File Identifier

Description

The file identifier (FID) is a 2-byte value that uniquely addresses a file within the file system of a SIM (Subscriber Identity Module) or UICC (Universal Integrated Circuit Card). In the context of 3GPP specifications, the file identifier is used to locate and access elementary files (EFs), dedicated files (DFs), and master files (MFs) that store subscriber-related data, network parameters, and application code. The file system on a UICC is hierarchical, with the master file at the root, dedicated files as directories, and elementary files as the actual data containers. Each file is assigned a file identifier, which is used in APDU (Application Protocol Data Unit) commands to select and read/write the file.

The file identifier is defined in 3GPP TS 21.905 (Vocabulary for 3GPP Specifications), TS 31.121 (UICC-terminal interface; USIM application test specification), and TS 31.829 (UICC-terminal interface; CSIM application test specification). These specifications ensure that UICCs from different vendors and network operators are interoperable with mobile equipment (ME). The file identifier is part of the ISO/IEC 7816-4 standard for smart cards, which 3GPP adopts for UICC file system structure. The 2-byte length allows for up to 65,536 unique file identifiers, which is sufficient for the current and foreseeable needs of UICC applications.

In operation, when the ME needs to read subscriber data such as the IMSI (International Mobile Subscriber Identity), it sends a SELECT command with the appropriate file identifier to the UICC. The UICC responds by making that file the current file, and subsequent READ commands can retrieve the data. The file identifier is also used in the UICC's internal file management, such as when creating or deleting files during personalization. The file identifier is distinct from other identifiers like the application identifier (AID), which identifies an application on the card, and the DF name, which is used for application selection.

The file identifier has evolved over time. In early GSM SIM cards, the file identifier addressed files in the SIM. With the introduction of the UICC in 3GPP Release 4, the terminology shifted to UICC, but the fundamental concept remained the same. The file identifier is also used in the IP Multimedia Services Identity Module (ISIM) and other UICC applications. In later releases, the file identifier continues to be a core element of the UICC file system, supporting new applications and services such as NFC, mobile payments, and IoT.

The file identifier is crucial for security and management. Access to files is controlled by access conditions, which are checked by the UICC before allowing read, write, or update operations. This ensures that sensitive data like authentication keys cannot be accessed without proper authorization. The file identifier also plays a role in remote management, where the network can update files on the UICC using over-the-air (OTA) mechanisms. Overall, the file identifier is a foundational identifier in the UICC ecosystem, enabling standardized and secure access to subscriber data.

Purpose & Motivation

The file identifier exists to provide a standardized way to address files within the UICC file system. Before its introduction, different smart card platforms used proprietary addressing schemes, which hindered interoperability between mobile equipment and UICCs from different vendors. The file identifier solves this by defining a common 2-byte identifier that all compliant UICCs and MEs understand, ensuring that any ME can access the necessary files on any UICC.

Historically, the file identifier was introduced in 3GPP Release 4 as part of the UICC specifications, although the concept originated in earlier GSM specifications for SIM cards. The shift from SIM to UICC brought a more flexible and secure platform for hosting multiple applications, and the file identifier was retained as the fundamental addressing mechanism. It enables the ME to retrieve subscriber identity, authentication parameters, and network configuration data without needing to know the physical layout of the card.

The file identifier also supports the evolution of UICC applications. As new services like ISIM, CSIM, and IoT-specific applications were introduced, they were assigned their own file identifiers within the UICC file system. This allows a single UICC to host multiple applications, each with its own set of files, without conflict. The file identifier remains a stable and essential element of UICC technology, underpinning the security and functionality of mobile networks.