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En ergy-Ef f ic ient C om put ing for Wi ld l ife Track ing:Design Tradeof fs and Ear ly Exper iences wi th ZebraNetPhi lo Juang, H idekazu O ki , Yong Wang t ,Margaret M artonosi,Li-Shiuan Pe h, and Daniel Rubenstein~

Depts. of Electr ical Engineering, tCom puter S cience, and ~Ecology and Ev ol. BiologyPrinceton U niversityA B S T R A C TOver the past decade, mobile computing and wire-less communication have become increasingly importantdrivers of many new computing applications. The fieldof wireless sensor networks particul arly focuses on appli-cations involving autono mous use of compute, sensing,and wireless communic ation devices for bo th scientificand commercial purposes. This paper exa mines the re-search decisions and design tradeoffs that arise whenapplyin g wireless peer-to-peer networking technique s ina mobile sensor network designed to support wildlifetracking for biology research.The ZebraNet system includes custom tracking collars(nodes) carried by animals under study across a large,wild area; the collars operate as a peer-to-peer networkto deliver logged data back to researchers. The collarsinclude global positioning system (G PS), Flash me m-ory, wireless transceivers, and a small CPU; essentiallyeach node is a small, wireless compu tin g device. Sincethere is no cellular service or broadcast communicationcovering the region where animals are studied, ad hoc,peer-to-peer routi ng is needed. Althoug h numerou s adhoc protocols exist, additional challenges arise becausethe researchers themselves are mobile and thus there isno fixed base station towards which to aim data. Over-all, our goal is to use the least energy, storage, and otherresources necessary to maintain a reliable system witha very high 'd ata homing' success rate. We plan to de-ploy a 30-node ZebraNet system at the Mpala ResearchCentre in central Kenya. More broadly, we believe thatthe d omain-ce ntric protocols and energy tradeoffs pre-sented here for ZebraNe t will have general applicabilityin other wireless azld sensor applications.1 . I N T R O D U C T I O N

Mobile computing and wireless communication arehigh-growth areas in the computer/communicationsarena. An increasing wealth of compu te capability isavailable in handheld systems, and improved supportfor wireless commun icatio n helps interconn ect these mo-bile platforms with each other, as well as with tether ed

Permission to make digital or hard copies of all or part of this wo rk fo rpersonal or classroom use is granted wit hou t f e e p r o v i d e d t h a t copiesa re not made or distrib uted for profi t or commercial advantage a n d t h a tcopies bear this notic e and the full citati on on the first page. To copyother wise, to republish, t O p o s t on servers o r to redistribute to lists,requires prior speci fic permission a nd/or a fee.ASPLOS X, 10/02, San Jose, CA, USA.Copyright 2002 ACM ISBN 1-58113-574-2-O2/OO10 .,.$5.O0

desktop c ompu ter s or servers. The m ain focus, of mo-bile computing has been on systems such as PDAs andtelephones inte nded for direct human use. Researchatte ntio n is increasingly focused, however, on systemswith mor e limited hu man interven tion; wireless sensornetworks are a key example. This paper e xamines theresearch decisions and im plem entat ion choices inherentin designing mobile compute /communi cation nodes forZebraNet, a wireless sensor network aimed at wildlifetracking.In general, s e n s o r n e t w o r k s a r e systems in which nu-merous compute and sensing devices are distributedwithin an envi ronment to be studied. Sensor networkshave been proposed for a ran ge of engineering, scientificand defense applications. While some sensor networkshave static sensor positions, we focus here on issues re-lated to dynamic sensor networks with mobile nodesand wireless commu nica tion between them. In fact, inour system, the sensor nodes are tracking collars carriedby the animals under study; wireless ad hoc network-ing techniques allow them to swap and store data in apeer-to-peer manner and to percolate it towards a mo-bile base station that sporadically traverses the area toupload data.An increasing focus of biology and biocomplexity re-search has been on gathering data and observations ona range of species, with a goal of unde rsta ndin g theirinterac tions and influences on each other. For example,it is important to know how human development intowilderness areas affects indi geno us species there. It isalso important to un ders tand the migration patterns ofwild animals and how they may be affected by changesin weather pa tter ns or pl ant life, by ifftroduction of non-native species, and by other influences. Learning suchdetails about animals requires both detailed long-termposition logs as well as other biometric data such asheart rate, body temperature, and frequency of feeding.Despite the importance of detailed data on animalmovements and their relationship to weather, huma ndevelopment and other pattern~, insufficient data cur-rently exists. Furthe rmore, da ta collection technologym also qmte hmlt ed. For the mot part, current; wildlifetracking studies rely on fairly simple technology. For ex-ample, man y studies rely on collaring a sample subset ofanimals with simple VHF t rans mitte rs [10]. Researchersperiodically drive through (or fly over) an area with areceiver antenna, and listen for pings from previously-collared animals. Once an animal is found, researcherscan observe its behavior and log its observed position.The limits to such studies should, however, be fairlyapparent. First, da ta collection is infrequent and maymiss many "interesting events". Second, data collectionis often limited to daylight hours, but animal behavior

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an d m o v em en t s i n n ig h t t im e h o u r s can b e q u i t e d i f f e r -en t . T h i r d an d f in a lly , d a t a co l l ec t io n is im p o ss ib l e o rsev e re ly l im i t ed fo r r ec lu s iv e sp ec i es t h a t av o id h u m anco n tac t .B e c a u s e o f t h e l i m i t a t i o n s o n s i m p l e V H F - a i d e d v i s u a lo b se rv a t io n s , m o re so p h i s t i ca t ed t r ack e r s a re s lo w ly b e -c o m i n g a v a i la b l e . T h e m o s t s o p h i s t i c a t e d t r a c k e r s c u r-r en t ly c o m m e rc ia l ly -av a i l ab l e u se g lo b a l p o s i t i o n in g sy s -t e m s ( G P S ) t o t r a c k p o s i t i o n a n d u s e s a t e l li t e up l o a d sto t ransfer data to a base s ta t ion [4 , 19 , 27] . These sys-t em s , h o w ev er , a l so su f fe r f ro m s ig n i f i can t l im i t a t io n s .T h e m o s t s o p h i s t i c a t e d t r a c k e r c u r r e n t l y a v a i l a b le o n l yk eep s a l o g o f 30 00 p o s i t i o n sa m p les a n d n o b io m et r i cd a t a [19 ]. B ecau se sa t e l l i t e u p lo ad s a re s lo w an d p o w er -h u n g ry , t h ey can o n ly b e d o n e in f r eq u en t ly. T h i s l im -i t s h o w o f t en p o s i t i o n sam p les can b e g a th e red w i th o u to v er f lo w in g th e 3 0 0 0 -en t ry lo g s to rag e . - -F u r th e rm o re ,d o w n l o a d s o f d a t a f r o m t h e s a t e l l i t e t o t h e r e s e a r ch e r sa re b o th s lo w an d ex p en s iv e ( r e sea rch e r s a re ch a rg ed b yt h e b i t ) , c o n s t r a i n i n g th e a m o u n t o f d a t a c o l l e c te d . F i -n a ll y , t h e s e s y s t e m s o p e r a t e o n b a t t e r i e s w i t h o u t s o l a rr ech a rg e , so w h en p o w er i s d ra in ed , t h e sy s t em i s u se l e s su n less i t i s r e t r i ev ed , r ech a rg ed , an d r e -d ep lo y ed .F ram in g w i ld l if e t r ack in g as a se n so r n e tw o rk s p ro b -l e m , t h e Z e b r a N e t p r o j e c t i s b u i l d i n g t r a c k i n g n o d e st h a t i n c l u d e a l o w - po w e r m i n i a t u r e G P S s y s t e m w i t h au s e r - p r o g r a m m a b l e C P U , n o n - v o l a t il e s to r a g e f or d a t alo g s , an d r ad io t r an sce iv e r s fo r co m m u n ica t in g e i th e rw i th o th e r n o d es o r w i th a b ase s t a t i o n . O n e o f t h e k eyt e n e t s o f Z e b r a N e t i s t h a t t h e s y s t e m s h o u l d w o r k in a r -b i t r a ry w i ld e rn ess l o ca t io n s ; w e d o n o t a s su m e th e p res -en ce o f f i x ed an ten n a to w ers o r ce l lu l a r t e l ep h o n e se r -v i ce . T h e s y s t e m t h e r e f or e u s e s p e e r - t o - p e e r d a t a s w a p st o m o v e t h e d a t a a r o u n d ; p e r i o d i c r e s e a r c he r d r i v e - b y s(o r fl y -o v er s ) can th en co l l ec t l o g g ed d a t a f ro m m an y an -im a l s d es p i t e en co u n te r in g r e l a t i v e ly f ew w i th in r an g e .W h i l e a d h o c s e n s or n e t w o r k s h a v e b e e n w i d e l y s t u d i e din th e ab s t r ac t , m u ch l e s s h as b een p u b l i sh ed ab o u t t h ech arac t e r i s t i c s o f m o b i l e sen so r n e tw o rk s w i th m o b i l eb ase s t a t i o n s an d r e l a t i v e ly few s tu d ies fo cu s o n b u i ld -in g r ea l sy s t em s . In p a r t i cu l a r , t h i s p ap e r o f f e r s sev e ra lu n iq u e co n t r ib u t io n s :

F i r s t , w e b e l i ev e w e a re t h e f i r s t t o s tu d y p ro to co l sfo r m o b i l e sen so r n e tw o rk s in w h ich th e " b ase"s t a t io n i s a l so m o b i l e . In o u r case , w e p resu m eth a t r e sea rch e r s w i l l u p lo ad d a t a w h i l e d r iv in g o rf ly in g b y th e r eg io n . A n d in f ac t , t h e b ase s t a t i o ni s av a i l ab l e o n ly sp o rad ica l ly , w h en r e sea rch e r s a reo u t d r iv in g a d a t a -co l l ec t io n lo o p . S eco n d , zeb ra - t r ac k in g is a d o m a in in w h ich th en o d e m o b i l i t y m o d e l s a re l a rg e ly u n k n o w n , an d infac t a r e u l t im a te ly th e r e sea rch g o a l . U n d e r s t an d -in g h o w , w h y , an d w h en zeb ras u n d er t ak e lo n g -t e rm m ig ra t io n s i s t h e m o s t p ress in g b io lo g ica lq u es t io n fo r t h i s w o rk . In e s sen ce , w e " b o o t s t r ap "m o b i l i t y m o d e l s b y u s in g cu r ren t , l e s s w e l l - r e f inedb io lo g y d a t a t o d es ig n o u r ea r ly p ro to co l s , w h ichc a n t h e n b e r e f i ne d a n d a d a p t e d a s t h e i n i t i a l d e -p l o y e d s y s t e m h e l p s u s l e a r n a b o u t z e b r a m o v e -m en t s , e sp ec i a l ly l o n g - t e rm m ig r a t io n s , i n m o red e ta i l . L ik e o th e r sen so r n e tw o rk s , Z eb raN e t ' s d a t a co l -l e c ti o n h a s s t y l i z e d c o m m u n i c a t i o n p a t t e r n s i nw h i c h d a t a c a n b e c o o p e r a t i v e l y f u n n el e d t o w a r d sa b ase s t a t i o n . We o p t im ize o u r p ro to co l s fo r t h i s" d a t a - g a t h e r i n g " c o m m u n i c a t i o n p a t t e r n a n d f o rth e h ig h d eg ree o f l a t en c y to l e ran c e in t h i s ap p l i -c a t i o n d o m a i n .

