Constructing a Rugged Outdoor Node
One of the biggest sources of signal loss in a wireless network is in the cable between the low power radio in each node and the antenna. High quality antenna cable is very expensive. In order to maximize signal strength and to minimize cost, it is best to put the node as close to its antenna as possible. This generally means putting the node outdoors on the rooftop on the the same mounting pole as the node itself.
Outdoor nodes need to have no moving parts, withstand extreme temperatures, withstand rain, snow, wind, sand, and other local climate conditions and be upgradable without physical access.
Single Board Computer
The first component of any outdoor node is the single board computer. This is a small form factor motherboard built for embedded computing applications. Typically a single board computer will be built to industrial standards and will be much more rugged than a typical consumer motherboard.
Typically a single board computer will have a processor (CPU), some memory, some compact flash storage (or a slot for it), one or two Mini-PCI or PCMCIA slots, one or more ethernet jacks, a serial port, and an input for DC power.
In order to run current versions of CUWiNware you will need a single board computer with an x86 CPU, preferably 486 or better running at 100 MHz or better. It will need at least 64MB of RAM and 64MB of compact flash.
The 64MB of compact flash is necessary to contain two full versions of the software which is currently around 32MB in size. The space for the second image is used for on-line upgrades of the node. It may be possible to build custom versions of CUWiNware with on-line upgrades disabled that would only require 32MB of compact flash.
Make sure there is an enclosure available for the board you choose!
We recommend Soekris Engineering's net4526 board which has 1 ethernet interface, 2 Mini-PCI slots, 64MB of on-board compact flash, and 64MB of on-board RAM. This board is small, powerful, rugged, and has good enclosures available.
As I write this (August 2005) a complete, quality, node can be bought for around $300, but it is the eventual goal of the CUWiN project to run on hardware that costs less than $100.
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The Wireless NIC
The three requirements for a wireless NIC are that it must have an external antenna connector, it must be compatible with the motherboard you have chosen, and it must use a chipset that is compatible with NetBSD in Ad-Hoc mode.
Interfaces between the motherboard and the wireless NIC include: Mini-PCI, PCI, PCMCIA (a.k.a. Cardbus or PC Card), and USB. Most rugged single board computers will use either PCMCIA or Mini-PCI cards.
Make sure there is a pigtail available for the card you choose!
We recommend Netgate's NL-3054MP Aries Mini-PCI 802.11b/g or SL-5354MP Mini-PCI 802.11a/b/g cards. These use the Atheros Chipset.
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The Enclosure
The enclosure you choose must allow your single board computer to be mounted. Many enclosures come with a special mounting plate that the manufacturer can custom drill to work with any single board computer that you specify. Many manufacturers make enclosures specifically for Soekris boards. The enclosure must have holes for an ethernet cable gland and and an N connector for the antenna connection.
Any enclosure for an outdoor wireless node should be rated at least NEMA Type 4x. Type 4X NEMA enclosures protect against falling dirt, rain, snow, blown dust/sand, splashing water, and ice.
The enclosure should be white in order to reflect rather than absorb external heat from sunlight. If the enclosure is plastic, it must be a type of plastic that doesn't break down when exposed to UV light.
It is vitally important that any and all holes on the enclosure be on the side that faces downwards. No matter how well sealed the hole, with mastic tape or with a cable gland and gasket, if it is on top water will pool around it and freezing/thawing will allow the water to seep in. After months or years of operation the node will fill with water and fail.
It is best not to mount the antenna directly onto the enclosure but to mount both the enclosure and the antenna on a mast. Antennas directly mounted on the enclosure put a huge amount of stress on the joint between the antenna and the enclosure and this can cause leaks.
There is some debate among community networkers as to the utility of a drain hole in the bottom of your node enclosure. If you trust that your node will remain absolutely water tight even after months or years of freezing and thawing then it may be best to not include a drain hole. A drain hole is for water that does get into the enclosure due to condensation/humidity or due to leaks. A desiccant package placed in a sealed node may do a better job of defeating small amounts of moisture. A drain hole may be susceptible to sand or dust in a desert environment and would definitely not be recommended in that case.
A very labor intensive but low material cost way to build an enclosure is to use a surplus military ammunition can painted white with a custom plexiglas mounting assembly. These cans are built to be very rugged and waterproof and are essentially free if you can find a source of them. They do, however, corrode after prolonged exposure to the elements and will eventually fail.
We recommend the NEMA-4x enclosures sold by Metrix Communications for use with Soekris 4526 boards.
