First, why promote wireless communication technology

China's current smart card devices (IC card consumer machines, attendance machines, access control machines, etc.) have been in large-scale development history since the mid-1990s, and have experienced more than 10 years of large-scale development history. During this period, smart cards have undergone several upgrades, from contact IC cards have evolved into contactless IC cards, from memory cards to logical encryption cards and CPU cards. The device functions have also undergone several upgrades. However, its networking method is developing very slowly. Among them, more than 80% of the devices connected to the PC still use the RS485/RS422 network; a few devices start to build the TCP/IP communication module, using the five types of cable networking, but the cost is higher, the networking distance Closer (100 meters). The individual system uses the CAN bus, the cost is moderate, the networking distance is about 1000 meters, and the communication speed is better at 19.2-100 kbps.

At present, the smart card device using RS485 network communication has a maximum communication distance of 1200 meters, a communication rate of about 9.6-19.2 kbps, a half-duplex mode, and a number of networks of less than 128. Due to the lag of the networking method, the network communication bottleneck of the smart card device is generated. In particular, it is necessary to quickly solve the construction convenience, communication speed, communication distance and the like.

At present, smart card devices are limited by the way of communication, which brings great restrictions on the installation of devices; for example, in some densely used places, such as campus classrooms, factory workshops, etc., it is necessary to lay intensive networking lines, usually 1-2 cables are laid in each classroom, and hundreds of information points are laid on a campus. The cost of laying the lines is high and the construction period is long. In particular, the maintenance work caused by the line fault in the later stage is large, and the efficiency of finding the fault point is low.

For some existing sites such as campuses, factories, etc., due to insufficient consideration in the initial stage of construction, network pipelines have not been reserved. Only pipelines, excavations, etc. can be considered for re-laying pipelines. As a result, construction costs have far exceeded smart cards. Total equipment investment.

At present, the research on ZigBee network has a broad application prospect on smart card devices. And has the following characteristics:

1. Mature technology, the basis of ZigBee network is IEEE 802.15.4;

2, low cost, the main controller of ZigBee network generally uses SoC (system on chip), the peripheral circuit is simple, and the interface with the main processor (MCU) of the smart card device is convenient. Existing smart card device architectures can be easily implemented without major upgrades.

3. No need to apply for a wireless channel.

4. The communication radius can reach several hundred meters under the line of sight. The repeater can be designed to expand the communication distance by multiple times (the current scheme can extend the communication radius by 6 times to 2000 meters).

5, communication speed is fast, ZigBee network communication speed can reach 250kbps, and even higher, its speed greatly exceeds the RS485 network.

6. Smart card devices using ZigBee wireless network communication The maximum number of device communication in a single network is 750. This number can also be expanded (currently the number of limit networking is 62,500), and its networking capability greatly exceeds wired RS485 network, CAN network, etc. .

7, wireless communication has no line restrictions, the signal can easily penetrate the conductive objects such as walls, glass, wood, etc. After the equipment is in place, the wireless network channel is formed immediately, and the construction speed is fast. Once the network is built, all smart card devices can scale to any mobile and limited (up to 750) devices as long as they are within the wireless signal coverage.

8, wireless communication has no line restrictions, the signal can easily penetrate the conductive objects such as walls, glass, wood, etc. After the equipment is in place, the wireless network channel is formed immediately, and the construction speed is fast. After the network is built, as long as the wireless signal coverage, all smart card devices can achieve any mobile and device expansion; even the ultimate customer can directly purchase wireless smart card devices, self-organizing, no professional company construction. Wireless communication technology has pushed the application of smart card devices to a new milestone.

9, wireless communication can greatly reduce the use of cables, the average use of 50 meters of wire per machine calculation, the annual sales of 10,000 sets of equipment to save 500,000 meters of cable, if the use of five types of lines, save more than 1 million yuan Cable cost; if it is calculated at a wiring cost of 10 yuan per meter, it will save more than 5 million wiring costs. Under the policy of building a conservation-oriented society in the central government, the popularity of wireless smart card devices has obvious social benefits.

Second, how to ensure that data is not interfered with during wireless communication

As we all know, when communicating wirelessly, the communication signal will inevitably be disturbed due to the surrounding electromagnetic environment. The ZigBee wireless network protocol has been designed with various anti-interference methods in mind. These anti-interference methods have also been approved by the international IEEE and have issued relevant standards. However, it is an indisputable fact that wireless communication signals will be interfered.

The topic we are going to discuss today is how to ensure that our swipe data is not lost and undisturbed when the wireless signal is disturbed. Readers who have used wireless networks will have this feeling: copying large files from one computer in the network to another (connected via a wireless network), the transmission speed is not constant, when there is signal interference, The speed will suddenly and slowly, but when the interference signal disappears, its transmission speed will recover; but the transmitted file will not be damaged because of the communication protocol.

Our communication protocol packs the data to be transmitted into one data frame. The communication data frame consists of frame number, frame header, frame length, and frame data check. The communication steps are as follows:

1. The computer first sends a query command to the machine. After receiving the computer's query command, the machine will respond to the computer with the current data status. The response content includes: machine number, model number, days of uncollected data, date of micro-acquisition data, number of uncollected data, number of communication frames to be divided into uncollected data, and so on.

2. When the computer receives the response frame of the machine and checks the check digit, it knows the data record of the machine; then the computer sends a command to collect data.

3. When the machine receives the command to collect data, it will pack and number the data to be transmitted, and then transmit the data to the computer one by one.

4. After receiving the last frame number (because the number of frames transmitted by the machine has been known before), the computer starts to check all the data frames received this time one by one, and then loses the data frame that failed the verification, and the data is lost. The intact data frames are sorted by number.

5. After the computer is finished, if the number of the received data frame is found to be incomplete, such as 100 frames received this time, the data of frames 10, 23, and 58 will be interfered after verification, and the computer will send a retransmission designation. Frame number" command.

6. After receiving the command of “Retransmit the specified frame number”, the machine retransmits the specified data frame: frames 10, 23 and 58 to the computer.

7. Repeat all the data frames of the 5th and 6th straight channels to be correctly transmitted.

8. The computer issues the “Marked Transferred Data” command. After receiving the command, the machine automatically records the credit card that has been correctly transmitted to the computer as the “transmitted” mark. Data with the "Transferred" tag will not be transferred to the computer again the next time new data is collected.

From the above communication process, we assume that the wireless signal is interfered when the machine transmits 100 credit card data to the computer, resulting in the destruction of frames 10, 23 and 58. The computer requires 10, 23, 58 when proofreading the data. The frame is retransmitted, and if it is disturbed again, the computer will also request retransmission of the interfered frame until it is fully received.

The above transmission process is the same as the principle of transmitting files using the wireless network. When the wireless network signal receives interference, it is only interfered with by a certain communication frame or multiple communication frames, but our communication protocol will check out. The interfered communication frame then automatically retransmits the disturbed data frame until it is all correct. Thus, when the wireless network signal is interfered, only data retransmission is generated instead of data loss. In the case of serious communication interference, we can only feel the communication speed is slow, not the data is lost or destroyed.

Third, wireless communication network topology

3.1 Standard wireless card network topology

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