Điện thoại di động mạng lưới Radio P6

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Trunked Mobile Radio and Packet Data Radio In addition to the public radio telephone service and the paging service, there are other radio services that are not accessible by the public. These radio systems, called the non-public land mobile radio network , have access to frequencies that cannot be used by the public but only by specific users or groups of users. Probably the best known non-public mobile radio service is analogue Private Trunked Mobile Radio (PTMR), which has been used for many years by large firms such as airlines, taxi and transport companies, the railways, and ports, as well...

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  1. 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) 6 Trunked Mobile Radio and Packet Data Radio In addition to the public radio telephone service and the paging service, there are other radio services that are not accessible by the public. These radio systems, called the non-public land mobile radio network , have access to fre- quencies that cannot be used by the public but only by specific users or groups of users. Probably the best known non-public mobile radio service is analogue Pri- vate Trunked Mobile Radio (PTMR), which has been used for many years by large firms such as airlines, taxi and transport companies, the railways, and ports, as well as by government departments and organizations responsible for security. What is characteristic of previous PTMR systems is that they have one radio channel that is used exclusively by all the mobile terminals of a specific user group. An analysis of conventional commercial radio sys- tems reveals a number of weaknesses that affect both the customer and the operator: • Because of too many PTMR users, the fixed allocation of radio channels in congested areas leads to a frequency overload. • Radio supply areas are too small. • There is the possibility of eavesdropping by unauthorized persons. • There is no link to the public telephone networks. • There is limited support of voice and data transmission. Frequency overload was the main reason for considering new radio systems and infrastructures. This led to the introduction of trunked mobile radio systems as the successors to analogue PTMR. Although it is not possible for trunked mobile radio systems to expand the frequency spectrum available, they are able to improve the quality of service both for the end user and for the network operator through the optimization of frequency utilization and increased channel use. Advances in trunked mobile radio technology have resulted not only in providing user groups with one channel as in PTMR but also in making a trunk of channels available jointly to a large number of users. A channel is allocated to the user by the system only when required, and then immediately withdrawn after use. Whereas
  2. 366 6 Trunked Mobile Radio and Packet Data Radio in PTMR a user would have to wait until a channel allocated to his user group was free, a trunked mobile radio user can start speaking as soon as any one of the channels in the channel group is free. In trunked mobile radio, traffic volume is divided evenly over all the available radio channels, with the trunking of the channels achieving a trunking gain, i.e., the loss probability pv becomes less and less as the number of channels in a group increases and each channel is constantly utilized (see Appendix A.2). The traffic capacity ² [in Erl./(MHz · km )], increases with the trunk group size. In addition to frequency economy, trunked mobile radio systems offer other advantages: • Low installation cost compared with separate radio control centres. • Radio supply areas corresponding to the economic areas of activity. • Higher range. • No undesirable eavesdropping by others. • Increased availability because of allocation of channels according to need. • Optional access to the public telephone network. • Expanded services because of selective calling, variable group calling and priority calls. • Improvement in quality of service in voice and data transmission. • Orderly call queuing operation. 6.1 The MPT 1327 Trunked Mobile Radio System The pacesetter in standardized trunked mobile radio systems was Great Britain, where the Ministry of Post and Telecommunications developed the trunked mobile radio standard MPT 1327/1343, which is also used in Ger- many as the technical standard for the first generation of (analogue) trunked mobile radio networks. Following are some of the services offered in an MPT 1327 trunked mobile radio network: • A normal call can be either an individual or a group call. • A priority call can be either an individual or a group call. • The mobile telephones called do not respond when they receive a recorded announcement.
