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Yesterday I received a Fire 7 (9th Gen) Tablet. According to the official datasheet, it supports dual-band 802.11n Wi-Fi. To my surprise, the SSID of my 5GHz Wi-Fi router didn't show up. After talking to support it was explained to me that the device only supports the channels 36, 40, 44, 48 on the 5GHz band. At the same moment, my router doesn’t allow me to specify the channel it is using.

So I talked to both support staff for the tablet and the router. The ISP who is the provider of my router is stated:

It's the tablet's fault it should be able to scan on all channels. Other devices can connect on the 5GHz band without problems.

The Amazon support, on the other hand, stated:

It’s the router's fault, it should be able to specify the channel it is using. The tablet can connect to other 5GHz Wi-Fis without problems.

So my question is: Which device violates the 802.11n protocol standards? Or is it totally valid that two devices which both support 802.11n on the 5GHz band aren’t able to connect?

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    I don't know formal details, but the idea of WiFi device only being able to operate on a few select channels is crazy, especially on the 5 GHz band where channels don't overlap.
    – gronostaj
    Nov 28 '19 at 10:55
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    Please edit your question: What is the make and model number of the Wi-Fi router and what country are you in? Nov 28 '19 at 14:46
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    Can you specify what country the router and tablet were sold for? It is not unusual to have different variants of the same equipment which support different channels based on the country they are designed for, for instance, so if you bought one of the two in a different country, then you may easily end up with this kind of issues (though it's not the only possible reason).
    – jcaron
    Nov 28 '19 at 22:40
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Who is violating the 802.11n WiFi standard?

Nobody. They just chose not to bother with all the complex regulations surrounding radio use. So they only support a subset of the available channels.

5 GHz channels 36, 40, 44 and 48 are the only channels that can be used without radar detection, power control etc, indoors, world-wide (well, almost). Many early 11ac(-ish) routers don’t support other channels at all, due to driver and hardware limitations.

From searching the net, it appears all sorts of Amazon devices have a long history of only supporting these channels for 5 GHz Wi-Fi. There is nothing you can do about that. It is also not unusual at all, unfortunately.

Unless you want to use large amounts of data, just use 2.4 GHz Wi-Fi. It has vastly superior range anyway.

If you do want to use 5 GHz Wi-Fi, you need a different/additional router/access point.

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    In some places (especially dense urban areas), the 2.4 GHz is close to unusable given the number of devices using it (or trying to).
    – jcaron
    Nov 28 '19 at 22:45
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    @jcaron I live in a normal (not dense) suburb and see at least 30 AP's on 2.4 GHz. Effectively unusable. The biggest ISP in the area has Wifi (a public hotspot SSID and a private SSID for the customer on different channels) on every router AND every TV set-top box (that doubles as AP) they provide. Random channel selection on each device. With router + upto 3 set-top boxes per customer there isn't a single channel without at least 2 AP's on it. (They only do channels 1-11, so 13 is somewhat usable.) 5Ghz is short-range enough that my own AP can over-power the competition.
    – Tonny
    Nov 28 '19 at 23:17
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    If the information about the supported channels wasn't provided to the OP before the purchase, they may still be able to return the device under the customer protection law. Nov 29 '19 at 2:32
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    @Tonny Not every SSID is really separate. For example the ISP hotspots on consumer routers you mention do not usually run on different channels. The hardware simply does not support it. // 2.4 GHz overpopulation is real, of course. Still, I had no problem with that even when I was still living in city.
    – Daniel B
    Nov 29 '19 at 7:38
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    @DanielB It is unusual but that ISP over here really does that. I have only 10 neighbors in range on 2.4 GHz. 8 of them on that ISP.
    – Tonny
    Nov 29 '19 at 16:21
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I'll say it's probably neither. Both of them being cheap crap doesn't necessitate either of them violating any standards.

As far as I know the 802.11 standards neither require a device to support all possible channels, nor that the selection of channel can be made by the user.

Personally I would blame the router, as it's too lame that it provides no means for the user to fix the selection of channel to one that can be used without DFS, while I think it's tolerable that a client device supports only those channels.

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    The router is also probably the easiest and cheapest to replace.
    – Barmar
    Nov 29 '19 at 18:14
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I looked into the question of channels and found it to be very complicated indeed, and also highly dependent on the country where you reside.

According to the article 802.11ac Channel Planning, the router is set to function on Channel Width of 20 MHz.

You will find the authorized channels for your country in the Juniper article
AX411 Access Point Country and Channel Support.

There are many rules and regulations in the various countries. For example, in Europe (or in ETSI jurisdiction) channels 36–64 are restricted for indoor use only.

If you have received a tablet that does not match the regulation in your country, you should return it, or demand one that is suitable.

If the tablet matches the regulation in your country, then the ISP is in the wrong and the router/modem is too restrictive.

You may find more information in Wikipedia List of WLAN channels.

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It is not strictly a violation of the standard, it's just that these devices use the cheapest, crappiest WiFi chips that exist, and these can't operate on all channels. As long as they operate on one channel, they're technically operating within the standard.
It's a well-known, annoying issue which has existed for many years, not limited to Amazon devices, but to many crappy low-end devices. Funnily, no such issue exists on the 2.4GHz band (at least I've never heard of that). Presumably that's because even the cheapest chipsets support all channels anyway.

The devices work perfectly well, only just... not in a usable manner on any usable band (i.e. a band where there aren't already 5-10 other routers in a city). That's bad luck, but not much you can do (well, nothing in fact, not just "not much").

Note that it's pretty obvious that Fire tablets operate on a razor and blades model where Kindle books as well as the movies that you are to buy on Prime Video are the blades. There is no way you could possibly buy an Amazon Fire for the price at which they're sold normally, legitimately, and in a no-cheat-no-fraud way.
So... the fact that there's crappy WiFi hardware inside shouldn't really come as a surprise. They are cheap devices sold way under price.

Looking at it from the positive side, you do not need to enable radar avoidance scanning for these channels, so there's faster startup and fewer forced channel changes to be anticipated in case there's an airport within 50-60km.
Also, the higher channels necessarily have less range and lower penetration (higher frequency). So, using a lower channel may be an advantage.

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"Violating a standard" is doing something explicitly forbidden in the standart OR refusing to do something explicitly required thereof.

Implementing a support for a particular channel in 5GHz band is optional.

Implementing some particular standard in two devices is a good step in making them interopeate, but not necessarily enough.

You just happen to have two devices whose (maybe) correct, but partial implementations do not overlap.

You may, or may not have success by installing alternative firmware in one of the devices or just rooting it and kicking the limitations out. YMMV and also you may be violating some local law doing so.

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