"Signalling in EPC/LTE" - Chapter 12 CSFB and SMSoGSs (sample) by Leliwa

12 CSFB and SMSoSGs

Chapter 12 CSFB and SMSoSGs SMSoSGs Topic

Page

Introduction ................................................................................................... 37 Protocol stack ................................................................................................ 39 SGs procedures.............................................................................................. 42 Location update for non-EPS services ........................................................ 42 Detach procedures ....................................................................................... 45 Paging for non-EPS services procedure ...................................................... 48 Service request procedure ........................................................................... 56 Non-EPS alert procedure ............................................................................ 59 MM information procedure......................................................................... 60 Tunnelling of NAS messages...................................................................... 61 Failure procedure ........................................................................................ 62 CSFB procedures .......................................................................................... 65 Mobile Originating call ............................................................................... 66 Mobile Terminating call ............................................................................. 70 References ...................................................................................................... 73

Signalling in EPC / LTE

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12 CSFB and SMSoSGs

Introduction [1, 3GPP 29.118], [2, 3GPP 23.272] The CS fallback (CSFB) in EPS enables the provisioning of voice and other CS-domain services (e.g. CS UDI video, LCS, SS) by reuse of CS infrastructure when the UE is served by E-UTRAN. A CSFB enabled terminal, connected to E-UTRAN may use GERAN or UTRAN to connect to the CS-domain. This function is only available in case E-UTRAN coverage is overlapped by either GERAN coverage or UTRAN coverage. CSFB function is realised by reusing Gs interface [3, 3GPP 23.060] mechanisms (defined for SGSN-VLR interworking) on the interface between the MME and the MSC/VLR. This interface is called SGs interface. The SGs interface connects the databases in the VLR and the MME 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 CS related procedures via the MME. Additionally, SGs interface supports SMS delivery via the CS core network (SMSoSGs) without CSFB functionality.. CSFB and IMS-based services are able to co-exist in the same operatorâ&#x20AC;&#x2122;s network.

Architecture The EPC architecture supporting CSFB and SMSoSGs in shown in Fig. 2-1. GERAN

SGSN

Gb Gs Iu

UTRAN

MSC SGs

E-UTRAN

server

MME

Figure 2-1 CSFB and SMSoSGs architecture

Signalling in EPC / LTE

UE The CSFB capable UE supports access to E-UTRAN/EPC as well as access to the CS domain over GERAN and/or UTRAN. The SMSoSGs capable UE supports access to E-UTRAN/EPC and may support access to the CS domain over GERAN and/or UTRAN. The support of SMSoSGs is mandatory for a UE that supports CSFB, whereas a UE that supports SMSoSGs is not required to support CSFB. These UEs support the following additional functions: •

Combined procedures for EPS/IMSI attach, update and detach.

•

CSFB and/or SMSoSGs procedures for using CS domain services.

From the UE's perspective there is no difference whether the MME provides SMSoSGs or by SMS in MME.

MME The CSFB and/or SMSoSGs enabled MME supports the following additional functions: •

Deriving a VLR number and LAI from the TAI of the current cell.

•

For CSFB, generating a TAI list such that the UE has a low chance of "falling back" to a cell in a LA different to the derived LAI (e.g. the TAI list boundary should not cross the LA boundary).

•

Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE.

•

Initiating IMSI or EPS detach.

•

Initiating paging procedure towards eNBs, as a result of received paging for CS services from MSC.

•

Supporting SMS procedures.

MSC The CSFB and/or SMSoSGs enabled MSC supports the following additional functions: • Maintaining SGs association towards MME for EPS/IMSI attached UE. • Supporting SMSoSGs procedures.

12 CSFB and SMSoSGs

E-UTRAN The CS fallback enabled E-UTRAN supports the following additional functions: •

Forwarding paging request for CS domain to the UE.

•

Directing the UE to the target CS capable cell considering the LAI for CS domain received from the MME.

•

To facilitate the configuration of TA boundaries with LA boundaries, the E-UTRAN can gather statistics (from the inbound inter-RAT mobility events of all UEs) of the most common LAs indicated in the RRC signalling.

•

Configuration to permit the operator to choose the target 'fallback' RAT and frequency.

For SMSoSGs, no specific E-UTRAN functionality is required.

RAN If the BSS receives indication from the MSC that the CS service was established as a result of CSFB the BSS may e.g. select to use RR Connection Release with redirect to send the UE back to E-UTRAN at release of the CS service.

Protocol stack [2, 3GPP 23.272] SGsAP is the protocol used across SGs interface to connect an MME to an MSC server. SGsAP is based on the BSSAP+ protocol. [1, 3GPP 29.118] SCTP (RFC 4960) is the transport layer of SGsAP messages. Semi-permanent SCTP associations are established between the MME and VLR, i.e. the SCTP associations remains up under normal circumstances. Transport network redundancy can be achieved by SCTP multi-homing between two end-points, of which one or both is assigned with multiple IP addresses. SCTP end-points shall support a multi-homed remote SCTP end-point.

Signalling in EPC / LTE

SGsAP

SCTP

MME

SGs

MSC server / VLR

Figure 2-2 SGs interface protocol stack MME and VLR shall support a configuration with a single SCTP association per MME/VLR pair. Configurations with multiple SCTP endpoints per MME/VLR pair may be supported. If multiple SCTP endpoints are configured and several SCTP associations are established between the MME/VLR pair, whether the VLR and the MME send and receive SGsAP messages via same or different SCTP associations for a given UE is up to implementation. Within the SCTP association established between one MME and one VLR, both MME and VLR shall reserve several stream identifiers, based on the INIT message exchange for the sole use of SGsAP procedures. The MME shall establish the SCTP association. The registered port number for SGsAP is 29118. The payload protocol identifier to be used for SGsAP is 0.

Figure 2-3 SCTP configuration for SGsAP

12 CSFB and SMSoSGs

SGsAP-LOCATION-UPDATE-REQUEST

SGsAP-DOWNLINK-UNITDATA

SGsAP-LOCATION-UPDATE-ACCEPT

SGsAP-UPLINK-UNITDATA

SGsAP-LOCATION-UPDATE-REJECT

SGsAP-RELEASE-REQUEST

SGsAP-TMSI-REALLOCATION-COMPLETE SGsAP-RESET-INDICATION SGsAP-EPS-DETACH-INDICATION

SGsAP-RESET-ACK

SGsAP-EPS-DETACH-ACK

SGsAP-UE-ACTIVITY-INDICATION

SGsAP-IMSI-DETACH-INDICATION

SGsAP-ALERT-REQUEST

SGsAP-IMSI-DETACH-ACK

SGsAP-ALERT-ACK SGsAP-ALERT-REJECT

SGsAP-PAGING-REQUEST SGsAP-MM-INFORMATION-REQUEST

SGsAP-PAGING-REJECT SGsAP-SERVICE-REQUEST SGsAP-UE-UNREACHABLE

SGsAP-STATUS

SGsAP-SERVICE-ABORT-REQUEST

Figure 2-4 SGsAP messages

TAI list and LAI allocation [2, 3GPP 23.272] For CSFB, the fallback procedure is likely to be faster if the network can allocate a LA to the UE that is the LA of the overlapping target RAT's coverage. For this situation, the MME should avoid allocating TAI lists that span multiple LAs of the target RAT (which may be contrary to the normal usage of the TA list concept). This can be achieved by: • configuring the E-UTRAN cell's TAI to take into account the LA boundary of the target RAT; • the MME being configured to know which TAIs are within which LA; and • the MME using the TAI of the current E-UTRAN cell to derive the LAI. The operator should be able to configure the MME as to whether it either: • provides normal usage of the "tracking area list" concept, or, • the TAI list allocation is adjusted, for CSFB mobiles, to provide "TAI lists that do not span multiple LAs".

