Cutting the Wires... (Part 2)

Copyright (c) 2000-2001 by Rich Morin
published in Silicon Carny, June 2000


Although it is possible to hack together your own cables, and even your build your own antennas, it may not be legal. So, the FCC might come knocking on your door if you do it wrong. Also, doing things right can reduce risks (e.g., from lightning). Thus, it may make sense to take a more conservative approach.

Building your own antennas and cables is appealing, as is buying economical parts "off the shelf". The practice could get you in hot water, however, if the FCC has to track down your system as the cause of some annoying interference.

As noted in last month's column, the FCC wants complete transmitting systems (including transmitters, antennas, and even cabling) to be checked for spurious radiation, etc. Even if a particular combo seems to "work just fine", it could be generating noise in other frequewncy bands.

So, a more conservative approach may be advisable. Fortunately, type-certified systems are starting to become available at reasonable prices. Let's sketch out a system that you might build, using components from Lucent's "Orinoco" line.

To start, you will need some sort of IP router. You can buy these from Lucent (or other firms), but you could also depend on a local computer to fill this need. Most Unixish systems can perform this function, along with firewalling and other protective duties.

Bear in mind, however, that using a general-purpose OS for boundary protection makes your entire network vulnerable to performance problems or security holes in the OS. Thus, a dedicated router may be a better choice unless you are very confident of your OS security.

If the router already has a free PCMCIA slot, you can plug a WaveLAN card directly into it. If not, you will need to install an adapter. Alternatively, you may be able to find a "base station" that speaks 10-BaseT, a PCI-based tranceiver card, etc.

The RF (Radio Frequency) connection of the WaveLAN card is made via a "pigtail" cable, which the card to a "lightning protector". Finally, a "low-loss" cable runs out to a high-gain antenna. Here are some (approximate) list prices:

    ISA/PCI to PCMCIA adaptor              70
    WaveLAN Gold card                     200
    "pigtail" cable                        95
    Lightning protector                   165
    Low-loss cable                         85 to 125
    Antenna                               125 to 345
                                          ==========
    TOTAL                                 740 to 980

In short, you're looking at $750 to $1000 per node, depending on the type and location of your antenna. Cable cost goes up (moderately) with length: $85 buys 20' of cable, $105 buys 50', and $125 buys 75'.

Antenna mounting hardware can be trivial to tricky, depending on the height, expected wind load, etc. My own mast ends 30 feet above my roof and must be able to endure 70 mph winds; I was quite happy to let professionals install it!

Antenna Choice

Lucent sells a pretty wide range of antennas. Here is their list, with suggested list prices:

    24 dBi Directional Parabolic Grid     200
    14 dBi Directional Yagi               195
    12 dBi Directional Wide Angle         295
    10 dBi Omni Directional               345
     7 dBi Omni Directional               225
     5 dBi Omni Directional (car)         125

The terms "Directional" and "Omni Directional" require a bit of clarification. Any antenna you are likely to buy is going to be somewhat directional. After all, why waste precious signal strength on birds and earthworms?

Thus, an "Omni Directional" antenna actually covers a "pancake-shaped" volume, rather than a sphere. Antennas that cover a sphere-shaped volume are called "isotropic radiators" and are mostly of interest as the basis for Effective Isotropic Radiated Power (EIRP).

EIRP, a measure of transmitted signal strength, is the amount of power a transmitter would need to produce the same signal strength through an isotropic radiator. The 24 dBi Directional Parabolic Grid listed above, for example, produces 24 dB (about 250 times) more signal strength than an isotropic radiator would with the same input.

In short, high dBi numbers are a Good Thing. On the other hand, if you are trying to reach receivers in different locations, you may need to use an Omni Directional antenna, an electronically steerable Directional antenna, or multiple Directional antennas.

Bear in mind, however, that any exotic antenna setup will need to be approved by the FCC (or whomever) if you want to stay legal. You can, however, use multiple transmitter/antenna combinations (one per link) without fear of hassles.

Before you order your antenna, be sure that you have an appropriate place to mount it. There needs to be a clear "line of sight" from your location to the other antenna. This means no buildings, trees, etc. (The reason for my own tall mast is a handsome stand of pine trees across the street. :-)

The antenna also needs to have enough room and support; the Lucent 24 dBi Directional Parabolic Grid measures 610 x 914 x 381 mm (24 x 36 x 15 in.) Subjecting a sail like that to heavy winds, without proper mounting, could cause some serious problems!

Design Issues

If you are planning to set up a radio-based "toaster net", you should plan for expansion, losses, and other issues. If half of the sites in the network rely on a given node, you had best be sure that it won't go away! Also, the bandwidth through that node could easily become a "choke point".

You may also encounter annoying variations in path quality. Just because one site can "hear" another, for instance, does not mean that the reverse will be true. Worse, if a site can "hear" more than one incoming site, you may encounter situations where a sending site cannot tell that another transmission is already underway!

In short, this sort of thing can easily become far trickier than laying out a local piece of Cat-5 cabling. On the other hand, the chance to get free of the TELCOs may be worth a bit of start-up hassle...

On final caveat may be in order. In discussing the certification process with a vendor, I was told that all of their systems were FCC certified, but that they "had no control over what an integrator might do". It turns out that assorted integrators (i.e., contractors) commonly purchase equipment from multiple hardware vendors and assemble them as wireless networking solutions.

Even if an integrator had the technical expertise, it is unlikely that they would spend the effort to go through the laborious type certification process. Thus, the resulting system may be no more legal than one cobbled together by an amateur.

So, if you want to be sure you're in the clear, make sure that the vendor has type certified the precise system you're installing (transmitter, cables, antenna, and all). The responsibility for running a legal site is yours; assurances from contractors won't keep you out of trouble if the FCC comes knocking on your door...

Resources

Aironet Wireless Communications (part of Cisco)
http://www.aironet.com/default.asp
http://www.aironet.com/products/building_building.asp
    
BreezeCOM
http://www.breezecom.com
http://www.breezecom.com/Products/Antennas/ant_access_overview.htm

Enterasys Networks (part of Cabletron Systems)
http://www.cabletron.com
http://www.enterasys.com
http://www.enterasys.com/technologies/wireless/outdoor
 
Lucent Technologies
http://www.lucent.com
http://www.lucent.com/wirelessnet
http://wwwdb.lucent.com/bcs/syst.main?p_id=65
http://www.wavelan.com/products/productdetail.html?id=10
http://www.wavelan.com/products/productdetail.html?id=31

Nortel Networks
http://www.nortelnetworks.com
  
Proxim
http://www.proxim.com
      

About the author

Rich Morin (rdm@cfcl.com) operates Prime Time Freeware (www.ptf.com), a publisher of books about Open Source software. Rich lives in San Bruno, on the San Francisco peninsula.