F in a l ly , w e ex am in e en e rg y t r ad eo f f s i n d e t a i l , u s -i n g re a l s y s t e m e n e r g y m e a s u r e m e n t s f o r Z e b r a N e tp r o t o t y p e h a r d w a r e i n o p e r a t i o n .In co n s id e r in g Z eb raN et , a n u m b er o f i n t e res t in g r e -sea rch q u es t io n s a r i se . H o w to m ak e th e co m m u n ica -t i o n s p ro to co l b o th e f f ec t iv e an d p o w er -e f f i c ien t ? T ow h a t e x t e n t c a n w e r e l y o n a d h o c , p e e r - t o - p e e r t r a n s -f e r s i n a sp a r se ly - co n n ec ted sp a t i a l l y -h u g e sen so r n e t -w o rk ? A n d f in a lly , h o w can w e p ro v id e co m p re h en s iv et r ack in g o f a co l l ec t io n o f an im a l s , ev en i f so m e o f t h ean im al s ax e r ec lu s iv e an d r a re ly a re c lo se en o u g h to h u -m a n s t o h a v e t h e i r d a t a l o g s u p l o a d e d d i r e c t l y ? T h i sp a p e r g i v es q u a n t i t a t i v e e x p l o r a t i o n s o f . t h e d e s i g n d e -c i s io ns b eh in d so m e o f t h ese q u es t io n s . In ad d i t i o n ,w e g i v e in i t i a l s y s t e m s e x p e r i e n ce s a n d p o w e r m e a s u r e -m e n t s f o r o u r Z e b r a N e t p r o t o t y p e . M o r e b r o a d ly , b ysu m m ar iz in g ea r ly ex p er i en ces w i th Z eb raN e t , w e f ee lth a t t h i s p ap e r o f f e r s p ro to co l i d eas t h a t sh o u ld b e r e l -ev an t t o a w id e se l ec t io n o f r e sea rch e r s i n t h e w i re l e s sa n d a d h o c n e t w o r k i n g d o m a i n.T h e r e m a i n d e r o f t h i s p a p e r i s s t r u c t u r e d a s f o ll ow s .S e c t io n 2 d e s c r i b e s t h e p r o b l e m d o m a i n a n d m e t r i c s o fi n t e r e s t i n m o r e d e t a i l . W h i l e u l t i m a t e l y b i o l o g is t s wi s hto p l ace Z eb rm N et - s ty l e n o d es o n a r an g e o f sp ec i es i nan eco sy s t em , o u r f i r s t g o a l i s t o d ev e lo p a co l l a r d e -s ig n an d p ro to co l t h a t w o rk s w e ll w i th zeb ras . F o r

th i s r easo n , S ec t io n 3 d i scu sses t h e so c i a l s t ru c tu res an dm o v e m e n t p a t t e r n s f o r z e b ra s t h a t w e u se w h e n d e s i g n -i n g p r o t o c o ls a n d r e a s o n i n g a b o u t w e l l - s u it e d m o b i l i t ym o d e l s fo r o u r ap p l i ca t io n . F o l lo w in g th i s , S ec t io n 4g iv es an o v e rv i ew o f t h e Z eb raN e t t r ack in g n o d e an dco l l a r d es ig n , S ec t io n 5 d i scu sses Z eb raN e t p ro to co l s ,an d S ec t io n 6 r ep o r t s t h e i r e f f ec t iv en ess an d en e rg y e f -f i c ien cy . S ec t io n 7 r e l a t e s o u r w o rk to o th e r p ro j ec t sin sen so r n e tw o rk s , en e rg y -e f f i c i en t m o b i l e sy s t em d e -s ig n an d o th e r d o m ain s . F in a l ly , S ec t io n 8 su m m ar ize so u r r e su l t s , d i scu sses o u r fu tu re p l an s , an d o f fer s co n -clusions.2. Z E B R A N E T D E S I G N G O A L S

T h e Z eb raN et p ro j ec t i s a d i r ec t an d o n g o in g co l l ab -o r a t i o n b e t w e e n r e s e a r c h e r s i n e x p e r i m e n t a l c o m p u t e rsy s t em s an d in w i ld l if e b io lo gy . T h e w i ld l if e b io log i s t sh av e a r t i cu l a t ed th e t r ack e r ' s o v e ra l l d es ig n g o a l s a s : G P S p o s i t i o n s a m p l e s t a k e n e v e r y th r e e m i n u t e s . D e ta i l e d ac t iv i ty l o g s t ak en fo r 3 m in u tes ev e ryh o u r 1 y ea r o f o p e ra t io n w i th o u t d i r e c t h u m an in t e rv e n -t io n . (T h a t i s , w e sh o u ld n o t co u n t o n t r an q u i l i z -in g an d r e -co l l a r in g an an im a l m o re th an o n ce p e ry ea r . ) O p era t io n o v e r a w id e r an g e (h u n d red s o r t h o u -san d s o f sq u a re k i lo m ete r s ) o f o p en l an d s . Wep l a n t o d e p l o y ou r s y s t e m a t t h e M p a l a R e s e a r c hC en t re i n cen t r a l K en y a [2 5 ] . N o f ix ed b ase s t a t i o n s , a n t en n as , o r ce l lu l a r se r -v i ce . ( A n y u n g u ard e d eq u ip m en t , l a rg e o r sm a l l ,i s t o o l i k e l y t o a t t r a c t a t t e n t i o n a n d u n f o r t u n a t e l y ,v a n d a l i s m . ) Wh i l e l a t en c y i s n o t c r i t i ca l , a h ig h su ccess r a t e fo reventually d e l iv e r in g a l l l o g g ed d a t a i s im p o r t an t . For a ze bra co l lar , a weigh t l im i t o f 3 -5 lbs is rec-o m m e n d e d . S m a l l e r a n i m a l s m a y n e e d e v en l ow e rw e ig h t l im i t s .

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T h e t h r e e - m i n u t e d u r a t i o n b e t w e e n p o s i t i o n s a m -p les is m o t iv a t ed b y b io lo g ica l r e sea rch th a t sh o w sth a t t h e in t e rv a l i s l o n g en o u g h to r eco rd s t a t i s t i ca l ly -i n d e p e n d e n t b e h a v i o r a n d y e t f r e q u e n t e n o u g h t o l o gsu f f ic i en t d a t a p o in t s o v e r t im e [1] . In ad d i t i o n , o n cep er h o u r , t h e u n i t w i l l l o g d e t a i l ed in fo rm at io n fo r ad u ra t io n o f 3 fu l l m in u tes . U l t im a te ly , t h i s d e t a i l edi n f o r m a t i o n m i g h t i n c l u de s e v e r a l p o s i ti o n e s t i m a t e s ,t e m p e r a t u r e i n f o r m a t i o n , w e a t h e r d a t a , e n v i r o n m e n t a ld a t a , a n d b o d y m o v e m e n t s t h a t w i ll s e r v e a s s i g n a t u r e so f b eh av io r ; i n o u r i n i t i a l sy s t em h e re , h o w ev er , w e fo cu sso le ly o n p o s i t i o n d a t a .O v era l l , t h e k ey g o a l i s t o d e l iv e r b ack to t h e r e -sea rch e r s a v e ry h ig h f r ac t io n o f t h e d a t a co l l ec t ed o v e rt h e m o n t h s o r y e a r s t h a t t h e s y s t e m i s i n o p e ra t i o n . A sa r e su l t , Z eb ra N e t m u s t b e q u i t e p o w er -e ff i c i en t , m u s tb e d e s i g n e d w i t h a d e q u a t e d a t a l o g s t o r a g e , a n d m u s tb e ru g g ed to en su re r e l i ab i l i t y u n d er t o u g h co n d i t i o n s .2 .1 Z e b r a N e t P r o b l e m S t a t e m e n t

H av in g s t a t ed ab o v e th e b io lo g i s t s ' d es ig n g o a l s , w en e x t t u r n t o t h e i m p l i c a t i o n s of t h o s e g o a ls o n t h e e n -g i n e e r 's t a s k a t h a n d .T h e p r i m a r y f i gu r e o f m e r i t f o r o ur d e s i g ns i s t h a tt h e s u c c e s s r a t e a t d e l i v e ri n g p o s i t i o n d a t a t o t h er e s e a r c h e r - - a m e t r i c w h i c h w e r e f e r t o a s t h e data hom-ing r a t e - - s h o u l d a p p r o a c h 1 00 % . T h e e n g i n e e r i n g r e -sea rch p ro b lem s a r i se f ro m sev e ra l i s su es .F o r ex am p le , a s sh o w n in S ec t io n 4 , w e ig h t l im i t s o ne a c h n o d e t r a n s l a t e a l m o s t d i r e c t l y t o c o m p u t a t i o n a len e rg y l im i t s . T h i s i s b ecau se th e w e ig h t o f t h e b a t -t e r y a n d s o l a r pa n e l d o m i n a t e s t h e t o t a l w e i g h t o f aZ eb raN et n o d e . A s a r e su l t , o u r co l l a r an d p ro to co l d e -s i g n d e c i s io n s m u s t m a n a g e t h e n u m b e r a n d s i ze o f d a t at r a n s m i s s i o n s r e q u i r e d . W e m u s t a l s o m a k e s y s t e m d e -s ig n ch oices t h a t l im i t t h e r an g e o f t r an sm iss io n s , s in cet h e r e q u i r e d t r a n s m i t t e r e n e r g y i n c r e a s e s d r a m a t i c a l l yw i t h t h e d i s t a n c e t r a n s m i t t e d . F i n a ll y , w e m u s t l i m i tt h e a m o u n t o f st o r a g e n e e d e d t o h o l d p o s i t i o n lo g s. A tro u g h ly 6 K B p er d ay , a s in g le an im a l ' s p o s i t i o n d a t au s e s r e l a t i v e l y l i t t l e s t o ra g e . B u t i f m a n y r e d u n d a n tc o p i es a xe s t o r e d a n d s w a p p e d , t h e s t o r a g e r e q u i r e m e n t sc a n s c a l e a s O ( N 2 ) . A l t h o u g h t h e e n e r g y c o s t o f s t o r -a g e i s s m a l l c o m p a r e d t o t h a t o f tr a n s m i s s i o n s , i t s t il lb eh o o v es u s t o d ev e lo p a s to rag e -e f f i c i en t d es ig n .B e c a u s e o f l i m i t e d t r a n s c e i v e r c o v e r a g e a n d a b a s es t a t i o n o n l y s p o r a d i c a l ly - a v a i l a b le , Z e b r a N e t m u s t f o r-w a r d d a t a t h r o u g h o t h e r n o d e s in a p e e r - t o - p e e r m a n -n e r an d s to re r e d u n d an t co p ies o f p o s i t i o n lo g s in o th e rt r ack in g n o d es . S ec t io n 5 d i scu sses o u r p ro to co l ex p er -h n en t s fo r o p e ra t in g in a sy s t em w i th m o b i l e sen so r sa n d b a s e s t a t i o n , a s w e ll a s b a n d w i d t h a n d s t o r a g e c o n -s t r a in t s .S o m e o f t h e k ey ch a l len g es in Z eb raN e t co m e f ro m th es p a t i a l a n d t e m p o r a l s c a l e o f t h e s y s t e m . I n t e r m s o ft e m p o r a l s c a l e , k e e p i ng a s y s t e m r u n n i n g a u t o n o m o u s l yfo r m o n th s a t a t im e i s ch a l l en g in g ; i t r eq u i r es s ign i f i -c a n t d e s i g n - t i m e a t t e n t i o n t o b o t h h a r d w a r e a n d s o f t-w are r e l i ab i l i t y . We a l so p l an w o rk (n o t d i scu ssed h e re )to im p lem en t o n - th e - f ly so f tw are u p d a t es w h ich w i l l f a -c i l i t a t e b u g f i x es a n d p a r a m e t e r t u n i n g a f t e r t h e c o l-l a r s a r e d ep lo y ed . In t e rm s o f sp a t i a l s ca l e , Z eb ra N e ti s a l so ag g ress iv e ; i t i s t h e sp ec i f i c i n t en t o f o u r sy s -t e m t o o p e r a t e o v e r a n a r e a o f hu n d r e d s o r t h o u s a n d so f sq u are k i lo m ete r s . B ecau se o f t h e l a rg e d i s t an ce s i n -v o l v ed a n d s p a r s e s e n s o r c o v e ra g e , e n e r g y / c o n n e c t i v i t yt r ad eo f f s b eco m e k ey .T h e c h a l l en g es an d i s su es o u t l i n ed h e re co m e to g e th e rin a sy s t em d es ig n th a t t ack les sev e ra l o p en p ro b lem s .N a m e l y , Z e b r a N e t ' s p r o t o c o l p r o m i s e s g o o d c o m m u n i -

