Question

How UE latch to the UTRAN in CSFB?Say UE connected to LTE and when UE wants to make an call then it has to move to UTRAN, in this scenario how UE leaves LTE and connects to 3G/2G. can anyone explain in detail with call-flow.

Answer 1

Hi jaganathan,

Find the call flow below:

Step
1
UE <---> SS
< Power On >
Cell 1

2
UE <---> SS
< PRACH and RACH Response >
Cell 1

3
UE ---> SS
RRC Connection Request
Cell 1

4
UE <--- SS
RRC Connection Setup
Cell 1

5
UE ---> SS
RRC Connection Setup Complete + Attach Request + PDN Conn Req
Cell 1

6
UE <---> SS
< NAS : Security Mode Establishment >
Cell 1

7
UE ---> SS
< RRC : Security Mode Establishment >
Cell 1

8
UE <--- SS
RRC Connection Reconfiguration + Attach Accept + ...
Cell 1

9
UE ---> SS
RRCConnectionReconfigurationComplete / Attach Complete / ...
Cell 1

10
UE <---> SS
< Packet Data Transaction >
Cell 1

11
SS

LTE Network Gets CS Notification from WCDMA NW

12
UE <--- SS
CS Service Notification with Paging Identity
Cell 1

13
UE ---> SS
Extended Service Request
Cell 1

14
UE <--- SS
RRC Connection Release with Redirected Carrier Info
Cell 1

15
UE <---> SS
< RRC Connection Setup with cause of terminatingConversationCall >
Cell 2

16
UE ---> SS
Routing Area Update Request
Cell 2

17
UE <---> SS
< Authentication >
Cell 2

18
UE <---> SS
< Voice Call Setup >
Cell 2

19
UE <---> SS
< End Voice Call >
Cell 2