  3. 6.1 The MPT 1327 Trunked Mobile Radio System 367 • A conventional central station call in which a radio unit wishing to make a call is not immediately allocated a channel but is required to wait until the central station sets up the call at a convenient time. • A conference call in which additional users can participate in a setup call. • An emergency call, which can be either a voice or a data call placed by an individual or by a group. • A data call can take place between different signalling systems, and is either an individual call or a group call that is transmitted either as a normal or a priority call. • Call forwarding or call diversion to another user or group is possible. • Status messages can be interchanged between different radio units or be- tween radio units and the system, whereby there are 30 different special- purpose messages available. • Radio telegrams are up to 184 bits long and can be interchanged between the radio units or between the radio units and the system. • A short telephone call permits access to a private branch exchange and to the public telephone network. In a trunked radio network a distinction is made between two different types of radio channel: the control channel and the traffic channel. All switching- related organizational functions between the system controller and the mobile radio devices are carried out over the control channel through the exchange of data. The main tasks of the control channel include: • Notification of call requests • Establishment and termination of calls • Allocation of communications channels to mobile stations Trunked radio systems can be operated as local systems with only one base station or as area-wide (cellular) systems with cell sizes from 3 km up to 25 km, e.g., in metropolitan areas with a 6 km diameter of the cells. The basic structure of a cellular MPT 1327 trunked radio network consists of several cells, each with a radio base station (transceiver, TRX), a trunked system controller (TSC) and a central node, the master system controller (MSC), which also implements the gateway to the public telephone network or to the branch exchange networks (see Figure 6.1). The TSC controls a radio cell and manages the traffic channels and their allocation to the mobile stations when a call is made. Since roaming is allowed in a multiple-cell trunked radio network, the TSC also maintains a home and
  4. 368 6 Trunked Mobile Radio and Packet Data Radio TRX Mobile Mobile User User Central Station Antenna Switch Antenna Switch Antenna Switch TRX TRX TRX 1 TRX 1 TRX TRX 1 TRX TRX TRX 2 2 2 3 TRX 3 TRX 3 TRX 4 4 4 TRX TRX TRX 20 20 20 Sender- Sender- Sender- Receiver Receiver Receiver Trunked System Controller Trunked System Controller Trunked System Controller Area 1 Area 2 Area 3 Node MSC OMC Telephone System Central Terminal Central Terminal Figure 6.1: Principle structure of a trunked mobile radio network visitor location register of all the subscribers allocated to the radio cell or who are temporarily operating in the cell. If a call is in progress during a cell change, it is not taken over by the new cell but is broken off; handover is not supported. Operating and maintenance centres (OMC), which monitor the system, carry out statistical evaluations and record charges, are coupled to the MSC. In addition to the MPT 1327 standard described, which defines the sig- nalling protocols between the TSC and the mobile devices, the following stan- dards are also of importance: • MPT 1343 specifies the operations of the terminal equipment and defines the functions for system control and access to the traffic channel. • MPT 1347 specifies the functions of the fixed network of the system as well as directives on the allocation of identity numbers. • MPT 1352 describes the procedures for checking the conformity of the network elements of different manufacturers.
  5. 6.1 The MPT 1327 Trunked Mobile Radio System 369 Trunked radio networks can operate in any frequency band suitable for mobile communications. In Europe trunked radio networks operate in the 80–900 MHz range. One example is the Chekker network operated by Deutsche Telekom AG in Germany in accordance with the MPT 1327 standard in the 410–418 MHz (uplink) and 420–428 MHz (downlink) frequency bands. Up to 20 radio chan- nels, each with a 12.5 kHz bandwidth, are available per cell. One channel can normally service 70–80 users. The maximum transmitter power per base station is 15 W. Messages are transmitted digitally on the control channel, whereas with the MPT standard user information is transmitted on the traffic channels in analogue. Mobile stations use the control channel in half-duplex mode, whereas the base station transmits on this channel in duplex mode. The necessary signalling data is exchanged on the allocated traffic channel during a user connection. Phase-shift keying (PSK) modulation has been selected for speech modula- tion. Fast-Frequency Shift Keying modulation (FFSK) is used for data. The transmission rate for signalling data is 1.2 kbit/s; the data transmission rate possible is 2.4 kbit/s. Systems with a small number of channels can employ a technique allowed by the MPT protocol in which the control channel can be used as a commu- nications channel if the need arises. Mobile stations in a trunked radio system access the control channel in accordance with a random access method, called the S-ALOHA protocol. In a trunked radio network a call is set up through a series of steps. All checked-in radio units follow the sequence of operations on the control channel in standby mode. When a call request is made, indicated by a keystroke on the mobile terminal to the central station, the central station checks the availability of the subscriber terminal being called and informs the respective subscriber, in some cases through a paging signal over the control channel. If the subscriber called answers, a free traffic channel is automatically assigned to the respective parties. The maximum call duration in the Chekker service is 60 s (billing for the service is on a monthly fixed basis). When a call has been completed, the terminals switch back to the control channel. In the event that all the radio channels are occupied, an automatic queuing buffer system ensures that radio channels are allocated on an orderly basis, depending on waiting time or priority. In Germany the federal postal and telecommunications ministry has pro- vided four trunked radio network licences for commercial communications: Licence type A Trunked radio networks for regional areas (metropolitan ar- eas), which were stipulated by the licensor before the invitation to tender (e.g., Chekker). Licence type B Other regional areas proposed by the licensee.
