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Let us consider a typical VoLTE call performed between two UEs: UE1 and UE2. To enable a VoLTE call, the UEs are required to be registered with both the LTE EPC and IMS for the allocation of the necessary resources, for example, EPS bearers with required Quality of Service (QoS). Several steps are involved in a VoLTE call over the IMS as described below:

 UE Registration with EPC and Allocation of Default Bearer with QoS Class Identifier (QCI) = 5

Figure 6.3 below illustrates the allocation of a default bearer to each UE as part of its ATTACH procedure requested to the core network. The allocated default bearer has the QCI = 5, Priority =1, refer to TS 23.203 [33], with nonguaranteed bit rate (NGBR) that is used to exchange IMS signaling messages between the UE and IMS. The dotted lines in Figure 6.3 indicate that some of the messages as part of the overall ATTACH procedure (which are exchanged between the UE and the EPC) are not shown in this figure. One such message is the Evolved Packet System Session Management (ESM) Information request, from EPC to UE, and its response, from UE to EPC. Using the ESM information response message, the UE provides the P‐CSCF’s Access Point Name (APN) in the case of the IMS.

 UE Registration with IMS through Authentication

The UEs register with the IMS using the default bearer with QCI = 5 that was allocated during their registrations with EPC. To register with the IMS, a UE uses the SIP signaling message that is transferred as the user plane data over the LTE/EPS toward the IMS. P‐CSCF is connected with the P‐GW of the LTE/EPC. For a UE, the proxy CSCF is the entry and exit server in the IMS. A UE sends the APN of the P‐CSCF server in the SIP: REGISTER message to the IMS. The UE sends its authentication response details in the REGISTER message again to the IMS following the 401: Unauthorized request message from the IMS; see Figure 6.4.

Figure 6.3 Allocation of default bearer with QCI = 5 for IMS signaling.

Figure 6.4 Illustration: registration of UE in an IMS.

The end‐to‐end message flow for a typical registration of a UE in an IMS is ‐ UE < ‐>eNodeB<− > S‐GW < ‐>P‐GW < ‐>P‐CSCF. Though not shown in Figure 6.4, once the P‐CSCF receives the registration request from a UE, it is further processed by the I‐CSCF, HSS, S‐CSCF server and responds with the SIP: 200 OK messages to the registered UE.

 VoLTE Call over IMS

UEs that are EPC and IMS registered can establish a voice‐over IMS to facilitate a voice call between two subscribers. The SIP signaling messages flows associated with a typical VoLTE call between two UEs – UE1 and UE2 – over IMS are illustrated in Figure 6.5. Only the P‐CSFC is shown in the call flow below, though the I‐CSCF and S‐CSCF also take part in an IMS call.

In Figure 6.5, the E‐UTRAN and EPC/NAS‐related signaling messages that are exchanged with the UEs are not shown for simplicity of the flows. The INVITE method carries the calling and called party’s number and is used to establish a session between the two UEs. As shown in this figure, the EPS allocates a dedicated bearer with QCI = 1, Priority = 2, refer to TS 23.203 [33], with a guaranteed bit rate (GBR) over which the voice conversation takes place between the users. The establishment of a dedicated bearer for voice conversation purposes is intimated to the called UE through SIP:UPDATE method, following which it sends the SIP:RINGING status to the caller UE.

Figure 6.5 Illustration: VoLTE call between IMS registered UEs.

The IMS for VoLTE described above will be also the foundation for the voice services over the 5G system with essential extensions added, as part of the 3GPP Release 15, over some of the existing logical interfaces, such as the Rx, Sh.