But the end users can send & receive messages when there's no 3G/4G data signal, just good old GSM. Admittedly it's pretty rare to have a GSM single and no data, but it can happen
I was talking more about what happens after the SMS reaches the tower. How is that message delivered to the cell tower that the receiver is affiliated with, especially if that tower is owned by a different operator?
They traverse the SS7 signaling network. This is the network that routes voice calls on the POTS lines.
Oh interesting...but it also sounds like SIGTRAN provides signaling using SCTP associations over the Internet Protocol?
Same idea just much older protocols. No security, plain text, easy to spoof just like caller ID which also uses SS7.
It looks like they go over the cell network rather than the Internet, but they have lots of the same concepts. SMSCs act like routers, for example.
But in some circumstances is that PLMN/PTSN interconnectivity just an “application layer” on top of existing IPv4/v6 infrastructure...similar to how VoIP/SIP works?