Tài liệu Điện thoại di động mạng lưới Radio P9 pdf

9 DECT ∗ Besides cellular mobile radio networks that are primarily envisaged for use outdoors, systems that have been specifically designed for use in buildings are also important. In recent years cordless telephones with a range of a few hundred metres have become increasingly popular in private households. There is (along with CT2/CAI) a digital alternative to these analogue devices that offers better voice quality and a greater security against eavesdropping, as well as other advantages: the DECT system. The abbreviation DECT originally stood for Digital European Cordless Telecommunications, but to underline its claim of being a worldwide stan- dard for cordless telephony DECT today stands for Digital Enhanced Cordless Telecommunications. This standard was specified by the European Telecom- munications Standards Institute (ETSI) in 1992. A DECT network is a mi- crocellular digital mobile radio network for high user density and primarily for use inside buildings. However, an outdoor application is also possible. DECT systems allow complete cordless private branch exchanges to be set up in office buildings. Calls can be made over the normal subscriber line as well as between mobile stations through the DECT base station subsystem. When a staff member leaves his office, he usually can no longer be reached for incoming calls over a wireline telephone connection, although he may only be located in another part of the building. But if he uses a DECT terminal, he can continue to be reached under his normal telephone number wherever he may be anywhere on the premises. To enable users to continue to receive calls after they have left the DECT coverage area, ETSI specified an interface between GSM and DECT (see Section 9.16). The DECT standard permits the transmission of voice and data signals. Consequently cordless data networks can also be set up on a DECT basis. The use of ISDN services (Integrated Services Digital Network) is also pos- sible. Users are able to move freely within different cells without risking an interruption to their calls. The handover process switches calls from one radio cell to the next without interrupting the call. In outdoor areas the maximum distance between base and mobile station is approximately 300 m; in buildings, depending on the location, it is up to 50 m. Larger distances to the base station can be bridged through the installation of appropriate base stations using the relay concept (see also Section 9.12). ∗ With the collaboration of Christian Plenge and Markus Scheibenbogen Mobile Radio Networks: Networking and Protocols. Bernhard H. Walke Copyright ©1999 John Wiley & Sons Ltd ISBNs: 0-471-97595-8 (Hardback); 0-470-84193-1 (Electronic) 460 9 DECT The first time DECT systems were presented to the public was at the CeBIT exhibition in Hanover in March 1993. Since then, the costs for a DECT mobile have been decreased to be com- parable to those of analogue cordless telephones. 9.1 Possible Applications of DECT Systems The size of a DECT system determines how it is installed. The labels large and small are relative and relate to the number of mobile stations to be operated within the DECT coverage area. A DECT system is capable of automatically localizing up to 1000 subscribers in one location area (LA). With a larger number of users additional location areas are required that fall under the administration of the DECT system (see Sections 9.1.2 and 9.16.1.3). 9.1.1 DECT Fixed Networks DECT fixed networks are normally designed as dedicated private branch ex- changes. If the DECT systems are installed by a network operator, each customer- specific DECT system can be allocated its own location area. All DECT location areas are then interconnected over a backbone ring and administered centrally by a DECT system, as illustrated in Figure 9.8. Each customer has his own DECT fixed station (DFS) with his own identification of the location area. Despite the efficient location administration of DECT systems, it was planned that each customer would be allocated his own location area to en- sure that the channel capacity of other customers would not be affected by the calling activity of his own mobile stations. Private home base stations These offer a possibility of using the DECT sys- tem in small private households (see Figure 9.1). The home base station supplies the entire area of a house and can support one or several mobile stations that are supplied by the base station. The home station con- sists of a DECT fixed system (DFS), which controls the system, a simple database (DB) for user administration and a fixed part (FP), which pro- vides the radio supply to the mobile stations. An interworking unit (IWU) is provided for connection to an external network [20]. Wireless private branch exchanges In terms of installation, central systems with a DFS to which several FPs are connected are suitable for large private households or small companies (see Figure 9.2). Fixed terminals can also be connected. The system is administered by the DECT fixed system. The correct location area of the mobile stations is stored in a database (DB), and an IWU enables a connection to the external networks. 