2015 IEEE Vehicular Networking Conference (VNC)
Improving QoS on High-Speed Vehicle by Multipath Transmission Based on Practical Experiment Ping Dong*, Xiaojiang Du†, Tao Zheng*, Hongke Zhang* *
School of Electronic and Information Engineering, Beijing Jiaotong University, Beijing 100044, China Department of Computer and Information Sciences, Temple University, Philadelphia, PA 19122, USA
[email protected],
[email protected]
†
the limitation of user devices, onboard users can hardly have a good experience if they can only visit the Internet via one wireless network, e.g. laptops with WiFi, which may be subjected to coverage gaps and signal attenuation.
Abstract—With the boom of modern high-speed vehicles and various wireless technologies, providing stable and high-quality Internet services on board is becoming a rising trend for both passengers and operators. However, a major challenge is that the vehicle-to-ground communication links may suffer from insufficient bandwidth, long round-trip time, and high packet loss rate. Although multiple wireless networks, such as WiFi, 3G and 4G, may be available along the track, onboard users can hardly have a good experience if they can only visit the Internet via one wireless network because of the limitation of the user devices, e.g. laptops with WiFi. The wireless network supported by a device may be subjected to coverage gaps and signal attenuation. In this paper, we first measure the link qualities of multiple existing 3G and 4G technologies, which are candidates for the vehicle-to-ground communication, in a typical high-speed environment on high-speed trains in China. Then, a concurrent multipath transmission scheme is proposed, which provides transparent and effective Internet services for the users through multiple available wireless technologies, without requiring the participation of the user devices in any multipath signaling. Experiments show that the scheme can improve QoS (Quality of Service) of Internet on board by increasing the available bandwidth and reducing the packet loss rate.
To solve the problems, some proposed onboard schemes [3] designed mobile routers with multiple interfaces for vehicle-toinfrastructure communications. These interfaces may support various wireless technologies, such as WiFi, cellular network, and satellite technology. For example, FIFTH (Fast Internet for Fast Train Hosts) and MOWGLY (MObile Wideband Global Link sYstem) projects in Europe provide Internet services for trains by satellites and fill the coverage gaps by WiFi. Recently, with the rapid development of 3G and 4G wireless techniques, which may provide higher bandwidth and lower transmission latency at less cost, the cellular-based solutions [4] for highspeed vehicular networks are generating more interest. To allow multiple interfaces to work cooperatively to provide more stable and higher quality onboard services, a mobile router is required to have the ability of monitoring the link status of heterogeneous wireless interfaces. In addition, the mobile router should implement some “intelligence” to select the best interface(s) to transmit data in the vehicle-to-ground communications. Some previous works [5] [6] performed measurements for existing mobile communication networks, but they look a bit dated with the rapid development of mobile technologies. NEMO (Network Mobility) [7], which is an IETF standard for mobile networks, does not support multiple interfaces. Although SCTP [8] supports multi-homing, it does not transmit data on multiple paths simultaneously; instead, it selects a primary data transmission path with enabling transparent fail-over between redundant network paths. Recently MPTCP has been presented to transmit data on multiple paths simultaneously. However, some research [9] shows that the throughput of MPTCP may decrease significantly when there are some bottle-neck paths.
Keywords—mobile Internet; vehicle; high-speed; wireless
I. INTRODUCTION With the rapid development of modern high-speed vehicles and the boom of various wireless technologies, providing stable and high-quality mobile Internet services on board is becoming a rising and inspiring trend for both passengers and operators today. Japan, Germany and France are rapidly deploying their national high-speed rail networks [1]. Also for China, according to a statistics [2] published by the National Railway Administration in April 2015, the number of passengers has reached 2.357 billion in 2014. Meanwhile, a report from WiFi Alliance, the certification authority of WiFi devices, shows that about 4.5 billion WiFi products are in use all over the world today. Thus, it is becoming crucial to provide mobile Internet services to passengers on the highspeed vehicles.
In this paper, we introduce a scheme of concurrent multipath transmission based on multiple interfaces in highspeed mobility. Our contributions are as follows:
However, several special issues associated with high-speed mobility [3], such as high penetration losses for signals, severe Doppler shift, and frequent handoffs, are faced by the mobile network systems for high-speed vehicles. These problems lead to the instability of popular Internet services and result in poor QoS (Quality of Service). Although multiple wireless networks, such as WiFi, 3G, 4G, may be available along a track, due to
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(1) We perform practical experiments to evaluate the link status of multiple 3G and 4G mobile networks (EV-DO, FDDLTE/HSPA+, TD-LTE) under different status (static and on high-speed trains). (2) We propose a concurrent multipath transmission scheme, which provides transparent and effective Internet
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Campus Server in Network our university
China Unicom FDD-LTE/ HSPA+
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2015 IEEE Vehicular Networking Conference (VNC)
China Mobile TD-LTE
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services for the users through multiple available wireless technologies, without requiring the participation of the user devices in any multipath signaling.
