Signalling in EPC/LTE

12 CSFB and SMSoSGs
35
ChapterChapterChapterChapter 11112222
CSFB and SMSCSFB and SMSCSFB and SMSCSFB and SMSoSGsoSGsoSGsoSGs
TopicTopicTopicTopic PagePagePagePage
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
37
IntroductionIntroductionIntroductionIntroduction
[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 operator’s
network.
ArchitectureArchitectureArchitectureArchitecture
The EPC architecture supporting CSFB and SMSoSGs in shown in Fig. 2-1.
Figure 2-1 CSFB and SMSoSGs architecture
MSC
server
UTRAN
E-UTRAN
GERAN SGSN
MME
CS
S3SGs
Gs
S1
A
Iu
Iu
Gb

38
UEUEUEUE
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.
MMEMMEMMEMME
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.
MSCMSCMSCMSC
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
39
EEEE----UTRANUTRANUTRANUTRAN
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.
RANRANRANRAN
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 stackProtocol stackProtocol stackProtocol 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.

40
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
SGsAP
SCTP
IP
L2
L1
SGs
SGsAP
SCTP
IP
L2
L1
MME MSC server /
VLR

12 CSFB and SMSoSGs
41
Figure 2-4 SGsAP messages
TAI list and LAI allocationTAI list and LAI allocationTAI list and LAI allocationTAI 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".
SGsAP-LOCATION-UPDATE-REQUEST
SGsAP-LOCATION-UPDATE-ACCEPT
SGsAP-LOCATION-UPDATE-REJECT
SGsAP-TMSI-REALLOCATION-COMPLETE
SGsAP-EPS-DETACH-INDICATION
SGsAP-EPS-DETACH-ACK
SGsAP-IMSI-DETACH-INDICATION
SGsAP-IMSI-DETACH-ACK
SGsAP-PAGING-REQUEST
SGsAP-PAGING-REJECT
SGsAP-SERVICE-REQUEST
SGsAP-UE-UNREACHABLE
SGsAP-SERVICE-ABORT-REQUEST
SGsAP-DOWNLINK-UNITDATA
SGsAP-UPLINK-UNITDATA
SGsAP-RELEASE-REQUEST
SGsAP-RESET-INDICATION
SGsAP-RESET-ACK
SGsAP-UE-ACTIVITY-INDICATION
SGsAP-ALERT-REQUEST
SGsAP-ALERT-ACK
SGsAP-ALERT-REJECT
SGsAP-MM-INFORMATION-REQUEST
SGsAP-STATUS

42
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 proceduresSGs proceduresSGs proceduresSGs procedures
Location update for nonLocation update for nonLocation update for nonLocation update for non----EPS servicesEPS servicesEPS servicesEPS 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;
MSC
server
MME
SGs
SGs
association
TAI list, LAI
LAI
CSFB
inter-RAT
mobility
TAI list
LAI
no need
for LAU

12 CSFB and SMSoSGs
43
• 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).
Figure 2-6 Location update for non-EPS services procedure (not started)
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.
Attach / TAU Request
(combined EPS/IMSI attach
or combined TA/LA update)
ULR
ULA
(NAM=PS / CS & PS, MME Registered for SMS)Attach / TAU Accept
(combined EPS/IMSI or
combined TA/LA, SMS only,
non broadcast LAI)
Attach / TAU Complete
MME MSC
server
HSS
Attach / TAU Complete
Attach / TAU Accept
(combined EPS/IMSI or
combined TA/LA,
- / SMS only, TMSI, LAI)
ULR / ULA (NAM=CS&PS)
Attach / TAU Request
(combined EPS/IMSI attach or
combined TA/LA update,
- / SMS only, old LAI, TMSI status,
TMSI based NRI container)
MME MSC
server
HSS
LOCATION-UPDATE-REQUEST
Upd. Loc.
LOCATION-UPDATE-ACCEPT
MME name,
IMSI, IMEISV, TMSI status,
new LAI, old LAI, TAI, E-CGI,
TMSI based NRI container,
selected CS domain operator
IMSI, LAI, new TMSI or IMSI

44
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-LOCATION-
UPDATE-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.
1 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
45
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-LOCATION-
UPDATE-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 proceduresDetach proceduresDetach proceduresDetach procedures
Explicit IMSI detach from EPS servicesExplicit IMSI detach from EPS servicesExplicit IMSI detach from EPS servicesExplicit 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.
Figure 2-8 Explicit IMSI detach from EPS services
EPS-DETACH-ACK
Detach Accept
Detach Request
(EPS detach)
MME MSC
server
EPS-DETACH-INDICATION
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
SGSN
ISR
Detach
Notification
(PS detach)
UE remains attached,
implicit detach timer restarted

