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eSIM

(Embedded SIM) A chip that is equivalent in function to a Mini or Micro SIM card used to identify the telephone number in a cellphone. With the traditional SIM card, changing phone numbers or carriers requires replacing the card in the device's SIM tray. However, an embedded SIM is built in and programmed remotely by the carrier. Google's Pixel 2 phones and the Apple Watch 3 with Cellular were some of the first devices to employ eSIMs.

eSIMs are also used in wireless machine-to-machine devices that have permanent identities. See SIM card.



SIM card

(Subscriber Identity Module card) A smart card that gives a cellphone its phone number and customer identity. Introduced in 1991 for GSM phones, most cards are removable. Satellite and LTE phones also use SIM cards. Activated by the cellular carrier, the SIM card contains a unique ID, authentication codes, network-specific data and the customer's phone contacts. See IMSI.

As long as users stay with the same carrier, they can generally switch their phone number and calling plan to a different phone by moving the SIM card. For travelers, prepaid SIM cards are available for unlocked phones, and phones that hold more than one SIM are also offered (see multi-SIM). In the early 1990s when cellphones were large, the first SIMs were the size of a credit card (1FF). See eSIM, SIM lock, R-UIM and smart card.




Mostly Nothing
A SIM is just a chip behind contacts. The rest is cardboard. Used for a while were Micro SIMs in a Mini SIM form factor (right).



                   Size   Thick   Form
  SIM Card         (mm)    (mm)   Factor

  Full Size        86x54   .76    1FF
  Mini (Standard)  25x15   .76    2FF
  Micro            15x12   .76    3FF
  Nano             12x9    .67    4FF
  Embedded          6x9   up to 1





Mini SIM
This Mini SIM was removed and flipped over to show the contacts and the six pins they press against.






Open the Case or Slide Out the Tray
Access to a SIM card is either from the inside by taking the back off or by inserting a pin into the side of the phone that releases the tray.






Prepaid SIM Cards
For travelers with unlocked Android and other GSM cellphones, prepaid SIM cards can be purchased.






Mini or Micro
A SIM card may come in both Mini and Micro SIM formats that can be punched out as required.






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