Wireless local area networks (WLANs) can reduce the cost of network infrastructure, enabling users to enjoy mobile network applications and achieve high efficiency, high quality and low cost services.
introductionWireless local area networks (WLANs) can reduce the cost of network infrastructure, enabling users to enjoy mobile network applications and achieve high efficiency, high quality and low cost services. Mobility allows users to freely change positions while using devices such as handheld computers or data loggers, greatly facilitating the need to constantly move positions while working. These devices provide real-time access to data stored in a central database. Radio Frequency Identification (RFID) is a non-contact automatic identification technology. Its basic principle is to use the RF signal and spatial coupling transmission characteristics to realize automatic recognition of the identified object. It has the characteristics of large data storage, readable and writable, strong penetrating power, long recognition distance, fast recognition speed, long service life and good environmental adaptability. In addition, it is the only automatic identification technology that enables multiple tags to be read simultaneously. This paper studies a combination system called WLAN+RFID (hereinafter referred to as a combined system) consisting of WLAN and RFID. It is based on wireless local area network and radio frequency identification technology. It utilizes the wireless transmission of WLAN and the non-contact automatic identification of RFID to provide users with faster and more flexible identification services.
1 WLAN + RFID combination system applicationThe RFID device consists of an electronic tag (referred to as a tag, a tag) and a reader (reader). In practical applications, the label is attached to the surface or inside of the identified item. When the identified item passes the readable range, the reader automatically takes out the appointment identification information in the label in a contactless manner, thereby realizing automatic identification. The function of an item or automatic collection of item identification information. The Handheld Reader is one of the readers that staff can quickly identify the goods to be found from the pile.
Take Lucent Technologies' WaveLAN series products as an example. WLAN devices include: wireless access bridge (Access Point, AP), products include Orinoco AP-1000 and Orinoco Wave POINT-II, wireless network card OrinocoPCCard, antenna, network The bridge setup software OrinocoWaveManagerAP v6.00, the wireless network card driver OrinocoClientv6.0 and the roaming protocol Wave AROUND.
In the outdoor open-air large cargo yards at port terminals, stations or logistics centers, due to the large number of sites and the variety of materials, traditional manual operations consume a lot of manpower and time, and are prone to errors. Establish a combined system to identify the goods and wireless access to the central database, so that the entire processing of goods receipt, delivery and inventory is very convenient and fast, and the error rate is greatly reduced. When the staff inspect, store, register, select, locate and transport the goods, they and the position of the forklift need to be moved. Install multiple APs on the roof of the yard building or at the relevant location so that the WLAN's wireless signal covers the entire yard. The handset can be roamed within the coverage area and connected to the inventory system of the yard via WLAN, thereby solving the operator's mobility problems.
At the time of receipt, the staff carried the handset close to the cargo. Within the reading range, the tag data can be displayed on the LCD screen of the handset and entered the central database via WLAN. Staff can also enter other relevant data on the handheld's small keyboard, which also enters the central database via WLAN. Next, the system feeds back the location and related data of the goods back to the handset and displays them on the display. The staff and the forklift driver follow the instructions to deliver the goods to the storage location. At the time of shipment, the employee arrives near the storage location of the goods according to the shipping list provided by the inventory management system. As long as the label of the goods enters the reading range of the handset, the handset will identify the goods to be sought from the pile of goods and display the data of the goods on the display. Counting goods is the same as receiving and shipping. The inventory management system keeps track of the entire process of receiving, shipping and inventory, so that accurate inventory reports can be generated in real time and conveniently. A read head is installed at the exit of the yard to identify the shipped goods. When the truck is about to pass through the exit, the reader identifies the label of all the goods. If all are goods on the shipping list, the reader will give a voice prompt and automatically open the gate of the channel through the control system to release the goods. At the same time, the inventory management system database automatically updates the data. Conversely, if there are goods that are not allowed to be released, including hidden in the packing box of the goods that are allowed to be shipped out, the reading head will automatically recognize them, close the gate of the passage, and issue a prompt. Therefore, the application combination system can also play the role of anti-theft monitoring for goods.
2 RFID and its data communication2.1 RFID readers and electronic tags
The reader consists of a control system, a communication interface, a microstrip antenna, and a power supply. The handset is a reader that is suitable for handheld use by the user. It works in exactly the same way as other readers and readers. In addition to the four modules that a typical reader has, it also has a liquid crystal display and a keyboard.
The power supply voltage of the handset is powered by rechargeable battery, 6 V or 9 V DC; the operating system can use WinCE or other operating system; data storage 32 MB flash memory, 32 MB memory: the antenna is an internal antenna or probe detector; the communication interface can be Select RS232 and other interfaces as well as 802.11 interfaces. The handset can also be added to the barcode scanning module to provide both RFID identification and barcode scanning.
The electronic tag consists of modules such as data storage, data processing, communication interfaces, microstrip antennas, and power supplies. The tag writes information about the ID code and the object. Labels are classified into passive tags and active tags according to different forms of power supply. The power supply of the passive tag is obtained from the RF signal emitted by the reader, so the reader has a higher transmission power and the recognition distance is closer. Current low-voltage and low-power power supply technologies can solve the disadvantages of passive tags requiring high transmit power of the reader. Active tags rely on their own micro-battery, so the reader's transmit power requirements are lower, and the system's recognition distance is far. Compared with active tags, passive tags have the advantages of low cost, no maintenance, high reliability and long life. In the combined system, passive tags and active tags can be applied.
