Here's how an SMS message travels through the GSM network. The description is slightly simplified and doesn't go into technical details.
- The GSM handset sends the message to the SMS Center (SMSC), which stores the message for delivery
- The SMSC send an acknowledgement about the received message to the sender
- The SMSC tries to send the message to the receiving handset until the message times out
- When the receiving handset has received the message, it sends an acknowledgement to the SMSC, which deletes the message
1. ---> 3. --->
Sender SMSC Receiver
2. <--- 4. <---