Why Are Mobile And SIM Separated In GSM Telecommunication System?

In GSM (Global System for Mobile Communication) telecommunication system a SIM (Subscriber Identification Module) and a mobile phone handset are designed separately to give user a choice to change their mobile phone handsets. A SIM card is designed to save user's subscription information and phone book. This gives user the liberty of retaining his or her information even after switching mobile phone handsets. Other than this, a user is also able to change Network operators if they tend to be unhappy with the service or any reason what so ever.
However in some countries, Network operators block SIM cards so that a user can only use that particular SIM card with  that particular mobile phone handset. This is called SIM locking. This is done in countries where Network operators tend to sell mobile on a subsidiary price and target their sales from the subscription and user's activity.
This happens regularly in the United States, Canada, Europe and Australia. However in countries like Belgium and Hong Kong, it is considered illegal. Another Popular Mobile Communication standard is the CDMA which is getting popular to its own distinct advantages. However CDMA as of now does not provide the feature of unlocking CDMA Phone, which means that each time you wish to change your handset, you would have to change your telephone number or each time you wish to change your network provider you would have to change your handset.
Therefore GSM technology provides a great advantage over other standards which makes it the most widely used standard all over the world.

GSM or Global System for Mobile Communications is a leading cellular system and is accepted worldwide. This system uses a narrowband called TDMA which stands for Time Division Multiple Access.  This system enables the user to receive as many as eight calls simultaneously, on the same radio frequency. TDMA is responsible for allowing this to happen. It is a technology that delivers wireless service by dividing radio frequency in to different time slots. It then allots slots to multiple calls. This is how a user can receive multiple calls on a single radio frequency.

You are required to use SIM cards differently in GSM systems because this system is world wide and is acceptable in over a 100 countries in the world. It needs service providers every where to support this system. So, different regions have different service providers, all of them support the GSM system and you are left with several choices of GSM mobile service providers that you can choose from.