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S-TMSI SAE Temporary Mobile Subscriber Identity

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The S-TMSI (SAE Temporary Mobile Subscriber Identity) is a temporary identity used in the 3GPP (3rd Generation Partnership Project) system, specifically in the context of the System Architecture Evolution (SAE) and the Evolved Packet System (EPS). It is primarily used in LTE (Long-Term Evolution) and 5G networks.

The purpose of the S-TMSI is to provide a temporary identifier for a mobile subscriber within the network. It is assigned by the serving network to uniquely identify a subscriber during their session or connection establishment. The S-TMSI allows the network to manage and track the subscriber's activities without exposing their permanent identity, which is typically the International Mobile Subscriber Identity (IMSI).

Here's a step-by-step overview of how the S-TMSI is assigned and used within the network:

  1. Initial Attach: When a mobile device connects to a network for the first time or after being idle for a certain period, it initiates an attach procedure. During this procedure, the mobile device sends an Attach Request message to the network.
  2. Temporary Mobile Identity: In response to the Attach Request, the network generates a temporary identifier called the S-TMSI. This S-TMSI is unique within the serving network and is associated with the subscriber's context in the network. The S-TMSI is temporary and can change between attach procedures to provide additional privacy.
  3. Attach Accept: The network sends an Attach Accept message to the mobile device, containing the S-TMSI and other necessary information for establishing the session.
  4. S-TMSI Mapping: The network maps the S-TMSI to the subscriber's permanent identity, the IMSI, in its internal database. This mapping allows the network to correlate the temporary identifier with the subscriber's actual identity for billing, authentication, and other network functions.
  5. Session Management: Once the attach procedure is successful, the S-TMSI is used by the network to identify and manage the subscriber's session. For example, when the subscriber performs subsequent activities like data transmission or mobility events (handover), the S-TMSI is used to maintain continuity and ensure the proper handling of the subscriber's session.
  6. S-TMSI Reallocation: The S-TMSI can be reallocated when the subscriber detaches from the network or the network decides to change the temporary identifier for security or optimization purposes. Upon reattachment, a new S-TMSI may be assigned to the subscriber.

The S-TMSI serves several important purposes in the network. Firstly, it helps to preserve the subscriber's privacy by allowing the network to use a temporary identifier instead of their permanent IMSI. Secondly, it reduces signaling overhead since the S-TMSI is shorter than the IMSI, making it more efficient for network operations. Lastly, it enables efficient session management by providing a unique identifier for tracking the subscriber's activities within the network.

Overall, the S-TMSI plays a crucial role in the temporary identification and management of mobile subscribers in LTE and 5G networks, offering a balance between privacy, efficiency, and network functionality.