用户身份模块(SIM) 卡插入移动电话。 SIM卡用于唯一标识网络用户。 SIM 卡用于存储消息和联系信息。它还存储有关它所连接的塔的信息以及订户的身份验证密钥。 SIM 卡有多种变体。 “K”代表特定 SIM 卡中可用于存储的内存千字节。
SIM 卡的早期变体包含 32 KB 内存，而较新版本具有 64、128 和 256K 内存。 SIM 卡上通常会提及此内存信息。内存空间越大，越新，功能越多。较新的版本用于 3G、4G 和 LTE 互联网连接。最新版本、128K 及更高版本具有跟踪系统，以防手机丢失。
64K 和 128K SIM 卡的区别：
| S.No. | 64K | 128K | |||
|---|---|---|---|---|---|
| 1. | It was very popular before 2003. | It has become more popular than 64K SIM cards after 2003. | |||
| 2. | It can be used to store contact information of about 500 people. | It can be used to store contact information of about 600 or more people (approximate value, as it depends upon the provider). | 3. | It is used for 3G and 4G data services. | It is used for 4G and later data services. |
| 4. | It does not have computer synchronization and recovery in case of theft or loss of mobile devices. | It has the feature of computer synchronization and recovery in case of theft or loss of mobile devices. | 5. | The network operators can run less mobile network operations on it because it has lesser storage. | The network operators can run more mobile network operations on it because it has more storage. |
| 6. | It has 64 Kilobytes of storage space. | It has 128 Kilobytes of storage space. | |||
| 7. | It may or may not have SMS lock, auto roaming and direct access to social network accounts, depending on the povider. | It has SMS lock, auto roaming and direct access to social network accounts. |