Temporary Identity used in Next update

Description

Temporary Identity used in Next update (TIN) is a parameter stored in the UE's EPS Mobility Management (EMM) context, as defined in 3GPP TS 23.401 and TS 24.301. It indicates the type of temporary identity that the UE shall use in the next Attach or Tracking Area Update (TAU) procedure. The TIN can take one of several values: GUTI (Globally Unique Temporary Identity), P-TMSI (Packet Temporary Mobile Subscriber Identity), RAI (Routing Area Identity), or TMSI (Temporary Mobile Subscriber Identity). The TIN is used to ensure that the UE and the network have a consistent understanding of which temporary identity is valid and should be used for subsequent mobility management procedures.

The TIN is particularly important in networks that support interworking between EPS (Evolved Packet System) and GPRS/UMTS. When a UE moves between EPS and GPRS/UMTS coverage, it may need to use different types of temporary identities. The TIN parameter helps the UE determine which identity to use in the next update procedure, avoiding unnecessary signaling and ensuring that the network can correctly identify the UE. For example, if the TIN is set to GUTI, the UE will use the GUTI in the next Attach or TAU. If the TIN is set to P-TMSI, the UE will use the P-TMSI and RAI in the next Routing Area Update (RAU) procedure.

The TIN is managed by the UE and is updated based on the outcome of mobility management procedures. When the UE performs an Attach or TAU and receives a new GUTI from the network, it may update the TIN accordingly. Similarly, when the UE performs a RAU in GPRS/UMTS and receives a new P-TMSI, it may update the TIN. The TIN is also used during inter-system changes to determine whether the UE should perform a combined Attach/TAU or a separate procedure. The parameter is stored in the UE's non-volatile memory and persists across power cycles.

In 5G networks, the concept of TIN has been extended with the introduction of the 5G-GUTI and the 5G-S-TMSI. The UE's 5GMM (5G Mobility Management) context includes a similar parameter to indicate which temporary identity to use in the next Registration procedure. The principles remain similar to those in EPS, ensuring consistent identity handling across generations. The TIN is defined in TS 24.301 for EPS and in TS 24.501 for 5G, with the latter specifying the 5G equivalent.

The TIN is a critical parameter for ensuring seamless mobility and interworking between different radio access technologies. Without a consistent TIN, the UE and network could use mismatched identities, leading to failed procedures, unnecessary signaling, and degraded user experience. The TIN is therefore an essential part of the UE's mobility management context and is carefully managed by the UE's protocol stack.

Purpose & Motivation

The Temporary Identity used in Next update (TIN) exists to solve the problem of identity consistency during mobility management across different radio access technologies (RATs) and core networks. In networks that support interworking between EPS and GPRS/UMTS, a UE may have multiple temporary identities (e.g., GUTI for EPS, P-TMSI for GPRS/UMTS). The TIN parameter tells the UE which identity to use in the next Attach, TAU, or RAU procedure, ensuring that the network can correctly identify the UE and that the procedure succeeds.

Before the introduction of TIN, UEs and networks could experience inconsistencies in identity handling, leading to failed mobility procedures and unnecessary signaling. For example, a UE moving from LTE to UMTS might use a GUTI in a RAU procedure, which the UMTS network would not recognize, causing the procedure to fail. The TIN parameter addresses this by allowing the UE to select the appropriate identity based on the current RAT and the network's capabilities.

The TIN was introduced in Rel-8 as part of the EPS specifications, alongside the introduction of the GUTI and the need for interworking with legacy GPRS/UMTS networks. It has been carried forward through all subsequent releases, with extensions for 5G in Rel-15. The parameter remains essential for ensuring seamless mobility and interoperability in multi-RAT networks.