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Due to a UE's intersystem changes from E‐UTRAN/EPS to GERAN/UTRAN (UMTS) and vice versa, the new SGSN or new MME requires to retrieve the UE information from the old SGSN or old MME. Apart from this, identity mapping is also required at the UE end if it wishes to perform a mobility management procedure in the target system.

Consider that an E‐UTRAN capable UE leaves a GERAN/UTRAN coverage area and enters into an E‐UTRAN area and wants to perform an EPS ATTACH mobility management procedure toward the LTE/EPC network. Also, assume that the UE does not have a valid GUTI at present. In this case, the UE maps the existing GERAN/UTRAN domain identities into the E‐UTRAN domain corresponding identities and sends those identities as EPS Mobile Identity in the Attach request message, along with the Old GUTI Type as “Mapped GUTI” to the MME. The GERAN/UTRAN network identities mappings to E‐UTRAN network identities are shown in Table 5.1.

Figure 5.6 shows graphically the mapping of LTE/EPS M‐TMSI from the GERAN/UTRAN P‐TMSI. In the current intersystem change scenario, the visiting and new MME performs the reverse mapping of the identities shown in Table 5.1, and based on these, the new MME is able to contact the old SGSN to retrieve the UE information.

Table 5.1 GERAN‐UTRAN to E‐UTRAN‐EPC: identities mapping.

GERAN‐UTRAN	E‐UTRAN‐EPC
Mobile Country Code (MCC)	Mobile Network Code (MNC)
Mobile Network Code (MNC)	Mobile Network Code (MNC)
Location Area Code (LAC)	MME Group ID
Routing Area Code (RAC)	Bit 23rd to 16th of M‐TMSI
8 Most Significant Bits of NRI	MME Code
29th to 24th bits of P‐TMSI	29th to 24th bits of M‐TMSI
15th to 0th bits of P‐TMSI	15th to 0th bits of M‐TMSI

Figure 5.6 Illustration: UE identities mapping: GERAN/UTRAN to E‐UTRAN.

For a UE's intersystem change from E‐UTRAN to GERAN/UTRAN area, the procedure for identities mapping is almost the same. For more information, refer to TS 23.003 [30].

 LTE/EPS: Indication of Native and Mapped Identity

In the LTE/EPS, the CN elements are resolved and identified using their corresponding identity, i.e. GUTI for MME and P‐TMSI/ Routing Area Identity (RAI) for the SGSN. These identities (GUTI and P‐TMSI/RAI) used and assigned to a UE can be a native or mapped one. Native or mapped identity type or its nature is determined as follows:

1 Explicit indication in the Attach request and tracking area update/routing area update request message from UE to MME /SGSN.

2 Indication by setting the MSB of LAC and MME group Id.MSB of MME Group = 1 when a native GUTI is used; MSB of MME Group = 0 when the GUTI is mapped from P‐TMSI and RAC.MSB of LAC = 1 when P‐TMSI/RAC is mapped from a GUTI; MSB of LAC = 0 when a native P‐TMSI/RAC is used.

We have discussed and illustrated only a few network identities in the preceding sections. In fact, mobile communications systems based on GSM, GPRS, UMTS, and LTE systems use numerous network identifiers across the protocol layers and interfaces. Refer to the corresponding 3GPP specification for the different network identities and their components. Later, Section further describes some of the identities associated with the mobility management procedures of GSM, GPRS, LTE/EPS, and 5G systems.