ID-1 Form Factor SIM

Description

The ID-1 SIM is a physical form factor of a Universal Integrated Circuit Card (UICC) that hosts a SIM application. Defined in 3GPP TS 21.905 and based on the ID-1 card format specified in ISO/IEC 7816-1, it has the dimensions of a standard credit card: 85.6 mm × 53.98 mm. This was the original SIM form factor, introduced in the early days of GSM, and is often referred to as the "full-size" SIM. The ID-1 form factor was first standardized in 3GPP Release 4, although it was used in practice before that.

Architecturally, the ID-1 SIM is a smart card with an embedded integrated circuit chip that contains a microprocessor, memory, and a serial interface compliant with ISO/IEC 7816-3. The SIM application on the UICC stores subscriber identity information such as the IMSI, authentication keys (Ki), and network-specific parameters. When inserted into a mobile equipment (ME), the ME reads the SIM to authenticate the subscriber to the network and to derive encryption keys for securing the radio interface. The ID-1 form factor is designed to be easily handled and inserted into early mobile phones, which were larger and had more space for the card.

In operation, the ID-1 SIM performs the same functions as other SIM form factors: it authenticates the subscriber using the AKA protocol, stores subscriber data, and provides a secure environment for applications. The card communicates with the ME via APDU commands as defined in ISO/IEC 7816-4. The ME acts as a mediator between the SIM and the network, forwarding authentication challenges and responses.

The ID-1 form factor was the first SIM size used in GSM networks, but as mobile phones became smaller, the industry moved to smaller form factors: the ID-000 (plug-in) SIM, followed by Micro-SIM and Nano-SIM. The ID-1 SIM is now largely obsolete for consumer mobile devices, but it is still used in some specialized equipment, such as early car phones, industrial modems, and test equipment. Its large size made it impractical for handheld devices, but it offered durability and ease of handling.

From a standardization perspective, the ID-1 SIM is specified in 3GPP TS 21.905. The definition was initially "The SIM having the format of an ID-1 card (see ISO 7816-1 [24])." in Release 4 and 5, and was revised in Release 6 to "A UICC having the format of an ID-1 card (see ISO 7816-1 [24]) that contains a SIM." This change reflected the architectural shift from a standalone SIM to a UICC hosting multiple applications. The definition has remained stable since Release 6, with only minor editorial adjustments.

Purpose & Motivation

The ID-1 SIM exists to provide a removable, secure element for subscriber authentication in mobile networks, using a form factor that was already established in the smart card industry. Before the SIM, mobile networks lacked a standardized, secure method for identifying and authenticating users. The ID-1 form factor was chosen because it was based on the widely used ISO 7816 ID-1 card standard, which was already prevalent in banking and other sectors. This allowed the telecom industry to leverage existing card manufacturing and reader technology, reducing costs and accelerating deployment.

The ID-1 SIM was the original SIM form factor for GSM, but its large size (the same as a credit card) made it impractical for the shrinking mobile phones of the 1990s. This motivated the development of the smaller ID-000 (plug-in) SIM, which quickly became the dominant form factor. The ID-1 SIM addressed the need for a secure, removable subscriber identity module, but its physical size limited its application to larger devices. It paved the way for subsequent form factors and established the concept of a removable secure element that could be transferred between devices.

Today, the ID-1 SIM is largely of historical interest, but it remains relevant in certain legacy and specialized applications. Its existence highlights the evolution of SIM technology from large, credit-card-sized cards to tiny embedded chips, driven by the relentless miniaturization of mobile devices.