Introducing Probability in RFID Reader-to-Reader Anti-collision

2009 Eighth IEEE International Symposium on Network Computing and Applications Introducing Probability in RFID Reader-to-Reader Anti-collision Filippo Gandino, Renato Ferrero, Bartolomeo Montrucchio, Maurizio Rebaudengo Dipartimento di Automatica ed Informatica Politecnico di Torino Torino, Italy Email: {filippo.gandino, renato.ferrero, bartolomeo.montrucchio, maurizio.rebaudengo}@polito.it Abstract Nowadays, several kinds of applications based on Radio Frequency Identification (RFID) employ a large number of tags and readers, involving collision problems. A relevant group of reader-to-reader anti-collision protocols are based on time division. Normally these protocols do not require special readers or additional entities; their main challenge is the collision resolution, since after a collision, readers have to choose a new time slot trying to avoid new collisions. It is observed that after a collision, its slot is often unoccupied, therefore this paper proposes to introduce the slot change probability as an additional parameter, in order to reduce the number of readers that change slot and the number of colliding transmissions. A new version of Distributed Color Selection (DCS) [1] is presented, analyzed and compared with well-known protocols based on time division. The simulation analysis shows that the average time required to transmit may be reduced by over 10%. Keywords-RFID; reader-to-reader collision; DCS; time division. I. INTRODUCTION A typical RFID system [2] includes a reader, which creates an electromagnetic field, and some passive tags, which get the power supply from the reader field. The RFID tags hold a memory that stores an unambiguous identification code (ID) and potentially a rewritable user memory. A reader can read and write the tag memories in its interrogation range. Nowadays, RFID is a well-known technology for automatic identification, and it is largely employed for different applications such as health care [3], traceability [4], and robot localization [5]. Usually, these applications require a large number of tags and readers operating close together, so the proximity of different electromagnetic fields at the same frequency could generate electromagnetic interferences. The interference range, defined as the maximum distance at which the field of a reader can alter other transmissions, is a critical parameter to be properly considered. When two readers, within the reciprocal interference range, try simultaneously to communicate with different tags, the two transmissions collide. Figure 1 shows an example of two tightly coupled readers. Their interrogation fields are not overlapped, but the readers are inside the reciprocal interference ranges. The reader-to-reader collision must be carefully addressed, otherwise the RFID network could be ineffective. In recent years different anti-collision protocols have been proposed. In 2003 Waldrop et al. presented Distributed Color Selection (DCS) and Colorwave [1] [6]. These protocols use time division in order to organize the transmissions and to minimize collisions. The transmissions are composed by rounds divided in timeslots called colors. DCS presents few special requirements. Colorwave requires readers that can manage an additional transmission among RFID readers, but it is more adaptable and more suitable to heterogeneously deployed reader networks. These protocols present an acceptable efficiency and provide a high probability that readers do not collide twice consecutively. More recently other protocols, as HiQ [7] and Pulse [8] [9], improved time performance, but they introduced stricter requirements, as a communication channel dedicated to the network management. This paper proposes to consider, as additional parameter, the probability in collision resolution for time division protocols. The novelty is that the probability P determines whether a reader changes timeslot after a collision, or it keeps the same. A new version of DCS with the new collision resolution subroutine is presented. In DCS after every collision a reader changes timeslot randomly, so the slot of the collision often becomes unoccupied and if the new colors are occupied, then the change produces new collisions. While in the probabilistic version, after a collision, readers change timeslot with probability P. The paper analyzes the proposed protocol, and it studies the effects of P. The proposed version has been simulated in order to evaluate the time performance and the effects of the proposed parameter P in comparison