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II. NETWORK CHARACTERISTICS OF MOBILE TECHNOLOGIES
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Fig. 1 shows our experimental environment. We measured the network characteristics of 3G and 4G technologies by establishing communications between the clients, the mobile router and the server. There are three major mobile network providers in China, i.e., China Telecom, China Unicom and China Mobile. EV-DO (3G), FDD-LTE/HSPA+ (4G/3G) and TD-LTE (4G) are the technologies adopted by their networks, respectively. We evaluated all these technologies in our tests. The mobile router, with multiple MiniPCI-E 3G/4G cards, was placed on a high-speed train and could visit the Internet through multiple networks simultaneously. Note that for China Unicom, the card may change its mode between FDD-LTE and HSPA+ automatically due to the incomplete coverage of FDDLTE. Each interface was allocated IP addresses dynamically when the train moved along the rail. The Server was placed in the Information Center of our university and could visit the Internet through our campus network.
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From Fig. 2, we observe that the RSSIs of all the 3G/4G technologies keep stable at a relative high level in a static state. However, as shown in Fig. 3, the RSSIs fluctuate remarkably and can even be 0 at some time. Fortunately, we can also observe that different 3G/4G wireless networks can complement each other well at some geographic coordinates.
During the experiments, we recorded the RSSI (Received Signal Strength Indicator), RTT (Round-Trip Time) and packet loss rate of different networks. For comparison, we performed the experiments both in our laboratory in a static state and on a high-speed train with the speed of 300km/h.
RTT was obtained by a modified “ping” program. We modified the source codes by adding a timestamp in ICMP messages, and recorded the RTT in a MySQL database in Linux (Ubuntu 14) every 5 seconds. Fig. 4 shows the RTT between the static client and the server. The RTTs are relatively low (around 100ms~200ms) and stable for all the networks in a static state, while from Fig. 5 we can conclude that in the high-speed environment, the RTTs become much larger and unstable.
The RSSIs were obtained by sending AT commands to the 3G/4G cards plugged into the mobile router and clients. The AT command returns a value between 0 and 31, which is a relative quality of the received signal and unique to each cellular technology. We use the values to evaluate the stability of each technology under different status.
During the experiments for packet loss rate, we got similar trends with RTT in a static state and on a high-speed train.
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2015 IEEE Vehicular Networking Conference (VNC)
TABLE I. COMPARISON BETWEEN STATIC AND HIGH-SPEED EVIRONMENT
Average RSSI Average RTT Average packet loss rate
EV-DO FDD-LTE/HSPA+ TD-LTE EV-DO FDD-LTE/HSPA+ TD-LTE EV-DO FDD-LTE/HSPA+ TD-LTE
High-Speed Train 17.7 15.6 18.3 1255ms 763ms 1163ms 11.6% 9.2% 13.6%
Internet
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Fig. 6 Overview of the Concurrent Multipath Transmission Scheme
new assigned addresses whenever a handover occurs. When a SCTP tunnel is adopted, the access router can be informed of the new addresses by SCTP messages, while when a UDP tunnel is adopted, Mobile IP or a self-define message can be used to notify the access router about the changing of addresses.
Table I shows the average RSSI, RTT and packet loss rate for EV-DO, FDD-LTE/HSPA+ and TD-LTE in the experiments. According to the results, we can conclude that the network characteristics in a high-speed environment are quite different from those in a static environment.
The multipath management module is responsible for establishing logic tunnels on wireless links. It selects and changes the modes of the tunnels according to link conditions. It also changes the parameters of tunnels according to the changing of link parameters, e.g., lifetime parameter in PRSCTP. Three kinds of tunnel modes are designed: (a) PR-SCTP [8] Tunnel, which can provide partial transmission reliability for data on an unstable wireless link. (b) UDP Tunnel, which can maximize the transmission of data without considering the loss of packet. When UDP tunnels are applied, the upper layer applications are responsible for the reliability of data. The advantage of a UDP tunnel is that it can maximize the survival of the tunnel in the atrocious wireless environments. (c) Mixed Mode, which establishes PR-SCTP tunnels on some links and UDP tunnels on the others, so that the two kinds of tunnels can play to their strengths. Note that each tunnel is independent of other tunnels in all the tunnel modes.