  6. 370 6 Trunked Mobile Radio and Packet Data Radio Licence type C Trunked radio networks for local, geographically tightly re- stricted areas. Licence type D Countrywide trunked radio network for mobile data radio applications. Trunked radio networks are divided into two categories based on user type: • Public networks, which are operated by an operating company, whose users include small to medium-sized firms (e.g., towing services, haulage firms, other services). • Private networks, which are operated by large groups, such as port au- thorities, automobile manufacturers, airline companies and the police. 6.2 MODACOM Trunked radio networks based on the MPT 1327 standard cannot satis- factorily support data transfer (status messages, radio telegrams and data calls). Mobile radio networks were therefore developed for connecting data terminals with an ITU-T X.25 interface to their data processing systems (host). Examples of these proprietary data networks include MOBITEX (Swe- den/England), COGNITO (England), ARDIS (USA) and the MODACOM network (e.g., Germany since 1992). MODACOM is a public mobile radio service that was specifically devel- oped to provide frequent, high-quality and cost-effective data transmission and support access to the public X.25 network. Data transfer in this sys- tem is particularly frequency-economic, because it is transmitted digitally and packet-switched, and, compared with other services, is very economical for low-volume transmission. Because of the direct, bi-directional data trans- fer between data processing systems and mobile data terminals, MODACOM can provide considerable cost savings in operational organization. The MODACOM network was initially developed for operation outdoors, and, in contrast to GSM, was not planned to be used across borders. It is directed towards customers who would benefit from services being expanded from the wired, packet-switched data network to the mobile area. Applications for the MODACOM service include: • Database access by mobile terminals over the public X.25 network • Scheduling applications • Dispatching services, e.g., for shipping and haulage companies • Telemetry applications such as emission tests, burglar alarms and pa- rameter requests from vehicles • Service and maintenance, i.e., remote diagnostics, fault searches or elim- ination, access to inventory and stock data
  7. 6.2 MODACOM 371 6.2.1 Services in the MODACOM Network After a terminal has been switched on, it searches for a free channel in the designated channel grid and is checked into the system. After it has been checked in, a continuous virtual connection exists over which control signals are exchanged from time to time. These are not transmitted as data packets until actual data is ready to be sent. Network management ensures that a user terminal has continuous send and receive capabilities within the overall MODACOM network as well as access to the following services or performance characteristics: • Transmission of status messages or file transfer (bi-directional) • Communication between mobile subscribers themselves • Intermediate storage of data through the mailbox service • Connection to other data services/networks • Closed user group • Support of group calls • Automatic acknowledgement of receipt of data sent • Roaming • Secure data transfer • Password query, personal identification and authorization 6.2.2 The MODACOM Network Structure The MODACOM system has a cellular structure in which each cell is served by a base station (BS). The cell radius in urban areas is 8 km and in rural areas it is 15 km. Therefore the radio coverage area in a radio data network (RDN) consists of at least one BS that is connected to the area communications controller (ACC) over a direct data link at 9.6 kbit/s (see Figure 6.2). The ACC is a switching computer that controls and coordinates the base stations attached to it. One or more radio areas and the ACC responsible for the areas together form a domain. MODACOM terminals are allocated to the ACC with the coverage area in which they are most frequently likely to be active (home domain). Mobile terminals (MT) also operate outside their home domain and are allowed to move across domain boundaries, in which case they are handed over from ACC to ACC (handover ). Communication between ACCs takes place—unnoticed by the user—over the public X.25 network. The transition from the radio data network (RDN) to the X.25 data network materializes over one or more nodes (ACC|G, G = gateway). This transition is implemented indirectly over dialled-up switched virtual circuits (SVCs) or permanent virtual circuits (PVCs). A network administration host (NAH) is responsible for the configuration and the monitoring of the radio network.
  8. 372 6 Trunked Mobile Radio and Packet Data Radio X.25 User Terminal BS MT ACC G X.25 User Terminal BS MT ACC G X.25 User Terminal BS MT ACC G Network X.25 Administration BS Host 9.6 kbit/s (NAH) Radio Data Network (RDN) X.25 Data Network RD-LAP (Air Interface) Figure 6.2: The architecture of the MODACOM system 6.2.3 Technical Data The MODACOM system was allocated in Germany the frequency ranges 417– 427 MHz (uplink) and 427–437 MHz (downlink), with a duplex separation of 12.5 kHz. These frequency bands are divided into 12.5 kHz bandwidth channels. The 4-FSK technique is used for modulation. The transmitter power is 6 W for the base station and a maximum of 6 W for the mobile terminal. Data packets are transmitted over the air interface in accordance with the radio protocol RD-LAP (Radio Data Link Access Procedure), which alter- natively allows connection-oriented or connectionless communication for syn- chronous dialled calls in half-duplex mode between MT and host. At the radio interface the RD-LAP protocol is based on the Slotted Digital Sense Multi- ple Access (DSMA) channel access method, and incorporates the following characteristics: • Maximum packet length is 512 bytes, with shorter packets also allowed. • If a channel is occupied, the packets to be sent are deferred and the timing of the next transmission attempt is determined at random (non- persistent behaviour). • If a channel is free, transmission is with the probability p (p-persistent behaviour) • Connection-oriented transmission.