with standard DCS and Colorwave. The achieved results demonstrate that the average time required to transmit can be reduced by over 10%. The introduction of probability in collision resolution does not require additional features and it provides better time performance. The remaining of the paper is organized as follows: in Section II the concept of efficiency is explained and the performance evaluation approach is introduced. In Section III the main previously proposed reader to reader anti-collision protocols are described, while in Section IV the characteristics of the proposed protocol are described. In Section V the theoretical and the simulation analysis are presented. Finally, 978-0-7695-3698-9/09 $25.00 2009 IEEE DOI 10.1109/NCA.2009.45 250

Figure 1. Reader to Reader Collision in Section VI some conclusions are drawn. II. PERFORMANCE EVALUATION APPROACH This section analyzes the concept of efficiency for anticollision protocols. The previously proposed methods are analyzed, and the new approach is presented. Some parameters that will be used in this paper are: Waiting Time (WT), which corresponds to the time interval between a transmission request and the transmission; it involves Average Waiting Time (AWT), Waiting Time Variance (WTV), Maximum Waiting Time (MWT), Attempted Transmissions (AT); Successful Transmissions (ST). A. State-of-the-Art Evaluation Approaches In [10], Engels and Sarma state that the goals of reader-toreader anti-collision protocols are to minimize the time span required to let all readers communicate at least once (minimize MWT), and to schedule all readers to communicate as often as possible (maximize ST). In [6], the authors consider the requirements of real-time applications as inventory detection, so they propose the goal of scheduling readers to communicate as often as possible (maximize ST). In order to evaluate different configurations of the proposed protocol, the total successful transmissions performed by a set of readers have been considered. This evaluation method is used in order to compare two protocols called DCS and Colorwave. Moreover, in [1], ST AT is considered, in order to evaluate different configurations of DCS. Birari and Iyer [8] [9] use two parameters for the evaluation of anti-collision protocols: the throughput ( ST time ), and the efficiency ( ST AT ). In [7] the considered target function is to maximize the number of readers simultaneously communicating (maximize ST). B. Proposed Evaluation Approach In the majority of actual RFID applications, the readers may have different goals. Some readers have to continuously detect the tags in a specific location, some others have to communicate with a specific tag. In both these cases the goal of the anti-collision system is to schedule all readers to communicate as soon as possible (minimize WT). Also if the goal is to detect all the tags in the area covered by the network, independently from their exact location, the goal is to minimize the maximum interval between two transmissions (minimize MWT), which could hide some tags. Therefore, we state that the most effective definition of performance corresponds to the minimization of W T. More in detail, a protocol should minimize: AWT, in order to schedule readers to communicate as soon as possible, WTV, in order to minimize the quantity of readers with low performance, MWT, in order to avoid large gaps between two transmissions of the same reader. III. RELATED WORK Several approaches have been proposed to address the reader-to-reader collision problem [11]. A comparison among proposed schemes has been presented in [12]. According to this comparison, the protocols with better throughput are characterized by heavy requirements, such as high management overhead, more energy and a dedicated control channel. This section describes the main relevant anti-collision protocols that are characterized by limited requirements. A. ETSI EN 302 208-1 V1.2.1 The ETSI EN 302 208-1 V1.2.1 standard [13] involves the optional use of a protocol named Listen Before Talk, which is based on Carrier Sense Multiple Access (CSMA). In this protocol readers check if the channel is unoccupied before transmitting. However, collisions are possible when the gap between the beginning of two transmissions is small. The limited duty cycle provides all the RFID readers the opportunity to transmit. This protocol requires readers that check if the channel is occupied. B. Distributed Color Selection (DCS) In DCS [1] [6] each communication round is composed by time slots. Each RFID reader can communicate only during its time slot. When a transmission collides, it is stopped, the reader randomly chooses a new timeslot, and reserves it, sending a specific signal named kick. When the reserved slot is used by some neighbors, having received a kick, they choose a new slot and try at using it without reservation. The communications are divided in rounds. Each round is composed by a preset number of timeslots. Each timeslot 251

