SMS works on a store-and-forward basis. Instead of being sent directly to the recipient, SMS messages travel through several important nodes before reaching the recipient.
The SMS message is submitted to your wireless service provider's SMS Center.
After the message is processed internally, the SMS Center sends a request to the Home Location Register (HLR) and receives the routing information for the recipient.
The SMS Center sends the message to the Mobile Switching Center (MSC).
The MSC collects the recipient's information from the Visitor Location Register (VLR) and, sometimes, proceeds with an authentication operation.
The MSC forwards the message to a Mobile Server.
If the recipient has a Treo or VisorPhone, the SMS is placed on the SIM card until the handheld picks up the message and transfers it to the internal memory on the unit.
This is how Treos and VisorPhones can store hundreds of SMS messages, unlike ordinary GSM mobile phones that store only a few SMS messages.
Treo or VisorPhone owners may encounter the error message "Your SIM contains at least one SMS message. Do you want to move the message to your Visor?"
The MSC returns the outcome of the Forward Short operation to the SMS Center.
The SMS Center reports delivery status of the short message back to the sender.