46
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-EPS-
DETACH-INDICATION message.
Implicit detach timerImplicit detach timerImplicit detach timerImplicit 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.
ExpExpExpExplicit IMSI detach from nonlicit IMSI detach from nonlicit IMSI detach from nonlicit IMSI detach from non----EPS servicesEPS servicesEPS servicesEPS 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.
Figure 2-9 Explicit IMSI detach from non-EPS services
IMSI-DETACH-ACK
Detach Accept
Detach Request
(IMSI detach or combined
EPS/IMSI detach)
MME MSC
server
IMSI-DETACH-INDICATION
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
SGSN
ISR
Detach
Notification
(Complete Detach,
Comb. PS/CS Detach)
UE
detached

12 CSFB and SMSoSGs
47
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-DETACH-
INDICATION 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 nonImplicit IMSI detach from nonImplicit IMSI detach from nonImplicit IMSI detach from non----EPS servicesEPS servicesEPS servicesEPS 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.
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".
IMSI-DETACH-ACK
MME MSC
server
IMSI-DETACH-INDICATION
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
UE
detached
no contact
UE
detached

48
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 servicesImplicit IMSI detach from EPS servicesImplicit IMSI detach from EPS servicesImplicit 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".
VLR shall perform the procedures described in subclause 5.4.3.
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-EPS-
DETACH-INDICATION message.
Paging for nonPaging for nonPaging for nonPaging for non----EPS services procedureEPS services procedureEPS services procedureEPS 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
49
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].
MME MSC
server
PAGING-REQUEST
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
UE in
EMM-IDLE state
Paging
(UE Paging Identity,
list of TAIs,
CN Domain: CS / PS)
Paging
SGSN
ISRPaging
CS Paging
Indication
Ts5 (2 to 20s)
SERVICE-REQUEST /
Initial UE NAS message

50
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-PAGING-
REQUEST 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].
Figure 2-12 Paging for non-EPS CS services (EMM-CONNECTED)
MME MSC
server
PAGING-REQUEST
VLR name,
IMSI, TMSI,
LAI,
Service indicator: CS call,
Channel needed,
CLI, SS code, LCS indicator, LCS client id.
UE in
EMM-CONNECTED state
CS Service Notification
(Paging identity, CLI, SS Code,
LCS indicator, LCS client identity)
SERVICE-REQUEST

12 CSFB and SMSoSGs
51
Figure 2-13 Paging for non-EPS SMS service (EMM-CONNECTED)
MT CS services delivery via an alternative MME in the MME poolMT CS services delivery via an alternative MME in the MME poolMT CS services delivery via an alternative MME in the MME poolMT 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-UDATE-
REQUEST, SGsAP-RESET-INDICATION or SGsAP-RESET-ACK
messages.
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.
MME MSC
server
PAGING-REQUEST
VLR name,
IMSI, TMSI,
LAI,
Service indicator: SMS,
Channel needed,
UE in
EMM-CONNECTED state
SERVICE-REQUEST
MSC
server
MME
MME name =
mmec01.mmegi0001.mme.epc.
mnc003.mcc260.3gppnetwork.org
MME pool = mmegi0001.mme.epc.mnc003.mcc260.3gppnetwork.org
MME
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

52
Additional paging casesAdditional paging casesAdditional paging casesAdditional paging cases
Paging for CS callPaging for CS callPaging for CS callPaging 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):
• 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;
• 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").
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
MME MSC
server
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
UE
detached

12 CSFB and SMSoSGs
53
the SGs cause IE "UE unreachable". The state of the SGs association does not
change at the MME.
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].
Figure 2-17 Paging for non-EPS service (PPF & ISR & LAI)
MME MSC
server
PAGING-REQUEST
PPF = false
SGSN
no ISR
no contact
mobile
reachable timer
UE-UNREACHABLE
IMSI
SGs cause = UE unreachable
UE SGs-ASSOCIATED
UE attached
UE SGs-ASSOCIATED
CS Paging
Indication
MME MSC
server
PAGING-REQUESTPPF = false
SGSN
ISR
no contact
mobile
reachable timer UE attached
LAI

