An Improved Anti-collision Algorithm in RFID System

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1 An Improved Anti-collision Algorithm in RFID System Leian Liu, Zhenhua Xie, Jingtian Xi, Shengli Lai School of Electronic and Information Engineering South China University of Technology Guangzhou, , China gudu420(4163com Abstract-Collision is a familiar problem in a RFID (Radio * When there is a barrier in front of a barcode, the Frequency Identification) system. How to solve this problem barcode can not be read by a reader, but a tag can be effectively is very important to the whole RFID system. A RFID read due to the different working style; system is always made up of two indispensable components: one * A tag can be scanned by a reader more quickly than a is the tag, the other is the reader. Collisions occur when there are barcode; so many tags within the interrogation zone of a reader * A tag has a much longer life than a barcode especially communicating with the reader synchronously. Binary search when the working condition is extremely bad; algorithm is an important anti-collision algorithm to solve the * Using tags is much safer than using barcodes. collision problems. But the main disadvantage of any existing Because of merits mentioned above, RFID systems arouse algorithm based on binary search algorithm is the rate of much interest. Several big companies such as IBM, Sun and identifying the identification codes (IDs) of tags is slow. In this Microsoft have launched such programs. So in recent years, paper, an improved anti-collision algorithm is proposed. Using RFID systems develop very fast and have been used in many this algorithm will recognize more tags in less time compared areas. Identity identification and assets management are two with other existing algorithms. areas in which RFID techniques are used widely and maturely. Electronic tags are embed into lots of cards (ID cards, passports Keywords-wireless RF communication, RFID, anti-collision and employees' cards, etc) to identify people's identity, such as algorithm, multiple access access control management, access permission checking-up, etc. In the assets management area, a successful case is using I. INTRODUCTION RFID techniques to manage passengers' luggage in an airport. The RFID system is an important branch of the automatic Preventing from fakes is another use of RFID techniques. In identification system. In a RFID system, wireless RF this area, because electronic tags have lots of merits (tags can communication techniques are used. The omnipresent barcode be identified very quickly and are very difficult to be forged labels that triggered a revolution in identification systems and also the price of a tag is very low), difficulties of faking are some considerable time ago, are being found to be inadequate greatly increased. Fakers may shrink back from difficulties. in an increasing number of cases [1]. Now here comes RFID. Commercial supply chain is one of the most widely area in Compared with the barcode, a RFID system has many merits which RFID techniques are used. By supply chain, goods in the as below: chain can be run after and people can get the information of * Tags can be used repeatedly and the information stored them anywhere and anytime. in them can be reprogrammed, but barcodes can not; A RFID system is always made up of three components (we * The information stored in a tag is much larger than that can see this clearly in the following figure 1). in a barcode;

$by which the reader can get the information stored in Data "FID reader / \T <*==> Clock Energy Antenna of the reader /on Antenna of the tag D td manqgemjbht gygltm Data management system Network$