9.1 Possible Applications of DECT Systems 461 IWU ISDN PSTN GSM IWU DB = Portable PartPP FP = Fixed Part = Interworking Unit = Data Base PP PP FP DB DFS DFS = DECT Fixed System Figure 9.1: Private home base station IWU DB ISDN PSTN GSM FP FP PP PP DFS Figure 9.2: Wireless private branch exchanges Public Telepoint systems These systems provide DECT mobile stations ac- cess to the public telephone network over “other”FPs at public sites (see Figure 9.3). Possible application areas are public facilities with high user density such as airports, train stations and city centres. These private and public DECT systems consist of a number of FPs that are admin- istered by a DECT fixed system (DFS). Access to the public telephone network can be provided by a network interface (With DECT, PPs in principle can also be reached in the area of a Telepoint. The restriction to outgoing calls is dictated by the respective licensing agreement). Wireless local rings This is a ring to which a terminal adaptor (TA) and all the terminals are linked (see Figure 9.4). The terminal adaptor pro- vides a connection over the radio interface to one of the public FPs in the area. The terminals (e.g., telephone, facsimile) are linked to the ter- minal adaptor over conventional lines. If necessary, the terminal adaptor 462 9 DECT ISDN PSTN GSM PP PP FP FP Airport Railway Station FP City Centre IWU DB DFS Figure 9.3: Public Telepoint system TA ISDN PSTN GSM IWU TA = Terminal Adapter FP DB DFS Figure 9.4: Wireless local rings IWU ISDN PSTN GSM DB FP PP PP DFS Figure 9.5: Neighbourhood Telepoint establishes a radio connection to the next FP. The FP is administered in the same way as with the public Telepoint service. Neighbourhood Telepoint This is a combination of the public Telepoint ser- vice and a private home base station (see Figure 9.5). Private households do not have their own FP; instead one FP supplies a number of differ- ent private households on the basis of the Telepoint principle. The FPs 9.1 Possible Applications of DECT Systems 463 IWU SCU SCU Backbone Ring ISDN PSTN GSM PP PP FP FP PP PP FP FP SCU = Subsystem Control Unit DB DFS Figure 9.6: Private branch exchanges with ring and central DFS are installed so that several households will always be supplied by one FP. A DECT fixed system connected over a network interface with an interworking unit (IWU) to the public telephone network manages the FPs. Because of the way they are designed in terms of size and subscriber capac- ity, the following systems are also suitable for use by companies with scattered operating locations. One or more nodes can be provided for DECT at each company location. Each implementation of the system must be flexible and tailored to the needs of the customer in each individual case. Private branch exchanges with ring and central DFS These private branch exchanges consist of a backbone ring with a number of switching nodes (see Figure 9.6). Each switching node consists of a Subsystem Control Unit (SCU) and several base stations, i.e., FPs. One of the nodes con- tains the DFS which is responsible for all the nodes connected to the backbone ring and controls them from a central function. The DFS also contains the home database (HDB) and the interworking unit (IWU), which constitutes the external connection. All signalling data transmit- ted in this system must be transmitted over several nodes (SCUs) as well as the backbone ring. Transmission channels of the network that are no longer available for transporting user data are used for this pur- pose. A disadvantage is that the entire network suffers if the central system breaks down. Some relief is provided by the decentral systems, which ensure that there is a reduction in signalling traffic and that only 464 9 DECT SCU IWU SCU Backbone Ring IWU ISDN PSTN GSM VDBHDB VDBHDB HDB = Home Data Base VDB = Visitor Data Base DFS DFS PP PP FP FP FP FP PP PP Figure 9.7: Private branch exchanges with ring and distributed DFS the respective mobile stations are affected when the subsystem breaks down [20]. Private branch exchanges with ring and decentral DFS These are