8.2 Short Message Service (SMS)

23.2723GPPCircuit Switched (CS) fallback in Evolved Packet System (EPS)Release 17Stage 2TS

8.2.1 General

The procedures for SMS in this specification apply only if the UE is EPS/IMSI attached and the Non Access Stratum signalling based SMS transfer is chosen by the UE and/or the home PLMN for delivering short messages.

This clause describes both the mobile originating and mobile terminating SMS over SGs procedures in EPS. SMS support is based on the connectionless SGs reference point between the MME and the MSC Server and use of NAS signalling between the UE and the MME, i.e. no CS Fallback is performed for SMS.

This clause also describes the SMS procedures between the UE and the MME for UEs that use only PS and SMS services but no CS Fallback via GERAN and/or UTRAN.

The SMS protocol entities are reused from the existing MS/UE and MSC implementations. This means that the SMS over SGs procedures reuse the different protocol layers as defined in TS 23.040 [14].

NOTE. With SMS over SGs, the MSC/VLR produces the Call Detail Record. The stage 3 changes to the CDR for SMS over SGs were made in a manner that should permit an unmodified Release 7 entity to receive the CDR. However, for correct operation of, e.g. customer care services, when using an unmodified Release 7 entity to receive the CDR, the VPLMN should ensure that the TAC values do not overlap with the LAC values.

8.2.2 Mobile originating SMS in Idle Mode

The following sequence flow shows the delivery of mobile originating SMS in idle mode. The message flows between the ME/UE and MSC/VLR are also broadly applicable to the Memory Available Notification.

Figure 8.2.2-1: Mobile originating SMS in idle mode

1. The combined EPS/IMSI attach procedure as described in clause 5.2 has been performed earlier.

2. A mobile originating SMS is triggered and the MS/UE is in idle mode. The MS/UE initiates the UE triggered Service Request procedure, which is defined in TS 23.401 [2]. The UE indicates its S‑TMSI in the RRC signalling. If the UE and the MME accept to use Control Plane CIoT EPS Optimisation, then to enable SMS transfer the Service Request procedures defined in TS 23.401 [2] are not used for MO SMS, but instead UE and MME shall be using Control Plane CIoT EPS Optimisation.

For E-UTRAN, if there is a Service Gap timer running in the MME for the UE and the MME is not waiting for a MT paging response from the UE, the MME rejects the Service Request by discarding any NAS data PDU and sending a Service Reject message to the UE with an appropriate cause. The MME may also provide UE with a Mobility Management Back-off timer set to the remaining value of Service Gap timer (see TS 23.401 [2], clause 4.3.17.9).

NOTE 1: The above Service Gap enforcement is for UEs that do not support Service Gap Control. UEs that do support Service Gap Control will not invoke this procedure when the Service Gap timer is running in the UE.

3. The MS/UE builds the SMS message to be sent as defined in TS 23.040 [14] (i.e. the SMS message consists of CP-DATA/RP-DATA/TPDU/SMS-SUBMIT parts). Following the activation of the Radio Bearers, the SMS message is encapsulated in an NAS message and sent to the MME.

4. The MME forwards the SMS message to the MSC/VLR in an Uplink Unitdata message. In order to permit the MSC to create an accurate charging record, the MME adds the IMEISV, the local time zone, the Mobile Station Classmark 2, and the UE’s current TAI and E‑CGI.

4a. The MSC/VLR acknowledges receipt of the SMS to the UE.

5.-8. These steps are performed as defined in TS 23.040 [14]. The SMS message is forwarded to the SC that returns a delivery report message.

9. The MSC/VLR forwards the received delivery report to the MME associated with the MS/UE in a Downlink Unitdata message.

10. The MME encapsulates the received delivery report in an NAS message and sends the message to the MS/UE.

11, 12. The UE acknowledges receipt of the delivery report to the MSC/VLR.

13. The MSC/VLR indicates to the MME that no more NAS messages need to be tunnelled.

The MME should not use the SGs Release Request message as a trigger for the release of S1 resources.

NOTE 2: This is because the MME does not know whether the Service Request performed in step 2 was solely for the purpose of SMS, or, was for SMS and user plane data, or, whether or not the mobile has additional SMSs to send.

