Sensors & Transducers 2013 by IFSA http://www.sensorsportal.com A Improved Frame Slotted Aloha Protocol with Early End in Mobile RFID Systems 1 Xiaowu Li, 2 Zhenjiang Wang, 3 Xin Ren, 4 Yuyan Liu, 5 Qing Zhao School of Information and Technology, Kunming University, Puxin Road 2, Kunming, Yunnan, 650214, China Tel.: 1 +86-18213567364, 2 +86-18687183559, 3 +86-18314163283, 4 +86-087165098421, 5 +86-13759410453 1 E-mail: lxwlxw66@126.com, zjwangkm@126.com, renxin66@126.com, 454342120@126.com, 30505860@qq.com Received: 22 May 2013 /Accepted: 19 July 2013 /Published: 31 July 2013 Abstract: Frame slotted ALOHA (FSA) protocol is a classic tag anti-collision protocol. Similar to most existing anti-collision protocols, The FSA initially aims at tag identification of static scenarios, that is, all tags keep still during the tag identification process. However, in many real scenarios, tags generally move along a fixed path in the reader coverage area, that is, tags stay the coverage area only for a restricted time (sojourn time). Obviously, tag loss is inevitable in some mobile RFID scenarios. For example, tag quantity is too large or their moving speed is too high. Like almost existing tag identification protocols, FSA is not concerned with optimal management of lost tags. In this paper, we propose an improved FSA algorithm with early end which can decrease tag loss by early termining idle slots, collision slots and slots that are occupied by lost tag. Simulation results show that the proposed protocol can significantly reduce the numbers of lost tags in mobile RFID scenarios. The idea of the paper is beneficial for redesigning other existing tag anti-collision protocols so as to make these protocols adapt to mobile RFID systems. Copyright 2013 IFSA. Keywords: RFID, Anti-collision, Frame slotted ALOHA (FSA), Early end, Tag identification protocol, Tag loss rate (TLR). 1. Introduction One of radio frequency identification (RFID) advantages over traditional bar code is that RFID can support simultaneous multi-tag identification without any contact. So, RFID technique is applied to various sectors, such as supply chain management, traceability and emergency management. The majority of RFID systems are composed of some RFID readers and many passive transponders, called tags. Passive tags get their power supply from the electromagnetic field of the reader. When the multiple tags simultaneously transmit their signals to the reader, collisions will happen. So far, many collision resolution protocols have been proposed for static scenarios, that is, no tag enters or leaves the reader coverage area during the process of tag identification. These algorithms can be grouped into two broad categories: tree-based algorithms and aloha-based algorithms [1 6]. In ALOHA-based systems, the channel time is divided into frames. Each frame in turn is divided into several time slots. Within a frame, each tag randomly selects a time slot and transmits its identifier to the reader in that slot. 82 Article number P_1254
Note that, within each frame there exist three kinds of slots: (1) the idle slot where no tags reply; (2) the successful slot where only one tag replies; and (3) the collision slot where multiple tags reply. In tree-based systems, anti-collision protocols are based on binary trees and every root-to-leaf path in these trees represents a unique tag ID. Compared with ALOHAbased algorithm, tree-based algorithms promise deterministic identifications, but need a high number of reader-to-tag commands. moving velocity (assumed constant) and tag sojourn time S (given in slots) which can be derived from mentioned main parameters and the time interval for one slot. Mobile RFID systems shown in Fig. 2 are our research scenarios. Fig. 2. The mobile RFID system diagram. Fig. 1. Early end for idle slots. To decrease tag identification time, ISO-18000-6A and EPCglobal Class 1 [8] presented frame slotted ALOHA with early end, as shown Fig. 1. From this figure, we can see that all idle slots are terminated early, which can decrease identification delay effectively Besides, in many practical scenarios, tags attached to items usually move along the fixed path in the reader coverage area [3, 9-11], that is, tags stay the coverage area only for a certain time (sojourn time). Since the reader has limited sojourn time to identify the passing tags, it is critical to decrease the number of tags which leave