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SwissEx:WifiFrom SwissExperimentWifi
Long distance Wifi Links
This work has been carried out under the 3rd party funding of the Hydrosys EU FP7 project
BackgroundIn alpine areas, mobile network coverage is often limited (see Swisscom coverage - Swisscom is the provider used by most SwissEx partners). In order to get the data to a point where coverage is available, or in high data rate cases, where HSPA or UMTS coverage is available, SLF are developing and trialling a multi-hop wifi link. This link is to be mobile enough that it can be deployed in the event of a short term experiment to allow real-time data transfer. Two types of equipment have been implemented:
This isn't cutting edge as far as telecoms is concerned, but these types of link are not often used in environmental research and descriptions on the internet of how to build this type of router (especially low power) are few and far between. The instructions below form an initial, rough guide which will be made more detailed at a later stage. HardwareThe hardware chosen for these routers was the PC Engines Alix boards. Various boards have been tested and have all provided good results. The Alix 2 boards are good for indoor use, due to their i/o capabilities, though the Alix3d boards were the boards most used due to the availability of ruggedised outdoor enclosures. Various wireless cards from PC engines were tested (Compex and Wistron) were tested, all with good results - some are 5GHz compatible, some are more powerful etc. etc. 5.8GHz was chosen as the long distance transmission frequency as this frequency, although generally attenuated more, is less absorbed by precipitation (contrary to common sense, but apparently there is a notch in the absorbtion spectrum at this frequency) - something which has to be taken into account in the alps. *prices at time of writing - costs here are for the ruggedised waterproof version and for the equipment that we used, which is not necessarily the cheapest, you may make it cheaper with other Alix boards/cases/wireless cards from pc engines.
Total cost = 199.08 CHF If you want long distance links: we used various antennae from http://www.wifi-link.com/, depending on whether you need to connect to laptops/wide area coverage/long distance etc. etc. GPRS/3G Wifi access pointRuggedised wifi hotspots with high gain directional antennae have been effectively used to provide internet access to entire valleys where mobile phone access is not available. Two types of wireless access points were built, one using Voyage Linux and one using Mikrotik RouterOS. A description of how to build the Linux devices is provided here. We eventually abandoned the Linux devices as, although linux laptops worked reliably with it, Windows and Mac devices were only working intermittently. We replaced the software on these devices using Mikrotik RouterOS. Although the Alix3d boards described above were used in combination with a 3G USB stick for demonstrating this capability, it has been since found that the Routerboard RB411UAHR combined with the outdoor enclosure would provide a cheaper, fully integrated system, with the advantage that the Routerboard comes with a RouterOS licence. The RouterOS software comes with an installer to install the OS to a Compact Flash card. Once installed, you must use a null-modem cable in combination with Hyperterminal to connect to the board and set up an ip on the ethernet port. The latest start-up instructions say that the ethernet port is preconfigured, but we do no know if this is the case for a non-routerboard board (e.g. Alix). Once configured, you can connect to the router through the ethernet port and bring up the router page in your browser by entering the ip of the ethernet port. From here, you can then download the Winbox software which significantly speeds up configuration. Open the Winbox software and enter the ip of the router (under the IP > Addresses tab). Once the software has opened, you can enable your wireless cards and any other ethernet ports using the 'interfaces' tab and add their ips. Using the PPP tab, you can enable your 3G interface. In our case, we were using the Sierra wireless compass 885 usb modem, which was immediately recognised. We then used the AT&T connection instructions, replacing various parameters with the Swisscom ones. The next step is to set up a NAT for the PPP and your output interface(s). This can be found under the firewall settings and should be set to masquerade. The last step is to add a DHCP server to your output interface(s). This can also be done very simply in winbox in the DHCP server settings under the IP tab.
Ruggedised routers for long distance wifi linksThe wireless routers should be set up as for the 3G Hotspot, but omitting the PPP and if appropriate the DHCP settings. Once the ips of the interfaces have been set up, you need to add the routes of the system under the IP > routes tab. La Fouly Wifi Hotspot Test  In August 2010, the wifi hotspot described above was used with a 24dB 2.4GHz wifi antenna to cover the whole of the valley floor with wifi access from a laptop. The router itself was placed on a peak, where 3G access was available and from where most of the valley floor was visible. In 2011, the same experiment will take place, with the addition of a. a solar power source and b. a second antenna to give access to the valley floor to the east of the router (the antenna in the pictures is facing south).
Davos Router Test NetworksDavos Radar data linkWifi equipment such as that described below is in operational use to retrieve data from the LTE Radar and to link the radar to the internet so that it can be remotely operated from anywhere in the world. Such equipment allows the high data rates required to return the data (not currently achievable using mobile networks) as well as being operable in any area regardless of the mobile telephony infrastructure. The link currently operates over a 2.6km stretch, providing data rates of approx. 16Mbits/s. This initially seems to be regardless of weather conditions, although performance plots will be posted here as they become available. The network scenario is shown below:  This network is attached to the radar at one end and the server PC at the valley end. The server is connected (on a second network card) to the internet. Data is regularly transferred from the radar to the server PC, where it is stored. As internet data upload rates do not currently suffice to transfer this amount of data continuously, certain periods of interest will be picked out of this data set for further processing. Update 21/02/2011These links have now been extended to provide a downlink from the Wannengrat fieldsite (which will eventually also provide wireless internet access in the field) and a link to Dorfberg for retrieval of data from and operation of the under-snow radar. Sustained data rates will be posted here when they become available. Contact: Nicholas Dawes
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