Turns out though, it might not be as easy as I initially made out. In particular, I was noticing the expected switchover as I walked down the corridor in the middle of my house:
... was simply not happening. I would be "stuck" on Channel 3 (red) or Channel 9 (green) based on whatever my Mac had woken up with.
Lots of Googling later, and the simplest diagnostic tool on the Mac turns out to be Wireless Diagnostics -> Info - take a snapshot, turn off that AP, and wait for the UI to update. Then stick them side by side and eyeball them:
I wasted quite some time following a wild goose because of the differing Country Codes - it's not really something you can change in most consumer AP/routers so I thought I was in trouble, until I discovered that "X1" really just means "not broadcast" so I decided to ignore it, which turns out to be fine.
Now the other thing that was most definitely not fine was the different Security policies. I thought I had these set up to match perfectly, but as you can see, MacOS thought different (pun not intended).
When is WPA2 Not WPA2?
The D-Link AP (on Channel 9, on the right in the above screenshot) was supposedly in "WPA2 Personal" mode but the Mac was diagnosing it as just WPA v1. This is most definitely enough of a difference for it to NOT seamlessly switch channels. Even more confusingly, some parts of the MacOS network stack will report this as WPA2. It's quite tricky to sort out, especially when you have access points of different vintages, from different manufacturers, who use different terminology, but what worked for me (on the problematic D-Link) was using Security Mode WPA-Personal together with WPA Mode WPA2 plus explicitly setting the Cipher Type to AES and not the default TKIP and AES:
This last change did the trick for me, and I was able to get some automatic channel-switching, but the Mac was still holding on to the Channel 3 network for much longer than I would have liked. I could even stand in the same room as the Channel 9 AP (i.e. in the bottom-left corner of the heat map above) and not switch to Channel 9.
Performance Anxiety
The clue, yet again, is in the Info window above. In particular, the Tx Rate field. It would seem that rather than just naïvely choosing an AP based on signal strength, MacOS instead (or perhaps also) checks the network performance of the candidate AP. And look at the difference between the newer dual-band Linksys on Channel 3 (145 Mbps) and the D-Link on Channel 9 (26 Mbps)!
There are plenty of ways to increase your wireless data rate, the most effective being switching to be 802.11n exclusively, as supporting -b and -g devices slows down the whole network, but (as usual) I hit a snag - my Brother 2130w wireless (-only) laser printer needs to have an 802.11g network. As it lives in the study, a couple of metres from the D-Link AP, I'd had the D-Link running "mixed mode" to support it.
Printers were sent from hell to make us miserable
As it turns out, the mixed-mode signal from the Linksys at the other end of the house is good enough for the printer (being mains-powered it's probably got very robust WiFi) and so I could move the D-Link to "n-only". But there was a trick. There's always a trick ...
You need to make sure the printer powers up onto the 802.11g network - it doesn't seem to be able to "roam" - which, again, makes sense - it's a printer. It knows to join a network called MILLHOUSE and will attempt to do so - and if the "n-only" network is there, it'll try to join it and never succeed.
So powering down the n-only AP, rebooting the printer, checking it's online (ping it), then powering the n-only AP back up again should do the trick.
Summary
Moving the D-Link AP to n-only doubled the typical Tx Rate at the front of the house to around 50 Mbps, the result being that the Mac now considers Channel 9 to be good enough to switch to as I move towards that end of the house. It still doesn't switch quite as fast as I'd like, but it gets there, and doesn't drop any connections while it switches, which is great.
Here's the summary of the setup now:
| Living Room | Study | |
|---|---|---|
| Device | Linksys X6200 | D-Link DIR-655 |
| IP Address | 10.240.0.2 | 10.240.0.3 |
| Channel | 3 | 9 |
| Mode | b/g/n | n-only |
| Band | 2.4 GHz | 2.4 GHz |
| Bandwidth | 20 MHz | 20 MHz |
| Security | WPA2 Personal | WPA Personal (sic) |
| WPA Mode | n/a | WPA2 Only |
| Cipher Type | n/a | AES Only |
Stand by for even more excruciating detail about my home network in future updates!
