GSM Originating SMS Call

This scenario describes the session setup for a GSM originating SMS. This sequence diagram describes the SMS signaling and data transfer between the mobile subscriber and the SMS service center. Short messages are carried on GSM signaling channels rather than traffic channels: the SDCCH (Standalone Dedicated Control CHannel) when the MS is idle, or the SACCH (Slow Associated Control CHannel) during a call. Thus they can be received while the user is talking. In this flow the MS is idle, so it establishes an SDCCH using the RR connection establishment procedure.

SMS Protocol stack consists of:

  1. SM Application layer (AL)
  2. SM Transfer layer (TL): SM-TL transfers SM-AL messages. SM-TL messages are called Transfer Protocol Data Units (TPDUs).
  3. SM Relay layer (RL): SM-RL provides services to transfer TPDUs and corresponding delivery report for the SM-TL. SM-RP is the protocol between peer SM-RL entities at MS and MSC. SM-RP messages are Relay Protocol Data Units (RPDUs).
  4. SM Connection Management sub-layer (CM-sub). CM-sub layer protocol, Short Message Control Protocol SM-CP provides services to SM-RL and communication between peer Short message Control entities, SMCs.
  5. The MSC forwards the short message over MAP (carried on TCAP) to the SMS interworking MSC (SMS-IWMSC), which passes it to the SC.

Errata: The diagrams below say "SC talks to MSC via TCAP/MAP". MAP (over TCAP) runs between the serving MSC and the SMS-IWMSC, which passes the message to the SC. They also call the SDCCH a traffic channel; it is a dedicated control channel. The text on this page is correct. Corrected diagrams are tracked internally.