2 Overview of Short Message Service (SMS) support
24.0113GPPPoint-to-Point (PP) Short Message Service (SMS) support on mobile radio interfaceRelease 17TS
The purpose of the Short Message Service is to provide the means to transfer messages between a GSM PLMN Mobile Station (MS) and a Short Message Entity via a Service Centre, as described in 3GPP TS 23.040 [2]. The terms "MO" ‑ Mobile Originating ‑ and "MT" ‑ Mobile Terminating ‑ are used to indicate the direction in which the short message is sent.
The present document describes the procedures necessary to support the Short Message Service between the MS and the MSC or SGSN or MME or SMSF and vice versa, as described in 3GPP TS 23.040 [2].
The procedures are based on services provided by the Mobility Management sublayer as described in 3GPP TS 24.007 [4]/3GPP TS 24.008 [5] for CS in A/Gb mode and CS/PS services in Iu mode, 3GPP 24.301 [10] for CS/PS services in S1 mode, 3GPP TS 24.501 [15] for N1 mode and the Logical Link Control layer described in 3GPP TS 44.064 [6a] for GPRS services. For CS/PS service in S1 mode, depending on network configuration and UE subscription data, network may choose to use either packet-switched service or circuit-switched service to provide SMS service. If circuit-switched service is used instead of packet-switched service, then the messages are tunneled through the MME. In this case the network also uses procedures described in 3GPP 29.118 [12].
2.1 Protocols and protocol architecture
In Iu mode only, integrity protected signalling (see 3GPP TS 24.008 [5], subclause ‘Integrity Protection of Signalling Messages’ and in general, see 3GPP TS 33.102 [5b]) is mandatory. In Iu mode only, all protocols shall use integrity protected signalling. Integrity protection of all SMS signalling messages is the responsibility of lower layers. It is the network which activates integrity protection. This is done using the security mode control procedure (3GPP TS 25.331 [5a]).
The hierarchical model in figure 2.1a shows the layer structure of the MSC and the MS in A/Gb mode. The hierarchical model in figure 2.1c shows the layer structure of the SGSN and the MS in Iu mode. The hierarchical model in figure 2.1d shows the layer structure of the MSC and the MS in S1 mode. The hierarchical model in figure 2.1e shows the layer structure of the MME and the MS in S1 mode.
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MSC |
MS |
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SM‑AL |
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SM‑TL |
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SM‑RL |
SMR |
< SM‑RP protocol > |
SMR |
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CM‑sublayer |
SMC |
< SM‑CP protocol > |
SMC |
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MM‑sublayer |
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RR‑sublayer |
Figure 2.1a/3GPP TS 24.011: Protocol hierarchy for circuit-switched service
The hierarchical model in figure 2.1b shows the layer structure of the SGSN and the MS in A/Gb mode.
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SGSN |
MS |
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SM‑AL |
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SM‑TL |
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SM‑RL |
SMR |
< SM‑RP protocol > |
SMR |
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CM‑sublayer |
SMC |
< SM‑CP protocol > |
SMC |
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LLC‑sublayer |
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GRR‑sublayer |
Figure 2.1b/3GPP TS 24.011: Protocol hierarchy for GPRS in A/Gb mode
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SGSN |
MS |
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SM‑AL |
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SM‑TL |
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SM‑RL |
SMR |
< SM‑RP protocol > |
SMR |
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CM‑sublayer |
SMC |
< SM‑CP protocol > |
SMC |
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GMM‑sublayer |
Figure 2.1c/3GPP TS 24.011: Protocol hierarchy for packet-switched service in Iu mode
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MSC |
MS |
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SM‑AL |
SM‑AL |
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SM‑TL |
SM‑TL |
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SM‑RL |
SMR |
< SM‑RP protocol > |
SMR |
SM‑RL |
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CM‑sublayer |
SMC |
< SM‑CP protocol > |
SMC |
CM‑sublayer |
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SGs layer |
EMM-sublayer |
NOTE: SM-CP messages are transferred between the MSC and the MS through the MME. The protocol stack on the MME is not shown.
