NB-IoT packet loss

[allard.p]

I'm testing a simple application based on the UDP/CoAP example.  It samples some peripherals and ADC channels, these values are then packed into a CoAP message. This CoAP message is sent trough the socket api (UDP), and the application waits for a response. I've run some tests using different power saving settings. The current version uses PSM in between samples and uses RAI to prevent long time spent in the 'active timer'.

A observation we made after analyzing the logs is the high rate of timeouts/packet loss. We declare a packet lost if the uplink CoAP packet, or the downlink CoAP ACK message is lost. We got a packet loss of around 3% over around 1000 messages (multiple devices, connected to the same cell). Questions:

• I known UDP is a unreliable protocol by design, but I'm wondering what packet loss numbers I should expect? What numbers are other users experiencing?
• Is there some way to identify the cause of these lost packets, using the PCAP/modem log files for example?

The devices all have a Taoglas MFX3 antenna connected, the RSSI is around -85dBm (%CESQ: 54,2,21,3).

[mcallistertad]

3% packet loss sounds on the high side with good signal quality - is this consistent across requests and responses? What is the connection density - are they operating in an urban environment, how many devices are connected to the same cell, and how often do they transmit? When they transmit, do they all transmit simultaneously? Latency may be a factor It would be interesting to log the round trip time and see if this coincides with packet loss/ time of day.

From what I've seen, NB-IoT uplinks had a higher rate of success. CoAP is flexible with retransmission schemes, so this may not be a problem. 

sdk-nrf v1.9 and the nRF connect desktop app support PCAP trace capture.