is composed by a kick phase and a transmission phase. The identification of a timeslot is called color. During the kick phase, each reader assigned to the current color which collided at the previous transmission sends a kick. During the kick phase of its color, each reader that receives a kick changes color. During the transmission phase, each reader with the current color that has to read tags executes a transmission. If the transmission collides then the involved readers stop themselves and randomly choose a new color. At the subsequent round the readers will send a kick, in order to reserve the timeslot. In DCS when more than one neighbor transmits a kick during the same slot, all the transmitting readers also receive the kick, and they choose a new color. The protocol requires readers that sense collisions, send signals (kicks) without additional information, and sense kicks also when they are transmitting a kick. A synchronization system for timeslot phases is also required. C. Colorwave An evolution of DCS is represented by Colorwave [1] [6]. This protocol introduces a variable quantity of timeslots that compose a round (µ), differently from DCS where the number of timeslots is fixed. The value is dynamically changed in order to increase the efficiency of the RFID network. When the number of collisions is high, µ rises, when it is small µ decreases. This protocol requires a special kick transmission, which states the change to a new µ. The kick phase is divided in two subphases, where standard kicks are sent during the first one, and µ-kicks during the second one. In order to manage changes in the number of colors for round, Colorwave introduces two couples of thresholds, one for the increases and the other for the decreases. Each reader counts its percentage of successful transmissions. When the percentage exceeds the second threshold of a couple, the reader changes µ and during the second kick subphases it communicates the change to its neighbors; if a reader exceeded the first threshold of a couple and it receives a µ- kick compliant with the exceeded threshold, then it changes µ and communicates the change to its neighbors. The variable number of colors allows Colorwave to find autonomously a good configuration. The protocol is suitable for heterogeneously deployed networks, since each reader can employ a different µ suitable to its area. Furthermore, it is more suitable than DCS to mobile readers, since its configuration is updateable. The protocol has the same requirements of DCS and it requires also readers that send signals (µ-kicks) with the value of the new µ, and that receive µ-kicks. IV. INTRODUCING PROBABILITY IN DCS In DCS, after sensing a collision, all the involved readers choose a new color and reserve it. However, when a large part of the timeslots are just used, a change of color could generate a second change without timeslot reservation, so a probable collision between two readers occurs. Furthermore, the kicked reader would not transmit during the reserved round, and during the subsequent collision round, so it would wait two rounds before transmitting. After a collision between two readers, all the involved nodes will change their color, so both the readers will reserve a new color. When the majority of colors are used, both the new timeslots could be engaged, so two readers would change their color. Therefore, this double color change could generate two second generation collisions. Furthermore, when both the colliding readers choose a new slot the old one become unoccupied. With the introduction of the new parameter P, which represents the probability of colliding readers to change color, after a collision between two readers 3 cases are possible: 1) no reader changes its color, so at the subsequent round the involved readers will receive a kick and they will change the color without reservation; 2) one reader changes its color, so at the subsequent round one reader will transmit with the previous color, and the second reader will reserve a new color, possibly requiring another reader to change; 3) both the readers change color, this case corresponds to the standard DCS collision