RTR 4/2010 36 Fast, wireless communication along the whole length of trains thus to contribute a certain intelligence of its own. If the system is then suitably programmed and use is made of powerful management software for monitoring and controlling the hard-wired and wireless network, we should be well on the way to a solution. 3 Ethernet for industrial and railway applications Belden, as a manufacturer of both Ethernet hardware suitable for use in industrial environments and powerful management software, offers such a solution from a single source under its brand name of Hirschmann™. Its management software uses LLDP to recognise the topology of the network in the train. After it has done that, it is then possible to assign numbers flexibly to coaches, seats and so on. Redundancy mechanisms that have been thoroughly tested in industry support the reliability of the network. Hirschmann WLAN access points, moreover, support the common railway standards and can handle the latest generation of the WLAN standard. This IEEE802.11n standard makes provision for transmission rates of up to 300 Mbit/s and a marked improvement in transmission stability. With these prerequisites, the use of WLAN for transmitting video becomes a feasible option. This new standard is supported, for example, by the Hirschmann BAT300 access points (Fig. 3) and access clients, which are available as an IP40 version (BAT300-Rail) and an IP65/67 one (BAT300-F).The integrated multiple-input and multiple-output (MIMO) technology ensures a stable wireless connection even in the presence of disruptive influences, such as reflections. The devices come in a compact metal housing and are suitable for use within a temperature range of –30°C up to +50°C (or even +55°C in the case of the BAT300-F). Thanks to these properties, it is possible to establish fast, stable infrastructure networks, wireless-distribution systems (WDSs) or point-to-point connections even in harsh environments. Whereas the BAT300-Rail, which can be slotted onto DIN rails, has been designed for use inside switching cabinets, the BAT300-F may be positioned either indoors or outdoors. This latter version, whose housing is also resistant to salt water, can, for example, be fastened directly to the outer wall of a wind converter using magnetic shoes. Both versions incorporate a wireless module functioning in the 5-GHz waveband (BAT300-Rail) or, alternatively, in the 5-GHz or 2.4-GHz waveband (BAT300-F). Fast roaming ensures that there is no interruption in the connection when moving from one wireless cell to another. The antenna interfaces have N sockets (BAT300-F) or manded for installations in trains. These requirements are tough, and it is not every device that satisfies them. The biggest challenge of all, however, is caused precisely by the unlimited possibilities of wireless network themselves. Electromagnetic waves do not suddenly stop at the outside walls of trains. Only a very short distance has to be overcome by the connection between two coaches or two parts of the train. The most important “malfunction” to prevent is the creation of unwanted links between two neighbouring trains. It is equally necessary, of course to ensure the security of the link, so that no illicit users are able to gain access to the train’s internal network (Fig. 2). Modern WLAN devices are capable of satisfying these demands. Authentication and encryption are mechanisms described in the WLAN standard, IEEE802.11i. Nearly all modern products support this security standard. Solving the second of the above-mentioned tasks, i.e. establishing the correct and unequivocal link between one portion of the train and the other, calls for the devices to possess properties that are more advanced than the standards. The system software with which the WLAN access points are equipped is able to support the fine tuning and verification of the connections and Up until the present, the WLAN standard has been subdivided into four variants, ranging from the ageing, but still common IEEE802.11b standard, with up to 11 Mbit/s gross data rate, through to the brand new IEEE802.11n standard, which currently offers up to 300 Mbit/s and which is even going to offer up to 450 Mbit/s with its forthcoming enhancement. Data rates of this magnitude are certainly sufficient for implementing the desired range of functions and services to be offered onboard trains and are even ready to handle future requirements, such as electronic rear-view mirrors or video cameras of the line ahead as convenience services for passengers. Wireless networks are suitable for retrofitting, and it is not even necessary to modify the existing cables and couplings. In practice, however, there are various technical difficulties to be overcome in setting up a WLAN-based network. Fitting in the WLAN access points and, in particular, the antennas often constitute a challenge in existing trains. Every square centimetre of space will already be accounted for in some way. In this sort of situation, small devices that are easy to mount by mechanical means are naturally advantageous. It is also beneficial to be able to reduce the number of devices, for instance by integrating several WLAN interfaces in a single access point. The devices must, of course, comply with all the standards that are deFig. 2: Encrypted point-to-point WLAN connection, here from coach to coach, incorporating antennas with a low scatter. This broadband wireless connection renders easily disrupted, low-performance mechanical couplings and transmission cables superfluous Fig. 3: Hirschmann™ BAT access points and antennas are suitable for use in trains
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