Subscriber Identity Module (SIM), usually known as a SIM card, functions as a tinny smart card that carries crucial details about the user's telecommunication service.

At its core, SIM card functions as a key to accessing a cellular network, allowing mobile phone owners to send and receive calls. In addition, it similarly aids in keeping important personal contacts, text messages, and even in some cases data related to the cellular network.

There are basically three types of SIM cards available in the market: standard, micro, and nano. Each of these variants vary in size, but produce similar functionality.

Every SIM card comes unique by an individual IMSI (International Mobile Subscriber Identity) more info number, which is comprised of a fixed length numeric code. This identifies each subscriber within the telecom network

Moreover, every SIM card comes configured with a specific Authentication Key (Ki), which serves for ensuring communication over the service provider's network.

Switching to these SIM cards' functions, they hold details such as your contacts, service information, and security-related information. Apart from these, they allow services including call forwarding, call waiting, and voicemail.

In conclusion, it's clear that SIM cards are crucial parts of our mobile communication. They are like the identity cards for our phones, making them function correctly. Knowing your SIM information helps in tackling any issues related to your telecommunication service.

Moreover, understanding your SIM card can also assist in making choices about upgrading your phone, switching service plans, and even traveling internationally.