(PSTN), as shown in Figures 2.1 and 2.3, connecting the GSM and UMTS Release 99 network with the PSTN.
External IP data network, e.g. Internet, as shown in Figures 2.2, 2.3, and 2.13, connecting the GPRS and UMTS Release 99 network with the Internet.
Packet domain communications networks.
Interworking among the networks is realized through a particular logical interface that connects their gateway or anchoring network element. For example, in Figures 2.2 and 2.3, GPRS and UMTS PS core network is connected with the Internet through the Gi interface. Similarly, in Figure 2.4, the LTE/Evolved Packet Core (EPC) network is connected with the Internet through the SGi interface.
In the next sections, we will discuss the interworking among the packet domain communications networks, i.e. LTE/EPS, GPRS, and UMTS networks. The interworking of these networks can be realized in the following ways:
Interworking through enhanced network elements
Interworking through legacy network elements
6.2 Interworking Through Enhanced Network Elements
It was mentioned in Section 2.3.2 that the legacy GPRS and UMTS PS core network element Serving GPRS Support Node (SGSN) exists in LTE/EPC network also. The SGSN is the central interworking network element for the LTE/EPC, GPRS, and UMTS PS domain core networks. Functionality wise, the LTE/EPC SGSN was upgraded to support interworking and mobility management capabilities between the LTE/EPC and GPRS/UMTS PS domain networks. This is in addition to the legacy functions that are performed by the SGSN for a GPRS and UMTS network. A scenario where the GSM, GPRS, UMTS, and LTE networks may be deployed by an operator through their interworking can be found in Figure 1(b) TS 23.002 [29].
In addition to the existing connections with the legacy GSM and UMTS network elements, the LTE/EPC SGSN also interconnects with the Mobility Management Entity (MME), Serving Gateway (S‐GW) of an EPC network. Similarly, the S‐GW is a centralized core network element for the interworking of user data/traffic between the LTE/EPC and the UMTS PS domain core networks.
Figure 1(b) TS 23.002 [29] contains a basic interworking configuration where the voice call services may be provided through the GSM network (left side); voice and data services through the UMTS network (middle part); only data services through the LTE/EPS network (right side) during its initial rollout. This high‐level interworking configuration may appear to be complex in the first place because of the numerous network elements that are interconnected through various logical interfaces as indicated by the bold and thin solid lines. Look at this figure from the UE/MS, in the bottom‐up approach, toward the core network. Identify the access network and code network followed by the CS domain and PS domain network. From Figure 1(b) TS 23.002 [29], it is observed that the interworking among the GSM, GPRS, UMTS, and LTE/EPS networks is available at the core network level only, and no interworking is available at the radio access network level.
To offer voice as well as data services through an LTE/EPS network only, the operator requires to deploy additional IP transport‐based network components along with the EPC network shown in Figure 1(b) TS 23.002 [29]. On the other hand, an operator may choose to provide LTE/EPS data services only while it may continue to provide a voice call through the legacy GSM, UMTS network. Such provision requires interworking capabilities among the networks and their components deployed by an operator.
In the next sections, we will discuss the following interworking features that facilitate an LTE/Evolved‐UMTS Terrestrial Radio Access Network (E‐UTRAN) network registered UE and its user to make a voice call over an LTE/EPS network, in coexistence with the legacy GPRS Edge Radio Access Network (GERAN) and UMTS terrestrial radio access network (UTRAN).
Voice‐over IP Multimedia Subsystem (IMS) or voice‐over LTE (VoLTE),
Single Radio Voice Call Continuity (SRVCC), and
Circuit‐Switched Fallback (CSFB).
For these features to work, both the CS domain and PS domain core network elements, Mobile Switching Centre (MSC) and SGSN, are upgraded from its legacy networks, i.e. GSM/GPRS, UMTS. The network elements are upgraded in terms of new functions and procedures that are to be performed by the respective network element. The new functions and procedures are specified in the form of a new logical interface between two network elements. The dark dashed lines in the following illustrative figures show the existing, as well as the new logical interfaces for realizing a particular interworking feature through signaling or user traffic, flows between two network elements.
6.2.1 Interworking for Voice Call Through IMS: VoLTE
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.
6.2.1.1 IP Multimedia Subsystem (IMS)
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