resolution. Case 1 is worse than DCS, since the involved readers will lose a second round. Case 2 is better than DCS, since one reader probably will not produce second generation collisions. Case 3 corresponds to DCS. According to the value of P, each case has a characteristic frequency, so a proper value of P maximizes the number of occurrences of Case 2. The variables of the protocol are: color i, the index of the time slot that the i th reader can use for transmissions; µ, the number of time slots in a round; kickflag i, the boolean flag, true when the i th reader requires a kick; transflag i, the boolean flag, true when the i th reader requires a transmission; The introduction of P in DCS requires a new collision resolution subroutine. The new version of DCS is composed by the following subroutines. Beginning of a timeslot. New timeslot: i : color i = (color i + 1) mod (µ); if (reader i th has to read tags) then transflag i = true; Kick phase. Kick sending: if (kickflag i = true AND color i = 0) 252

then reader i th sends the kick; and kickflag i = false; Kick resolution: if (reader i th receives a kick AND color i = 0) then color i = random(µ 1) + 1; Transmission phase. Transmission: if (transflag i = true AND color i = 0) then reader i th transmits and transflag i = false; Collision resolution: if (transmission of reader i th collides) then color i = random(µ); and kickflag i = true; transflag i = true; Remark 1. The current color is always equal to 0. Remark 2. In the proposed version, when more than one reader transmits a kick during the same slot, all the transmitting readers also receive the kick, and so they will choose a new color. V. EXPERIMENTAL ANALYSIS As introduced in Section II, the most relevant parameter here considered for the evaluation of the time performance of an anti-collision protocol for RFID networks is WT. The main novelty due to the introduction of P is the collision resolution subroutine, and its effects on WT must be carefully analyzed. A. Collision Between Two Readers Observing one collision between two readers, it is possible to understand effects of P on the protocol. Figure 2 shows the probability of each case described in Section IV, according to P. Starting from P = 1, a short decrease of P corresponds to: a slight rise of Case 1 (negative case); a considerable grow of Case 2 (positive case); a sharp fall of Case 3 (intermediate case). The effects of the different cases on the time performance of the protocol can be evaluated according to two measures: second generation collisions (γ), that represent the number of readers involved by second generation collisions produced by a collision; fairness (φ), that represents the fraction of readers that can transmit the round after sensing a collision. An optimal value of P may produce a considerable fall of γ and a slight decrease of φ. The negative effects of the decline of φ should be widely repaid by the reduced number of collisions. Figure 2. B. Waiting Time Color Change After a Collision Between Two Readers The actual index for evaluating the time performance of a RFID reader-to-reader anti-collision protocol is represented by WT. However, γ and φ are two important factors that affect WT. The reduction of γ produced by P decreases AWT and WTV; the reduction of φ increases WTV. Considering a case without collisions, the performance of time division protocols are only related to µ, where the rise of µ corresponds to the decline of AWT. However, AWT is affected by the collisions. When a reader randomly chooses a color, the probability that the color is occupied corresponds to the percentage of occupied colors, so the decrease of µ corresponds to the rise of the collision probability. In time division protocols a reader that should transmit can: 1) normally transmit, so the Standard Waiting Time (SWT) for occasional transition requests is: 0 SWT µ (1) since the request can start at any moment between the starting point of the correct timeslot of a round, and the starting point of the same timeslot of the subsequent round; instead, for consecutive transition requests, we have: SWT = µ 1 (2) since the request is ready at the end of the correct timeslot of the each round; 2) collide, so the additional Collision Waiting Time (CWT) is 1 CWT µ (3) 3) receive a kick, so the additional Kick Waiting Time (KWT) is 253