set up similarly to the systems with a central DFS (see Figure 9.7) and consist of a backbone ring that interconnects a number of nodes. A control unit is located at each of these nodes and several fixed stations are connected to a node. This system operates as a decentral structure because each control unit is controlled by its own DECT fixed system. Furthermore, each DECT fixed system has an interworking unit (IWU) that connects it to the external networks. The data is also stored at decentral locations. Sub- scriber administration is carried out in a home database (HDB), from which important data for operations can be retrieved over the network. For the purposes of reducing the signalling required when a mobile sub- scriber is roaming the network, certain information about each sub- scriber who moves out of the coverage area of his home database is written to the visitor database (VDB) of the new coverage area. The network provides access to the data in the visitor database, and the backbone ring is unloaded of responsibility for the signalling. If a DFS suffers a breakdown, only the subscribers moving in the respective cov- erage area will be affected [20]. Private branch exchanges with direct connection of FP These branch ex- changes have the same decentral structure as the decentral private 9.1 Possible Applications of DECT Systems 465 IWU SCU IWU SCU Backbone Ring ISDN PSTN GSM VDBHDB VDBHDB PP PP PP PP DFS DFS FP FP FP FP Figure 9.8: Private branch exchanges with direct connection of FP branch exchanges (see Figure 9.8). The difference between them is that the fixed stations of these private branch exchanges are directly con- nected to the respective DFS, which has the advantage that there is a reduction in the signalling traffic within a node and consequently a higher capacity for user data [20]. 9.1.2 Data Storage Databases can be set up in different ways. Three different types are presented below: SS 7-MAP The SS 7-MAP (Common Channel Signalling System Number 7, Mobile Application Part) is a method used by public cellular networks such as GSM900 and GSM1800. User data is stored in two registers: a home location register (HLR) and a visitor location register (VLR). Incoming calls are routed over the HLR to the VLR and from there to the mobile terminal. Outgoing calls only use the VLR. Access of the HLR is not required for these calls. X.500 Another method is the storage of data based on the standardized reg- ister service ITU-T X.500. The database is distributed physically over different locations but logically centralized. The data therefore gives the impression of being stored in a Directory Information Base (DIB). All data is organized as objects in a hierarchical Directory Information Tree (DIT), in which each branch can be filed in a physically separate Directory System Agent. A Directory User Agent accesses the individual 466 9 DECT objects. The search for the data takes place in a chain of interlinked databases. A critical factor of this method is the time involved in access- ing the data, but this is less of a problem for private branch exchanges with a limit on subscriber numbers. Telecommunication Management Network (TMN) A further method for data storage is TMN based on ITU-T M.30. A management system is controlled by a “manager” that realizes the management system using objects containing important data. These objects are stored in a hierar- chical MIB database (management information base). This MIB can be distributed physically over different locations. The management system is able to reproduce the HLR and VLR databases and store all location area data in one object. In comparison with public networks, only a limited number of users are managed in private networks such as DECT branch exchanges. The data for these users can usually be administered in one register. Moreover, it is easier to change objects in the frequently changing organizational structure of office environments than in conventional databases. This makes the HLR/VLR concept less suitable for private branch exchanges than the other alternatives. A comparison between X.500 and TMN shows that X.500 is more flexible because it is not required to copy the HLR/VLR principle. 9.2 The DECT Reference System The DECT Reference System describes the defined logical and physical com- ponents of the DECT system, the interfaces between the different units and the connection points to other networks. The global logical structuring of a local DECT network is explained below. This is followed by examples of different physical implementations. 