8.2.3 Mobile originating SMS in Active Mode

Mobile Originating SMS in active Mode procedure is specified by reusing the Mobile Originating SMS in Idle Mode with the following modification:

– The established signalling connection between the MS/UE and the MME is reused for the transport of the SMS message and the delivery report (i.e. the UE triggered Service Request procedure defined in step 2 is skipped).

8.2.3a Multiple Mobile originating SMSs

In clause 3.2 of TS 24.011 [28], the simultaneous transmission of more than one MO SMS/notification per domain is prohibited.

If the UE has more than one SMS/notification to send, the subsequent SMS/notification is sent at step 11 of clause 8.2.2 and the acknowledgement of the delivery report for the previous SMS/notification is not sent.

8.2.4 Mobile terminating SMS in idle mode

The following sequence flow shows the delivery of mobile terminating SMS in idle mode.

Figure 8.2.4-1: Mobile terminating SMS in idle mode

1. The combined EPS/IMSI attach procedure as described in clause 5.2 has been performed.

2‑4. The SC initiates transfer of mobile terminating SMS. The HLR is requested for routing number for SMS services and the SMS message is forwarded to the MSC/VLR where the MS/UE is CS attached.

5. The MSC/VLR sends a Paging (IMSI, VLR TMSI, Location Information, SMS indicator) message to the MME.

6. The MME initiates the paging procedure by sending the Paging (as specified in TS 23.401 [2]) message to each eNodeB with cells belonging to the tracking area(s) in which the UE is registered. The UE is paged with its S‑TMSI.

7. The MS/UE is paged by the eNodeBs.

8. The UE sends a Service Request message to the MME. The UE indicates its S‑TMSI in the RRC signalling. The MME sends the S1-AP Initial Context Setup Request message to the eNodeB and the eNodeB establishes the Radio Bearers. If the UE and the MME accept to use Control Plane CIoT EPS Optimisation, then to enable SMS transfer the Service Request procedures defined in TS 23.401 [2] are not used, but instead UE and MME shall be using the Control Plane CIoT EPS Optimisation.

8a. The MME sends a Service Request message to the MSC. In order to permit the MSC to create an accurate charging record, the MME adds the IMEISV, the local time zone, the Mobile Station Classmark 2, and the UE’s current TAI and E‑CGI.

9a. The MSC/VLR builds the SMS message to be sent as defined in TS 23.040 [14] (i.e. the SMS message consists of CP‑DATA/RP‑DATA/TPDU/SMS‑DELIVER parts). The MSC/VLR forwards the SMS message to the MME in a Downlink Unitdata message.

9b. The MME encapsulates the SMS message in a NAS message and sends the message to the MS/UE.

9c, 9d. The MS/UE acknowledges receipt of the SMS message to the MSC/VLR.

10. The MS/UE returns a delivery report as defined in TS 23.040 [14]. The delivery report is encapsulated in an NAS message and sent to the MME.

11. The MME forwards the delivery report to the MSC/VLR in an Uplink Unitdata message.

12‑13. These steps are performed as defined in TS 23.040 [14]. The delivery report is forwarded to the SC.

14-15. In parallel to steps 12-13, the MSC/VLR acknowledges receipt of the delivery report to the MS/UE.

16. The MSC/VLR indicates to the MME that no more NAS messages need to be tunnelled.

The MME should not use the SGs Release Request message as a trigger for the release of S1 resources.

NOTE: Although the MME could use the RRC establishment cause (set to mt-Access) sent in the S1-AP Initial UE Message in step 8 to deduce that the MS/UE sent the Service Request solely for receiving the SMS), the MME does not know whether the MT-SMS might cause the initiation of user plane traffic or an MO SMS. So, for simplicity of MME implementation, the normal eNodeB procedures should be used to initiate the release of S1 resources.

8.2.4a Mobile Terminated SMS in extended Idle mode DRX and in PSM

8.2.4a.1 General

Mobile terminated SMS in extended idle mode DRX or in PSM can be achieved via different deployment options depending on operator needs:

– Deployment Option 1:

– Based on MME rejection of paging message from MSC/VLR with no impact to SMS-GMSC.