the coverage area without being identified. Systems involving moving tags are generally called mobile RFID systems [3, 9-10]. A typical example of them is the portal of a warehouse equipped with RFID reader to automatically identify passing tags and update the central database with the captured information. Another typical example of them is a conveyor belt in a factory or a logistic center where objects attached with RFID tags have to be identified [9]. In these situations, tags usually move along a fixed path at a constant speed and can enter or leave the reader coverage area during the process of tag identification. Fig. 2 shows the mobile RFID system diagram and illustrates some parameters: the reader coverage area length, the tag As mentioned previously, in the mobile RFID systems, some tags may leave the coverage area unidentified. We refer to such a tag as a lost tag. In some situations (e.g. the tag moving velocity is too great, etc.), the phenomenon of lost tags is inevitable. How to decrease the number of lost tags is the critical research issue in mobile RFID applications. Therefore, we rate tag loss ratio (TLR) as a critical performance metric, which is defined as the quotient between the number of lost tags and the total number of tags entering the coverage area [3, 10-11]. Obviously, TLR is related to other many system performance metrics, such as throughput, system efficiency and the identification delay [4]. For example, if throughput of protocol A is higher than protocol B, then TLR of protocol A is usually lower than protocol B. Compared with other performance metrics, the users of RFID systems put more emphasis on TLR and it reflects the performance of mobile RFID system very well. So, we only focus on the performance metric TLR in the paper. In mobile RFID systems, there exists a special slot, named lost tag slot, which is different from the idle, collision and successful slot. The reason for its advent is that one or more tags that select the slot to communicate with reader have left the reader s field unidentified when the reader identifies them. Successful and collision slots could switch to lost slots when tags that select them leave the coverage area unidentified, as shown in Fig. 3. So, in the mobile RFID systems, there are four types of slots: idle, collision, successful and lost tag slots. In this paper, we will present an improved frame slotted ALHOHA (FSA) protocol with early end which can decrease tag loss by early termining idle slots, collision slots and slots that are occupied by lost tag in the mobile RFID systems. The proposed protocol is named EARLY-END protocol. The remainder of this paper is organized as follows. In Section 2 we briefly review the related 83
work in the area. Then, in Section 3 we offer EARLY-END protocol in the mobile RFID systems. Section 4 provides the simulation results. Finally, Section 5 concludes. belt carrying tag sets and proposed a dynamic frame length ALOHA to improve the system performance. The proposed protocol can improve to some extent the system performance of mobile RFID systems. However, the scenarios studied in [9] are similar to that in [11] and thus they are also not our research scenarios. In this paper, we develop an improved frame slotted ALOHA protocol with early end in mobile RFID systems which is devoted to avoid the wastage due to the lost tag slots. Fig. 3. The illustration of lost slots. 2. Related Works The tag recognition rate is a conception that is similar to TLR since it also involves unidentified tags. In [13], Vogt first modeled the identification process of framed slotted ALOHA as a Markov chain, and asserted that only by performing multiple read cycles, a high tag recognition rate can be reached. Unlike our research scenarios, tag recognition rate is based on the static scenario. In order to compute TLR, many researches based on Markov chain were done. In [11], Vales-Alonso et al. focused on TLR of single tag set passing the coverage area in a limited time. In the scenarios, no more tag sets enter the coverage area until the previous one has left. However, the scenarios are different from our research scenario. In [10], Vales-Alonso et al. offered a Markov model for the analysis of mobile RFID systems. The big drawback of applying the Markov approach to mobile RFID scenario is that, as the dimension of the problem increases, TLR requires too much computation. Furthermore, the model did not consider the wastage due to the lost tag slots. Alcaraz et al. [3] provided dynamic systems model for mobile RFID systems implementing FSA as the collision resolution protocol. In this model, both TLR and the optimal frame size can be easily obtained. However, the model is only suitable for analyzing TLR of FSA. In addition, the model did not consider the wastage due to the lost tag slots. There are other works that focus not only on computation of TLR but also on performance improvement of mobile RFID systems. In [9], Xie et al. presented a probabilistic model for a conveyor Fig. 4. Idle and lost tag slots terminated early. 