  9. 6.2 MODACOM 373 Table 6.1: Technical parameters of MODACOM packet radio network Frequency ranges 417–427 MHZ and 427–437 MHz Duplex separation 10 MHz Channel grid 12.5 kHz Modulation 4-FSK Radiation power 6 W ERP Air interface Open standard RD-LAP Data transfer Digital, packet-oriented Bit rate 9.6 kbit/s net Message length Max. 2048 bytes Packet size Max. 512 bytes Block size 12 bytes Response time Approx. 1.5 s Channels/carriers One data channel per carrier Channel access Slotted DSMA Forward error correction Trellis coding with interleaving Error detection code CRC-check sum Bit-error ratio Better than 10−6 , typically 10−8 • Reservation is not possible. The RD-LAP protocol includes error detection and correction as well as procedures for message segmentation and reassemling after receipt. Layer 2 is able to process messages of a maximum length of 2048 bytes segmented into four data packets each 512 bytes in length and transmitted at 9.6 kbit/s. The data packets are automatically acknowledged by the network, and in the case of error a transmission attempt is repeated three times. CRC check sum procedures (cyclic redundancy check ) are used for error detection. Data is transmitted with forward error correction procedures using trellis coded mod- ulation and interleaving with a bit error ratio less than 10−6 . The technical parameters of the MODACOM system are summarized in Table 6.1 [15]. 6.2.4 Different Connection Possibilities in the MODACOM Radio Data Network 6.2.4.1 Connection of Two Mobile Terminals This type of connection (messaging) allows the exchange of free message texts with manual or automatic acknowledgement between two mobile terminals. Simplified message routing to a third terminal is possible. A terminal is dialled through the manual input of a terminal address, which for simplification can be associated with an alias table of names or abbrevi- ations. The messages are provided with an appropriate header. The system accepts the message, converts it per the sender’s request and transmits it to
  10. 374 6 Trunked Mobile Radio and Packet Data Radio MT Radio X.25 - PDN X.25 X.25 Data PAD PAD Data Network (RDN) SVC Terminal RD-LAP Equipment X.25 (DTE) X.3 Partial Support X.29 Figure 6.3: Outgoing individual call the addressee terminal. If the terminal is not available, the message is then temporarily stored in the MODACOM box. 6.2.4.2 Connection to Fixed Network Connections between MT and the wireline fixed network are exclusively car- ried out over the public X.25 network. The MODACOM system supports three types of connection betwen host and MT [17]: Type 1: Outgoing individual call These calls can be used to interrogate pub- lic databases, etc. An outgoing individual call is always initiated by an MT and used for interactive communication with the dialled host. The connection between MODACOM system and host is set up as a switched virtual channel (SVC) in the X.25 network. Figure 6.3 illustrates a Type 1 connection over a PAD (packet assembly disassembly). In order to set up the connection, the MT sends a special data packet that contains the ITU-T Rec. X.121 address of the target host. The connection between MT and X.25 network is implemented through X.3- PAD functions and the SVC connection in the wireline network. The PAD links asynchronous (start/stop) terminals to an X.25 host. The MODACOM system emulates only a subset of the X.3 and X.29 interfaces, whereas the X.28 specifications, which describe the config- uration of a PAD by the asynchronous terminal, are not required. A special data packet is sent by the MT in order to terminate a connec- tion. A connection can also be terminated by the host using the normal resources of the X.29 specification. Type 2: Incoming individual call These connections are based on exclusive switched virtual channels (SVC) that can only be set up by the host. Figure 6.4 illustrates an example of an incoming individual call. Since the public X.25 network is only able to connect data terminal equipment (DTE) with an X.25 interface, the MODACOM network RDN or its gateway node G is connected to the network as an X.25 subscriber. Every incoming call requires a connection and termination
  11. 6.2 MODACOM 375 MT Radio Data X.25 X.25 Data X.25 - PDN Terminal Network G X.25 Equipment (RDN) SVC (DTE) RD-LAP MT A A Radio Data MT DTE DTE B Network B (RDN) One Channel per MT Connection C MT C Data Terminal Equipment (DTE) Figure 6.4: Incoming individual call MT One Channel for Multiple User A A MT Connections Process A MT B SCR DTE DTE SCR B B C C MT C Radio Data Network Data Terminal (RDN) Equipment (DTE) Figure 6.5: Pooling procedure. When a connection is established, the MT is allocated an SVC connection, which must be managed exclusively by the MT if it makes further connections. The number of MTs that can be served by a host is equal to the number of SVCs that are available between the host and the MODACOM system. The combination of the SVC and logical connection to the MT is stored in the MODACOM system to enable incoming data to be routed to the responsible MT on the basis of the FIFO method. Type 3: Pooling This is the standard connection between a host and the MODACOM system in which a large number of (e.g., >100) MTs are connected to a host through an SVC or a PVC (see Figure 6.5). Standard context routing (SCR) is used in pooling calls in order to dis- tinguish between the data packets of the different MTs in a time-division