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The Antenna
Antennas must be made for the band (or frequency) that you are using: 2.4GHz (802.11b/g) or 5.8GHz (802.11a). They should use an N connector for the cable connection. The N connector is most appropriate for the high frequency signals used by 802.11x.
Choices in antenna selection include: omni vs. directional, manufactured vs. homebrew, and amount of gain.
Typically mesh nodes will use omnidirectional antennas. This is recommended for most cases. These transmit and receive equally in any direction towards the horizon. This allows the formation of arbitrary meshes without having to aim (or re-aim after a high wind) antennas. It also means that there may be "wasted" radio energy that causes interference for nearby nodes.
You may want to choose a directional (sector or beam) antenna if you are creating a long distance point-to-point link within the mesh or if you know for sure that no nodes will ever be placed on the null side of the directional antenna. Sector antennas focus all radio energy in an area of a wide angle (e.g. 60 or 120 degrees). Beam antennas focus radio energy into an even tighter beam. Directional antennas come in many types such as Yagi, Sector, Dish, and Patch and Panel.
In a very dense network the interference caused by nearby nodes using omnidirectional antennas can cause a major drop in throughput. This problem can be addressed by replacing key omni nodes with multiple nodes with sector antennas. Collectively the nodes form 360 degree coverage and are linked via their wired ethernet cables.
There are many designs on the internet for homebrew antennas that you can make from common household items and cheap hardware using simple tools. The antennas can be pretty time intensive to build and their quality will vary greatly. The easiest designs are for directional beam antennas. Homebrew omnidirectional antennas are very difficult to build.
Every antenna will include a "gain" rating. This is a measure of how much the antenna increases the signal power in the direction that the antenna points. For omnidirectional antennas a high gain means that less power is radiated "up" or "down" and more power is radiated "outwards". For a directional antenna a high gain means less power is radiated out the "back" and more out the "front" and higher directional gain also means a tighter beam. Gain is logarithmic so every increase of 3dB gain means a doubling of power.
The best antenna cable to use for microwave applications such as 802.11x is LMR-400. The antenna cable is a major source of signal loss so it is very important to use only short runs of high quality cable with N connector ends. Installing your own ends on raw cable is very time consuming and requires special tools.
For most generic mesh applications we recommend the HyperLink Technologies HGV-2409U 2.4GHz 8dbi omnidirectional antenna. This antenna features a flared base which makes it less susceptible to wind-shear than other similar antennas from other manufacturers.
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Mast and Mounting Hardware
Microwave signals are extremely dependent on line of site. The higher your antenna can be the more likely other nodes are to have line of sight to it.
Generally, you can use an antenna mast on a rooftop to achieve height. If you live in a tall building you may be able to just place the node in a window. If you have access to a radio tower then you can achieve the best heights.
There are several ways to mount an antenna mast on a rooftop. For flat roofs there is a great non-destructive flat roof mounting platform that is held in place by cinderblocks. There are special mounts for gables and chimneys, as well as tripod mounts.
Any hardware designed for television antenna mounting will suffice for a mesh node antenna.
We recommend the Radio Shack ratchet style Chimney Mount (cat no 15-839) and any 5 foot antenna mast typically sold for television antennas.
Grounding and Lightning Arrest
Tall metal polls on your rooftop can be a major fire hazard as well as a risk to all electronic equipment connected to the node in any way. It is important to follow proper grounding guidelines to protect your safety and your equipment.
Lightning can travel down either of the 2 conductors in the antenna cable and they can't both be directly connected to ground. In order to protect this signal from lightning you need to install an inline gas-discharge lightning arrestor and connect that to ground.
See the ARRL Antenna Handbook for tips on grounding antennas.
Outdoor Cat-5e Cable and Power over Ethernet Injector
The cable from the node into the user's house is a CAT-5 ethernet cable which also carries the node's power. Although it is more expensive, you should get special outdoor CAT-5 cable. The outdoor cable's jacket will not break down in UV light and it is filled with a waterproof gel that prevents condensation-related corrosion of the conductors. Regular CAT-5 cable will still work, but may need replacement after a few years.
In order to send power to the node over the ethernet cable you will need a power over ethernet (PoE) injector. Many power over ethernet injectors claim to provide protection against reversing the cables (so that you are sending power back into your LAN rather than up into your node) but this is not implemented on any PoE injectors that we have used. Be careful that you connect the power side to the node and the data side to your LAN.