c a t i o n b e h a v i o r o n m o b i l e s e n s o r s p e r c o l a t i n g d a t a t o -w ard s a m o b i l e b ase s t a t i o n . S eco n d , Z eb raN e t ex -p lo res d es ig n i s su es fo r sen so r s t h a t a r e m o re co a r se -g ra in ed th an m an y p r io r sen so r p ro p o sa l s . T h e l a rg e rw e ig h t l im i t s an d s to rag e b u d g e t s a l l o w u s to co n s id e rd i f f e ren t p ro to co l s w i th im p ro v ed l ev e rag e fo r sp a r se ly -co n n ec ted , p h y s i ca l ly -w id es p read sen so r s .3 . A D A Y I N T H E L I FE O F A Z E B R A

Mo b i l i t y m o d e l s a re a t t h e co re o f d es ign d ec i s io n sfo r m a n y m o b i l e n e tw o rk s . Mo b i l i t y m o d e l s h e lp toab s t r ac t h o w fas t an d h o w o f t en u se r s ( an d th e re fo re ,w i re l e s s n o d es ) m o v e , i n w h a t d i r ec t io n , an d w i th w h a tfo rces o f a t t r ac t i o n o r r ep u l s io n . L ik ew ise , t o d es ig nZ e b r a N e t , w e a l s o n e e d t o u n d e r s t a n d h o w t h e n o d e sw i l l m o v e , a s t h i s c r i t i c a l ly a f f ec ts h a rd w are , p ro to co lan d o v era l l sy s t em d es ig n . U l t im a te ly , w e w i sh to d e -p lo y se t s o f Z eb r aN e t co l l a r s o n a r an g e o f sp ec i es t h a tsh a re t h e sam e eco sy s t em : zeb ras , l i o n s , w i ld d o g s , an dev en l a rg e m am m als su ch as e l ep h an t s . T h i s a l l o w s b i -o l o gi s ts t o g a t h e r f u n d a m e n t a l i n t e r -s p e c i e s d a t a t h a t i scu r ren t ly w o efu l ly l ack in g . F o r t h i s p ap e r , h o w ev er , w efo cu s o n zeb ras . We in c lu d e th i s sec t io n to g iv e sp ec i f i c sa b o u t z e b r a m o t i o n a n d s o c i a l s t r u c t u r e t h a t i m p a c t o u rsy s t em d es ig n ch o ices .3.1 Social Structure and C ol laringA p p r o x i m a t e l y 35 ,0 00 z e b r a s r a n g e w i d e l y o ve r t h e4 0, 00 0 s q u a r e k i l o m e t e r s t h a t c o m p r i s e t h e L a i k i p i ae c o s y s t e m of c e n t r a l K e n y a . U n d e r s t a n d i n g h o w t h e yu s e t h e l a n d s c a p e r e q u i r e s c o l l a r i n g r e p r e s e n t a t iv e i n d i -v i d u a l s a n d c h a r a c t e r i z i n g t h e i r f i n e - g r a i n e d m o v e m e n t san d b eh av io r s o v e r l a rg e sca l es . F o r tu n a te ly , t h e so c i a ls t r u c t u r e o f s o m e z e b r a s p e c i e s e n a b le s u s t o c o l la r o n l ym a l e s a n d y e t s t i ll g a t h e r i n f o r m a t i o n o n t h e r a n g i n gb e h a v i o r o f l a r g e s u b s e t s o f t h e p o p u l a t i o n .T w o s p e c i e s o f z e b r a s i n h a b i t t h e L a i k i p i a e co s y s-t e m . O n e , t h e G r e v y ' s z e b r a ( E q u u s g r e v y i ) f o r m s l ar g el o o s e l y - b o n d e d h e r d s . T h e o t h e r , m o r e c o m m o n , P l a i n szeb ra (E . b u rch e l l i ) fo rm s t i g h t -k n i t u n i -m a le , m u l t i -f em ale b reed in g g ro u p s . T h ese so -ca l l ed "h a rem s" a rech a rac t e r i zed b y 4 -5 f em ales an d th e i r y o u n g o f f sp r in gl iv in g in c lo se a s so c i a t io n w i th a s t a l l i o n fo r l o n g p e r i -o d s o f t im e , o f t e n m a n y y e a r s. F e m a l e s t y p i c a l l y i n i t i a t em o v e m e n t s b u t t h e m a l e o f t e n a d j u s t s t h e d i r e c t i o n a n dsp eed o f m o v e m en t o f t h e g ro u p [34 ]. T h u s b y co l l a r -i n g o n ly t h e m a l e w e c a n e f f ec t i ve l y t r a c k t h e m o v e m e n to f 10 -12 in d iv id u a l s , v as t ly r ed u c in g th e n u m b er o f co l -l a r s r e q u i r e d a s w e t r y t o c h a r a c t e r i z e t h e m o v e m e n t so f e n t ir e p l a i n s z e b r a p o p u l a t i o n s .A l t h o u g h p l a i n s z e b r a s l i v e in t i g h t - k n i t b r e e d i n gg ro u p s , t h ese g ro u p s o f t en co a l esce an d fo rm m o d er -a t e l y s t a b l e l o n g - t e r m h e r d s . T y p i c a l l y h a r e m g r o u p sco a lesce in to h e rd s a t w a te r in g p o in t s b e fo re em b ar k -in g o n m o v e m en t s t o n ew g raz in g g ro u n d s . E n ro u te ,h a r e m s s o m e t i m e s j o i n o r l e a ve t h e se h e r d s d e p e n d i n go n t h e s t r u c t u r e o f t h e h a b i t a t , t h e q u a l i t y o f t h e v e g e -t a t i o n a n d t h e c o m p o s i t i o n o f i n d i v i d u a l h a r e m g r o u p s[2] . C lea r ly , h e rd s a re m o re a m o rp h o u s th an th e sm a l l e rh a r e m g r o u p s , b u t t h e y l a s t l o n g e r t h a n a m e r e t e m p o -rax y ag g reg a t io n . S u ch d y n a m ics p resen t a ch a l l en g -in g p ro b le m to eco lo g i s t s t ry in g to u n rav e l t h e i r cau ses ,b u t w i ll a c t u a l l y a s s is t Z e b r a N e t i n p r o p a g a t i n g p o s i -t i o n lo g s ac ro ss t h e l an d scap e to w ard s a m o b i l e b ases t a t i o n .3 .2 Mo v e m e n t P a t t e rn s

Z e b r a m o v e m e n t c a n b e c h a x a c t e r i z e d i n t e r m s o fth ree m a in s t a t e s : g raz in g , g raze -w a lk in g , an d f a s t -m o v in g . Z eb ras sp en d m o s t o f t h e i r t im e g raz in g , b o th

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G R A Z I N G G R A Z E -W A L K I N G F A S T M O V I N G

F i g u r e 1 : T h r e e - t i e r e d m o b i l i t y m o d e l .1 8 . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . - ' 3

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3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 67 36 41 43 45 47 49net dis tance t reve led (meters)

F i g u r e 2 : D i s t r i b u t i o n o f z e b r a m o v e m e n t s o b -s e r v e d b y f i e l d b i o l o g i s t s .

d ay an d n ig h t . Z eb ras p re fe r t o g raze in a reas o f sh o r tb u t r ap id ly -g ro w in g g rasses . T h ese a re as o f fe r h ig h en -e rg e t i c g a in s an d lo w r i sk s o f p red a t io n . Wh i l e g raz -in g o n sh o r t g rass sw ard s , zeb ras t y p ica l ly ex h ib i t l o wm o v e m e n t r a t e s a n d h i g h t u r n i n g a n g le s .A t o th e r t im e s , zeb ras w a lk d e l ib e ra t e ly , w i th h ea d slo w ered , c l i p p in g v eg e ta t io n as t h ey m o v e . T h ese l a t t e rm o v em en t s a re r e fe r r ed to a s " g raze w a lk in g " an d a rec h a r a c t e r i z e d b y h i gh e r s t e p r a t e s a n d s m a l l e r t u r n i n gan g les t h an th o se fo r fo cu sed b o u t s o f g raz in g .F i n a l ly , e i th e r d u e t o p r e d a t o r s o r b e c a u s e a n a r e a ' sv eg e ta t io n h as b een ex h au s t ed , zeb ras w i l l o ccas io n a l lym o v e m u ch m o re q u ickly , fo r l o n g er d i s tan ces , w i th t h e i rh ead s r a i sed b ecau se th ey a re n o t g raz in g . We ca t eg o -r i z e t h is a s t h e f a s t - m o v i n g s t a t e .F i g u r e 1 i l l u s t r a t e s t h e s e t h r e e m o d e s o f z e b r a m o v e -m e n t a b s t r a c t l y , w i t h t r a n s i t i o n p r o b a b i l i t i e s b e t w e e nt h e m . T h e s p e e d d i s t r i b u t i o n s i n e a c h m o d e a n d t h ep r o b a b i l it i e s of t r a n s i t i o n i n g b e t w e e n e a c h s t a t e a r e d e -r iv ed th ro u g h f eed b ack f ro m b io lo g i s t s a s d esc r ib ed b e -low.