Signalling in EPC / LTE

MSC

LAI

server

CSFB inter-RAT mobility

SGs

MME

TAI list

SGs association

no need for LAU

TAI list, LAI

Figure 2-5 TAI list and LAI allocation (CSFB) The MME may use alternative approaches for LAI and TAI list allocation. In particular, this is appropriate for: • the case of SMS over SGs without overlapping GERAN/UTRAN coverage; and • the case when not all MSCs in the VPLMN support the SGs interface. In these situations, one approach is to configure the MME to allocate a default (e.g. non-broadcast) LAI which is associated with a VLR that supports the SGs interface. If "SMS in MME" is used for a UE, then the MME shall allocate a non-broadcasted LAI (not associated with any VLR) and a reserved TMSI, if needed, for that UE.

SGs procedures Location update for nonnon-EPS services [1, 3GPP 29.118] The location update for non-EPS services procedure is a general procedure used by UEs which are configured to use CSFB and SMSoSGs, or SMSoSGs only. This procedure allows UEs and the network to perform: • combined IMSI attach for EPS and non-EPS services or for SMS only; • IMSI attach for non-EPS services or for SMS only if the UE is already IMSI attached for EPS services;

12 CSFB and SMSoSGs

• normal location update procedure to the VLR if the UE is IMSI attached for both EPS and non-EPS services, or for SMS only; or • allocation of new TMSI to an UE. The location update for non-EPS services procedure in the SGs interface is always started as a consequence of a direct action from the UE (i.e. combined Attach and TAU procedures). MSC

MME

server

HSS

Attach / TAU Request (combined EPS/IMSI attach or combined TA/LA update)

ULR

Attach / TAU Accept

(NAM=PS / CS & PS, MME Registered for SMS)

ULA

(combined EPS/IMSI or combined TA/LA, SMS only, non broadcast LAI)

Attach / TAU Complete

Figure 2-6 Location update for non-EPS services procedure (not started)

MME Attach / TAU Request (combined EPS/IMSI attach or combined TA/LA update, - / SMS only, old LAI, TMSI status, TMSI based NRI container)

MME name, IMSI, IMEISV, TMSI status, new LAI, old LAI, TAI, E-CGI, TMSI based NRI container, selected CS domain operator

MSC server

HSS

ULR / ULA (NAM=CS&PS) LOCATION-UPDATE-REQUEST

Upd. Loc.

LOCATION-UPDATE-ACCEPT

Attach / TAU Accept (combined EPS/IMSI or combined TA/LA, - / SMS only, TMSI, LAI)

IMSI, LAI, new TMSI or IMSI

Attach / TAU Complete

Figure 2-7 Location update for non-EPS services (started) The location update for non-EPS services procedure is initiated with a combined Attach or a combined TAU procedure. On receipt of an NAS EMM ATTACH REQUEST message or a NAS EMM TRACKING AREA UPDATE REQUEST message from the UE, the MME handles the EPS related as in the case of a non-combined procedure.

Signalling in EPC / LTE

When the MME receives the S6a Update Location Answer (ULA) message containing subscription data from the HSS, the MME determines whether it needs to perform location update for non-EPS services procedure. The MME sends response message to the UE without starting the location update for non-EPS services in case the MME is configured with ability not to perform registrations with a VLR and the Network Access Mode (NAM) subscription data indicates that the subscription is for PS only or CS and PS but the MME is registered for SMS for this UE1. Otherwise, the MME shall start the location update for non-EPS services procedure and shall wait for the outcome of both location update procedures towards the VLR and the HSS before sending the response message to the UE2. The MME derives the LAI from the current TAI where the UE is located, as an exception; if the MME received a combined Attach request or combined TAU request from the UE with an indication for "SMS only", or the network only supports "SMS only", the MME may allocate a default LAI specifically configured for that case. Then, the MME derives the VLR name from the LAI which was determined and sends to that VLR the SGsAP-LOCATIONUPDATE-REQUEST message For networks supporting the feature “Intra Domain Connection of RAN Nodes to Multiple CN Nodes”, commonly known as “MSC/VLR in pool” the selection of the VLR in the MME follows the same rule as the selection of the VLR in the SGSN [4, 3GPP 23.236]. When a VLR receives an SGsAP-LOCATION-UPDATE-REQUEST message, the VLR checks whether the IMSI is known. If the IMSI is not known, the VLR retrieves the MM context of the UE from the HSS. If the location update is accepted by the VLR and, if necessary, by the HSS, the VLR shall move the SGs association to the SGs-ASSOCIATED state and send an SGsAP-LOCATION-UPDATE-ACCEPT message to the sending MME.

When the MME supports both SMS in MME feature and SMS over SGs, depending on UE capability and/or network configuration and/or “PS and SMS only” subscription, it can be preferred to provide SMS services via SMS in MME to avoid the VLR registration. 2 When the combined Attach or TAU procedure is for both EPS services and non-EPS services and the MME is not registered for SMS for the UE and the UE has "PS and SMS only" subscription, the MME starts the location update for non-EPS services procedure for SMSoSGs only.

12 CSFB and SMSoSGs

If the MME receives an SGsAP-LOCATION-UPDATE-ACCEPT message from the VLR, the MME moves the state of the SGs association to SGs-ASSOCIATED. The MME shall wait for the outcome of the location update for non-EPS services procedure towards the VLR before sending a response to location update procedure to the UE. If the VLR included the mobile identity IE in the SGsAP-LOCATIONUPDATE-ACCEPT message, the MME shall relay the information received to the UE. If the mobile identity IE contains a new TMSI, this will cause the UE to perform a TMSI reallocation procedure. In this case, the MME shall send to the VLR the SGsAP-TMSI-REALLOCATION-COMPLETE message when the MME receives the ATTACH COMPLETE or the TRACKING AREA UPDATE COMPLETE message from the UE. If the Mobile identity information element contains an IMSI, this will cause the UE to deallocate its TMSI.

Detach procedures Explicit IMSI detach from EPS services This procedure is used by the MME to indicate to the VLR that the UE has been detached from EPS services and therefore the SGs association between the MME and the VLR has to be deactivated. The procedure applies to EPS detach indication initiated by the UE or by the network [5, 3GPP 24.301]. The procedure is also used by the MME to indicate to the VLR when a combined or periodic TAU procedure has been rejected by the MME or the MME receives the GTP S3 DETACH NOTIFICATION from the SGSN [6, 3GPP 29.274], when ISR is activated.

SGSN

ISR

Detach Notification (PS detach)

MME

MME name, IMSI, IMSI detach from EPS service type: • Net. init. IMSI detach from EPS / • UE init. IMSI detach from EPS / • EPS services not allowed

MSC server

Detach Request (EPS detach)

EPS-DETACH-INDICATION EPS-DETACH-ACK

Detach Accept

UE remains attached, implicit detach timer restarted

Figure 2-8 Explicit IMSI detach from EPS services 45

Signalling in EPC / LTE

When a VLR receives an SGsAP-EPS-DETACH-INDICATION message, the VLR shall send an SGsAP-EPS-DETACH-ACK message to the sending MME. If the VLR’s implicit detach timer is not running then the VLR shall set and restart the implicit detach timer upon reception of an SGsAP-EPSDETACH-INDICATION message.

Implicit detach timer When a UE is IMSI attached for EPS and non-EPS services, the VLR shall stop any implicit detach timer. Instead the MME uses the "Paging Proceed Flag" to determine the likely availability of the UE to the network. Upon reception of the periodic TAU, the MME does not report to the VLR, and the MME shall not change the state of the SGs association. When the UE performs a detach only for EPS services or the MME performs an implicit detach for EPS services, and the VLR's implicit detach timer is not already running, the EPS detach indication to the VLR shall cause the VLR's implicit detach timer to be restarted from its initial value.

Explicit Explicit IMSI detach from nonnon-EPS services This procedure is used by the MME to indicate to the VLR that the UE has performed IMSI detach from non-EPS services and therefore the SGs association between the MME and the VLR has to be deactivated. The procedure applies only to IMSI detach request, combined IMSI and EPS detach requests from the UE or GTP S3 DETACH NOTIFICATION [6, 3GPP 29.274] message from an SGSN.

