initiated by an MS toward the CN. A TMSI is used by the MSC as the identity of an MS during the air interface Layer 3 mobility management‐related procedures such as sending a paging request message to the MS to notify about an incoming call.
A TMSI has an end‐to‐end significance and has 32 bits in length. TMSI with all 1's is not a valid one. A new TMSI is also allocated to a mobile device on entering a new location area under a different VLR where the MS performs a location area update procedure with the CN. The VLR maintains an association between the IMSI of the mobile device and the allocated TMSI. For multi‐RATs, i.e. GPRS Edge Radio Access Network (GERAN), UTRAN, and E‐UTRAN, capable MS/UE, a TMSI is allocated as a result of the combined mobility management procedures performed by a UE/MS toward the CN.
5.3.2 GPRS System Temporary Identities
Packet TMSI (P‐TMSI)
A P‐TMSI is allocated by the GPRS PS domain SGSN to protect a mobile user's identity while communicating over the air interface. A P‐TMSI is used as the identity of an MS during the GPRS air interface Layer 3 mobility management‐related procedures such as the routing area update, paging request, and an attach request.
A P‐TMSI has an end‐to‐end significance and has 32 bits in length. The first two Most Significant Bit (MSB) bits (31, 30) of a P‐TMSI are always set to 1's. A new P‐TMSI is allocated to a mobile device on entering a new routing area under a different SGSN where the MS performs a routing area update procedure with the CN. The SGSN maintains an association between the IMSI of the mobile device and the allocated P‐TMSI.
Network Resource Identifier (NRI)
An NRI is a CN‐level identifier and is used to identify a GSM MSC or SGSN or an LTE/EPS MME. Several MSCs or SGSNs or MMEs can be configured in a pool to serve mobile users in a load‐balanced way. An NRI is assigned and identifies uniquely an individual CN element, i.e. MSC, SGSN, or an MME, out of all the CN elements configured in a pool.
An NRI has an end‐to‐end significance and has a length from 0 to 10 bits; NRI having length 0 means the feature is not used at the CN end. More about the CN element pool and their selection are described later in Chapter 7.
Temporary Logical Link Identity (TLLI)
A TLLI identifies a logical link at the Logical Link Control (LLC) layer between the MS and the SGSN of a GPRS system. At the GPRS LLC layer level, an MS is uniquely identified by a temporary logical link identifier (TLLI). A TLLI is constructed from a P‐TMSI that is allocated by the SGSN which has an end‐to‐end significance and is used across the MS, BSC, and the SGSN.
5.3.3 LTE/EPS System Temporary Identities
MME TMSI (M‐TMSI), S‐TMSI
An M‐TMSI is allocated by the LTE/EPS MME to a UE. An S‐TMSI is constructed from an M‐TMSI and the MME code, which is 8 bits in length. Using the S‐TMSI, MME used to send a paging message to a UE. S‐TMSI has an end‐to‐end significance.
For a multi‐RATs capable UE/MS that supports the GSM, GPRS, UMTS, and LTE systems, a UE/MS will perform the combined registration procedures toward the respective CN. The concerned CN element will assign and allocate the three TMSIs accordingly, i.e. TMSI, P‐TMSI, and S‐TMSI, to a UE. For example, to notify an incoming CS call to an E‐UTRAN registered with the Circuit Switched Fall‐back (CSFB) feature, the MSC will send the paging request message to the MME with TMSI. The MME will send the paging command to the UE using the S‐TMSI. The UE will send the CS paging response with TMSI allocated by the MSC/VLR.
Network Identities for LTE/EPS Network elements
Various identities are used to identify network elements or logical objects, at the protocol layer level, of an LTE/EPS network. Such network identities, as elucidated in Example 5.1 below, may be assigned as part of a periodical network planning or maintenance processes or can be generated and assigned by a network element dynamically to another network element as part of a protocol layer procedure.
Example 5.1 Network Identities of LTE Network Elements
Figure 5.2 illustrates the fundamental identity assigned to some of the network elements of an LTE/EPS PLMN.
Figure 5.2 Illustration: identities for LTE network elements.
Several network identities also contain the PLMN Id. The purposes of the network identities shown in Figure 5.2 are described below:
E‐UTRAN Cell Identity (ECI) (28 bits in length): It is a unique cell identity and cannot be repeated in a PLMN. It is defined as the SIBType1. CellIdentity; refer to TS 36.331 [94]. An ECI together with the PLMN Id constitutes an E‐UTRAN Cell Global Identity (ECGI).
eNB Id (28 bits length): It identifies an eNodeB within a PLMN.
Radio Network Temporary Identifier (RNTI) (16 bits length): It identifies a UE having a Radio Resource Control (RRC) connection and also can be scheduled for data transmission/reception.
MMEC (8 bits length): MME Code – identifies MME with the MME group.
MMEGI (16 bits length): MME Group Id – identifies the MME Group Id of that an MME belongs to.
Globally Unique Temporary Identity (GUTI)
In the case of the LTE/EPS system, a mobile device UE is uniquely identified by a GUTI that is allocated by the MME. A GUTI is similar to a P‐TMSI and is used as the identity of a UE during the LTE/EPS mobility management‐related procedures executed between a UE and the MME. A GUTI can be a native or mapped as described later in Section 5.5. A GUTI is constructed from several fundamental network identities as illustrated in Example 5.2.
Example 5.2 LTE/EPS Globally Unique Temporary UE Identity (GUTI)
A GUTI is an LTE/EPS network‐level identity that has end‐to‐end significance and is used to provide unambiguous identification of a UE. The construction, as well as its hierarchical structure of a GUTI, is shown in Figure 5.3.
