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Figure 1(b), TS 23.002 [29], presents a basic interworking of the GSM, UMTS, and LTE/EPS networks that can be deployed by an operator with entirely new and upgraded/enhanced legacy network elements, e.g. SGSN, MSC, and RNC. However, another operator may choose to interwork its LTE/EPC network with the legacy GSM, GPRS, and UMTS core network elements, e.g. SGSN and MSC, only as its initial deployment. To realize successful interworking under such a scenario with legacy network elements, the EPC network elements will be required to support the necessary interfaces/protocols also that are used by the legacy core network elements, e.g. SGSN and MSC. One such interworking scenario is shown in Figure 6.10, reproduced from TS 23.401 [39].

Figure 6.10 Interworking of LTE/EPS with GERAN and UTRAN through legacy network elements.

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.

In Figure 6.10, only the representative figure for the GERAN and UTRAN networks along with the legacy SGSN is shown. The Gn interface works on top of the GTP and is used by the legacy GPRS and UMTS SGSN to communicate with the GGSN of a GPRS and UMTS network. The LTE/EPC MME communicates with the legacy SGSN over the Gn interface. LTE/EPC S‐GW also interfaces with the legacy SGSN through the Gn interface. The EPC HSS communicates with the SGSN through the existing Gr interface that works on top of the SS7 signaling protocols. On the other hand, the HSS communicates with the MME through the S6a interface that works on top of IP transport.