D i s t a n c e M o v e d . F i g u r e 2 s h ow s z e b r a m o v e -m en t d a t a co l l ec t ed b y f ie ld b io lo g i s t s [2] . T h e h i s -to g ram sh o w s h o w o f t en d i f f e ren t n e t m o v em en t s w ereo b se rv ed . E ach d a t a sam p le i s n e t d i s t an ce m o v ed in ath ree - m in u te in t e rv a l s in ce th e l a s t o b se rv a t io n . (T h eth ree -m in u te i n t e rv a l i s ch o sen b ased o n em p i r i ca l b io -lo g ica l s tu d ies t h a t sh o w i t s su i t ab i l i t y for s t a t i s t i ca l ly -v a l id sam p l in g o f an im a l m o v em e n t s [1] .) We d e f in e n e td i s t a n c e m o v e d a s t h e n e t d i s t a n c e f r om t h e b e g i n n i n go f t h e th ree -m in u te i n t e rv a l t o t h e en d . T h a t i s , i f a ze -b ra m o v ed t en m e te r s f ro m i t s o r ig in a l p o s i t i o n an d t h encam e b ack ag a in , a l l i n t h ree m in u tes , i t s n e t d i s t an cem o v ed w o u ld b e ze ro .B e c a u s e t h e d a t a w a s c o l l e c t e d b y a s t a t i o n a r y o b -se rv e r , F ig u re 2 ' s d a t a i n c lu d es m o s t ly g raz in g an d g razew alk in g o b se rv a t io n s . T h e tw o ty p e s o f m o t io n can b ed i s c e r ne d b y t h e b i m o d a l n a t u r e o f t h e d i s t r i b u t i o n .T h e f i r s t m o d e , g raz in g , h as a h i s to g ram p eak g rap h eda t 2 m an d a m ean n e t -d i s t an ce o f 3 . 1m . T h e seco n dm o d e , g raze -w a lk in g , r an g es f ro m 1 0 -2 0 m , h as a p eakg r a p h e d a t a p p r o x i m a t e l y 1 4 m , a n d h a s a m e a n v a l u eo f 1 3 .0 m . T h e f ew o u t l i e r s i n t h e d i s t r i b u t io n in d ica t ep o in t s w h ere th e zeb ra m ay h av e sen sed d an g er an d f l ed .O v era l l , i t i s c l ea r t h a t zeb ras t en d to m o v e v e rys lo w ly ; a s t h ey sp en d m o s t o f t h e i r t im e s im p ly g raz in g,

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0 5 10 15 20 25 30 ~ 40 45 50 55 60 65 70 75 80 85 90 95 1(~0 105 110 115 120net angle turned (degrees)

F i g u r e 3 : D i s t r i b u t i o n o f z e b r a t u r n i n g a n g l e so b s e r v e d b y f i e l d b i o l o g i s t s .

t h e i r n e t d i s t a n c e m o v e d t e n d s t o b e v e r y s m a l l . T h i sh in t s t o u s t h a t r o u t in g p ro to c o l s w h ich in t e l l i g en t ly ex -p lo i t p as t l i n k h i s to ry in fo rm at io n m ay b e f ru i t fu l .T u r n i n g A n g l e . A n o t h e r f a c e t o f m o v e m e n t is d i -r ec t io n . F ig u re 3 g iv es f i e ld d a t a o n n e t t u rn in g an g le .S im i l a r t o n e t d i s t an ce m o v ed , t h e n e t t u rn in g an g lei s d e f in ed as t h e ab so lu t e v a lu e o f t h e an g le b e tw eent h e s t a r t o f t h e t i m e i n t e r va l a n d t h e e n d o f t h e t i m ein t e rv a l . I f t h e zeb ra m o v e d 3 60 d eg rees w i th in th reem in u tes , i t s n e t t u rn in g an g le w o u ld b e ze ro . T h e m ax -im u m tu rn in g an g le is t h e re fo re 1 8 0 d eg rees . We u set h e s e d i s t a n c e a n d d i r e c t io n h i s t o g r a m s t o g u i d e m o b i l -i t y m o d e l s fo r s im u la t io n s in S ec t io n 6 .W a t e r S o u r c e s a n d D r i n k i n g . Z e b ra s a r et e r m e d a w a t e r - d e p e n d e n t h e r b i v o r e b e c a u s e t h e y s e e ko u t w a te r t o d r in k o n a d a i ly b as i s . A g a in b ased o n o b -s e r v a ti o n s , o u r m o b i l i t y m o d e l s a s s u m e t h a t z e b r a s h e a dfo r w a te r so u rces ab o u t o n ce p e r d ay . O n ce th e re , t h eyd r in k r e l a t i v e ly q uick ly . A n d o n ce th e i r t h i r s t i s s a t i -a t e d , t h e i r m o v e m e n t i s a g a i n i n d e p e n d e n t o f t h e w a t e rs o u rc e u n t i l t h e n e x t d a y. W e a s s u m e i n o u r m o d e l st h a t t h e s o u r c es o f w a t e r a r e r a n d o m l y d i s t r i b u t e d , a n dt h a t t h i r s t y z e b r a s c a n e a s i l y ( b u t n o t i n s t a n t l y ) f i n dth e i r w ay to an ad eq u a te so u rce .S l e e p . Z e b r a s t e n d n o t t o h a v e l o n g p e r i o d s o fm o t io n less s l eep . U n l ik e ca rn iv o res , w h ich a re eq u ip p edw i th s ig n i f i can t d e fen se m ech an i sm s , zeb ras r e ly o nk eep in g w a tch an d f l ee in g f ro m p red a to r s . T h ere fo reo u r m o d e l s a ss u m e t h a t z e b r a s m a i n t a i n t h e i r m o b i l i t y

p a t t e rn 2 4 h o u r s a d ay .4 . C O L L A R DESIGN

F ig u re 4 sh o w s a p h o to g ra p h o f t h e co re o f a Z e -b r a N e t p r o t o t y p e n o d e: t h e e v a l u a t i o n b o a r d fo r t h eG P S - M S I E ( c o n t a in i n g a G P S , F l a s h R A M , a n d C P U ) ,a sh o r t r an g e r ad io , an d a l o n g r an g e r ad io w i th i t sp ack e t m o d em . (T h e p h o to d o es n o t sh o w th e p ack ag -in g , b a t t e r i e s , so l a r a r r ay , an d p o w er m an ag em en t c i r -cu i t s . ) T h i s sec t io n g ives an o v e rv i ew o f t h e t r ac k in gco l l a r n o d e d es ign . T h e b lo ck d i ag ra m in F ig u re 5 i ll u s -t r a t e s t h e d i f f e r e n t c o m p o n e n t s a n d t h e i r i n t e r a c t i o n sw i th o n e an o th e r .T o m in im ize th e p a r t co u n t an d o v era l l s i ze an dw eig h t o f t h e sy s t em , w e u se a s in g le -ch ip m in ia tu reG P S s o l u t i o n f r o m p B l o x : G P S - M S 1 E [ 5 ] . T h e G P S -MS 1 E i s a 1 2 -ch ax mel G P S rece iv e r cap ab le o f g e t t i n g ap o s i t i o n u p d a t e e v e r y s e c o n d ( t h o u g h w e g e t t h e m l e s sf r eq u en t ly ) . I t h as an in t eg ra t ed 2 0 Mh z H i t ach i S H 13 2 -b i t m ic ro p ro ces so r a s w e l l a s I /O su p p o r t . We u seth e S H 1 fo r d a t a cap tu re an d p ro to co l co n t ro l ; i t i s t h eo n l y p r o g r a m m a b l e C P U i n t h e Z e b r a N e t no d e . T h eG P S - M S I E a l s o h a s a b u i l t i n 1 M B F l a s h R A M m o d -u le ; 6 4 0 K B i s av a i l ab l e fo r u se r d a t a w h i l e t h e r e s t i su sed to s to re t h e f i rm w are .U s i n g t h e G P S - M S I E ' s m i c r o p r o c e s s o r , w e p e r i o d i -c a l l y o b t a i n t h e p o s i t i o n c o o r d in a t e s a n d s t o r e t h e m i n

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1

- F ig u r e 5: B l o c k d i a g r a m o f Z e b r a N e t n o d e d e -s i g n .

its on-board flash RAM. GPS readings are accurate towithin 5-10m; this is more than sufficient for our pur-poses. Assumin g tha t we store 30 coordinates per hour,each hour requires a little over 240 bytes of space. Thisimplies that 640KB of storage is equivalent to approx-hnate ly 110 collar-days worth of data. Further more,we plan to compress the data by representing mostof the coordinates as offsets from two reference pointsper hour. Assuming a compression rate of abo ut 36~ ,640KB of Flash is then capable of storing 300 collar-days of data.The processor also coordinates the communicationsover the two radios. We chose to use two radios so wecan have broa d control over tradeoffs in energy vs. com-munic atio n range. First, the Linx Technologies SC-PAseries [18] is a data radio wit h a r ang e of only 100 me-ters but very low power consumption. Second, we usea slow but higher-power data radio and packet modemfor longer-range (Skm) transfers. The sho rt-ran ge radiois power-efficient for peer transfe rs when zebras are con-gregating by water sources, while the longer-ra nge radiois necessary for commu nica ting to the base stati on overthe large area studied with relatively few tracking col-lars.Shor t Ra nge Rad io Pro toc o l . While the Pico-Packet packet mode m handles error correction, collisiondetection, and packetization for data sent on the longrange Tekk data radio, the same for data transmissionover the Linx radio must be performed by the ZebraNetfirmware. Short range radio packets have a maxi mumsize of 300 bytes and a 16 bit CRC provides error check-

ing. T he Linx radio also requires a MAC protocol sincenone is provided in the hardware. While malay stan-dard protocols such as Aloha, Slotted Aloha, CSMA,and MACA [16] are available, ZebraNet has require-ments and resources that differ from typical wirelessad-hoc networks. Whe n doing peer search, collars mustavoid collisions by selecting designated senders one-a t-a-time. Fortunat ely, we can implemen t a unique collision-avoidance protocol that takes advantage of the fact th atGPS gives our networked system an extremely accurateand precise synchroniz ed clock. (The system has net-worked timing with 30-50ns precision and 30ns RMSaccuracy.) By broadcasting peer to peer search queriesin non-overlapping predetermined time slots that repeatevery 10 seconds or so, we can elim inat e collisions. Th eminim um length of the time slots is dictated by theavailability of CPU time and the time needed to switchbetween receive and tr ans mit modes on the Linx radios.CPU availability is an issue since the GP S-MS1 E C PUis also run nin g time-critic al tasks related to GP S tra ck-ing. Because of this, we work with 100-200ms time slotsin our ini tial devices. Thi s gives us collision-free opera -tion for 50 or 100 collars, with 200ms and lOOms timeslots respectively.Wi re l e s s N e t w o rk i n g A l t e rn a t i v es . F req uencyrange regulations affect the choice of radio for ourprotot yping purposes. For example, there are high-performance radios manufactured by MaxStreaan thatoperate in the 900Mhz and 2.4Ghz ISM bands that arelicense-free in the US [22]. In Kenya , however, we wouldhave to use the shorter-range (and only very recentlyavailable) 2.4Ghz unit, the 24xstream. Nevertheless, ithas a range of up to 4kin line of sight with low powerconsumption (1.2W transmi tting, 0.3W receiving). Be-cause of its power advantages, we may switch in the nea rfuture to using this radio for long-range transraissions.Finally, we also considered using an OEM wirelessEthe rne t (802.11b) module [35] instead of the short-range radios. The potentia l advantages of 802.11bwould be very high data throughput and the abilityto abst ract away details of wireless commu nica tion in-cluding collision detection and avoidance, error detec-tion, etc. There are disadvantages, however. The GPS-MS1E's serial ports only support speeds up to 33.8Kbps.This becomes such a severe bottleneck that unless wechoose another I/O method, we lose most of the speedgain of Ether net. Wit hou t the speed gain, our power re-quirem ents would go up by 16-25X per un it dat a tr ans-ferred [35]. We could solve the I /O bottlenec k by addinga separate microcontroller and storage, instead of rely-ing on those provided by pBlox, but this would furtherincrease the energy requirements of an Ethe rnet-basedchoice and also would increase per-node hardwar e costs,size, an d complexity.Ener gy Is sues and P ower Supp ly . Table 1 g ivescurrent cons umption of the ZebraNet node operatingin different modes. All figures in the tab le are cu rrentdrains on the 3.6V power supply. The cu rrent figures arebased on actual lab measurements of the current con-sump tion of indivi dual devices, but the aggregate cur-rent dr ains on the 3.6V supply for each mode were calcu-lated assuming the use of 70% efficient DC-DC yoltageconverters with the appropriate output voltages power-ing devices tha t run on voltages higher tha n 3.6V. (T hecollars require DC-D C voltage converters with regulate doutputs , especially for the long-range radio whose am-perages are highly variable.)Current drains range from a low of less than lmAwhen the system is in stand-by mode (most of the time),to a high of 1.622A when the system is traammitting