– Deployment Option 2:

– Based on deployment Option 1, with additional optimization for avoiding additional signalling to HLR/HSS, consisting on signalling and handling of Maximum Retransmission Time.

– Deployment Option 3:

– The supervision timers to acknowledge receipt of the SMS message at the SMS-GMSC and at the MSC/VLR may be configured by the operator with a value (e.g. 45 minutes) longer than the maximum possible extended idle mode DRX cycle allowed in the network and the maximum possible time that the network allows the UE to continue to remain in power saving state.

8.2.4a.2 Procedure for Deployment Option1 and Deployment Option 2

The sequence flow described in Figure 8.2.4a.2-1 shows the delivery of mobile terminating SMS when the UE is using extended idle mode DRX with a DRX cycle value, or when the UE is in Power Saving Mode (as defined in TS 23.682 [48]) that can cause mobile terminating SMS retransmissions for Deployment Option 1. It also shows the additional optimizations in case of Deployment Option 2.

Figure 8.2.4a.2-1: Mobile terminating SMS in extended idle mode DRX (Deployment Options 1 and 2)

1. The combined EPS/IMSI attach procedure as described in clause 5.2 has been performed, and extended Idle mode DRX has been negotiated.

2 4. The SC initiates transfer of mobile terminating SMS. The HLR is requested for routing number for SMS services and the SMS message is forwarded to the MSC/VLR where the MS/UE is CS attached. The SMS-GMSC may include in case of Deployment Option 2 the Maximum Retransmission Time in the Forward Short Message to indicate that it is capable to retransmit the Short Message until a maximum retransmission time.

5. The MSC/VLR sends a Paging (IMSI, VLR TMSI, Location Information, SMS indicator, in Deployment Option 2, SM Delivery Timer, SM Delivery Start Time) message to the MME. For Deployment Option 2, if the Forward Short Message received from the SMS-GMSC includes the Maximum Retransmission Time IE, the VLR should forward the received Maximum Retransmission Time IE in the Paging message to indicate to the MME that the SMS GMSC is capable to retransmit the Short Message until the indicated maximum retransmission time. In Deployment Option 2, if the Forward Short Message received from the SMS-GMSC includes the SM Delivery Timer and SM Delivery Start Time IEs, the VLR may include these IEs in the Paging message to indicate to the MME the time until which the VLR supports buffering the Short Message.

6a. If the UE is considered unreachable for paging (as specified in TS 23.682 [48]), the MME sends UE unreachable message (cause that indicates UE Temporarily Unreachable and for Deployment Option 2 Requested Retransmission Time, SM Buffer Request Indicator) to the MSC/VLR.

For UEs using extended idle mode DRX, the MME proceeds with step 6a, steps 6b-14 as shown in Figure 8.2.4a.2-1 and described below are performed.

For UEs in Power Saving Mode, the MME proceeds with step 6a, steps 7a-9, and steps 11-14 as shown in Figure 8.2.4a.2-1 and described below are performed.

For Deployment Option 2, if the UE is using a power saving mechanism such as extended idle mode DRX or Power Saving Mode and the Paging message includes the Maximum Retransmission Time IEs, the MME may return an UE Unreachable message to that VLR with the cause UE Temporarily Unreachable and with the Requested Retransmission Time IE requesting the SMS GMSC to retransmit the Short Message at a later time prior to the Maximum Retransmission Time. In that case, the MME shall set the NEAF, so as to notify the VLR of any subsequent UE activity prior to the requested SM retransmission time.

In Deployment Option 2, if the UE is using extended idle mode DRX or PSM and the Paging message includes the SM Delivery Timer and SM Delivery Start Time IEs, and if the UE is expected to respond to paging after the paging guard timer but before the time indicated by the SM-Delivery-Timer and SM-Delivery-Start-Time IEs, the MME should page the UE and return an UE Unreachable message to that VLR with cause UE Temporarily Unreachable and with the SM Buffer Request Indicator to request the VLR to buffer the Short Message.

