I came across an interesting question on the RPi SE recently. The question solicited an approach to accessing multiple WiFi networks/Access Points in an automated fashion; i.e. no reboot or manual intervention required. I'd never tried this before, but the idea intrigued me. I cobbled together an answer. Unfortunately, I don't have access to an environment that allows for full verification of my approach, and so I am posting it here for a couple of reasons:
- To get a bit wider distribution - in the hope that perhaps an interested reader may give this a whirl & provide some negative feedback toward verification and improvement
- As a solicitation for potential applications... for example, a mobile drive-by remote sensor data collection - farms, wildlife, inaccessible areas, etc. 'Pie-in-the-sky' stuff I suppose, but remote sensing has interested me since I worked on a
TinyOSapplication for combat casualty care years ago.
What follows is mostly the answer I posted to RPi SE. I'll augment this based on interest, and availability of resources:
"Simultaneous" WiFi connections are possible only with hardware & drivers that support it. AFAIK, there is no available RPi hardware that does.
A "sequential" solution is possible, and illustrated below. This solution employs the wpa_cli app (part of wpa_supplicant) to rotate WiFi AP priorities using set network <id> priority <n> followed by a reassociate to switch to the higher priority network id/AP. This is a rather obtuse approach IMHO, but necessary to avoid the select_network option that disables the other networks.
As I understand it, this requires both an interface (hardware) and a driver that supports dual-channel management. Assuming this is good information (and I feel it is) the iw list command will inform whether or not your system currently supports "simultaneous connections".
On my RPi (a 3B+ running bullseye), my result is shown below, and as expected. The <=1 output effectively means that the "simultaneous" option is not available on my 3B+.
$ iw list | grep -A 4 'valid interface combinations'
valid interface combinations:
* #{ managed } <= 1, #{ P2P-device } <= 1, #{ P2P-client, P2P-GO } <= 1,
total <= 3, #channels <= 2
* #{ managed } <= 1, #{ AP } <= 1, #{ P2P-client } <= 1, #{ P2P-device } <= 1,
total <= 4, #channels <= 1
Vivek Gite's blog post may guide you in a search for a USB NIC that supports "simultaneous".
The REFERENCES below may provide some useful background on this 'sequential' process. The 'sequential' solution works - this statement based on my experiments, the REFERENCES below and other research. The caveat being that the APs used in my experiments were configured, and are administered by me. The procedure is outlined below in two steps:
- Declare both networks in
/etc/wpa_supplicant/wpa_supplicant.conf
I've created thiswpa_supplicant.conffile as a typical example :
ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev
update_config=1
country=GB
network={
ssid="MyWiFiAP"
psk="mypasswd"
}
network={
ssid="MyIoTAP"
psk="mypasswd2"
}
- The interactive mode of
wpa_cliis used below; know that all of these commands may be written as stand-alone commands, suitable for scripting:
$ wpa_cli
# ... # preliminaries...
> list_networks
network id / ssid / bssid / flags
0 MyWiFiAP any [CURRENT]
1 MyIoTAP any
# The above result shows the two SSIDs configured in wpa_supplicant.conf
> get_network 1 priority
0
> get_network 0 priority
0
# The above results show the *priority* of the two SSID/network ids (0 & 1)
> set_network 1 priority 2
OK
# Assign a higher priority to network id 1 (MyIoTAP)
> reassociate
OK
... a series of CTRL_EVENTS are listed ...
# Connect to the higher priority network (MyIoTAP)
> list_networks
network id / ssid / bssid / flags
0 MyWiFiAP any
1 MyIoTAP any [CURRENT]
# after `reassociate`, connection has moved to higher priority network/SSID
# to restore the original connection to MyWiFiAP:
# use the `set_network` priority commands above,
# follow that with another `reassociate`
# Note: when priorities of all networks are equal,
# wpa_supplicant defaults to the one with the strongest signal
> quit
# terminates the wpa_cli interactive session
$ And that's it... the alternate/sequential wifi connections process. You should verify this works with your network configuration manually as I've shown above. Once verified, you may use the equivalent stand-alone commands to automate the process in a script - or experiment with wpa_cli running in daemon mode. Once you've got this working, the next step may be to read all about iw, enhance the crude script here, deposit your WiFi APs in your area of interest, put your RPi in your car or bike, and go collect some data.