The SGSN or the HLR/HSS may not be able to deliver the SM to the MS. This may for example happen when the MS is not attached to GPRS, when the radio channel conditions are bad, or when the Mobile-terminated SMS is barred.
When the SGSN cannot deliver the SM to the MS, the SGSN sets the Mobile station Not Reachable for GPRS flag (MNRG), and returns a failure report to the SMS GMSC. Based on the routeing information received from the HLR/HSS, the SMS GMSC shall do one of the following:
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If an MSC/VLR is available for the MS, the SM is forwarded to the MS via the MSC/VLR. A successful delivery report shall be returned to the SM SC.
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If an MSC/VLR is not available for the MS, the Message Waiting Indication information in the HLR/HSS shall be updated and an unsuccessful delivery report shall be returned to the SM SC.
Figure 97 illustrates one possible traffic scenario when neither the SGSN nor the MSC is able to deliver the SM.
Step 1.
The short message service centre determines it shall send an SM to an MS. SM SC forwards the SM to a SMS GMSC.
Step 2.
SMS GMSC examines the destination MS Address, and sends a Send Routeing Info For Short Message message to the relevant HLR/HSS.
Step 3.
HLR/HSS checks the subscriber data (e.g. ODB data and Call Barring Info) for the MS to determine whether the MS is allowed to receive the SMS. If the Mobile-terminated SMS is barred, the HLR/HSS returns a Send Routing Info for Short Message Error message with an appropriate cause. If the Mobile-terminated SMS is not barred, the HLR returns a Send Routeing Info For Short Message Result message to the SMS GMSC. The Result contains an SGSN Number and an MSC Number. The result also contains the SGSN Diameter identity if Gdd is supported.
Step 4.
SMS GMSC forwards the SM to the SGSN.
Step 5.
SGSN attempts to transfer the SM to the MS, but fails.
Step 6.
SGSN sets MNRG and returns a Forward Short Message Result message to SMS GMSC indicating unsuccessful delivery of the SM.
Step 7.
SMS GMSC selects an alternative route for the SMS, and forwards the SM to the MSC/VLR.
Step 8.
MSC/VLR attempts to transfer the SM to the MS, but fails.
Step 9.
The MSC/VLR requests the setting of the NGAF at the SGSN.
Step 10.
VLR sets MNRF and returns a Forward Short Message Result message to the SMS GMSC indicating unsuccessful delivery of the SM.
Step 11.
SMS GMSC sends a Report SM Delivery message to the HLR/HSS.
Step 12.
HLR/HSS updates its Message Waiting Indication fields and returns a Report SM Delivery Result message to the SMS GMSC.
Step 13.
SMS GMSC returns a Failure Report to the SM SC indicating unsuccessful delivery of the SM.
CAMEL is not supported by SGSNs using EPS interfaces; otherwise CAMEL procedure calls shall be performed according to
TS 23.078:
C1)
CAMEL_T_SMS_INIT.
The procedure returns as result
"Continue".
C2)
CAMEL_T_SMS_FAILURE.
This procedure does not return a result.
C3)
CAMEL_T_SMS_INIT.
The procedure returns as result
"Continue".
C4)
CAMEL_T_SMS_FAILURE.
This procedure does not return a result.
Figure 69 shows that the SGSN sends a Ready for SM (MS Reachable) message to the HLR/HSS when the MS becomes reachable and MNRG is set in the SGSN. The SGSN indicates also to the MSC/VLR when the MS becomes reachable and NGAF is set in the SGSN. If the MNRF is set at the MSC/VLR, the MSC/VLR sends a Ready for SM (MS Reachable) message to the HLR/HSS. Reception of a Ready for SM message or Update Location message when MNRG is set in the HLR/HSS shall trigger the SMS alert procedure as defined in
TS 23.040.
MNRG remains set in the SGSN independently of whether the MSC/VLR was successful in delivering the SM or not.
This means that the SGSN in certain cases sends a Ready for SM message to the HLR/HSS when an MS becomes reachable via the SGSN, even if no SM is waiting. This causes a small amount of duplicate signalling between the SGSN and the HLR/HSS only.