CS domain services are the services that can be offered today in GSM-UMTS networks. Examples of such services are: voice and its supplementary services (e.g. call waiting, call forwarding), USSD, LCS, SMS, E911, LI, and even CS DUI video, etc. This rich set of CS domain features and capabilities are the result of years of standardization works in 3GPP and operators investments to their GSM-UMTS network.
In EPS, richer features/services can be offered to the end-user together with voice via IMS. While this is the case for EPS, it is challenging for some operators to launch EPS with data and voice/IMS from day one. Hence, these operators need a migration path to allow them to start from EPS with data only and allow the reuse of CS domain services until they get to the point where IMS voice can be added to the EPS.
Such migration path is possible with CS Fallback (CSFB) feature. CSFB is introduced in 3GPP Rel-8 to allow an UE in EPS to reuse CS domain services by defining how the UE can switch its radio from EUTRAN access to other RAT (e.g. GERAN/UTRAN/1xRTT access) that can support CS domain services. In addition, CSFB specification TS 23.272 also defines how the SMS is transferred to the UE natively via EPS from the MSC. It should be noted that this type of SMS delivery mechanism is defined in CSFB specification but the UE is not falling back to GERAN/UTRAN/1xRTT access.
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). This is based on network configuration and deployment option. For 1xRTT CSFB feature, CSFB can be done with RRC connection release with redirection information or 1xSRVCC based signaling (known as enhanced 1xCSFB). 1xRTT CSFB UE may also have dual-Rx/dual-Tx or Dual-Rx/Single-Tx capability. Dual-Rx/dual-Tx 1xRTT CSFB UE can simultaneously transmit and receive on both EPS and 1x at the same time. This allows the UE to obtain 1x voice service from 1xRTT system while maintaining the data stream over EPS at the same time. This is also based on network configuration and deployment option, and UE capability. Dual-Rx/Single-Tx 1xRTT CSFB UE allows simplification in EPS network deployment because there is no coordination is required between the E UTRAN and 1xRTT network (i.e. S102 is not required).
After the UE is redirected to GERAN/UTRAN/1xRTT access via one of the above procedures, the existing CS setup procedure is taken over for the remaining of the call.
In Rel-9, IDLE mode camping mechanism is enhanced in the EPS and GPRS to allow the network to influence the UE’s RAT camping policy so that a CSFB UE will select GERAN/UTRAN access when it is in IDLE condition. The intention is to minimize the occurrence of CSFB procedure from EPS to allow the UE to invoke the CS domain services directly from GERAN/UTRAN as much as possible. On the other hand, this requires additional intelligence in the cell reselection policy in the GERAN/UTRAN access in order to move the UE in active state to EPS to enjoy the fast PS access when appropriate. There are also optimization enhancements to Rel-9 for speeding up the overall CSFB procedure.
As indicated earlier, SMS delivery via CS Domain is also defined as part of the CSFB feature. UE can utilize this feature after it has successfully attached itself to the CS domain. It should be noted that EPS has the option to support only the SMS feature and not the CSFB feature which redirect the UE to another RAT. For GERAN/UTRAN CSFB, MME can indicate this condition by having an SMS-only indicator to the UE during their combined EPS/IMSI Attach/TAU procedure. For 1xRTT CSFB, this indication is not specified, as the 1xCS registration procedure is transparent to the EPS. UE receiving the “SMS-only” indicator will not invoke the CSFB request and should not expect any CS paging coming from EPS.
When interworking with a 3GPP MSC, SMS is delivered via the SGs interface. For MO-SMS, UE first establishes a NAS tunnel to transfer the SMS PDU to MME. MME then transfer these SMS PDU over to MSC via the SGs. MT-SMS works the same way by having the MME establish a NAS tunnel to UE over E-UTRAN access.
When interworking with 1xMSC, the UE establishes a CDMA2000 tunnel with the 1xCS IWS via EPS and SMS is delivered via that tunnel. EPS is transparent to this process.
3GPP also defines the CSFB UE in voice-centric and data-centric mode of operation in TS 23.221. Voicecentric CSFB UE will always attempt to find a RAT where voice services can be supported. In the example of UE receiving an SMS-only or “CSFB Not Preferred” indication from the network during combined EPS/IMSI attach procedure, the voice-centric UE will autonomously switch to UTRAN/GERAN access if coverage is available so voice service is possible to this user. With a data-centric mode of operation, the CSFB UE will not switch to UTRAN/GERAN given the same scenario with the SMS-only indication from the network and will forgo the voice services or CS domain services altogether. This is because the data-centric mode UE wants the best possible PS access and voice is not the determining factor to move away from EPS.
Article from "4G Broadband Evolution: 3GPP Release 10 and beyond"