2 by which the reader can get the information stored in Data "FID reader / \T <*==> Clock Energy Antenna of the reader /on Antenna of the tag D td manqgemjbht gygltm Data management system Network Figure 1. The structure of RFID system The first one is the transponder or can also be called the tag, which is located on the object and usually has a special identification code by which the tag can be identified. The tag can be divided into two different parts: the antenna by which the tag can extract energy from the reader and an electronic microchip which controls the tag to work effectively. The second one is the interrogator or can also be called the reader. The reader includes two different parts: the antenna and the reader. In addition, many readers are fitted with an additional interface, such as RS232, RS485, etc, to enable them to forward the data received to another system (PC, robot control system, etc) [1]. Depending upon the design and the technology used, the reader may be a read or read/write device. But usually, it is called as "reader". The reader transmits signals to "wakeup" the tag and transmits instructions to the tag and then receives the return data stored in the tag. The useful data (the data that have been filtrated by the reader) can be used by the data management system by the network. The last one is the data management system. The function of the data management system is storing information and managing data. The data management system can communicate with the reader which is located in the remote place and can get the information of the tag anytime. In conclusion, the function or aim of a RFID system is to identify tags, that is, to get the identification codes of these tags and then get the data stored in tags. At last, the data which have been filtrated are stored and managed by the data management system. The working process of the RFID system is as follows: * The reader transmits interrogation signals at first; * If a tag is within the interrogation zone of the reader,becit e becoes ctiv an the comuncate wih th reder the tag. From all the above, we can see that if there are many tags within the interrogation zone of the reader, and they communicate with the reader at the same time, the reader will not know which one it should "talk" with at first. The reader will get the wrong information of the tags if the problem which is called as "collision" is not dealt with properly. To deal with the collision problem in a RFID system, several correlative commands are needed. These commands are called as anti-collision commands or anti-collision algorithms which are discussed in this paper. In the mechanics of communication, communication collision problems always exist. In the same time, lots of methods have been found in order to solve these problems. Generally, these methods can be divided into four parts: SDMA (Space Division Multiple Access), FDMA (Frequency Division Multiple Access), CDMA (Code Division Multiple Access) and TDMA (Time Division Multiple Access). But unfortunately, all these methods can not be used in a RFID system directly because they are much too complicated. To solve collision problems in a RFID system, just like in a communication system, there are always three methods: spatial domain method, frequency domain method, and time domain method. But in most RFID systems, time domain method which means every tag communicates with the reader in the different time is used because it is easier to be put in practice. In general, time domain method has two branches: one is assured algorithm, the other is un-assured algorithm. What is discussed in this paper belongs to assured algorithm. 1. SOME ExiSTIG ANTI-COLLISION ALGORITHMS Solving the collision problem is the basic and the most important task in a RFID system. That is, only there is no collision, can a RFID system work properly. So how to solve this problem and how to solve this problem effectively are crucial to the whole RFID system. Until now, there are some methods as follows: A. Bit Arbitration Based Anti-collision Algorithm [2] In this algorithm, all unidentified tags in the interrogation azone of the reader are active at the beginnng. All active tags will take part in the arbitration process. Newly entering tags

3 will not take part in the arbitration process if the arbitration the broken lines in figure 2. Then the reader can know process is in process. The pivotal idea of this algorithm is: collisions occur in these two bits. In binary search algorithm, When there is a request from the reader, all active tags send tags whose first collided bit is logical "1" do not respond to the value of a given bit of their identification codes. Then tags the reader's next request. But tags whose first collided bit is will be divided into two groups according to the given bit. The logical "O" send their identification codes to the reader's next reader will choose one group to continue another bit request. After each request, the collided bits become less. arbitration step and then inquire of next bit until the The searching process keeps going until the identification identification code of a tag is recognized. The identification code of a tag is recognized. When the identification code of time of one tag of bit arbitration based algorithm is one tag is recognized, the searching process will be repeated independent of the number of tags in the interrogation zone of from the beginning until the identification codes of all the tags the reader, but linearly dependent of the identification code are recognized. length. So if there are n tags in the interrogation zone of the C g. C. Anti-collision Algorithm Balsed on Jumping and Dynalmic reader and the length of every tag's identification code is m, Searching[4] the total number of iterations of bit arbitration based algorithm This algorithm is the same to binary search algorithm is mxn. except for two aspects. They are: B. Binary Search algorithm [3] * After the identification code of a tag is recognized, the In this algorithm, Manchester code is used in order to process of anti-collision algorithm based on jumping recognize the bit where there is a collision. In Manchester and dynamic searching is not just repeated as binary code, the ascending edge is coded as logical "O" and the search algorithm. The code which the reader will send descending edge is coded as logical "1". So when the reader next is not the same as that the previous process sent at receives an identification code and the states of the code do the beginning. It is the second code (counting not change in some bits, the reader can know where collisions backwards) that the previous process sent. Then the occur (we can see this in figure 2). third code (counting backwards), the fourth code (counting backwards), etc, until the identification codes ID of tagl: of all the tags are recognized. In the first Request command, the reader sends a code "1" instead of sending a maximal code whose length is ID of Tag2: the same to the tags' identification codes and every bit F; of the code is logical "1". In the next Request command, the reader sends the known bits instead of Received ID by the reader: lox lix 11 sending all the bits as do in binary search algorithm. ILJTLfL W VLF -f-1vl VL...VI..Like binary search algorithm, the searching process is from the top bit to the bottom bit. If the higher bits of Figure 2. Collisions in received ID the tags' identification codes are the same to the known In figure 2, there are two tags. These two tags will answer bits, the corresponding tags will send their the reader when the reader sends a Request command to them identification codes to the reader. at the first time. We can see from figure 2 that the values of bit5 and bit2 of tagl's identification code are logical "O" and III. THE IMPROVED ANTI-COLLISION ALGORITHM logical "I" respectively, but those of the same bits of tag2's The proposed anti-collision algorithm is an improved one, identification code are logical "1" and logical "0" respectively, compared with binary search algorithm and anti-collision thus the states of the identification code that the reader algorithm based on jumping and dynamic searching. Its goal is receives will not change in these two bits which are shown by to enhance the speed of identifying tags when collisions occur

