Reliable Wireless Connections for Fast-Moving Rail Users Based on a Chained Fog Structure

ARTICLE IN PRESS JID: INS [m3Gsc; June 30, 2016;9:36 ] Information Sciences 0 0 0 (2016) 1–17 Contents lists available at ScienceDirect Information Sciences journal homepage: www.elsevier.com/locate/ins Reliable wireless connections for fast-moving rail users based on a chained fog structure ∗ Tian Wang a, , Zhen Peng a, Sheng Wen b, Yongxuan Lai c, Weijia Jia d, Yiqiao Cai a, Hui Tian a, Yonghong Chen a a College of Computer Science and Technology, Huaqiao University, Xiamen, Fujian Province, PR China b School of Information Technology, Deakin University, Melbourne Burwood, Victoria, Australia c School of Software, Xiamen University, Xiamen, Fujian Province, PR China d Department of Computer Science and Engineering, Shanghai Jiao Tong University, Shanghai, PR China a r t i c l e i n f o a b s t r a c t Article history: Currently, 3G and 4G networks provide customers with high-speed wireless services al- Received 15 November 2015 most everywhere. However, the wireless connection is often unstable and unreliable, espe- Revised 13 June 2016 cially for fast-moving end users (e.g., those on trains and buses). To investigate the sever- Accepted 23 June 2016 ity of this problem, we conducted real experiments on fast-moving trains to investigate Available online xxx the quality of 3G connections. From the results, we found that 1) from the temporal per- Keywords: spective, the 3G connections were not stable and suffered from frequent disruptions of Reliability connectivity, and 2) from the spatial perspective, the connections that were established Fast-moving rail in different train compartments were largely independent. These two findings motivate Fog structure us to propose a brand-new fog computing structure, which acts as an intermediate layer Cascade shifting flow problem between the end users and the 3G infrastructure. This new fog structure introduces a series of mutually chained network gateways that are located in different compartments. This structure addresses the aforementioned problem of unstable connectivity and thus ensures reliable wireless service for fast-moving users, such as passengers on trains. We performed a series of theoretical and empirical analyses to evaluate the performance of the newly proposed structure. All of the experimental results suggest that our proposed fog structure greatly improves the reliability of wireless connections on fast-moving trains. © 2016 Elsevier Inc. All rights reserved. 1. Introduction The popularity of various wireless networks has dramatically increased in recent years. New communication technologies, such as 3G and 4G networks [10,29] , are providing customers with high-speed wireless services. For example, 3G networks support data rates of up to 14.4 Mbps on the downlink and 5.76 Mbps on the uplink for stationary users. They also have reduced latency of nearly 50 ms [36] . As a result, broadband applications can be easily accessed by stationary users in most areas with acceptable signal quality [19] . However, the proliferation of mobile devices, such as smartphones and tablets, and their associated applications has considerably increased along with the growing availability of wireless connectivity [24] . With rising demands for real-time data, such as videos and live streams, customers strongly desire consistent connections with high-speed transmission capability. Considering Hong Kong as an example, an average of 4.5 million people use ∗ Corresponding author. E-mail address: [email protected] (T. Wang). http://dx.doi.org/10.1016/j.ins.2016.06.031 0020-0255/© 2016 Elsevier Inc. All rights reserved. Please cite this article as: T. Wang et al., Reliable wireless connections for fast-moving rail users based on a chained fog structure, Information Sciences (2016), http://dx.doi.org/10.1016/j.ins.2016.06.031 ARTICLE IN PRESS JID: INS [m3Gsc; June 30, 2016;9:36 ] 2 T. Wang et al. / Information Sciences 0 0 0 (2016) 1–17 trains (MTR lines) to commute on workdays each week [23] . Train passengers often use their smartphones to watch online videos on services such as YouTube [30] and play games such as Hearthstone [27] . These online video services and games typically have stringent performance requirements, including sufficient connectivity time, short delay, and high bandwidth [37] . For example, online games require consistent network coverage to smoothly deliver streaming data. Support for mobile applications via wireless networks has recently received significant research attention [12,13,25] . In this paper, we focus on the application scenario of wireless services in public transport systems. To investigate whether current wireless networks (e.g., the 3G network) can support the use of wireless services on fast-moving vehicles, we conducted a