SGSN

ISR

Detach Notification (Complete Detach, Comb. PS/CS Detach)

Detach Request (IMSI detach or combined EPS/IMSI detach)

Detach Accept

MME

MME name, IMSI, IMSI detach from non-EPS service type: • Explicit UE init. IMSI detach from non-EPS / • Combined UE init. IMSI detach from EPS and non-EPS / • Implicit network init. IMSI detach from EPS and non-EPS

IMSI-DETACH-INDICATION IMSI-DETACH-ACK UE detached

Figure 2-9 Explicit IMSI detach from non-EPS services

MSC server

12 CSFB and SMSoSGs

When an MME receives a Detach Notification message for a UE from an SGSN and an SGs association for the UE exists, the MME shall check the cause and detach type indicated. If the cause is indicating "IMSI Detach only", the MME shall send an SGsAP-IMSI-DETACH-INDICATION message to the VLR indicating "Explicit UE initiated IMSI detach from non-EPS services". If the cause is indicating "Complete Detach" and Detach type is indicating "Combined PS/CS Detach", the MME shall send an SGsAP-IMSI-DETACHINDICATION message to the VLR indicating "Combined UE initiated IMSI detach from EPS and non-EPS services". When a VLR receives an SGsAP-IMSI-DETACH-INDICATION message, the VLR shall send an SGsAP-IMSI-DETACH-ACK message to the sending MME and mark the UE as detached. If the detach type received from the UE indicated IMSI only detach or combined EPS/IMSI detach not due to switch off, the MME shall wait for the reception of the SGsAP-IMSI-DETACH-ACK message before sending the confirmation of the detach to the UE.

Implicit IMSI detach from nonnon-EPS services This procedure is used by the MME to indicate when an internal MME timer mechanism has caused the MME to delete the EMM context of an UE or mark its EMM context as detached.

MME

MSC server

no contact

IMSI-DETACH-INDICATION IMSI-DETACH-ACK UE detached

UE detached

Figure 2-10 Implicit IMSI detach from non-EPS services When the implicit IMSI detach from non-EPS services procedure is started for a UE by the above mentioned internal MME timer mechanism, the MME shall send an SGsAP-IMSI-DETACH-INDICATION message to the VLR indicating "Implicit network initiated IMSI detach from EPS and non-EPS services". 47

Signalling in EPC / LTE

When a VLR receives the SGsAP-IMSI-DETACH-INDICATION message, the VLR shall send an SGsAP-IMSI-DETACH-ACK message to the sending MME. If the VLR does not have a signalling connection for the UE, the VLR shall mark the UE as detached.

Implicit IMSI detach from EPS services This procedure is used by the MME to indicate when an internal MME timer mechanism has caused the MME to delete the EMM context of an UE or mark its EMM context as detached. This procedure only applies to UEs for which there is an SGs association at the MME and the network operating in NMO I and supporting ISR. When the implicit IMSI detach from EPS services procedure is started for a UE by the above mentioned internal MME timer mechanism, the MME shall send an SGsAP-EPS-DETACH-INDICATION message to the VLR indicating "Network initiated IMSI detach from EPS services". When a VLR receives an SGsAP-EPS-DETACH-INDICATION message, the VLR shall perform the procedures described in subclause 5.4.3. When a VLR receives an SGsAP-EPS-DETACH-INDICATION message, the VLR shall send an SGsAP-EPS-DETACH-ACK message to the sending MME. If the VLRâ&#x20AC;&#x2122;s implicit detach timer is not running then the VLR shall set and restart the implicit detach timer upon reception of an SGsAP-EPSDETACH-INDICATION message.

Paging for nonnon-EPS services procedure This procedure is used by the VLR to send an SGsAP-PAGING-REQUEST message to a UE. This procedure applies to UEs that are simultaneously attached for EPS services and non-EPS services, or for EPS services and SMS only. The VLR shall handle the timers, queuing and retransmission for sending the SGsAP-PAGING-REQUEST message on the SGs interface in the same way that it handles the sending of a PAGING message on the A or Iu interface. The VLR shall be configured to send paging messages on both the SGs and the A/Iu interface. The VLR may apply implementation specific rules for sending the paging on the A/Iu interface; e.g. paging on the A/Iu interface may be limited to cases when the UE does not respond to a first paging on SGs interface.

12 CSFB and SMSoSGs

Paging

ISR

SGSN

MME CS Paging Indication Paging

Paging (UE Paging Identity, list of TAIs, CN Domain: CS / PS)

VLR name, IMSI, TMSI, LAI, Service indicator: CS call / SMS, Channel needed, CLI, SS code, LCS indicator, LCS client id. Additional paging indicators: CSRI flag

MSC server

PAGING-REQUEST UE in EMM-IDLE state

Ts5 (2 to 20s)

SERVICE-REQUEST / Initial UE NAS message

Figure 2-11 Paging for non-EPS services (EMM-IDLE) When a VLR has to page a UE, the VLR shall check whether the VLR has a SGs association for that UE and sends SGsAP-PAGING-REQUEST messages to the MME. In this message, the VLR includes the Service indicator IE which will be used to indicate the type of CS service. For SMS, SMS indicator is used. For all the other CS services, CS call indicator is used. If the Calling Line Identification (CLI) [7, 3GPP 24.081] of the service is available in the VLR, the VLR may include the CLI IE. If the paging is due to a NW-initiated Call Independent SS procedure [8, 3 GPP 24.010], the VLR may include the SS code [9, 3GPP 29.002]. If the paging is due to a MT Location Request [10, 3 GPP 24.030], the VLR shall include LCS indicator and may include LCS client identity [9, 3GPP 29.002]. When a MME receives a SGsAP-PAGING-REQUEST message from a VLR, the MME shall first check if the UE is known by the MME. The handling of the paging request depends on the state of the SGs association, the EMM context variables at the MME, and the Service indicator information element in the SGsAP-PAGING-REQUEST message. If the UE is considered to be IMSI attached for EPS and non-EPS services (i.e. the SGs association is not in the state SGs-NULL) and no NAS connection exists towards that UE, the MME shall page the UE based on the location information stored in the MME, i.e. in all TAs of the stored list. If the MME has activated ISR for the UE, the MME shall forward the paging request to the associated SGSN [6, 3GPP 29.274].

Signalling in EPC / LTE

When a NAS signalling connection exists towards the UE, and the Service Indicator IE in the SGsAP-PAGING-REQUEST message indicates "CS call indicator", the MME sends the CS SERVICE NOTIFICATION message to the UE through the NAS signalling connection, including the CS service related parameters (CLI, SS code, LCS indicator and LCS client identity), received from the VLR. If the Service Indicator IE in the SGsAP-PAGINGREQUEST message indicates "SMS indicator" and the MME accepts the paging request, the MME need not take any action towards the UE. The VLR stops the paging procedure on expiry of timer Ts5 (2 to 20s) or on receipt of a SGsAP-SERVICE-REQUEST message from the MME. [11, 3GPP 24.008] The VLR also stops the paging procedure on receipt of an SCCP connection establishment containing the Initial L3 message from the UE via the A or Iu interface. If the paging response is received via the A or Iu interface from a LA which differs from the one stored in the VLR, the VLR shall move the SGs association to the SGs-NULL state after the UE has been authenticated successfully. UE sends this paging response as a result of receiving paging request with IMSI and with CN domain indicator set to "CS" [5, 3GPP 24.301].

