DL - 4G Americas

In the following outline, the functions of various CSFB architectural 144 elements are explored further.
� Mobility Management Entity (for GERAN/UTRAN CSFB)
o Multiple PLMN selection and reselection for the CS domain
o Deriving a VLR number and LAI from the TAI of the current cell and based on the
selected PLMN for CS domain, or using a default VLR number and LAI
o For CS fallback, 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)
o Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE
o Initiating IMSI detach at EPS detach
o Initiating paging procedure towards eNodeB when MSC pages the UE for CS services
o Supporting SMS procedures with UE and MSC via SGs
o Rejecting CS Fallback call request (e.g. due to O&M reasons)
� Mobility Management Entity (for 1xRTT CSFB)
o It serves as a signaling tunneling end point towards the 3GPP2 1xCS IWS via S102
interface for sending/receiving encapsulated 3GPP2 1xCS signaling messages to/from
the UE
o Handling of S102 tunnel redirection in case of MME relocation
o 1xCS-IWS (terminating S102 reference point) selection for CSFB procedures
o Buffering of messages received via S102 for UEs in idle state
� MSC for GERAN/UTRAN
o Maintaining SGs association towards MME for EPS/IMSI attached UE
o Supporting SMS procedures via SGs to EPS
o In order to speed up the potential LAU procedure during CS fallback the MSC may be
configured to lower the frequency of Authentication, TMSI reallocation and Identity check
for UEs that are EPS/IMSI attached via the SGs interface
� MSC for 1xRTT
o Maintaining association towards 1xIWS for 1xRTT attached UE
o Support 1xSRVCC procedure for enhanced 1xCSFB procedure
144 Requirements related to ISR and CSFB interworking is outside the scope of this section and can be found in 3GPP TS 23.272.
www.4gamericas.org February 2011 Page 127