What is a SIM card?

According to Ericsson’s Mobility report nearly 8 billion mobile devices are connected to cellular networks worldwide. This is more than the entire population on earth and it will not stop there. As stated in the same report, every quarter an additional 53 Mio subscription will be added. All these subscriptions use a SIM card, this tiny little peace of plastic with a micro chip on it. And all this SIM cards are inserted in Smartphones, tablets, laptops and many other devices. The SIM card is required to get connected to the cellular network. But what is its purpose of a SIM card?

What is a SIM Card Used For?

A SIM cards is a piece of plastic which holds a micro-chip. It is basically the “ticket” or “key” to get access to a cellular network. Without a SIM card, devices will not be able to connect to a cellular network. It holds all the required information to get access to the network and identify the subscriber and its device.

 

Which information is stored on a SIM card?

SIM cards store network-specific information enabling a subscriber and device to authenticate and getting identified on the cellular network. The most important information is:

  • ICCID: Each SIM is internationally identified by its integrated circuit card identifier (ICCID). The ICCID is printed or engraved on the SIM cards, but also stored on the SIM chip itself. It is a between 20 and 22 digits long. It mainly serves as the visible identifier of a SIM card.
  • IMSI: The international mobile subscriber identity (IMSI) is a unique number, usually fifteen digits, associated with Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) network mobile phone subscriber. It is stored on the chip on the SIM and is usually not visible to the subscriber. The IMSI is the key information on the SIM cards. It is used by the cellular network to identify the subscriber and device. Based on the IMSI, the operator grants the subscriber and its device access to the network and services. On the operator side, the IMSI is registered in a database, together with subscriber information and the selected price plan. This eventually decides which network and services a subscriber can use and how much he is going to be charged. In the usually 15-digit number of an IMSI, the first three digits represent the Mobile Country Code (MCC). The next two or three digits represent the Mobile Network Code (MNC). The next digits represent the mobile subscriber identification number (MSIN).
  • Authentication Key (Ki) is a 128-bit value used in authenticating the SIMs on a GSM mobile network (for USIM network. Each SIM holds a unique Ki assigned to it by the operator during the personalization process. The Ki is also stored in a database on the operator’s network. This delivers a pretty good protection against faking a SIM card.
  • Localation Area Identity (LAI): The SIM stores network state information, which is received from the cellular network. Operator networks are divided into Location Areas, each having a unique LAI number. When the device changes locations, it stores the new LAI to the SIM and sends it back to the operator network with its new location. If the device is power cycled, it takes data off the SIM and searches for the prior LAI.
  • Various other carrier-specific data such as Operator-Specific Emergency Number, the SMSC (Short Message Service Centre) number. Others such as subscribers contact information can be stored on the SIM card as well. But with the rise of smartphones, this has become obsolete for the most users.

 

Which size of SIM cards are available?

Unfortunately, not all devices demand the same form factor. That’s why cell phone SIM cards come in 4 form factors:

  • 1FF 85.6mm × 53.98mm × 0.76 mm – no longer used in modern day devices
  • 2FF 25mm x 15mm x 0.76mm – called Mini SIM
  • 3FF 15mm x 12mm x 0.76mm – called Micro SIM
  • 4FF 12.3mm × 8.8mm × 0.67mm – called Nano SIM

This is an example of a SIM card supporting all form factors.

Are there Cellular devices without SIM cards?

Yes, there are cellular devices without SIM cards. But to fully understand this, you need to know the difference between GSM carriers and CDMA carriers.

GSM stands for “Global System for Mobile Communications,” while CDMA stands for “Code Division Multiple Access.” The difference between the two, in the simplest of terms, is the SIM card (for GSM networks) and the lack of a SIM card (for CDMA networks, although that’s changing). CDMA is mainly used in the US where as GSM is used almost worldwide (including the IS) these days.

So, for all GSM type of phones, a SIM card is required, whereas for CDMA in most of the cases no SIM card is required. Anyway, for most people this is less relevant as CDMA networks are not so widespread.

Who issues SIM cards?

Each SIM is issued by a service provider who has access to an IMSI range. The MCC and MNC of the IMSI shows from which country and operator a SIM card was issued. A service provider can for example be a cellular operator or an MVNO (Mobile virtual network operator) or a reseller.

What about eSIM?

The eSIM, the e stands for electronic or embedded – will replace the plastic SIM card with a virtual embedded equivalent in the device that cannot be removed from the device. It is a rewritable SIM card that is built into devices.

There are a couple of advantages:

  • No need to plug a SIM into a device anymore. All you need to do is download your “virtual” SIM. As your device might be able to host several virtual SIM’s, you can easily swap providers at your fingertips. Subscribing with a new service provider becomes very easy and can be done all online without visiting any store.
  • Manufactures gain more space and flexibility in a device. This leaves more room for innovation and will eventually also help lower the cost.
  • At last but not least, eSIM will certainly push competition in the market which is good news for the users.

One of the major drawbacks is still the lack of availability of devices and provider offering services based on eSIM. A good example for eSIM is certainly the Apple Watch 3 or the latest Microsoft Surface series with integrated LTE module. However, due to the lacking standard the eSIM is not yet very widespread.

 

This article was sponsored by Qynamic.

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