9.2.1 Logical Grouping of DECT systems The logical groups of the DECT network are organized according to their functionalities with the intervening interfaces D1, D2, D3 and D4, which, however, do not describe how they are physically implemented (see Figure 9.9). 9.2.1.1 Global Network The global network supports the national telecommunication services. It carries out address conversion, routing and relaying between the individual connected socalled local networks. The global network is usually a national (sometimes an international) network. Examples of local networks include: • Public Switched Telephone Network (PSTN) 9.2 The DECT Reference System 467 DECT Network D3 D3 D3 D3 Portable Application Portable Application Portable Application Portable Application Portable Application Portable Application Portable Application Portable Application Portable Radio Termin. Portable Radio Termin. Portable Radio Termin. Portable Radio Termin. Portable Radio Termin. Portable Radio Termin. Portable Radio Termin. Portable Radio Termin. D4 D4 D4 D4 D4 D4 D4 D4 Global Network Local Network Local Network Fixed Radio Termination Fixed Radio Termination Fixed Radio Termination Fixed Radio Termination D1 D1 D2 D2 HDB VDB HDB VDB D2 D2 Figure 9.9: DECT Reference System: Logical grouping • Integrated Services Digital Network (ISDN) • Packet Switched Public Data Network (PSPDN) • Public Land Mobile Network (PLMN) 9.2.1.2 Local Networks Each local network provides a local telecommunications service. Depending on the actual installation, this can vary from a simple multiplexer to a highly- developed complex network. If the subordinate DECT fixed radio termination (FT) does not have a switching function then the local network must take over this function. Yet it should be noted that there can be a difference between logical definition and physical implementation, e.g., it is possible for several networks with their functions to be combined into one unit. A local network converts the global identification numbers (e.g., ISDN num- bers) to the DECT-specific IPUI (International Portable User Identity) and TPUI (Temporary Portable User Identity). The following networks are often found under the local network: • analogue or digital Private Automatic Branch Exchange (PABX) • Integrated Services Private Branch Exchange (ISPBX) • IEEE 802 LAN: local area network based on IEEE 802 All the typical network functions must be embedded outside the DECT system and occur either in a local or in a global network. Similarly to GSM, 468 9 DECT the HDB and VDB are required for controlling inter-DECT mobility, i.e., al- lowing users to use their mobile stations to move within different independent DECT areas (see Section 3.2.1.3). Incoming calls are automatically routed to the subsystem in which the user is currently located. When a user moves from one network to another, a new entry for the current VDB is made in the HDB. 9.2.1.3 DECT Network The DECT network consists of base and mobile stations, and connects users to the local fixed network. No application processes are defined for the system, and it only functions as a multiplexing facility. A DECT system always has only one network address per user, i.e., mobile station, and (from a logical standpoint) consists of one or more fixed radio terminations (FT) and many portable radio terminations (PT) that have been allocated to it. Fixed Radio Termination The FT is a logical grouping of all the functions and procedures on the fixed network side of the DECT air interface. It is responsible for: • layer-3 protocol processes in the C-(control) layer (except for mobility) • layer-2 protocol processes in the U-(user) layer • layer-2 switching (routing and relaying) in the respective DECT network Except for handover and multicell management, the FT contains no switch- ing functions. Although it can manage a large number of call entities, it cannot establish a direct connection between two users. This can only be carried out outside the logically delineated area of the FT in the local network. Portable Radio Termination and Portable Application These two parts con- stitute the logical groups on the mobile side of the DECT network. Whereas the portable radio termination with all its protocol elements for OSI layers 1, 2 and 3 is defined in the standard, it is up to the manufacturer of the equipment to define the acceptable application. Therefore it is not standardized. 9.2.2 Physical Grouping of DECT Systems Whereas the logical structure of the DECT network is clearly defined, the physical grouping can assume different forms. It is adapted to each cus- tomer’s requirements, and therefore can be conceptualized as a single base station with up to 12 simultaneously communicating mobile stations if it is equipped with one transceiver or as an independent switching centre for office buildings. The logical interfaces D1, . . . ,D4 are thereby partially integrated into a common physical unit and consequently can no longer be clearly ap- portioned (see Figure 9.10). [...]