Analysis of Energy-Conserving Access Protocols for Wireless Identification Networks

Transcription

1 From the Proceedngs of Internatonal Conference on Telecommuncaton Systems (ITC-97), March 2-23, Analyss of Energy-Conservng Access Protocols for Wreless Identfcaton etworks Imrch Chlamtac a, Chara Petrol b, Jason Red c a The Unversty of Texas at Dallas, Rchardson, T, U.S.A., fchlamtac,radhag@utdallas.edu b Unverstà d Roma La Sapenza, Italy, petrol@ds.unroma1.t c Boston Unversty, Boston, MA, U.S.A., red@bu.edu Abstract A myrad of applcatons such as rado frequency dentfcaton (RFID) and smart card networks are emergng n whch nodes are desgned for extremely low-cost, large scale applcatons such that the replacement of batteres s not feasble. Energy conservaton therefore becomes a major constrant. Classcal access protocols are ether not energy conservng or lead to unacceptable delays. In a prevous paper we presented three classes of energy-conservng protocols. In ths paper we descrbe analytc models to descrbe and evaluate ther performance. 1 Introducton Rado Frequency Identfcaton Devces (RFIDs), such as warehouse dentfcaton tags and ntellgent ID cards, referred to here as IDETs, are examples of a new world of applcatons whch use small, nexpensve devces for whch battery conservaton s a crtcal system parameter. A typcal IDET s composed of a number of nterconnected base statons communcatng over a shared wreless channel to a large number of small, low-cost, wreless nodes or tags. These tags usually contan some sort of mcroprocessor, power source n the form of a battery, capactor or solar cell, a rado frequency recever, possbly a transmtter, and some support logc. The range of potental uses for RFID tags s extremely large. By the year 2, the RFID market s expected to expand 2 fold to over $5 bllon [3]. Some examples of current uses for RFID tags are: tags attached to the ears or worn around the necks of lvestock for use n locaton trackng, smart tags used n warehouses to automatcally and quckly track nventory, and the numerous tag companes targetng the retal market for electronc shelf labels and automatc purchasng. There are four fundamental characterstcs of RFID wreless nodes whch make RFID networks dstnct from other wreless systems: 1. Scale: There may be a very large number, perhaps thousands, of wreless nodes per base staton. 2. Cost: Due to the large number of extremely nexpensve tems to tag, nodes must be extremely nexpensve, often on the order of only a few dollars. 3. Sze: odes must be very small. The sze of a pack of cards wll be the maxmum sze for many applcatons. 4. Traffc: Communcaton s typcally based on short, smple messages. Transmsson speeds are usually low, on the order of tens of klobts per second n order to mnmze cost and power of transmsson. From these characterstcs follow a number of mportant observatons whch further defne the unque constrants of an RFID network. Most mportantly, as n all moble computng applcatons, the battery s energy s a lmted and scarce resource, whch s not expected to ncrease n potental more than 3% n the near future [5]. Especally demandng of energy s any uplnk transmsson as t can typcally use twce as much energy as recepton [4]. It s possble to use spread spectrum modulaton such that the base staton ndrectly provdes the energy for lmted uplnk communcaton, but ths requres that any uplnk traffc be base staton ntated. Furthermore, lmted unlcensed bandwdth and smplcty of the tag means that all tags must share the same broadcast band. For all of the above consderatons, these types of systems requre new access protocols whch are desgned around these unque constrants and provde a combnaton of two mportant factors: low delay and low energy requrements. The allowable delay s an applcaton dependent constrant. For example, trackng the movement of tags across the cells wthn a system requres updates to be performed wthn a short, bounded, amount of tme. The system s already constraned by the speed of the shared channel and has to manage a potentally large number of tags. Therefore, t s extremely mportant for access protocols to not add sgnfcantly to the transmsson delay. Low energy consumpton s the second requrement that the access protocol must satsfy. The large number of nodes makes t economcally mpossble to replace or recharge the batteres. Therefore tags must be desgned so as to requre a mnmum of energy for operaton. In order to conserve battery lfe, the tag can enter a sleep state where the CPU s n a low power mode and rado recepton s dsabled. In contrast wth ths, the awake state, n whch the CPU operates at full energy and