in succession. In this algorithm, just like binary search existing algorithms mentioned above, the next Request algorithm, there are some stipulations: command directly equals to the old one plus one.

4 in succession. In this algorithm, just like binary search existing algorithms mentioned above, the next Request algorithm, there are some stipulations: command directly equals to the old one plus one. Keep * The encoding style must be Manchester code. Only on sending Request command after having it added to using Manchester code can a reader find each collision one, until all the tags in the interrogation zone have bit easily and recognize all the tags in its interrogation been identified. zone at last. We can see the process clearly from the following example. * Several commands are used. The first one is Request Assuming there are 8 tags whose identification codes are command. In the beginning, a reader sends a request (tagl), (tag2), (tag3), command. Tags will reply if they meet a certain (tag4), (tag5), (tag6), condition. The second one is Select command. If the (tag7), and (tag8) in the interrogation reader receives an identification code and there is no zone of the reader, the process of the algorithm is as follows: collision, the tag which has been identified will be To begin with, the reader sends a Request command (1), and selected by the reader, and then other commands will all the tags in the interrogation zone of the reader will answer be executed (e.g. the Read/Write data command). The the reader. According to the principle of Manchester code, the third one is Read/Write data command. Data stored in reader receives the code 1OxxxOO1, so the reader can know the selected tag will be read or changed according to that collisions occur in bits D5, D4, and D3. The higher bits Read/Write data command. The last command is where collisions occur are D5 and D4. According to this Unselect command. If a tag acts upon this command, it algorithm, D5 and D4 are set to zero. Bits that are higher than will enter a "sleeping" state, in which the tag will not D5 will not be changed, so D7 D6=10. answer any command sent by the reader unless it Then the reader sends a Request command (1000). The leaves the interrogation zone of the reader and enters reader receives the code 1OOOxOO1. Because there is only one again. bit where the collision occurs, it is no need to send Request * The key point of this algorithm is all the tags must command again. The reader can get the identification codes of answer the reader at the same time, so that tags can tagl and tag2. Then the reader runs Read/Write command to send their identification codes to the reader deal with tagl and tag2. At last, the reader runs Unselect synchronously. Only under such condition can the command in order to turn tagl and tag2 into "sleep" state. algorithm work well. Next, the reader sends a Request command (1001). The All of the above are the same with binary search algorithm reader receives the code 1001xOO1. Now D3 is the only bit and anti-collision algorithm based on jumping and dynamic where the collision occurs. Similarly, the reader can get the searching, but the proposed improved algorithm in this paper identification codes of tag3 and tag4. Then the reader deals has two major differences compared with the other two with tag3 and tag4 and turns them into "sleep" state at last. algorithms as follows: Furthermore, the reader sends a Request command (1010). * Due to the mutually exclusive value of a binary bit, if The reader receives the code 101OxOO1. D3 is the only bit there is only one collided bit no matter where it is, the where the collision occurs. So the reader can get the reader does not need to send Request command again identification codes of tag5 and tag6. Then the reader deals and can automatically identify two tags once. with tag5 and tag6 and turns them into "sleep" state at last. * Different search algorithm will be used if collisions Finally, the reader sends a Request command (1011). The occur in succession. After successive collisions have reader receives the code 1011xOO1. D3 is the only bit where been detected, every correlative bit but the last in the collision occurs. So the reader can get the identification Request command will be set to zero. Then two tags codes of tag7 and tag8. Now all the tags are recognized by the which collided at the last bit of the collision sequence reader. The whole process is described in the following table will be identified synchronously. Contrasted with the 1.