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C o l l a r S t a t eS t a n d - b yP o s i t io n S a m p l i n g a n d S t o r a g eP e e r D i s c o v e r y / T r a n s f e r O n l yB ase D i sco v ery O n lyS i m u l t a n e o u s P e e r a n d B a s e S e a r chT r a n s m i t t i n g D a t a t o B a s e

D e v i c e a n d M o d eA llG P S - M S I E , A c t i ve A n t e n n aG P S - M S 1 E + S h o r t - r a n g eG P S - M S 1 E + L o n g - r a n g e ,G P S - M S 1 E + S h o r t - r a n g e + L o n g - r a n g eG P S - M S 1 E + L o n g - r a n g e

C u r r e n t d r a i no f 3 . 6 V s u p p l y< l m A1 7 7 m A1 7 7 m A4 3 2 m A4 6 9 m A1 6 2 2 m AT a b l e 1 : E n e r g y m e a s u r e m e n t s f o r a Z e b r a N e t n o d e i n d i ff e r e nt s t a t e s o f o p e r a t i o n .

I t e mp B l o x G P S - M S I E S i ng le - ch i p G P S / C P UL i n x S C - P A S h o r t - r a n g e R a d i oL o n g - r a n g e R a d i o a n d P a c k e t M o d e m1 4 S o n y L i th iu m -I o n P o ly m e r C e l l s: (U P 5 0 3 7 5 9 A H ) 3 . 7v , 1 A H ce l l sS o la r A r r ay - U n i so [a r U S F 5 f l ex ib l e 5 w a t t

8 g ram s2 0 g ram s296 g rams540 g rams

T o t a l 1 , 1 5 1 g r a m s ( 2 . 5 4 l b s) [T a b l e 2 : W e i g h t m e a s u r e m e n t s f or d i f fe r e n t c o m p o n e n t s o f a Z e b r a N e t n o d e .

u s in g it s p ack e t m o d em an d lo n g - ran g e rad io . O u r g o a li s t o h av e a p o w er su p p ly sy s t em in w h ich th e b a t t e r y i sr ech a rg ed f ro m a so l a r a r r ay , b u t i n w h ich th e b a t t e rycan o p er a t e t h e s y s t em fo r 5 fu l l d ay s b e tw een r ech a rg esi f n eed ed . We co n se rv a t iv e ly a s su m e th a t i n t h o se 5 d ay swe wil l do the fo l lowing :

3 0 p o s i t i o n sam p les p e r h o u r , 2 4 h o u r s ev e ry d ay. 6 ( t o t a l ) h o u r s p e r d ay o f sea rch in g for p ee r n o d esa n d t r a n s f e r r i n g d a t a b e t w e e n t h e m o v e r l o w -p o w er sh o r t - r an g e r ad io . 3 h o u r s o f sea rch in g for t h e m o b i l e b ase s t a t i o n u s -in g th e lo n g - ran g e r ad io p e r d ay . T o sav e en e rg y ,th e 3 h o u r s o f b ase s t a t i o n sea rch o v e r l ap in t im ew i th th e 6 h o u r s o f p ee r sea rch an d p ee r t r an s -f e r b ecau se in b o th m o d es , t h e r e l a t i v e ly p o w erh u n g r y C P U m u s t b e o n a n y w a y . 6 40 k i l o b y t e s t r a n s m i t t e d t o m o b i l e b a s e s t a t i o nd u r in g 5 d ay p e r io d

T o o p e r a t e w i t h t h e a b o v e a s s u m p t i o n s , w e n e e d a1 3. 5 A m p ere -h o u r b a t t e ry w i th a v o l t ag e g rea t e r t h an o req u a l t o 3 . 6 v o l t s . A r ead i ly -av a i l a b l e , easy - to -u se l ead -a c i d b a t t e r y w i t h a p p r o p r i a t e c a p a c i t y w o u ld w e i gh f o u rp o u n d s . S in ce th i s i s t o o h eav y , w e a re o p t in g in s t eadfo r L i th iu m - io n p o ly m er ce l l s , w h ich h av e th e h ig h es ten e rg y d en s i ty ev en am o n g l i t h iu m io n ce ll s . A s in-d i ca t ed in T ab le 2 , t h e r eq u i r ed en e rg y cap ac i ty w i thth i s b a t t e ry t ech n o lo g y w i l l w e ig h ab o u t 2 8 7 g ram s o rab ou t 0 .631bs. Tab le 2 sum ma riz es the weigh ts for a l lt h e k ey co m p o n en t s i n a Z eb ra N e t n o d e . A t t h i s p o in t ,t h e h eav ie s t s ing le co m p o n en t i s t h e f l ex ib l e am o r p h o u ssi l icon so lar cel l arr ay [37]. At 540 g ram s (1 .181bs) , i tco n t r ib u tes ab o u t h a l f o f t h e to t a l co l l a r w e ig h t . (R ig idso la r ce l l a r r ay s w o u ld b e ch eap er , l i g h t e r an d h av eg rea t e r p o w er g en e ra t io n e f f i c ien cy, b u t f l ex ib le am o r -p h o u s s i l i c o n a r r a y s a r e b e t t e r a t w i t h s t a n d i n g r u g g e den v i ro n m en t s . )

C u r r e n t S t a t u s . W e h a v e b u i l t t w o p r o t o t y p eco p ies o f Z eb raN e t n o d es , w h ich a re cu r re n t ly o p e ra -t i o n a l i n t h e l ab . In p a r t i cu l a r , t h ey can n o w au to m at -i c a l ly s a m p l e G P S c o o r d i n a t e s a n d s t o r e t h e m i n F l a s hR A M . I n a d d i t i o n , t h e y c a n u s e t h e s h o r t - r a n g e w i r e -l e s s r ad io to sea rch fo r pee r s , an d to ex ch an g e d a t a w i than o th e r co l l a r .

5. P R O T O C O L D E S I G NT h e g o a l i n Z e b r a N e t i s t o g a t h e r d a t a c o l l ec t e d a teach co l l a r b ack to t h e b a se s t a t i o n . S in ce n o t ev e ry co l -l a r i s w i t h i n r a n g e o f t h e b a s e s t a t io n , d a t a c a n n o t b esen t d i r ec t ly . In s t ead , i t h as t o h o p i t s w ay to w ard s th eb ase s t a t i o n , u s in g o th e r co l l a r s a s i n t e rm ed ia t e h o p s .I n Z e b r a N e t , a l l n o d e s e x c e pt t h e b a s e s t a t i o n a r e d a t aso u rces , w h i l e t h e b ase s t a t i o n a lo n e i s a d a t a s in k . T h i s" d a t a g a t h e r i n g " t r a i t c o n t r a s t s w i t h t h e g e n e r a l e n d -t o -e n d c o m m u n i c a t i o n p r e v a l e n t i n m a n y w i r e d a n d w i r e -l e s s n e tw o rk s , w h ere ev e ry n o d e can b e a so u rce an d /o rs ink .In ad d i t i o n , Z e b raN e t n o d es a re m o b i l e . T h e n o d esm o v e a r o u n d a l m o s t c o n s t a n t l y ( a l b e i t sl o w ly ) . T h eb ase s t a t i o n i s a l so m o b i l e , d ep en d in g o n th e ro u te t ak enb y r esea rch e r s i n t h e i r v eh ic l e s . F u r th e rm o re , t h e b ases t a t io n i s o n ly ac t iv e so m e o f t h e t im e , w h en r e sea rch e r s

a r e d r i v i n g a r o u n d g a t h e r i n g d a t a . I n t h e d u r a t i o n t h a ta b ase s t a t i o n i s i n ac t iv e , t h e n e tw o rk es sen t i a l l y h asn o k n ow n d e s t i n a t i o n w h e r e d a t a s h o u l d b e s e nt . T h e s ech arac t e r i s t i c s , co u p led w i th th e h ig h l a t en cy to l e ran ceo f Z eb raN e t , ca l l for sp ec i a l i zed p ro to co l s .5.1 Flooding Protocol

A s i m p l e a p p r o a c h t o m o v e d a t a b a c k t o t h e b a s es t a t io n i s t o f l o o d d a t a t o a l l n e ig h b o rs w h en ev er t h eya re d i sco v ered . F ig u re 6 sh o w s th e p seu d o -co d e fo r t h ef lo o d in g p ro to co l . I f t h e n o d es m o v e ex ten s iv e ly an dm ee t a f a i r n u m b er o f o th e r n o d es , t h en g iv en en o u g ht i m e , d a t a w i l l e v e n t u a l l y m i g r a t e b a c k t o t h e b a s e . I nth i s w ay , a h ig h p e rcen tag e o f t h e d a t a ev en tu a l ly m ak esi t b ack to b ase .T h e b ase s t a t i o n d o es n o t n ecessa r i l y h av e to co m ein to co n tac t w i th a l l t h e n o d es in t h e sy s t em ; in s t ead ,co m in g in to co n tac t w i th j u s t a f ew n o d es m ay b een o u g h . In d eed , i t can b e in fe r r ed th a t b y id en t i fy in ga f ew h ig h ly - in t e rac t iv e n o d es , i . e . n o d e s t h a t m e e t al a rg e n u m b er o f o th e r n o d es , w e can co l l ec t a su b s t an t i a la m o u n t o f d a t a r e a d il y .W h i l e f l o o d i n g c a n p o t e n t i a l l y r e t u r n t h e h i g h e s t s u c -cess r a t e i n a p ee r - to -p ee r n e tw o rk , t h e l a rg e am o u n t o fd a t a f l o o d ed th ro u g h th e n e tw o rk can l ead in so m e s i t -u a t i o n s t o e x o r b i t a n t d e m a n d s f o r n e t w o r k b a n d w i d t h ,s to rag e cap ac i ty , an d en e rg y .

