Description
The Short File Identifier (SFI) is a crucial addressing mechanism within the smart card file system architecture defined by 3GPP, primarily for the Universal Integrated Circuit Card (UICC) which hosts the USIM application. It is a compact, 2-byte (16-bit) value that uniquely identifies an Elementary File (EF) within the context of its parent Dedicated File (DF), typically the DF for a specific application like the USIM (DF_GSM or DF_Telecom). The file system on a UICC is hierarchical, resembling a tree structure with a Master File (MF) at the root, Dedicated Files (DFs) as directories, and Elementary Files (EFs) as leaves containing actual data. While files can also be selected using longer, absolute file identifiers (FIDs), the SFI provides a shorter, more efficient alternative for frequent access.
Architecturally, the SFI is assigned during the personalization or application creation phase of the UICC. Each EF that is expected to be accessed regularly by the mobile terminal (ME) is assigned an SFI. The ME (like a mobile phone) uses the SFI in specific commands, such as READ BINARY or UPDATE BINARY, to quickly reference the desired EF without needing to navigate the full file path from the MF. This is managed through the Application Protocol Data Unit (APDU) command interface between the ME and the UICC. When the ME wants to read, for example, the EF storing the International Mobile Subscriber Identity (IMSI), it can issue a command specifying the SFI value associated with that EF, leading to faster and more power-efficient data retrieval.
How it works involves a mapping table, often defined in the USIM specification, that correlates SFI values to specific EFs and their purposes. For instance, SFI '6F07' is standardized for the EF containing the IMSI. The key advantage is efficiency in terms of command overhead and processing time on the constrained microcontroller of the UICC. Using a full FID might require multiple SELECT commands to navigate the directory tree before accessing the EF, while the SFI allows direct access if the correct DF (application) is already selected. This is particularly important during critical procedures like network registration, where speed and reliability are paramount. The SFI system ensures that essential telecommunication data can be accessed rapidly and predictably, forming a low-level but vital part of the subscriber identity and service management infrastructure.
Purpose & Motivation
The SFI was created to optimize the interaction between the Mobile Equipment (ME) and the smart card (UICC/USIM) in 2G/3G systems and beyond. Early smart card systems used full file paths for access, which required multiple command exchanges. Given the constrained processing power of early smart cards and the need for swift network access procedures (like power-on registration), this overhead was problematic. The primary problem the SFI solves is the reduction of communication latency and command complexity for accessing frequently used data files.
Its creation was motivated by the need for a fast, standardized addressing scheme for essential subscriber and network data. By assigning well-known, short identifiers to critical files (e.g., IMSI, ciphering keys, phonebook entries), the mobile phone could access this data with minimal APDU commands. This directly improved the user experience by reducing the time to register on the network after power-on. It also conserved battery life by minimizing the active communication time between the phone's baseband processor and the UICC. Furthermore, it provided a cleaner abstraction for handset manufacturers; they could write software that referenced 'SFI for IMSI' rather than managing complex file selection logic. The SFI addressed the limitations of the more generic, but slower, file selection mechanism using full File IDs (FIDs), creating an optimized pathway for performance-critical data access within the standardized smart card ecosystem.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (32 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-4, normative work from Rel-15.
In Release 15, the specification text formally defined the "Short File Identifier (SFI)" as a 5-bit abbreviated name for a file within a directory on the UICC. This definition was provided as part of broader updates to USIM management procedures for 5GS. The release also included updates to other USIM data, such as the Equivalent HPLMN list and configuration data for Mission Critical Services.
- Subscription identifier privacy suppport TS 31.102CR0778
- USIM Service Table update for PDU session call control support TS 31.102CR0786
- Allow configuration of MCS (Access Identity 2) via USIM. TS 31.102CR0794
- Mission Critical Services configuration data update to USIM TS 31.102CR0808
- Enhance USIM OPL configuration to support 3 bytes TAC when in NG-RAN. TS 31.102CR0818
- Updates to USIM management procedures for 5GS TS 31.102CR0806
+ 1 more changes
In Release 16, the Short File Identifier (SFI) function itself was not updated, but the release introduced several new USIM configuration and storage capabilities. These included dedicated storage for a non-3GPP access security key (KSEAF), lists for PS Data Off configuration and Trusted non-3GPP access networks, and a new dedicated file (DF_SAIP) for SUCI calculation in eUICCs. These additions expanded the USIM's role in managing network access policies and security parameters.
- Support for USIM configuration of RLOS PLMN list TS 31.102CR0847
- URSP storage in USIM TS 31.102CR0861
- Specify storage for a potentially separate KSEAF for non-3gpp access on the USIM TS 31.102CR0864
- USIM configuration of RLOS allowed MCC list TS 31.102CR0881
- Support for Trusted non-3GPP access networks list by USIM TS 31.102CR0891
- Dedicated AID for USIM Applications with non-IMSI based SUPI Types TS 31.102CR0897
+ 4 more changes
In Release 17, the SFI function saw corrections to the file identifiers for specific USIM elementary files, namely EFKAUSF_DERIVATION and EFMCHPPLMN. Additionally, a correction was made to the file identifier for the EF5G_PROSE_UIR file. These updates ensured the proper 5-bit abbreviated naming for these files within the UICC directory structure.
- Introduce a USIM file to store pre-configured CAG information list TS 31.102CR0904
- SOR-CMCI storage in USIM TS 31.102CR0917
- Addition of USIM files for the indication of whether disaster roaming is enabled in the UE, disaster roaming wait range, disaster return wait range and applicability indicator for disaster roaming PLMNs list provided by VPLMN. TS 31.102CR0938
- Adding eDRX parameters in the USIM for NG-RAN TS 31.102CR0943
- 5G NSWO (Non-Seamless WLAN Offload) configuration support in the USIM compromised proposal. TS 31.102CR0946
- Support of 'No E-UTRA Disabling In 5GS' in USIM TS 31.102CR0947
+ 4 more changes
In Release 18, the SFI (Short File Identifier) function itself was not directly modified. The release instead introduced new Elementary Files (EFs) to be stored on the USIM, such as the EF for IMS Data Channel configuration and the EF for Access Control to GBA_U_APIs. These additions would utilize the existing SFI mechanism, a 5-bit abbreviated name for a file in a directory on the UICC, for their identification and access within the USIM's file structure.
- 5G Security Parameters extended storage on USIM (Mandating Service n°133 to be enabled when Service n°123 is enabled) Rel18. TS 31.102CR1014
- Add EF of Access Control to GBA_U_APIs to the USIM TS 31.102CR1007
- Add EF of IMS Data Channel configuration to the USIM TS 31.102CR1006
- Correction of DF Identifier for 5MBSUECONFIG TS 31.102CR0984
In Release 19, the specification for the Short File Identifier (SFI) function itself was not updated. The release instead introduced a new backward compatibility procedure for the USIM, specifically handling cases where a USIM lacks the extended security parameter storage in the EF_5GAuthKeys file. This ensures proper network access and security operations even when this specific file, identifiable by its SFI, is not present on the card.
- Backward compatibility handling of USIM without extended security parameter storage in EF_5GAuthKeys - Rel19 TS 31.102CR1074
Explore further
Broader topics and technologies where SFI plays a role.
Defining Specifications
3GPP specifications that define or reference SFI, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 31.102 vj40 | USIM Application Specification | Rel-19 |
| TS 31.103 vj00 | ISIM Application Specification | Rel-19 |
| TS 31.121 vi50 | UICC-terminal interface test specification | Rel-18 |