After realising the need for faster data transfer, we started looking at the ESP32 and the various protocols it supports. Although using layer 4 protocols like TCP or UDP is possible, the overheads of handling connections to an ad-hoc network could be problematic. We thus examined ESP-NOW, a protocol defined by Espressif which sends data over raw 802.11 packets directly without the need for devices to be on the same WLAN network.

Packet Capture of ESP-NOW data being sent and the resulting acknowledgement packets

Code was written to test the effects of the following parameters on the throughput of ESP-NOW:

1. Wi-Fi Data Rate
2. Broadcast or Unicast:
   1. Sending packets directly to a peer identified by their MAC address results in the peer sending an Acknowledgement packet back
   2. Broadcasting packets (send to MAC FF:FF:FF:FF:FF:FF) will not cause Acknowledgement packets to be sent, potentially speeding up transfer.

The test code transmits 250 byte (maximum supported packet length) ESP-NOW packets as fast as possible from a device (henceforth the master) to another device (henceforth the slave) for 3 seconds, sleeps for 10 seconds, then repeats.

The slave device then transmits the number of packets received at 1 second intervals back to the master device, which then prints out this number.

The following data rates have been observed between two ESP32-DevKitC’s approximately 1m apart in a standard household environment (ie with Wi-Fi interference):

We can observe that sending broadcast instead of unicast packets speeds up data transmissions slightly. Increasing the Wi-Fi bit rate also has a significant impact on data rates, but comes at the cost of significantly more packet loss (packets not acknowledged by peer).

Quick testing in outdoor conditions (ESP32-DevKitC placed at a distance of 20m with line-of-sight visibility) shows similar (or slight better) data rates at WIFI_PHY_RATE_1M_L and WIFI_PHY_RATE_11M_L when compared to the results above. Data rates of up to 549250 bytes/s were observed at WIFI_PHY_RATE_24M. Almost all packets were lost at WIFI_PHY_RATE_54M.

12 bit data sampled at 80Hz results in 553Mbit or 70875 kbyte of data, which would take approximate 145s to transmit at 500000 bytes/s.

These data rates were also observed using the on-board PCB antenna. An alternative option is to connect an external antenna to ESP32-WROOM-32U modules, which is likely to improve the data rate.