How doe SIK determine if a link is being maintained

I am trying to understand better how the SIK radio determines whether there is a valid link or not. Are there messages that the radios send periodically to one another? If this is the case, where in the code could I find the details of these codes (i.e., what is transmitted, and how frequently?)

No file or test is named in the issue. Search the SiK C firmware for periodic radio transmissions and link-validity checks, then trace the relevant message definitions and timing. Done means documenting where these checks and messages are implemented, what they transmit, and how frequently they occur.

Written by the indexing model from the issue text.