54
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".
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 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.
Detach Notification
MME MSC
server
PAGING-REQUESTPPF = false
SGSN
ISR
no contact
mobile
reachable timer
UE SGs-ASSOCIATED
UE attached
no LAI
MME MSC
server
PAGING-REQUEST
PAGING-REJECT
IMSI
SGs cause = IMSI unknown
UE
unknown
no CSRI
MME-reset =
false

12 CSFB and SMSoSGs
55
Figure 2-20 Paging for non-EPS service (UE unknown, MME-reset)
Paging for SMSPaging for SMSPaging for SMSPaging 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".
MME is set to "false" and the CS restoration indicator 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 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 VLRHandling in the VLRHandling in the VLRHandling 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].
MME MSC
server
PAGING-REQUESTUE
unknown
IMSI, LAI/-
MME-reset =
true
Paging
(UE Paging Identity,
list of TAIs)
Paging

56
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 procedureService request procedureService request procedureService 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-PAGING-
REQUEST message from the VLR, based on the UE’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 SGsAP-
PAGING-REJECT message to the VLR with the SGs cause information
element indicating "Mobile terminating CS fallback call rejected by the user".
(CN Domain = CS)
(CLI, SS Code, LCS ind., LCS client id.)
MSC
server
PAGING-REQUEST
Service indicator: CS call
PAGING-REJECT
Paging
(MT CSFB call rejected by the user)
Extended Service Request
MME
SGs cause:
MT CSFB call rejected by the user
UE SGs-ASSOCIATED
UE SGs-ASSOCIATED
UDUB

12 CSFB and SMSoSGs
57
If the UE was in EMM-IDLE mode, the MME shall send the SGsAP-
SERVICE-REQUEST message to the VLR when the UE enters EMM-
CONNECTED mode.
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 EMM-
CONNECTED modes.
MSC
server
PAGING-REQUEST
SERVICE-REQUEST
Paging
(Extended) Service Request
MME
UE EMM-IDLE
UE EMM-CONNECTED
MSC
server
PAGING-REQUEST
SERVICE-REQUEST
MME
UE EMM-CONNECTED

58
Figure 2-23 Service request (CSFB call rejected by the user)
Service abortService abortService abortService abort procedureprocedureprocedureprocedure
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.
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 SGsAP-
SERVICE-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.
(Cause = CS service temporarily not available)
Service Reject
MSC
server
PAGING-REQUEST
SERVICE-ABORT-REQUEST
Paging / CS service notification
Extended Service Request
MME
REL

12 CSFB and SMSoSGs
59
NonNonNonNon----EPS alert procedureEPS alert procedureEPS alert procedureEPS 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.
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 SGsAP-
ALERT-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-UE-
ACTIVITY-INDICATION message towards the VLR and reset the NEAF.
ALERT-ACK
UE activity
ALERT-REQUEST
Alert MME Notification
MME MSC
server
NEAF
SGSN
ISR
UE Activity Notification
Alert MME Ack.
UE Activity Ack.
UE activity
UE-ACTIVITY-INDICATION

60
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 procedureMM information procedureMM information procedureMM 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).
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-INFORMATION-
REQUEST 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).
MSC
server
MM-INFORMATION-REQUEST
MME
EMM information
full name for network,
short name for network,
local time zone,
universal time and local time zone,
network daylight saving time

12 CSFB and SMSoSGs
61
TTTTunnelling of NAS messagesunnelling of NAS messagesunnelling of NAS messagesunnelling 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 procedureUplink unitdata procedureUplink unitdata procedureUplink 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 SGsAP-
UPLINK-UNITDATA message to the VLR.
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 UE’s current TAI and E-CGI to the SGsAP-UPLINK-UNITDATA
message.
Downlink unitdata procedureDownlink unitdata procedureDownlink unitdata procedureDownlink 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-UPDATE-
PRESENT, 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.
Uplink NAS transport
MSC
server
DOWNLINK-UNITDATA
MME
UPLINK-UNITDATA
Downlink NAS transport
NAS message container:
SMS
SMS
SMS
SMS
RELEASE-REQUEST

62
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 procedureRelease procedureRelease procedureRelease 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 procedureFailure procedureFailure procedureFailure procedure
VLR failure procedureVLR failure procedureVLR failure procedureVLR 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.
Figure 2-28 VLR failure procedure
When the VLR restarts, the VLR shall send an SGsAP-RESET-
INDICATION message to all the MMEs connected to the VLR by the SGs
interface.
RESET-INDICATION
MSC
server
RESET-ACK
MME
Detach request
reset
(Detach type: IMSI detach)