  12. 376 6 Trunked Mobile Radio and Packet Data Radio multiplexed virtual connection. An SCR header that contains the logical destination address of the MT or the host and other applications-related parameters is added to each X.25 data packet. To identify an MT, the host must decode the SCR header that has been received and add it be- fore each packet when it is transmitting messages, to ensure that each message is received by the right MT. In a switched connection (SVC) between the MODACOM system and the host, the connection must first be set up by the host. A PVC (permanent virtual circuit) connec- tion that requires no time for setting up or terminating a connection is suitable for users with high traffic volumes. 6.2.4.3 Group Calls Along with the individual call, group calls are another feature of pooling connections. Each MT can respond to up to seven group addresses; thus up to seven different groups can be formed within a pool of MTs. A group call is only transmitted once, and there is no acknowledgement of receipt by the terminal or storage in the MODACOM box. Application software is available that enables a serial group call to be initiated in which the same message is sent to MTs in succession. With this procedure each call is acknowledged or temporarily stored in the MODACOM box. 6.2.5 Roaming and Handover In the MODACOM system MTs are not restricted to a particular area and, through constant accessibility by the host, can move about freely in the radio coverage area. Logical connections are supported by a handover procedure when there is a changeover from one radio cell to another. If the terminal establishes that the field strength of the selected radio chan- nel is too low or the bit-error ratio is too high, it initiates a roaming process and assigns itself to a new base station. The MT thereby searches for a new radio channel, evaluates the quality and the availability of the radio channel on the basis of status messages that are regularly sent by the base station, and, if the channel is considered satisfactory, transmits a registration packet to register itself with the relevant base station. Two types of roaming are differentiated in the MODACOM system: Roaming within the home ACC area in which messages coming from the host are merely rerouted to the other base station. Roaming between two ACC areas occurs when an MT leaves its ACC area and registers in another ACC area. The visited ACC checks the au- thorization of the MT with the home ACC and, if the MT is accepted, exchanges all the data required for the operation of the MT with the home ACC. The MT is then registered in the visitor register of the new ACC. All data relating to the MT is then forwarded from the home ACC
  13. 6.3 The TETRA Trunked Mobile Radio System 377 to the visited ACC. The visited ACC in turn reroutes all messages from the MT to the home ACC. 6.3 Second-Generation Trunked Mobile Radio Systems: The TETRA Standard∗ Despite the introduction of GSM throughout Europe, it is expected that the subscriber numbers for trunked mobile radio systems will continue to grow steadily, possibly reaching around five million by the year 2000. None of the first-generation trunked radio systems currently on the market offers satisfactory voice and data services or is technically capable of dealing with the anticipated number of subscribers. In an effort to harmonize the trunked radio market in Europe, and taking all these factors into account, ETSI decided in 1988 to produce a standard for a digitial, Pan-European trunked radio network. The first working title for this system, which was developed by the Technical Subcommittee RES 06, was MDTRS (Mobile Dig- ital Trunked Radio System). However, in late 1991 the new name TETRA (Terrestrial Trunked Radio) was introduced for MDTRS. Two families of standards have been produced for TETRA (see Table 6.2): • Voice plus Data Standard (V+D) • Data only (Packet Data Optimized Standard, PDO) ∗ With the collaboration of Martin Steppler Table 6.2: The series of the TETRA standard Series Content V+D 01 General network description and 02 Definition and description of air interface PDO 03 Definition of interworking function 04 Description of air interface protocols 05 Description of user interface 06 Description of fixed network stations 07 Security aspects 08 Description of management services 09 Description of performance characteristics V+D 10 Supplementary services—Level 1 11 Supplementary services—Level 2 12 Supplementary services—Level 3
