Description
The M-Temporary Mobile Subscriber Identity (M-TMSI) is a core network identifier used within the Evolved Packet System (EPS) for mobility management. It is assigned by the Mobility Management Entity (MME) to a User Equipment (UE) during the attach procedure or a subsequent tracking area update. The M-TMSI's primary function is to serve as a temporary alias for the permanent, subscription-based International Mobile Subscriber Identity (IMSI), thereby enhancing subscriber privacy over the radio interface. Architecturally, the M-TMSI is a 32-bit value that is used within the Non-Access Stratum (NAS) protocol exchanged directly between the UE and the MME. How it works is integral to the EPS attach and paging processes. Upon initial network attachment, if the UE provides its IMSI, the MME authenticates the subscriber and may assign a new M-TMSI. This M-TMSI is then sent to the UE encrypted within NAS signaling. For subsequent communications, such as a service request or response to a paging message, the UE includes the M-TMSI in the NAS message instead of the IMSI. When the MME needs to page a UE (e.g., for an incoming call), it uses the M-TMSI to construct the paging identity. The S-TMSI, which is a shortened version used for radio efficiency, is actually a concatenation of the MME Code (MMEC) and the M-TMSI. The MME maintains the mapping between the allocated M-TMSI and the subscriber's IMSI and other context in its database. Its role is critical for reducing signaling overhead (shorter than IMSI) and for implementing identity confidentiality, as the sensitive IMSI is rarely transmitted in clear text after initial registration. The M-TMSI is valid within the assigned MME's area; if the UE moves to an area served by a different MME, a new M-TMSI may be assigned.
Purpose & Motivation
The M-TMSI was created to address two key problems in mobile networks: subscriber identity privacy and signaling efficiency. The permanent IMSI, if transmitted frequently in clear text, could be used to track a user's location and activities. The purpose of the M-TMSI is to obfuscate this permanent identity on the radio interface after initial authentication. It solves the privacy issue by providing a temporary, frequently changing identifier for most NAS signaling transactions. Furthermore, the 32-bit M-TMSI (and its derived S-TMSI form) is shorter than the up-to-15-digit IMSI, leading to more efficient use of radio resources in paging messages and signaling procedures. Historically, temporary identifiers like TMSI existed in GSM and UMTS. The M-TMSI is the EPS equivalent, adapted for the all-IP, flatter architecture of LTE/EPC. Its introduction in Release 8 was motivated by the need to maintain and enhance subscriber privacy protections within the new 4G system architecture, while also optimizing the signaling load for massive numbers of connected devices. It allows the network to uniquely identify a UE for mobility management purposes without constantly exposing the subscriber's permanent credentials.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (42 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the handling of the M-TMSI was updated to support interworking with 5G, specifically allowing the use of a 5G-GUTI for the creation of an ATTACH REQUEST message. This included mechanisms for a UE in single-registration mode without an N26 interface to correctly include its identity, such as a GUTI, in the ATTACH REQUEST. Furthermore, the release clarified procedures for identity transmission and GUTI mapping, ensuring proper UE status and identity handling during mobility between 4G and 5G systems.
- EPS mobile identity and UE status in the ATTACH REQUEST message TS 24.301CR3028
- Update of UE identity used for attach in S1 mode for single-registered UE without N26 TS 24.301CR3070
- Clarification on eDRX handling with assigning a new GUTI TS 24.301CR2895
- Correction on transmission failure of IDENTITY RESPONSE message TS 24.301CR3078
- Correction on inclusion of GUTI for UE in single-registration mode TS 24.301CR3119
- GUMMEI mapped from 5G GUTI with indication at 5G interworking TS 24.301CR3108
+ 1 more changes
In Release 16, the M-TMSI's role within the GUTI was enhanced to support UE radio capability ID assignment and deletion via the GUTI reallocation procedure. This provided a new mechanism for the network to manage UE radio capability information directly through the established identity management procedure. No other specific changes to the M-TMSI itself are detailed in the provided context beyond this new association with radio capability ID handling.
In Release 17, key enhancements for M-TMSI handling introduced the concept and procedures for a negotiated "IMSI Offset" to support MUSIM devices in EPS, primarily during Attach and Tracking Area Update (TAU) procedures. This included mechanisms for the network to store an Alternative IMSI in the MM Context and to handle the IMSI Offset during periodic TAUs and when TAU COMPLETE is not received. Furthermore, clarifications were made on paging restrictions and the inclusion of an Additional GUTI IE in TAU requests for inter-RAT changes between N1 and S1 modes.
- Introducing IMSI Offset to Attach and TAU procedures for MUSIM handling in EPS TS 24.301CR3527
- Negotiated IMSI offset and TAU COMPLETE TS 24.301CR3614
- Negotiated IMSI offset when TAU COMPLETE is not received by network TS 24.301CR3625
- Storing of alternative IMSI TS 24.301CR3679
- Clarification on removing condition of paging restriction and IMSI Offset TS 23.401CR3652
- Adding Alternative IMSI to the MM Context in the MME TS 23.401CR3677
+ 13 more changes
In Release 18, changes to M-TMSI-related handling were introduced, including UE identity management during a USIM removal in an Attach or TAU procedure and a correction to the determination of the mobile reachable timer based on user plane activity. Furthermore, enhancements ensured consistent value naming for the GUTI type within the UE's temporary identity management framework.
- Include DN-specific identity in PDN CONNECTIVITY REQUEST TS 24.301CR3861
- Indicating the SDNAEPC DN-specific identity in the protocol configuration options TS 24.301CR3886
- Correction to determination of periodic timer, mobile reachable timer based on UP and start of UP TS 24.301CR3996
- Abnormal case for SDNAEPC related to DN-specific identity TS 24.301CR4011
- UE identity handling in case of a USIM removal during an Attach or TAU procedure TS 24.301CR4024
- Custom throttling to temporary failed ESM procedure TS 24.301CR4053
+ 1 more changes
In Release 19, enhancements to the M-TMSI function were made through updates to the mobile reachable timer and implicit detach timer adjustments for NAS-level congestion control. Furthermore, the release specified new UE actions upon GUTI deletion and introduced RAT utilization control as a factor during GUTI reallocation procedures. These changes refined identity management and timer handling for UEs in idle mode.
- The adjustment of mobile reachable timer or the implicit detach timer TS 24.301CR4343
- Invalid EPS bearer identity in the Data container IE in the EMM TRANSPORT message TS 24.301CR4494
- Correction on term 4G-GUTI TS 24.301CR4180
- RAT utilization control in GUTI reallocation TS 24.301CR4124
- UE actions upon GUTI deletion TS 24.301CR4442
- Correction to the mobile reachable timer TS 24.301CR4656
Explore further
Broader topics and technologies where M-TMSI plays a role.
Defining Specifications
3GPP specifications that define or reference M-TMSI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.401 vj50 | Evolved Packet System (EPS) Stage 2 Description | Rel-19 |
| TS 24.301 vj60 | NAS protocol for Evolved Packet System | Rel-19 |
| TS 24.801 v810 | CT1 SAE NAS Aspects for EPC | Rel-8 |