Content for TS 29.118 Word version: 18.0.0

3.1 Definitions 3.2 Abbreviations 4.1 General 4.2 SGs association at the VLR 4.3 SGs association at the MME
...

1 Scope |R16| p. 9

CS Fallback in the Evolved Packet System (EPS) enables the provisioning of CS-domain services (e.g. voice call, Location Services (LCS) or supplementary services) by reuse of CS infrastructure when the UE is served by E-UTRAN. Additionally, SMS delivery via the CS core network is realized without CS fallback.

The present document specifies the procedures and the SGs Application Part (SGsAP) messages used on the SGs interface between the Mobility Management Entity (MME) in the EPS and the Visitor Location Register (VLR), to allow location management coordination and to relay certain messages related to GSM circuit switched services over the EPS system.

The present document also specifies the use of Stream Control Transmission Protocol (SCTP) for the transport of SGsAP messages.

The present document is applicable to the MME in the EPS and to the VLR. The functional split between the MME and the VLR is defined in TS 23.272.

2 References |R16| p. 9

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
For a specific reference, subsequent revisions do not apply.
For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

[1]

TR 21.905: "Vocabulary for 3GPP Specifications".

[2]

TS 22.101: "Service aspects; Service principles".

[3]

TS 23.003: "Numbering, addressing and identification".

[4]

TS 23.007: "Restoration procedures".

[5]

TS 23.018: "Basic call handling; Technical realization".

[5AA]

TS 23.078: "Customised Applications for Mobile network Enhanced Logic (CAMEL) Phase 4; Stage 2".

[5A]

TS 23.081: "Line identification supplementary services".

[5B]

TS 23.082: "Call Forwarding (CF) supplementary services".

[6] Void.

[6A]

TS 23.236: "Intra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodes".

[7]

TS 23.272: "Circuit Switched Fallback in Evolved Packet System; Stage 2".

[7A]

TS 23.251: "Network Sharing; Architecture and Functional Description".

[7B]

TS 23.401: "GPRS enhancements for E-UTRAN access".

[8]

TS 24.008: "Mobile radio interface Layer 3 specification; Core network protocols; Stage 3".

[9]

TS 24.010: "Supplementary services specification; General aspects".

[10]

TS 24.011: "Point-to-Point (PP) Short Message Service (SMS) support on mobile radio interface".

[11]

TS 24.030: "Location Services (LCS); Supplementary service operations; Stage 3".

[12]

TS 24.081: "Line Identification Supplementary Services - Stage 3".

[13]

TS 24.082: "Call Forwarding (CF) supplementary services; Stage 3".

[14]

TS 24.301: "Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3".

[15]

TS 29.002: "Mobile Application Part (MAP) specification".

[15A]

TS 29.011: "Signalling interworking for supplementary services".

[16]

TS 29.018: "Serving GPRS Support Node (SGSN) - Visitors Location Register (VLR) Gs interface layer 3 specification".

[16A]

TS 29.060: General Packet Radio Service (GPRS);GPRS Tunnelling Protocol (GTP) across the Gn and Gp interface.

[17]

TS 29.272: "MME and SGSN Related Interfaces Based on Diameter Protocol".

[17A]

TS 29.274: "3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3".

[17B]

TS 32.250: "Telecommunication management; Charging management; Circuit Switched (CS) domain charging".

[18]

TS 32.422: "Telecommunication management; Subscriber and equipment trace; Trace control and configuration management (CM)".

[19]

TS 36.331: "Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC) protocol specification".

[20]

RFC 791 (September 1981): "Internet Protocol".

[21] Void.

[22]

RFC 2460 (December 1998): "Internet Protocol, Version 6 (IPv6) Specification".

[23]

RFC 4960 (September 2007): "Stream Control Transmission Protocol".

[24]

TS 22.067: "enhanced Multi Level Precedence and Pre-emption service (eMLPP); Stage 1".

[25]

TS 23.067: "enhanced Multi-Level Precedence and Pre-emption service (eMLPP); Stage 2".

