From SwissExperiment

Long distance Wifi Links

Contents 1 Long distance Wifi Links 1.1 GPRS/3G Wifi access point 1.2 Ruggedised routers for long distance wifi links 1.3 Davos Test Networks 1.3.1 Davos Radar data link

In 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, a and a long distance wifi router.

• GPRS/3G Wifi access point - Allows equipment in the field to be connected to the internet over the mobile phone network, either directly, or via a long distance link (where mobile connection is not available in the area).
• Wifi Router - Ruggedised (and cheaper, non-ruggedised) routers which can be connected to various antennae to create long distance links.

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.

GPRS/3G Wifi access point

A ruggedised hotspot has now been completed and will be field tested with a long distance 2.4GHz wifi link this week, providing devices in the field (in this case the Hydrosys devices) with internet access where mobile phone network coverage is not available. Pics to come later in the week.

A description of how to build these devices is provided here.

Ruggedised routers for long distance wifi links

The 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 - something which has to be taken into account in the alps.

For most purposes, Mikrotik RouterOS was used to reduce setup time. However, for other purposes, such as the connection of the USB modem, installation of Voyage Linux is required.

More information on building the wireless routers will be added soon.

Davos Test Networks

Davos Radar data link

Wifi 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.

Legality of wifi link

Contact: Nicholas Dawes