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i. At each scan for neighbors,2. if node is withi n range of the base station,3. send data to base station;4. delete this data, since it has successfully

reached the base station;5. else6. send data to all neighbors;

F i g u r e 6 : Ps eu d o - co d e fo r t h e f l o o d i ng p ro t oco l .1 . At each scan fo r ne ighb ors ,2 . i f node i s wi th i n range of the base s ta t i on ,3 . send da ta to base s ta t io n ;4 . d e l e t e t h i s d a t a , s i n c e i ~ h a s s u c c e s s f u l l y

r e a c h e d t h e b a s e s t a t i o n ;5 . i n c r e m e n t h i e r a r c h y l e v e l ;6. e l s e7 . check h ie ra r chy l e v e l s o f n e i g h b o r s ;8 . s e n d d a t a t o n e i g h b o r w i t h h i g h e s t l e v e l ,b r e a k i n g t i e s r a n d om l y ;9 . decay h ie ra r chy leve l a f te r D scans ;

Fi g u re 7 : Ps eu d o -co d e fo r t h e h i s t o ry -b as ed p r o -t o c o l .

5.2 History-based protocolRather than flooding data to all neighbors, we alsoconsider a simple protocol tha t intelligently selectsnodes to send to based on prior communication pat-terns. Naturally, a good targe t no de is one that will ul-timately relay the data to the base station. Our history-based protocol encodes the likelihood of a node beingin range with the base station by assigning each nodea hierarchy level based on its past success at transfer-ring data to the base station. The higher the level, thehigher the probability that this node is within range ofthe base station. The intuition behind this is that nodesthat were previously within range of the base stationwill still be close by, so they will be able to relay thedata back to the base sta tion either directly (if theyare still in range) or indirectly through minimal othernodes. This protocol th us biases the selection of a node

based on history.Each node re member s its own current hierarchy level.Each time a node scans for neighbors, it requests the hi-erarchy level of all its neighbors. It th en sends the datait has collected to the neighbor with the highest hier-archy level, with ties randoml y broken. Whe n a nodecomes within range of the base station, its hierarchylevel gets increased. Conversely, when a node is out-of-range from the base station, its hierarchy level getsdecayed over time at a rate of one level per every Dscans. Tha t is, if D is 5, we decrement the hierarchylevel by 1 every 5 consecutive scans where it is beyo ndthe base stat ion' s range. At the start, all nodes startoff at t he sa me lowest hierarch y level of zero. The pseu-docode of the proposed protocol is shown in Figure 7.Clearly, the success of the history-based unicast rout-ing protocol depends on t he mobility of the base stati onand nodes. If the network changes very dynamically, anode that was previously near the base station may nolonger be the best communication target. Then, theproposed protocol may mis-direct traffic frequently andget a poor homing success rate.6 . E X PE R I M E N T A L R E S U L T S

In this section we will describe our simu lation environ-ment and our protocol evaluation. We first present dataon the ideal case; from here, we will constrain two major

factors--s torage and b andw idth- -and then present en-ergy tradeoffs betwe en different protocols. Finally , wesimulate our proposed design and show the results--success rate and energy consump tion- -of our design.6.1 ZNetSimArmed with facts and field observations about zebrabehavior and reasonable assumptions of the terrain andoperating characteristics of the Mpala Research Cen-ter in Kenya, we construc ted a zebra mobilit y modeland simulation enviro nment for Zebra_Net. Our simula-tor, ZNetSim, takes user-defined storage and ba ndwi dthconstraint s, and retu rns two metrics: (i) success rate,which is the percentage of data that gets back: to base,and (ii) energy consumption. We developed ZNetSim inC, and it currently stands at 5941 lines of code.Mob il i ty Mode ls . At the s tart of each s imula-tion, we randomly place 50 zebras and 10 water sourcesacross a 20kmX20km map. As this is savanna, thereare no major mount ains or canyons that might hinderherd movements, animal interactions, or networking in-teractions, so we assume unobstr ucted communications.Once the zebras and wat er sources are placed, t:he map isset into motion. Th e zebra movements are based on thethree-tier mobil ity model shown in Figure 2. Each ze-bra independently selects speed and turni ng angles suchthat aggregate thre e-minute movements match the dis-trib ution s in Figures 2 and 3. Unless otherwise stated,the zebras move at a base speed of 0.017m/s whe n graz-ing, four times faster at 0.0723m/s when graze-walking,and nine times faster at 0.155m/s when fast-moving.Communication events are simulated on 30 second gran-ularity.Once per day at a random time, the simulated ze-bras become "thirsty." Wh en thirsty, a zebra movesas if in "graze walking" mode---i.e, faster and moredeliberately--towa rds the nearest watering hole. (Wepresume that they know the location of the nearest wa-tering hole from any point on the simulated grid.) Fi-nally, since field data indicates that zebra movementstend to be similar 24 hours per day, our simulator treatsnighttime the same as da yti me- -an endless cycle of eat-ing and walking. While predator s do range across theareas under study, the zebra mortality rates due topredators are low enough that we ignore them for thesesimulations.We compared ZNetSim's mobility model with that ob-served by biologists and found our di strib ution to matchalmost exactly with Figures 2 and 3, with the discre pan-cies being simply round ing error. The ba se station itselffollows a rectangu lar route from (5kin, 5kin) to (15kin,15kin) in the 20kin by 20kin map. The base moves threehours per day, between 2 p.m. and 5 p.m., and movesat 8m/s, or roughly 30km/h. Once three hours are up,it goes off-line immediately, but restar ts the next dayfrom this same location.Simu la t i on Metho dolog y . Our s imulat ions con-sist of four communica tion phases tha t occur within 30minutes every two hours, i . e . from 12:00-12:30, 2:00-2:30, 4:00-4:30, etc, over an enti re month. This time lineis arrived at due to the power constraints discussed inSection 4, as collars are limited to six hours per dayof searching for peers and tran sferri ng data. The fourphases are:

. Peer Discovery: All nodes first enter a mode wherethey use their short-range receivers to search forneighbors within range.a Base Discovery: Likewise, nodes with a separatelong-range radio will query to see if they are within

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range of the base station. Since the nodes do notknow when the base station will be available, basediscovery is done from noon till midnight everyday. This is typically overlapped with Peer Dis-covery to save power. Peer Transfer: Upon finding one or more nodeswithin range, one collar initiates data transfers.Once this node has finished its transfer, anothernode begins, till the en d of the 30 minutes. The or-dering of these collar selections is random in our

simulator, but future protocols may try to opti-mize this order. Base Transfer: After successfully finding the basewithin range, collars upload all stored data to thebase station. With our long range radio, we as-sume that total bandwidth can be shared, so allradios within range of the base can transfe r at thesame time, dividing the ban dwid th equally. Oncethe data entries are transferred, they are deletedfrom the collar to free up storage.

We assume peer and base discoveries take 30 secondsand peer and base transfers are dependent on the avail-able bandwidth and the amount of data to be trans-ferred. In all transfers, nodes send their own data firstbefore forwarding other nodes' data. Once the 30 min-utes communication interval is up, all discoveries andtransfers immediately cease. Unless otherwise men-tioned, we use a single radio in all simulations. T his letsus more clearly illustrate the effect of radio range on net-work performance. Finally, we note that for simplicitywe ignore the irregular and asymmetric characteristicsof radio ranges as discussed in [9].Dele t ions wi th l imi t ed s to rage . Wi th l imi tedstorage, a node prioritizes its own data over that col-lected from others. So, if a dat a point comes in an dthere is no free space to store it in memory, the nodefirst deletes the oldest data point belonging to anot hernode. If none are available, it will then delete its ownoldest data point. In this way, the system prioritizes themost recent timestarnped points; the data points thathave been around the system the longest --an d thus hadthe highest probability of being already transferred tobase---are the first to be evicted to mak e room for newerincomin g points. Similarly, a node 's own data is alwayslast to be evicted, and in that case only for newer pointsof itself.Once a data point has beefi transmitted to base, itis added i nto a "delete list." The del ete list is a datastructure that indicates a particular point is now obso-lete and can be erased. Like regular data points, deletelists are also transferre d between nodes. Unlike regulardata points, delete lists do not contain full data points.In peer to peer transfer, upon receiving a data point, ifit is alrea dy in the delete list, it is discarded. In addi-tion, once every hour, the nodes "scrub" their memoriesof data points in the delete list.6.2 N etwork connectivityAs ZebraNet relies on animal movements to createan ad hoc network, how these zebras move and interactcritically determines the topology and connectivity ofthe network, which influences the performance of rout-ing protocols. Hence, before we evalu ate the protocols,we first characterize network connectivity. There aretwo measures of connectivity:

Direct connectivity: This counts neighbors en-countered directly by each node. Tha t is, given

0 o . . . . . "

. ' g ~ 1

t, , 0 ~ i

rad io range (metere)

F i g u r e 8 : A v erag e p e r cen t ag e o f d i s t i n c t n e i g h-b o r s en co u n t e r ed d i r ec t l y .[-~,-o.os7m;s -,e- 0.1 ~m, , "-~-0.2,S7m,',}

i -

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rncllo rag e (meters)

F i g u r e 9 : A v e r a g e p e r c e n t a g e o f d i s t i n c t n o d e se n c o u n t e r e d i n d i r e c t l y , t h r o u g h p ee r - t o -p ee r r e -l ay ing .

a circular radio range of radius r, a collar i is aneigh bor of collar j if collar i is withi n r me ters ofcollar j. This is a good indication of the mobilityof nodes and their interactions with each other.Indirect connectivity: In addition to direct neigh-bors, indirect connec tivity includes nodes that arereachable via multihop relay through neighborsand neig hbors' neighbors. This is a good indica-tion of how peer-to-peer networking will work.

In a mobile ad hoc network, radio range radius r andthe mobility of the nodes significantly impact networkconnectivity. We thus s imulate the mo bility of zebrasat varying r a nd move ment speeds, over a month ofsimulated movement. Figures 8 and 9 plot the averagepercentage of distinct nodes zebras encounter ed directlyand indir ectly respectively, averaged over the total num-ber of collars. T he figures show that as radio range andmovement speed increase, direct and indirect networkconnecti vity rise. This is intuitiv e, since a wider radiusr increaseS the prob ability of other zebras falling withinrange. Likewise, a faster-movi ng animal covers moreground a nd thus increases the chance of meeting- otheranimals. Figure 8 shows that, using direct neighborsonly, 100% connectiv ity is attain ed at aroun d 12km ra-dio range for the fastest (0.267m/s) move ment speed. IfZebraNet protocols rely solely on direct connectivity toget data back to the base station, they require a verywide 12km radio range t hat practically covers the entire20km by 20km map.Since radio energy cons umpti on increases significantlywith radio range (following a square-law or more) apower-efficient network should also tap indirect con-nectivity through peer-to-peer communication. For the

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I'-'i -direct --I--hisl --i,--fll:cd i

" ' i iradio range (meters)

F i g u r e 1 0 : S u c c e s s r a t e w i t h i n f i n i te s t o r a g e a n db a n d w i d t h .

s a m e m o v e m e n t s p e e d o f 0 . 2 6 7m / s , F i g u r e 9 s ho w sth a t u s in g in d i r ec t n e ig h b o r r e l a t i o n sh ip s , t h e n e tw o rkach iev es 1 0 0 % co n n ec t iv i ty w i th r ad io r an g es o f l e s sth an 2 ,0 0 0 m e te r s . H en ce , p ee r - to -p ee r p ro to co l s axea b l e t o e x p l o i t i n d i r e c t c o n n e c t i v i t y t o r e d u c e r a d i oran g es in sp a r se ly -c o n n ec t ed sen so r n e tw o rk s , r ea l i z in ga h u g e r ed u c t io n in p o w er co n su m p t io n . T h ese r e su l t ss u p p o r t t h e p o t e n t i a l b e n e f i ts o f a p e e r - t o - p e e r p r o t o c o lin Z eb raN e t . T h e y a l so p o in t t o t h e l ik e ly r ad io r an g esw e w i l l n eed to su p p o r t . T h e su b sec t io n s th a t fo l lo wev a lu a t e p ro to co l i s su es i n m o re d e t a i l .6 . 3 P r o t o c o l E v a l u a t i o n s

