Communication-based train control (CBTC) network is an automated control network for railways that ensures the safe operation of rail vehicles using data communications. CBTC is based on two important technologies that marked profoundly the development of our society in the last century: railways and communication technologies. It is a modern successor of traditional railway signaling systems that provide a limited control using track circuits, interlockings and signals. In most CBTC networks, data between trains and trackside equipments are transferred bidirectionally by wireless communication networks, such as global system for mobile communications-railway (GSM-R) and wireless local area network (WLAN). For urban mass transit systems, WLAN is a better choice due to the available commercial-off-the-shelf equipments. WLAN-based CBTC has been deployed in many real systems, such as New York City Canarsie Line, Beijing Metro Line 10 from Siemens, and Las Vegas Monorail from Alcatel. We will focus on WLAN-based CBTC networks in this article.
Communication-based train control networks have stringent requirements for wireless communication availability and latency. Whereas in commercial wireless networks, less service availability and long latency mean less revenues or/and poor quality of services (QoSs); in CBTC networks, less service availability could cause train derailment, collision or even catastrophic loss of life or assets. Therefore, it is important to ensure the wireless communications are available when they are needed, and the latency is minimized in CBTC networks. Furthermore, in recent years, there have been significant developments of high speed train systems around the world (e.g., China railway high-speed (CRH) systems with the maximum speed of 352 km/h), which introduce new non-trivial challenges to the CBTC designs in the high speed environment.
Most existing WLAN-based CBTC networks are using traditional IEEE 802.11 technologies, such as 802.11a/b/g. However, IEEE 802.11a/b/g WLANs were not originally designed for high speed environments. Particularly, when a train moves away from the coverage of a WLAN access point (AP) and enters the coverage of another AP along the railway, a handoff procedure occurs, and this handoff process may result in communication interruption and long latency. The handoff procedure can be divided into four steps, namely probing (also referred to as scanning), channel switching, authentication and association. This whole procedure may take up to several hundreds milliseconds.
There are several schemes proposed in the literature to decrease WLAN handoff latency. Fitzmaurice and Mishra et al. have shown that over 90% of the time in the handoff process is spent in the scanning stage. Therefore, most of previous works in optimizing WLAN handoffs focus on making the scanning process more efficient. A SyncScan technique is proposed in, in which appropriate time synchronization is required between APs and clients. A topology inferencing technique in both clients and APs is proposed in to improve the scanning process. A cooperative handoff framework is proposed in to utilize mechanism for information sharing to reduce the delays during the scanning/probe phases. In, a fast handoff scheme that skips all mentioned stages is proposed, where handoff is controlled and prepared by the access network and is triggered by sending a hop request message to the mobile station (STA). There are some schemes using multi-radio in mobile clients trying to reduce the WLAN handoff latency. Adya et al. proposed a protocol to allow multi-radio mobile nodes in a mesh network to potentially establish two separate wireless links between a pair of nodes. This work primarily focuses on improving efficiency of wireless mesh networks, which is different from the CBTC networks considered in this article.
It is necessary to look at the handoff management at multiple layers of the protocol stack, not just at the data link layer as considered in the past. Indeed, the handoff management problem can be solved at transport layer[17–20]. For example, stream control transmission protocol (SCTP)[21, 22], a new IETF-standardized transport layer protocol in addition to transmission control protocol (TCP) and User Datagram Protocol (UDP), can be used to solve the handoff management problem. The multi-homing, multi-stream and partial reliable data transmission features of SCTP are especially attractive for applications that have stringent performance and high reliability requirements. Compared to other handoff management approaches, transport layer schemes have many advantages, such as improved throughput and latency performance. Moreover, no third party other than the endpoints participates in handoff process, and no modification or addition of network components is required, which makes transport layer approaches attractive in WLAN-based CBTC networks, where commercial-off-the-shelf equipments are widely used.
Although some works have been done for the handoff management in CBTC networks, most of them are focused on handoff protocols and network architectures, and handoff decision policy issues (i.e., when to execute handoff) are largely ignored in CBTC networks. However, due to the high mobility environment, as well as the high availability and low handoff latency requirements, handoff decision policy issues are very important in designing CBTC networks, which will significantly affect the overall system performance.
In this article, we study the handoff decision policy issues in CBTC networks based on SCTP and IEEE Std 802.11p-2010 WLANs[24
], which is an emerging technology for vehicular communication networks. To the best of our knowledge, the design of handoff management in CBTC networks based on SCTP and IEEE 802.11p WLANs has not been done in previous works. The distinct features of the proposed scheme are as follows.
We propose a handoff management scheme based on SCTP and IEEE 802.11p WLANs to provide high communication availability and low latency in CBTC networks.
We formulate the handoff decision problem as a stochastic Semi-Markov decision process (SMDP), which has been successfully used to solve finance and admission control problems, among others. This article focuses on the application of SMDP to the handoff decision problem in CBTC networks.
Minimizing the handoff latency is one of the objectives in the proposed scheme. In addition, since multimedia information, such as train schedule, weather forecast, live news, sports and finance, is more and more popular in railway communication networks, we also consider maximizing the SCTP throughput in our scheme.
Extensive simulation results are presented. It is illustrated that the proposed scheme can significantly decrease the handoff latency and improve SCTP throughput in CBTC networks.
The rest of this article is organized as follows. The 802.11p and SCTP based CBTC network with the corresponding handoff procedure is presented in Section “The proposed CBTC network based on SCTP and IEEE 802.11p”. The SMDP based handoff decision model, optimality equation, and value iteration algorithm are described in Section “Optimal Handoff the CBTC network using SCTP and IEEE 802.11p WLANs”. Some implementation issues are given in Section “Implementation issues”. Simulation results are presented and discussed in Section “Simulation results and discussions”. Finally, we conclude our study in Section “Conclusions and future work”.