2 From the Proceedngs of Internatonal Conference on Telecommuncaton Systems (ITC-97), March 2-23, the rado frequency crcutry s actve, can typcally use 1 tmes as much energy. Among exstng protocols, classcal random access protocols are not energy conservng. Whle determnstc protocols lead to unacceptable delays. In [2], we addressed the problem of desgnng communcatons protocols whch operate under an energy constrant, n whch the fracton of tmeslots n whch tags need to be n the actve (awake) state s mnmzed and the access delay meets the applcatons constrants. We ntroduced three classes of protocols: grouped-tag TDMA, drectory and pseudo-random. In ths paper we present analytc models whch use nfnte Markov chans and the theory of M/D/1 queues wth vacatons for further evaluatng the performance of these protocols. All three classes of protocols represent a dramatc mprovement over classcal approaches. In the remander of the paper we frst descrbe the tag network model, followed by a descrpton of the protocols. In the ensung analyss of energy consumpton and access delay, we derve the system behavor for both unformly and nonunformly dstrbuted traffc destnatons. We follow wth a detaled evaluaton of the protocols and conclude wth a summary. 2 etwork Model and Protocols Descrpton We consder a sngle cell system where a base staton communcates wth tags through a rado channel of bandwdth B. The communcaton s packet-orented. We assume the tme to be slotted and the base staton s transmssons to be synchronzed to the begnnngs of slots. The packet length c s constant, and exactly one packet can be transmtted durng one slot. In ths analytc model, we do not explctly treat transmsson errors. We defne an access protocol as consstng of two components: a transmsson schedulng strategy at the base staton whch n each slot selects a packet for transmsson from the arrval queue, and a wake-up schedule at each tag whch determnes the slots n whch the tag s awake. In general, the transmsson schedulng strategy can take nto account dfferent parameters: the number of packets n the queue, the packets ages, as well as the wake-up schedules of ther destnatons. In the protocols dscussed here the oldest packet crteron s generally adopted to help meet the applcaton delay requrements. We next present and compare three classes of protocols for constructng effcent wake-up schedules: groupedtag TDMA protocols, drectory protocols, and pseudo-random protocols. 2.1 Grouped-Tag TDMA Protocols Classcal TDMA can be adapted for use n energy-conservng envronments n the followng way: we dvde tags nto m dsjont groups, wth the cardnalty of each group dfferng by at most one tag, and assgn (reserve) each slot of the TDMA cycle to a unque group. Ths ncreases the average energy consumpton per slot by the cardnalty of the groups, but decreases the average delay snce there s a greater probablty that a tag wll be awake soon after a packet for t has arrved at the base staton. The optmal selecton of ths group sze wll ensure the best energy and delay performance of ths class of protocols. 2.2 Drectory Protocols In our drectory protocol, the base staton always wats for a group of k packets n the queue to accumulate. The base staton then transmts a lst or drectory of the k packet destnatons before transmttng the packets n the subsequent slots. The tags are all awake durng the transmsson of the drectory, and can therefore schedule ther wake-up slots to concde wth the broadcast of ther packets. When there s no group beng currently transmtted, the tags wake up perodcally every v slots n order to gve the base staton an opportunty to start the transmsson of a new group. The choce of the parameter k depends on the load and must take nto account the trade-off between the ncrease n the delay due to a larger k and the energy savngs from more nfrequent broadcastng of drectores. Addtonally, the parameter v should depend upon k and the load. A tag system wth small value of v and a low load wll have the tags wakng up frequently untl enough packets have accumulated at the base staton, whle a large value of v wll ncur an ncrease n delay before the start of group s transmsson. 2.3 Pseudo-Random Protocols The pseudo-random protocols are a class of protocols based on determnstc (pseudo-random) schedules whch preserves the power of randomzaton for farness, whle provdng the advantages of determnsm,.e., the base staton s ablty to predct tags state n each slot. In ths class of protocols all tags run the same pseudo-random number generator and determne ther state (awake or asleep) at each slot based on a probablty p and the stored state of the random number generator. In order to avod a complete overlap of the wake-up schedules, the pseudo-random generator of each tag s ntalzed usng a unque seed, whch s known at the base staton. Therefore, by usng the same pseudo-random number generator t s possble for the base staton to determne the schedules of the tags t wants to transmt to. The base staton can ntate changes n the value of p as a functon of the load, the number of tags, etc. 3 Analyss To compare the performance of the varous protocols proposed, we consder the behavor of a sngle cell tag system. Snce the tme needed to successfully receve a packet, gven that the tag s awake, s exactly one slot, then the slot duraton s = b +, wth b denotng the packet transmsson tme ( c B ) and the propagaton delay. The presented evaluaton utlzes the followng defntons:

3 From the Proceedngs of Internatonal Conference on Telecommuncaton Systems (ITC-97), March 2-23, T ; the average watng tme n slots experenced by a packet n the system from arrval at the base staton to successful recepton at the tag. E; the average percentage of slots n whch a tag s awake. L; the average number of packets n the system. The energy measure proposed does not take nto account the contrbuton durng slots where a tag s asleep. The energy used whle n ths state s consumed over the lfetme of the tag, whether the tag s beng used or not. Therefore only the percentage of awake slots s necessary for comparng dfferent energy-conservng access protocols. We assume that the packets arrve at the base staton accordng to a Posson process wth nterarrval rate. Each packet s addressed to a sngle destnaton selected from ether a unform or a Gaussan dstrbuton. The latter s ntroduced n order to model the heterogeneous nature of the traffc whch s common to many tag applcatons. We use the followng notatons: ; the mean arrval rate expressed n packets per slot ( = (b + )). ; the mean servce rate expressed n packets per slot. ; the utlzaton factor. 3.1 Analyss of the Grouped-Tag TDMA The access delay for packets whose destnatons are n dfferent groups s ndependent. Therefore, for each group, we can assocate a dfferent queung system whose Posson traffc stream has rate. Every tme the queue s empty, the server goes on vacaton for one TDMA cycle (m slots). Otherwse the servce tme of a packet s constant and equals 1. Thus, the average watng tme n the queue for a packet destned for group s equvalent to that of an M=D=1 queung system wth vacatons [1]: W = 2 + m (1;) 2 m = 2(1; m) Let T x denote the average number of slots, as a functon of the parameter x, durng whch a packet s watng n the system. Then: T x P m = =1 =1+ P m =1 (W +1) m 2(1; m) It remans to calculate the. These quanttes depend on the destnaton dstrbuton utlzed. Under a unform destnaton dstrbuton the percentage of the global traffc dedcated to each group s proportonal to the number of tags n that group. Thus, = < : d m e b m c modm > modm We next consder the case of when the packets destnatons are chosen accordng to a Gaussan dstrbuton wth a densty functon f x, average and varance 2. = p, where p denotes the probablty of choosng a packet n the -th group. p s the area of the regon bounded above by f x and the nterval assocated to the -th group, normalzed by the area of the usable part of the Gaussan (.e., the one correspondng to the nterval [1::: ]). So p = F x(d m e);f x((;1)d m e) F x();fx() modm F x(modmd m e+(;modm)b m c) F x();fx() ; > modm F x(modmd m e+(;1;modm)b m c) F x();fx() where F x (y) s the cumulatve dstrbuton functon of a Gaussan dstrbuton. 3.2 Analyss of the Drectory Protocol Let a = j c dloge k be the number of dfferent records whch ft nto a slot. Then, the servce tme of a group s k + k slots, where k ndcates the tme needed for transmttng the drectory and s gven by a k. When a cycle ends and no completed groups are n the queue, the tags go to sleep for an dle nterval of v slots. Thus, a server can start processng a new group f and only f there s at least one completed group n the queue and ether an dle or a servce cycle ended n the prevous slot. We compute the average watng tme T of a packet n the system as: T = W p + T g ; W s = k;1 2 + T g ; ( k;1 2 ) ;W p s the average watng tme of a packet n the queue before ts group s completed and s = k;1 2. ;T g s the average watng tme of a group n the system. ;W s s the average tme between the successful recepton of a packet and the completon of ts group s transmsson. W s = k;1 2. To compute T g we construct the followng analytc model. We observe the system at the regeneraton ponts embedded at the begnnng of each slot and model the system as a dscrete tme, nfnte Markov chan M =< S < j h> P < j h> < j h > >. Each state S < j h> denotes the number of completed groups n the system, the number j of non-grouped packets n the queue, and the ndex h of the current slot n ether the dle ( h v ; 1) or the servce (v h v + k + k ; 1) cycle. ote that wthn ether a servce or dle cycle we have only to ncrease the poston of the slot and keep track of the arrvals, whle durng the last slot of a cycle we also have to decde the nature (busy/dle) of the next nterval and possbly ndcate the ext of a group from the system. We obtan the followng transton probabltes:

