Bandwidth drop with more than 1024 bytes with TCP or UDP sockets (nRF9160 DK)

[Isaia Spinelli]

Hello,

I am doing tests on nRF9160 DK and I observe that if I send more than 1024 bytes on a TCP or UDP socket, the bandwidth drops.

Here are some numbers :

for TCP : 

Payload size [B]	Throughput [KB/s]
1000	15.32
1023	15.61
1024	15.57
1025	3.17
1050	3.18

for UDP : 

Payload size [B]	Throughput [KB/s]
1000	24.19
1023	25.44
1024	25.01
1025	12.37
1050	12.58

We can see that the bitrate drops after 1024 bytes. Do you have any idea why ? and if it is possible to avoid that ?

I am using :

- nRF COnnect SDK v1.4.99-dev1

- nRF9160 DK

- BSD library (CONFIG_BSD_LIBRARY=y) & standard POSIX 

Here is the loop I'm using to send the payload : 

The parameter SIZE_BUFFER represents the size of payload.

Thanks 

[Hakon]

Can you do the tests again with modem traces from when the bandwidth drops and when it is fine?

[Isaia Spinelli]

Hello, of course : 

save_trace.zip

a ZIP directory is fine ?

[Hakon]

Thank you, this is just fine.

[Hakon]

Also, the modem team commented;

"i'd recommend to move to NCS1.5.0 which contains modemlib which has the needed fixes for memory configurations. bsdlib has known problems. 

modemlib config for tx and rx mem size can be changed based on need also, e.g. what Modem Shell is using

CONFIG_NRF_MODEM_LIB_SHMEM_TX_SIZE=16384
CONFIG_NRF_MODEM_LIB_SHMEM_RX_SIZE=8192

TCP buffer length should be multiples of 708bytes (modem max MSS) and UDP 1200bytes (to fit into expected MTU), these have been verified with NCS1.5.0 at least."

[Isaia Spinelli]

I understand now, thank you for everything!

[Hakon]

Here are some comments about the trace;

"The difference in throughput comes outside from modem. In 1024B, the first 10 packets are sent from application to modem in 0.5 seconds. The first 10 packets in 1025B log takes 2,5 seconds to arrive from application to modem.

Customer should check the functionality of the application. Why it is feeding data to modem slower when packet size increases from 1024 Bytes."

[Isaia Spinelli]

Thanks for news,

" Why it is feeding data to modem slower when packet size increases from 1024 Bytes." This is part of my first question.

Like we can see in the picture on the first post, it's a while loop and there isn't difference in my application between feeding data to modem with 1024B or 1025B, but the size of the buffer. But its slower when packet size increases from 1024 Bytes...

Anyway, since i moved to NCS1.5.0, the problem is no longer there.