USIM parameters (files) refer to a set of data stored on a USIM (Universal Subscriber Identity Module) card, which is used in 3G, 4G, and 5G networks. These files contain various information and configurations that help manage a mobile device's connection to the network. Some key USIM files include:

• IMSI (International Mobile Subscriber Identity): A unique identifier for the user on the network.
• Authentication Keys (e.g., Ki): Used for secure communication and authentication.
• EF files (Elementary Files): Store SMS, contacts, and network settings.
• PLMN (Public Land Mobile Network) List: Determines preferred and forbidden networks.

These files are structured according to 3GPP TS 31.102 and provide the necessary data for functions like authentication, network selection, and secure communication.

NOTE : I would mostly focus on EF files in this note, but I would not explain every EF files defined in the specification. I would just list some of EF files that are familiar to me. For further details of each of these parameters or for EF files not listed here, refer to TS 31.102

EF_ACC : ACC stands for Access Control Class. The access control class is a parameter to control the access attempts. 15 classes are split into 10 classes randomly allocated to normal subscribers and 5 classes allocated to specific high priority users. When UE sees any Access Control settings from Network (e.g, ac-Barring setting in SIB message), UE refer to this UICC parameter to figure out which Class it belongs. Refer to 31.102 4.2.15 EFACC  for the detailed data format)

EF_ECC : ECC stands for Emergency Call Codes. This field contains the list of Emergency Call Codes. the emergency call code is of a variable length with a maximum length of 6 digits. Each emergency call code is coded on three bytes, with each digit within the code being coded on four bits as shown below. If a code of less than 6 digits is chosen, then the unused nibbles shall be set to 'F'( Refer to 31.102 4.2.21 EFECC  for the detailed data format)   

EF_EHPLMN : EHPLMN stands for Equivalent HPLMN (Home PLMN). It contains the list of PLMNs which can be regarded as a Home PLMN. The items are listed in order of decreasing priority, meaning that the first PLMN in the list has the highest priority and the last item has the lowest priority. (Refer to 31.102 4.2.84 EFEHPLMN for the detailed data format, refer to 23.122 4.4.3.1.1 for the determination of the priority in cell selection)

EF_HPLMNwAcT : PLMNwACT stands for 'HPLMN with Access Technology'. Basically there is where your HPLMN and list of avaiable Access Technologies are stored. This parameter contains the list of (PLMN, Access Technology) pairs. These PLMNs are listed in decreasing order of priority, meaning the first PLMN has the highest priority.(Refer to 31.102 4.2.54 EFHPLMNwAcT for the detailed data format, refer to 23.122 4.4.3.1.1 for the determination of the priority in cell selection).

EF_PLMNwACT: PLMNwACT stands for 'User Controlled PLMN with Access Technology'. This parameter contains the list of (PLMN, Access Technology) pairs. This information is determined by the user and defines the preferred PLMNs of the user in priority order.

These PLMNs are listed in decreasing order of priority, meaning the first PLMN has the highest priority. This parameter plays important roles to determine which PLMN should be selected first under various cell selection, reselection test. (Refer to 31.102 4.2.5 EFPLMNwAcT for the detailed data format, refer to 23.122 4.4.3.1.1 for the determination of the priority in cell selection)

EF_OPLMNwACT : OPLMNwACT stands for Operator controlled PLMN selector with Access Technology. This parameter is set by Network Operator. This is where Network operator put another networks which has roaming agreenment with. This parameter contains the list of (PLMN, Access Technology) pairs. These PLMNs are listed in decreasing order of priority, meaning the first PLMN has the highest priority. (Refer to 31.102 4.2.53 EFOPLMNwACT for the detailed data format, refer to 23.122 4.4.3.1.1 for the determination of the priority in cell selection)

EF_HPPLMN: OPLMNwACT stands for "Higher Priority PLMN search period" (Be careful. This is a Timer value, not a PLMN)

This parameter specifies the interval of time between searches for a higher priority PLMN (Refer to 31.102 4.2.6 EFHPPLMN for the detailed data format)

One typical example for this parameter is as follows

• i) UE is registered to a VPLMN cell. (It means that UE is not registered to cell with the highest priority)
• ii) Now UE is in Idle mode.
• iii) Since UE is not in connection to the highest priority cells, UE periodically search for the higher priority cell.

Now the question is how often UE search for the higher priority cells. This is determined by EF_HPPLMN. The basic periodicity is 6 min and EF_HPPLMN specifies the multiples of the 6 min. For example, if EF_HPPLMN = 2, UE searchs for the higher priority cells every 12 mins (2 x 6 min).

How about if EF_HPPLMN = 0. In this case, UE never searches the higher priority cells.

EF_LOCI: LOCI stands for 'Location Information'. This field is the one that stores TMSI value.

When the domain preference is set to be CS Only or CS + PS, UE gets RPLMN from this field.

This field stores following informations.

•   Temporary Mobile Subscriber Identity (TMSI);
•   Location Area Information (LAI);
•   Location update status.

One thing you have to notice here is the 'location update status'. This part indicate one of the following status.

• 0 0 0 : updated.
• 0 0 1 : not updated.
• 0 1 0 : PLMN not allowed.
• 0 1 1 : Location Area not allowed.
• 1 1 1 : reserved.

(Refer to 31.102 4.2.17 EFLOCI (Location Information) for the detailed data format)

EF_PSLOCI : This field is the one that stores P-TMSI

When the domain preference is set to be PS only, UE gets RPLMN from this field.

EF_LRPLMNSI : LRPLMNSI means 'Last Registered PLMN Selection Indication.

• If this value is set to be 1, UE attempt to camp on Home PLMN when power on.
• If this value is set to be 0, UE attempt to camp on Home LRPLMN when power on.

(Refer to 31.102 4.2.86 EFLRPLMNSI (Last RPLMN Selection Indication) for the detailed data format)

EF_EPSNSC : This field contains EPS NAS Security Context Information. K_ASME(Access Security Management Entity) is master key derived from CK/IK and used to derive subsequent Keys. (Refer to PDCP Integrity Process section for Key Relations). This is a LTE specific field.

EF_EPSLOCI : This field contains EPS location information such as GUTI(Globally Unique Temporary Identifier), Last visited TAI (Tracking Area Identity), EPS Update Status. This is a LTE specific field.

EF_CBMI : CBMI stands for Cell Broadcast Message identifier selection. This EF contains the Message Identifier Parameters which specify the type of content of the cell broadcast messages that the subscriber wishes the UE to accept. Any number of CB Message Identifier Parameters may be stored in the USIM. No order of priority is applicable.

(Refer to 31.102 4.2.14 for the detailed data format)

Reference

• ETSI TS 131 102 Characteristics of the Universal Subscriber Identity Module (USIM) application