  14. 378 6 Trunked Mobile Radio and Packet Data Radio The TETRA V+D standard is envisaged as the successor to existing first- generation trunked radio networks, whereas the PDO standard defines a second-generation packet radio system. Both standards use the same physical transmission technology and largely the same transmit/receive equipment. European-wide standardization is forcing the issue of interoperability, i.e., manufacturer-independency of terminal equipment in the TETRA network, as well as interworking between different TETRA networks and the fixed net- works. Local and regional voice and radio data applications are being replaced by a European trunked radio system that covers all voice and data services and satifies current requirements for bit rates and transmission delay. The main application areas for TETRA are fleet management, telemetry, service companies and for communication within government departments and orga- nizations responsible for security. Network operators, legislators, manufacturers and users were included in the standardization process to ensure that the ETSI-TETRA standard would have the chance of widespread implementation in the European market. The first TETRA products were available in late 1996. In 1997 the system was able to offer individual and group calls and data and other services described in detail in Section 6.3.2. 6.3.1 Technical Data on the TETRA Trunked Radio System The trunked radio system TETRA can be used as a local or a multicell net- work. Since the transmitter power of the terminal equipment is 1 W, 3 W or 10 W, the maximum cell radius in rural areas is limited to 25 km. Several frequencies in the ranges between 380 MHz and 470 MHz and 870 MHz up to 933 MHz have been allocated to the frequency bands for the uplink and the downlink (see Table 6.3). The possibility of using the 1.8 GHz band is being examined. The TETRA system uses π/4-DQPSK modulation and offers a gross bit rate of 36 kbit/s in a single 25 kHz channel. With an average quality of service guaranteed by the channel coding, the net bit rate is at 19.2 kbit/s. Without channel coding it is possible to achieve a maximum net bit rate of 28.8 kbit/s (see Table 6.4). With V+D four TDMA voice or data channels are available per carrier; with PDO there is no circuit-switched communication and a statistical multiplexing of packets is employed instead. Slotted-ALOHA (with reservation in data transmission) is used as the ac- cess procedure with V+D. In TETRA PDO a choice can be made between the access methods Slotted-ALOHA with reservation and Data Sense Multiple Access (DSMA), depending on traffic load. With V+D the frame structure consists of four 510-bit time slots per frame, 18 frames per multiframe and 60 multiframes per hyperframe, the latter representing the largest time unit and taking approximately one minute (see Section 6.3.4.2). With the PDO protocol the length of an information block is 124 bits protected by convolutional coding with a code ratio 2/3 and transmitted con-
  15. 6.3 The TETRA Trunked Mobile Radio System 379 Table 6.3: Technical data on TETRA in Europe Frequencies Uplink: 380–390 MHz, Downlink: 390–400 MHz Uplink: 410–420 MHz, Downlink: 420–430 MHz Uplink: 450–460 MHz, Downlink: 460–470 MHz Uplink: 870–888 MHz, Downlink: 915–933 MHz Channel grid 25 kHz Modulation π/4-DQPSK Bit rate 36 kbit/s gross; 19.2 kbit/s net (in 25 kHz channel) Channels/carriers V+D: 4 TDMA voice or data channels in 25 kHz PDO: Statistical multiplexing of packets Access methods V+D: TDMA with S-ALOHA on the random access chan- nel (reservation offered with packet data) PDO: S-ALOHA with reservation or DSMA, depending on traffic load Frame structure V+D: 14.17 ms/slot; 4 slot/frame; 18 frame/multiframe; 60 multiframe/hyperframe; slot length: 510 bit PDO: Uplink and downlink use 124-bit length blocks that are protected by FEC with code rate 2/3; continuous trans- mission on downlink, discontinuous transmission on uplink Neighbouring −60 dBc channel protection Connection setup <300 ms circuit-switched; <2 s connection-oriented Transmission delay V+D: <500 ms in a connection-oriented service, of a 100 byte refer- <3–10 s in a connectionless service, depending on trans- ence packet mission priority PDO: <100 ms with a 128-byte message tinuously on the downlink, discontinuously on the uplink. An exact profile of the channel coding for V+D is given in Section 6.3.4.3. The time required to establish a connection should not exceed 300 ms for a circuit-switched call or 2 s for a connection-oriented transmission of packet data (Connection Oriented Network Service, CONS). The transmission delay of a 100-byte reference packet in connection-oriented transmission with V+D should be a maximum of 500 ms; with connectionless transmission, depending on the respective transaction priority, it should be a maximum of 3 s, 5 s or 10 s. With PDO a transit delay of a maximum of 100 ms has been stipulated as the upper limit for connection-oriented services for a 128-byte reference packet. 6.3.2 Services of the TETRA Trunked Radio System The TETRA system provides packet data services, which are offered by the PDO and V+D standards, and circuit-switched data and voice services, which are only available with the V+D standard. The packet-oriented services dif- ferentiate between the following types of connections:
  16. 380 6 Trunked Mobile Radio and Packet Data Radio • Connection-oriented packet data transmission in accordance with ISO 8208 Connection-Oriented Network Service (CONS) and services based on ITU-T recommendation X.25. • Connectionless packet data transmission in accordance with ISO 8473 Connectionless Network Service (CLNS) for acknowledged point-to- point services and/or TETRA-specific acknowledged point-to-point and non-acknowledged point-to-multipoint services. Circuit-switched voice can be transmitted unprotected over bearer services or (preferably) protected over teleservices (see Table 6.4). The teleservices for voice transmission offer five different types of connection: Individual call Point-to-point connection between calling and called sub- scribers. Group call Point-to-multipoint connection between calling subscriber and a group called through a common group number. The call is set up quickly because no confirmation is required. The communication takes place in half-duplex mode through the activation of a push-to-talk switch. Direct call (Direct mode, DM) Point-to-point connection between two mo- bile devices with no use of the infrastructure. A mobile station estab- lishes a connection with another mobile station without the services of a base station, maintains the connection and takes over all the functions needed for local communication normally handled by the base station. Frequency ranges not normally used in the network are used for this purpose. At least one of the stations must have a connection to a base station on another channel [1]. For example, a connection can be es- tablished between two subscribers one of which is not operating in the supply area of the base station. Acknowledged group call Point-to-multipoint connection between a calling subscriber and a group called through a shared group number in which the presence of the group members is confirmed to the calling subscriber through an acknowledgement. If one of the group members is not present or is on another call, the TETRA infrastructure informs the calling subscriber. If not enough members in the group can be reached, the caller can decide whether to discontinue or to maintain the call. An option is to have the group members who initially were not available to switch into the conversation later. Broadcast call Point-to-multipoint connection in which the subscriber group dialled through a broadcast number can only hear the calling subscriber. The bearer services and teleservices provided in the standard for the pro- tocol stack V+D and PDO are listed in Table 6.4. The TETRA system supports the following data and text services:
  17. 6.3 The TETRA Trunked Mobile Radio System 381 Table 6.4: Bearer services and teleservices for V+D and PDO in the TETRA stan- dard TETRA V+D PDO Bearer 7.2–28.8 kbit/s circuit-switched, unprotected speech Ö – services or data 4.8–19.2 kbit/s circuit-switched, minimally protected Ö – data 2.4–9.6 kbit/s circuit-switched, highly protected data Ö – Connection-oriented packet transmission (point-to- Ö Ö point) Connectionless packet transmission in standard for- Ö Ö mat (point-to-point) Connectionless packet transmission in special format Ö Ö (point-to-point, multipoint, broadcast) Tele- 4.8 kbit/s speech Ö – services Coded speech Ö – • Group calls • Emergency call messages • Status messages • Electronic mail (e-mail) • Data messages • Facsimile and videotex In addition, different supplementary services are offered, e.g.: • Indirect access to PSTN, ISDN and PBX over a gateway. • List search calls (LSC) in which subscribers or groups are called in the order of sequence of the entries in a list. • Include calls in which by dialling the respective number an additional subscriber can be included in an existing telephone call. • Call forwarding and call diversion. • Call barring of incoming or outgoing calls (BIC/BOC). • Call authorized by dispatcher (CAD), in which a request is made to place a particular kind of call. • Call report (CR), allowing the telephone number of a calling subscriber to be recorded so that the called subscriber can return the call later. • Calling number/connected line identification (CLIP, COLP). This func- tion can be prevented using calling/connected line identification restric- tion (CLIR). Talking party identification is also possible.
