1991 年，一家名为 Giesecke & Devrient 的公司推出了用户身份模块或 SIM 卡。它们用于存储用户的个人信息和详细信息，用于连接到服务。只要设备支持特定的用户识别模块卡，就可以将 SIM 卡转移到不同的设备上。它们有多种格式，例如 32K、64K、128K 和 256K。
“K”代表特定 SIM 卡中可用于存储的内存千字节。随着数量的增加，内存空间也增加了，并且适应了更好的互联网连接技术。 128K、256K等最新版本，增加了手机丢失追踪功能。较早的版本，如 32K 和 64K，存储空间非常少，并且可以在卡上存储很少的消息。
32K和64K SIM卡的区别：
| S.No. | 32K | 64K | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1. | It was popular during the 1990s. | It was available after 2001. | |||||||||
| 2. | It is not used much in today’s world as it has become outdated. | It is used in present times. | |||||||||
| 3. | It can be used to store the contact information of lesser people of about 250. | It can be used to store the contact information of more people of about 500 (approximate value, as it depends upon the provider). | 4. | It is used for accessing 2G data services. | It is used for accessing 2G and 3G versions of data services. | 5. | It cannot be used for tracking phone in case of theft or loss. | It can be used to track the phone in case of theft or loss. | 6. | The network operators can run few mobile network operations on it because it has very less storage compared to 64K SIM cards. | The network operators can run more mobile network operations on it because it has more storage. |
| 7. | It has 32 Kilobytes of storage space. | It has 64 Kilobytes of storage space. | |||||||||
| 8. | It does not have SMS lock, auto roaming and direct access to social network accounts. | It has SMS lock, auto roaming and direct access to social network accounts. |