5 TABLE I The searching process of the improved anti-collision algorithm correct rate of being read by the reader is not as high as people wish and the rate of identifying the identification codes of tags 1st search 2 nd search 3rdsearch 4th search 5th search is not very fast. So how to solve the collision problems and enhance the rate of identifying the tags is crucial to the whole Request RFID system. It is urgent to find new ways to solve these Received 10xxool problems. In this paper, an improved anti-collision algorithm ID is proposed. Compared with other existing algorithm, it has Tagl great advantages especially when the number of collisions Tag becomes larger. And the fact is that the probability that Tag collisions occur rapidly increases with the number of tags. In Tag the final analysis, it is reasonably better to use this algorithm TagS I l oo l Tag o Tag to deal with more tags and it is of great significance to the development and application of RFID system. REFERENCES [1] Klaus Finkenzeller, RFID Handbook: Fundamentals and Tag Applications in Contactless Smart Cards and Identification, 2nd ed. John Wiley & Sons Ltd, From the table 1, we can know that the total number of [2] Marcel Jacomet, Adrian Ehrsam, and Urs Gehrig, "Contactless Identification Device With Anticollision Algorithm," IEEE Computer iterations of the improved anti-collision algorithm is 5. But if Society CSCC99, Conference on Circuits, Systems, Computers and anti-collision algorithm based on jumping and dynamic Communications,Jul. 4-8, Athens, [3] Klaus Finkenzeller, RFID Handbook: Radio-Frequency Identification searching is used, the total number of iterations will be 15. Fundamentals and applications, John Wiley & Sons Ltd, Else if binary search algorithm is used, the total number of [4] Songsen Yu, Yiju Zhan, Zhiping Wang, and Zhongping Tang, "Anti-collision Algorithm Based on Jumping and Dynamic Searching iterations will be larger. Compared with binary search and Its Analysis," Computer Engineering, Vol.31, pp.19-20, algorithm and anti-collision algorithm based on jumping and dynamic searching, the total number of iterations of the improved anti-collision algorithm is greatly decreased. In fact, successive collisions mean colliding in succession both physically and logically. The simplification in this paper aims to address the essence of the proposed algorithm easily. In fact, physical succession of collided bits is not necessary, because physically discrete collisions can be regarded as logically successive collisions. So all the conclusions deduced above are established all the same. Using the algorithm proposed in this paper, we can demonstrate the conclusion that if n-bit successive collisions (physically) are detected, 2n tags can be recognized at best with only 2n-1+1 commands transmitted. IV. CONCLUSION RFID systems are used in many areas today and are becoming more and more important. But when RFID techniques are concerned, because the working condition of the tags and the reader is very complicated, to the tags, the

Novel Tag Anti-collision Algorithm with Adaptive Grouping

Wireless Sensor Network, 2009, 1, 475-481 doi:10.4236/wsn.2009.15057 Published Online December 2009 (http://www.scirp.org/journal/wsn). Novel Tag Anti-Collision Algorithm with Adaptive Grouping Abstract