Figure 3. Effects of P on AWT with AN = 12.01 and NV = 13.57 Figure 4. Percentage difference of AWT respect to standard DCS 1 KWT µ (4) All the times are expressed in time slots. The WT of a transmission request is equal to SWT, with the possible addiction of some CWTs and KWTs. Without collisions WTV is equal to the variance relative to a random distribution, but CWT and KWT increases it. The goal of P is to decrease the occurrences of CWT and KWT. However, P < 1 increases the number of couples of readers that both do not change color, so the number of readers that receive a kick after a collision could rise. The main compositions of WT are: 1) SWT, the reader can normally transmit without collisions or kicks; this is the shortest WT composition; 2) SWT+CWT, the reader collides and then it changes color with reservation and transmits; if the new color is occupied by a reader that is transmitting, then this reader has one of the subsequent compositions; 3) SWT+KWT, the reader receives a kick, then it changes color without reservation and transmits; 4) SWT+KWT+CWT, the reader receives a kick, then it changes color without reservation, collides, changes color with reservation and transmits; 5) long compositions, where the reader receives a sequence of consecutive kicks (..+KWT+KWT+..) and/or a kick immediately after a collision (..+CWT+KWT+..). The goal of a suitable P is to reduce the occurrences of the compositions with CWT and KWT. C. Experimental Simulations Simulations of standard DCS, of Colorwave, and of the new version of DCS were performed on several kinds of RFID networks. The characteristics of the simulated networks are: total number of readers: 250; number of neighbors for readers: average (AN) and variance (NV); method of reader deployment: random; frequency of reading requests: 100%. Each protocol configuration was simulated 10 times for 100000 timeslots. The behavior of standard DCS (P = 1.00), of the proposed version, and of Colorwave have been simulated according to several networks with AN between 3 and 15. In order to show this behavior the results of the simulations related to a network with AN = 12.01, NV = 13.57 will be detailed. Figure 3 shows AWT reached by standard DCS (P = 1.00), the proposed version with P = 0.8, P = 0.7, and Colorwave with the thresholds respectively equal to 20%, 45% 45%, and 50% of successful transmissions. The results are shown according to µ between 9 and 20, since this interval includes the best configurations of all the simulated protocols. Colorwave, which does not have a static µ, has been simulated according to different starting values of µ. The value of AWT reached by this protocol fluctuates between 15.75 and 16.03, independently from the starting values of µ. It is observed that the value of AWT reached by standard DCS, after a gradual decline with small µ, falls to 15.52 with µ = 16. From µ = 17 the values of AWT grow steadily, providing AWT P 1. The values of AWT reached with P = 0.8 and P = 0.7 have a behavior similar to standard DCS. The difference in percentage with respect to standard DCS are shown in Figure 4. From µ = 9 to µ = 14, there is a limited reduction of AWT, but for µ = 15, both the configuration with P = 0.8 and P = 0.7 reach a reduction over 14%. From µ = 17 the values of AWT grow steadily and are similar for all the configuration of DCS. These graphs show that with a proper configuration of P 254

Figure 5. Effects of P on WTV with AN = 12.01 and NV = 13.57 Figure 7. Effects of P on MWT with AN = 12.01 and NV = 13.57 Figure 6. Percentage difference of WTV respect to standard DCS and µ it is possible to reach AWT better than colorwave and standard DCS. All the graphs of AWT, according to the simulated networks present the same fall. Values of P lower than 1 allow reaching a more considerable fall, for a lower value of µ. Moreover, also the other values of AWT reached with P < 1 next to the fall are lower than the corresponding values reached by standard DCS. Figure 5 shows WTV reached by the same protocols, previously described. The value of WTV reached by this protocol is stable, independently from the starting values of µ. The values of WTV reached by standard DCS and with P = 0.8 and P = 0.7 decrease significantly up to just over 0. The difference in percentage with respect to standard DCS are shown in Figure 6. With small µ, standard DCS reaches better WTV, but between µ = 15 and µ = 18, P = 0.8 and P = 0.7 provide a reduction of WTV over 20%. With colorwave WTV is very high, because this protocol allows that some readers transmit much more often than some others, providing different performances. Figure 7 shows MWT reached by the same protocols, previously described. It is observed that the behavior of MWT is quite similar to WTV. In the following, the performances of DCS according to