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what is a sim card

What is a SIM Card & What does it do?

Author: Anthony Lee

Phone & Tablet Expert

sim card types explained

The SIM card is an integral part of every mobile phone. With the help of the SIM card, the PIN, the phone number and the individual contacts. It serves to identify the user in the network – “SIM” stands for “Subscriber Identification Module”. Besides, the mobile operators provide the user with a mobile phone and data connections using the SIM card.

What is a SIM Card?

A SIM card is like a small processor with internal memory: it is used to assign a cell phone or smartphone to a cellular network. Access to the network is authorized and the voice and data transmission are encrypted using number codes stored in the SIM card. Temporary network-related data can also be saved on the SIM card.

The phone contacts, a memory for SMS and the last dialed phone numbers are also stored on the SIM card. A so-called SIM toolkit can be used to store small programs on the SIM card, for example, messaging services from which one can get the current information sent to them. The SIM toolkit is used to top up or query the prepaid credit. The plus point for such small programs that are stored directly on the SIM card is that they can be used regardless of the smartphone.

This way, the SIM card can be used in a new cell phone without any apparent change in functionality. The manufacturers are now working on SIM cards that have particularly large storage space – similar to the SD card from the digital camera. This memory could then be used for music, pictures or other documents, for example.

SIM cards age

The ravages of time are also gnawing at SIM cards: They don’t have an unlimited lifespan, because they “age” every time the SIM card registers in the mobile network. Most manufacturers say that the lifespan is around 100,000 such authentication processes. Some SIM cards have a kind of life counter inside, which reacts to every switch on and off. When the counter reaches zero, the card is automatically deactivated.

What is a Dual SIM Card?

Use two SIM cards and thus two different tariffs in parallel with a cell phone, be reachable via two numbers and significantly reduce the cell phone bill – no problem thanks to dual SIM cell phones. If you have a dual SIM cell phone, you can insert two different SIM cards at the same time. Before each call, SMS or Internet access, the user can decide which of the two cards should be used.

How does dual SIM work?

Dual-SIM phones can hold two SIM cards through a double slot. This means that the user can be reached via two numbers and can use two tariffs from two different providers at the same time. The two independent cell phone connections can be used alternately or simultaneously and switched off individually – as long as the cell phone is equipped with several transmitting and receiving units.

When is dual SIM worthwhile?

There are primarily financial advantages for users: if you combine two network cards, you can choose the strength of each tariff. With so-called tariff sharing, for example, a card with unlimited data can be used for fast surfing on the Internet, while the other uses a low-cost tariff for making calls. Before each cell phone is used, the consumer then decides which card to use – depending on whether he wants to make a call, send an SMS or surf the Internet.

The advantage: The user remains reachable with the main card via the usual phone number and can always use the cheapest alternative for outgoing calls or browsing. Even those who regularly stay abroad can save if they use a local and a foreign prepaid card at the same time.

Another option for using dual SIM results from the combination of two connections. For example, if you always have a second cell phone with you for professional reasons, you can use the second SIM card to only make business calls – without having to carry an annoying additional mobile device with you.

The dual SIM offers are becoming more attractive

The use of a dual SIM device is becoming more and more attractive due to the increasing diversity and the improved range of devices. While only a few years ago hardware had to be cut back, today the equipment of the dual SIM smartphones or cellphones is getting better and better. Besides, there is now a very decent selection of attractive mid-range devices. As the focus of the dual SIM market has historically been in Asia, top models with dual SIM are also available in Europe and the USA, which unfortunately still have to be avoided.

What is a Micro-SIM?

Since cell phones and smartphones are now getting smaller and thinner, there is also a miniature version of the SIM card – the micro-SIM. This version of the SIM card is only 15 x 12 millimeters in size. For comparison: a common SIM card has the dimensions 25 x 15 millimeters. There are no significant differences concerning the chip, except for the internal memory. Despite the smaller format, this is about 50 percent larger. Apple’s iPad was one of the first devices to use a micro SIM card.

Apple as the founder of the micro-SIM card

It is not surprising that micro and nano-SIM cards are becoming more and more popular since one of the leading manufacturers of modern cell phones is responsible for the creation of these small SIM cards. The main reason for this is the launch of the iPhone and iPad. The previous card models were a bit too big for these small devices. The micro-SIM card is by no means the end of development – the smallest variant, the nano-SIM card, is just under 40 percent smaller.

For a long time, it is no longer just Apple that is concentrating on the use of micro-SIM cards. Other providers are now offering these smaller cards to activate the tariffs so that every user can purchase the right card for their needs.

Turn a SIM card into a micro SIM card

Another interesting fact is that micro-SIM cards do not always have to be supplied as such. They are often sent as a normal SIM card to enable compatibility and possible use in other devices. Changing the format is usually easy to do. Breaking out is possible as well as cut out with the help of a punch. This costs only a few euros and guarantees clean and safe punching out of the mold. The remnants of the cut should not be thrown away, however, to convert the micro-SIM card into a SIM card if necessary.

What is a Nano-SIM Card?

