5 GHz VS 2.4 GHz

This really is a great explanation, I have a question, I have recently installed a new router, which has 5 and 2.4 GHz, I have really got issues with a MBA early 2014 which connects to the router using ch 100 (5GHz) - the reason I know this is that I have a MBP early 2015 in the same room as the MBA - the MBP connects and the MPA will not - I took the MBA closer to the router - sweet all ok - this is why I thought it is a frequency issue - taking the MBA upstairs (1 ft of concrete floor will introduce about 20 dB of attenuation or more) it would not connect - I checked the channel and found it was a 5 GHz channel - channel 100 - so of course it will not work (not thru that concrete) - so how do I get the MBA to go back to 2.4?? h e l p

Are you carrying the MBA from your downstairs to upstairs turned on or in standby.. it is still connected to 5ghz and refuses to drop the connection even if it is very poor.

You can turn off the wireless.. or turn off the laptop and restart it.. by default the Mac will connect to the strongest signal, assuming the router is running identical names on both 2.4ghz or 5ghz.. otherwise just manually select the 2.4ghz signal via its name.. (router using different wireless names for both signals).

It can be also worthwhile doing diagnostics.. the inbuilt wireless diagnostic in Mac OS is very good.

see Check for Wi-Fi issues using your Mac - Apple Support

Signal strength is just "one side of the equation" for signal quality. The other side is signal noise. Subtracting noise from the strength gives you the Signal-to-Noise Ratio (SNR) value. This value is one of a number of others that can help to determine signal quality.

I put together a user tip related to this. Although it was written on where to best place an extending AirPort base station. The principles still apply for any wireless devices. So, for your case, you could make these SNR measurements (with your notebooks) at any location you take them to get an idea on how "good" the Wi-Fi signal is at that point.

Ref: AirPort - Optimal Base Station Placement

apart from that then 2.4Ghz is also a pretty busy band

bluetooth, landline wireless phones (dect) and much other wireless tech use it which can cause interference

and microwave ovens is suppose to send noise on it when operating

most routers like you use both and let you bridge them because many older devices only do 2.4

Hi thanks for your help! I am carrying the MBA upstairs while the computer is still on - I have rebooted the MBA but it will not connect to the router (same SSID for both 2.4 and 5) - My MBP sat next to me connects on 2.4 no problems, but the MBA refuses to! Under diagnostics, scan, I can see that both the MBA and MBP see the router in 2.4 and 5 at similar signal levels - no clue??

Ok.. in the MBA clear out all the saved wireless names.. dump the whole lot.. power off and restart the MBA and then reconnect to the wireless from your upstairs location.. put in the password.. and see which it connects to.

This is often required as wireless is still 80% black magic.. !!

Ok done that - did not connect - I have made a quick comparison between the MBP and MBA Network settings, what I have noticed on the page with Location at the top (and on the left the various networks, bluetooth wifi etc) my MBP has Wi-FI and the MBA has Wi-Fi 1 and Wi-Fi 2???

upstairs on the MBA the signal strength of the 2.4 and 5 GHz signals are -85 (5) and -80 (2,4) on the MBP -83 (5) and -73 (2,4) - I am not sure what the threshold is for good reception but it could be that the MBA simply does not have enough signal strength upstairs to connect to 2,4??

... I have switched off the 5GHz on my router and moved the MBA downstairs - connects to 2.4GHz - my presumption now is that my wireless adapter in the MBA is somehow not as effective as the MBP - both upstairs, one receives an adequate signal from 2.4 the other doesn't - i googles signal strengths required for WiFi and it indicates that around -70dBm is required for reliable comms, above this and packets get lost.... my -80 upstairs in therefore not going to work - is there any way of 'cleaning' up the wireless adapter in the MBA ?

next test - connected to 2.4 downstairs (MBA) and walked upstairs with it still on - it was quite happy on the 2.4 upstairs - now the darn MBA is showing a signal strength of -73db - in exactly the same spot as earlier, where it showed -80dB and would not connect - that radio 80% technology and 20% black magic is is well known to me as my day job is that of a radio engineer - there are many things which can affect the radio field in a house, but this one had me very worried my wireless adapter in the MBA had an issue! BTW, I put the Mac into sleep mode, once it awoke it connected to the 2.4 without a hitch...

I work on -60db as signal you need.. although you can get away with lower signal in a very quiet (RF) environment the problem is stability of connection.

The best solution is to simply have more AP units in the house.. connected by ethernet at best or powerline (EOP or homeplug in US) adapters.

The other way is using a wireless repeater.. but this is no where near as good.. especially if the house uses heavy construction materials. So any repeater needs to be placed at around the halfway point .. not distance but db loss. They will always cause lots of speed degradation.. due to the nature of using using wireless. As radio engineer you would understand very well.

Modern mesh products have become popular in places where there is no access to running ethernet in walls. These are expensive but the better ones like Netgear Orbi use a second channel to provide backhaul link which makes them a lot faster.

I have lived in this house for the last 17 years, I guess for the past at least 12 years we have used a wireless router - all over the house - all 3 floors (the router is in the same position for the last 12 years) - we have managed quite well without a WiFi repeater - MBA have been in our house for I guess at least 7 - 8 years without a WiFi repeater (we have 4 MBA and 1 MBP) - the only change was the installation of a new WiFi router (a week ago) which has taken us from the dark ages of 1MB download, to 15MB download (3 years ago) to 50 MB download - and this is the first router with both 2.4 and 5GHz - and I guess this is where the trouble has started (we live in a German house, built like a Panzer - the floors are probably close to a foot or so of steel reinforced concrete). I have now switched off the 5GHz and all seems stable - what is curios is the the MBA took to channel 100 pretty close to the router, and when going upstairs to the next floor, the MBA would not release channel 100 - not even after a reboot - it would not log onto the 2.4 channels - simply kept trying to remain on 100 - it was not until switching of 5GHz did the MBA decide to log onto 2.4!