For a robotic application, I would like to use a direct wireless link between a control computer and a distant robot. With minimum latency, and delay.
Expressif proposed ESPNOW, a way to link two ESP8266 or ESP32 using vendor specific action frame of 802.11. This is way faster than UDP on regular WIFI. My first test show less than 300us to transmit 200Bytes of data !
A solution is to use two of this module, but then we need to use another link between the computer and one of the modules. We don't want to use USB because of the possible jitter and delay it introduces.
Since some wireless card support monitoring and injection of raw packets, it should be possible to use directly the WiFi card of the control PC !! But how??
This project will keep track of my investigation, and any advise is warmly welcome.
Ideally, at the end, we could have a ESP-NOW driver for linux that allow to efficiently connect our favorite ESP chip to a control PC.
To implement the ESP-NOW Protocol, we first need to understand how it works in details.
The datasheet doesn't say much on the really low level, especially on the acknowledgment behavior.
To understand what ESP-NOW does, I use Wireshark with my PC Wifi card in monitoring mode:
> sudo ifconfig wlan0 down
> sudo iwconfig wlan0 mode monitor
> sudo ifconfig wlan0 up
> sudo wireshark
With one ESP8266 sending ESP-NOW to a non existing MAC adress, I got the following packets:
The same packet is sent multiple times, with a decreasing datarate :
2 tries at 6Mb/s
2 tries at 2Mb/s
7 tries at 1Mb/s (maybe this last number of tries depends on a timeout...)
First bad news: ESP-NOW continues to send the packet again and again for 20ms !
I hope their is a way to disable this resend behavior... Indeed, for a robotic control, we prefer to wait for the next up-to-date packet than getting an outdated one, and jamming the channel...
Now, if I turn on the destination ESP8266, the packets are sent only once, and a quick acknowledgement frame is sent back in (about 60us):
I guess this acknowledgment frame will be generated by the low level MAC layer, so we shouldn't take care of managing it...
The next test is then to send ESP-NOW packets to my laptop MAC address, and check that ACK are sent automatically even in monitoring mode...
Hum... After a quick test, when I Sending a packet to my laptop's MAC address, it does't send an ACK back. Maybe the monitor mode disable this in the MAC layer ?
Thomas Flayols
bafeigum
Alexander