A beginner's guide to SIM cards

A beginner’s guide to SIM cards

When you acquire a new phone, its packaging will always contain a SIM card.

Whether you’re transferring an existing number or setting up a new account, SIM cards underpin every mobile phone’s functionality.

But what are these compact plastic microchips, and how do they identify a specific phone?

A brief history lesson

A Subscriber Identity Module gives each smartphone its unique identification, by allocating a phone number to it.

(In the UK, it will always be an 11-digit number starting in 07.)

This number is programmed into the microchips visible on every card – which are fairly rudimentary by modern standards.

That’s because the telecommunications protocol used in SIM cards dates back to the early 1990s, when digital mobile phones were first launched.

Digital devices offered superior signal strength and reliability compared to analogue protocols of the late 1980s.

An International Mobile Subscriber Identity was embedded into every SIM card, ensuring calls were directed to the correct destination.

These storage chips also contained enough space for the names and telephone numbers of key contacts – a feature we still rely on while upgrading or replacing our phones.

A final component was a 128-bit password, used to unlock encrypted digital data once it was received over the airwaves – thereby preventing eavesdropping or call interference.

Size matters

SIM cards only came in one size back in 1992, when the Nokia 1011 became the first GSM digital mobile phone on sale in the UK.

Resembling a credit card, full-size SIMs measured over eight centimetres long and five centimetres wide – though handsets were big enough to accommodate them back them.

Today, there are three smaller variants, still packaged within a credit-card sized piece of plastic in a nod to yesteryear (and to prevent them getting lost).

Certain handsets only accept particular card sizes, which is important if you’re planning to transfer an existing SIM into a new smartphone:

  1. Mini-SIM. Far smaller than their predecessors, thumbnail-sized mini-SIMs appeared in 1996 as smartphone bodies became too small to accept full-size cards.
  2. Micro-SIM. By 2003, technological advances in Europe heralded the micro-SIM – 40 per cent shorter than the mini, but fully backwards compatible.
  3. Nano-SIM. Roughly half the area of a micro, the nano-SIM debuted in 2012. Some consumers find them fiddly when inserting into handset slots.

Manufacturers have attempted to circumvent compatibility issues by producing three-in-one cards, known as triple or combi SIMs.

All relevant data is stored in a microchip within the nano part, which can be detached from the two larger frames.

This makes a single SIM suitable for all three types of handset slots, since a divide remains between the adoption of micro or nano variants. Mini-SIM devices are also still in use.

Apple devices have all made the switch to nano, and Google exclusively use nano technology as well.

As a general rule, newer phones from other manufacturers feature nano slots, while older smartphones still rely on micro cards.

In terms of mobile operators, every network listed on SIM Only Deals will be able to offer all three types of SIM cards.

You’ll see these different chip sizes beside each deal we offer.

Changing identities

It’s interesting to observe how swapping one SIM card for another changes a phone’s identity.

Some consumers like to use a separate SIM while abroad, for instance.

This process is child’s play in dual-SIM smartphones, which support two different numbers (potentially from rival networks) on a single device.

Dual-SIM phones tend to be from niche manufacturers like Asus and Honor, though the Sony Xperia XA1 Plus can maintain two identities simultaneously.

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