FabFi Technical Summary
Overview
FabFi is a user-extensible long range point-to-point and mesh hybrid-wireless broadband transmission infrastructure. It operates on the simple principle that by focusing the energy from a weak off-the-shelf WiFi transmitter into a narrow beam, one can communicate directionally for very long distances (physics is cool!). In practice, we mount commercial wireless routers on a fabbed RF (Radio Frequency) reflector with a wire mesh surface that redirects the RF energy. Reflector gain depends on the materials used and the size of the reflector, but has been measured as high as 15dBi with some of the current designs.
A single wireless link in the FabFi system consists of two reflectors with attached wireless routers. Similarly, two routers can be linked with a wired connection. A single router can be linked to both wired and wireless connections at the same time. The system is configured for individual links to be combined in numerous ways, creating links that cover very long distances or service many users. A key component of this linking is called "meshing". A mesh network is one where any device can be connected to one or more other neighbor devices in an unstructured (ad-hoc) manner. Mesh networks are robust and simple to configure because the software determines the routing of data automatically in real-time based on sensing the network topology. Traditional mesh networks are limited in scale because they rely on single radio, wireless-only connections and omni-directional antennas. By using directed wireless links and wired transfers whenever possible, the Fabfi system is optimized for building very large-scale static (as opposed to mobile) mesh networks. With Scale comes the potential for robust digital communities within a region without dependence on high-bandwidth local uplinks, which are expensive and unavailable in many places. Check out the animation for a little more detail (2MB, might take a while to load):
FabFi reflectors use the property of parabolic shapes (Y=cX^2) that a when a vector travelling perpendicular to a parabola's directrix hits the surface of the parabola it is reflected to the parabola's focal point. (see Mathworld for more on this...) By attaching a RF reflective material such as window screen or chicken wire to a frame that forms the shape of a parabola in three dimensions and then attaching our wireless router to the reflector at the focal point we can precisely concentrate and direct the RF energy coming from the router in transmission and efficiently collect RF energy from the paired router in reception.
Building Reflectors
An essential component of the FabFi system is it's flexibility to be implemented with whatever materials are locally available. All that's required is the ability to print out a 2D design file and create the pieces out of whater material you can find. If you have a Fab Lab, you can use a laser cutter or CNC wood router to create reflectors directly from wood, metal or acrylic, but there's no reason they can't be molded from clay, carved from stone or chiseled out of a block of ice as long as there's a way to attach a metallic RF reflective surface to the front.
Three different reflector designs were implemented in Jalalabad during the inital deployment in January 2009: a large 4' wooden version, a 2' wooden version and an 18" acrylic version. Reflective surface materials included chicken wire, woven stainless steel mesh and window screen.
Needs in the field subsequently drove the development of modified reflector designs with integrated weatherproofing and and fastner-less assembly. These new designs debuted in the summer of 2010.
It was not long afterward, however, that network users began designing and building their own reflectors out of locally sourced scrap materials. While still in need of significant refinement, these reflectors are clear physical signs of technology transfer and local human-capital development in the technology domain. They also cost less than $3US!
FabFi uses an open source 3rd party firmware called openWRT on all of its routers. Taking advantage of openWRT's linux-based flexibility, FabFi devices can run a wide range of network monitoring and self-diagnostic tools, some of which are compiled on the real-time stats page The linux kernel also allows automated configuration scripts, which can be found our "technical project wiki". Automated configuration has been steadily improving since the bygone days of the FabFi 1.0 release. We now support multiple routers, inclding theLinksys WRT54GL, which can be had for anywhere between $50 and $65 depending on available promotions; and the ASUS WL-520GU, which is closer to $35.
PowerIn developing places, reliable power is an ongoing challenge. Conveniently, both of our currently supported devices will run on 12VDC, and can be easily powered directly from a car or small engine battery. A car batteries and a couple of inexpensive chargers function as reliable UPS devices on two major distribution hubs in the Jalalabad network, powering a bank of routers for nearly two days without city power. Future development is planned for a bare-bones 12V-12V UPS that can be integrated into installations by plugging the provided 100-240VAC switching power brick into the fabbed UPS and the UPS into the router. Wind and other locally harvested powered charging circuits are a parallel FabLab project.
The Fab Future
Despite te cobbled-together aesthetic, Fabfi has proven incredibly reliable in Afghanistan's harsh climate (it reaches 130degF in Jalalabad in the summer with regular sandstorms).
In nearly 18 months of deployment, we can still count the hardware failures on one hand. To our surprise, the biggest challenge so far has been uplink bandwidth.
With nearly e-society still in its infancy within the country, users still pull most of their content from outside the country, keeping the system's satellite connection to the WWW at the throughput limit nearly 20 hours a day.
With the release of fabfi 3.0 in April 2010, we hope to facilitate the development of local content through a dynamic web dashboard, and optimize our proxy server to serve more content locally.
It won't be long until we begin to address scaling in a big way either...
Follow our progress with these and other projects on the FabFi Blog.
