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.
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