6 Voice Transport in LTE
6.1 Voice over LTE

Voice communication is in LTE natively supported only using IMS services, with specific profiles for control and media planes.

IMS is an access-independent overlay to existing network architectures, guaranteeing seamless service continuity, not only for voice, but also e.g. for video application. The first version of IMS was standardized in 3GPP release 5, with many enhancements specified in subsequent releases. IMS needs to be implemented on both the network as well as the device side, whereas rollout of IMS in commercial networks was slower than originally expected.

image
IMS subsystem in LTE

For LTE, the IP-CAN IP (Connectivity Access Network) would be composed of the EPS and the E-UTRAN.

The call session control functions are the core components of the IMS. There are three CSCF:

The HSS (Home Subscriber Server is the main subscriber database used within IMS. The IMS HSS provides details of the subscribers to the other entities within the IMS network, enabling users to be granted access or not dependent upon their status.

The AS (Application Server) provides specific IP applications such as messaging.

The IMS calls for VoLTE are processed by the subscriber's S-CSCF in the home network. The connection to the S-CSCF is via the P-CSCF. Dependent upon the network in use and overall location within a network, the P-CSCF will vary, and a key element in the enablement of voice calling capability is the discovery of the P-CSCF.

The continuity of voice calls needs to be guaranteed by handover to a legacy technology such as GSM. This is achieved by a feature called SRVCC (Single Radio Voice Call Continuity).