What infrastructures are required for airlines to send PNL messages?
It's not just one. Private networks are used such as the IP-based ARINC AviNet, AFTN Teletype, etc.

For the specifics and standards, that information is not freely available (>\$3000). From IATA manuals, go to the Passenger tab, then the Security​ &amp; Facilitation section. Or click here.


  IATA's Passenger Data Exchange Support Plus package provides governments with all necessary standards, helpfully grouped in a single purchase.


The following example is for France (may not be current):





There is also a presentation on the IATA website you may want to download and check.