2.2 RFID data communication
RFID data communication takes place via RF transmission between the reader and the tag. The transmission of information between the reader and the tag shall be in accordance with the selected communication protocol. Label first speaking (TTF) and reader first speaking (RTF) are two anti-collision protocol methods of RFID. TTF means that the tag can send its own ID code actively without the instruction of the reader. RTF refers to the instruction that the tag receives the reader before it can be sent. Compared with the two, TTF has the characteristics of fast recognition and stability, and can adapt to the situation of high-speed transmission and dynamic change of the number of tags. For passive tags, it needs to convert the energy of the reader's RF signal to the power it needs, so RTF is generally used. Active tags do not require the reader to provide energy, so RTF or TTF can be used.
A big feature of RFID is that multiple tags are read at the same time, that is, one reader can dock multiple tags. The reader simultaneously transmits signals to each tag in a radio broadcast manner, and each tag simultaneously receives the signals; each tag transmits a signal to the reader in a multi-access manner. When TTF is used, all tags randomly send their own ID codes repeatedly, and different tags are read by the reader at different times. When RTF is adopted, the reader first issues an isolation instruction to a batch of tags in the reading range, leaving only one tag in an active state, and establishing a conflict-free communication connection with the tag; after the communication ends, the tag is instructed to enter a sleep state. Then complete the reading of one label according to the above.
Part 15 (Revised) of the FCC's Radio Spectrum Regulations allows wireless network products to operate in the ISM band without approval. The ISM includes three frequency bands: 902 to 928 MHz, 2.4 to 2.4835 GHz, and 5.725 to 5.85 GHz. Currently, WLANs in the 902MHz band are deployed in the United States and are not globally applicable. 2.4 GHz is the only unlicensed band that is globally applicable. The world's leading WLAN products now operate in the 2.4 GHz band.
The frequency bands for wireless communication between the tag and the reader are: low frequency below 135 kHz, high frequency 13.56 MHz, ultra high frequency 869 MHz, 902 to 928 MHz and microwave 2.4 GHz, 5.8 GHz. The communication distance between the low frequency and the high frequency is close, and the data transmission rate is slow. The communication distances of high frequency, ultra high frequency and microwave are far, and the data transmission rate is fast.
Currently, RFID has two technical standards: the EPC standard in Europe and the United States and the UID standard in Japan. The wireless bands used by the two are different. The EPC standard uses UHF 902 to 928 MHz, and the UID standard uses 2.45 GHz and 13.56 MHz.
In a combined system, WLAN and RFID must coexist, and the RF signals of the two do not interfere with each other. Frequency segmentation is the best and easiest option. Therefore, the WLAN in the combined system uses the 2.4 GHz band, and the RFID uses the 902 MHz band. They are all open bands that can be used freely, but the transmit power is subject to regulatory restrictions.
4 Modulation technology used in the combined systemAs of now, DSSS and FHSS compare with existing product parameters, DSSS has better advantages in applications that require optimal reliability, and FHSS dominates in applications that require low cost. DSSS uses the full-band data transmission, which is faster, and has the performance advantages of anti-interference and anti-noise ability, anti-fading ability, concealment and confidentiality, and does not interfere with the same frequency system.
Because DSSS has the above advantages, the WLAN in the combined system uses DSSS. DSSS's high data transmission rate can meet the data transmission rate requirements of large freight yards; good anti-interference performance adapts to the more complex propagation environment in the goods yard, such as multipath interference caused by signal fading caused by object movement; DSSS The transmission distance is relatively long, and the number of APs set in the same area of ​​the goods yard can be small.
The WLAN in the combined system uses the IEEE 802.11b standard. 802.11b is an extension of 802.11 that specifies the 2.4 GHz band. It adds two new rates to the physical layer of the 802.11 protocol: 5.5 Mbps and 11 Mbps. In order to achieve this goal, DSSS was selected as the only physical layer transmission technology of the technology, because FHSS can not increase the speed without violating the FCC principle. The DSSS modulation scheme is DBPSK at 1 Mbps, DQPSK at 2 Mbps, CCK at 5.5 Mbps and 11 Mbps, and QPSK is used as the modulation technique. RFID RF transmission can be inductively coupled, also known as load modulation, commonly used in low-frequency and high-frequency identification systems at close range. RF transmission can also be performed by electromagnetic backscatter coupling, also known as backscatter modulation, which is commonly used in UHF (eg 902 MHz) and microwave (eg 2.4 GHz) identification systems at longer distances. In the combined system, the RFID uses backscatter modulation and operates at a frequency of 902 MHz.
The working principle of backscatter modulation is: when the passive tag sends data back to the read head, the tag controls the antenna switch according to the data signal to be sent back, so that the tag antenna is in impedance mismatch or impedance matching state, so the tag reflection The energy of the reader/writer is also correspondingly large and small, representing "0" and "1" in the binary signal.
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