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SIM Card

Chika C Uchendu
Last Updated May 27, 2022 4:21 am

A SIM card (Subscriber Identity Module) card is a tiny, portable memory chip or integrated circuit containing unique information that identifies it to a specific mobile network.

SIM cards store relevant information needed in identifying and authenticating subscribers on mobile telephony devices to receive calls, send SMS messages, or connect to mobile internet services with their mobile devices. In addition, SIM cards are also used in satellite phones, smartwatches, computers, and cameras.

SIM cards and GSM

Sim Card

The SIM card is fundamental to the Global System of Mobile Communication (GSM) network. Developed and overseen by the European Telecommunications Standards Institute (ETSI), GSM is used to describe protocols for second-generation (2G) cellular networks. It is the primary network type used all over the world. Users can swap SIM cards from one GSM phone to another without loss of data. The new phone will connect to the GSM network that the SIM card is tied to, e.g., T-Mobile or AT&T, without requiring approval. 

The mobile SIM is a necessity for all GSM phones. A notable exception is mobile devices that operate on a Code Division Multiple Access (CDMA) network—an additional network type used in the United States and provided by carriers like Verizon Wireless, Virgin Mobile, and Spring. Phones using CDMA may have SIM Card or SIM card slots in line with the LTE standard requirement or for use with foreign GSM networks. 

Types of SIM card

SIM card sizes compared.
Variation in SIM sizes. (US Mobile)

Different SIM card sizes exist with different memory capacities ranging from 32KB to 128KB. This memory space is independent of the card’s size. Also, the same unique piece of information regarding ICCID, IMSI, etc., are stored on the integrated circuit regardless of the size. The first SIM cards (full SIM) were considerably larger than those used today. . With a dimension of 85 mm x 53 mm, they’re roughly the size of a credit card and have gone out of use. Other popular types of SIM and their sizes are:

  • Mini-SIM:25 mm x 15 mm
  • Micro-SIM: 15 mm x 12 mm
  • Nano-SIM: 12.3 mm x 8.8 mm

Phones using Nano-SIMs include Samsung Galaxy S6 & S7 and iPhone 5 & later versions. Samsung Galaxy S4 & S5, iPhone 4 and 4S use large Micro-SIM cards. 

With a 6mm x 5mm dimension, the embedded SIM (eSIM) is the next generation SIM. Most phone models made after 2018 use eSIM. 

How a SIM card works

Once you buy a new GSM phone, you need to connect it to a mobile phone network to make calls. If you already have a SIM that you’re using in your old phone, you can insert it into the new phone and start making calls, texting, etc. 

However, if you’re a new subscriber, or don’t have a SIM card from your previous phone, you need to sign up for an account plan with your favorite network to be able to make calls.  During sign-up, you provide vital information about yourself and agree to pay a certain amount monthly as stipulated in the plan.  This information is stored in the company’s database. 

After signing up, your network operators will provide you a SIM card which you’ll then use to activate the plan and start using your phone. The SIM card links your account to your phone and, through the International Mobile Subscriber Identifier (IMSI), identifies your phone to the network.  That way, networks can link what you do on your handset to your account and charge you accordingly.

Features of a SIM card

The tiny integrated circuit at the back of the SIM contains its primary features. These include:

  • Integrated Circuit Card Identifier (ICCID): The ICCID  is 19-digits long and is the primary account number. It incorporates sections like Individual Account Identification (IAI), Issuer Identification Number (IIN), checks digits, etc.
  • International Mobile Subscriber Identity (IMSI): The IMSI is 15 digits long and occupies 64-bit space. It is used in identifying an individual operator’s network. It encompasses Mobile Country Code or MCC (the first three digits), the Mobile Network Code or MNC (2 to 3 digits next to the MCC), and Mobile Subscriber Identification Number or MSIN (remaining digits after the MNC).
  • Authentication Key or Ki: The Ki occupies a 128-bit space in the SIM memory and is used to authenticate your phone on the mobile network. It is assigned by the network operators during personalization and is unique to each SIMs. The authentication key is stored in the operator’s database.
  • Location Area Identity (LAI): The LAI is a series of digits representing a specific usage location. The operator’s mobile network is divided into smaller locations referred to as Local Area. Each location has its unique identification number.
  • Personal Identification Number (PIN) and Personal Unblocking Key (PUK): Both of these numbers serve security purposes.  Phone networks set a default PIN which you can change to a more secured digit that you can easily remember. 

Enabling PIN protection means that you have to enter your PIN whenever your phone is switched on. The PIN prevents unauthorized access to your phone. Entering the wrong PIN three times in a row blocks the SIM card. You then will need a PUK number from the network operators to unblock it. Entering the PUK incorrectly ten times in a row permanently disables the SIM card.

Benefits of SIM cards

SIM cards are beneficial in two key ways.

Communication access: While you can still snap pictures, listen to downloaded music, and watch videos without a SIM, making calls, sending SMS messages, and connecting to mobile internet services like 3G, 4G, and 5G becomes impossible. However, you can still connect to WiFi. 

Information storage: The SIM card stores text messages, up to 250 contacts, and other vital information used in identifying subscribers, authenticating them, and providing security. Other information stored includes a Short Message Service Centre (SMSC) number, Service Provider Name (SPN), Service Dialing Number (SDN), Value Added Service (VAS), etc. By leveraging the SIM Tool Kit standard, basic applications like chatting, cell broadcast, phonebook backup, location-based services are also possible