latency. Finally, we conduct extensive simulations to show how TRENS conserves energy and performs better than other grid-based schemes.

A hybrid efficient routing protocol for sink-oriented ad hoc sensor networks

Since most sensor networks expose a funnelling effect caused by the congestion phenomenon, where routing transit flows towards the base station through a distinctive multihop, many-to-one pattern, we propose a Hybrid Efficient Routing Protocol (HERP) with a power awareness algorithm for sink-oriented mobile ad hoc sensor networks to boost the network fidelity and prolong the network lifetime of mobile communications. The results show that HERP can boost network fidelity up to 20% under intense traffic in a localised sink-oriented mobile ad hoc sensor network and effectively prolong the network lifetime of mobile communications.

VA-TCP: a vertical handoff-aware TCP

A vertical handoff occurs when a multi-homed mobile host roams from a network to another heterogeneous network. Nevertheless, the network before and after a vertical handoff usually have drastic different characteristics. In this paper, we proposed an end-to-end based Vertical-handoff Aware TCP, called VA-TCP, which promptly adapts TCP behavior during a vertical handoff. When entering a new network, on the basis of packet-pair algorithm, VA-TCP dynamically estimates the new network bandwidth and round-trip time by using out-of-band ICMP messages. After that, by the estimated bandwidth and round-trip time, VA-TCP adjusts the TCP parameters including congestion window size, slow-start threshold, retransmission timer and RTTs to respond to the new network environment. From the experimental result, the TCP throughput by the proposed VA-TCP scheme is around 2 times that by the original TCP.

BLAS: Block-level adaptive striping for solid-state drives

Increasing the degree of parallelism and reducing the overhead of garbage collection (GC overhead) are the two keys to enhancing the performance of solid-state drives (SSDs). SSDs employ multichannel architectures, and a data placement scheme in an SSD determines how the data are striped to the channels. Without considering the data access pattern, existing fixed and device-level data placement schemes may have either high GC overhead or poor I/O parallelism, resulting in degraded performance. In this article, an adaptive block-level data placement scheme called BLAS is proposed to maximize the I/O parallelism while simultaneously minimizing the GC overhead. In contrast to existing device-level schemes, BLAS allows different data placement policies for blocks with different access patterns. Pages in read-intensive blocks are scattered over various channels to maximize the degree of read parallelism, while pages in each of the remaining blocks are attempted to be gathered in the same physical block to minimize the GC overhead. Moreover, BLAS allows the placement policy for a logical block to be changed dynamically according to the access pattern changes of that block. Finally, a parallelism-aware write buffer management approach is adopted in BLAS to maximize the degree of write parallelism. Performance results show that BLAS yields a significant improvement in the SSD response time when compared to existing device-level schemes. In particular, BLAS outperforms device-level page striping and device-level block striping by factors of up to 8.75 and 7.41, respectively. Moreover, BLAS achieves low GC overhead and is effective in adapting to workload changes.

TARS: An Energy-Efficient Routing Scheme for Wireless Sensor Networks with Mobile Sinks and Targets

Wireless sensor networks (WSNs), which are operated with strict resources, offer promise for surveillance technology including target tracking and environmental monitoring. In some sensor network applications, aggregating and delivering sensing data from mobile targets to mobile sinks is necessary. Nevertheless, such mobility can cause the established routes non-optimal and even broken, and require repeated route recovery procedures. To address this issue, previous TRENS routing protocol presented a tracking-assisted routing scheme to avoid the need for frequent rerouting. In this paper, we propose an enhancement of TRENS, called TARS, that also deals with the routing issue for WSNs consisting of multiple mobile targets and sinks. Compared with TRENS, TARS proposes an enhanced tracking technology and a lightweight shortcutting approach to decrease redundant messages, so as to increase the efficiency of routing procedures and to optimize the routing paths in the context of dynamic topology. Besides, TARS introduces a time-scheduling method to control the radio channels of idle nodes to save more energy. Comparing TARS with previously proposed grid-based schemes through extensive simulations, we unveil that our scheme conserves more energy and achieves better performance in aspect of specified evaluation metrics.

An Agile Vertical Handoff Scheme for Heterogeneous Networks

In this paper, on the basis of end-to-end principle, we propose a mobility management scheme to solve the NAT issue. Furthermore, since different communication technologies own different characteristics, during a vertical handoff, a well-behaved mobility management system should adapt an ongoing connection to different communication systems. In this paper, we also propose a handoff-aware TCP adaptation scheme, called CWND-restore. To verify the feasibility of our approach, we implement a prototype system on Linux. The experimental results show that our mobility management scheme can work under various mobility cases. Furthermore, our proposed CWND-restore scheme can offer higher TCP throughput during a vertical handoff. For example, the TCP throughput by proposed adaptation scheme is at most 2.3 times the original TCP

Reversible data hiding in dual stego-images using frequency-based encoding strategy

Reversible data hiding methods can embed secret data into multimedia to prevent attacks from hackers. Reversible data hiding methods based on multiple stego-images have been proposed extensively in recent years. This type of method can embed large amounts of secret bits into several stego-images. However, data embedding may cause serious distortion of stego-images. To solve this problem, Lu et al. proposed a center-folding strategy to reduce secret digits to narrow down the distance between the original pixel and the stego pixel in 2015. Although the center-folding strategy works well, it can still be improved. In their proposed scheme, the maximum secret digit seriously damages the image quality of the stego-image. If the maximum secret digit occurs often, then the visual quality of the stego-image decreases quickly. In this paper, we propose a frequency-based encoding method to solve this problem. The secret digit that occurs most frequently is encoded as the minimum absolute digit, thereby decreasing the frequency and level of modification. Experimental results showed that the proposed method can achieve a greater peak signal-to-noise ratio value than previous methods, thereby confirming that the proposed method is effective.