Should I set WiFi channel to auto or manual?

Most devices will select the best channel on boot up only, and generally speaking they aren't good at it.

If you are using 2.4 GHz band, I recommend you to use channels 1, 6, 11 (wide bandwidth) to avoid interference, though they aren't the very best for all cases.

For the optimal results use this WiFi Analyzer app, it will tell you the best channel to use for both bands, even if nearby WiFis are not turned on all the time you can always open the app multiple times during the day, then write down the best channels, and select the common best one.

Why Japan is the only country that supports WiFi ch.14?

Japan is notoriously forward thinking when it comes to technology applications. They may have freed up this space from legacy technology or maybe never used it at all as much of what they do has their own standards for technology.

Where as the rest of the world and the US is very slow to discontinue old services. So many bands around the world are tied up by old technology that depended on the upper echelons of the 2.4ghz frequency.

The link above comments on global military use, we may never truly know the real reason many bands are reserved other than for "Military Use".

Trust me there are many many frequency ranges reserved for government use.

How can I tell if I need a new router?

Find out what internet speed you pay for from your Internet Service Provider.

Run a website based bandwidth speed test.

Make notes of the results.

Speedof. And others are good sites to test, avoid testing provided by your ISP - it is inaccurate for your purposes.

Run the test if possible from an Ethernet connected device (a network cable connected game console, desktop/laptop, or other devices connected by Ethernet that can provide a bandwidth benchmark). A score dramatically less (more than 50% less) than ISP stated bandwidth merits consistent testing and then a complaint to your ISP.

Next run the bandwidth test from wifi, standing right next to the wireless router (a smart phone works best for this stage of testing). A much slower speed at this stage indicates an aging or weak wifi router, power off and on the wifi router, if results are similarly poor compared to your Ethernet, try a different testing device and then contemplate replacement.

Now move as far away as you can while keeping your wifi testing equipment in line-of-sight of the wifi router. Run the bandwidth test again at this time. If results are still within 80% of original results at 35m then proceed. Otherwise, change testing devices and contemplate a new router if results are consistently poor.

Now go to an area of the structure that has one wall between the testing equipment and wifi router. Run test. If results are significantly lower you may need to switch testing devices or contemplate a router replacemect, or a mesh network to assist in penetrating heavy walls (or run all Ethernet).

Also check for signal obstructions like a microwave in between you and the router, as these can impede or interrupt the wifi connection.

Why do WiFi channels have overlapping widths?

Each channel in the 2.4 GHz frequency is usually 20 Mhz wide, which is the channel width. What it means is, a device cannot project a radio signal at EXACTLY 2.412 GHz (well theoretically it could, but then you can't squeeze much data in it with such a narrow spectrum).

So in practice, Channel 1 (2.412) actually occupies frequencies from 2.402 all the way to 2.422 - 20MHz Channel

The reason for the channel width is to host more data 'sub-carriers' meaning the 20Mhz width frequency is further divided into smaller partitions (52 sub-carriers), each sending its own data in parallel. This in turn maximizes data throughput. (Note that previous WiFi technologies used Frequency Hopping, so the channel width is the channel 'within' the hops could occur, google up FHSS if you're interested)

Imagine taking the whole visible light spectrum and dividing them into 7 colours of the rainbow, each colour transmitting a different stream of bits, each colour will he considered a subcarrier, and the entire visible light spectrum the entire channel width.

Best WiFi channel for 2.4 GHZ band?

As for the 2.4 GHz band the best channel to use is the one that is not overused by your neighbors, channels 1,6,11 have wider bandwidth so they suffer less from interference, though they are NOT the best ones to choose in all cases.

For the optimal results, I recommend you to use this WiFi Analyzer app.

This app will analyze your nearby WiFi networks and will till you the best channel for your case in order to avoid interference.