Mobile Station Identifier

Description

Mobile Station Identifier (MSID) is a foundational, high-level concept in 3GPP standards referring to any identifier that uniquely designates a Mobile Station (MS), now commonly known as User Equipment (UE), within the context of a cellular network. It is not a single, specific identifier with a defined format, but rather a categorical term encompassing the various unique names or numbers assigned to a UE or its subscription. These identifiers are crucial for all network operations, including registration, authentication, mobility management, charging, and security procedures.

The architecture for MSIDs spans multiple network domains. Identifiers are stored in various network entities: the UE itself (in the USIM or device memory), the Radio Access Network (RAN), the Core Network (CN), and often in external databases like the Equipment Identity Register (EIR). The 'how it works' is specific to each type of MSID. For instance, the International Mobile Subscriber Identity (IMSI) is used by the core network to fetch subscriber authentication data from the HSS. A Temporary Mobile Subscriber Identity (TMSI) is assigned by the Mobility Management Entity (MME) or Serving GPRS Support Node (SGSN) to provide subscriber identity confidentiality on the radio interface, replacing the IMSI after initial registration.

Key components of the MSID framework include permanent identifiers (like IMSI and International Mobile Equipment Identity or IMEI), temporary identifiers (like TMSI, GUTI, and S-TMSI), and radio network identifiers (like C-RNTI used for scheduling within a cell). The role of MSIDs is to provide unambiguous addressing at different layers and for different purposes: permanent subscription identity for management and roaming, temporary identity for air interface protection, temporary network layer identity for tracking area routing, and temporary cell-level identity for radio resource control. The network seamlessly maps between these identifiers as a UE attaches, moves, and communicates.

Purpose & Motivation

The concept of the Mobile Station Identifier exists to address the fundamental requirement of uniquely naming and addressing a mobile endpoint in a vast, global network. Early mobile systems needed a way to distinguish one subscriber's phone from another for call routing, billing, and basic network access. The purpose of defining MSID as a generic term is to provide a clear architectural placeholder and classification for all the specific identifiers that were developed, each solving a particular sub-problem of identification.

Different MSIDs were created to solve specific limitations. The permanent IMSI solved the problem of globally unique subscription identification for roaming. However, sending the permanent IMSI frequently over the air posed a security risk (e.g., tracking, identity capture). This limitation motivated the creation of temporary identifiers like TMSI, which provide identity confidentiality. Similarly, the IMEI was created to identify the device hardware independently of the subscription, helping to address problems like stolen device blocking. The evolution of MSIDs reflects an ongoing effort to balance the needs of unique identification, operational efficiency, subscriber privacy, and security across increasingly complex network architectures and services.

Key Features

• Umbrella term for all UE/subscription identifiers in 3GPP systems
• Encompasses permanent identifiers (IMSI, IMEI) and temporary identifiers (TMSI, GUTI)
• Essential for core network procedures like authentication, mobility management, and charging
• Critical for radio network procedures like paging and connection scheduling
• Includes identifiers with different scopes: global, network, tracking area, and cell-level
• Underpins key network features including security (identity confidentiality) and roaming

Evolution Across Releases

Defining Specifications