MME

VLR name, IMSI, TMSI, LAI, Service indicator: CS call, Channel needed, CLI, SS code, LCS indicator, LCS client id. Additional paging indicators: CSRI flag

MSC server

PAGING-REQUEST CS Service Notification (Paging identity, CLI, SS Code, LCS indicator, LCS client identity)

UE in EMM-CONNECTED state

SERVICE-REQUEST

Figure 2-12 Paging for non-EPS CS services (EMM-CONNECTED)

12 CSFB and SMSoSGs

MME

VLR name, IMSI, TMSI, LAI, Service indicator: SMS, Channel needed, Additional paging indicators: CSRI flag

MSC server

PAGING-REQUEST UE in EMM-CONNECTED state

SERVICE-REQUEST

Figure 2-13 Paging for non-EPS SMS service (EMM-CONNECTED) MT CS services delivery via an alternative MME in the MME pool [1, 3GPP 29.118],[12, 3GPP 23.007] If the VLR detects that the MME serving the UE is no longer in service3 and the VLR supports MT CS services delivery via an alternative MME in the MME pool the VLR shall send the SGs-PAGING-REQUEST message to one alternative MME in the same MME pool. In that case, the VLR shall set the CS restoration indicator in the Additional paging indicators IE. The VLR may know the set of MMEs pertaining to the same MME pool by local configuration or by checking the MME Group ID within the MME name that MMEs signal to the VLR in the SGsAP-LOCATION-UDATEREQUEST, SGsAP-RESET-INDICATION or SGsAP-RESET-ACK messages.

MME name = mmec01.mmegi0001.mme.epc. mnc003.mcc260.3gppnetwork.org

MSC

MME mmec01.mmegi0001.mme.epc. mnc003.mcc260.3gppnetwork.org

server

VLR name, IMSI, TMSI, LAI, Service indicator: CS call / SMS, Channel needed, CLI, SS code, LCS ind., LCS client id. Add. paging indicators: CSRI flag

MME mmec02.mmegi0001.mme.epc. mnc003.mcc260.3gppnetwork.org

MME pool = mmegi0001.mme.epc.mnc003.mcc260.3gppnetwork.org

Figure 2-14 MT CS services delivery via an alternative MME

3 The VLR can detect that an MME is no longer in service if there are no SCTP associations in service with that MME.

Signalling in EPC / LTE

Additional paging cases Paging for CS call If the Service Indicator IE in the SGsAP-PAGING-REQUEST message indicates "CS call indicator" and the UE is considered to be IMSI attached for EPS and non-EPS services (i.e. the SGs association is not in the state SGs-NULL), the MME shall page the UE based on the location information stored in the MME, i.e. in all TAs of the stored list. If the UE is marked as IMSI detached for EPS services or IMSI (implicitly or explicitly) detached for non-EPS services (i.e. the state of the SGs association is SGs-NULL): â&#x20AC;˘ if the UE is in the EMM-DEREGISTERED state and if the MME supports MT CS services delivery via an alternative MME in the MME pool and the CSRI is set in the SGsAP-PAGING-REQUEST message, the MME shall send the paging request with the location information provided by the VLR. If no such location information is provided, the MME may either page the UE in all the TAs served by that MME or in the TAs served by the MME and by the VLR, or reject the paging request per operator policy; â&#x20AC;˘ otherwise, the MME shall return an SGsAP-PAGING-REJECT message to that VLR indicating in the SGs cause IE the detach circumstance ("IMSI detached for EPS services", "IMSI detached for non-EPS services" or "IMSI implicitly detached for non-EPS services"). MSC

MME

server

UE detached

PAGING-REQUEST PAGING-REJECT IMSI SGs cause = IMSI detached for EPS services / IMSI detached for non-EPS services / IMSI implicitly detached for non-EPS services

Figure 2-15 Paging for non-EPS service rejected If the UE is marked as unreachable, indicated by Paging Proceed Flag (PPF) set to "false" [3, 3GPP 23.060], and the ISR is not activated, the MME shall return an SGsAP-UE-UNREACHABLE message to that VLR indicating in

12 CSFB and SMSoSGs

the SGs cause IE "UE unreachable". The state of the SGs association does not change at the MME. no ISR

SGSN MME

UE SGs-ASSOCIATED

mobile reachable timer

MSC server

UE attached

no contact PPF = false

PAGING-REQUEST UE-UNREACHABLE IMSI SGs cause = UE unreachable

UE SGs-ASSOCIATED

Figure 2-16 Paging for non-EPS service (PPF & no ISR) If the UE is marked as unreachable, indicated by PPF set to "false", and the ISR is activated, the MME shall not return an SGsAP-UE-UNREACHABLE message to that VLR and if the SGsAP-PAGING-REQUEST message does not include the LAI, then the MME shall send GTP S3 DETACH NOTIFICATION message to the associated SGSN; otherwise the MME shall forward the paging request to the associated SGSN in GTP S3 CS PAGING INDICATION [6, 3GPP 29.274]. ISR

SGSN MSC

MME mobile reachable timer no contact PPF = false

CS Paging Indication

server

UE attached

PAGING-REQUEST LAI

Figure 2-17 Paging for non-EPS service (PPF & ISR & LAI)

Signalling in EPC / LTE

ISR

SGSN MME mobile reachable timer

UE SGs-ASSOCIATED

MSC server

UE attached

no contact

PAGING-REQUEST

PPF = false

Detach Notification

no LAI

Figure 2-18 Paging for non-EPS service (PPF & ISR & no LAI) If the UE is not known and the "MME-Reset" restoration indicator at the MME is set to "false" and the MME does not support MT CS services delivery via an alternative MME in the MME pool; or the MME supports MT CS services delivery via an alternative MME in the MME pool and the CSRI is not set in the SGsAP-PAGING-REQUEST message; the MME shall return an SGsAP-PAGING-REJECT message to that VLR indicating in the SGs cause information element "IMSI unknown". MME-reset = false UE unknown

MME

no CSRI

MSC server

PAGING-REQUEST PAGING-REJECT IMSI SGs cause = IMSI unknown

Figure 2-19 Paging for non-EPS service (UE unknown, no CSRI) If the UE is not known and if the "MME-Reset" restoration indicator at the MME is set to "true"; or the MME supports MT CS services delivery via an alternative MME in the MME pool and the CSRI is set in the SGsAPPAGING-REQUEST message; the MME shall page the UE in all the TAs served by the MME that can be mapped to the LAI in the LAI IE. In case the SGsAP-PAGING-REQUEST message does not include the LAI, the MME may page in all the TAs served by the MME, or the TAs served by the MME and by the VLR or reject the paging request per operator policy.

12 CSFB and SMSoSGs

MME-reset = true IMSI, LAI/-

MME UE unknown

MSC server

PAGING-REQUEST

Paging

(UE Paging Identity, list of TAIs)

Figure 2-20 Paging for non-EPS service (UE unknown, MME-reset) Paging for SMS If the Service Indicator IE in the SGsAP-PAGING-REQUEST message indicates "SMS indicator" and the UE is considered to be IMSI attached for EPS and non-EPS services or IMSI attached for EPS services and "SMS only", the MME shall page the UE based on the location information stored in the MME. If the UE is marked as IMSI detached for EPS services or IMSI (implicitly or explicitly) detached for non-EPS services, or as unreachable, the MME shall proceed as specified for the case when the service indicator indicates "CS call indicator". If the UE is not known and the "MME-Reset" restoration indicator at the MME is set to "false" and the CS restoration indicator is not set in the SGsAP-PAGING-REQUEST message; the MME shall return an SGsAPPAGING-REJECT message to that VLR indicating in the SGs cause information element "IMSI unknown". If the UE is not known and the "MME-Reset" restoration indicator at the MME is set to "true"; or the MME supports MT CS services delivery via an alternative MME in the MME pool and the CS restoration indicator is set in the SGsAP-PAGING-REQUEST message; the MME shall page the UE in all the TAs served by the MME that can be mapped to the LAI in the LAI IE. In case the SGsAP-PAGING-REQUEST message does not include the LAI, the MME may page in all the TAs served by the MME, or the TAs served by the MME and by the VLR or reject the paging request per operator policy. Handling in the VLR When the VLR receives the SGsAP-PAGING-REJECT message with the SGs cause information element indicating "MT CSFB call rejected by the user", the VLR shall trigger User Determined User Busy (UDUB) [13, 3GPP 24.082].