The successor to the micro-SIM is the even smaller nano-SIM, which enables smartphone manufacturers to build even flatter housings. Nano-SIM is the name of a fourth-generation SIM card. The nano-SIM card is only 12.3 x 8.8 x 0.67 mm in size. The difference to the previous SIM card models, such as micro, mini or multi-SIM, is that the nano-SIM consists almost exclusively of the chip (processor & memory).

In June 2012, Apple’s design proposal was adopted as the standard. The cards are backward compatible – older SIM cards can be cut to size. The iPhone 5 and the Galaxy SIII work with such cards.

How does a nano-SIM card differ from other SIM cards?

In its core functions, a nano-SIM card is no different from its predecessors. Only the size has been reduced so that at 12.3 x 8.8 mm with a thickness of 0.67 mm it is 40 percent smaller than a micro-SIM. This space-saving is extremely important for modern smartphones, because the smaller the components, the more space there is for additional hardware. For example, it would be possible to install a larger battery that supplies energy-hungry features such as LTE. Thanks to the established SIM standard, backward compatibility is guaranteed, which means that an adapter turns a nano-SIM into a micro-SIM card.

Get a nano-SIM card from providers

Anyone who signs a mobile phone contract will receive a SIM card from the provider. For a long time, this was a micro SIM card that was simply inserted into the cell phone. Most network providers now offer nano-SIM cards to choose from when signing a contract, but if this is not the case, the customer faces a problem: Modern smartphones such as the iPhone or Nokia Lumia absolutely need a nano-SIM, but they do the included micro-SIM is simply too big for the nano-slot.

Cutting the SIM card is strongly discouraged

Even if there are numerous guides and instructions, you should refrain from cutting them yourself. Since the micro-SIM differs from the nano-SIM not only in width and length but above all in thickness, these fractional millimeters would have to be planed out. On the one hand, the card itself can be damaged, on the other hand, metal chips can touch electronic contacts when inserted into the smartphone and cause a short circuit.

An immense risk if you compare the costs for a new purchase of the mobile phone and the price for a nano-SIM – especially since most providers provide free cards. Anyone who lends a hand and cuts out or cuts the SIM card, can lose the guarantee under iPhone and Co. and runs the risk of having to pay a high price at a later date.

Which smartphones need a nano-SIM?

Apple prevailed in the development of the new SIM card standard against the competitors from Nokia, Motorola and RIM (Blackberry), which is why all manufacturers now offer smartphones with a nano-SIM slot. This is mandatory among others for Apple starting from the iPhone 5, the HTC One (M8), the Motorola Moto X and the Nokia Lumia 1520.

What is an eSIM?

An eSIM is a SIM card permanently installed in the end device. The “e” stands for “embedded”. eSIM cards are the latest generation of SIM cards and are programmed by mobile phone providers as required.

What are the advantages of an eSIM?

  • Smaller than a plastic SIM card
  • Simplified change of mobile phone contracts
  • Use in small devices such as smartwatches
  • Use of eSIM devices even without a smartphone


In the mobile radio industry, there is increasing talk of an electronic standard for SIM cards. The eSIM is a SIM card (“embedded SIM”) built into the smartphone, and its main advantage is the programmable – and thus simplified – change of contract. Critical voices, on the other hand, warn of a loss of control by the customer and the abolition of the freedom of choice. Apple is a pioneer in its use. With the release of the iWatch Series 3, Apple launched a smartwatch that optionally integrated an eSIM. The second major device manufacturer Samsung also wants to support the new generation of SIM cards.

The eSIM in use: flexible and fast

The stationary SIM cards integrated into the mobile device bring some simplifications. eSIM cards can be controlled by radio. In this way, the provider or the tariff can be changed without changing the card – and that within a few minutes. While the standard SIM cards are linked to a mobile network provider, the built-in eSIM cards remain assigned to the device. This makes switching to a new smartphone easier: Users no longer have to pay attention to whether the card fits in the new device, maybe not the wrong size, or how the card slot can be opened.

Anyone who previously used two or more SIM cards in a smartphone (dual SIM or multi-SIM) would no longer be dependent on the appropriate hardware. The eSIM could simply be registered with several providers and used as required. Or the other way around: An eSIM could be activated and used on different devices – as soon as it is connected to the smartphone via Bluetooth, for example. Abroad, changing providers and networks would also be more flexible. So tourists can easily activate a suitable internet package with a local provider – and save roaming costs or additional prepaid cards.

Criticism: weakened competition expected

In the coming years, both standards, the eSIM, and the conventional SIM card will initially exist in parallel. Because the introduction of the eSIM costs a lot of money, big names like Apple and Samsung are likely to expand their pioneering role. Critics fear that the built-in eSIM card will restrict competition among mobile operators and take away the freedom of choice for the customer.

Because the device manufacturers could only activate their eSIM cards for the use of certain tariffs or network operators, but block the rest – which would mean a significant intervention in the competition. So far, customers themselves have decided on the combination of a mobile phone contract and the device used – thanks to the insertable SIM card.

Prepaid cards, with which users can make cheap and, above all, anonymous calls today, would also be out of date. It remains to be seen how the legislator will react to these challenges and which regulations will be made.

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