On the construction of 350 megabytes trunking communication network of public security
Since the 350 trillion police cluster communication system has served the public security department in the early 1990s, it has played a huge role in various major security work, emergency rescue and disaster relief and daily police activities. The rapid flexibility of its command and dispatch is unmatched by any public network mobile communication system, which has been fully proved in practice.
The status of public security cluster communication network construction The wireless communication of the public security department was basically based on conventional communication methods in the late 1980s and early 1990s. With the maturity of the MPT1327 signaling system cluster system, large-scale Large-scale cluster system construction. At present, in the economically developed eastern regions, the police cluster system basically covers the administrative regions above the prefecture-level cities. In the western region, more than 70% of the cluster systems are still blank.
At present, the cluster interphone systems used in various places are all cluster systems of the MPT1327 signaling system, and most of the systems are constructed in accordance with the industry standard "GA176-1998" formulated by the Ministry of Public Security. Some cluster systems built in the early stage are also gradually Improve or upgrade to this standard.
Although the digital trunking system is currently hot on various media and some commercial systems are in operation, the author believes that for the public security department, the construction of the police trunking system should still be dominated by the analog trunking system in recent years. Because: first, the analog cluster system has experienced more than ten years of application, running-in and improvement, and its use function should be better than the current digital cluster system; second, the analog cluster system has been localized long ago, so its price is vast What the public security department can accept is that, because the coverage of the analog trunking system is larger than that of the digital trunking system, so the same coverage is obtained in the same city. The price advantage of the analog trunking intercom system is very obvious; the digital trunking system is mainly to solve Due to the problem of frequency congestion, the State Commission for Non-Commissioning has allocated the 350 to 370 trillion frequency band specifically for use in police wireless communications. There are a total of 160 pairs of dual-frequency points for use in trunking systems, and 40 pairs of dual-frequency points are reserved. A reasonable reuse plan can fully satisfy the use of most public security departments in the next few years. Of course, the application of digital trunking system is an inevitable trend of the development of electronic communication technology, but it is not suitable for promotion and use in public security departments at present and in the next few years.
The demand for the construction of public security cluster communication network can be discussed from three aspects:
First, the networking mode of the cluster network;
The networking mode of the trunking network mainly includes the number of mobile terminals to be equipped by the user, the number of base stations, the number of channels of each base station, the location selection of each base station, the height of the antenna of each base station, and the connection method between each base station and the switching control center, etc. .
The number of mobile terminals directly determines the number of channels of the base station. Table 1 is a comparison table of the number of channels of the single base station cluster system and the number of mobile terminals. The table is calculated according to the empirical formula A = M.AA. A is the total traffic volume of the system, which is determined by the number of channels and the call loss probability of the system. The call loss probability of dispatched calls is usually 10%. From these two indicators, the total traffic volume of the system can be obtained by checking the Irish C table ; M is the number of mobile user terminals of the system; AA is the average traffic of mobile user terminals, usually 0.008Erl, which means that the average user occupied channel length is 28 seconds, and the average call is 1 time per hour.
Table I:
The number of base stations mainly depends on the geographical environment of the user's city and the coverage required. Generally, large and medium-sized cities have many high-rise buildings and the electromagnetic environment is not very good, so we can take the experience value of the project, that is, the coverage radius of the handheld machine is about 10 kilometers. If the service radius of the trunking system is required to be greater than 10 kilometers, multiple base stations are usually required.
The choice of the location of the base station mainly considers whether the electromagnetic environment of the selected location meets the requirements, whether the conditions of the equipment room meet the requirements, and whether it has a sufficient height of the tower and other factors. Because many public security departments are currently constrained by economic conditions and do not have the ability to build infrastructure, choosing a place with installation conditions as a base station is a good solution.
The trunking system belongs to a large-area wireless communication system. The height of the wireless installation directly affects the coverage distance of the system. Table 2 shows the empirical values ​​for the prediction of the height of the base station antenna and the coverage distance of the base station. The calculation of the coverage prediction is based on the theory of radio wave propagation in mobile communication engineering and the engineering experience formula of radio wave propagation loss. The prediction table is suitable for areas with relatively flat terrain, such as urban suburbs with flat terrain, and is not suitable for many tall buildings. Inside the city.
Table II: 
The connection methods of each base station and the switching control center usually have two methods of optical fiber and microwave. If the base station site is selected in the urban area and the optical cable is available, it is best to use optical fiber connection. If the base station site is selected on the mountain, microwave equipment can be used in places where there is no optical cable. Either fiber or microwave will eventually convert the E1 interface to the analog trunk interface required for base station connection. If conditions permit, it is best to use all fiber optic connections. Fiber optic relay equipment runs stably and basically does not require maintenance. Microwave equipment will be affected by weather (such as rain attenuation) and electromagnetic environment, so the benefits of using fiber optics are self-evident. .