3. Early-end Protocol The ideal of EARLY-END protocol is if the reader detects no transmission after a small period of time, it terminates idle, collision and lost tag slots early using the End-slot command, as shown in Fig. 4. From the figure, the time spent by idle, collision and lost tag slots are decreased effectively. The method of EARLY-END protocol can be summarized as follows: 1) At the beginning of a frame, the RFID Reader sends the Frame-begin command with an integer parameter 'L'. Upon hearing this command, unsilenced tags generate a random number from 0 to L - 1. Those generating '0' reply immediately. 2) If only one tag replies, the reader can identify it and send back the Silence command. Upon hearing this command, the replied tag will be silenced, that is, it will not respond to future commands, while the other tags decrease their counter values by 1 and contend the channel if the counters reach 0. 3) If the reader detects a collision, the reader will send back the End-slot command early. In this paper, time cost of a collision slot is equal to 0.6 times of that of a successful slot. Upon hearing this command, all the unsilenced tags will decrease their counters by 1 and contend the channel if their counters reach 0. 84
4) If no tag replies, the reader will send back the End-slot command early. This means that an idle slot or a lost tag slot has occurred. In this paper, we assume that time cost of an idle and a lost tag slot is equal to 0.2 times of that of a successful slot. Upon hearing this command, all the unsilenced tags will decrease their counters by 1 and contend the channel if their counters reach 0. 5) After one frame ends, the Reader will begin a new frame by sending the Frame-begin command if some tags were collided in the previous frame. Through these steps, EARLY-END protocol can terminate idle, collision and lost tag slots early under the mobile RFID systems. 4. Simulation and Results Now, we begin to evaluate the proposed protocol. Because TLR can be derived by mathematical model only for FSA and CSMA so far [8] [10], we offer a simplified mobile RFID experiment model. The model comprises 3 groups of mobile tags, as shown in Fig. 5 where T1 denotes the arriving interval between the 1st and 2nd groups of tags, T2 denotes the arriving interval between the 2nd and 3rd groups of tags, N1, N2 and N3 respectively denote the number of tags in the 1st, 2nd and 3rd groups of tags, S denotes the tag sojourn time. Notice that in the paper, all parameters related to time, such as T1, T2 and S are measured in slot. Parameters S have been defined in section 1. Below, we compare EARLY-END protocol with FSA without early-end feature [14] which is similar to EARLY-END protocol except that it is not characterized by early end. Our simulation based on Monte Carlo technique and the simplified mobile RFID experiment model. when the number of tags in the second group N2 is equal to 50 tags, TLR of our method is nearly 3 percent while that of FSA is 42 percent. In the Fig. 6 (b), we survey the relationship between TLR and the tag sojourn time S by changing S (corresponding to change of the tag moving speed or the reader coverage area length). Related parameters in the experiment are: N1=70, N2=70, N3=70, T1=50, T2=80. We can find that TLR of both protocols decreases as S increases. The reason is that the reader has more time to identify tags as S increases. Compared with FSA, EARLY-END protocol has better performance. For example, when tag sojourn time S is larger than or equals 350 slots, TLR of the proposed protocol is zero while that of FSA is about 26 percent. (a) TLR vs. the tag density. (b) TLR vs. the sojourn time model Fig. 5. Simplified mobile RFID experiment mode. In the Fig. 6 (a), we survey the relationship between TLR and the tag density by changing the number of tags N2 in the second group. Related parameters in the experiment are: N1=70, N3=70, T1=50, T2=80, S=230. We can find that TLR of both protocols increases as the tag density increases. The reason is that the reader must identify more tags in the identical interval. Compared with FSA, EARLY- END protocol has better performance. For example, (c) TLR vs. tag arrival rate. Fig. 6. TRL of IVAEEMR and Vogt protocols. In the Fig. 6 (c), we survey the relationship between TLR and tag arrival rate by changing T1. 85
