Detailed Analysis of GSM Communication Service System

　　GSM communication service system can use two ways of short message and GPRS according to the different data transmission methods. As far as the short message service is concerned, since the short message service is the only service in GSM that does not require the establishment of an end-to-end path, it provides good conditions for using the short message service to transmit GPS information. There are generally two ways of SMS service: ① "point-to-point" working mode; ② "point-to-center" working mode. As far as GPRS service is concerned, due to the difference between GPRS (General Packet Radio Service) and GSM circuit switching, it belongs to the data communication system of packet switching, which has the advantages of "always-on, high-speed transmission, fast login, and traffic charging", which is especially suitable for For applications with burst data like vehicle positioning system.

　　(1) "point-to-center" service model

　　The so-called "point-to-center" service mode is the communication method between the vehicle-mounted unit and the monitoring center. There are three communication methods:

　　1. Directly forwarded by SMS service center:

　　Introduction to GSM short message technology and its characteristics: Short message service (SMS) is an important part of GSM technology application. It has some outstanding features such as: 140 bytes of data can be transmitted at a time, and the content of the data can be characters or numbers. It can be transmitted end-to-end in the GSM network, and can also send short messages to an end station from outside the GSM network (such as the Internet); Channel transmission, which does not conflict with the voice channel, can be transmitted even if the terminal is in a call state; in the process of short message transmission, the process of establishing and releasing the call connection is not performed; the MSC has a short message storage function, and the terminal equipment is turned off when , you can keep the message valid for a certain period of time, etc. Using these characteristics and the performance of two-way transmission, the information collection and remote control of the equipment of the collection station can be easily realized, that is, the realization of telemetry and remote control.

　　The short message service is the only service in the GSM/DCS1800 system that does not require the establishment of an end-to-end service channel. A point-to-point short message is an alphanumeric string in any form, transmitted over a digital control channel. When idle, it occupies the independent dedicated control channel (SDCCH), and the information rate is 782b/s; when talking, it occupies the slow associated control channel (SACCH), and the channel rate is 383b/s. In order to avoid too long time delay and too much load on these competing access channels, the maximum frame length of each short message is 140 bytes (160 characters according to ASCII character 7bit encoding). Therefore, a Transport Protocol Data Unit (TPDU) of a short message can be sent or received at any time, regardless of whether voice or data communication is in progress. Since the public GSM network has realized networking and roaming on a global scale, the establishment of a GSM system does not require the establishment of a dedicated communication network, so the short message application with real-time data transmission function will be able to transmit various detection and monitoring data signals and control commands. It can be widely used in remote monitoring, positioning and navigation, personal communication terminals, etc.

　　Both the vehicle-mounted unit and the monitoring center use a mobile phone-like SMS sending and receiving communication module. The vehicle-mounted unit SMS sending and receiving communication module sends the short message together with the destination station to be sent to the short message service center (SMSC), and then the SMSC forwards it to the monitoring center for short message sending and receiving communication In the same way, the short message sending and receiving communication module of the monitoring center will also send the short message including the central processing unit and the destination station to be sent to the SMSC, and then the SMSC will forward it to the vehicle-mounted unit.

　　In the short message service system, the SME server is responsible for establishing the data communication connection with the short message service center, each monitoring center and the remote database; after converting the data format of the data frame sent by the SMSC, it is entered into the central database according to the frame type; and then the data is sent to the It converts the format of the command frame sent by each monitoring platform, and sends the converted message frame to the SMSC. The number of messages that need to be billed is entered into the database as the basis for user billing; if encryption and decryption are required, data encryption and decryption work is performed.

　　The biggest advantage of this short message service mode is that the short message sending and receiving communication module is cheap, without additional network overhead, and the communication cost is low; but the short message service mode sometimes has a long delay, and the data processing capacity of the short message sending and receiving communication module of the monitoring center is relatively low. Low, only for low-vehicle situations.

　　2. DDN dedicated digital circuit or Internet forwarding:

　　When the short message is upstream, the short message module of the vehicle unit sends the short message including the GPS information and the destination station information to the SMSC in the communication service center, and then forwarded by the communication service center, and finally routed through the DDN dedicated digital circuit or the Internet. Go to the monitoring center for processing; when the short message is down, the monitoring center will route the short message including the command sent by the central processing unit and the differential correction signal and the destination station information to be sent to the communication service center through the DDN dedicated digital circuit or the Internet, and then Sent to SMSC, and finally forwarded by SMSC to the SMS module of the vehicle-mounted unit.

　　The communication service center is the core of this part of the system. It is usually located in the computer room near the mobile phone switch and is connected to other parts through high-speed data lines. It is a separate computer network (Novell network), consisting of GSM switches, SMSC short message service center (or dedicated No. 7 signaling protocol processing unit), routers, WAN ports, workstations, etc. It has to complete 4 basic functions:

　　(1) Two-way data communication with the vehicle-mounted unit is realized by means of short messages.

　　(2) It has a connection port with the wide area network, which can realize the connection between the wireless mobile communication network and the wired computer network.

　　(3) According to the requirements of the command and monitoring center or the user, the system can complete certain functions, such as mute, display call charges, etc.

　　(4) Vehicle information database and maintenance functions.

　　The advantages of this short message service mode are strong data processing capability and small delay (usually in the range of 2~10s), which is suitable for situations where there are many vehicles. However, this short message service mode has a relatively large network overhead, so it is necessary to build a router by itself and apply for an IP address; if the DDN private line is adopted from the perspective of security, there is also the overhead of renting a circuit.

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