Second, the scheduling and fault weakening functions that can be achieved by the cluster network;
The Ministry of Public Security issued the police cluster system standard "GA176-1998" has specified the scheduling function of the cluster system in detail, but with the continuous improvement of user needs, some new functions have been proposed. Here is a brief explanation.
Voice call scheduling function. The voice call scheduling function mainly uses single call, group call, team call, full call, emergency call, transfer call, inclusive call, automatic call back in busy queue, etc. Among them, group calls are the most important dispatch calls. Temporary group calls and open group calls are group calls that are often used in large-scale security activities. With the emergence of multiple base station systems and the networking of multiple systems, the system is required to be able to realize group calls across multiple base stations and across multiple systems. Is it possible to realize group calls across multiple base stations and across multiple systems? An important indicator of the technological advancement of the system. In addition, the most prominent advantage of the cluster system is that the call connection establishment time is shorter. At present, the call connection time of group calls across multiple systems can be within 700 milliseconds.
The network scheduling function of the system and computer network technology have been widely used in various fields, and the cluster communication system is no exception. The single PC terminal management mode of the cluster system is no longer suitable for the overall needs of the alloy shield project. The establishment of the 110 linkage command center network is an important part of the "technical strong police". The 350-megabyte cluster system is an important branch of the network, which requires it to adopt a management system with a network structure and TCP / IP protocol through the network interface. Connected to the network platform of the 110 linkage command center, the 110 linkage command center can realize remote command scheduling and remote monitoring of the system, and at the same time, it is convenient for users or manufacturers to remotely maintain and manage the cluster system. With a simple practical case, you can understand the importance of network scheduling in cluster systems. Some people call 110 to call the police. The location of the person ’s alarm can be displayed on the dispatch terminal of the 110 dispatcher through the data provided by the telecommunications department. The dispatcher clicks the alarm location with the mouse, and the surrounding patrol car can be displayed. An idle patrol car can realize a three-way call between the alarm person, the dispatcher and the patrol person. The call patrol personnel are dispatching calls initiated through the network interface.
The fault weakening function of the cluster system. The fault weakening function of the cluster system can be discussed from two aspects: the topology structure of the network and the software design of the network. The cluster system usually has two structures: centralized control and branch control, and each structure has its own advantages. The centralized control system has an absolute advantage in connection time, and the work of each base station of the branch control system is not affected by the switching control center and the relay link. If the topology structure of the cluster system can combine the two, then it is a perfect system. The network topology structure that combines centralized control and branch control is also one of the important means to realize the networking of city and county cluster systems. In terms of software design, signaling channel timing switching technology or failover technology can be used to ensure the normal operation of the signaling channel. It can also automatically close the failed channel by detecting the interference of the receiver and the normality of the transmitter's transmitted power. and many more. General cluster systems can do this: the failure of the wired interface does not affect the conversation between wireless users, the failure of the voice switching unit does not affect the conversation of the wireless users of this base station, and the failure of the control center or link does not affect the work of the sub-base station or the sub-base station is weakened as a routine way of communication.
Third, the value-added regional services that the cluster network can provide.
After the construction of the trunking network is completed, if it can only provide the pure voice service of the network, it cannot play its due role. Here we discuss the value-added services of cluster systems from the aspects of system networking, mobile data transmission, and interconnection of cluster systems with conventional systems.
System networking, the service scope of each cluster system is limited, generally limited to local use, but with the rapid development of economy, transportation, and information technology, the scope of police activities can no longer be limited to a certain area, such as a police car driving On the highway, it may leave its own jurisdiction or base station coverage within a few minutes. If the walkie-talkie can network a cluster system in a certain region or province, then the scope of use of mobile terminals will be expanded exponentially, which greatly facilitates police officers to go out to perform tasks. System networking is mainly to connect the voices of different systems and update the location information of mobile user terminals in time. Over the past few years since the implementation of the Golden Shield Project, the public security private data communication networks in various provinces have basically been established. There are two main forms, one is the transmission network, and the other is the IP data network. The combination of data and images. The establishment of a public security dedicated data communication network provides a physical basic channel for the networking of the cluster system. We can set up a province-wide cluster system voice tandem switching center in the provincial office, and the voice of each sub-system will be received by the provincial office through the voice channel provided by the dedicated data network. Through the data interface provided by the dedicated data network, the network management system of each subsystem is formed into a dedicated local area network to automatically update the user's location registration information and cross-system call information.