... Network D2 Fixed Radio Term D3 Portable Radio Term Portable Appl D4 Example Domestic FP PSTN Figure 9.11: Domestic telephone conguration PSTN Global Network PBX D1 Local Network D2 POT Fixed Radio Term D3 Portable Radio Term Portable Appl D4 Example PBX PSTN Common control RFP Fixed Part Figure 9.12: DECT-PBX conguration PSTN Global Network D1 Local Network D2 Fixed Radio Term POT D3 Portable Radio Term Portable... 9.13: Radio local loop conguration for PSTN PSTN NT1 NT2 U Global Network D1 Local Network D2 Fixed Radio Term T D3 Portable Radio Term TE1 S D4 Portable Appl Example ISDN Terminal Local Exchange Common control Cordless Terminal Adapter ISPBX RFP Fixed Part Figure 9.14: Radio local loop conguration for ISDN ISDN Terminal 472 MSC 9 A BSS Um MT S DECT TE TA R Global Network D1 Local Network D2 Fixed Radio. .. can contain one or more logical groups of the xed radio termination with common control An FP can be divided into two physical subgroups: Radio Fixed Part (RFP): responsible for only one cell in the network Radio End Point (REP): corresponds to a transceiver unit in the RFP 9.2.2.2 DECT Mobile Device (Portable Part) The two logical groups portable radio termination and portable application are physically... can be integrated into the FP or into the PBX Radio Local Loop A DECT system can also be incorporated as a local access network into the PSTN (see Figure 9.13) In this case the radio connection is transparent to the user The users wireline telephone is linked to a Cordless Terminal Adapter (CTA), which carries out the radio transmission to the RFP In the radio in the local loop (RLL) area, suppliers... Because with DECT the quality of the transmission medium (radio) is continuously changing and channel access is a complicated function that must be carried out frequently, the data link layer has been divided into the two sublayers Data Link Control (DLC) and Medium Access Control (MAC) Figure 9.16 relates the DECT reference model to the corresponding ISO/OSI layers Above the MAC layer the functions... the transmission channels over the radio medium At the same time it has to share the medium with many other mobile stations that are also transmitting Interference and collisions between communicating base and mobile stations are largely avoided thanks to the decentrally organized use of the available dimensions location, time and frequency (see Figure 9.17) Each dimension contains several possibilities... Each station receives a protected, periodically recurring portion of the overall transmission rate of a frequency According to Figure 9.18, the frame and time slot structure of the DECT system is explained in the physical layer The transmission capacity of each frequency is divided into 10 ms long periodically recurring frames, each of which has a length corresponding to the duration of 11.520 bits This... Primary Access Rights Identier (PARI) and its Radio Fixed Part Number (RPN), the MAC layer of the base station generates its own Radio Fixed Part Identity (RFPI), which is transmitted to the terminal QT System Information and Multiframe Marker This channel, used once in each multiframe, is only transmitted by the base station, and therefore is used for indirect synchronization on the multiframe cycle... The protocol stack directly above the MAC layer divides into two parallel parts Similarly to the MAC layer, extensive error protection in the C-plane of the data link layer improves the reliability of the data transmission Along with a point-to-point service, the C-plane oers a broadcast service to the network layer above it The U-plane is responsible for processing user data on the radio section, with... available and evaluates the quality of the received signal 9.4 9.4.1 Detailed Description of Services and Protocols Physical Layer (PHL) A physical channel is the physical transmission path between two radio units Radio transmission places a great demand on transceivers to ensure that there is good reception of the usage signal The receiver sensitivity for the required bit error ratio (BER) of 0.001 is 83 dBm . Portable Radio Termin. Portable Radio Termin. Portable Radio Termin. Portable Radio Termin. Portable Radio Termin. Portable Radio Termin. Portable Radio Termin Portable Radio Termin. D4 D4 D4 D4 D4 D4 D4 D4 Global Network Local Network Local Network Fixed Radio Termination Fixed Radio Termination Fixed Radio Termination