Making 802.11 Take The Longshot

Simone from O'Reilly pointed out the continuing developments with Rob Flickenger & Co's efforts with 802.11. This time around they're trying to make a five mile to homes, via a 20.9 mile boost antennae. Fun for the whole wireless family.

Comments

Hemos is new to the English language

[novarese]

This time around they're trying to make a five mile to homes, via a 20.9 mile boost antennae.

For those of you in the audience who just said to your self "what the fuck is he talking about?", a quick translation/clarification from the actual story:

This series of articles tracks our progress in trying to use the 802.11b protocol to create a link from Sebastopol to a hilltop tower 20.9 miles north, and from there on to some homes 5 miles across a valley.

Not that I blame Hemos, I know how hard it must be to "edit" these submissions.

FCC rules for 802.11 and fines up to $250,000

[Totally_Lost]

Everyone should consider that it is illegal to mix and match arbitrary 2.4Gz gear, unless you employ the services of what the O'Reilly guys call "an FCC-approved installation geek at $200 per visit" who are required to make sure the installation is legal as required by Part 15 Regs.

While it is pretty easy to buy off the self antennas for 2.4GHz on Ebay or mail-order, there are very few vendors who sell certified configurations approved by the FCC for use with specific 802.11 vendors gear. Companies like Winncom and Hyperlink have done the required FCC certifications (which typically cost $15-30K) and can sell kitted systems - radio, cable, antennas to allow self installation of an FCC legal configuration. Otherwise, attempting to save a few hundred bucks is likely to net $250,000 in fines per installation as soon as a competitor (either in the wireless business or your trade) attempts to level the playing field with a complaint to the FCC.

When I was at Networld/Interop last year several of the wireless vendors were quick to note FCC raids in the south brought on by ISP's home brewing illegal configurations that violated the specs.

We have been doing what the O'Reilly boys are trying for several years now, and have a strict policy in our customer base to purchase approved kits from Winncom AND to use professional installation to make sure our network remains legal.

Multipoint links out to 25 miles are not that difficult - but it's also not a piece of cake using commercial hill top radio sites - especially when microwave links, AM/FM/TV broadcast stations, Cell Operators, Paging Operator, 400/800MHz moble repeaters, and other high power transmitters are on or near the tower. These transmitters all product Broad Band Noise (white noise) in their power amplifiers which greatly increase the noise into the 802.11 radio's reciever - often making the receiver deaf to your signals at distances further than a few miles - no amount of filtering will solve this problem, only careful trial and error placement of your antenna will minimize, but not remove the problem. Secondary to this, is the fact that snow, fog, rain, hail at the site will cause reflection/defraction of the broadband noise back into your antenna making the reciever deaf under cetain weather conditions that would not otherwise impair the link.

Also there is a problem with the transmit energy from co-located gear at the site mixing in your amplifier and radio front end, producing harmonic products of your radio's IF frequencies, which go right thru the filters and deafen your radio for as long as the RF carriers are present. We run with 110dB of band pass filter between a 7dBi omni and amplifier/radio to suppress this "Intermod". Reciently one of the PCS cell carriers doubled the number of channels at two sites we share, and seriously disrupted our repeaters for 30-90 minutes at a time - until we identified the source of the problem and added an additional 40dBi of band pass filter. We still see several second hits off the 3rd and 5th harmonics of 400/800Mhz pagers/mobile communications due to the extremely high EIRP they transmit at (several hundred watts). Even though their out of band energy is legal (50-70dB down from their primary carrier) it deafen's our recievers for brief periods due to near-far problems (IE our incoming signal is down around -80dBm - their harmonics are -40 to -10 dBm at the repeaters antenna.

It has taken us 6 weeks of trial and error placement and equipment/cable tuning to make some hill top repeater sites usable. This is not a binary problem where it just plugs and plays after hooking up some wires.

So in short, Metropolitan Area Networks built from off the shelf 802.11 wireless lan gear using repeaters over commercial radio hill tops sites isn't always easy to make links more than 2-3 miles work reliably. Even using this this gear shorter distances across roof tops is often difficult due to diffraction losses caused by the buildings and trees, and other Fresnel zone violations resulting in multipath interference that varies with temperature and weather. On some of our longer (IE weaker) links we have even seen Solar radition interference patterns on radios with west facing dishes as the sun sets at times. And other links installed in the winter, where a tree 90' away, and 40' to the side of the beam, leafed out and disrupted the link. And where standing waves from metal building 400' away at 10 O'clock to the beam, nulled out the beam at certain times of the day probably due to the metal siding warping in the heat and changing the pattern reflected back at the side of the dish antenna.

So do your homework before tring to save a few bucks buy 3rd party 802.11 gear from vendors that have not done the required certification with each radio you have. The several million in fines if you get caught is not a savings. You probably want to get a spectrum analyzer for the 2.4GHz band before you start that has a calibrated dBm vertical scale - like the HP8559s and later series of SA's. You will also need a display which offers "Max Hold", with an 8559A this means an 853A digial display, not a 180 series display. Trying to debug metropolitan area wireless lan's without being able to see the spectrum is nearly impossible - and horribly frustrating.

For more information of microwave path issues, check out:
http://www.tapr.org/tapr/html/ve3jf.dcc97/ve3jf.dc c97.html

For FCC regs see:
http://www.fcc.gov/oet/info/rules/
Also checkout the fines section elsewhere on the site.
Extracted from current FCC Regs, Part 15

PART 15 - RADIO FREQUENCY DEVICES
Subpart C - Intentional Radiators Section
Section 15.203 Antenna requirement.
An intentional radiator shall be designed to ensure that no antenna other than that furnished by the responsible party shall be used with the device. The use of a permanently attached antenna or of an antenna that uses a unique coupling to the intentional radiator shall be considered sufficient to comply with the provisions of this Section. The manufacturer may design the unit so that a broken antenna can be replaced by the user, but the use of a standard antenna jack or electrical connector is prohibited. [...] Further, this requirement does not apply to intentional radiators that must be professionally installed, such as perimeter protection systems and some field disturbance sensors, or to other intentional radiators which, in accordance with Section 15.31(d), must be measured at the installation site. However, the installer shall be responsible for ensuring that the proper antenna is employed so that the limits in this Part are not exceeded.

Section 15.204 External radio frequency power amplifiers and antenna modifications.
(c) Only the antenna with which an intentional radiator is authorized may be used with the intentional radiator.