III. A CONCURRENT MULTIPATH TRANSMISSION SCHEME A. Overview Fig. 6 shows an overview of our concurrent multipath transmission scheme. It is proposed to provide more stable Internet services for the onboard users by multiple available wireless technologies, without requiring the participation of the user devices in any multipath signaling. In the scheme, one or more mobile routers are deployed on a high-speed vehicle. Moreover, a mobile router can connect to the Internet by accessing multiple external wireless networks that are available along the track, including WiFi, 3G, 4G, satellite. The mobile router then provides transparent Internet services for the users onboard. An access router is a device located on the ground. It visits the Internet via wired links, and maintains logic tunnels with multiple mobile routers.
In the data layer, three scheduling modes are available:
The logic tunnels are established between a mobile router and an access router. All the data transmitting from the user devices to the Internet will be routed through the tunnels, and then be forwarded to the Internet by the access routers. NAT (Network Address Translation) is deployed at the access routers. We choose network layer for performing scheduling between different tunnels due to its flexibility.
(a) Arbitrary Mode, which forwards the IP packets equally on all the available links. The arbitrary mode can balance the throughput among all the links. (b) Fee-Based Mode, in which the packets are preferentially transmitted to the low-cost link. This mode is designed for the operators. In order to reduce operating costs, when multiple wireless links are available, the one with lowest cost is preferred to transmit data.
B. Working Schemes Fig. 7 introduces the proposed scheme in detail. In the control layer, the mobility management module maintains the available IP addresses allocated by multiple wireless networks. In addition, a mobile router will inform the access router of the
(c) Client-Server Pair Mode, which is based on the IP-pair of (client IP, server IP) and forwards the IP packets belong to a same IP-pair on a same link. More packets are scheduled on the links with higher bandwidth. The packets from a client can
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2015 IEEE Vehicular Networking Conference (VNC)
be forwarded on different links. Due to the fact that different links may have different transmission delays, this mode can reduce the out-of-order of packets between a client-server pair as much as possible. In practice, the total bandwidth provided to users is usually less than the sum of the bandwidth of each link because: (1) an outer header specific to the tunnel mode is added before the original IP header, and (2) the out-of-order of packets may lead to retransmission of some packets. Our scheme is open and other scheduling algorithms may also be adopted. C. Experiments on a High-Speed Train We performed some experiments on a high-speed train to evaluate the scheme. In the experiments, the data was generated by real applications. All the data was captured and stored in a MySql database.
Fig. 8 Bandwidth Aggregation of Multiple Paths
First, we made an experiment about the performance of multipath transmission. Two EV-DO cards, two FDDLTE/HSPA+ cards and two TD-LTE cards were installed in a mobile router. The UDP tunnels, together with Client-Server Pair scheduling mode, were used to aggregate the available bandwidth of the six links and provide the total bandwidth to the onboard devices. The data rate of each of the six links and the total data rate are shown in Fig. 8. We can observe that the concurrent multipath transmission scheme can remarkably improve the overall throughput more than each of the individual networks, and can also make up the coverage gaps of individual wireless network well. By this way, our scheme can provide more stable and higher QoS Internet services for users, and thus reduce the influences of fast motion.
Fig. 9 Comparison the packet loss rate of PR-SCTP and UDP tunnels
ACKNOWLEDGMENT This work was supported in part by the Fundamental Research Funds for the Central Universities No. 2014JBM004 and No. 2015JBM001, in part by Beijing Higher Education Young Elite Teacher Project under Grant No. YETP0534.
Second, to show the difference between a PR-SCTP tunnel and a UDP tunnel, we performed an experiment and made statistics of the packets loss rates of the two kinds of tunnels. We sent ICMP packets at a high rate (one packet per 10 milliseconds) in both PR-SCTP and UDP tunnels, and recorded the packet loss rate of both tunnels every five seconds. Fig. 9 shows the results. For a UDP tunnel that does not retransmit the lost packets, the packet loss rate of the UDP tunnel is almost as same as that of the original wireless link. In contrast, a PR-SCTP tunnel will partially retransmit the lost packets so that its loss rate is lower than that of the UDP tunnel.
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However, please note that when the packet loss rate of a link is high, the packets waiting to be retransmitted may fill up the sending buffer of a PR-SCTP tunnel, so that the new packets can hardly be transmitted. In the real test, we had observed that the PR-SCTP tunnel might stop working under some severe link conditions.
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IV. CONCLUSION In this paper, some practical measurements of the existing 3G and 4G technologies are performed in a typical high-speed environment on high-speed trains in China. Then, a concurrent multipath transmission scheme is proposed, which provides transparent Internet services for the onboard users by multiple available wireless networks. Experiments show that the scheme can improve QoS of Internet services on board by increasing the available bandwidth and reducing the packet loss rate.
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