Mobile data transmission and mobile data query are an indispensable and important part of current police activities. Police officers working in the frontline often need to query information such as license plate number, driver's license, permanent population, temporary population, and criminal suspect information in real time. Inquiry of this information can make full use of the air transmission channel provided by the 350-megabyte cluster system, without any changes to the system and terminal equipment, just add a micro modem and handheld computer to the mobile terminal, and add a wireless gateway interface to the user database. . The speed of the mobile data transmission system currently in use is 1200bit / s, and it takes about 8 seconds to query a 200-word message. The biggest benefits of mobile data transmission with the help of a 350 megabyte cluster system are saving money, fast networking, convenient query and strong practicability. This method is fully capable of mobile data transmission under 2K bytes. Technology development is endless, more advanced modulation and demodulation technology may increase the rate of data transmission, but we still have to measure how cost-effective.
The trunking system is interconnected with the conventional system. In many places, conventional communication equipment is still being used in large amounts. The 150, 350, and 450 megabands are available. We can add cluster interface control equipment to these conventional relay stations to give the cluster interface The device has a cluster ID number and several participating group numbers. The cluster interface device converts the DTMF call signal sent by the conventional terminal into MPT1327 format signaling and forwards it to the cluster system, so that the conventional terminal can call the cluster terminal. The cluster terminal can call the conventional terminal by calling the ID number of the cluster interface device. Figure 2 shows a schematic diagram of the interconnection between conventional systems in different frequency bands and the cluster system. From the figure, it can be seen that the cluster terminals can schedule any conventional terminal, and the conventional terminals can also communicate with each other through the cluster system.
Several ways to expand the coverage of the cluster system Single-zone multi-base station, divide the base station of the cluster system into several base stations, pull each base station with microwave, optical fiber and other relay links and place them in different locations to expand The coverage of the entire system, the scheduling function of each sub-base station is the same as the main base station, so it is the most complete solution to expand the cluster coverage. We call it a single-zone (same system station number PFIX) multi-base station system.
Intelligent extension base station, which is a networking method that extends the coverage of the cluster system. It is an intelligent base station that uses a frequency-controllable link channel machine to connect to the remote MPT1327 cluster system and solve local cluster coverage. It is a solution that is easy to install, does not require microwave relay equipment, and invests in a low-cost cluster system to extend coverage. It is mainly suitable for high mountain base stations or tunnels where microwave equipment cannot communicate. The intelligent extension base station is composed of a controller, a local wireless channel and a link channel. The controller of the extension station does not supervise the mobile user, and all registration and call signaling are turned to the donor base station. When the station works, the overhead of the donor base station will increase, so the channel number configuration of the extension station should not be too much. When using the extension station, it should be noted that the frequency of the local channel should be as far as possible from the frequency of the donor base station, the larger the interval, the better, and the frequency group without third-order intermodulation with the donor base station is used to reduce the mutual interaction between the local channel and the link channel interference. Because the extension base station uses a different frequency from the donor base station, user terminal usage in overlapping coverage areas is not affected. The extension station has limitations in the scheduling function, and cannot realize functions such as open grouping and continuous channel update.
The cluster intelligent extension system, as the name suggests, is a self-contained cluster system. It adds intelligent wireless interface equipment on the basis of the cluster system. The extension system is connected to the main system to form a centralized system Network topology structure combining control and sub-control. Wired calls directly enter the local PBX. Only cross-system calls are initiated to the main system, while the extension system can block unwanted cross-system group calls. A mobile user can automatically roam between the main system and the extended system, but the previous topic is that the user is a legitimate user of both systems. Since the extension system is connected to the main system in the MPT1327 signaling format, it can be connected to any MPT1327 cluster system of different models. Tingshen system can refresh the location information of roaming mobile users by querying. The extension system is mainly suitable for the construction of the county-level cluster system. It is interconnected with the upper-level cluster system through the wireless link channel, and the autonomous system and the link machine can also be pulled away by wire. The wireless signal is not connected. Figure 3 shows the schematic diagram of the interconnection between the intelligent extension system and any type of main system. If the extension system and the main system are the same type of system, you can remove the link machine and directly connect to the main system through the cable.
Summary The above discussed the construction of the 350 megabyte trunking system for public security from several aspects. The system construction has flexible and diverse networking methods. When planning a cluster network, users should first fully understand their actual conditions, the equipment and technical support of their vendors, and work out the most suitable and cost-effective networking solution. I hope these points of view can provide some help to the construction of public security 350 megabyte trunking system.
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