Brief Introduction to the SMS System

The SMC is a service processing system independent of the mobile network. It is responsible for submitting, storing, and forwarding short messages (SMs). It implements the interconnection with such networks as the public switched telephone network (PSTN),
integrated services digital network (ISDN) and packet switched public data network (PSPDN) to transfer SMs between extended short message entities (ESMEs) and mobile stations (MSs).

The SMC supports the SMC running mode and the FDA running mode. The license decides which running mode is used in the SMC.

In the SMC running mode, the SMC stores and forwards the SMs.
In the FDA running mode, the SMC does not store the SMs. Instead, the SMC delivers the SM once only. When a delivery fails, the system forwards the SM to the SMC with the function of storing SMs.



SMS System in the GSM Network
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.

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 SMC. After receiving the SMs delivered by the SMC, 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.

After receiving the SMs delivered by the SMC, 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.

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.

Position of the SMS System in the GSM Network
The position of the SMS system in the GSM network is as shown in the circle.
The physical entities related to the SMS system are the MSC, VLR and the HLR. Their
functions in the SM transmission are as follows:
�� MSC
− The SMs submitted by the MS pass the BSS and reach the MSC. Then the MSC
forwards them to the corresponding SMS system.
− After receiving the SMs delivered by the SMS system, 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 VLR stores the temporary information of a subscriber roaming in the local MSC
area. The subscriber must pass the authentication through the VLR before sending an SM,
and the MSC queries the route of a subscriber through the VLR before delivering an SM.
�� HLR
The HLR is a database for mobile subscriber management. It stores the subscription
information, position information, mobile station international ISDN number (MSISDN),
and international mobile station identity (IMSI) of a subscriber. Before delivering SM,
the SMS system finds which MSC the subscriber belongs to through the HLR.

Functions of the SMS System

This section describes the functions of the SMS system.
The functions of the SMS system are as follows:
�� Providing value-added services (VASs) and obtaining profits
As a basic telecom service of the mobile communication network, the SMS enables
subscribers to transmit information through SMs and obtain such useful information as
stock market, weather, sports and bank information, which greatly facilitates subscribers in getting information and gets subscribers into a better and closer relationship with operators. At the same time, operators can get direct revenue by providing VASs based on the SMS service.
�� Improving quality of service (QoS)
Mobile subscribers have high requirements for timely information transfer. In many
cases such as channel busy, masking or network coverage problems, communication is
difficult and information cannot be exchanged. With the SMS system, subscribers can
get information at the earliest time and smooth information transmission is ensured.
�� Improving connectivity and reducing congestion of the mobile network
According to statistics, over 30% of calls or faxes cannot be charged due to busy line or no reply, resulting in the waste of network resources. As a solution to this problem, the
SMS system can connect incoming calls manually or automatically and record the
information subscribers want to transmit, thus dredging the GSM network. In addition,the SMS system can reduce false load which is caused by repetitive dialing to an MS switched off, thus reducing network congestion.