[26]

TS 23.216: "Single Radio Voice Call Continuity (SRVCC); Stage 2".

[27] Void.

3 Definitions and abbreviations |R16| p. 10

3.1 Definitions p. 10

For the purposes of the present document, the terms and definitions given in TR 21.905 apply. Additionally the following definitions of TS 24.301 apply:

Non-EPS services
SMS only

For the purposes of the present document, the following terms and definitions given in TS 23.272 apply:

CS fallback
SMS over SGs
SMS in MME

For the purposes of the present document, the following terms and definitions given in TS 23.251 apply:

Common PLMN
Gateway core network (GWCN)

For the purposes of the present document, the following terms and definitions given in TS 24.301 apply:

In NB-S1 mode

3.2 Abbreviations p. 11

For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905.

LCS

Location Services

MME

Mobility Management Entity

NEAF

Non-EPS Alert Flag

SCTP

Stream Control Transmission Protocol

SGsAP

SGs Application Part

SMS

Short Message Service

PSI

Provide Subscriber Information

4 Description of the SGs association between a VLR and an MME |R16| p. 11

4.1 General p. 11

CS fallback function and SMS delivery via the CS core network is realized by reusing Gs interface mechanisms as defined in TS 29.018 on the interface between the MME in the EPS and the VLR. This interface is called SGs interface.

The SGs interface connects the databases in the VLR and the MME. The procedures described in the present document are used to co-ordinate the location information of UEs that are IMSI attached to both EPS and non-EPS services. The SGs interface is also used to convey some circuit switched related procedures via the MME.

The basis for the interworking between a VLR and an MME is the existence of a SGs association between those entities per UE. The SGs association is applicable to UEs which are configured to use CS fallback and SMS over SGs, or SMS over SGs only. The SGs association is not applicable if the subscriber data indicates that the subscription is for packet only. The SGs association is also not applicable if the MME is registered for SMS for the UE as specified in TS 23.272.

In NB-S1 mode, the SGs association is also applicable to UEs supports NB-S1 mode only to use SMS over SGs only.

The behaviour of the VLR and the MME entities related to the SGs interface are defined by the state of the SGs association for a UE. Individual SGs association states are maintained at both the VLR and the MME for each UE.

4.2 SGs association at the VLR p. 11

4.2.1 General p. 11

The states associated to the SGs interface in the VLR are specified in subclause 4.2.2 and the state diagram at the VLR is shown in Figure 4.2.2-1. The state diagram does not include the message error handling specified in clause 7.

4.2.2 States at the VLR p. 12

SGs-NULL

There is no SGs association with an MME for the UE and therefore the VLR considers that the UE is IMSI detached for EPS services. In this state no SGsAP-MM-INFORMATION-REQUEST messages are sent to the MME. The VLR may initiate paging on the SGs interface if the "Confirmed by Radio Contact" restoration indicator in the VLR is set to "false" (see TS 23.007). Any message from the MME is ignored except SGsAP-LOCATION-UPDATE-REQUEST, SGsAP-IMSI-DETACH-INDICATION and SGsAP-EPS-DETACH-INDICATION.

LA-UPDATE-PRESENT

The VLR has received an SGsAP-LOCATION-UPDATE-REQUEST message from the MME. In this state, the VLR may be waiting for the outcome of the Update Location procedure from the HSS, if the IMSI is not known in the VLR. For UEs which are configured to use CS fallback and SMS over SGs, or SMS over SGs only, the VLR sends SGsAP-PAGING-REQUEST messages via the SGs interface.

SGs-ASSOCIATED

The VLR considers that the UE is attached to both EPS and non-EPS services. For UEs which are configured to use CS fallback and SMS over SGs, or SMS over SGs only, the VLR sends SGsAP-PAGING-REQUEST messages via the SGs interface. The VLR can perform the MM information procedure.

