802.11 channel 14

This article in Daily Dot’s Kernel magazine, “The mystery of WiFi channel 14”, was recently posted in /r/TIL. This led me to discover that there apparently exists a conspiracy theory as to why WiFi channel 14 is not permitted in the United States.

This made me irrationally angry.

I have hence dug up the relevant FCC and even ITU regulations to explain why it’s not mysterious at all that channel 14 is not permitted.

Let us begin. What is channel 14? The 802.11 channels go like this:

(source: Wikimedia Commons. CC-BY-SA 3.0)

Channel 14 is all the way on the right there, spaced 7 MHz farther apart than usual.

It’s not just channel 14, for the record. As Wikipedia points out, channels 12 and 13 are generally not used either, because anything operating more than very low power will almost certainly cross into the restricted zone.

Wait. The restricted zone? Yes, there’s a restricted zone starting at 2483.5 MHz extending to 2500 MHz, per 47 CFR §15.205. No Part 15 device can transmit in this zone. The question becomes why. Part 15 doesn’t say.

So we look in the FCC allocation table (PDF warning). There’s conveniently an entry for 2483.5–2500 MHz:

Screen Shot 2016-05-29 at 12.43.46 AM

Let’s break it down. First, notice that this frequency band is allocated to a bunch of radio services mentioning satellites, in particular space-to-Earth communications. We then get a shitload of references to various notes. Let’s go through each of them. (Some of these notes do not appear in the FCC document and need to be searched for in the full ITU Radio Regulations. As of writing, it’s available here.)

5.150 defines the ISM bands. In particular, one of them is 2400–2500 MHz.

5.398 “In respect of the radiodetermination-satellite service in the band 2483.5-2500 MHz, the provisions of No. 4.10 do not apply.”

4.10 “Member States recognize that the safety aspects of radionavigation and other safety services require special measures to ensure their freedom from harmful interference; it is necessary therefore to take this factor into account in the assignment and use of frequencies.”

5.402 “The use of the band 2483.5-2500 MHz by the mobile-satellite and the radiodetermination-satellite services is subject to the coordination under No. 9.11A. Administrations are urged to take all practicable steps to prevent harmful interference to the radio astronomy service from emissions in the 2483.5-2500 MHz band, especially those caused by second-harmonic radiation that would fall into the 4990-5000 MHz band allocated to the radio astronomy service worldwide.”

US41 “In the band 2450-2500 MHz, the Federal radiolocation service is permitted on condition that harmful interference is not caused to non-Federal services.”

US319 “In the bands 137-138 MHz, 148-149.9 MHz, 149.9-150.05 MHz, 399.9-400.05 MHz, 400.15-401 MHz, 1610-1626.5 MHz, and 2483.5-2500 MHz, Federal stations in the mobile-satellite service shall be limited to earth stations operating with non-Federal space stations.”

US380 “In the bands 1525-1544 MHz, 1545-1559 MHz, 1610-1645.5 MHz, 1646.5-1660.5 MHz, and 2483.5-2500 MHz, a non-Federal licensee in the mobile-satellite service (MSS) may also operate an ancillary terrestrial component in conjunction with its MSS network, subject to the Commission’s rules for ancillary terrestrial components and subject to all applicable conditions and provisions of its MSS authorization.”

US391 “In the band 2495-2500 MHz, the mobile-satellite service (space-to-Earth) shall not receive protection from non-Federal stations in the fixed and mobile except aeronautical mobile services operating in that band.”

NG147 “In the band 2483.5-2500 MHz, non-Federal stations in the fixed and mobile services that are licensed under 47 CFR parts 74, 90, or 101, which were licensed as of July 25, 1985, and those whose initial applications were filed on or before July 25, 1985, may continue to operate on a primary basis with the mobile-satellite and radiodetermination-satellite services, and in the sub-band 2495-2500 MHz, these grandfathered stations may also continue to operate on a primary basis with stations in the fixed and mobile except aeronautical mobile services that are licensed under 47 CFR part 27.”

 

So. The reason channel 14 is restricted is because it conflicts with the allocation of 2483.5–2500 MHz to the mobile-satellite and radiodetermination-satellite services. In particular, that band is used for space-to-Earth communications, meaning any WiFi operation in that band would almost certainly overpower any satellite signals.

mystery solved…

 

Edit: One last note. On top of clearly doing absolutely no research whatsoever, the Daily Dot leaves us with this: “Though the channel is banned the consequences of using the restricted channel are not specified. It is considered a felony due to its illegality though it seems unlikely that the FCC will come knocking on your door.” The FCC takes its regulations extremely seriously: even in the Amateur Radio service, repeated willful interference can often lead to a forfeiture order well into the five figures. Should you ever cause interference to one of the satellite services mentioned above by operating a WiFi access point on channel 14–enough to be noticed, anyway–you most certainly can expect a knock on your door.

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