SIM cards and communication

A mobile phone/handset (or User Equipment, UE for short, as per the mobile standards) is a small computer, tailored to interact with you using touch on the screen as input. In order to be useful, the UE is connected to the outside world in two ways;

  • WiFi – a data only type of connection, based on one of the 802.11 standards
  • Mobile operator connection, based on one of the 3GPP standards

The mobile connection

The mobile connection is to the service provided by your mobile operator (or “carrier” as they tend to say, even if this more correctly is the player on the market that connects operators). With this subscription you get voice, messaging and data.

  • Voice is a dedicated service using the “phone number” (formally the E.164 number).
  • SMS is a text based communication that uses the same phone number for addressing.
  • Data is also provided
  • MMS and Circuit Switched Video are two services obsoleted by development. Technically MMS is available in 2G and 3G networks, and CS video in 3G only.

The service is provided inside the operators service area, and using the mean of roaming (the operator has an agreement with other operators to allow it’s customers access to the service area of the contracted partner), service is extended beyond the operators own service area.

Key here is that the SIM card is the key to authenticate yourself towards the operator. Think of it like a user name password implemented in a chip (typically also called a “hard token”). The SIM manifests the key to the mobile subscription.

=> Without the SIM card in the handset, you cannot access the mobile network.
=> Using another another SIM card, you have setup your handset to use that connection (including the phone number associated with that SIM card)

One can add that 4G/LTE is a data only to topology. That means that the voice service in 4G networks basically is an operator provided OTT service.

WiFi Connection

The WiFi connection is, as said, data only. Services that uses the data bearer (wifi or the mobile operators data bearer) for communication is called Over The Top (OTT) services. Herre you find Viber, WhatsApp, Hangouts, Facebook Messenger, WeChat, LIME, Zoom, BlueJeans and the plethora of other communication tools which allow instant messaging, voice calling and video.

SIM cards and form factors

SIM cards come in different sizes. The old standard was the credit card size, which Motorola was the last to abandon. Then you have the mini/standard, micro and nano ones.

Apple was pioneering usage of the smaller form factors, but nowadays the nano is commonly used with all manufacturers. There seems to be an erroneous misconception that there are special iPhone SIMs. There is not.

In addition we have just seen the advent of eSIM. This is the technology where the SIM sits inside the handset and can be updated with the cryptographic keys. Once installed, the work logically the same as the physical SIMs.

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