In Deployment Option 2, if the VLR included the SM-Delivery-Timer and SM-Delivery-Start-Time IEs in the Paging message, and if the UE Unreachable message indicates a request to buffer the Short Message in the VLR, the VLR shall buffer the Short Message until the time indicated in the Paging message or until receipt of any earlier SGs Service Request message. Steps 7a-9 and 12-14 are not performed.

Otherwise, if the UE is considered reachable for paging, steps 6-16 of clause 8.2.4 are performed.

6b. The MME sends the paging request just before the occurrence of next paging occasion. This step is not performed for UEs in PSM.

NOTE: Steps 7-9 are as per legacy behaviour of MT SMS delivery failure as defined in TS 23.040 [14].

7. The MSC/VLR sends a failure report to the SMSC-GMSC with an Absent Subscriber indication as defined in TS 23.040 [14]. For Deployment Option 2, if the UE Unreachable message includes the Requested Retransmission Time IE, the VLR shall forward the received Requested Retransmission Time IE in the Failure Report to the SMS-GMSC. In that case, the MSC/VLR shall not set the MNRF flag, and steps 8-9 and 12-13 are not performed.

7a. The MSC/VLR shall, in parallel with step 8, set its MNRF and send an SGs interface Alert Request message to the MME, as described in clause 8.2.5c. The MME shall set the NEAF so as to notify the MSC/VLR of any subsequent UE activity.

8. When the SMS-GMSC receives the absent subscriber indication from MSC/VLR it sends an SM-DeliveryReportStatus with absent subscriber indication to HSS in order to be notified when the UE becomes reachable, according to TS 23.040 [14].

9. The SMS-GMSC, in parallel with step 8, sends a failure report to the SC, according to TS 23.040 [14].

10. The MS/UE is paged by the eNodeBs. This step is not performed for UEs in PSM.

11. The UE sends a Service Request message to the MME. The UE indicates its S-TMSI in the RRC signalling. The MME sends the S1-AP Initial Context Setup Request message to the eNodeB and the eNodeB establishes the Radio Bearers. The network should ensure the UE is connected long enough that it allows the SC to reattempt to deliver the MT SMS message while the UE is still active. If the UE and the MME accept to use Control Plane CIoT EPS Optimisation, then to enable SMS transfer, the Service Request procedures defined in TS 23.401 [2] are not used, but instead UE and MME shall use Control Plane CIoT EPS Optimisation procedure defined in TS 23.401 [2].

For UEs in PSM, Service Request, or Data Transport in Control Plane CIoT EPS Optimisation or Tracking Area Update, as defined TS  23.401 [2], is performed when the UE wakes up from its power saving state.

12. The MME sends UE-Activity-Indication (IMSI) to the MSC/VLR. For Deployment Option 2, if the UE is using a power saving mechanism (such as extended idle mode DRX or PSM), the MME includes the Maximum UE Availability Time IE in the UE-Activity-Indication message to indicate the time until which the UE is expected to be reachable.

12a. The MSC/VLR sends Ready-For-SM Service to the HLR. For Deployment Option 2, the MSC/VLR includes this information to the HLR if received in step 12.

13. The HLR initiates Alert-SC function towards the SC as defined in TS 23.040 [14]. For Deployment Option 2, the HLR includes the Maximum UE Availability Time IE in the Alert-SC if received at step 12a.

14. The SC initiates a new attempt to deliver the MT SMS according to clause 8.2.5. For Deployment Option 2, the SC prioritizes the retransmission of pending Mobile Terminated Short Messages to UEs using a power saving mechanism, i.e. UEs known to have a Maximum UE Availability Time.

8.2.4a.3 Procedure for Deployment Option 3

The following sequence flow described in Figure 8.2.4a.3-1 shows the delivery of mobile terminating SMS when the UE is using extended idle mode DRX (as defined in TS 23.682 [48]) with a DRX cycle value, or when the UE is using PSM, that can cause mobile terminating SMS retransmissions for Deployment Option 3.

Figure 8.2.4a.3-1: Mobile terminating SMS in extended idle mode DRX (Deployment Option 3)

1-5. Steps 1-5 as described in clause 8.2.4 are performed.

