Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 137

The IMS is the core network infrastructure backbone for offering common IP‐based multimedia, e.g. audio, video, voice, and services to subscribers over an LTE/EPS IP transport network. The IMS contains the core network elements and interfaces, interconnecting them for routing of a VoLTE call either to a 2G/3G network or PSTN network or within an IMS network accordingly. The reference architecture, the network elements, and various interfaces of an IMS system are shown in Figure 6.2. For more information on this reference architecture, refer to TS 23.228 [36]. The IMS was standardized for mobile communications networks and was introduced as part of the 3rd Generation Partnership Project (3GPP) Release 5 architecture to provide voice calls along with other multimedia services.

The main network elements of an IMS is the call session control function (CSCF) and the media gateway control function (MGCF). As shown in Figure 6.2, the CSCF performs several roles: Proxy CSCF, Interrogating CSCF, and Serving CSCF.

 Proxy‐CSCF (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].