Signalling in EPC / LTE

MME Paging (CN Domain = CS)

CS Service Notification

UE SGs-ASSOCIATED

MSC server

PAGING-REQUEST Service indicator: CS call

(CLI, SS Code, LCS ind., LCS client id.)

SGs cause: MT CSFB call rejected by the user

Extended Service Request

PAGING-REJECT

(MT CSFB call rejected by the user) UDUB UE SGs-ASSOCIATED

Figure 2-21 MT CSFB call rejected by the user On receipt of an SGsAP-PAGING-REJECT message with the SGs Cause IE in the SGsAP-PAGING-REJECT message does not indicate "MT CSFB call rejected by the user", the SGs association is moved to the SGs-NULL state. If the SGs Cause IE in the SGsAP-PAGING-REJECT message indicates "IMSI detached for EPS services" the VLR shall send the paging message on the A/Iu interface. On receipt of an SGsAP-UE-UNREACHABLE message the paging procedure for that paging request towards the MME is stopped. The state of the SGs association at the VLR is unchanged.

Service request procedure After the reception of an SGsAP-PAGING-REQUEST message from the VLR, the MME will use this procedure to indicate to the VLR that a NAS signalling connection exists between the UE and the MME. The procedure can be invoked, by the MME, either upon reception of a Service Request message from the UE or directly after receiving the SGsAP-PAGINGREQUEST message from the VLR, based on the UEâ&#x20AC;&#x2122;s EMM mode. When receiving the SGsAP-PAGING-REQUEST message, the MME shall check whether the UE, for which the paging is sent, is in EMM-IDLE or EMM-CONNECTED mode. If the UE was in EMM-CONNECTED mode, the MME shall immediately create and send an SGsAP-SERVICE-REQUEST message to the VLR. If the UE subsequently rejects the CS fallback call, the MME shall send the SGsAPPAGING-REJECT message to the VLR with the SGs cause information element indicating "Mobile terminating CS fallback call rejected by the user".

12 CSFB and SMSoSGs

If the UE was in EMM-IDLE mode, the MME shall send the SGsAPSERVICE-REQUEST message to the VLR when the UE enters EMMCONNECTED mode. MSC

MME

server

UE EMM-IDLE

PAGING-REQUEST

Paging (Extended) Service Request

SERVICE-REQUEST UE EMM-CONNECTED

MSC

MME

server

UE EMM-CONNECTED

CS Service Notification

PAGING-REQUEST SERVICE-REQUEST

Figure 2-22 Service request The MME shall set the service indicator in the SGsAP-SERVICE-REQUEST message equal to what was received in the SGsAP-PAGING-REQUEST message. Additionally, in order to permit the VLR to create an accurate charging record, the MME shall add the IMEISV, the UE Time Zone, the Mobile Station Classmark 2, and the UE's current TAI and E-CGI to the SGsAP-SERVICE-REQUEST message. Upon reception of the SGsAP-SERVICE-REQUEST message, the VLR shall consider the paging procedure as successful. If the paging procedure is for SMS, the VLR shall then start the delivery of the SMS message(s). [5, 3GPP 24.301] Upon reception of a paging indication, a UE that is IMSI attached for non-EPS services shall stop timer T3346, if it is running, and initiate a service request procedure. If the paging is received in EMM-IDLE mode, the UE shall respond immediately. If the paging is received as a CS SERVICE NOTIFICATION message in EMM-CONNECTED mode, the UE may request upper layers input i.e. to accept or reject CS fallback before responding with an EXTENDED SERVICE REQUEST. The response is indicated in the CSFB response information element in the EXTENDED SERVICE REQUEST message in both EMM-IDLE and EMMCONNECTED modes.

Signalling in EPC / LTE

Figure 2-23 Service request (CSFB call rejected by the user)

Service abort procedure This procedure can be invoked by the VLR to abort a mobile terminating CS fallback call during call establishment. The procedure applies to UEs that are simultaneously attached for EPS services and non-EPS services, but not to UEs attached for EPS services and SMS only. MSC

MME Paging / CS service notification

server

PAGING-REQUEST SERVICE-ABORT-REQUEST

REL

Extended Service Request Service Reject (Cause = CS service temporarily not available)

Figure 2-24 Service abort procedure If the VLR decides to abort a MT CSFB call for which it has sent an SGsAP-PAGING-REQUEST message to the MME, and the VLR has not received an SCCP connection establishment containing the Initial L3 message from the UE via the A or Iu interface, the VLR shall send the SGsAPSERVICE-ABORT-REQUEST message to the MME. When the MME receives the SGsAP-SERVICE-ABORT-REQUEST message from the VLR, the MME shall set the Call Cancelled Flag to "true". If the MME receives an NAS EMM EXTENDED SERVICE REQUEST message from the UE with Service type set to "MT CSFB" and CSFB response set to "CSFB accepted by the UE" and the Call Cancelled Flag is set to "true", the MME shall set the Call Cancelled Flag to "false" and will reject the CS fallback call [5, 3GPP 24.301]. When the MME receives the SGsAP-SERVICE-ABORT-REQUEST message after the UE has accepted the CS fallback call, the MME shall discard the SGsAP-SERVICE-ABORT-REQUEST message. 58

12 CSFB and SMSoSGs

NonNon-EPS alert procedure This procedure is used by the VLR to request from an MME an indication when any signalling activity from the UE is detected. This procedure can be invoked at any time by the VLR. ISR

SGSN MSC

MME NEAF

Alert MME Notification

server

ALERT-REQUEST ALERT-ACK

Alert MME Ack. UE activity

UE Activity Notification UE Activity Ack. UE activity

UE-ACTIVITY-INDICATION

Figure 2-25 Non-EPS alert procedure The VLR may start the Non-EPS alert procedure at any time. When the VLR wants to request from an MME that further activity from a UE is reported by the MME, the VLR shall send an SGsAP-ALERT-REQUEST message to that MME. Upon receipt of an SGsAP-ALERT-REQUEST message from the VLR and if the IMSI is known in the MME, the MME shall reply with an SGsAPALERT-ACK message and set the Non-EPS Alert Flag (NEAF). If the MME has activated ISR for the UE, the MME shall send an GTP S3 ALERT MME NOTIFICATION message to the associated SGSN [6, 3GPP 29.274] If an SGsAP-ALERT-REQUEST message is received for an IMSI that is unknown at the MME, the MME shall return an SGsAP-ALERT-REJECT message to the VLR indicating the SGs cause information element value "IMSI unknown". The MME shall report to the VLR upon detection of any activity in E-UTRAN from the UE if the NEAF is set. If the MME detects EPS signalling that leads to a procedure towards the VLR, the MME shall follow this procedure and reset the NEAF. If the MME detects activity that does not lead to any procedure towards the VLR, the MME shall send an SGsAP-UEACTIVITY-INDICATION message towards the VLR and reset the NEAF.

Signalling in EPC / LTE

Upon receipt of a GTP S3 UE ACTIVITY NOTIFICATION message from the SGSN, the MME shall reply with a GTP S3 UE ACTIVITY ACKNOWLEDGE message, send an SGsAP-UE-ACTIVITY-INDICATION message to the VLR and reset the NEAF flag.

MM information procedure The MM information procedure is performed between the VLR and the MME via the SGs interface if the target UE for the MM information procedure is IMSI attached to both EPS and non-EPS services (i.e. the state of the SGs association is SGs-ASSOCIATED).

MME EMM information

MSC server

MM-INFORMATION-REQUEST full name for network, short name for network, local time zone, universal time and local time zone, network daylight saving time

Figure 2-26 MM information procedure If for the target UE for the MM information procedure the state of the SGs association in the VLR is SGs-ASSOCIATED, the VLR may initiate the MM information procedure by transferring an SGsAP-MM-INFORMATIONREQUEST message to the MME. If an SGsAP-MM-INFORMATION-REQUEST message is received for a UE, dependent on operator preference the MME shall either: • check and update the contents of the received MM information IE. The MME then sends the resultant contents of the MM information IE to the UE, using an NAS EMM INFORMATION message [5, 3GPP 24.301]; or • discard the SGsAP-MM-INFORMATION-REQUEST message (in this case the MME can send an NAS EMM INFORMATION message, with contents generated locally by the MME).