series of real experiments in trains running over a distance of more than 100 km in total during a period of 3 months. We found that 1) from the temporal perspective, the 3G connections were not stable and suffered from frequent disruptions of connectivity, and 2) from the spatial perspective, the connections that were established in different train compartments were largely independent. These two findings suggest that when the bandwidth of one device is low or the connection has been broken, other devices may still be functioning well with their wireless connections. This inspired us to design a structure that can help to share communication capabilities among neighboring devices through multiple links. This structure can minimize connection disruptions and improve the reliability of connectivity. Accordingly, we propose a novel fog structure [35] for providing wireless services, specifically for fast-moving users on trains. This fog structure is an intermediate layer between the end users and the 3G/4G infrastructure (e.g., Node Bs) and addresses the problem of unstable connectivity problem. This structure consists of a few end devices called Personal Gate- ways ( PGs ) [26] , such as set-top boxes and access points (APs) [38] , deployed at various positions on a train. The PGs are connected and can share their communication capabilities to provide wireless service to different end users on the train. The deployment of our fog structure will provide the following benefits. First, the PGs can have more powerful signal re- ceivers embedded inside. The PGs will therefore establish more reliable connections with Node Bs to support services for end users, especially in fast-moving vehicles. Second, because the connected users and the required network resources may be different for each PG , the network resources can be redistributed among PGs to improve their efficiency of use. Based on the newly proposed fog computing structure, we studied the Cascade Shifting Flow (CSF) problem and ex- amined the wireless services provided to fast-moving users on trains. By mathematically formulating the CSF problem, we were able to solve this problem by limiting the maximum shifting hops of the communication flows, thereby minimizing the maximum delay while maximizing the throughput. Furthermore, we designed a realistic and localized algorithm for flow shifting that collects information from neighboring PGs . The main contributions of this paper as follows: 1. We conducted extensive experiments in the real environment of interest (i.e., trains) to examine the reliability of 3G/4G connections for fast-moving users. The results suggested that the connectivity was not stable or reliable. 2. A new fog computing structure, which consists of a series of chained fog devices, is proposed to improve the reliability of wireless services under fast-moving conditions. 3. We theoretically studied the CSF problem and developed an approximately optimal solution to maximize the availability and reliability of throughput while minimizing the communication delay. 4. Extensive evaluations were performed to validate the effectiveness of the proposed method. The results suggest that the method achieves high throughput, a lower packet loss rate and low communication delay. The remainder of the paper is organized as follows: Section 2 presents a review of related work. Section 3 introduces our empirical study of 3G/4G connectivity for fast-moving users. We also introduce our new fog computing model in this section. Section 4 discusses the CSF problem, and Section 5 presents our evaluations, followed by the conclusions of the paper in Section 6 . 2. Related work As a commonly used solution for reliable and fast wireless communication, 3G cellular networks and their related im- proved techniques, such as HSPA , have garnered wide attention [11] . In [21] , Liu et al. reported a large-scale empirical field study of the performance of 3G mobile systems in China. Their measurements were performed in five cities and included three different 3G standards. They observed that for voice service, indoor performance is better than outdoor performance. Specifically, fast movement can lead to an unsteady wireless environment, thereby causing performance degradation in 3G networks. However, their study did not include HSPA . In [18] , Laner et al. investigated the 3G uplink delay based on measurements performed in an operational HSPA network. They provided models for the latency of single network entities and the accumulated delay. Their results showed that the delay is strongly dependent on the packet size, with random compo- nents depending on synchronization issues. In [40] , Xu et al. showed that the performance of downloads in 3G/ HSPA mobile networks can be significantly degraded by

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