6a. If the UE is considered unreachable for paging due to using extended idle mode DRX or PSM (as specified in TS 23.682 [48]), the MME sends a Service Request message to the MSC/VLR. If the MME proceeds with step 6a, steps 6b-16 as shown in Figure 8.2.4a.3-1 and described below are performed.

Otherwise steps 6-16 of clause 8.2.4 are performed.

6b. The MSC/VLR builds the SMS message to be sent as defined in TS 23.040 [14] (i.e. the SMS message consists of CP DATA/RP DATA/TPDU/SMS DELIVER parts). The MSC/VLR forwards the SMS message to the MME in a Downlink Unitdata message. The MME stores the received SMS message.

7a. The MME sends the paging request just before the occurrence of next paging occasion. This step is not performed for UEs in PSM.

7b. The MS/UE is paged by the eNodeBs. This step is not performed for UEs in PSM.

8. The UE sends a Service Request message to the MME. The UE indicates its S TMSI in the RRC signalling. The MME sends the S1-AP Initial Context Setup Request message to the eNodeB and the eNodeB establishes the Radio Bearers. If the UE and the MME accept to use Control Plane CIoT EPS Optimisation, then to enable SMS transfer the Service Request procedures defined in TS 23.401 [2] are not used, but instead UE and MME shall use Data Transport in Control Plane CIoT EPS Optimisation procedure defined in TS 23.401 [2].

For UEs in PSM, Service Request, or Data Transport in Control Plane CIoT EPS Optimisation or Tracking Area Update, as defined TS 23.401 [2] is performed when the UE wakes up from its power saving state.

9. The MME encapsulates the SMS message in a NAS message and sends the message to the MS/UE.

9c-16. Steps 9c-16 as described in clause 8.2.4 are performed.

8.2.5 Mobile terminating SMS in Active Mode

Mobile terminating SMS in Active Mode procedure is specified by reusing the Mobile Terminating SMS in Idle Mode with the following modification:

– There is no need for the MME to perform Paging of the MS/UE after step 5. MME continues with step 8a (i.e. steps 6 to 8 are skipped). The MME immediately sends a Downlink Unitdata to the UE.

– The MME also includes the E-CGI and TAI in step 9d.

8.2.5a Multiple Mobile terminating SMSs

In clause 3.2 of TS 24.011 [28], the simultaneous transmission of more than one MT SMS per domain is prohibited.

If the MSC/VLR has more than one SMS to send, the subsequent SMS is sent in a Downlink Unitdata message after step 14 and instead of the Release Request in step 16 of clause 8.2.4. i.e. the MSC/VLR does not need to send another SGs Paging message.

8.2.5b Simultaneous Mobile terminating and Mobile originating SMSs

The above clauses on mobile originating and mobile terminating SMS handling in active and idle mode can be reused such that no special treatment is needed for this case.

8.2.5c Unsuccessful Mobile terminating SMS delivery attempt

As specified in clause 3.2.8 of TS 23.040 [14], setting the Mobile Station Not Reachable Flag (MNRF) in the MSC/VLR is mandatory. However, when using the SGs interface, the MSC/VLR has delegated the ‘implicit detach’ functionality to the MME (and/or, if Network Mode of Operation 1 is in use in GERAN/UTRAN, to the SGSN).

If an SGs based MT SMS delivery attempt fails, the MSC/VLR shall set its MNRF and send an SGs interface Alert Request message to the MME. Upon receipt of Alert Request message, MME shall set its Non-EPS Alert Flag (NEAF) and if ISR is activated, the MME shall then send an S3 interface Alert-MME-Request message to the SGSN. SGSN shall set the S3 SMS Alert Flag (SSAF).

If the MME operator knows (e.g. because it is in the HPLMN) that the receiving UE’s HPLMN deploys SMS-Router, and if the receiving UE’s HPLMN uses both SMS via MSC and SMS via SGSN, then the MME need not send the Alert-MME-Request message to the SGSN for that UE.

NOTE: The receiving UE’s HPLMN should ensure that the SMS-Router in the receiving UE’s HPLMN only returns SMS-Router address to the SMS-GMSC of the sender UE’s PLMN.

