Internet Traffic Performance in High Speed Trains 1

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1 Internet Traffic Performance in High Speed Trains 1 Dmitry Sivchenko a, Bangnan Xu a, Gerd Zimmermann a, Sven Hischke b a T-Systems, Technologiezentrum, Am Kavalleriesand 3, D Darmstadt, Germany b Deutsche Telekom AG, Friedrich-Ebert-Allee 140, D Bonn, Germany dsivchenko@web.de; Bangnan.Xu@t-systems.com Abstract: Internet is increasingly important to our work and daily life. With the fast development of wireless technologies Internet can now be accessed anywhere and anytime. This paper investigates the Internet traffic performance in high speed trains, considering various wireless channels to connect the high speed train and wireless access network. Wireless link parameters such as bandwidth, propagation delay and error rate have a considerable impact on the quality of Internet access. The traffic performance of http, ftp, cbr and is evaluated intensively by the Network Simulation Tool NS2. Keywords: Wireless Internet Access, traffic performance, high speed trains, traffic modelling, NS2 1. INTRODUCTION Internet access is increasingly important for the business trip and private life. With the great success of wireless technologies, it is possible to provide the users Internet access anywhere and anytime. In comparison to wired link, wireless medium has its own characteristics. High error rate, larger link delays (i.e. round trip times) and smaller link bandwidth of the wireless medium result in limitations on connection quality. The problem becomes more challenging if high mobility must be dealt with such as in the high speed train. This paper investigates the Internet traffic performance for the scenario shown in Figure 1, where IEEE WLAN is used within the train and various wireless channels may be used to connect the train gateway to the access infrastructure. The wireless connection between the train and the access infrastructure is the bottleneck of Internet Access of the users in the train. The influence of parameters such as bandwidth, propagation delay and error rate of this wireless link on the Internet access is studied in this paper. The paper is organised as follows: Section 2 describes the traffic models to be used. Computer simulation using NS2 is discussed in Section 3. Section 4 presents simulation results. A final conclusion in Section 5 closes this paper. Fig. 1. Internet Access in High Speed Trains 1 This work is partly supported by the German Minister for Education and Research under IPonAir Project P27/1

2 2. TRAFFIC TYPES The most expected traffic type in the networks is the traffic of the http applications. The web traffic is an incoming traffic for users, e.g. downloading requested html pages with different sizes from http servers in the Internet. According to [3] this traffic type takes up to roughly 75% of all data transfers in the Internet. The other general services in the Internet are ftp, real-time communications (based on the cbr traffic) and applications. We assume, that in our application scenario the ftp and cbr traffic are incoming to users only. The uplink bandwidth in wireless channels is generally much smaller than that of the downlink. Therefore it is not possible to provide users a large uploading data rate and it is not considered that users upload any large data files using ftp. For the same reason the uplink cbr services are not considered, as the delay for a realtime traffic is too large. The traffic of applications is similar to the ftp traffic, but the size of messages is generally smaller than that of data files transferred with ftp. However, the incoming traffic usually uses an application like Outlook Express (ftp traffic) or web-mail (http traffic). Therefore only outgoing traffic from mobile users is evaluated. Each traffic type has its own quality requirement. For http traffic it is important for users to get a requested html page as soon as possible. The average response time defined as the time it takes to complete an html page transfer once the page transfer is initiated is the essential parameter to be evaluated. Most of html pages are expected to be transferred within this time to the accessing user. For ftp and users the effective throughput is of most interest. The effective throughput is defined as the ratio of the number of the data bits in the transported file to the time it takes to transmit it [4]. So the effective throughput refers to how fast a data file using ftp connection can be sent on the wireless link. The time needed to transmit a long data file using ftp or an message can be estimated using these throughput values. Packet delay is the parameter used to define the quality of a cbr connection. Some results are compared to an ISDN reference user with standard 64 Kbit/s data channel. The provided Internet Access in the train must have at least the same quality as the ISDN channels that clients usually have at home. 3. COMPUTER SIMULATION The traffic performance is intensively studied using Network Simulator NS2 [1]. A structure shown in Figure 2 is used to analyse the traffic performance. Fig. 2. Simulated network The simulated network consists of one cbr server (node 0), one ftp server (node 1), two http servers (nodes 2 and 3), one server (node 4), one router (node 5), one wireless access router (AR, node 6) and various amount of mobile nodes (MN) accessible through the wireless access router (MNs are not shown on the network structure). The main goal of the simulations is to determine the traffic performance that may be provided to users in high speed trains depending on wireless link parameters and number of active users in the train. The wired links are with the data rate of 100 Mbit/s and the link delay of 2ms. The access network P27/2

