190937694 csfb-call-flows

CSFB Call Flows From: Network Planning
Unit: Unit name Ver.:1.0
Owner: Voice Services
Page 2 of 12 Release Date: 7/11/2010
Table of contents
1 Introduction..............................................................................................................................3
2 Network Architecture for CSFB ............................................................................................3
3 SMS over SGs & CSFB Call flows......................................................................................4
3.1 Attach Procedure....................................................................................................................4
3.2 Mobile originating SMS ..........................................................................................................5
3.3 Mobile Terminating SMS ........................................................................................................7
3.4 Mobile originating call.............................................................................................................8
3.5 Mobile Terminating Call..........................................................................................................9
4 References.............................................................................................................................11
5 Abbreviations.........................................................................................................................11

1 Introduction
The CS FallBack (CSFB) in EPS enables the provisioning of voice and other CS-domain
services (e.g. CS UDI video, LCS, USSD) 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 overlapping with either GERAN coverage or UTRAN coverage.
The MO SMS and MT SMS are signalled over SGs and do not cause any CS Fallback to
GERAN/UTRAN RATs, and consequently does not require any overlapped GERAN/UTRAN
coverage.
2 Network Architecture for CSFB
The legacy 2G/3G network and the LTE network co-exist in mixed networks, residing
between the mobile customer’s User Equipment (UE) and the common core network, where
an MME (Mobility management Entity) is serving users while in LTE access, while for 2G/3G
an SGSN (Serving GPRS Support Node) is serving users when utilizing data services and an
MSC Server (Mobile Switching Center Server) when utilizing voice services. The MSC Server
connects to the carrier’s telephony network. To support CS Fallback signaling and SMS
transfer for LTE devices, the MME connects to the MSC Server.
The architecture in Figure below shows a simplified view of the parallel LTE and 2G/3G
networks.
The interface (SGs) between the MSC Server and the LTE Mobile Management Entity(MME)
enables the user’s device to be both CS and PS registered while on the LTE access network.
This interface also enables the delivery of CS pages via the LTE access, as well as SMS,
without having the device leave LTE.

3 SMS over SGs & CSFB Call flows
3.1 Attach Procedure
The attach procedure for the CS fallback and SMS over SGs in EPS is realized based on the
combined GPRS/IMSI Attach procedure.

1) The UE initiates the attach procedure by the transmission of an Attach Request
message to the MME. The Attach Type parameter indicates that the UE requests a
combined EPS/IMSI attach and informs the network that the UE is capable and
configured to use CS fallback.
2) Security procedures, registration and default bearer establishment as in ordinary EPS
Attach procedure.
3) The MME allocates a default LAI, which is configured on the MME and may take into
account the current TAI and/or E-CGI and whether the IMSI attach is for both CSFB
and SMS, or for SMS only. The MME derives a VLR number based on the allocated
LAI and IMSI. The MME starts the location update procedure towards the new
MSC/VLR upon receipt of the subscriber
data from the HSS in step 2.
4) The MME sends a Location Update Request (new LAI, IMSI, MME name, Location
Update Type) message to the VLR. MME name is the FQDN string.
5) The VLR creates an association with the MME by storing MME address.
6) The VLR performs the normal subscription checks for CS and if all checks are
successful VLR performs Location Updating procedure in CS domain.
7) The VLR responds with Location Update Accept (TMSI) to the MME.
8) The EPS Attach procedure is completed. Attach Accept message includes LAI and
TMSI. The existence of LAI and TMSI indicates successful attach to CS domain.
3.2 MobileoriginatingSMS

1) UE does combined attach. MSC stores the MME name and creates a association with
the MME for the UE.
2) After the EPS/IMSI attach, the UE triggers a service request to initiate a mobile
originated SMS procedure.
3) The SMS is encapsulated in a NAS message and sent to the MME.
4) The MME forwards the SMS to the MSC/VLR that acknowledges the receipt of the
SMS to the UE (step 4a).
5) MSC forwards the SMS to the SMS-IWMSC.
6) SMS-IWMSC forwards the SMS to the SC.
7) SC returns a delivery report which is forwarded back to the UE via MSC & MME
(steps 7-10).
11) The UE acknowledges the receipt of the delivery report to the MSC/VLR, and the
MSC/VLR indicates to the MME that no more NAS messages need to be tunnelled.
(step 11-13)

3.3 MobileTerminatingSMS
1) UE does combined attach. MSC stores the MME name and creates a association with
the MME for the UE.
2) The service center initiates the transfer of the SMS.
3) The HLR is requested for the routing number for SMS services and receives the VLR
number where the UE is registered.
4) SMS is forwarded to the right MSC/VLR where the UE is attached.
5) The MSC/VLR issues a paging to the MME.
6) The MME initiates a paging to each LTE base station with cells in the tracking area(s)
in which the UE is registered.
7) eNB pages the UE.
8) Upon successful paging, the UE sends a SERVICE REQUEST message to the MME
which in turn issues a service request to the MSC/VLR (step 8a).
9) The MSC/VLR builds the SMS and forwards it to the MME which encapsulates the
SMS in a NAS message to the UE (steps 9a,9b,9c,9d).
10) The UE acknowledges the receipt of the SMS to the MME.

