Pings
Previously reported: ACARS will store messages until it can establish contact.
It tries later by sending a ping. If there is no response it tries again later.
Previously reported: it tries every half hour.
Other communications devices will have different retry times - mostly much shorter. The airwaves will be full of pings on all digital frequencies but normally only the ones that establish contact by getting a response are then followed by data.
Service assets acknowledge pings addressed to them (thereby establishing the link) and service the ensuing data.
Surveillance assets potentially record the pings, any responses and any ensuing data.