Providing broadband Internet access to railway passengers is a challenge. Given the current emerging trials and early commercial releases, it is only a matter of time before best-effort Internet on a limited number of trains will become a reality. The satellite-based communication systems were the first solutions on the market but they lack uplink connectivity from train to satellite. These systems can combine satellite communication with GSM or GPRS receivers to ensure continuous up-and downlink coverage along the entire train trajectory [1] . The first commercial bidirectional satellite communication system (4 Mb/s down and 2 Mb/s up), which offers high-speed Internet to high-speed trains, has been realized [2] . However, measurements on a typical high-speed train with Internet connection via satellite show that the achieved bandwidth of approximately 1 Mb/s is low and the measured latencies (approximately 616 ms) are high, making real-time communication impossible. The observed latencies are high due to the fact that the signal has to travel four times the distance an Earth satellite travels (twice for query and twice for answer). Currently, WiFi is also used as a technology to deliver broadband Internet to trains. Japan Telecom has announced that they have successfully demonstrated stable wireless broadband Internet connections onboard a train using WiFi hotspots placed along the railways. However, besides the good results with WiFi and good mobility support in the Mobile WiMAX standard, the aggregation network is still responsible for the transport of data traffic from the fast moving users to the service providers' networks and vice versa. Because of the fast moving aspect, this configuration has to be done in a timely fashion. We propose that the transport of data traffic between the service providers' domains and the WiFi and WiMAX antennas be done by means of tunnels in the aggregation network. The challenge is to design telecom networks in such a way that high-bandwidth services which require a high level of quality of service (QoS) -such as multimedia content delivery, video phoning, and on-line gaming -can be provided. These tunnels are needed in order to guarantee the required QoS. Since the management of the tunnels in the aggregation network is an important aspect, it is tackled in detail in this article. As presented in  Delivering broadband services to fast moving users (e.g., in trains) involves both wired and wireless networks. In this article we demonstrate that a hierarchical wired Ethernet aggregation network in combination with Ethernet-based wireless access networks is an interesting approach for realizing broadband access to fast moving users. The focus of the article is on fast and autonomic setup and removal of tunnels in the Ethernet aggregation network for providing broadband services to trains. The resulting moving tunnels realize seamless connectivity for the users on a train with an efficient resource reservation in the aggregation network. A hybrid approach is presented, which allows tunnel-setup times to be minimized and provides accurate tunnel-setup triggers. A testbed has been set up to evaluate the hybrid approach and the results of this work are intended to be used as a guideline for network service providers on how to set up an Ethernet-based aggregation network for fast moving users. Managing Ethernet Aggregation Networks for Fast Moving Users 79 the following section, Ethernet is a strong candidate for delivering broadband services to fast moving users. This article describes an overall solution for mobility in Ethernet-based networks. Different telecom operators are already using Ethernet in the aggregation networks, and Ethernet is also becoming the standard for other networks (e.g., access networks). In order to measure our solution in a real-life case, we focus on an architecture enabling broadband Internet for commuters on trains. The architecture consists of a core network, an aggregation network, and different access networks. This article introduces a hierarchical Ethernet aggregation network. Based on the hierarchical structure of the Ethernet aggregation network, a hybrid approach for fast tunnel setup is further discussed. We clarify our solution, highlighting the management of this network. Then the evaluation results of the solution are presented. This is followed by a brief presentation on how an Ethernet-based aggregation network for fast moving users can be deployed. This choice for Ethernet in the aggregation network is motivated by the fact that telecom operators tend (mainly for economical reasons) toward networks consisting of standard QoSaware Ethernet switches. With the introduction of the IEEE 802.1s Multiple Spanning Tree Protocol (MSTP), the bandwidth efficiency can be improved by maintaining multiple trees instead of a single tree, which is the case with the IEEE 802.1D Spanning Tree Protocol (STP) and the IEEE 802.1w Rapid Spanning Tree Protocol (RSTP). Almost every commercial switch is IEEE 802.1q/p compliant, that is, they all support virtual LAN (VLAN) technology and are QoS-aware (based on priority scheduling). VLANs provide a way of separating the physical topology in different logical networks and can be used to define end-to-end tunnels in the network. For