Copy of original 3GPP image for 3GPP TS 29.118, Fig. 4.2.2-1: State diagram at the VLR

Figure 4.2.2-1: State diagram at the VLR
(⇒ copy of original 3GPP image)

4.3 SGs association at the MME p. 12

4.3.1 General p. 12

The MM context variables associated to the SGs interface in the MME are specified in subclause 4.3.2 and states associated to the SGs interface in the MME are specified in subclause 4.3.3. The state diagram at the MME is shown in Figure 4.3.3-1. The state diagram does not include the message error handling specified in clause 7.

4.3.2 MM context variables at the MME p. 13

VLR-Reliable:

Boolean set to "false" when the MME has received a reset indication from the VLR. The MME may:

upon reception of a combined tracking area update request from a UE that is still attached for non-EPS services, perform immediately the location update for non-EPS services procedure; or
upon reception of a periodic tracking area update request from a UE that is still attached for non-EPS services, dependent on network configuration and operator policy,
- perform a network initiated detach with detach type "IMSI detach" immediately after the completion of the periodic tracking area update procedure; or
- perform immediately the location update for non-EPS services procedure.

MME-Reset:

Boolean set to "true" when the MME restarts after a failure. The "MME-Reset" restoration indicator is unique within an MME and it applies to all the MM contexts stored in the MME.

4.3.3 States at the MME p. 13

SGs-NULL

There is no SGs association with a VLR for the UE and therefore the MME considers that the UE is IMSI detached for non-EPS services. In this state the MME accepts SGsAP-PAGING-REQUEST messages to UEs only if:

the "MME-Reset" restoration indicator in the MME is set to "true"; or
the MME supports the CS restoration indicator set in the Additional paging indicators information element in the SGs-PAGING-REQUEST message.

LA-UPDATE-REQUESTED

The MME has sent an SGsAP-LOCATION-UPDATE-REQUEST message to the VLR. In this state the MME waits for the outcome of the Update Location for non-EPS services procedure at the VLR before sending the response to the UE. In this state the MME accepts SGsAP-PAGING-REQUEST messages.

SGs-ASSOCIATED

The MME stores an SGs association for the UE. In this state the MME performs the location update for non-EPS services procedure towards the VLR, e.g. when the location area is changed. All conditions describing when to trigger the location update for non-EPS services procedure are listed in subclause 5.2.2.2.

Copy of original 3GPP image for 3GPP TS 29.118, Fig. 4.3.3-1: State diagram at the MME

Figure 4.3.3-1: State diagram at the MME
(⇒ copy of original 3GPP image)

4.3.4 State transitions in the MME without SGsAP signalling p. 14

During the tracking area update procedure or routing area update procedure (see TS 23.401) and if the SGs association is not SGs-NULL, the MME may change the state of the SGs association to SGs-NULL upon receipt of a SGSN Context Acknowledge (see TS 29.060) or Context Acknowledge message without ISR activation (see TS 29.274) from the target network.

During the inter-system handover from S1 mode to A/Gb mode or Iu mode, or handover from S1 mode to S1 mode (see TS 23.401) and if the SGs association is not SGs-NULL, the MME may change the state of the SGs association to SGs-NULL upon receipt of a Forward Relocation Complete (see TS 29.060) or Forward Relocation Complete Notification message without ISR activation (see TS 29.274) from the target network.

If the MME receives a normal attach request, attach request for emergency bearer services or a normal tracking area update request from the UE in state SGs-ASSOCIATED, the MME shall change the state of the SGs association to SGs-NULL.

If it is required by network configuration or operator policy, the MME shall change the state of the SGs association to SGs-NULL if the following conditions are fulfilled:

the MME receives an EXTENDED SERVICE REQUEST message with service type set to "mobile originating CS fallback emergency call or 1xCS fallback emergency call";
the state of the SGs association is not SGs-NULL; and
the MME determines that the UE is not available for PS services at the target RAT as described in TS 23.272.

4.3.5 State transitions in the VLR without SGsAP signalling p. 14

Based on configuration, during PS to CS SRVCC from E-UTRAN (see TS 23.216) and if the SGs association is not SGs-NULL, the VLR may change the state of the SGs association to SGs-NULL upon receipt of a Relocation Complete/Handover Complete message from target RNS/BSS.