  18. 382 6 Trunked Mobile Radio and Packet Data Radio • Call waiting (CW), indicating to the subscriber who has called while the talking party was on another call. • Call holding, connect-to-waiting, allowing a subscriber to interrupt his current call to take another call and then to continue with the original call. • Short-number addressing (SNA), allowing a user to make a call using an abbreviated calling number. The TETRA infrastructure converts the abbreviated number to the subscriber number. • Priority calls. • Priority calls with interruption. • Access priority. • Advice of charge (AoC) is a service that indicates the cost of a call to a subscriber either during or after a call. • Discrete eavesdropping of a conversation by an authorized person. • Ambience listening (AL), allowing the user of a mobile device to be restricted to emergency calls only. • Dynamic group number allocation. • Transfer of control (TC), permitting the initiator of a multipoint con- nection to transfer control of a call to another person involved in the call. • Area selection (AS), allowing an authorized user to select the cell for setting up a call or a subscriber currently being served to determine the cell. • Late entry (LE) is an invitation to potential subscribers in a multipoint connection to be switched into an existing call. Series 10, 11 and 12 of the V+D standard contain a complete compilation of the supplementary services, along with detailed definitions and descriptions [10, 11, 12]. 6.3.3 Architecture of the TETRA Standard 6.3.3.1 Functional Structure of the TETRA System With few differences, the TETRA system is structured like the GSM one (see Figure 6.6). It has the following three subsystems: • Mobile station • Line station • Switching and management infrastructure
  19. 6.3 The TETRA Trunked Mobile Radio System 383 Air Interface Switching and Management Intersystem Infrastructure Interface Line- Mobile Station Station Line Station Man- Data Man- Network Base Interface Network Machine Termination Machine ISDN Termination Interface Interface Base User Network Station Network User Interface Termination Termination Interface PSTN ISDN PDN PBX Figure 6.6: The architecture of the TETRA system Mobile station (MS) The mobile station (MS) comprises all the subscriber’s physical equipment: the radio telephone and the interface that the subscriber uses to access the services. As in GSM, a mobile station consists of two parts: the telephone device, which contains all the necessary hardware and software for the radio interface, and a Subscriber Identity Module (SIM), which contains all the subscriber- related information. The SIM can either be in the form of a smart card the size of a cheque card or permanently mounted in the device. The first version has the advantage that it allows a quick change in ownership of the mobile station. The third option is to key in a login code to convey the subscriber- specific information. In this case too the mobile unit is not restricted to one particular user. In addition to the subscriber’s identification, each mobile device has a TETRA equipment identity (TEI) that is specific to the device. This num- ber is input by the operator; only the operator is able to bar the use of the device or release it for use. This means that a stolen device can be disabled immediately and unauthorized access is virtually impossible. The following numbers and identities are allocated to ensure that a mobile station can be uniquely addressed and managed: • TETRA Subscriber Identity • Short Subscriber Identity (TSI) (SSI) • TETRA Management • Mobile Network Identity Identity (TMI) (MNI) • Network Layer SAP Addresses (NSAP) The TSI consists of three parts: a Mobile Country Code (MCC) that contains the country identification, a Mobile Network Code that identifies the respective TETRA network, and a Short Subscriber Identity (SSI) that identifies the subscriber. When a connection is to be established within the home network,
  20. 384 6 Trunked Mobile Radio and Packet Data Radio only the SSI is used as the address. This reduces the volume of signalling data. The TMI is used for the management functions of the network layer. The NSAP is employed for the addressing of external, i.e., non-TETRA networks, and is optional. For instance, it can be used to establish a connection to ISDN. Similarly to GSM, mobile telephones can be installed in vehicles or used as portable/hand-portable devices. All the standard services listed in Sec- tion 6.3.2 can be accessed by a mobile station. Supplementary services are offered by the network operator or must be contracted at the same time as the mobile phone contract so that the user can access them. Line station (LS) In principle a line station is structured in the same way as the mobile station, but with the switching and management infrastructure connected over ISDN. For example, the owner of a company that operates a fleet of lorries would use a line station as the central station for his network. A line station offers the same functions and services as a mobile station. Switching and management infrastructure (SwMI) The switching and management infrastructure (SwMI) forms the local control unit of the TETRA system. It contains base stations that establish and maintain communication between mobile stations and line stations over ISDN. It carries out the required control tasks, allocates channels and switches calls. It carries out authentica- tion checks and supports the relevant databases such as the Home Data Base (HDB), which contains the telephone numbers, the equipment numbers and the subscribed basic and supplementary services for each individual subscriber in the home network, as well as the Visited Database (VDB), which contains information on visitors to the network copied from their HDB. It also handles call charging. 6.3.3.2 Interfaces of the TETRA System Subscriber interface of the mobile station A TETRA mobile station is called a mobile termination (MT). Its functions cover radio channel resource and mobility management, speech and data coding/decoding, transmission security as well as data flow control. The following versions are used: • MT0 (Mobil Termination Type 0 ) contains the named functions with the support of non-standardized terminal interfaces that contain the terminal equipment functions (see Figure 6.7). • MT2 (Mobil Termination Type 2 ) also supports the named functions, and has an RT interface for terminal equipment based on the TETRA standard (see Figure 6.7). The terminal equipment (TE2) is directly accessible by the subscriber, and corresponds to comparable function groups of the GSM and ISDN concept.
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