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To provide both the voice and data services to subscribers over an LTE/EPS, an IMS may be deployed and interworked through the SGi interface further with the LTE EPC network. The provision for providing voice services to subscribers over the LTE system and IMS is called the VoLTE. One such network deployment scenario providing VoLTE services is illustrated in Figure 6.1.

Along with the LTE/EPS network and its IMS configuration, the handset must be also VoLTE capable to enable a user to make a voice call over an LTE network. A VoLTE‐capable UE can work on different networks, i.e. GSM, UMTS, and LTE using different Radio Access Technologies (RATs).

Figure 6.1 Illustration: LTE network and IMS interworking for VoLTE call.

As shown in the above interworking scenario, a voice call can be facilitated to a user through the legacy GSM, UMTS network, or LTE/EPS IMS. A UE indicates its preference for a voice call through the Voice domain preference and UE’s usage setting IE in the ATTACH Request message, TS 24.301 [46], TS 24.008 [45], that is sent to the core network during its initial registration of a UE. The same IE is also sent to the core network as part of the Routing Area Update (RAU) (GPRS, UMTS) and Tracking Area Update (TAU) (LTE/EPS) request to indicate its voice domain preference by a UE. An LTE UE that does not support the voice‐over IMS feature will indicate the preference as the “CS voice only”. In such a case, the UE will use the CS fallback method of providing a voice call facility to a user.

An LTE UE that support the voice‐over IMS feature will indicate the preference as the “IMS PS voice only”. All these interworking possibilities of providing voice and data services depend on the capabilities of a UE and its usage setting, which is described in the next section. Below we briefly describe the IMS and its network elements that are interworked with an LTE/EPC to provide a VoLTE call facility to subscribers.