Figure 2.1d/3GPP TS 24.011: Protocol hierarchy for circuit-switched service in S1 mode
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MME |
MS |
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SM‑AL |
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SM‑TL |
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SM‑RL |
SMR |
<¾¾¾¾¾¾ SM‑RP protocol ¾¾¾¾¾> |
SMR |
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CM‑sublayer |
SMC |
<¾¾¾¾¾¾ SM‑CP protocol ¾¾¾¾¾> |
SMC |
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EMM‑sublayer |
Figure 2.1e/3GPP TS 24.011: Protocol hierarchy for packet-switched service in S1 mode
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SMSF |
MS |
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SM‑AL |
SM‑AL |
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SM‑TL |
SM‑TL |
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SM‑RL |
SMR |
< SM‑RP protocol > |
SMR |
SM‑RL |
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CM‑sublayer |
SMC |
< SM‑CP protocol > |
SMC |
CM‑sublayer |
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N20-layer |
5GMM-sublayer |
NOTE: SM-CP messages are transferred between the SMSF and the MS through the AMF. The protocol stack on the AMF is not shown.
Figure 2.1f/3GPP TS 24.011: Protocol hierarchy in N1 mode
The CM‑sublayer, in terms of the Short Message Service Support, provides services to the Short Message Relay Layer.
On the MS‑side the Short Message Relay Layer provides services to the Short Message Transfer Layer. The Short Message Relay Layer is the upper layer on the network side (MSC or SGSN or MME), and the SM‑user information elements are mapped to TCAP/MAP.
The peer protocol between two SMC entities is denoted SM‑CP, and between two SMR entities, SM‑RP.
Abbreviations:
SM‑AL Short Message Application Layer
SM‑TL Short Message Transfer Layer
SM‑RL Short Message Relay Layer
SM‑RP Short Message Relay Protocol
SMR Short Message Relay (entity)
CM‑sub Connection Management sublayer
SM‑CP Short Message Control Protocol
SMC Short Message Control (entity)
MM‑sub Mobility Management sublayer
GMM-sub GPRS Mobility Management sublayer
RR‑sub Radio Resource Management sublayer
LLC-sub Logical Link Control sublayer
GRR-sub GPRS Radio Resource sublayer in GSM
EMM-sub EPS Mobility Management sublayer
5GMM-sub 5G Mobility Management sublayer
2.2 Use of channels (A/Gb mode only)
Table 2.1/3GPP TS 24.011 summarizes the use of channels for the short message service for circuit switched in A/Gb mode. Arrows indicate changes of channel.
Table 2.1/3GPP TS 24.011: Channels used for short message transfer over circuit switched in A/Gb mode
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Channel dependency |
Channel used |
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TCH not allocated |
SDCCH |
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TCH not allocated ‑> TCH allocated |
SDCCH ‑> SACCH |
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TCH allocated |
SACCH |
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TCH allocated ‑> TCH not allocated |
SACCH ‑> SACCH opt. SDCCH³ |
The short message service for GPRS shall be supported by a PDTCH.
2.3 Layer 2 SAPI 3 handling for circuit switched in A/Gb mode
General rule:
– the Radio Resource Management (RR reference 3GPP TS 44.018 [7]) in the Mobile Station and on the network side (i.e. in the BSC) shall establish the acknowledged mode of operation on SAPI 3 whenever needed, i.e. when a message requiring SAPI 3 transfer shall be transmitted;
– RR shall control the layer 2 also for SAPI 3, and keep knowledge of the mode;
– the network side may initiate release of the acknowledged mode for SAPI 3 either explicitly (by the use of DISC‑ and UA‑frames, see 3GPP TS 44.006 [3]) or indirectly by channel release (see 3GPP TS 44.018 [7]).
This means:
– the Mobile Station side will initiate establishment of SAPI 3 acknowledged mode in the case of mobile originating short message transfer;
– the network side will initiate establishment of SAPI 3 acknowledged mode in the case of mobile terminating short message transfer;
– the network side may choose to keep the channel and the acknowledged mode of operation to facilitate transfer of several short messages for or from the same Mobile Station. The queuing and scheduling function for this should reside in the MSC.
2.4 Layer 2 (LLC) GPRS support (A/Gb mode only)
It shall be possible for a GPRS-attached MS of any class (A, B, C) to send and receive short messages over GPRS radio channels.
GPRS shall use the unacknowledged mode of LLC frame transfer as described in 3GPP TS 44.064 [6a], and shall use SAPI 7 to identify the SMS Logical Link Entity within the LLC layer.
A description of the different GPRS MS classes can be found in 3GPP TS 23.060 [3a], and a brief overview is given below:
– class A/B MSs may be able to send and receive short messages using either the MM sublayer (using SACCH or SDCCH) or the LLC layer (using PDTCH);
– class C MSs may be able to send and receive short messages using only the LLC layer (using the PDTCH). The capability for GPRS-attached class-C MSs to receive and transmit SMS messages is optional.
