1. Ultra-Flash CSFB(SRVCC based CSFB)
SA WG2 Meeting #100  S
November 2013, San Francisco, USA
Ultra-Flash CSFB(SRVCC based CSFB)
Source Company: Huawei, HiSilicon, Bharti, China Unicom, Deutsche Telekom

2. Content Ultra-Flash CSFB introduction Ultra-Flash CSFB Discussion
Summary

3. What is Ultra-Flash CSFB?
MME
LTE
G/U
SGs
MSC
SGSN
MME
Redirection
LTE
G/U
SGs
MSC
SGSN
MME
Redirection
LTE Sv
SRVCC Inter
RAT Handover Sv
SRVCC Inter
RAT Handover
VoLTE
w/ SRVCC
CSFB
SRVCC IRAT HO
SRVCC IRAT HO
SRVCC Session Transfer
SGSN
G/U
MSC
IMS
Ultra-Flash CSFB, SRVCC Session Transfer is not needed as it is a CS call setup procedure rather than VoLTE to 2/3G hand over .
Ultra-Flash CSFB requires UE to support standard SRVCC feature.
Ultra-Flash CSFB
Ultra-Flash CSFB is a solution used SRVCC IRAT handover to accelerate CSFB

4. Ultra Flash CSFB with SRVCC capable UE
Per UE’s SRVCC capability and without a QCI=1 bearer, eNB triggers SRVCC HO during CSFB
MME does not delete a QCI=1 bearer as the SRVCC procedure is due to CSFB and there is no QCI=1 bearer
(1) The MSC does not initiate the Session Transfer procedure and only triggers a normal CS Handover preparation
(2) the MSC sends Disconnect to clear the dummy call instance due to SRVCC procedure

5. Ultra Flash CSFB with SRVCC capable UE - MO
2017/3/22

6. Huawei Analysis
As shown in green lines, the UE start CS call and CC sublayer allocate (TI=0,TI Flag=0) CS voice session.
The MM sublayer find out the UE in E-UTRAN side and indicate EMM sublayer to initiate ESR procedure
The LTE AS layer forward HO CMD to GERAN/UTRAN AS layer
GERAN/UTRAN AS layer access the GERAN/UTRAN and SRVCC handling is triggered
MM sublayer gives an indication to the CC sublayer to establish CS call(TI=0,TI Flag=1)due to SRVCC per step b6.
And derives GERAN/GERAN security contexts for the CS domain from the current EPS security context
Upon receiving step b6, CC sublayer allocates(TI=0, TI Flag=1) for SRVCC and proceed with (TI=0,TI Flag=0) call setup procedure by sending CM Service Request and Setup with TI=0.
MSC send disconnect(TI=0,TI Flag=1) to clear the dummy call object (TI=0, TI Flag = 1) in UE side.

7. Ultra Flash CSFB with L->U SRVCC - MT
Per UE’s SRVCC capability and without a QCI=1 bearer, eNB triggers SRVCC HO during CSFB
MME does not delete a QCI=1 bearer as the SRVCC procedure is due to CSFB and there is no QCI=1 bearer
(1) The MSC does not initiate the Session Transfer procedure and only triggers a normal CS Handover preparation
(2) the MSC sends Disconnect to clear the dummy call instance due to SRVCC procedure
(3)SRVCC MSC send setup(TI=1) to the UE

8. Ultra Flash CSFB with SRVCC capable UE - MT
2017/3/22

9. Huawei Analysis
As shown in green lines, the UE(LTE part) receive CSFB paging call and trigger the CSFB procedure accordingly.
The LTE AS layer forward HO CMD to GERAN/UTRAN AS layer
GERAN/UTRAN AS layer access the GERAN/UTRAN and SRVCC handling is triggered
MM sublayer gives an indication to the CC sublayer to establish CS call with TI =0 due to SRVCC per step b6.
And derives GERAN/GERAN security contexts for the CS domain from the current EPS security context
CC sublayer allocate TI=0 CS session per step b6
MM sublayer sends paging response to MSC and MSC sends setup(TI=1) to the UE as TI=0 is already in CC sublayer.
MSC further sends Disconnect(TI=0) to the UE in order to release the TI=0 CS session

