Tài liệu Điện thoại di động mạng lưới Radio P6 ppt

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 A channel is allocated to the user by the system only when required, and then immediately withdrawn after use.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 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) 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 p v 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 [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. 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 368 6 Trunked Mobile Radio and Packet Data Radio Trunked System Controller Trunked System Controller Trunked System Controller Antenna Switch Antenna Switch Antenna Switch Mobile User Mobile User TRX TRX TRX TRX TRX TRX TRX TRX TRX TRX 1 2 3 4 20 4 3 2 1 MSC Area 3Area 2 Node Area 1 2020 4 3 1 2 TRX TRX TRX TRX TRX TRX OMC Telephone System Central Station Sender- Sender- Sender- Receiver Receiver Receiver 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. 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. 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 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. 372 6 Trunked Mobile Radio and Packet Data Radio User Terminal User Terminal User Terminal Network Administration Host (NAH) MT MT BS BS ACC G GACC ACC G BS BS MT Radio Data Network (RDN) RD-LAP X.25 X.25 X.25 X.25 (Air Interface) X.25 Data Network 9.6 kbit/s 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. 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 374 6 Trunked Mobile Radio and Packet Data Radio Radio (RDN) Network Data (DTE) Terminal Data Equipment RD-LAP PAD X.25 X.25 - PDN PAD X.25 SVC X.3 Partial Support X.29 X.25 MT 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 [...]... The radio interface in Section 6.3.4.2 6.3.4 The radio interface at reference point Um is discussed The Voice+Data Protocol Stack This section will cover the Voice+Data protocol stack in general This will be followed by detailed discussion of the radio interface, the physical layer and the link layer, with emphasis on functions, service elements, data structures and states 386 6 Trunked Mobile Radio. .. services—Level 3 378 6 Trunked Mobile Radio and Packet Data Radio The TETRA V+D standard is envisaged as the successor to existing firstgeneration 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,... 1) mod 4a 1 4 4 − (MN + 3) mod 4a 1 4 1 4 1 4 TN 6.3 The TETRA Trunked Mobile Radio System 393 394 6 B(m) Trunked Mobile Radio and Packet Data Radio S(k) Phase Transition Generation s(t) Modulation Symbol Generation D/A Modulation Filter g(t) Differential Encoding M(t) Frequency Translation Modulation Figure 6.13: Block diagram of the modulation process then the uplink and the downlink SCHs are also... 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 channel 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... Mobile Radio and Packet Data Radio Information Bits in MAC Blocks Type 1 Bits in Type 1 Blocks Block Coding Block-Coded Bits Tail Bits Type 2 Bits in Type 2 Blocks Convolutional Coding Convolutional-Coded Bits Type 3 Bits in Type 3 Blocks Interleaving Interleaved Bits Type 4 Bits in Type 4 Blocks Scrambling Type 5 Bits in Type 5 Blocks Multiplex Blocks Figure 6.15: Diagram of TETRA channel coding Since... >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 distinguish between the data packets of the di erent MTs in a time-division 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... videotex In addition, di erent 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...6.2 MODACOM 375 MT Radio Data Network (RDN) X.25 G X.25 X.25 - PDN X.25 SVC RD-LAP Data Terminal Equipment (DTE) MT A A Radio Data Network (RDN) MT B DTE DTE B One Channel per MT Connection MT C C Data Terminal Equipment (DTE) Figure 6.4: Incoming individual call MT A MT One Channel for Multiple MT Connections A B SCR DTE DTE User Process A SCR B B C C MT C Radio Data Network Data Terminal... channel coding are divided into two di erent blocks (Block Number 1 (BKN1), BKN2) for this In addition, there are four downlink bursts: a Normal and a Synchronization Continuous Downlink Burst (NDB, SB) and a Normal and a Synchronization Discontinuous Downlink Burst, each of which occupies an entire time slot This di erentiation is made because the base station can chose between continuous mode and discontinuous... ACC 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 . 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. areas. Licence type D Countrywide trunked radio network for mobile data radio applications. Trunked radio networks are divided into two categories based on