HLR Redundancy Solution

Home Location Register (HLR) bears a crucial position in the mobile network, where a wireless user´s identity is assigned for record purpose. The ZXG10-HLR/AUC system, ZTE´s HLR, is composed of the Common Channel Processing Module, the Service Processing Module, the Database Module and the Operation and Maintenance Module. For considering the security of data, the redundancy structure of ZTE´s HLR system has been developed to provide higher reliability. Currently, there are two solutions applicable to deal with HLR redundancy: 1+1 and N+1 schemes. ZTE´s redundancy solutions to HLR have been successfully adopted in several projects home and abroad.

More information at:
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The most important telecommunication resources for every company are the data. However, a number of disasters such as flood, fire, lightening strike, earthquake and power failure, equipment failure, computer virus and man-made errors may seriously affect the security of data. Therefore, the security of data has become an extremely important issue in the eyes of most companies.
    Redundancy system is an effective way to guarantee data security. The most important point of a redundancy system is to provide backup for the data. It backs up the subscriber information, as well as the logical data information of the system service. The subscriber information backup is dedicated to keeping the integrity of data, while the logical data information backup ensures that the service will be recovered in a short time.
As the center for subscription and location information of the subscribers, Home Location Register (HLR) bears a crucial position in the mobile network, where a wireless user´s identity is assigned for record purpose. If HLR fails to work well, it could disastrously produce a ripple effect to the entire network and even lead to the failure of mobile service.  Therefore, it is quite necessary for operators to implement the high-level data backup and recovery seurity of their networks. ZTE´s redundancy solutions to HLR introduced here can meet the necessity of the operators.

1 ZTE´s HLR System
The ZXG10-HLR/AUC system, ZTE´s HLR, is composed of the Common Channel Processing Module (CPM), the Service Processing Module (HSM), the Database Module (HDM) and the Operation and Maintenance Module (OMM). Modular design is adopted for each unit, and TCP/IP protocols are used for communications. Figure 1 illustrates the hardware structure of ZXG10-HLR/AUC.

    (1) CPM : Common Channel Signaling Processing Subsystem
    CPM is the SS7 processing module of ZXG10-HLR/AUC. It provides the SS7 link connection with other network entities of Public Land Mobile Network (PLMN). It performs the Message Transfer Part (MTP), Signaling Connection and Control Part (SCCP), and Transaction Capabilities Application Part (TCAP) functions of SS7. In addition, it provides an interface of the service processor.

    (2) HSM: Service Processor Subsystem
    The service processor of ZXG10-HLR/AUC is the center for performing the functions of the whole ZXG10-HLR/AUC system. It uses the TCP/IP protocols to communicate with the database server, the SS7 front-end processor and the OMM.

    (3) HDM: Database Subsystem
    The ZXG10-HLR/AUC system database adopts the cluster technology and the configuration of a shared disk array. The database management uses multiple modes to meet different requirements.

    (4) OMM: Operation and Maintenance Subsystem
    It is used for ZXG10-HLR/AUC management, including authority management, data configuration, security management, performance statistics, fault management, diagnosis testing, subscriber equipment tracing, signaling tracing, version management, file management, service observation, clock management and database management.

2 Redundancy Solutions to HLR System
For considering the security of data, the redundancy structure of the HLR system has been developed to provide higher reliability. Currently, there are two solutions applicable to deal with HLR redundancy: 1+1 or N+1 schemes.

2.1 1+1 Redundancy Scheme
The 1+1 redundancy scheme means that there is a master HLR and a slave HLR installed. If the 1＋1 scheme is utilized, data in the master and slave HLRs shall be identical, and synchronization mechanism is applied between the master and slave HLRs to implement real-time data synchronization.

    A Business and Operation Support System (BOSS) is connected to each HLR. However, the BOSS system can merely manipulate the master HLR in each pair of the master and slave，for the connection between the BOSS and the slave HLR is screened. The data synchronization between the master and slave HLRs is fulfilled by the internal synchronization mechanism.

    Figure 2 illustrates the 1+1 scheme with master and slave HLRs in the same city.

2. 2 N+1 Redundancy Scheme
The N+1 scheme means that one HLR can provide redundancy backup for n HLRs for security. It supports up to 20 million static and 2 million dynamic subscribers. If the N+1 scheme is used, in case some master HLRs fail, the redundancy HLR will take over their services. It automatically starts the takeover procedure and the service will resume in short time.

    The BOSS system is connected to every HLR. It manipulates the master HLRs and the HLR in redundancy center simultaneously. Therefore, the data synchronization is implemented by the BOSS system.

    If one HLR fails, the HLR at redundancy center will take over its services. In this case, dynamic data can only be collected after the HLR system at redundancy center sends HLR RESET message to each Visit Location Register (VLR) and subscribers have executed the related operations.

    Figure 3 illustrates the N+1 scheme.

2.3 Comparison Between 1+1 and N+1 Schemes
    (1) Service Handover
    In the 1+1 scheme, if the master HLR fails, the slave HLR can resume services immediately. Therefore, it has no impacts on the services.

    In the N+1 scheme, if a master HLR fails, subscribers in the failed HLR cannot be called in a period of time because the HLR at redundancy center only synchronizes static data in normal time. The subscribers´ dynamic data are not synchronized.

    (2) Data Backup
    In the 1+1 scheme, subscribers´ data synchronization between the master HLR and the slave HLR at the redundancy center does not involve the BOSS. However, users´ subscription information is synchronized to the HLR at redundancy center by the BOSS in the N+1 scheme. For instance, the BOSS synchronizes user subscription, user deletion and user data modification to the HLR at redundancy center.

    (3) Investment and Construction
    The 1+1 scheme requests several HLRs to match existing HLRs for data backup so that data in the master and slave HLRs are exactly same.

    The N+1 scheme only requests one set of HLR at redundancy center to back up data in all HLRs.

3 Applications of ZTE´s Redundancy Solutions to HLR
ZTE´s redundancy solutions to HLR have been successfully applied in several projects. The operators have sung high praise for their good quality and reliability. The 1+1 redundancy scheme and N+1 scheme can be selected according to real situation and requirement.

    ZTE´s N+1 redundancy system has been used successfully in the Shandong project of China Mobile, the biggest mobile operator in the world. In this project, one redundancy HLR was installed for the backup of all the HLRs in the entire Shandong province. All these HLRs were from different equipment vendors. With strong compatibility, ZTE´s N+1 HLR solution effectively and efficiently implemented the backup of those HLRs from other vendors in the same network. Meanwhile, it provided the highest security to benefit the operator. It was configured up to 20 million static and 1 million dynamic subscribers.

    Another application of the N+1 scheme was in Guangdong Province of China Telecom. In this project, one redundancy HLR was installed for five master HLRs in four areas of Guangdong, China.

    The N+1 scheme has also been applied in international markets.

    The 1+1 redundancy system has been successfully used in Guangzhou, the provincial capital of Guangdong of China. In this project, one backup HLR is installed for the backup of the master HLR.The 1+1 scheme has also been implemented in some international markets.

Manuscript: 2005-05-30