10. Ultra-Flash CSFB VS NW-Initiated CS Services while UE is in IMS VoIP session in 8.4.2.2

11. NW-Initiated CS Services while UE is in IMS VoIP session in 8. 4. 2
NW-Initiated CS Services while UE is in IMS VoIP session in Vs Ultra-Flash CSFB
NW-Initiated CS Services while UE is in IMS VoIP session
Ultra-Flash CSFB 1
MME
Need to delete QCI=1 bearer during SRVCC
Do not delete QCI=1 bearer during SRVCC as there is no QCI=1 bearer. 2
MSC
Initiates Session Transfer to move VoLTE to 2/.3G CS
(1)MSC does not Initiates Session Transfer during SRVCC procedure.
(2) Send Disconnect to clear due to SRVCC HO procedure.
(3) MSC sends Setup message to UE with TI value of the voice call to "001" and TI flag value as in mobile terminated call since UE. 3
eNB
Based on UE measurement reports and UE’s SRVCC capability and QCI = bearer, decides to trigger an SRVCC handover to UTRAN/GERAN.
Based on UE measurement reports and CS Fallback Indicator and UE’s SRVCC capability , decides to trigger an SRVCC handover to UTRAN/GERAN. 4
BSC/RNC
No impact 5 UE
Standard VoLTE/SRVCC UE
Standard CSFB/SRVCC UE

12. Content Ultra-Flash CSFB introduction Ultra-Flash CSFB Discussion
Summary

13. Call Setup Time Comparison(Fall back to GSM)
Ultra-Flash CSFB to GSM
SRVCC/PS HO
Authentication/encryption/UE Capacity identify
CS RAB establishment
UE access to 2G cell
Read SI
Basic call procedure
2G call 2G
Ultra-Flash CSFB
SRVCC
IRAT HO
Per Huawei lab testing, L->G measurement takes about ms.
If DRX based measurement is used, L->G measurement could takes about 200ms i.e. the real measurement time is (800/40)*6=120ms that can be done in DRX 256ms with short active time. R8
R9(Flash)
Ultra-Flash call Ultra-Flash
GSM Call
LTE call LTE,
both MO/MT fall back
Based on Huawei Lab testing

14. Call Setup Time Comparison(Fall back to UMTS)
Ultra-Flash CSFB to UMTS R8
PSHO
Ultra-Flash call Ultra-Flash
UMTS Call
Ultra-Flash CSFB
3G call 3G
Based on Huawei Lab Test
SRVCC/PS HO
Authentication/encryption/UE Capacity identify
CS RAB establishment
UE access to 3G cell
Read SI
Basic call procedure
SRVCC
IRAT HO
PS HO
LTE call LTE,
both MO/MT fall back

15. Why Ultra-Flash CSFB Achieve the Shortest Call Setup ?
SRVCC MSC
MME Sv
SGs 3
BSC/RNC
eNB 1 2 4
Both Sv and SGs interfaces supported, SRVCC IRAT HO was used to accelerate CSFB
Ultra-Flash CSFB Reduce Call Setup Time in Following 4 Steps
CSFB
Ultra-Flash CSFB 1
Cell Selection
Use redirection mechanism to access target 2/3G cell
Use SRVCC or PS IRAT handover to access target cell 2
CS RAB Establishment
Start CS RAB setup procedure during the CS call setup procedure in 2/3G
Skipping the CS RAB setup procedure, as the RAB is pre-allocated during the SRVCC IRAT Handover procedure 3
Authentication / encryption/UE Capacity identify
Authentication/encryption/UE Capacity identify are mandatory
Authentication/encryption/UE Capacity identify were done during SRVCC handover, reduce extra time consumption 4
Signaling transport channel(Only GSM)
SD(stand-alone dedicated control channel)，Shared by 8 users
TCH(Traffic Channel)，Single-user exclusive

