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(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: | ||
Line 12: | Line 127: | ||
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. |