12 CSFB and SMSoSGs
63
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 restartVLR fails without restartVLR fails without restartVLR 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:
• 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
• 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.
Figure 2-29 VLR fails without restart
MSC
server
MME
Detach request
VLR not in
service
(Detach type: IMSI detach)
UE SGs-ASSOCIATED
MSC
server
TAU request
UE SGs-ASSOCIATED

64
MME failure procedureMME failure procedureMME failure procedureMME 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 SGsAP-
RESET-INDICATION message to all the VLRs connected to the MME by
SGs interfaces4.
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:
• 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 SGs-
NULL state; or
• 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.
MSC
server
MME
failure
RESET-INDICATION
RESET-ACK

12 CSFB and SMSoSGs
65
HSS failureHSS failureHSS failureHSS 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.
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-ACTIVITY-
INDICATION message towards the VLR and reset the NEAF.
CSFBCSFBCSFBCSFB proceduresproceduresproceduresprocedures
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.
UE activity
Alert MME Notification
MME
MSC
server
NEAF
SGSN
ISR
UE Activity Notification
Alert MME Ack.
UE Activity Ack.
UE activity
UE-ACTIVITY-INDICATION
HSS
RSR
RSA
failure

66
Mobile Originating callMobile Originating callMobile Originating callMobile Originating call
[2, 3GPP 23.272] The procedure for MO call with PS HO support is
illustrated in Fig. 2-32.
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.
MME MSCBSC/RNC 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)

12 CSFB and SMSoSGs
67
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.
Figure 2-33 MO CSFB call without PS HO support (no ISR)
MME MSCBSC/RNC SGSN P-GW
❶ Extended Service Request
❷ CSFB Ind.
❸ Meas. Rep.
❺ S1 UE context release
❻ Location Area Update (if new cell’s LAI different from stored in the UE)
❼ Suspend
❹ CCO / Rel.
S-GW
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

68
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
69
"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 P-
GW(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.
6 This 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.

70
Mobile Terminating callMobile Terminating callMobile Terminating callMobile Terminating call
[2, 3GPP 23.272] The procedure for MT CSFB call in EMM-IDLE mode is
illustrated in Fig. 2-34.
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-PAGING-
REJECT 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.
MME MSC
❶ IAM
❿ Information flow may continue as described in MO call section
HSS GMSC
❷ SRI
PRN
❸ IAM
❹ Paging Req.
❺ Paging (CS)
❻ Paging (CS)
❼ Extended Service Request ❽ Service Req.
❾ Initial Ctx.
Setup (CSFB)

12 CSFB and SMSoSGs
71
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 CallMT Roaming Retry CallMT Roaming Retry CallMT 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

72
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.
Figure 2-35 Roaming Retry for CS fallback
Setup
LUP
CSFB
CSPAG
LUP
CANCLO
C
PRN
SRI
SRI
IAM
RCH
IAM
GMSC
MSC
MSC
MMEHSS
IAM

12 CSFB and SMSoSGs
73
ReferencesReferencesReferencesReferences
[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) – Visitor Location Register (VLR) SGs
interface specification
Specification Group Services and System Aspects; Circuit Switched
(CS) fallback in Evolved Packet System (EPS); Stage 2
Specification Group Services and System Aspects; General Packet
Radio Service (GPRS); Service description; Stage 2
Specification Group Services and System Aspects; Intra-domain
connection of Radio Access Network (RAN) nodes to multiple Core
Network (CN) nodes
Specification Group Core Network and Terminals; Non-Access-Stratum
(NAS) protocol for Evolved Packet System (EPS); Stage 3
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
Specification Group Core Network and Terminals; Line Identification
supplementary services; Stage 3
Specification Group Core Network and Terminals; Mobile radio
interface layer 3; Supplementary services specification; General aspects

74
Specification Group Core Network and Terminals; Mobile Application
Part (MAP) specification
Specification Group Core Network and Terminals; Location Services
(LCS); Supplementary service operations; Stage 3
Specification Group Core Network and Terminals; Mobile radio
interface Layer 3 specification; Core network protocols; Stage 3
Specification Group Core Network and Terminals; Restoration
procedures
Specification Group Core Network and Terminals; Call Forwarding
(CF) supplementary services; Stage 3
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
Specification Group Services and System Aspects; General Packet
Radio Service (GPRS) enhancements for Evolved Universal Terrestrial
Radio Access Network (E-UTRAN) access

Signalling in EPC/LTE

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