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Mạng lưới hệ thống con chuyển mạch NSS đóng một phần trung tâm trong tất cả các mạng di động. Trong khi BSS cung cấp truy cập vô tuyến cho MS, các yếu tố mạng khác nhau trong NSS giả định trách nhiệm cho các bộ hoàn chỉnh các chức năng kiểm soát và cơ sở dữ liệu cần thiết để thiết lập kết nối cuộc gọi bằng cách sử dụng một hoặc nhiều các tính năng này: mã hóa, xác thực, và chuyển vùng. Để đáp ứng những nhiệm vụ, NSS bao gồm những điều sau đây: • MSC (trung tâm. | 4 The Network Switching Subsystem The NSS plays the central part in every mobile network. While the BSS provides the radio access for the MS the various network elements within the NSS assume responsibility for the complete set of control and database functions required to set up call connections using one or more of these features encryption authentication and roaming. To satisfy those tasks the NSS consists of the following MSC mobile switching center HLR home location register authentication center AuC VLR visitor location register EIR equipment identity register . The subsystems are interconnected directly or indirectly via the worldwide SS7 network. The network topology of the NSS is more flexible than the hierarchical structure of the BSS. Several MSCs may for example use one common VLR the use of an EIR is optional and the required number of subscribers determines the required number of HLRs. Figure 4.1 provides an overview of the interfaces between the different network elements in the NSS. Note that most interfaces are virtual that is they are defined as reference points for signaling between the network elements. 31 32 GSM Networks Protocols Terminology and Implementation External connections Figure 4.1 The NSS. 4.1 Home Location Register and Authentication Center Every PLMN requires access to at least one HLR as a permanent store of data. The concept is illustrated in Figure 4.2. The HLR can best be regarded as a large database with access times that must be kept as short as possible. The faster the response from the database the faster the call can be connected. Such a database is capable of managing data for literally hundreds of thousands subscribers. Within the HLR subscriber-specific parameters are maintained such as the parameter Ki which is part of security handling. It is never transmitted on any interface and is known only to the HLR and the SIM as shown in Figure 4.2. Each subscriber is assigned to one specific HLR which acts as a fixed .
