SIM¶

Physical Form Factors¶

Before anything about the card's structure, "SIM" has referred to a shrinking series of physical cutouts of the exact same chip:

UICC Hardware Structure¶

The UICC (Universal Integrated Circuit Card) is the actual smart card hardware; SIM/USIM/ISIM are applications that run on it.² Physically, it's built like any ISO/IEC 7816 smart card:

The 8 physical contacts follow the ISO/IEC 7816-2/7816-3 layout:⁵

C1:  VCC                # (1)!
C2:  RST                # (2)!
C3:  CLK                # (3)!
C4:  (reserved / USB)   # (4)!
C5:  GND                # (5)!
C6:  VPP / SWP          # (6)!
C7:  I/O                # (7)!
C8:  (reserved / USB)   # (8)!

1. C1 — VCC — Supply voltage from the phone to the card (historically 5V, now typically 1.8V or 3V on modern low-power cards).
2. C2 — RST — Reset line; the phone pulses this to trigger the card's Answer To Reset (ATR) sequence.
3. C3 — CLK — Clock signal supplied by the phone; the card has no internal oscillator and derives timing entirely from this line.
4. C4 — Reserved/USB — Historically unused on GSM cards; some modern UICCs use it for a USB data interface.
5. C5 — GND — Ground reference.
6. C6 — VPP/SWP — Originally a programming voltage for early cards; on modern UICCs this is commonly repurposed as the SWP (Single Wire Protocol) line, used to connect the SIM to an NFC controller for contactless payments.
7. C7 — I/O — The single bidirectional serial line carrying every APDU command and response between phone and card — the entire SIM "conversation" happens over this one wire.
8. C8 — Reserved/USB — Paired with C4 for USB-capable cards.

Internally, the chip is a self-contained computer:

SIM vs. USIM vs. ISIM: Applications, Not Cards¶

A modern UICC typically holds multiple applications side by side, selected by their AID (Application Identifier):³

A phone requesting LTE service will SELECT the USIM application's AID; a VoLTE call setup separately selects the ISIM application — both coexisting on the same physical (or embedded) card.

The File System: MF, DF, EF¶

Everything the card stores is organized in a strict tree, defined by ISO/IEC 7816-4 and extended for telecom use by 3GPP TS 31.102:⁴

APDUs: How the Phone Talks to the Card¶

Every single interaction — selecting a file, reading data, running authentication — is one APDU (Application Protocol Data Unit) command/response pair over that single I/O line.⁸

Command APDU¶

Response APDU¶

Common Commands¶

eSIM: Same Model, Remote Provisioning¶

An eSIM is not a new kind of authentication or file system — it's the same UICC model, just soldered to the board as an eUICC (embedded UICC) and loaded with a profile over the air instead of by inserting a pre-programmed card.⁶

iSIM: One Step Further¶

An iSIM (integrated SIM) goes past eSIM by removing the separate chip entirely — the UICC's secure environment is implemented as an isolated, certified enclave directly inside the main cellular SoC, using the same processor die instead of a discrete piece of silicon.⁷ Functionally, it still runs the same USIM/ISIM applications over the same logical file structure — the difference is purely physical integration, aimed at further reducing board space, cost, and power for compact IoT devices.

Useful Resources¶

• 3GPP TS 31.102 — Characteristics of the Universal Subscriber Identity Module (USIM) application — the USIM file structure specification
• ISO/IEC 7816 — Identification cards, Integrated circuit cards — the base smart card physical/APDU standard
• GSMA — eSIM (Embedded SIM) — official eSIM/RSP resources