12 CSFB and SMSoSGs

Tunnelling of NAS messages The tunnelling of NAS messages procedure is used to encapsulate the NAS messages exchanged between the UE and the VLR. This procedure can be used by either the VLR or the MME depending on the direction of the NAS message.

Uplink unitdata procedure When the MME receives an NAS EMM UPLINK NAS TRANSPORT message [5, 3GPP 24.301] from a UE, the MME shall copy the value part of the NAS message container IE to the value part of the NAS message container IE of the SGsAP-UPLINK-UNITDATA message and send the SGsAPUPLINK-UNITDATA message to the VLR. NAS message container: SMS

Uplink NAS transport

MSC

MME

server

UPLINK-UNITDATA NAS message container: SMS

Downlink NAS transport

DOWNLINK-UNITDATA

RELEASE-REQUEST

Figure 2-27 Tunnelling of NAS (SMS) messages In order to permit the VLR to create an accurate charging record, the MME shall add the IMEISV, the UE Time Zone, the Mobile Station Classmark 2, and the UEâ&#x20AC;&#x2122;s current TAI and E-CGI to the SGsAP-UPLINK-UNITDATA message.

Downlink unitdata procedure When the VLR needs to send a NAS message to the UE, the VLR shall first verify whether or not it has an SGs association for the UE. If the state of the SGs association for the UE is SGs-ASSOCIATED and LA-UPDATEPRESENT, then the VLR continues with the procedure. The VLR shall build and encapsulate the NAS message into the value part of the NAS message container IE of an SGsAP-DOWNLINK-UNITDATA message and send the SGsAP-DOWNLINK-UNITDATA message to the MME.

Signalling in EPC / LTE

Upon reception of an SGsAP-DOWNLINK-UNITDATA message, the MME shall copy the value part of the NAS message container IE to the value part of the NAS message container IE of a NAS ESM DOWNLINK TRANSPORT.

Release procedure When the VLR determines that there are no more NAS messages to be exchanged between the VLR and the UE, or when a further exchange of NAS messages for the specified UE is not possible due to an error, the VLR shall send the SGsAP-RELEASE-REQUEST message to the MME, including the IMSI of the UE for which there are no more NAS messages to be tunnelled. For the SMS transport, the VLR can send the SGsAP-RELEASE-REQUEST message when the SMS transaction is complete (reception of a CP-ACK message for the MO case, sending of a CP-ACK message for the MT case).

Failure procedure VLR failure procedure This procedure is used by the VLR to inform the MMEs with an SGs association about the recovery from an internal failure that has affected the SGs association with the MMEs when the VLR fails with restart. This procedure is also used by the MMEs with an SGs association to a failed VLR in the VLR pool to perform the restoration for CS services for the affected UEs when the VLR fails without restart or fails for a long duration. The VLR recovery procedure is handled in such a way that the signalling load on the VLR and MMEs does not create any overload problem. reset

MME

MSC server

RESET-INDICATION RESET-ACK Detach request (Detach type: IMSI detach)

LOCATION-UPDATE-REQUEST

Figure 2-28 VLR failure procedure When the VLR restarts, the VLR shall send an SGsAP-RESETINDICATION message to all the MMEs connected to the VLR by the SGs interface. 62

12 CSFB and SMSoSGs

Upon receipt of an SGsAP-RESET-INDICATION message from the VLR, the MME is informed that all the SGs associations with that VLR for all the UEs registered in the MME are no longer reliable because the VLR has lost information about the state of the UEs and during the failure the VLR might have missed signalling messages. The MME shall set the "VLR-Reliable" MM context variable to "false" and sends an SGsAP-RESET-ACK message to the VLR. If the "VLR-Reliable" MM context variable is set to "false", upon reception of a combined TAU update request or a periodic TAU request from the UE that is attached for non-EPS service, the MME may requests the UE to re-attach for non-EPS services [5, 3GPP 24.301], or may alternatively immediately perform the location update for non-EPS services procedure towards the VLR.

VLR fails without restart If the MME detects that the VLR serving the UE is no longer in service and the MME supports restoration for CS services via an alternative VLR [12, 3GPP 23.007], upon reception of a combined TAU request or a periodic TAU request from the UE that is attached to the VLR not in service, the MME may: â&#x20AC;˘ request the UE to re-attach for non-EPS services and then select an alternative VLR that is in service for the UE during the subsequent combined TAU procedure, before performing the location update for non-EPS services procedure towards the selected VLR; or â&#x20AC;˘ select an alternative VLR that is in service for the UE and immediately perform the location update for non-EPS services procedure towards the selected VLR.

VLR not in service

MME

UE SGs-ASSOCIATED

MSC server

MSC

Detach request

server

(Detach type: IMSI detach)

TAU request

LOCATION-UPDATE-REQUEST

UE SGs-ASSOCIATED

Figure 2-29 VLR fails without restart

Signalling in EPC / LTE

MME failure procedure In the event of a failure at the MME which has resulted in the loss of the SGs association information on some UEs, the MME may send an SGsAPRESET-INDICATION message to all the VLRs connected to the MME by SGs interfaces4. failure

MSC

MME RESET-INDICATION

server

RESET-ACK

Figure 2-30 MME failure Upon receipt of an SGsAP-RESET-INDICATION message from the MME, the VLR is informed that all the SGs associations with that MME for all the UEs registered in the MME are no longer reliable because the MME has lost information about the state of the UEs for that VLR and during the failure the MME might have missed signalling messages. The VLR shall either: â&#x20AC;˘ set the "Confirmed by Radio Contact" restoration indicator to "false" in all the SGs associations containing the restarted MME and set the state of all the SGs associations containing the restarted MME to the SGsNULL state; or â&#x20AC;˘ keep the 'Confirmed by Radio Contact' restoration indication and the state of all the SGs associations containing the restarted MME unchanged. The option to not set the 'Confirmed by Radio Contact' restoration indicator to 'false' in all the associations containing the restarted MME reduces subsequent paging signalling the VLR can initiate by avoiding a complete search of the UE on the entire VLR area. The VLR shall then send an SGsAP-RESET-ACK message to the MME. If the "Confirmed by Radio Contact" restoration indicator is "false" the VLR may send paging messages on both the SGs and the A/Iu interface.

4 The option to not send any SGsAP-RESET-INDICATION message to all the VLRs connected to the MME by SGs interfaces reduces subsequent paging signalling initiated by VLRs by avoiding a complete search of the UE on the entire VLR area.

12 CSFB and SMSoSGs

HSS failure In the case of an HSS failure, the HSS informs the associated MMEs about the recovery from an internal failure that has affected the SGs association with the MMEs [14, 3GPP 29.272]. This information is used in the MME to trigger the VLR to perform a location update towards the HSS in order to restore the HSS subscriber data. ISR failure

SGSN MME

HSS RSR

NEAF

RSA

Alert MME Notification Alert MME Ack. UE activity

MSC

UE Activity Notification

server

UE Activity Ack. UE activity

UE-ACTIVITY-INDICATION

Figure 2-31 HSS failure Upon receipt of a HSS reset indication from the HSS, the MME shall set the Non-EPS Alert Flag (NEAF) for all registered UEs in the MME for which a valid SGs association with a VLR exists. Upon detection of any signalling activity from the UE, the MME shall report to the VLR if the NEAF is set for this UE. If the MME detects signalling that leads to a procedure towards the VLR, the MME shall follow this procedure and reset the NEAF. If the MME detects activity that does not lead to any procedure towards the VLR, the MME shall send an SGsAP-UE-ACTIVITYINDICATION message towards the VLR and reset the NEAF.

