Structure of the SMS-GSM System

Structure of the GSM System

This section describes the architectures of the GSM, CDMA, and TDMA networks, as well as the position and functions of the SMSC in the three networks.
The system architectures of the CDMA, TDMA, and GSM networks are quite similar. Each of the three network systems is composed of the following four parts:

• Network subsystem (NSS)
  
• Base station subsystem (BSS)
  
• Operation and maintenance center (OMC)
  
• A great many mobile stations (MSs)
  

There are interfaces between various subsystems and functional entities within the subsystems. The specifications in the CDMA, TDMA, and GSMnetworks stipulate standard interfaces and communication protocols through which the functional entities can implement communications coordinately. Adopting different interface specifications and protocols, the three mobile networks are independent of each other.

Figure 1 Mobile network

SMSC: Short Message Service Center	NSS: Network SubSystem
HLR: Home Location Register	AUC: Authentication Center
EIR: Equipment Identity Register	MSC: Mobile Switching Center
VLR: Visitor Location Register	OMC: Operation & Maintenance Center
BSS: Base Station Subsystem	DCN:Data Communication Net
BSC: Base Station Controller	PSTN: Public Switched Telephone Network
BTS: Base Transceiver Station	MS: Mobile Station

NSS

In the mobile network, the NSS mainly implements the switching function and database function. The database function is necessary for subscriber data management, mobility management and security management. The NSS includes the following functional entities:

• MSC
  The MSC controls calls, manages the communication service of MSs within the local network and with other networks (such as PSTN/ISDN/PSPDN, and other mobile networks), and provides the charging information. Besides, in order to set up call routes to MSs, the MSC can also function as the GMSC, that is, querying location information of MSs.
  Short messages submitted by the MS pass the BSS and reach the MSC. Then the MSC forwards them to the corresponding SMSC.
  After receiving the SMs delivered by the SMSC, the MSC queries the VLR and the HLR for the related routing and subscriber information, and then delivers them to subscribers through BSS.
• VLR
  The visitor location register (VLR) stores the relevant information of registered subscribers in the control area, and provides the mobile subscribers with necessary conditions for call connection. It can be regarded as a dynamic database.
  
• HLR
  The home location register (HLR) is the central database of a mobile network. It stores the information of all mobile subscribers in the control area. The HLR stores two kinds of information: subscriber data, and location information of MSs for setting up call routes to the MSs.
  
• AuC
  The AuC stores authentication algorithm and encryption key, which prevent illegal subscribers from accessing the system, thus ensuring the security of the mobile subscribers' communications through the radio interface.
  
• EIR
  The EIR stores the International Mobile Equipment Identity (IMEI.). Generally the AuC and the EIR are integrated physically.
  
  NOTE:
  

  Generally, the MSC and the VLR are integrated physically; and the AUC, EIR and HLR are integrated physically.

BSS

The BSS is the system equipment, which is controlled by the MSC and communicates with MSs in the specific radio coverage area. It is responsible for radio transmission and reception, and radio resource management. The BSS consists of the following two functional entities:

• Base station controller (BSC)
  
• The BSC is responsible for radio network resource management, cell configuration data management, power control, location and switchover. It implements powerful communication control function.
  
• Base transceiver station (BTS)
  
• The BTS is the radio interface equipment, which is controlled by the BSC. It handles the radio connection, conversion between radio signals and landline signals, radio diversity, radio channel encryption, frequency hopping, and so on.
  

OMC

The OMC is responsible for managing and controlling the whole GSM and CDMA networks.

MS

The MS is the mobile subscriber's device. It consists of the following two parts:

• Mobile terminal (MT)
  The MT implements such functions as voice coding, channel coding, information encryption, information modulation and demodulation, information transmission and reception.
  
• Subscriber identity module/user identity module card (SIM/UIM card)
  The SIM/UIM card is an intelligent card, which keeps the subscriber identity information and the management data related to the subscriber and network. An MT with an SIM/UIM card inside can access the telecom network.