3 in the train is IEEE WLAN that consists of mobile nodes and the wireless access router. The wireless channel between the train and land infrastructure, i.e. connection between node 5 and node 6, is simulated as an asymmetric link between the router and AR. Its downlink/uplink parameters such as bandwidth, error rate and link delay can be set according to the wireless technologies to be used. 3.1 Simulated traffic models Every user has one traffic connection, no handover occurs during the simulation time. The amount of active users in the train is varied between 1 and 50. The cbr server is used for generation of real-time streams such as video or speech traffic with the data rates of 128 kbit/s and 8 kbit/s respectively; the ftp server contains some data files of different sizes, these can be downloaded by users; every http server has a html page pool, users access html pages from these pools; the server is used to receive the outgoing traffic from users. Four traffic types are simulated using the described network servers and distributed among active users according to the models in Table 1. At least one cbr connection (128 kbit/s) is used if cbr connections are simulated. 3.2 Parameters of the traffic types Traffic http ftp cbr model down down down up 1 80% 10% 0 10% 2 70% 20% 0 10% 3 70% 10% 10% 10% 4 100% Table 1. Models for distribution of traffic types among active users The transport layer of http, ftp and traffic is TCP with the packet size of 1000 bytes. cbr traffic uses UDP with a packet size of 210 bytes. The packet interval of cbr traffic is varied according to the cbr data rate. Html pages with a data size according to [2] are generated in the pools and for stochastic evaluation a new random model for html pages is implemented in NS2. The data size of an html page consists of main object size and size of a number of embedded objects in the page. Distribution values of stochastic parameters of simulated traffic types are given in Table 2. Traffic parameter http main object size http embedded object size http number of embedded object http reading time Distribution Truncated Lognormal Truncated Lognormal Distribution function 2 1 (ln( x) µ ) f x = exp 2 σx 2π 2σ Mean = 11000; Variance = 2, Min = 1000;Max = Mean = 7000; Variance = 1 Min = 1000; Max = Uniform Min = 0; Max = 5 Exponential f x e x = λ λ, λ = http data file size Truncated Exponential λ = http Duration Truncated Exponential λ = http message size Truncated Exponential 5 λ = Start time value Truncated Exponential λftp = 0.05; λcbr = ; λ = 0.02; Table 2. Distribution values of traffic parameters P27/3

4 To smooth the link load at the start of simulations a random value with exponential distribution (start time) is used. Table 3 contains parameters of all simulated traffic types. Traffic type http ftp cbr Parameters Start, sec. 1 Interval, sec. av: 30 Page Size, Kb av: 55, min: 1, max: 550 Start, sec. av: 20, min: 5, max: 150 File Size, Kb av: 2000, min: 50, max: 5000 Start, sec. av: 30, min: 5, max: 150 Duration, sec. av: 90, min: 10, max: 150 Start, sec. av: 50, min: 5, max: 150 Size, Kb. av: 30, min: 1, max: 1000 Table 3. Parameters of traffic types 4. PERFORMANCE EVALUATION To investigate the Internet traffic performance in trains two wireless links connecting the train and the fixed infrastructure are used. The downlink/uplink propagation delay and bandwidth of the wireless channel are given according to the wireless technologies. The link delay of 3ms/3ms corresponds to an IEEE wireless channel and 500ms/100ms to the link delay of a satellite communications link with terrestrial return channel (mobile radio). 4.1 Packet delay of the cbr traffic To study the performance of cbr traffic that corresponds to real time traffic, we use the traffic model 3 (see 3.1). Figure 3 presents packet delay of the cbr traffic vs. downlink bandwidth of the wireless channel under various numbers of mobile users in a train. The uplink bandwidth is 25% of that of the downlink, and the link is assumed error-free. Fig. 3. cbr packet delay for link delays 3ms/3ms (downlink/uplink) From Figure 3 we can see that the link delay has a significant impact on the mean packet delay. Using satellite link with a link delay of 500ms/100ms, the mean packet delay is always larger than 0.5s. That means that satellite link is not suited to support any real time traffic no matter how high the downlink bandwidth is. With an IEEE b wireless link, 10 MNs can be supported keeping the mean packet delay lower than 0.5s with a downlink bandwidth of 512 kbit/s, while 30 MNs can be supported if the downlink bandwidth is P27/4

