This document describes CSFB (Circuit Switch Fallback), which allows LTE users to fallback to 2G/3G networks to make voice calls or SMS when out of LTE coverage. It outlines the network architecture and call flows for CSFB, including mobile terminating calls, SMS-MO, and SMS-MT. Key interfaces involved are LTE-Uu, S1-MME, Iu-CS, SGs. CSFB supports fallback to UTRAN or GERAN networks for circuit switched services when the UE is in E-UTRAN but not able to receive CS services over the LTE network.

1. CSFB (CIRCUIT switch Fall Back)

2. CSFB Network Architecture
UE
LTE -Uu
E -UTRAN MME
S1 -MME
UTRAN
GERAN
Um
Uu
MSC
Server
Iu -CS
A
SGs
UE
LTE -Uu
E -UTRAN MME
S1 -MME
UTRAN
GERAN
Um
Uu
MSC
Server
Iu -CS
A
SGs

3. CS Call Request in E-UTRAN, Call in
GERAN/UTRAN

4. Mobile Terminating call in Active Mode
- PS HO supported

5. UE eNodeB MME MSC SMS-IWMSC SC
1. Combined EPS/IMSI attach procedureand locationupdate for non-EPS
services procedure
2. UE triggeredService Request ( when UE is in idlemode)
3. UplinkNAS Transport
5. SGsAP UplinkUnidata(CPDATA)
4. UplinkNAS Transport
6. ForwardShortMessage
7. Message transfer
8. Deliveryreport
9. Deliveryreport
10. SGsAP DownlinkUnidata
11. DownlinkNAS Transport
12. Downlink NAS Transport
5a. SGsAP DownlinkUnidata (CPACK)
5a. Downlink NAS Transport
5a. Downlink NAS Transport
13. UplinkNAS Transport
14. UplinkNAS Unidata
16. Release Request
15. SGsAP UplinkUnidata(CPACK)
• SMS over SGs is
independent from CS
Fallback; meaning it
does not trigger CS
• Fallback to UTRAN or
GERAN. As no fallback
takes place, SMS over
SGs does not require
overlapped coverage
of LTE and legacy
technologies.
CSFB SMS-MO

6. CSFB SMS MT
UE eNodeB MME MSC SMS-IWMSC SC
11. SGsAP Uplink Unidata
3. SendRoutingInfoForSMS
4. ForwardShortMessage
2. Message transfer
13. Delivery report
6. Paging
9a. SGsAP Downlink Unidata (CPDATA)
Downlink NAS Transport
HLR
5. SGsAP Paging Request (SMS Indicator)
7. Paging
8. UE triggered Service Request procedure
8a. SGsAP Service Request (SMS Indicator)
10. Uplink NAS Transport
12. Delivery report
1. Combined EPS/IMSI attach procedure and location update for non-EPS
services procedure
9b. Downlink NAS Transport
9c. Uplink NAS Transport
9d. SGsAP Uplink Unidata (CPACK)
15. Downlink NAS Transport
14. SGsAP Downlink Unidata (CPACK)
16. Release Request

7. Reference(s) -:
3GPP TS 29.118 MME – Visitor Location Register (VLR) SGs Interface Specification
3GPP TS 24.301 Non-Access-Stratum (NAS) Protocol for Evolved Packet System
(EPS)
3GPP TS 23.018 Basic Call Handling
3GPP TS 23.002 LTE; Network architecture
3GPP TS 23.272 Circuit Switched Fallback in Evolved Packet System