Underground Broadband: Design of a Reliable WLAN Gap Filler Solution Tom Van Leeuwen, Ingrid Moerman, and Piet Demeester Department of Information Technology of Ghent University Gaston Crommenlaan 8 9050 Gent, Belgium Telephone: (+32 (0) 9 33 14901 Fax: +32 (0) 9 33 14899 Email: firstname.lastname@intec.ugent.be Abstract— Nowadays on-board wireless broadband connectiv- ity in mass transport vehicles, like trains and light rails, are being trialed around the world. These solutions depend on existing mobile network or satellite coverage to maintain a broadband internet connection to these vehicles. However these technologies lack coverage in underground areas or tunnels, and an alternative “gap filler” technology is needed to keep the train connected. In this paper we discuss the architecture and design of a WLAN based gap filler network solution for rail vehicles, which is able to provide a reliable broadband connection, even at high speed. I. I NTRODUCTION The provisioning of internet services on-board fast moving transportation vehicles, like trains or light rails, pushes the “Always Best Connected” paradigm to its limits. Inside such rail vehicles, the commuters connect to an on-board backbone network via Wireless LAN (WLAN) access points if they desire internet access. Typically, this backbone network is connected to a number of wireless broadband network in- terfaces that share an outdoor antenna system on the roof of the vehicle. Depending on required bandwidth and the coverage at its location, the vehicles connection manager chooses the best network connection from the technologies available. However, due to obstructions on the trajectory, it is not always possible to connect to a satellite or broadband cellular network, for example in case of tunnels. In Fig. 1, we have depicted the length of the major tunnels in a number of European countries in percentage of the track [1]. Add to this the number of underground stations and a train or metro spends a considerable time out of range of any cellular or satellite network. In order to cope with these long term network outages, “gap filler” technology needs to be installed inside these areas. Currently, two gap filler solutions have been documented. A first solution is based on repeaters of 3G or satellite radio signals. Inside the tunnel these repeaters capture the radio signals and bridge them onto a shielded cable. This cable is connected to a 3G or satellite modem outside the tunnel, which has a clear line of sight connection with its wireless network. To extend its coverage inside the tunnel, the repeater can also be connected to a “leaky feeder” cable. This cable has vertical slits at fixed distances through which the 3G signal leaks in 0 1 2 3 4 5 6 7 8 Austria Belgium Cyprus Czech Republic Denmark Estonia Finland France Germany Greece Hungary Ireland Italy Latvia Lithuania Luxembourg Malta Netherlands Poland Portugal Slovakia Slovenia Spain Sweden Switzerland UK Country Percentage of track Fig. 1. Tunnel length in percentage of track length (EU25 countries) and out. The advantage this repeater solution is that there is no need to install new interfaces on-board the train. However, a large number of expensive amplifiers and leaky feeder cables are necessary to cover a tunnel of several kilometers due to the environment’s strong signal fading and tunnel bends. Additionally the required cables are several centimeters wide, making them rigid and heavy. The installation cost of tunnel coverage can be reduced by deploying an 802.11 a/g Wireless LAN network inside this area. Again, leaky feeder cables are commercially available, but the signal loss in these cables at 2.4 GHz is about 10 dB/100m and even higher at 5GHz, which makes them unusable after about 100-150m. An alternative and more flexible solution is to use directional antennas. However, the main issue with using WLAN as an alternative broadband technology for fast moving vehicles, is the lack of a standardized reliable fast handoff protocol between the WLAN access points. In this paper we introduce the architecture and design of a WLAN based gap filler solution for rail vehicles, which is able to provide a reliable broadband connection with QoS guarantees, including fast handoff between access points. The rest of the paper is organized as follows. We first discuss the available research on the topic, after which we introduce the architecture of our solution. In the next paragraph, we focus on the analysis and optimization of (some) key network protocol parameters, such that a reliable system is obtained. We finish with a conclusion and future work. II. RELATED RESEARCH The two most well known European Information Society Technologies (IST) projects that study the technical require- ments for broadband on rail vehicles are FIFTH (FP5) [2]