The GSMS entity for GPRS class A/B MS is shown in figure 2.2. The GSMS shall communicate with the MM entity via the GMMSMS-SAP for GPRS Class A/B MO SMS, in order to ascertain which transport service to use.
SMS delivery via GPRS is normally a more radio resource efficient method than SMS delivery via CS in A/Gb mode. The delivery path for MO SMS is selected by the MS.
Figure 2.2/3GPP TS 24.011: GSMS entity for GPRS Class A/B MS
2.5 GSMS entity in Iu mode
It shall be possible for a PS-attached MS of any mode of operation to send and receive short messages in Iu mode.
A description of the different mode of operation UMTS MS can be found in 3GPP TS 23.060 [3a], and a brief overview is given below:
– CS/PS mode of operation MSs may be able to send and receive short messages using either the MM sublayer or the GMM sublayer;
– PS mode of operation MSs may be able to send and receive short messages using only GMM sublayer.
The GSMS entity for CS/PS mode of operation MS is shown in figure 2.3. The GSMS shall communicate with the MM entity via the GMMSMS-SAP for CS/PS mode of operation MO SMS in Iu mode, in order to ascertain which transport service to use.
The delivery path for MO SMS is selected by the MS.
Figure 2.3/3GPP TS 24.011: GSMS entity for CS/PS mode of operation MS in Iu mode
The Radio Resource Management shall use the ‘low priority’ class for the downlink transfer of SMS signalling messages. This means the core network shall set the SAPI value to 3 in RANAP Direct Transfer messages (see 3GPP TS 25.413 [8]) when sending SMS signalling messages over the Iu interface.
2.5A ESMS entity in S1 mode
It shall be possible for an MS that is attached to CS and PS or attached for PS to send and receive short messages in S1 mode.
A description of the different modes of operation for E-UTRAN MS can be found in 3GPP TS 24.301 [10]:
– if the MS is attached to CS and PS, an overview of SMS services in S1 mode can be found in 3GPP TS 23.272 [11]. In S1 mode, the network may choose to use either circuit-switched service or packet-switched service to provide SMS services (see subclause 2.1). If circuit-switched service is used by the network, then messages are tunnelled through the PS domain between the MS and the MSC, and otherwise have no impact on PS domain operation. If packet-switched service is used by the network, then messages are transferred between the MS and the MME.
– if the MS is attached for PS (see 3GPP TS 24.301 [10]), the MS may be able to send and receive short messages using only EMM sublayer. In this case, short messages are transferred between the MS and the MME.
NOTE: If the MS is attached for PS with CIoT optimizations and the MS has requested "SMS only" and the MS is in NB-S1 mode, the MS sends and receives short messages using only EMM sublayer.
The ESMS entity for CS/PS mode of operation MS is shown in figure 2.5A.
Figure 2.5A/3GPP TS 24.011: ESMS entity for MS in S1 mode
2.5B 5GSMS entity in N1 mode
It shall be possible for an MS in N1 mode that is attached to the 5GCN to send and receive short messages.
The 5GSMS entity for an MS in N1 mode is shown in figure 2.5B.
Figure 2.5B/3GPP TS 24.011: 5GSMS entity for MS in N1 mode
2.6 MS support for SMS over GPRS
If the MS is attached to GPRS and the circuit-switched domain, and an SMS transfer via GPRS fails either due to a reception of an RP-ERROR message with cause #69 or due to the complete lack of network response, then the MS shall take the following actions:
– The MS shall use the circuit-switched domain instead of GPRS for SMS transfer for an implementation dependent time. When a different PLMN is selected, if the MS preferred method is the sending of SMS over GPRS, the MS shall revert to trying an SMS transfer via GPRS.
– If the SMS transfer failed in both GPRS and the circuit-switched domain, the user shall be informed.
As an implementation option, the MS may also use the circuit-switched domain instead of GPRS for SMS transfer due to a reception of an RP-ERROR message with a cause different than #69.
2.7 MS support for device triggering short message
If the UE is attached for EPS services, supports non-IP PDN type as specified in 3GPP TS 24.301 [10] and supports the device triggering short message as specified in 3GPP TS 23.040 [2], then upon receiving a device triggering short message with the Application port addressing information element set to "trigger to establish a PDN connection of non-IP PDN type using the default APN" as specified in 3GPP TS 23.040 [2], the UE shall send to the ESM entity as specified in 3GPP TS 24.007 [4] an indication requesting establishment of a PDN connection for non-IP PDN type using the default APN.