T o f i r s t e s t ab l i s h a b ase l in e , F ig u re 1 0 sh o w s th e su c -c e s s r a t e o f d a t a r e t u r n e d t o t h e b a s e s t a t i o n f o r a n i d e a ln e t w o r k w h e r e t h e r e i s i n f in i t e s t o r a g e c a p a c i t y a n dn e t w o r k b a n d w i d t h . W e c o m p a r e t h r e e pr o t o c ol s . I na d d i t i o n t o t h e t w o p r o p o s e d p e e r - t o - p e e r p r o t o c o l s - -f l o o d in g a n d h i s t o r y - b a s e d - - w e a l so p l o t s u c c e s s r a t ef o r a p r o t o c o l t h a t s u p p o r t s n o p e e r - t o - p e e r t r a n s fe r san d o n ly a l lo w s d i r ec t t r an sm iss io n o f a co l l a r ' s d a t a d i -r e c t l y t o t h e b a s e . B o t h p e e r - t o - p e e r p r ot o c o l s ( f lo o d -i n g a n d h i s t o r y ) p e r f o r m b e t t e r t h a n d i r e c t t r a n s m i s -s io n , ach iev in g 1 0 0 % su ccess r a t e a t a r ad io r an g e o fa b o u t 6 k m a s c o m p a r e d t o l l k m r a d i o r a n g e n e e d e d f o r' d i r e c t ' . T h i s i s b e c a u s e t h e p e e r - t o - p e e r p r o t o c o l s a r eb e t t e r a b l e t o p e r c o l a t e d a t a f r o m r e c l u s i ve n o de s t h a td o n o t m e e t t h e b a s e s t a t i o n d i r e c tl y .We a l so see th a t fo r t h i s u n co n s t r a in ed se tu p , f l o o d in gp e r f o r m s b e t t e r t h a n t h e m o r e s e l e c ti v e h i s t o r y - b a s e dp r o t oc o l . W i t h n o c o n s t ra i n t s on s t o ra g e a n d b a n d -w id th , f l o o d in g w i l l h av e th e b es t p e r fo rm an ce o f an yp eer - to -p ee r p ro to co l , s in ce i t co m p le t e ly l ev e rag es th ein d i r ec t co n n ec t iv i ty o f a n e tw o rk , b y b ro ad cas t in g toev e ry n e ig h b o r .6 . 4 S t o r a g e C o n s t r a i n t s

A s s t o r a g e c a p a c i t y i s a p r o m i n e n t c o n s t r a i n t i n t h ed es ig n o f sen so r n o d es , w e n ex t i n v es t ig a t e t h e im p ac t o fl i m i t i n g t h e c a p a c i t y o f o n b o a r d m e m o r y . T o i l l u s t r a t eth e t r en d s , w e sh o w an ex t r em e case in w h ich s to rag e i sl im i t ed to 1 0 co l l a r -d ay s .A s sh o w n in F ig u re 1 1 , ev en w i th s to rag e sev e re lyc o n s t r a i n e d , - b o t h p e e r - t o - p e e r p r o t o c o l s p e r f o r m b e t -t e r t h a n d i r e c t t r a n s m i s s i on t o b a s e . T h i s i s s o m e w h a ts u r p r i s i ng s i n c e p e e r - t o - p e e r re q u i r es t h a t t h e s t o r a g eh a n d l e b o t h t h e c o l l a r ' s ow n d a t a a s w e ll a s t h a t o f i t sp e e r s. T h e s u c c es s o f t h e p e e r - t o - p e e r p r ot o c o l s c o m e sla rg e ly d u e to o u r d e l e t io n s t r a t eg y w h ich p r io r i t i zes aco l l a r ' s o w n d a t a o v e r o th e r s . T h i s h e lp s en su re th a t

I - e - d i re c t - - i -- h i sl ~ f l o o d ]

ao i " i| ,o - - - - , , . . . . . . . . . . . . . . . . . . ii ,o

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F i g u r e 1 1: S u c c e s s r a t e w i t h c o n s t r a i n e d [ s t o r a g ea n d i n f i n i t e b a n d w i d t h .I,-t-dirocl . = i - - h i s t -'-~llcc


10/12

~:-~--~reet .-m-~st .-J..-nood j9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .a

~3

r a d i o r e n g e ( m e t e r s )

F i g u r e 13: Normal ized e n e r g y c o n s u m p t i o n f o rt h e n o n - r e s o u r c e - c o n s tr a i n e d c a s e . T h e e n e r g yc o n s u m e d b y d i r e c t t r a n s m i s s i o n i s V e r y c l o s e t ot h a t d i s s i p a t e d b y t h e h i s t o r y - b a s e d p r o t o c o l .

When running our simulations without prioritizing lo-caL! colla r da ta over peers, flood ing does ev en worse, aseach collar wastes too much time tran smit tin g anothercollar's likely-redundan t data instea d of its own data.

6 .6 E n e r g y T r a d e o ff sBesides success rate, another metric of key interestin sensor networks is the energy consumption. Figure13 shows the energy consum ption for the protocols run-ning on a non-resource-con strained network. The peer-to-peer protocols are shown as energy costs normalizedto that of direct transmission, which is plotted as al-ways equal to one. Floodi ng's energy consum ption ismore than 8X that of direct trans mission at large radioranges. In constrast, the relative energy of the history-based protocol grows very slowly from 1.0X at lkm ra-dio range to 1.04X at a radio ran ge of 15kin. This isexpected, since flooding sends messages to everyone inrange, when only one copy is needed back at the base.Furthermore, flooding may perform many redundantswaps of data that has already been delivered to thebase in cases when the delete-list percolates only veryslowly back from the base station. History-based, on theother hand, sends its data to only one receiver. Thus,we see that while flooding typically gives the best per-formance in peer-to=peer networks with no constraintson storage and bandwidth, its real-life energy cost andbandwidth expectations are exorbitant for large radioranges. While flooding makes sense at low-radio-rangeand low-connectivity points in th e design space, it is apoor choice for the high-conn ectivity regime.

6 .7 F i n a l D e s ig n C h o i c e sThe trends summarized in this selection have helpedguide the design choices in the ZebraNet prototypenode, and in fact, led to our selection of two radios inZebraNet. The first radio is a low-power, short-range

(100m, 19.2Kbps) radio in tende d mainly for peer-to-peer communica tions. The second radio is higher-powerand longer range (8km at 2.4Kbps) and is intendedmainly for tra nsmi ttin g to base. With the uBlox chipproviding 640KB of user-accessible flash memory, stor-age is essentially uncons traine d. Simula ting a flood-ing protocol for short range and a direct protocol forlong range, our simul atio ns show an 83% success rate,with an estima ted 855kj (66 ampere-h ours) energy con-sumption per month. We are currently experimentingwith adaptive protocol variations that should increase

the protocol success rate while holding energy roughlyconstant.7. R E L A T E D W O R K

Sensor networks in general, and envir onmen tal sens-ing in particular, axe areas of considerable research in-terest. This section touches on some of the most salientrelat ed work for Zebra~Net, div ided into sec tions on envi-ronm enta l applications and wildlife, sensor node design,and protocol studies for sensor networks.E n v i r o n m e n t a l a n d W i l d l if e S e n s i n g Priorwildlife monit oring work for large ma mma ls has almostexclusively been s upport ed by relatively low-technologyVHF transceivers that periodically send out a ping sig-nal [10]. More recent impro vements have included GPS-based trackers, which have been used for tracking ofvarious animal s including birds [27] and sea turtles [4],but these rely on high-power transmitters that trans-mit data up to a satellite, and they operate off a non-recharged batte ry supply. Sensor networks have alsobeen proposed for intruder detection, tempera ture mon-itoring, and traffic control [7, 36]. Envi ronm enta l mon-itoring using sensor nodes with embedded processorsare also a focus of the habit at monito ring project [21].There, they plant the sensors statically in a grid-likefashion across two wildlife hab itat s. Thes e sensors iden-

tify animals when they move through the multiple sen-sors, a nd report observed phen omen a back to a basestation through peer-to-peer transfers through the sen-sor network. W hile this has issues in common with Ze-braNet, the key difference lies in mobility. In ha bita tmonitoring, sensor nodes are fixed, tracking a dynamicphenome na (moving animal), and reporting to a fixed-location base station. In ZebraNet, sensor nodes and thebase station itself are all mobile and only intermittentlyavailable for communication. Routing choices thus be-come more acute for ZebraNet.S e n s o r N o d e D e s i g n In the research arena, webear some resemblance to the TinyOS and TinyNet-workedDevices project [13]. Key differences here arethat the Smar t Dust "motes" are much more fine-grained than ZebraNet nodes, which include GPS anda 20MHz processor. Thus, the y are targ eted at differ-ent points in the node design space. Rang huna than etal. have also studied energy and other design issues insensor networks, discussing different node alternatives[32].More coarse-grained, the Hiker's Buddy work fromDuke looked at power-aware compu ting issues for a mo-bile "platform" inc luding a PDA and GPS [6]. This, onthe other h and, is actually more coarse-grained in termsof software an d energy consum ption than w hat we wishfor ZebraNet. It also is inten ded for direct human useand so did not consider peer-to-peer forwaxdlng of po-sition data to a base station archive.P r o t o c o l S t u d i e s Moving more specifically toZebra~Net's commu nic ati on mechanisms , Zebra~Net is amobile ad hoc network, a research area that has seen in-creasing atte ntio n in recent years. The zebras (nodes)move dynamic ally and arbitrarily, so the wireless inter-connectio ns between the nodes change continually. Ina mobile ad-hoc network, the routing protocol has todeliver messages quickly, in the face of unpredictabletopology changes. In addit ion, power efficiency is crit-ical. Numero us routing protocols have been proposed[33]. Some proactive ly search for rout es to all othernodes [26, 29], while others only look for a path when amessage ne eds to be delivered [30, 15]. In ZebraJNet, ourdestin ation (base statio n) is only sporadically available.Thus , cachin g route s (DSI% [15]) or signif icant link stat e