16. How Ultra-Flash CSFB Reduce Impact to Current Network
IMS Core
Proxy solution, reduce the impact to CS network
eNodeB
SRVCC MSC
BSC/RNC
VMSC
MME Sv
SGs
EPC
Naturally support VoLTE/SRVCC，without MME/MSC upgrade again
CSFB
Ultra-Flash CSFB 1
MSC
Upgrade MSC or Proxy（additional 1~2s delay）
Proxy solution avoid MSC upgrading（with only about 400ms additional delay） 2
TA/LA mapping
Need strictly planning, otherwise call setup time would be prolonged and call success rate decrease
Don’t need strictly planning, no impact to call setup latency and call success rate 3
Roaming Retry/Roaming Forwarding
All MSC must be upgraded, otherwise it will lead to call failure in case of TA/LA misalignment
Don’t need deploy Roaming Retry/Roaming Forwarding, no impact to call success rate 4
VoLTE/SRVCC evolution
MME/MSC must be upgraded again when deploying VoLTE/SRVCC
Sv interface is ready for VoLTE/SRVCC, avoid MME/MSC upgrading again
Ultra-Flash CSFB additionally requires SRVCC IRAT Handover and has no impact on Terminals and GERAN /UTRAN. However, Ultra-Flash CSFB requires some light software updates in eNB/MME/MSC Server.
Thanks to SRVCC IRAT Handover during CSFB procedure, Ultra-Flash CSFB can provide quicker &easier deployment:
(1)Operator only need to update certain (one in minimum) MSC Servers rather than all the MSC Servers surrounding the LTE coverage.
(2)The network is certainly know about which 2/3G cell is the best target for the handover, which means the strict TA/LA mapping is not needed and TA/LA misalignment is naturally resolved by SRVCC HO
(3)Therefore there is no need of MTRR or MTRF.
CSFB only requires a light SRVCC IRAT Handover rather than a full SRVCC deployment:
(1)Similar to the legacy CSFB, Ultra-Flash CSFB still relies on the legacy 2/3G CS domain to provide voice and has nothing to do with IMS and therefore there is no need of SRVCC Session Transfer procedure.
(2)Ultra-Flash CSFB is a call-setup procedure rather than the VoLTE to 2/3G CS call handover procedure and therefore there is no requirement in terms of voice interruption time i.e. less than 300ms.
Could update some (one at the minimum) MSC Servers rather than all the MSC Servers

17. How Rel-8 CSFB and Ultra-Flash CSFB co-exist
Add software patch on SRVCC feature and no impact on Rel-8 CSFB.
IMS Core
SRVCC MSC
SGs Sv
SGs
MME
MSC
BSC/RNC
eNB
Per b50, during combined Attach/TAU procedure
If UE supports SRVCC, MME selects a MSC/VLR which is both SRVCC and CSFB capable
If UE does not support SRVCC, MME selects a MSC/VLR which is CSFB capable
During the CSFB procedure, based on UE capability
If UE support SRVCC capability, eNB is to perform Ultra-Flash CSFB i.e. executing SRVCC HO
If UE does not support SRVCC capability, eNB is to perform Rel-8 CSFB

18. Content Ultra-Flash CSFB introduction Ultra-Flash CSFB Discussion
Summary

19. Ultra-Flash CSFB Value and Principle
Easy Deployment
3 ~ 5 s
Ultra-Flash
L->U 6 ~ 9 s
L->G s
R8 CSFB
Call Setup Time
Principle 4
MME 2
G/U
MSC
SRVCC UE
MSC
MME
eNB
SRVCC HO 1 3 2
SRVCC HO
Avoid to update RNC/BSC
Fall back GSM and UMTS Supported
LTE
In parallel UE fallback and call in MSC
Shorter than the Traditional CS Call Delay
Reuse SRVCC handover to accelerate CSFB and smooth evolution to VoLTE/SRVCC
Avoid to update 2G/3G Radio
Proposed to add deployment option in Rel-12 that enables faster and more reliable CSFB for UEs and networks supporting SRVCC.

21. VoLTE configured UE support Ultra-Flash CSFB Directly
MME
MSC
SGW
PGW
VoLTE
Ultra-Flash CSFB
IMS PS voice priority, CS Voice secondary is configured in UE
UE decide to use VoLTE or Ultra-Flash CSFB according to MME indication
If IMS has been deployed，MME indicate UE IMS Voice over PS session supported
If IMS has not been deployed，MME indicate IMS Voice over PS session NOT supported
Value：
If IMS has not been deployed, VoLTE/SRVCC UE can use Ultra-Flash CSFB
After IMS has been deployed, UE can use VoLTE directly without reconfiguration