Subsequently, if the UE makes radio contact with the SGSN and SSAF is set, the SGSN informs the MME with an S3 interface UE-Activity-Indication. Upon receipt of the S3 interface UE-Activity-Indication, or, if the UE makes radio contact with the MME, the MME sends an SGs AP UE-Activity-Indication message to the MSC/VLR. Upon receipt of an SGs AP UE-Activity-Indication message, or signalling on the A, Iu-cs or Gs interface for that UE, the MSC/VLR shall inform the HLR.

For Deployment Option 2, if the UE is using a power saving mechanism (such as extended idle mode DRX), the MME may include the Maximum UE Availability Time IE in the SGs AP UE-Activity-Indication message to indicate the time until which the UE is expected to be reachable. This information may be used by the SMS Service Center to prioritize the retransmission of pending Mobile Terminated Short Messages to UEs using a power saving mechanism.

8.2.5d Non-SMS Mobile terminating activity during SMS delivery

While one or more SMS is being transferred, other mobile terminating requests (e.g. an MT voice call) may arrive in the MSC/VLR. If this happens the MSC/VLR continues the SMS activities but shall also send the SGs Paging message for the non-SMS activity to the MME. The MME shall handle this SGs Paging message as if no SMS transfers are ongoing. Typically this should lead to the MME invoking the handover/call redirection to GERAN/UTRAN features and it may lead to disruption of the SMS delivery. The MSC/VLR and UE shall recover from any such SMS disruption using the normal RP and CP layer retransmission timers/mechanisms.

8.2.5e Non-SMS Mobile originating activity during SMS delivery

While one or more SMS is being transferred, other mobile originating requests (e.g. an MO voice call or USSD) may be requested by the user. If this happens the MS/UE continues the SMS activities but shall also send the Extended Service Request message for the non-SMS activity to the MME. The MME shall handle this Extended Service Request message as if no SMS transfers are ongoing. Typically this should lead to the MME invoking the handover/call redirection to GERAN/UTRAN features and it may lead to disruption of the SMS delivery. The MSC/VLR and UE shall recover from any such SMS disruption using the normal RP and CP layer retransmission timers/mechanisms.

8.2.5f Mobile Terminating SMS when ISR is active and SGs is active between MSC/VLR and MME

When the MME receives the SGs Paging message for SMS, and ISR is active, and the UE is in idle mode, the MME sends the S1 interface paging message to the E‑UTRAN (using the S-TMSI as temporary identity) and sends a CS paging message (SMS indicator) to the SGSN using the MSC TMSI as temporary identity (unless the MSC did not allocate a TMSI, in which case the IMSI is used for paging).

The UE is paged on E‑UTRAN and by the SGSN on GERAN and/or UTRAN. For GERAN A/Gb mode, the SGSN sends a PAGING CS message to the BSS (see TS 48.018 [30]) if "SMS in SGSN" is not used for this UE. For UTRAN, the SGSN sends a PAGING message to the UTRAN (see TS 25.413 [29]) with the CN Domain Indicator set to ‘CS domain’ and the Paging Cause set to ‘Terminating Low Priority Signalling’ if "SMS in SGSN" is not used for this UE. The UE responds on the cell on which it is camped. When camped on E-UTRAN, the UE responds to the MME. When camped on GERAN or UTRAN, the UE responds to the MSC.

8.2.6 Co-Existence with SMS over generic 3GPP IP access

If the home operator has deployed SMS over generic 3GPP IP access and/or SMS-Instant Messaging Interworking as defined in TS 23.204 [15], and has configured the network and the UE for using SMS over IP or SMS-Instant Messaging Interworking, then an SMS or IM will be delivered over EPS in any visited network whether or not the visited network supports SMS over generic 3GPP IP access.

If the home operator has not deployed SMS over generic 3GPP IP access and the UE fails to successfully complete the combined EPS/IMSI attach procedure in the visited network (i.e. the visited network supports SMS over generic 3GPP IP access and does not support SGs for SMS capability), then the UE cannot execute MT or MO SMS procedures in the visited network.