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I am trying to use a RPI Zero W2 to solve a problem I have with collecting files from an IoT device. Specifically, I don't know how to setup the Wi-Fi network(s). Any inputs to my questions would be appreciated.

Existing Setup:

  1. IoT device acting as a web server and hosting a Wi-Fi access point (say 10.10.x.x subnet).
  2. Wi-Fi access point providing internet access (say 192.168.x.x subnet).

My Requirement / Need a RPI Zero W2 to do the below:

  1. Connect to the IoT device (1) and download some log files using a REST API. (could be a few GB)
  2. Connect to the Wi-Fi access point (2) and upload the log files to some cloud storage.

Note 1: I don’t need to provide internet access to the IoT device. It is acting as a web server only and does not generate outgoing traffic.

Note 2: I can take care of the code to do the download and upload mentioned in point 1. & 2. Need guidance on how to setup the networks at this point.

Questions:

  1. How do I connect to two Wi-Fi access points simultaneously? I was planning to add a second Wi-Fi NIC using a USB dongle & OTG cable. Alternatively can these connections be made simultaneously on the single built-in Wi-Fi NIC?

  2. Would it be better to alternate connecting to the two access points? For example connect to the IoT device Wi-Fi AP, download files, disconnect from that AP, connect to the internet Wi-Fi AP, upload to internet, disconnect from the internet Wi-Fi AP, and then repeat.

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  • Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer.
    – Community Bot
    Apr 9 at 15:44

2 Answers 2

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Interesting questions.

N.B.: I can't replicate your network setup. Consequently, the following answers have not been fully verified. I feel the alternate/sequential connections approach will work for you assuming both APs (IoT & WiFi) are on functional networks.

Summary:

A "simultaneous" solution is possible only with hardware & drivers that support it. AFAIK, there is no RPi hardware that does.

An "alternate/sequential" solution is possible, and illustrated below. This solution employs the wpa_cli app (part of wpa_supplicant) to rotate AP priorities using set network <id> priority <n> followed by a reassociate to switch to the higher priority network id/AP. This rather obtuse approach is necessary to avoid the select_network option that disables the other networks.

WRT Q1: connect to 2 Wi-Fi access points simultaneously ?

As I understand it, this requires both an interface (hardware) and a driver that supports dual-channel management. You should run the iw list command on your Pi 0 W2 to determine if your system currently supports "simultaneous".

For my RPi (a 3B+ running bullseye), I get the result below where <=1 effectively means that the "simultaneous" option is not available for my 3B+. If you get <=1 result, then "simultaneous" is likely not available for you either. This may guide your selection of a suitable USB NIC if you want "simultaneous".

$ 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

WRT Q2: alternate/sequential wifi connections

I believe this will do what you need using the single wlan0 interface built into your Pi 0 2W. You may wish to do some reading (see REFERENCES below) before you begin, and get some background on this process. Based on my reading of this & other research, and under the assumptions stated above, this option will work. The procedure is outlined below in two steps:

  1. Declare both networks in /etc/wpa_supplicant/wpa_supplicant.conf
    You didn't show your wpa_supplicant.conf file, so I've created this one to serve as an 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"
}
  1. We use wpa_cli in its interactive mode below, but 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.


REFERENCES:

  1. RE: wlan0 vs. p2p-dev-wlan0
  2. ArchWiki wpa_supplicant documentation
  3. man wpa_cli on your RPi
  4. Linux Find Wireless WiFi Driver Chipset Information
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  • unfortunately my RPI zero w2 returns <=1. so will have to go with the alternating connections to each wifi AP.
    – Sachin
    Apr 14 at 5:16
  • @Sachin: "my RPI zero w2 returns <=1" - not surprising considering RPi priority is on low cost. On the bright side, administration of such a beast might be x-painful? I just skimmed through this recent, interesting post on Vivek Gite's Linux blog; if dual simultaneous is what you want, there may be some information there to help you find it. But in any case, please do let us know how this works for you.
    – Seamus
    Apr 24 at 21:56
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I am now able to switch between Wi-Fi networks with the following commands:

wpa_cli -i wlan0 set_network 0 priority 2
wpa_cli -i wlan0 set_network 1 priority 0
wpa_cli -i wlan0 reassociate

and able to switch back to the first Wi-Fi with:

wpa_cli -i wlan0 set_network 0 priority 0
wpa_cli -i wlan0 set_network 1 priority 2
wpa_cli -i wlan0 reassociate

If I didn't use the -i switch it would default to p2p-dev-wlan0 instead of wlan0.

Reference: Switching to a different wireless network when it is available

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