5 as high as 1024 kbit/s. If the downlink bandwidth is large enough so that data packets can be sent over the link at once, the mean packet delay cannot be reduced further. In this case the packet delay is proportional to the propagation delay, there is no wait time in the link queue. 4.2 Throughput of the ftp traffic Figure 4 shows the effective throughput of the ftp traffic per user (see Section 2) vs. downlink bandwidth of the wireless channel under various traffic models, number of active users in the train and wireless link delays. The transport layer for ftp is the TCP, therefore only acknowledgement packets (ACK) will be sent back to the ftp server. The size of these ACK packets is insignificant therefore the uplink data traffic is very small and simulated results do not differ with different uplink bandwidths (e.g. 25% or 50% of the bandwidth of the downlink), the uplink bandwidth of the presented results uses 25% of that of downlink. Error free link is assumed. Fig. 4. Throughput of the ftp traffic We can see that a larger ftp effective throughput per user can be achieved if the number of active users is smaller and the downlink bandwidth is larger. More active users mean more ftp connections. Due to the waiting time for ACK packets, the total throughput is reduced if more ftp connections are active at the same time. We can also see that the round trip time (RTT) is a very important parameter for the effective throughput. This is because the next data packet cannot be sent by the server if the transfer of ACK packets from the client to the server takes more time. Thus, the time needed to transmit the data file increases, and the mean effective throughput is reduced. From Figure 4 we can see that the effective throughput with wireless link of 500ms/100ms is much less that that with wireless link of 3ms/3ms. We can also observe an interesting result with a large downlink bandwidth the effective throughput will not be increased any more after a threshold of 1536 kbit/s. The reason is that the ftp effective throughput cannot be increased further by increasing of the downlink bandwidth due to the RTT. We can also see that the results of the traffic model 1 are slightly better than of the model 2. This is because the traffic model 2 has much more ftp traffic than traffic model 1. As ftp traffic uses most of the channel resources (transfer of large data files), the effective throughput of a particular ftp user is smaller if more ftp traffic exists. 4.3 Response time of http traffic WWW applications generating http traffic are mostly used in Internet. The traffic model 4 is used for simulations with the uplink bandwidth of 50% of the downlink one. The mean response time of an html page vs. number of active users under various downlink bandwidths without errors is shown in Figure 5. P27/5

6 Fig. 5. Response time of http traffic, error free The average response time of an http page for an ISDN user with a 64 kbit/s channel is 7.89 seconds. A response time lower than this value is better than that of the ISDN reference user. A comparison of ISDN quality with other scenarios reveals the approximate number of users that can be use Internet access at the same time with a quality similar to that of ISDN users. We can see that if the downlink bandwidth is larger than 768 kbit/s even 50 http users with large link delays have a better service quality than an ISDN user. The impact of errors in the wireless channel with the downlink bandwidth of 384 kbit/s is displayed in Figure 6. From Figure 6 we can see an interesting result that the error rate at a large link delay is much more significant than at the small delay. Thus with the bit error rate (BER) of 1e-5 the response time at 3ms/3ms link delay is not much changed, while at the link delay of 500ms/100ms the response time are not acceptable even at the error rate of 5e-6 BER. This is due to the TCP protocol. At the small link delay the client receives the resent packet and replies with the ACK much more quickly than at larger link delay. Therefore loss of a data packet has more significant impact at a large link delay. Fig. 6. Impact of channel errors on the response time of http traffic 4.4 Throughput of the traffic The effective throughput vs. downlink bandwidth under various numbers of mobile users in the train and different link delays is shown in Figure 7, error-free wireless link is assumed. The results are presented for the traffic model 1. P27/6

7 Fig. 7. Throughput of traffic The traffic is the only outgoing data traffic from users and its data packets will be transmitted over the wireless uplink towards the access infrastructure. The downlink bandwidth does not impact the effective throughput in this case. Hence the throughput with the uplink bandwidth of 50% is much larger than with 25% of the downlink bandwidth. With the same reason as described in Section 4.2 the effective throughput is also larger at the smaller number of users in the train and at a smaller wireless link delay. A saturation of the effective throughput at the 3ms/3ms delays is not reached as the maximal uplink bandwidth (only 1024 kbit/s) is not enough to provide the minimal RTT. 5. CONCLUSION The results presented in this paper reveal influence of the wireless link parameters on the Internet traffic performance that can be provided to users in a train. The widely used traffic types are evaluated in this paper. A desired service quality can be derived from the obtained results. From the simulation results we can conclude that the link delay has a significant impact on the traffic performance of cbr, ftp, http and traffic. A large link bandwidth is not enough to achieve a better traffic performance. The link delay has a significant impact on the mean packet delay of cbr traffic. If the downlink bandwidth is large enough so that data packets can be sent over the link at once, the mean packet delay of CBR traffic cannot be reduced further. The round trip time that is determined also by the link delay is a very important parameter for the effective throughput of the ftp traffic. Moreover, the error rate at a large link delay is much more significant than at the small delay with the ftp traffic. REFERENCES [1] The Network Simulator ns2, Start Page: nsnam/ns/ [2] N.K. Shankaranarayanan, Traffic Models for IEEE MBWA System Simulations (Baseline Draft), 16 July [3] D. Staehle, K. Leibnitz, P. Tran-Gia, Source Traffic Modeling of Wireless Applications, Research Report Series, Report No. 261, June [4] Technology Development Group, Loral CyberStar, Inc., TCP/IP Performance over Satellite Links - Summary Report, 29 March P27/7