Why does the raspberry lose network connection?

Question

linux

Greetings.
Installed a server on Raspberry.
Everything is cool, except that after a while of lack of work with the network, the raspberry "falls off". Any remote access to it is lost.
If you connect locally and start pinging a resource, everything returns to normal.
Suggest a direction for excavation. Perhaps this is a default mechanism that needs to be configured.
Thank you.

Answer 1

Examine syslog, messages, you will surely find something interesting

Answer 2

As an assumption, the network device to which the "raspberry" is connected cannot break the session due to idle time? If possible, sketch a diagram of how the raspberry is connected to the network.