Forth Estuary Experimental Data NETwork

Introduction to Amateur Radio MESH networking.

FEEDNET Mesh Node, Edinburgh

The mesh network comprises a collection of 2.4Ghz Wi-Fi routers that have been updated with OLSR mesh firmware. These routers, when switched on, will look for other routers in range. If found, they will link themselves to that router. If there is more than one router that they can “see” they will link themselves to all of them. Each router sends information to all the others on the network and they calculate the best route to use for any data that they are asked to pass. All of this takes place in a portion of the public Wi-Fi band that is shared with amateur radio. Our unattended nodes operate with standard remote closedown procedures acceptable to Ofcom.

Link distances of up to 20 miles have been routinely managed by the FEEDNET team and the current major nodes on the network are shown in the network map on the left. Mobile stations come and go and it is indeed part of our design to have some good strong fixed links covering a wide are which are available for mobile stations that may be making use of services acting as routing nodes themselves. These intermediate routing nodes link sites that are too far apart or that have obstructions between them. Think of them in simple terms as repeaters although in data communications jargon this is not exactly their precise operating mode..

Off-the-shelf firmware is available for some Linksys routers and some Ubiquiti routers.  The Ubiquiti routers are of particularly good quality having low noise RF sections.  They are designed to connect directly to an antenna using an N connector, thus eliminating all feed line loss which is  important at 2.4Ghz.  Other routers can interoperate with mesh but it is a specialised task to get them on the air and most participants will not do this. For the keen, some options are to use a Raspberry PI, Beagle board or other small Linux PC with a Wi-Fi adaptor in ad hoc mode or to configure OpenWRT and flash it to a modem that meets the necessary hardware requirements.

The paths at 2.4Ghz must to be line of sight. Occasionally its possible to get past a few leafy trees if there isn’t too much attenuation from them on an otherwise good route but this is the exception rather than the norm.  Key to success is the choice of antenna. High gain parabolic antennas that have forward characteristics of 24dBi are effective but cumbersome. They are very difficult to line up because there is no simple signal to monitor using any kind of S-meter.  We have experimented with 2.4Ghz video senders to provide a constant analogue signal in order to line antennas up before replacing the video sender with a router.   Parabolic antennas have high wind loading and do not make good temporary antennas for this reason.

Sector antennas ae our most useful antenna. They can typically have 90 to 180 degree beamwidth (3dB) yet still have forward gains of 17-22 dBi. They achieve these results by having very focused beams and almost no radiation vertically or off the back.  It is easy to erect such an antenna and just generally pointing it in the correct direction will give good results without further adjustment. When a fixed link base station uses this kind of antenna it can often cover a wide geographic range.  Panel antennas are useful too, These are more or less square in shape and very flat. Wind loading can be surprisingly high so careful assessment should be made, as it always should be with any large wifi antenna before deployment – particularly if there may be safety at stake.

Although the firmware is plug and play, it has been found that a surprising amount of know-how may be required to deploy a mesh network. The hamnet versions of MESH software do still have bugs in them and whilst they all work out of the box, to implement some of the more sophisticated applications might require someone who can work around these bugs and is familiar with Linux.  Not everyone in the group however needs to have this knowledge but obviously the more informed all operators are, the better. To that end, training sessions on general IP networking, node configuration/management and the use of applications such as file transfer, photograph editing, IRC and VOIP are worth carrying out. Practice makes perfect.

Applications are key. The mesh itself is of little value unless there are ways of making use of it. Any IP traffic that can be carried over the internet can be carried over the mesh network.  Examples or mesh applications are: web servers – perhaps for real time updates or as information repositories. Web cams – these can be fixed or hand carried. Real time pictures are easy to stream over the mesh. Still photography typically provides higher resolution than web cams and photographs taken with a mobile telephone can be uploaded to a file server on the mesh direct from the telephone if it is equipped with Wi-Fi.  Photos can be sent person-to-person if using IRC chat. Each node can carry its own IRC server or perhaps more usefully, a centrally placed node can perform this function for all, with other nodes being able to provide a backup function for times of failure. Email is a private email system running on a server on the mesh network, although it can be linked to the public email either through an HF link to a distant station (very slow – text emails only) or through an internet gateway. Public internet facilities can be made available to everyone on the network although obviously in terms of resilience during major incidents public communications facilities should never be relied on.

VOIP or Voice over Internet Protocol is a well established technology in use throughout the world.  It is easy to make VOIP work on a mesh network.  With one small Linux server running Asterisk, anyone on the mesh can connect a VOIP telephone and make use of the PBX for one-to-one private conversations, conference calls, voicemail and even make use of an outside line for calling out to the public network. FEEDNET makes use of the ubiquitous “dial 9 for an outside line” facility and for incoming calls from the public network uses an auto-attendant which asks callers to “please dial the extension that you require”.   One useful feature is that smart phones (Apple or Android) can connect wirelessly to the network and run a Softphone app. This provides smart-phone access to the mesh VOIP PBX and thereafter to the outside calling facilities. Whilst care must be taken about relying on outside calling during major incidents, many other more routine circumstances arise where this facility is useful – for example where there is no mobile coverage. In an area of zero mobile coverage RAYNET ops can continue to use their mobile telephones – whether they have a SIM card installed in the phone or not.