Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 135

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.