(P‐CSCF)
Figure 6.2 Reference architecture of an IMS.
Source: © 2015. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2015, 3GPP.
The information sent by a UE is received first at the P‐CSCF and is forwarded to the I‐CSCF.
Interrogating CSCF (I‐CSCF)
I‐CSCF contacts the Home Subscriber Server (HSS), and based on the response from the HSS, it forwards the call to the S‐CSCF.
Serving CSCF (S‐CSCF)
The S‐CSCF handles the registration of UEs, maintains sessions, and performs routing functions.
MGCF
The MGCF interconnects the IMS with PSTN or CS domain network element. For a mobile‐terminated (MT) call, the MGCF contacts the I‐CSCF. For more information on the above CSCFs, refer to TS 23.228 [36].
The signaling protocol that is used to exchange information between a UE and the IMS is known as the Session Initiation Protocol (SIP) which uses the IP transport network. IMS uses the SIP messages to perform various functions and procedures such as registration, call establishments and release, session management, authentication, security, and charging. Though the SIP messages exchanged between a UE and the IMS are signaling/control plane information in nature with respect to the IMS, they are treated as user plane data by the E‐UTRAN and its core network. The real‐time voice packets are transferred through other protocols called Real‐Time Transport Protocol (RTP) andRTP Control Protocol (RTCP). For more information on the IMS, the reader is recommended to refer to TS 23.228 [36].
6.2.1.2 UE Registration and Authentication
In a live network of an operator, once a UE is registered and authenticated by the LTE/EPC as part of the LTE/EPS ATTACH procedure, TS 24.301 [46], a VoLTE‐capable UE is also required to register and authenticate itself with the IMS before it can start using IMS services. For these purposes, SIP signaling messages are exchanged between the UE and the IMS. Example 6.1 below describes and illustrates the flow of signaling messages, starting from registration to call establishment, conversation, and its completion, for a typical VoLTE call between two mobile devices over an IMS.
Example 6.1 Illustration: UE Registration and VoLTE Call over IMS
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.