CSFB procedures Since the functionality of CSFB spans across multiple interfaces and interacts with a wide variety of other procedures, this section presents overview of same selected cases of the end-to-end CSFB procedures.

Signalling in EPC / LTE

Mobile Originating call [2, 3GPP 23.272] The procedure for MO call with PS HO support is illustrated in Fig. 2-32. MME

BSC/RNC

MSC

SGSN

S/P-GW

❶ Extended Service Request ❷ CSFB Ind. ❸ Meas. Rep. ❹ PS HO preparation and start of the execution phase (CSFB ind.) ❺ Location Area Update (if new cell’s LAI different from stored in the UE) ❻ CM Service Request (CSMO flag) ❼ CS call establishment ❽ PS HO (continuation of execution phase)

Figure 2-32 MO CSFB call with PS HO support ❶ The UE sends an NAS EMM EXTENDED SERVICE REQUEST for MO CSFB to MME. The UE only transmits this request if it is attached to CS domain (with a combined EPS/IMSI Attach) and can not initiate an IMS voice session (because e.g. the UE is not IMS registered or IMS voice services are not supported by the serving IP-CAN, home PLMN or UE). ❷ The MME sends an S1AP INITIAL UE CONTEXT REQUEST or S1AP UE CONTEXT MODIFICATION REQUEST message to eNB. The type of the message depends on whether the procedure is started for the UE in EMM-IDLE or EMM-CONNECTED state. The send message includes CSFB Indicator and LAI which indicates to the eNB that the UE should be moved to UTRAN/GERAN. The eNB shall reply with S1AP INITIAL UE CONTEXT RESPONSE or S1AP UE CONTEXT MODIFICATION RESPONSE message. ❸ The eNB may optionally solicit a measurement report from the UE to determine the target GERAN/UTRAN cell to which the PS HO or redirection will be performed. ❹ When the eNB knows that both the UE and the network support PS HO, the eNB triggers PS HO to a GERAN/UTRAN neighbour cell by sending an S1AP HANDOVER REQUIRED message to the MME5. In the following an inter-RAT HO from E-UTRAN to UTRAN or GERAN as described in 5 Even

in case both the UE and the network support PS HO, the eNB may choose to use a different inter-RAT mobility procedure.

12 CSFB and SMSoSGs

Chapter “S1-MME” begins. The eNB indicates in the Source RNC to Target RNC Transparent container that PS HO was triggered due to CSFB. As part of this HO, the UE receives an RRC MOBILITY FROM EUTRA COMMAND and tries to connect to a cell in the target RAT. ❺ If the LA of the new cell is different from the one stored in the UE (which is received as part of Combined Attach/TAU procedure in E-UTRAN), the UE shall initiate a LAU procedure. When the UE initiates a LAU the UE shall set the "follow-on request" flag in the LAU Request in order to indicate to the MSC not to release the Iu/A connection after the LAU procedure completion. The UE shall indicate to the target MSC that this is an originating call establishment as a result of CSFB by including the "CSMO" flag. Regardless of the NMO, the UE shall initiate a separate Location Area Update before initiating the RAU procedure instead of a Combined RA/LA Update procedure (to speed up the CSFB procedure). The RAU or Combined RA/LA Update will be performed later in step ❽. ❻ The UE sends a NAS MM CM SERVICE REQUEST to the MSC. The UE shall indicate to the MSC that this is an MO call establishment as a result of CSFB by including the "CSMO" flag. ❼ The MSC initiates the CS call establishment procedure. ❽ The UE performs any remaining steps of the inter-RAT handover from E-UTRAN to UTRAN or GERAN, e.g. RAU, if required. If the UE remains on UTRAN/GERAN after the CS voice call is terminated the UE performs normal mobility management procedures. The procedure for MO call without PS HO support is illustrated in Fig. 2-33. BSC/RNC

MME

MSC

SGSN

S-GW

P-GW

❶ Extended Service Request ❷ CSFB Ind. ❸ Meas. Rep. ❹ CCO / Rel. ❺ S1 UE context release ❻ Location Area Update (if new cell’s LAI different from stored in the UE) ❼ Suspend Suspend Notification / Acknowledge ❽ Delete Bearer Cmd. / Req. / Rsp. (GBR bearers) Suspend Notification / Suspend Acknowledgement (non-GBR) ❾ CM Service Request (CSMO flag) CS call establishment

Figure 2-33 MO CSFB call without PS HO support (no ISR)

Signalling in EPC / LTE

Steps ❶, ❷ and ❸ are the same as in case with PS HO support. ❹ If the UE and network support inter-RAT Cell Change Order (CCO) to GERAN and the target cell is GERAN, the eNB can trigger an inter-RAT CCO (optionally with NACC) to a GERAN neighbour cell by sending an RRC MOBILITY FROM EUTRA COMMAND. The inter-RAT CCO may contain a CSFB Indicator which indicates to UE that the CCO is triggered due to a CSFB request. If the UE or the network does not support inter-RAT PS HO from E-UTRAN to GERAN/UTRAN nor inter-RAT CCO order to GERAN or the network does not wish to use these procedures, the eNB can trigger RRC CONNECTION RELEASE with redirection to GERAN or UTRAN. If the UE and network support RRC connection release with redirection and Multi Cell System Information to GERAN/UTRAN, the eNB can trigger RRC CONNECTION RELEASE with redirection to GERAN or UTRAN and include one or more physical cell identities and their associated System Information. ❺ The eNB sends an S1AP UE CONTEXT RELEASE REQUEST message to the MME. If the target cell is GERAN and either the target cell or the UE does not support DTM the message includes an indication that the UE is not available for the PS service. The MME releases the UE Context in the eNB as well as all eNB related information in the S-GW. ❻ In case of inter-RAT CCO, the UE moves to the new cell in GERAN. The UE uses the NACC information and/or the broadcast System Information and when it has all of the necessary information to access the GERAN cell, establishes a radio signalling connection. In case of RRC release with redirection, the UE moves to the target RAT, identifies a suitable cell, receives the broadcast System Information and when it has the necessary information to access GERAN/UTRAN, establishes a radio signalling connection. In case of RRC connection release with redirection and Multi Cell System Information, the UE moves to the target RAT and identifies a suitable cell. The UE uses the Multi Cell System Information and/or the broadcast System Information and when it has all of the necessary information to access GERAN/UTRAN, the UE establishes the radio signalling connection. If target RAT is GERAN, after the establishment of the main signalling link, the UE enters either Dual Transfer Mode or Dedicated Mode. If the LA of the new cell is different from the one stored in the UE, the UE shall initiate a LAU regardless of the different NMO. The UE shall set the

12 CSFB and SMSoSGs

"follow-on request" flag in the LAU Request in order to indicate to the MSC not to release the Iu/A connection after the LAU procedure is complete. The UE shall indicate to the target MSC that this is an MO call establishment as a result of CSFB by including the CSMO flag. Further the UE performs any RAU. ❼ If the target RAT is GERAN and DTM is not supported or the UE does not support DTM, the UE starts the Suspend procedure [3, 3GPP 23.060]. This triggers the SGSN to send a GTP SUSPEND NOTIFICATION (TLLI, RAI) message to the old CN node identified by the RAI and TLLI. If ISR is not active, the RAI and TLLI refer to an MME. The MME returns an GTP SUSPEND ACKNOWLEDGE to the SGSN even though the GUTI cannot be derived from the P-TMSI and RAI pair. If ISR is active, the RAI and TLLI refer to the old S4-SGSN. In this case, if the serving SGSN is different from the old SGSN which has ISR association with MME, the old SGSN returns a GTP SUSPEND ACKNOWLEDGE to the serving SGSN. ❽If the S1-AP UE Context Release Request message, received from the eNB in step ❺, indicates that the UE is not available for the PS service in the target cell, the MME deactivates GBR bearers towards S-GW and P-GW(s) by initiating MME-initiated Dedicated Bearer Deactivation procedure [15, 3GPP 23.401], and starts the preservation and suspension of non-GBR bearers by sending a GTP SUSPEND NOTIFICATION message to the S-GW6. The S-GW sends GTP-C SUSPEND NOTIFICATION message to the PGW(s) when it receives the GTP SUSPEND NOTIFICATION from MME or S4-SGSN. If the S-GW receives two GTP SUSPEND NOTIFICATION messages for the same UE, it ignores the second one except for sending response. The MME stores in the UE context that UE is suspended status. If ISR is active, the (old) S4-SGSN stores in the UE context that UE is in suspended status. All the preserved non-GBR bearers are marked as suspended status in the S-GW and P-GW(s). The P-GW should discard packets if received for the suspended UE. ❾ The UE continues with the MO call setup procedure with sending NAS MM CM SERVICE REQUEST. The UE shall indicate to the MSC that this is an originating call establishment as a result of CSFB by including the "CSMO" flag. ❿ After the CS voice call is terminated and if the UE is in GERAN and PS services are suspended, then the UE shall resume PS services.