4 From the Proceedngs of Internatonal Conference on Telecommuncaton Systems (ITC-97), March 2-23, Case h 2 [:::v ; 1) S [v:::v + k + k ; 1) < : P < j h> < j h > = (h 6= h +1)_ ( <) A j ;j+( ;)k Case h = v ; 1 P < j v;1> < j h > = _(( = ) ^ (j <j)) otherwse ((h 6=)^ ( = )) _((h 6= v) ^ ( 1)) _( <) _ (( = ) ^ (j <j)) otherwse A j ;j+( ;)k Case h = v + k + k ; 1 P < j v+k+k ;1> < j h > = ((h 6=)^ ( = )) _((h 6= v) ^ ( 1)) _( <; 1) _ (( = ; 1) A j ;j+( ;)k+k ^(j <j)) otherwse ;A r s the probablty of r messages orgnated by a Posson process durng a slot and s gven by: ; ()r A r = e r! The steady state probablty < j h> of beng n the < j h > state s obtaned by solvng the followng system of equatons: =P T P P j P h =1 Let us denote g () the probablty of havng completed groups n the system: g () = j h < j h> The average number of completed groups n the system L g can then be gven by: L g = g () Fnally, applyng Lttle s theorem we can compute the average watng tme of a group n the system: T L gk g = The energy consumpton E can be computed as follows: E = Pr E + Pr b E b ;Pr s the percentage of slots belongng to dle cycles and s gven by: Pr = k;1 v;1 j= h= < j h> : ;Pr b s the percentage of busy slots, Pr b =1; Pr. ;E s the average energy consumpton per tag per slot durng an dle perod and s equal to 1 v. ;E b s the average energy consumpton per tag per slot durng a servce cycle. E b = k + k k + k : 3.3 Approxmate Analyss of the Pseudo- Random Protocol We analyze the protocol under the assumpton that all tags use the same awake probablty parameter p. To obtan an exact descrpton of the system behavor through a Markov chan, the states should nclude the number of packets addressed to each tag, snce the probablty of successfully transmttng depends on the number of unque packet destnatons n the queue. It s therefore mpossble to analyze systems of a realstc sze. We solve ths problem by makng use of Stern s ndependence assumpton [6], statng that at the begnnng of a tme slot each message draws a new destnaton from a gven unform dstrbuton. The Markov chan descrbng the system s then represented by the total number of packets at the begnnng of a slot. The transton probablty matrx P s gven, smlarly to the random access case, by P j P j = = A j P mn( ) j<; 1 k=1 R k (1 ; (1 ; p) k )A j = ; 1 P mn( ) k=1 R k (1 ; (1 ; p) k )A j;+1 + P mn( ) k=1 R k (1 ; p) k A j; otherwse ;R k s the probablty that packets are addressed to k dfferent destnatons and s gven by R k = k Pk;1 k j= (;1)j (k ; j) j : The steady-state probabltes are the solutons of the followng set of equatons =P T P =1 We can now compute the average number L of packets n the system L =

5 From the Proceedngs of Internatonal Conference on Telecommuncaton Systems (ITC-97), March 2-23, l =.5 l =.1 l =.5 l =.5 l =.1 l = Fgure 1: Grouped TDMA Protocol, Unform Destnaton Dstrbuton, Optmal group sze Fgure 3: Pseudo-Random Protocol, Unform Destnaton Dstrbuton.5 l =.5 l =.1 l =.5 Grouped-Tag TDMA, l =.5 Grouped-Tag TDMA, l =.1 Grouped-Tag TDMA, l = Drectory, l =.5 Drectory, l =.1 Drectory, l = Fgure 2: Drectory Protocol, Unform Destnaton Dstrbuton Fgure 4: Energy conservng protocols, Gaussan destnaton dstrbuton ( = 2 2 =1) and, usng the Lttle s result: T = L Fnally, as n the prevous secton, the average energy consumpton s obvously E = p. 4 Performance Fgures 1 through 5 plot quanttatve results of the equatons derved n prevous sectons. These plots were all generated by assumng 1 tags wth consecutve sequental ds are assgned to a sngle base staton. All of the plots nclude sets of data wth nterarrval rate parameters equal to :5, :1 and :2 arrvals per slot. To show the effect of heterogeneous destnaton dstrbutons we use two Gaussans, wth mean and 2 varance 1 and respectvely. The nfnte Markov chan of the pseudo-random protocol was approxmated by truncatng the transton probablty matrx after the frst 1 states whle up to 25 states were consdered for the drectory protocol wth the lmtaton k 2f1 ::: 1g and v 2f1 ::: 25g. Fg.1,2, 3 compare the performance of the three protocols proposed usng a unform destnaton dstrbuton. Fg.1 shows the trade-off between energy and delay for optmum values of group sze usng the grouped-tag TDMA protocol. We note that the performance s better for low loads because there are fewer packets avalable for transmsson for the same slot n a cycle. Snce tags schedules are cyclc and ndependent of packets n the base staton s queue (a tag does not know when the base staton may have a packet destned to t) some unnecessary energy can be used due to tags contnung to wake up cyclcally. The performance of the drectory protocol (Fg.2) depends on the choce of the parameters k and v for each partcular arrval rate.

