GSM Terminating SMS Call

In this call flow we will look at how a terminating SMS is handled in GSM. Setting up a terminating SMS session is a multi-step process:

  1. Interrogate the HLR to locate the subscriber
  2. Set up the SMS session and acquire radio resources
  3. Send the SMS
  4. Release the session and associated radio resources

Errata: The diagrams below route the SMS as if it were a voice call. They show the HLR sending MAP PROVIDE ROAMING NUMBER to the MSC/VLR and returning an MSRN to the SMS-GMSC. For SMS there is no roaming number. The SMS-GMSC sends MAP SEND ROUTING INFO FOR SM to the HLR, which returns the subscriber's IMSI and the address of the serving MSC/VLR. The SMS-GMSC then sends MAP FORWARD SHORT MESSAGE to that MSC, identifying the subscriber by IMSI (3GPP TS 23.040, TS 29.002). Step (1) in the diagrams also reads "Interrogate the MSC"; it is the HLR that is interrogated. Corrected diagrams are tracked internally.