the configuration of VLANs, we have two options: it can be performed automatically by means of the standardized GARP VLAN Registration Protocol (GVRP) or it can be done in a management-based way by contacting every network device separately, for example, by using the Simple Network Management Protocol (SNMP). In summary, the ease of use and the autoconfiguration of standard Ethernet, in combination with the recent advances in QoS support, are probably Ethernet's strongest features. The scalability issues are addressed in detail below. Why is IP not an option in the aggregation networks? If Layer 3 mobility is used in the aggregation network, the IP addresses of the users in the trains change each time the train connects to a next hop. By placing an IP router on the trains, we observe a moving IP behavior while the IP addresses of the users on the trains remain unchanged during the whole length of their trip. Although different research groups have proven that the Mobile IP protocol can be used for single moving users, it has not been proven that it also works for the scenario of the aggregation of a large number of fast moving users. We believe that for a large group of users,all moving together (e.g., in trains), an aggregated tunnel (or multiple tunnels) per train will be more manageable and efficient, even in the access networks, instead of a per user tunnel management. The overall architecture is depicted in Fig.  1 . At the top of the aggregation network, a service gateway (SGW) is installed. This device acts as the uplink toward the Internet. At the lower side of the aggregation network, different access border switches (ABSs) are installed. They establish the connection between the aggregation network and the different access networks. Considering the wireless connections between the moving trains and the antennas near the railroad, there are two options: WiFi and WiMAX. In the case where WiFi technology is chosen, each access network consists of different antennas, so it is possible that multiple trains are connected to one access network at the same time. Therefore, VLAN tunnels will be needed in the access networks as well. In the case of using WiMAX technology, only one antenna is needed per access network, so no further tunneling is necessary in the access networks when WiMAX is chosen. While commuters on the train are moving along the railroad trajectory, their attachment point to the aggregation network hops from one access network to another. In order to preserve the connection between the train and the core network, tunnels in the aggregation network should move with the trains. This is done for two reasons: • Seamless connectivity: Due to the fast moving aspect of the users, seamless connectivity is not assured. However, this can be assured by making on-time resource reservations in the aggregation network by means of moving tunnels. Since the users have a fixed connection with the network in the train, the overall connectivity is guaranteed by using the moving tunnels between the trains and the SGW. • Resource efficiency: By using tunnels in the aggregation network, the traffic goes directly from source to destination. For the sake of resource efficiency, the tunnels must only be set up at their due time. When the train does not need the tunnel anymore, because the train moved to the next hop, the deprecated tunnel is released and the resources are released again. It is often assumed that every train has multiple antennas on its roof and that each antenna it is using uses a single associated tunnel in the aggregation network. By using multiple tunnels per train, a continuous connection can be achieved, even during the handover process of one antenna. These tunnels on which the data connections of fast moving users will be mapped are VLAN-based tunnels which are responsible for the delivery to the correct connection point in the aggregation network. Different scalability issues make Ethernet difficult to use in large aggregation networks. Firstly, the use of spanning trees is inevitable as the spanning tree avoids loops in the network, and secondly, routing in Ethernet networks is based on the nonscalable MAC learning and forwarding principles. To avoid the latter, we do not use the different MAC addresses of all the users for routing the packets; instead, VLAN tunnels are used to route the packets in the aggregation network from source to destination. Each packet is routed through a VLAN tunnel, without learning its MAC address. Still, the spanning trees make Ethernet not scalable enough to use it in large aggregation networks. To solve this problem, a hierarchical approach is proposed. Different levels of the hierarchy are distinguished by defining the border switches' level n, BSw n, in the aggregation network. The aggregation network can be interpreted as different smaller aggregation networks, each bordered by SGWs or BSw n values at the upper part (toward the uplink) and ABSs or BSw n values at the lower side (toward the trains), as depicted in Fig. 2
Mobility management for fast-moving vehicles REF in Spanning Tree Protocol (STP) based Ethernet networks has also been evaluated in the past.
Managing Ethernet aggregation networks for fast moving users
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