6 From the Proceedngs of Internatonal Conference on Telecommuncaton Systems (ITC-97), March 2-23, Grouped-Tag TDMA, l =.5 Grouped-Tag TDMA, l =.1 Grouped-Tag TDMA, l = Drectory, l =.5 Drectory, l =.1 Drectory, l = Fgure 5: Energy conservng protocols, Gaussan destnaton dstrbuton ( = 2 2 = ) If we assume an optmum value for v, an ncrease n the arrval rate wll reduce the amount of tme t takes for a complete group of K packets to arrve, reducng the average delay for the packets n that group. However, ths requres the tags to be awake more often n order to receve ths group, thereby ncreasng the energy. When and group sze k are large, the sze of the drectory becomes prohbtvely large, and therefore the amount of energy used by the tags just to read the drectory can become a major factor. Fg.3 shows the energy vs. delay graph for the pseudorandom protocol gven a unform destnaton dstrbuton. The performance acheved s better than that of the drectory protocol but slghtly worse than that of the grouped-tag TDMA although ths dfference decreases as ncreases. Fg.4, and 5 compare protocol performance for the case of heterogeneous traffc. The drectory protocol s not affected by the destnaton dstrbuton. Instead, we show how the performance of the grouped-tag TDMA degrades rapdly under heterogeneous traffc. Packets belongng to a group wth a hgh probablty of traffc are severely delayed due to the hgh volume of localzed destnatons. The reorganzaton of the groups, even f possble, can be extremely tme-consumng. Snce the number of packets n the queue addressed to the same group ncreases as ether the load ncreases or the Gaussan dstrbuton becomes narrower, the performance of the protocol decreases n both these cases. For nether of these Gaussans s the performance of the pseudo-random protocol dscussed. Intutvely, we do not expect the performance to change dramatcally from the case of a unform destnaton dstrbuton. Snce the tag s schedules are the same, the energy wll not change, and the probablty of at least one packet n the queue wth an awake destnaton should not sgnfcantly change. Ths ratonal s backed by the smulaton results n [2] whch shows only a slght ncrease n delay for even the tghter Gaussan dstrbuton. 5 Conclusons In ths paper, we addressed the analyss of three types of wreless access protocols whch nclude an energy constrant: grouped-tag TDMA, drectory and pseudo-random. Careful selecton of protocol parameters addresses the goal of mnmzng the energy requred for recepton of packets whle meetng the applcaton delay constrants. A detaled analyss and plots of quanttatve data from ths showed a varety of performances under varyng loads. In partcular, the groupedtag TDMA protocol acheves the best performance for ether a low traffc load or a unform destnaton dstrbuton. When the destnaton dstrbuton becomes more realstcally clustered, other protocols such as the drectory or pseudo-random protocols need to be adopted. The pseudo-random protocol consstently out-performs the drectory protocol n both energy and delay for all loads and most destnaton dstrbutons. However, for cases where the destnaton dstrbuton becomes extremely clustered, t s apparent that the drectory protocol s the best soluton, unless dynamc handlng of the protocol parameters s ntroduced. References [1] Bertsekas, D. and Gallager, R. Data etworks, Prentce Hall, [2] Chlamtac, I. and Petrol, C. and Red, J. Energy- Conservng Access Protocols for IDETs Techncal Report 96-6, Boston Unversty. [3] Dunn, D. RFID Engne Chugs Onto Scene. Electronc Buyer s ews, February 5, 1996, pp 14. [4] Harrs, E.P., Depp, S.W., Pence,E., Krkpatrck, S., Sr- Jayantha, M., and Troutman, R.R. Technology drectons for portable computers. Proceedngs of the IEEE, 3(4):636 5, Aprl [5] Powers, R.A. Batteres for low power electroncs. Proceedngs of the IEEE, 3(4):67 93, Aprl [6] Stern, T. E. Packet Schedulng protocols n mult-beam communcaton satelltes. Proceedngs of the Internatonal Symposum on Informaton Theory, Italy, June 1979, pp