Support of Voice Services and SMS
The EPC is a packet-only core network. It does not have a circuit-switched domain, which is traditionally used for phone calls and SMS.
3GPP specified two solutions for voice:
- IMS: A solution for IMS Voice over IP was specified in Rel-7.
- Circuit-Switched fallback (CSFB): in order to make or receive calls, the UE changes its radio access technology from LTE to a 2G/3G technology that supports circuit-switched services. This feature requires 2G/3G coverage. A new interface (called SGs) between the MME and the MSC is required. This feature was developed in Rel-8.
3GPP specified two solutions for SMS:
- IMS: A solution for SMS over IP was specified in Rel-7.
- SMS over SGs: this solution requires the SGs interface introduced during the work on CSFB. SMS are delivered in the Non Access Stratum over LTE. There is no inter-system change for sending or receiving SMS. This feature was specified in Rel-8.
CSFB and SMS over SGs are seen as interim solutions, the long term being IMS.
Read more about this topic: System Architecture Evolution
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