From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband

1.5 The GSM Subsystems

A GSM network is split into three subsystems which are described in more detail below:

• The Base Station Subsystem (BSS), which is also called ‘radio network’, contains all nodes and functionalities that are necessary to wirelessly connect mobile subscribers over the radio interface to the network. The radio interface is usually also referred to as the ‘air interface’.
• The Network Subsystem (NSS), which is also called ‘core network’, contains all nodes and functionalities that are necessary for switching of calls, for subscriber management and mobility management.
• The Intelligent Network Subsystem (IN) comprises SCP databases that add optional functionality to the network. One of the most important optional IN functionality of a mobile network is the prepaid service, which allows subscribers to first fund an account with a certain amount of money which can then be used for network services like phone calls, Short Messaging Service (SMS) messages and, of course, data services via GPRS and UMTS as described in Chapters 2 and 3. When a prepaid subscriber uses a service of the network, the responsible IN node is contacted and the amount the network operator charges for a service is deducted from the account in real time.