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Wannengrat:CommunicationsFrom SwissExperimentCommunications
Long Distance CommunicationsThis is about efforts to establish a long range (from 5 to 30km), relatively high bandwidth link between a base station (power autonomous, on the mountain) and a data collection station (connected to the web in the valley). IntroductionWhen the data has been acquired and recorded by a digitizer, it needs to be sent to the lab, where it will ultimately become available for research. The issue is made complex by the requirements of working with remote measuring stations in the mountain. That means that the physical means of transferring the data from the experimental site to the lab must accommodate the following:
On the positive side, the requirements are very low by today's standard for several aspects:
Technologies in use or being tested on WannengratGSM modemThis is the standard solution for communications with mountain stations. The communications rates are relatively slow (56kbps) and when only a weak signal is available at the station, error bits cause communication rates to be significantly reduced. Costs are calculated according to the length of time taken for the communications. GPRS modemRadio link to an infrastructure. Needs coverage. Usually reduced bandwidth. Theoritically available up to 85.6kbps. Low power draw.
Costs are calculated according to the amount of data transferred, hence for small data volumes, this is ideal. A system is currently in operation by SLF, using modems with less power consumption than that shown. More information will be posted soon Wifi using high gain antennasThis technology is cheap and unlicensed (although regulated). It uses regular Wifi radios (and protocols) together with parabolic antennas. The technology is quite mature but has not seen any kind of a widespread use coupled with high gain antennas. Most of the work so far has been that of a few amateurs, although the uptake by commercial companies is increasing Useful links:
The longest distance covered using wifi (but with a temporary setup) was 279km (see File:Enlace Aguila Baul EN.pdf and slides). A 6km wifi link has been set-up between Davos and Wannengrat. This setup is detailed below:
Long Distance Wifi LinkAntennaThe first step is to select the appropriate antenna geometry. Among the possibilities are grid antennas, parabolic antennas and Yagi antennas. All these antennas focus the electromagnetic wave, which is a way to improve the efficiency of the system (the energy is not radiated omnidirectionaly but focused on a target). Even if a very directional antenna has some drawbacks (emitting and receiving antennas need to be aligned, and to remain aligned), the benefits should outweigh them: it is assumed that the alignment will still be feasible (since other people succeeded in doing it) and that we will be able to keep it even after an extended period of time exposed to wind, freeze, etc In order to avoid having to buy a radome (which is necessary for a parabolic antenna in order to improve its resistance to the wind), grid antennas have been selected. The following manufacturers are selling such antennas:
WavelengthThe second step is to decide on the wavelength to use:
GainThe third step is to calculate the required gain. A simple method uses the system performance calculator, although it can be much more accurately predicted using the RadioMobile software (an example of the output of the software is shown below). The goal is to minimize the emitted power (to reduce the power required at the remote station) and to compensate using a high gain antenna (which is bulky, but for a fixed station, we don't care). Moreover, there is usually no price difference between an 18dB grid antenna and a 27dB grid antenna... which means that if we need, say 24dB antennas and some 20dB antennas for some other applications, it will be much simpler to take all as 24dB antennas to keep inventory simple and to have better price cuts. It is also much better to over engineer the link to compensate for weather effects.
Wifi radioAnother component is needed in order to create a link: a radio that connects to serial or Ethernet and sends everything over Wifi. Such a radio can be a simple bridge (one device connected to the wifi link) or a router (several Ethernet devices connected to the wifi link). The lowest power consumption for a bridge is less than 3W (between 2.5 and 3 watts) while it is less than 6W for a full featured router. The advantage of a router is that it could (but it depends on router models) be used for more than simply routing all traffic on the wifi link: it could in some circumstances be used as a tiny but real field computer. A list of devices worth mentioning is available below, although those chosen by SLF are listed in the second table:
The boards selected by SLF for use on the Wannengrat test site were:
Test setupIn order to have as much margin as possible to evaluate the performances of the system, everything is overdesigned. On both sides of the link, the same 27dB, 5.8GHz, grid antenna is used. The ideal goal would be to be able to use such an antenna for the stations and to use a low gain, omnidirectional antenna at the receiving station (in the valley): thus the same receiving station could serve several stations. The initial setup however, uses a parabolic grid antenna at either end. Once this system has been optimised, we can evaluate (from the achieved SNR) whether such an omnidirectional setup is viable. Originally, the boards were set up using the Voyage Linux OS. This was however found to be limited as the Regional Code for the radio was set to 0 (hardcoded) and hence the radio power could not be increased sufficiently to obtain a link. A better method was found using the Router OS software, which allowed complete control of all the parameters, showed system performance and simplified the routing process. Site SurveyEquipment used:
GIS Radio Mapping: The image below shows a generated view of the terrain between the two sites, the yellow lines show the antenna pattern.
Conclusion: After successfully positioning the two sites and establishing a connection we were surprised to see that we achieved better results than simulated using a GIS Radio surveying software, this could be due to the accuracy in the positional data and antenna data. From the Radio Mobile simulation a maximum of 19.7dB was achieved, however on the 13th August 2008 (in good weather conditions) we were able to achieve a maximum of 25dB and average of 23dB. After finding the best possible position for both antennas and then tweaking the connection we were able to run data bandwidth tests. A maximum TCP throughput of 12.7Mbits was achieved (uncompressed) and 14.4Mbits with compression added. As the results are good, steps can be taken to improve the link even more, e.g changing the type of cable assemble, (N-Type connectors ), using higher quality and shorter cable [LMR400], pre-set the modulation on the both radios, Some notes on the radio spectrum regulations: After checking the various channels used in the 802.11a standard, it has been observed that the antenna used lies outside the channels available to the standard (Channel 157 (5785Mhz)), see http://en.wikipedia.org/wiki/List_of_WLAN_channels for a list of the available channels. As the link is a point-to-point link and does not, “Broadcast” in our case we should be ok, but we are not currently fully aware of the local country regulations. Channel 140 (5700Mhz) is the closest match to the antenna used within the 802.11a frequency range. Other available technologies (not in use on Wannengrat)LasersThis sets up an optical link, thus needing the receiver and the emitter to have line of sight. These systems can provide high bandwidth (10Mb to >1Gb) as well as cover relatively large distances. On the over hand, they are not very common. Might need a heater. Up to 119 miles by amateurs.
A solution could be to use an enclosure, open the gate for transmitting, then close it back and turn the laser off. Then the power per day would be very small. WimaxRadio link (close to Wifi), large distances, high bandwidth (not together). Up to 112km. Expect ~10Mb/10km. Low power requirements (5W on evaluation PCI board). Might necessitate a license. Not yet mature! (but might come into operation in 2007. Swisscom has the license for Switzerland but it seems they took it in order to kill it: no deployments that I have heard of) |