Related parameters in the experiment are: N1=70, N2=70, N3=70, S=230. From the figure, we can find that TLR of both protocols decreases as the tag arrival rate become slow. The reason is that the reader has more time to identify tags when the rate becomes slow. Compared with FSA, EARLY-END has better performance. When T1 is equal to 60 slots, TLR of EARLY-END protocol is nearly 4 percent while that of FSA protocol is equal to about 44 percent. In general, the simulation results demonstrate that compared with FSA, EARLY-END protocol can reduce TLR and improve mobile RFID system performance remarkably since it can terminate idle, collision and lost tag slots early. 5. Conclusions This paper introduces a term lost tag slot first in the mobile RFID systems. Then, EARLY-END protocol is proposed. The protocol can terminate idle, collision and lost tag slots, which decrease TLR effectively in the mobile RFID system. Moreover, in order to evaluate our method, we present a simplified mobile RFID experiment model, which can easily analyze the TLR of many tag anticollision protocols in the mobile RFID systems and avoid complicated mathematical computation of TLR Acknowledgements This work is supported by the National Natural Science Foundation of China (NNSF) under Grant 71073179 and Science Foundation of Kunming University of China under Grant XJ11L001. References [1]. M. He, S.-J. Haring, P. Fan, M. K. Khan, R.-S. Run, J.-L. Lai, R.-J. Chen. A fast RFID Tag Identification Algorithm Based on Counter and Stack, Expert Systems with Applications, Vol. 38, 2011, pp. 6829-6838. [2]. M.-K. Yeh, J.-R. Jiang and S.-T. Huang, Adaptive splitting and pre-signaling for RFID tag anticollision, Computer Communications, Vol. 32, 2009, pp. 1862-1870. [3]. J. J. Alcaraz, E. Egea-López, J. Vales-Alonso, J. García-Haro, Dynamic System Model for Optimal Configuration of Mobile RFID Systems, Computer Networks, Vol. 55, 2011, pp. 74-83. [4]. K. Finkenzeller, RFID Handbook: Radio-frequency Identification Fundamentals and Applications, Wiley, New York, 2002. [5]. C. P. Wong, Q. Feng, Grouping Based Bit-slot ALOHA Protocol for Tag Anti-collision in RFID Systems, Communication Letters, Vol. 11, No. 12, 2007, pp. 946-948. [6]. X. Jia, Q. Feng, C. Ma, An Efficient Anti-collision Protocol for RFID Tag Identification, IEEE Communication Letters, Vol. 14, No. 11, 2010, pp. 1014-1016. [7]. L. Zhu, T.-S. P. Yum. A Critical Survey and Analysis of RFID Anti-Collision Mechanisms, IEEE Communications Magazine, Vol. 5, 2011, pp. 214-221. [8]. D. K. Klair, K. Chin, R. Raad, A Survey and Tutorial of RFID Anti-Collision Protocols, IEEE Communications Surveys Tutorials, Vol. 12, No. 3, 2009, pp. 400-421. [9]. L. Xie, B. Sheng, C. C. Tan, H. Han, Q. Li, D. Chen, Efficient Tag Identification in Mobile RFID Systems, in Proceedings of the IEEE International Conference (INFOCOM' 10), San Diego, 2010, pp. 15-19. [10]. J. Vales-Alonso, M. V. Bueno-Delgado, E. Egea- López, J. J. Alcaraz-Espín, J. García-Haro, Markovian model for Computation of Tag Loss Ratio in Dynamic RFID Systems, in Proceedings of the 5 th European Workshop on RFID Systems and Technologies, Bremen, Germany, 2009, pp. 16-17. [11]. J. Vales-Alonso, M. V. Bueno-Delgado, E. Egea- López, J. J. Alcaraz-Espín, J. M. Pérez-Mañil, On the Optimal Identification of Tag Sets in Timeconstrained RFID Configurations, Sensors, Vol. 11, 2011, pp. 2946-2960. [12]. V. Sarangan, M. R. Devarapalli, and S. Radhakrishnan, A framework for fast RFID tag reading in static and mobile environments, Computer Networks, Vol. 52, 2008, pp. 1058-1073. [13]. H. Vogt, Efficient Object Identification with Passive RFID Tag, in Proceedings of the IEEE International Conference on Systems, 2002, pp. 98-113. 2013 Copyright, International Frequency Sensor Association (IFSA). All rights reserved. (http://www.sensorsportal.com) 86