PLMN Management

CoreESIM: Network Selection Algorithms & Protocol Integration

3 min read

The CoreESIM, functioning as a tamper-resistant Universal Integrated Circuit Card (UICC) in an embedded context, plays a critical role in the mobile equipment's (ME) network selection processes. These algorithms, standardized primarily by 3GPP, dictate how an ME acquires and maintains service with a Public Land Mobile Network (PLMN). The CoreESIM stores essential subscription and configuration data that directly influences these selection procedures, ensuring compliance with operator policies and user preferences.

Initial network selection typically involves the ME performing a scan across available radio access technologies (RATs) – GERAN, UTRAN, E-UTRAN, and NR – to identify broadcasted PLMNs. The CoreESIM provides the Home PLMN (HPLMN) identity and, if provisioned, a list of equivalent PLMNs (EPLMNs). The ME prioritizes these based on rules defined in 3GPP TS 23.122, which outlines the overall service access and selection principles.

Network Selection Algorithms and CoreESIM Interaction

Network selection algorithms operate in both automatic and manual modes, with the CoreESIM facilitating the necessary data points. In automatic mode, the ME attempts to select the HPLMN first. If the HPLMN is unavailable, or if roaming, the ME consults various PLMN lists stored and managed by the CoreESIM. These include:

  • Operator-Defined PLMN List (OPLMN): Provided by the home network operator, specifying preferred roaming partners and their respective RAT preferences.
  • User-Controlled PLMN List (UPLMN): Allows subscribers to define their own preferences, typically overriding OPLMN entries when present.
  • Forbidden PLMN List (FPLMN): Maintained by the ME based on network rejection causes (e.g., "PLMN not allowed" or "LA/TA not allowed"), preventing re-selection attempts to non-serving PLMNs. This list is stored on the CoreESIM and persists across power cycles, crucial for efficient selection.

The ME's selection process further involves evaluating signal strength and quality, RAT capabilities, and the presence of locally stored data such as the "last registered PLMN" and "user preferred RAT" information. The CoreESIM's profile, provisioned via Remote SIM Provisioning (RSP) according to GSMA SGP.22 specifications, contains the necessary files (e.g., EF_PLMNsel, EF_OPL, EF_HPLMN) that guide these decisions. Updates to the CoreESIM profile can dynamically alter the network selection behavior, for instance, by modifying OPLMN entries, HPLMN identity, or RAT preferences, enabling adaptable connectivity strategies.

The Non-Access Stratum (NAS) protocols, particularly those defined in 3GPP TS 24.008, govern the signaling between the ME and the Core Network during network registration and selection. The CoreESIM ensures the integrity and authenticity of the subscription data presented to the network, which is paramount for successful registration. Furthermore, the CoreESIM's secure environment protects the integrity of the FPLMN and other critical selection parameters, preventing unauthorized modification that could compromise network access or security. The selection process is a continuous loop, with the ME periodically re-evaluating available networks based on predefined timers and thresholds, ensuring optimal connectivity according to the CoreESIM's configured parameters and network availability, adhering strictly to 3GPP specifications.