2. Cellular Radio Network
3. GSM Carrier Frequencies
4. GSM Voice Codecs
5. GSM Network Structure
6. Subscriber Identity Moudule(SIM)
One of the key features of GSM is the Subscriber Identity Module, commonly known as a SIM card. The SIM is a detachable smart card containing the user's subscription information and phone book. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking and is illegal in some countries.
SIM LOCK
A SIM lock, simlock, network lock or subsidy lock is a capability built into GSM phones by mobile phone manufacturers. Network providers use this capability to restrict the use of these phones to specific countries and network providers. Generally, phones can be locked to accept only SIM cards based on the International Mobile Subscriber Identity, which has elements of:
* Mobile country code (MCC; e.g., will only work with SIMs issued in one country)
* Mobile network code (MNC; e.g., AT&T Mobility, T-Mobile, Vodafone, Bell Mobility etc.)
* Mobile station identification number (MSIN; i.e., only one SIM can be used with the phone)
Additionally, at least Nokia phones can lock group IDs which are used in voice group call service.
In most countries, most mobile phones are shipped with country and/or network provider locks.
Most mobile phones can be unlocked to work with any GSM, such as O2 or Orange (in the UK), but the phone may still display the original branding and may not support features of the new carrier. Most phones can be unbranded by uploading a different firmware version, a procedure recommended for advanced users only.
Locked phones may also have firmware installed on them which is specific to the network provider. For example, if you have a Vodafone or Telstra branded phone in Australia, it displays the relevant logo and may only support features provided by that network (e.g. Vodafone Live!). This firmware is installed by the service provider and is separate from the locking mechanism.