various P will be analyzed, when AWT falls. Table I shows the behavior of standard DCS (P = 1.00) and the proposed version with P between 0.5 and 0.9, when µ is quite higher than the average neighborhood. The introduction of P improves strongly AWT, decreasing the number of collisions and kicks with respect to the successful transmissions. It is possible to observe that the reduction of AWT, WTV, and MWT corresponds to the rise of the percentage of short WT compositions. Table II shows the differences (expressed in percentage) due to the introduction of P in the previous configuration. Observing as example the introduction of P = 0.7, it is found that AWT is reduced by 16.25%, WTV by 80.93% MWT by 5.11%, and φ by 6.70%. The number of collisions and active kicks are reduced by over 80%. VI. CONCLUSION The paper studied the reader-to-reader anti-collision problem. Several evaluation methods have been analyzed, and an evaluation approach based on the Waiting Time (WT) has been adopted. A new version of DCS has been proposed, with the introduction of probability into collision resolution. The proposed version has been analyzed according to the WT measure, and compared with division approaches. Simulations show that the new approach presents significant improvements in terms of time performance. The presented analysis shows that the introduction of P = 0.7 in DCS reduces AWT, WTV, and MWT. Although 255

Table I EFFECTS OF P ON DCS WITH µ = 12, AN = 10.01 AND NV = 9.31 P = 1.00 P = 0.90 P = 0.80 P = 0.70 P = 0.60 P = 0.50 collision 336888.3 231468.2 95064.3 51913.7 34187.8 71466.1 reading 1715922.5 1826770.4 1975702.2 2022996.8 2042393.4 1994636 Transmissions active kicks 369767.1 241157 94891.5 50026.6 32322.3 67189 successful 70.83% 79.45% 91.23% 95.20% 96.85% 93.50% φ 71.71% 72.22% 70.56% 66.90% 62.26% 55.14% MWT 213 199 189 202 214 228 WT AWT 13.6 12.7 11.7 11.4 11.2 11.6 WTV 52.0 37.1 16.4 9.9 7.4 19.5 SWT 1374746.3 1597094.8 1884691.2 1975744.2 2013134.8 1938534.1 SWT+CWT 114871.1 81213.8 33218.8 17584.3 10829.8 20104.9 WT Compositions SWT+KW 56023.4 35352.2 13028.2 6244.7 3573.8 6110.7 SWT+KWT+CWT 68258.5 45253 16994.1 7938.7 4171.9 6307.7 long compositions 102023.2 67856.6 27769.9 15484.9 10683.1 23578.6..+KWT+KWT+.. 58798.8 37587.4 14435.9 7338.2 4612.3 9271.6..+CWT+KWT+.. 71857.2 49032.3 21012.5 12411.9 9009.2 20903.4 SWT 80.12% 87.43% 95.39% 97.66% 98.57% 97.19% SWT+CWT 6.69% 4.45% 1.68% 0.87% 0.53% 1.01% WT Composition SWT+KW 3.26% 1.94% 0.66% 0.31% 0.17% 0.31% Percentage Distribution SWT+KWT+CWT 3.98% 2.48% 0.86% 0.39% 0.20% 0.32% long compositions 5.95% 3.71% 1.41% 0.77% 0.52% 1.18%..+KWT+KWT+.. 3.43% 2.06% 0.73% 0.36% 0.23% 0.46%..+CWT+KWT+.. 4.19% 2.68% 1.06% 0.61% 0.44% 1.05% Table II PERCENTAGE DIFFERENCE WITH RESPECT TO STANDARD DCS WITH µ = 12, AN = 10.01 AND NV = 9.31 P = 0.90 P = 0.80 P = 0.70 P = 0.60 P = 0.50 collision -31.29% -71.78% -84.59% -89.85% -78.79% reading 6.46% 15.14% 17.90% 19.03% 16.24% Transmissions active kicks -34.78% -74.34% -86.47% -91.26% -81.83% successful 12.16% 28.80% 34.41% 36.73% 32.01% φ 0.72% -1.60% -6.70% -13.17% -23.11% MWT -6.84% -11.63% -5.11% 0.52% 6.99% WT AWT -6.31% -13.90% -16.25% -17.15% -14.78% WTV -28.67% -68.51% -80.93% -85.68% -62.46% SWT 16.17% 37.09% 43.72% 46.44% 41.01% SWT+CWT -29.30% -71.08% -84.69% -90.57% -82.50% WT Compositions SWT+KW -36.90% -76.75% -88.85% -93.62% -89.09% SWT+KWT+CWT -33.70% -75.10% -88.37% -93.89% -90.76% long compositions -33.49% -72.78% -84.82% -89.53% -76.89%..+KWT+KWT+.. -36.07% -75.45% -87.52% -92.16% -84.23%..+CWT+KWT+.. -31.76% -70.76% -82.73% -87.46% -70.91% experimental results show that the fairness φ is often decreased, when the number of collision drops, the resulting percentage of long WTs is almost negligible with respect to the WT composed only by one ST. The main result achieved by this paper is that the introduction of the probability can improve the time performance of time division anti-collision protocols for RFID. ACKNOWLEDGMENT This work has been partially supported by the grant Nano-materials and -technologies for intelligent monitoring of safety, quality and traceability in confectionery products (NAMATECH) from Regione Piemonte. REFERENCES [1] J. Waldrop, D. Engels, and S. Sarma, Colorwave: an anticollision algorithm for the reader collision problem, in Communications, 2003. ICC 03. IEEE International Conference on, vol. 2, May 2003, pp. 1206 1210. [2] K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification. New York, NY, USA: John Wiley & Sons, Inc., 2003. 256

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