DL - 4G Americas

Figure B.4. 1xRTT CSFB Architecture in EPS (Based on 3GPP TS 23.272).
With CSFB, UE under EPS can enjoy the fast PS data access and can switch over to
GERAN/UTRAN/1xRTT access for CS domain services when needed. In addition, UE can also utilize the
SMS feature supported by CSFB architecture.
UE, which wants to use CSFB, must first register itself to the CS domain via EPS. For GSM-UMTS CSFB
feature, UE performs a combined EPS/IMSI Attach/TAU procedure. In the EPS Attach/TAU response
message, the network indicates back to the UE whether CSFB (including SMS) is supported, “SMS-only”,
“CSFB Not Preferred”, or none of these features are supported. “CSFB Not Preferred” is an indication to
allow data centric devices to continue reside in EPS and to allow CSFB (including SMS) features to be
used. On the other hand, a voice centric device receiving “CSFB Not Preferred” or “SMS-only” will
assume CSFB is not supported in this network and will try to reselect to other networks (i.e. 2G or 3G) to
obtain voice services. In 1xRTT CSFB features, the UE is aware that the network supports 1xCSFB by
examining the system information broadcast information over E-UTRAN access and performs the 1xCS
registration to the 1xRTT MSC via the CDMA2000 signaling tunnel between the UE (via EPS) and 1xCS
IWS. This 1xCS registration request and response is transparent to the EPS.
After the UE has successfully registered itself to the CS domain (and has received positive response from
MME that CSFB is possible in GERAN/UTRAN case), it can then request the MME to perform CSFB
procedures whenever it wants to use CS domain services (e.g. originating a voice call or answer to a
terminating voice call). Besides voice call, USSD, MO-LR, MT-LR, NI-LR, and call-independent
Supplementary Services procedures (e.g. activates CFB) can also trigger CSFB procedures. In the CS
terminating scenario, an active UE has the ability to reject terminating call request while it still resides in
EPS. This is particularly useful when the end-user is watching a streaming video under EPS and does not
want to answer a call from an unknown number to avoid any streaming disruption in the streaming video
due to unwanted CSFB procedures.
For the GSM-UMTS CSFB feature, EPS can perform the CSFB procedure with PS handover procedure,
RRC connection release with redirection information, or cell change order with NACC (for GERAN only).
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