What is SIM ?

SIM toolkit applications were initially written in native code using proprietary APIs. To provide interoperability of the applications, ETSI chose Java Card.

SIM cards are the unseen magicians of today's smartphones. They make connecting to networks and switching phones as easy as removing a small metal tray.

Additional standard size and specifications of interest are maintained by GlobalPlatform.

When you’re moving about from your country to another one, your SIM card is essentially the only thing that can negotiate and provide you a door to an overseas mobile service company. It in fact helps you to get a “new line” in the mere moments it takes you to deplane and grab your bags from the carousel belt.

If you buy a new phone, you can simply insert your existing SIM and keep on using your existing service as long as the new phone isn't locked to a different carrier.

Sure, some of the overseas data and line fees can be a nightmare. There are apps like Feel at Home to help you along the way, and get you a better deal too. You could alternatively call your local subscriber to find out what’s the best communications provider to use at the country you’re going to travel to.

SIM cards store network-specific information used to authenticate and identify subscribers on the network.

Back in 1991, a company called Giesecke & Devrient in Munich developed the world’s very first SIM card. They then sold 300 SIM cards to Radiolinja, which was a wireless network operator from Finland.

Likewise, if you travel internationally you can just buy a SIM card from a local carrier — as long as your phone isn't locked to a carrier.

The most important of these are the ICCID, IMSI, Authentication Key (Ki), Local Area Identity (LAI) and Operator-Specific Emergency Number.

SIM cards are fairly simple. They have practically no function without an accompanying smartphone and are increasingly reliant on third-party apps to address functionality, such as connecting to multiple networks or using more than one phone number.

Here’s the punch – the humble SIM now caters to billions of mobile devices and gadgets across the world!

The SIM also stores other carrier-specific data such as the SMSC (Short Message Service Center) number, Service Provider Name (SPN), Service Dialing Numbers (SDN), Advice-Of-Charge parameters and Value Added Service (VAS) applications. (Refer to GSM 11.11)

The next installation of SIMs are called eUICCs, which will be embedded in the phone itself. Its plus points include lesser logistical challenges, as it is able to network globally.

And because they are simple pieces of plastic, their benefit is largely subject to the will of the carrier.

SIM cards can come in various data capacities, from 8 KB to at least 256 KB. All can store a maximum of 250 contacts on the SIM, but while the 32 KB has room for 33 Mobile Network Codes (MNCs) or network identifiers, the 64 KB version has room for 80 MNCs

If you buy a phone that is locked to one carrier, putting a different carrier's SIM requires an unlock code, which is often expensive to obtain and confusing to enter.

There’s less distribution and installation costs and processes to deal with too. Its connectivity is expected to be kick-ass too, as it is enabled from the time it leaves the factory – you’ll just have to register your phone with a subscriber to get what you need!

This is used by network operators to store data on preferred networks, mostly used when the SIM is not in its home network but is roaming.

Now, this might sound laughable, but compared to the huge amount of space you’ve got going on your phone, your SIM card can store just about 64Kb in subscriber info, texts and contact details. Yup, it sounds sad, but it’s actually just enough for it to function.

The network operator that issued the SIM card can use this to have a phone connect to a preferred network that is more economic for the provider instead of having to pay the network operator that the phone 'saw' first

Typically the SIM lock is in exchange for a subsidized phone (so you can't buy a cheap phone on one carrier then switch over an use another before you've paid back the subsidy over the life of your contract).

If you are an uncle or auntie today, you’ll remember how your SIM card could run out of memory for either your SMSes or your contacts. Ah, memories.

This does not mean that a phone containing this SIM card can connect to a maximum of only 33 or 80 networks, but it means that the SIM card issuer can specify only up to that number of preferred networks

These days, Apple sells the iPhone SIM-unlocked in most countries, including the U.S., Canada and the U.K. While customers have to pay more upfront for an unlocked version of the iPhone, they can use the phone with any compatible SIM card, as long as the phone works with that carrier's network. The latest iPhone, the 6s, supports 23 LTE bands, which comprises most of the world's carriers.