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Added info on FCC wifi channel rules
(Created page with "We use three different methods to map out spectrum usage. = Wifi channel usage scan = Most wifi chipsets are capable of reporting: *How many access points are on each chann...")
 
(Added info on FCC wifi channel rules)
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Note: Legal info on this page is U.S. centric. Also, do not take any of this as legal advice. We are not lawyers.
Channel selection is not as straight-forward as one might expect. In selecting channels we take into account:
*Minimizing interference / staying off highly used channels
:Is a channel prone to interference or high use in a given area?
*Using a single channel for many-to-many communication in a given area
:Most of the mesh nodes have only one radio, so can only be on one channel at a time.
*Legality in geographical region
:Some channels are completely illegal in the U.S.
*Additional technical legal requirements
:Some countries require dynamic frequency selection for some channels
= Legal =
== 2.4 GHz ==
== 5 GHz ==
For 5 GHz, there are quite a few available channels, but many require the use of Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC).
=== Non-DFS channels ===
The following 40 MHz wide overlapping channels are allowed without DFS for _outdoor_ use:
*149 - 5745 MHz (5725-5765)
*153 - 5765 MHz (5745-5785)
*157 - 5785 MHz (5765-5805)
*161 - 5805 MHz (5785-5825)
Maximum power output for these channels at 20 or 40 mhz is [http://louise.hallikainen.org/FCC/FccRules/2011/15/407/ 1 watt], with the following caveats:
<blockquote>
  ...the peak power spectral density shall not exceed 17 dBm in
  any 1-MHz band. If transmitting antennas of directional gain greater
  than 6 dBi are used, both the maximum conducted output power and the
  peak power spectral density shall be reduced by the amount in dB that
  the directional gain of the antenna exceeds 6 dBi. However, fixed
  point-to-point U-NII devices operating in this band may employ
  transmitting antennas with directional gain up to 23 dBi without any
  corresponding reduction in the transmitter peak output power or peak
  power spectral density. For fixed, point-to-point U-NII transmitters
  that employ a directional antenna gain greater than 23 dBi, a 1 dB
  reduction in peak transmitter power and peak power spectral density for
  each 1 dB of antenna gain in excess of 23 dBi would be required. Fixed,
  point-to-point operations exclude the use of point-to-multipoint
  systems, omnidirectional applications, and multiple collocated
  transmitters transmitting the same information. The operator of the
  U-NII device, or if the equipment is professionally installed, the
  installer, is responsible for ensuring that systems employing high gain
  directional antennas are used exclusively for fixed, point-to-point
  operations. -- [http://louise.hallikainen.org/FCC/FccRules/2011/15/407/ FCC rules]
</blockquote>
*[http://en.wikipedia.org/wiki/List_of_WLAN_channels#5.C2.A0GHz_.28802.11a.2Fh.2Fj.2Fn.2Fac.29.5B16.5D Wikipedia info on 5 ghz and DFS requirements]
*[http://louise.hallikainen.org/FCC/FccRules/2011/15/407/ FCC 5 ghz rules]
The following 20 MHz wide non-overlapping channels are allowed without DFS for _indoor_ use:
*36 - 5180 MHz (5170-5190)
*40 - 5200 MHz (5190-5210)
*44 - 5220 MHz (5210-5230)
*48 - 5240 MHz (5130-5250)
Maximum power output for these channels is 50 mW (!) at 20 or 40 mhz, with the following caveats:
<blockquote>
  ...the peak power spectral density shall not exceed 4 dBm in any
  1-MHz band. If transmitting antennas of directional gain greater than 6
  dBi are used, both the maximum conducted output power and the peak
  power spectral density shall be reduced by the amount in dB that the
  directional gain of the antenna exceeds 6 dBi. -- [http://louise.hallikainen.org/FCC/FccRules/2011/15/407/ FCC rules]
</blockquote>
=== DFS channels ==
None of the DFS channels allow more than 250 mW output power. The TPC requirement only applies if the EIRP is more than 500 mW.
The following channels with DFS are allowed for indoor or outdoor use:
*52 - 5260 MHz (5250-5270)
*56 - 5280 MHz (5270-5290)
*60 - 5300 MHz (5290-5310)
*64 - 5320 MHz (5310-5330) (possibly 5310-5350?)
*100 - 5500 MHz (5490-5510) (possibly 5470-5510?)
*104 - 5520 MHz (5510-5530)
*108 - 5540 MHz (5530-5550)
*112 - 5560 MHz (5550-5570)
*116 - 5580 MHz (5570-5590)
*132 - 5660 MHz (5650-5670)
*136 - 5680 MHz (5670-5690)
*140 - 5700 MHz (5690-5710)
Some, if not all of the Atheros AR9xxx chipsets support DFS. It looks like Linux support for DFS in ad-hoc mode has been implemented:
*[http://www.spinics.net/lists/linux-wireless/msg111894.html Patch for DFS support in ad-hoc mode for Linux ath9k driver]
It may be that we _can_ use these channels for point to multi-point rooftop links. More research is required.
Maximum power output for these channels is 250 mW at 20 or 40 mhz, with the following caveats:
<blockquote>
  ...the peak power spectral
  density shall not exceed 11 dBm in any 1 megahertz band. If
  transmitting antennas of directional gain greater than 6 dBi are used,
  both the maximum conducted output power and the peak power spectral
  density shall be reduced by the amount in dB that the directional gain
  of the antenna exceeds 6 dBi. -- [http://louise.hallikainen.org/FCC/FccRules/2011/15/407/ FCC rules]
</blockquote>
= Spectrum analysis / spectrum usage mapping =
We use three different methods to map out spectrum usage.
We use three different methods to map out spectrum usage.


= Wifi channel usage scan =
== Wifi channel usage scan ==


Most wifi chipsets are capable of reporting:
Most wifi chipsets are capable of reporting:
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See the [[Mesh/Software_tools|software tools]] page for suggestions on using this type of scan.
See the [[Mesh/Software_tools|software tools]] page for suggestions on using this type of scan.


= Atheros spectral scan =
== Atheros spectral scan ==


The Atheros AR92xx and AR93xx chipsets have a spectral scan mode that outputs FFT data for the entire range of the chip.
The Atheros AR92xx and AR93xx chipsets have a spectral scan mode that outputs FFT data for the entire range of the chip.
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