sim full form

What is the full form of sim card? 

Sim card is an electronic small chip. A GSM mobile devices uses sim card for communication with the mobile operator and rest of the world. The full form of sim is subscriber identity module. As the full form says that it identifies a mobile subscriber globally. It comes with many sizes. In start of gsm network, sim card was in very big size and mobile devices too. Now with the more compact devices, sim size also reduced over the time. There nano and macro sim cards.  Sim have inbuilt memory and small programs called applets.  Sim stores following:

Text messages. 
Phone Contacts.
Authentication Keys
IMSIs and ICCID
Applets  and other data.

IMSI and SIM :

A sim is nothing without an identity. IMSI assigned by the mobile operator to the sim card.  Which is an unique global identifier. Each mobile operator have its own range of IMSIs.  The another important thing sim stores are authentication keys. Keys are used for generating authentication vectors at the time of mobile registration and may be on call setup.  The communication over radio channel is encrypted used authentication vectors for security.

ICCID and SIM:

A sim may have multiple IMISs, roaming sim is one of the example. Based on roaming location a particular imsi gets active.  ICCID is an another unique identifier for the chip , the ICCID number is assigned by a standard body. It is printed on the sim card.

eSIM or embedded sim:

As mobile devices now supporting multiple sim cards. Each operator have its own sim cards, so if you change the mobile service provider. There is need to change the sim card as well. Each sim may have multiple imsis, all should be preconfigured.  But this leads to load a very high number of IMSIs at the time of manufacturing. Which is practically impossible as chip has limited memory.  This limits the roaming options for the roaming sim card providers. eSIM is a new option, where sim card is builtin with in the mobile device . Apple is one of the device manufacturer who has launched the phone with eSIM. With esim any IMSI can be provisioned over the air. This enables the support of very high number of identities and no change in chip when changing mobile operator.

SIM and CDMA :

CDMA is another network for mobile communication as similar to GSM.  But is less in use. In cdma there is no sim card, all information is present in the device for network operations.  This leads to a serious issue , when some one wants to change the device but the the phone number.