11) MME relays the acknowledge from the UE to the MSC/VLR.
12) MSC/VLR issues the delivery report to the SMS-SMSC.
13) The delivery report is forwarded to the service center.
14) The MSC/VLR acknowledges the receipt of the delivery report to the UE via MME
which encapsulates it into a NAS message (step 15).
16) MSC/VLR indicates to the MME that no more NAS messages need to be tunnelled.
3.4 Mobileoriginatingcall

1) The UE which has done combined attach and registered in both MME and MSC
sends an Extended Service Request (CS Fallback Indicator) to MME. CS Fallback
Indicator indicates MME to perform CS Fallback.
2) The MME sends an S1-AP Request message to eNB that includes a CS Fallback
indicator. This message indicates to the eNB that the UE should be moved to
UTRAN/GERAN.
3) The eNB may optionally solicit a measurement report from the UE to determine the
target GERAN/UTRAN cell.
4) The eNB triggers connection release with redirection to UTRAN.
5) The UE establishes CS signalling connection in the target RAT and sends CM
Service Request message. The simultaneous support of packet data bearers
depends on selected RAT and additional features like e.g. DTM.
6) The UE initiates the CS call establishment procedure.
7) The UE may trigger the RA update procedure when the sending of uplink packet data
is possible.
3.5 MobileTerminatingCall

1) The MSC where the SGs association for the customer exists receives an incoming
voice call for the customer after SRI from the HLR.
2) The MSC send a CS Paging (IMSI, VLR TMSI, Location Information, and optional
Caller Line Identification) message to the MME over the SGs interface. The TMSI (or
IMSI) received from the MSC is used by the MME to find the S-TMSI which is used as
the paging address on the radio interface. If the UE is in Idle mode the MME pages
the UE in all the TAs, the UE is registered to.
In active mode the MME reuses the existing connection to relay the CS Page to the
UE.
The eNB forwards the paging message to the UE. The message contains a suitable
UE Identity (i.e. S-TMSI or IMSI) and a CN Domain indicator and Caller Line
Identification if available and needed.
3) The UE establishes an RRC connection or reuses the existing connection to send an
Extended Service Request (CS Fallback Indicator, Reject or Accept) to MME. The
MME sends the SGs Service Request message to the MSC.
14
4) MME sends S1AP Initial UE Context Setup or S1AP Request message to eNB that
includes CSFB indicator. This message indicates to the eNB that the UE should be
moved to UTRAN.
5) The eNB may optionally solicit a measurement report from the UE to determine the
target UTRAN cell.
6) The eNB triggers connection release with redirection to UTRAN.

7) If the UE obtains LA/RA information of the new UTRAN/GERAN cell (e.g. based on
the system information or redirection info) and the LA/RA of the new cell is different
from the one stored in the UE, it performs a Location Area Update or a Combined
RA/LA procedure if the target system operates in NMO I
8) The UE establishes CS signalling connection in the target RAT and sends Paging
Response message. The simultaneous support of packet data bearers depends on
selected RAT and additional features like e.g. DTM.
9) MSC stops the paging response timer and establish the CS connection.
4 References
1) Circuit Switched (CS) fallback in Evolved Packet System (EPS); Stage 2 (3GPP TS
23.272 version 10.4.0 Release 10)
2) Mobility Management Entity (MME) - Visitor Location Register (VLR) SGs interface
specification (3GPP TS 29.118 version 10.9.0 Release 10)
5 Abbreviations
3G 3-rd Generation
3GPP 3rd GenerationPartnershipProject
CS CircuitSwitched
CSFB CircuitSwitchedFallback
EPS EvolvedPacketSystem
E-UTRAN Evolveduniversal terrestrial radioaccessnetwork
eNB e-NodeB
GPRS General PacketRadioService
GERAN GSM EDGE radioaccess network
GSM Global systemformobile communications
HLR Home locationregister
HSS Home SubscriberServer
HTTP HypertextTransferProtocol
IP InternetProtocol
IMSI International mobilesubscriberidentity
LTE Long termevolution
MME Mobilitymanagemententity
MSC Mobile switchingcenter

NAS Non-accessstratum
PDN Packetdata network
P-GW Packetdata networkgateway
PLMN Publiclandmobile network
PS PacketSwitched
RAN Radioaccess network
SC Service center
SGSN ServingGPRSsupportnode
S-GW Servinggateway
SMS Short Message Service
SMS-GMSC GatewayMSC for short message service
SMS-IWMSC InterworkingMSCforshort message service
TMSI TemporaryMobile SubscriberIdentity
UMTS Universal mobile telecommunicationssystem
USIM Universal subscriberidentitymodule
VLR VisitorLocationRegister

190937694 csfb-call-flows

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