step can not be triggered by the Suspend procedure from step ❼since the full GUTI can not be derived from the P-TMSI and RAI included in the GTP-C SUSPEND NOTIFICATION message.

6 This

Signalling in EPC / LTE

Mobile Terminating call [2, 3GPP 23.272] The procedure for MT CSFB call in EMM-IDLE mode is illustrated in Fig. 2-34. MME

MSC

❷ SRI

PRN

❻ Paging (CS)

❺ Paging (CS)

❼ Extended Service Request

❹ Paging Req.

GMSC

HSS

❶ IAM

❸ IAM

❽ Service Req.

❾ Initial Ctx. Setup (CSFB) ❿ Information flow may continue as described in MO call section

Figure 2-34 MT call in EMM-IDLE state ❶ GMSC receives IAM. ❷ GMSC retrieves routing information of the terminating UE by MAP SEND ROUTING INFO (SRI) operation. ❸ GMSC sends IAM to the MSC on the terminating side. ❹ The MME receives a SGsAP-PAGING-REQUEST (IMSI, TMSI, LAI, CS call indicator, CLI) message from the MSC over a SGs interface. IMSI is used by the MME to find the S-TMSI. If the TMSI and the LAI are received from the MSC/VLR, the S-TMSI is used as the paging address on the radio interface. If the TMSI is not received from the MSC/VLR, the IMSI shall be used as the paging address on the radio interface. If location information is reliably known by MME (i.e. MME stores the list of TAs), the MME shall page the UE in all the TAs. If the MME does not have a stored TA list for the UE, the MME may use the location information received from the MSC to page the UE7. If the MME receives a Paging Request message for an UE which is considered as detach for EPS services, the MME sends the SGsAP-PAGINGREJECT message to the MSC with an appropriate cause value. This rejection triggers the MSC to page the UE over A or Iu-cs interface.

7 This procedure takes place before step ❸, immediately after MSC receives MAP_PRN from HSS, if pre-paging is deployed.

12 CSFB and SMSoSGs

❺ If the MME did not return an "SMS-only" indication to the UE during Attach or Combined TA/LA Update procedures, the MME sends a S1AP PAGING message to each eNB. The Paging message includes a suitable UE Identity (i.e. S-TMSI or IMSI) and a CN Domain Indicator that indicates which domain (CS or PS) initiated the paging message. In this case it shall be set to "CS" by the MME. ❻ The radio resource part of the paging procedure takes place. ❼ The UE establishes an RRC connection and sends an NAS EMM EXTENDED SERVICE REQUEST for MT CSFB to MME. ❽ The MME sends the SGsAP-SERVICE-REQUEST message to the MSC containing an indication that the UE was in idle mode (and hence, for example, that the UE has not received any CLI). In order to avoid the calling party experiencing a potentially long period of silence, the MSC may use the the idle mode indication as a trigger to inform the calling party that the call is progressing. ❾ MME sends S1AP INITIAL UE CONTEXT SETUP containing CSFB Indicator, to indicate the eNB to move the UE to UTRAN/GERAN. The eNB shall reply with S1AP INITIAL UE CONTEXT SETUP RESPONSE message. ❿ The information flow may continue as described in MO call section.

MT Roaming Retry Call [2, 3GPP 23.272] MT Roaming Retry Call applies to a MT call while the called mobile is simultaneously moving from an old to a new MSC, if the GMSC, the HLR and the old terminating VMSC support the MT Roaming Retry procedure. In that case, upon receipt of an ISUP IAM message which was proceeded by a MAP Cancel Location procedure, the old VMSC instructs the GMSC to resume terminating call procedure by sending a MAP Resume Call Handling (RCH) message. The GMSC then releases the ISUP connection to the old VMSC, terminate any open CAP dialogue, and retry the terminating call setup towards the new MSC by sending an additional SRI to the HLR. This second SRI request leads to obtaining a roaming number from the new MSC towards which the call can then be delivered (possibly after new CAMEL interactions). The similar procedure is used for Roaming Retry for CS fallback. There are only two differences in this procedure compared to the Mobile Terminating Roaming Retry Call procedure described earlier. The first difference is that the paging message triggers the CS fallback including a location update in the

Signalling in EPC / LTE

new RAT. This functionality is already supported in the CS fallback flows for terminating calls and no additional functionality is needed. The second difference is that the UE may send a page response message after receiving Location Update Accept message. The new MSC ignores or rejects the page response message. MSC

GMSC

SRI

O C

CA NC L

SRI

IAM

CS PAG

H RC

PR N

IAM

MME

HSS

CS F

P LU

IAM

MSC

Figure 2-35 Roaming Retry for CS fallback

p tu Se

12 CSFB and SMSoSGs

References [1, 3GPP 29.118] 3GPP TS 29.118 V12.0.0 (2013-03); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Mobility Management Entity (MME) â&#x20AC;&#x201C; Visitor Location Register (VLR) SGs interface specification [2, 3GPP 23.272] 3GPP TS 23.272 V11.4.0 (2013-03); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Circuit Switched (CS) fallback in Evolved Packet System (EPS); Stage 2 [3, 3GPP 23.060] 3GPP TS 23.060 V12.0.0 (2013-03); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; General Packet Radio Service (GPRS); Service description; Stage 2 [4, 3GPP 23.236] 3GPP TS 23.236 V11.0.0 (2012-09); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Intra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodes [5, 3GPP 24.301] 3GPP TS 24.301 V12.0.0 (2013-03); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 [6, 3GPP 29.274] 3GPP TS 29.274 V12.0.0 (2013-03); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3 [7, 3GPP 24.081] 3GPP TS 24.081 V11.0.0 (2012-09); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Line Identification supplementary services; Stage 3 [8, 3GPP 24.010] 3GPP TS 24.010 V11.0.0 (2012-09); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Mobile radio interface layer 3; Supplementary services specification; General aspects

Signalling in EPC / LTE

[9, 3GPP 29.002] 3GPP TS 29.002 V12.0.0 (2013-03); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Mobile Application Part (MAP) specification [10, 3GPP 24.030] 3GPP TS 24.030 V11.0.0 (2012-09); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Location Services (LCS); Supplementary service operations; Stage 3 [11, 3GPP 24.008] 3GPP TS 24.008 V12.0.0 (2012-12); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 [12, 3GPP 23.007] 3GPP TS 23.007 V12.0.0 (2013-03); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Restoration procedures [13, 3GPP 24.082] 3GPP TS 24.082 V11.0.0 (2012-09); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Call Forwarding (CF) supplementary services; Stage 3 [14, 3GPP 29.272] 3GPP TS 29.272 V12.0.0 (2013-03); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Core Network and Terminals; Evolved Packet System (EPS); Mobility Management Entity (MME) and Serving GPRS Support Node (SGSN) related interfaces based on Diameter protocol [15, 3GPP 23.401] 3GPP TS 23.401 V12.0.0 (2013-03); Technical Specification; 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access