[CUWiN-Dev] Rugged Node Construction DOCO:

[Sascha Meinrath]

here's the text for the rugged node construction -- again, any feedback 
and suggestions are most welcome.

--sascha

                                                 Building a Rugged Outdoor Node

Introduction

       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 must be able to withstand extreme temperatures, rain, snow, wind, sand, and other local
       climate conditions, be upgradable without physical access, and should have no moving parts.

       There are seven basic components to an outdoor node:
        o  The single board computer
        o  The Network Interface Card (NIC)
        o  The node enclosure
        o  The antenna
        o  Mast/mounting hardware
        o  Grounding/lightning-arrest hardware
        o  Outdoor CAT5 cable
       Finally, you must build your node. To assist you in your endeavors, CUWiN has created an illustrated,
       step-by-step construction guide.

   The Single Board Computer

       The first component of any outdoor node is the single board computer. Simply put, this is a small
       motherboard designed for permanent or semi-permanent computing applications. A single board computer will
       typically be built to industrial standards and will be much more rugged than the average consumer
       motherboard.

       A single board computer will usually 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.

       To run current versions of CUWiNware you will need a single board computer with an x86 CPU, preferably 486
       or better running at 100Mhz or faster. It will need at least 64MB of RAM and 64MB of compact flash.

       The 64MB of compact flash is neccessary 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, which would require only
       32MB of compact flash.

       Before you settle on a board, make sure you can find an enclosure 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 fits a variety
       of good enclosures.

       Related Links:
             Soekris Engineering

   The Wireless NIC

       There are three requirements for a wireless NIC. It must have an external antenna connector, it must be
       compatible with your chosen motherboard, and its chipset must be compatible with NetBSD in Ad-Hoc mode.

       Possible 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 pigtails are 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.

       Related Links:
             Netgate's Mini-PCI Resources Page

   Enclosure

       The node enclosure is simply a box that protects your sensitive hardware. Whatever enclosure you choose, it
       must have holes for an ethernet cable gland and and an N connector for the antenna connection. It must also
       allow your single board computer to be mounted. Many manufacturers make enclosures specifically for Soekris
       boards. Other enclosures come with a special mounting plate that the manufacturer can custom drill to work
       with any single board computer that you specify.

       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.

       To help keep your single board computer from overheating, the enclosure should be white, which will reflect
       heat from the sun rather than absorb it. If the enclosure is plastic, be aware that it must resist UV
       light. Many types of plastic will break down when exposed to UV light.

       It is vitally important that any and all holes be on the bottom of the enclosure. Holes in the top of the
       enclosure, no matter how well sealed, will allow water to pool around the seal. The freeze/thaw cycle will
       eventually allow the water to seep in, causing your node tofail.

       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 mounted directly on the enclosure put a great deal of stress on the joint
       between the antenna and the enclosure, which 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. A drain hole is for water that does get into the enclosure due to condensation, humidity,
       or leaks. If you trust that your node will remain absolutely watertight, even after months or years of
       freezing and thawing, then it may be best not to include a drain hole.

       In a well-sealed node, a dessicant package placed inside the enclosure node may do a better job of
       defeating small amounts of moisture. A drain hole may also be susceptible to sand or dust in a desert
       environment and would not be recommended in that case.

       We recommend the NEMA-4x enclosures sold by Metrix Communications for use with Soekris 4526 boards.

       For those on a tight budget, a military surplus ammunition can with a custom plexiglass mounting assembly
       can be turned into a very sturdy enclosure. These cans are built to be rugged and waterproof, and are quite
       inexpensive. They do, however, corrode after prolonged exposure to the elements, and adding the mounting
       assembly is very labor intensive.

       Related Links:
             Metrix Communications Enclosures

   Antenna/Cable

       You must build your antennas for the band that you are using-2.4GHz (802.11b/g) or 5.8GHz (802.11a). Use an
       N connector for the connection between the cable and the antenna. 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.

       Mesh nodes will typically use omni-directional antennas. This is recommended for most cases.
       Omni-directional antennas transmit and receive equally in any direction towards the horizon. This allows
       the formation of arbitrary meshes without having to aim antennas (or re-aim after a high wind). 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 within the 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 omni-directional antennas can cause a
       major drop in throughput. This problem can be addressed by replacing key omni nodes with multiple nodes
       using sector antennas. Collectively, the nodes form 360 degree coverage and are linked via their wired
       ethernet cables.

       The internet offers a multitude of designs for homebrew antennas that can be built using common household
       items and cheap hardware, using only simple tools. While homebrewed hardware can be a very effective
       solution for those on a budget, be sure to research the quality of the antenna and factor the (often
       hidden) cost of labor into the equation. The easiest designs tend to be for directional beam antennas, as
       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 omni-directional 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." Higher directional gain also
       means a tighter beam. Gain is logarithmic, so every increase of 3dB gain means a doubling of signal power.

       LMR-400 antenna cable is the best choice for microwave applications such as 802.11x. 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.

       We recommend the HyperLink Technologies HGV-2409U 2.4GHz 8dbi omnidirectional antenna for most generic mesh
       applications. This antenna features a flared base which makes it less susceptible to windshear than similar
       antennas from other manufacturers.

       Related Links:
             Hyperlink Technologies 2.4GHz antennas The Original Pringles Can Antenna

   Mast/Mounting Hardware

       Microwave signals are extremely dependent on line of site. The higher you can place your antenna,the more
       likely it is that you will achieve line of site between your nodes.

       Generally, you can use an antenna mast on a rooftop to acheieve height. If you live in a tall building you
       may be able to simply place the node in a window. Ideally, you will be able to place your node on a radio
       tower.

       There are several ways to mount an antenna mast on a rooftop. For flat roofs, you can use an excellent
       non-destructive flat roof mounting platform that is held in place by cinderblocks. There are also 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/Lightning Arrest

       Mounting a tall metal pole on your rooftop can create a major fire hazard as well as a risk to all
       electronic equipment connected to the node in any way. It is vitally 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, due to the design of the
       antenna, they can't both be directly connected to ground. To protect this signal from lightning you will
       need to install an inline gas-discharge lightning arrestor and connect that to ground.

       See the ARRRL Antenna Handbook for more tips on grounding antennas.

   Outdoor CAT-5 Cable and PoE Injector

       The cable travelling 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 than standard ethernet cable, it is important to get special
       outdoor CAT-5 cable. The outdoor cable's jacket will not break down in UV light and is filled with a
       waterproof gel that prevents condensation-related corrosion of the conductors.

       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.

   Step by Step Guide to Building a CUWiNware Metrix Box

       Illustrated Guide