Cell Phone - What Is SIM?

Yes - we are all in a rather excited mood after engadget reported that iPhone could be unlocked and we could use other SIMs than that provided by AT&T. I am not a iPhone user, and I do not much care about it. But it made me remember of the days as communication engineering student, and I thought I would share it here.

What is SIM?

SIM or Subscriber Identity Module is a IC (Intergrated Circuit) chip that is used to identify a GSM subscriber with in a network. GSM is considered the second generation in mobile phone telephony protocol - more on that in a later post.

Pushing aside all these technical lingo aside, what does this mean to us users? SIM allows us to change a phone - the device without changing our number, with out changing our subscription to a particular network. It also means that, it allows us to change our network and still keep using our phone.

Technical Details

The SIM cards come in two sizes. One is the size of a credit card, and another - the more popular, almost stamp-sized, 25mm by 15mm chip. It has memory on chip. Depending on the version, it varies from a handful of kBs to 64 kBs.

Other than user data, the important function of the SIM is to hold several identification keys. These keys or numbers are ICCID, IMSI and Ki (or Authentication Key). These are securely stored keys - they are subjected to encryption algorithm and is used by the cell phone to identify itself to the network while placing and connecting a call.

The SIM also stores what is called LAI or Location Area Information. This plays a major part in executing the 'cell hopping'. Simply put, it is used by the network to handle calls when the phone moves from one network coverage area to another.

So, even if I do not use either iPhone or the AT&T network services, as a engineer at heart, I cheer a bit at this occassion.

It is one thing to use the technology to produce intricate legal bindings (and some times, it is necessary too), but we got to draw the line somewhere. I draw mine here.