**Anatomy of a SIM Card**

**Anatomy of a SIM Card**

Let’s break down what’s stored inside that tiny chip:

1️⃣ **ICCID (Integrated Circuit Card Identifier)**:
– A **19-20 digit serial number** etched into the SIM.
– Example: `8944 50 1234567890 1` → *Country code (89), Network code (44), Unique ID, Checksum*.

2️⃣ **IMSI (International Mobile Subscriber Identity)**:
– A **15-digit code** that identifies *you* globally.
– Structure: `MCC (Mobile Country Code) + MNC (Mobile Network Code) + MSIN (Your unique ID)`.

3️⃣ **Authentication Key (Ki)**:
– A **128-bit secret key** stored securely on the SIM and the carrier’s database.
– Used to encrypt your data and verify your identity (so hackers can’t clone you! ).

4️⃣ **SMS & Contacts Storage**:
– Older SIMs could store **up to 250 contacts** and **50 texts** (nostalgic, right? ).

5️⃣ **Network Data**:
– Preferred networks, roaming settings, and service codes (like *#100# for balance checks).

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⚙️ **How a SIM Works: Step-by-Step**

1️⃣ **Insertion & Boot-Up** :
– When you power on your phone, the SIM sends its **ICCID** and **IMSI** to the nearest cell tower.

2️⃣ **Network Authentication** :
– The carrier’s server checks if your IMSI matches their database.
– A **challenge-response** happens: The network sends a random number (RAND), and the SIM uses its **Ki key** to generate a signed response (SRES). If it matches, you’re in!

3️⃣ **Encryption Setup** :
– After authentication, the SIM and network agree on a **temporary session key (Kc)** to encrypt your calls, texts, and data.

4️⃣ **Roaming Magic** :
– If you travel, your SIM uses **Preferred Roaming Lists (PRL)** to connect to partner networks abroad.

5️⃣ **eSIM Evolution** :
– **eSIMs** (embedded SIMs) are *software-based*. No physical swapping! Just scan a QR code to activate a new plan.

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**SIM Types & Sizes**
Over the years, SIMs shrank to fit sleeker devices:
– **Full-Size (1991)**: Credit-card-sized (for old-school car phones!).
– **Mini-SIM (1996)**: The classic “SIM card” (15mm x 25mm).
– **Micro-SIM (2003)**: Trimmed edges (12mm x 15mm).
– **Nano-SIM (2012)**: Almost all-chip (8.8mm x 12.3mm).
– **eSIM (2016)**: No physical card—built into your phone!

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**Security: Why SIMs Are Crucial**
– **Anti-Cloning**: The **Ki key** is *never transmitted*—it stays locked in the SIM.
– **PIN/PUK Codes**: Extra layers of protection.
– **Remote Management**: Carriers can update SIM settings over-the-air (OTA).

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** Final Fun Fact**: The first SIM was launched in **1991** by German company Giesecke+Devrient for Finland’s Radiolinja network. Since then, over **20 billion SIMs** have been produced!

Disclaimer – This post has only been shared for an educational and knowledge-sharing purpose related to Technologies. Information was obtained from the source above source. All rights and credits are reserved for the respective owner(s).

Keep learning and keep growing

Source: LinkedIn

Credits: Mr. Ahmed Alnemer