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(A O D V [3 0 ] ) w i l l b e i n e f f ec t iv e , b ecau se cach ed d a t am a y g u i de d a t a u n n e c e s s a r i l y t o t h e b a s e s t a t i o n w h e ni t i s d o w n . F u r th e rm o re , f r eq u en t n o d e m o v em e n t s m ayt r ig g e r w as t e fu l cach e f lu sh es an d ro u te r e -d i sco v ery . O ft h e m a n y p r o p o s e d a d h o c p r o t o c o l s , o u r Z e b r a N e t p r o -to co l m o s t c lo se ly r e sem b les ep id em ic ro u t in g [38 ].G e n e r a l m o b i l e a d h o c n e t w o r k p r o t o c o l s t a r g e t a r b i -t r a r y d a t a f lo w p a t t e r n s b e t w e e n m u l t i p l e s o ur c e s a n dd es t in a t io n s . In Z eb raN e t , d a t a f l o w s e i th e r f ro m a l lt h e zeb ras (n o d es ) t o a s in g le d es t in a t io n (b ase s t a t i o n )i . e . d a t a g a th e r in g ; o r o ccas io n a l ly , f ro m a s in g le so u rceto a l l t h e n o d es , i .e . b ro ad c as t . T h ese d a t a f l ow p a t -t e rn s a s s o c i a t e Z eb raN e t c lo se ly w i th th e sen so r n e t -w o rk s su b c lass o f m o b i l e ad -h o c n e tw o rk s [7, 31 , 8] . Ins e n s o r n e t w o r k s , d a t a i s g a t h e r e d f r o m n u m e r o u s d i s -t r i b u t e d s e n s or s t o a b a s e s t a t i o n , s o d a t a t o o a g g r e g a t e sf ro m m an y n o d es to a s in gle d es t in a t io n . S im i l a r ly , t h eb a s e s t a t i o n b r o a d c a s t s t h e i n f o r m a t i o n i t i s i n t e r e s t e din to all sensors [14].T h e u n i q u e c o m m u n i c a t i o n c h a r a c t e r i s t i c s o f s e n s o rn e tw o rk s h av e l ed r e sea rch e r s t o s tu d y sp ec i f ic ro u t -in g a lg o r i th m s fo r t h em , s in ce ro u t in g p ro to co l s p ro -p o sed fo r g en e ra l m o b i l e ad -h o c n e tw o rk s d o n o t w o rkw el l [8] . H o w ev er, t h e ro u t in g a lg o r i th m s p ro p o se d th u sfar [12 , 17 , 28] assume stat ic sensor networks, i . e . , net -w o rk s w h ere th e sen so r s d o n o t m o v e o n ce th ey a re d e -p l o y ed . A l g o r i t h m s a ls o a s s u m e t h a t t h e b a s e s t a t i o ns t a y s a t a f i x e d l o c a t io n . B a s e d o n t h e t a x o n o m y p r o -p o sed in [3 6] , h o w ev er , Z eb r aN e t i s a d y n am ic sen so rn e tw o rk ; i t s n o d es a re m o b i l e , an d so i s t h e b ase s t a -t i on . T h i s s u b - c a s e h a s n o t p r e v i o u s l y b e e n s t u d i e d i nd e ta i l .A n o t h e r i n t e r e s ti n g a r e a o f r e s e a r c h i s o n c o n n e c t iv -i t y an d co v erag e p ro b lem s in w i re l e s s ad h o c n e tw o rk s .Wh i l e co v erag e i ssu es in ce l lu a r n e tw o rk s h av e b een w e l ls tu d ied , i s su es o f co n n ec t iv i ty an d " c r i t i ca l m ass" fo rm o b i l e ad h o c n e tw o rk s a re s t i l l o p en to p ics i n b o th th e -o ry an d sy s t em s [11 , 23 , 24 ]. T h u s f ax, co m p u t a t io n a lg e o m e t r y o r r a n d o m g r a p h t h e o r y t e c h n i q ue s h a v e b e e nap p l i e d to g lo b a l v i ew s o f n e tw o rk to p o lo g y . O u r w o rkin Z eb raN et h as fo cu sed o n s to ch as t i c s tu d ies b ased o nd e t a i l e d m o b i l i t y m o de l s . F i n a l ly , t h e r e h a s a l s o b e e nw o r k o n h i g h l ev e l d a t a p r o c e s s i n g a n d p r o g r a m m i n g i ns e n s o r n e tw o r k t o r e d u c e b a n d w i d t h o r s t o r a g e n e e d s [ 3,14, 20].8. C O N C L U S I O N ST h is p ap er d i scu sses t h e d es ig n t r ad eo f f s an d ea r lyex p er i en ces i n b u i ld in g a l o w -p o w er w i re l e s s sy s t em fo rp o s i t i o n t r ack in g o f w i ld l if e . B y u s in g p e e r - to - p ee r n e t -w o r k i n g t e c h n i q u e s , o u r s y s t e m c a n f o r w a r d d a t a t oa r e s e a r c h e r ' s m o b i l e b a s e s t a t i o n w i t h o u t a s s u m i n gth e p resen ce o f an y ce l lu l a r p h o n e se rv i ce o r w id e ly -a v m l a b l e t e l e c o m m u n i c a t i o n s s u p p o r t .W e p r e s e n t i n i t i a l d e s i g n i d e a s , m e a s u r e m e n t s , a n dw e i g h t e s t i m a t e s , a n d w e d i s c u s s h o w b a t t e r y a n dw e i g ht l i m i t s t r a n s l a t e i n t o e n e r g y a n d s t o r a g e l i m i t sfo r o u r sy s t em an d i t s p ro to co l s .A l t h o u g h o u r p r o t o c o l d e v e l o p m e n t i s s ti l l ve r y m u c hu n d e r w a y, w e f ee l t h a t t h e e a r l y p r o t o c o l d a t a t h e p a p e rp ro v id es m ay b e g en era l ly u se fu l t o t h e ad h o c n e tw o rk -i n g a n d s y s t e m s c o m m u n i t ie s . I t r e p r e s e n t s n ew s t e p sin p ro to co l s fo r m o b i l e sen so r n e tw o rk s , an d o f fe r s i n -s i g h t s i n t o h o w s t o r a g e a n d e n e r g y l i m i t s m a y i m p a c tp r o t o c o l d es i g n. W e a r e c u r r e n t l y m a k i n g f u r t h e r p r o t o -co l im p ro v em en t s t h a t w i l l i n c lu d e : ( i ) p o s i t i o n -b as ed ,in ad d i t i o n to h i s to ry -b a sed ro u t in g , ( i i ) s e l f - ad ap t iv ed ec i s io n s o n th e n u m b er o f n o d es to fo rw ard to i n t h eh i s to r y -b ase d ap p ro ach , ( i i i ) b e t t e r su p p o r t fo r d iv e r sem o b i l i t y m o d el s . I n p a r t i c u l a r , b y h a v i n g p r o t o c o l s t h a t

w e l l - su p p o r t n o d es o f d i sp a ra t e sp eed s , w e w i l l b e ab l eto co l l a r an d s tu d y d iv e r se se t s o f sp ec i es w i th in th es a m e e c o s y s t e m . F i n a l ly , we n o t e t h a t o u r h i s t o r y - b a s e dap p ro ach cu r ren t ly i s s t a t e l e s s ( i t t r an s fe r s t h e in fo r -m a t io n as p a r t o f t h e p ee r d i sco v ery p ro cess ) ; w e a rec o n s i d e ri n g s t a t e - b a s e d a p p r o a c h e s t h a t m i g h t d e c r e a s ep ee r d i sco v ery t im e .O v era l l , ad h o c n e tw o rk in g i s p resen t ly a v e ry ac t iv eresea rch a rea . O u r w o rk o n Z eb raN e t m ak es a s ig n if -i can t co n t r ib u t io n to t h a t d o m ain b y o f fe r in g d e t a i l edsy s t em s- l e v e l p e r sp e c t iv es o n h o w to b u i ld l o w -p o w erp ee r - to -p e e r sy s t em s th a t o p e ra t e e f f ec t iv e ly an ( ! ax e o p -t i m i z e d t o t h e c h a r a c t e r i s t i c s o f a p a r t i c u l a r a p p l i c a t i o nd o m a i n .9. A C K N O W L E D G M E N T SW e t h a n k C h i d o z ie E n y i n n a , T i n g L i n , K i n a r i P a t e l ,K a r e n T a n g , a n d J e r e m y W a l l f or t h e ir c o n t r i b u t i o n s t ot h e Z e b r a N e t p r o j e c t a n d f o r t h e i r c o m m e n t s o n i d e a sr e l a t e d t o t h is p a p e r . J u l i e B u e c h n e r ' s s e n i or th e s i sp r o j e c t p r o v i d e d t h e a n i m a l o b s e r v a t i o n d a t a i n S e c -t i o n 3 . We a l so th an k th e an o n y m o u s r ev i ew ers fo r t h e i rc o m m e n t s o n th e p a p e r . T h e Z e b r a N e t p r o j e c t i s s u p -p o r t e d i n p a r t b y a n N S F I T R g r a n t . M p a l a R e s e a r c hC e n t r e i s a d m i n i s t e r e d b y t h e M p a l a R e s e a r c h T r u s t , ac o l l a b o r a t i o n o f P r in c e t o n U n i v e r si t y , t h e S m i t h s o n i a aI n s t i t u t i o n , t h e N a t i o n a l M u s e u m s o f K e n y a , t h e K e n y aW i l d l i f e S er v ic e , a n d t h e M p a l a W i l d l i f e F o u n d a t i o n .1 0 . R E F E R E N C E S

[1 ] J . A l tm a n n . O b se rv a t io n a l s tu d y o f b eh av io r :s a m p l i n g m e t h o d s . B e ha v i o r , 49:227-267, 1974.[2] J . B e u ch n er . Mo v em en t ru l e s o f a f r i can u n g u la t e s .P r i n c e t o n U n i v e r s i t y S e n i o r U n d e r g r a d u a t eT h e s i s . D e p t . o f E c o l o gy a n d E v o l u t i o n a r yB io lo gy . A d v i s ed b y P ro f . D an R u b en s t e in . , Ju n e2002.[3 ] P . Bonnet , J . E . Gehrke, and P . Seshadx i .T o w a r d s S e n s o r D a t a b a s e S y s t e m s . I n Proc .Se c o n d In t e r n a t i o n a l C o n f e r e n c e o n M o b i l e Da t aM a n a g e m e n t , Jan . 2001 .[4 ] C ar r ib ean C o n se rv a t io n C o rp o ra t io n .h t t p : / / w w w . c c c t u r t l e . o r g / s a t w e l c . h t m . 2 0 0 2 .[ 5 ] P . E g g e n b u r g e r . G P S - M S 1 E M i n i a t u r e G P SR e c e i v e r M o d u l e D a t a S h e e t . u - B l o x A G . U R L :h t t p : / / w w w . u - b l o x . c h / , O c t . 2 0 0 1 .[6] C. E l l is . The Case fo r Higher- leve l Powe rM a n a g e m e n t . I n Proc . HotOS, Mar. 1999 .[7 ] D . E s t r in , D . C u l l e r , K . P i s t e r , an d G . S u k h a tm e .C o n n e c t i n g t h e P h y s i c a l W o r l d w i th P e r v a s i v eN etw o rk s . In P e r v a s i v e C o mput i n g , Jan . 2002 .[8 ] D . E s t r in , R . G o v in d ran , J . H e id em an n , az ldS . K u m a r . N e x t C e n t u r y C h a l le n g e s: S c a l a b l eC o o rd in a t io n in S en so r N e tw o rk s . In Proc . F i f thA C M / I E E E I n t ' l C o n f . o n M o b i l e C o m p u t i n g a n dN e t w o r k i n g , Aug. 1999.[9 ] D . G an esan , B . K r i sh n am u r th y , A . Wo o ,

D . C u l l e r , D . E s t r in , an d S . Wick er . A n em p i r i ca ls t u d y o f e pi d e m i c a l g o r i t h m s i n l a r g e s c a l em u l t i h o p w i r e le s s n e t w o r ks , u n d e r s u b m i s s i o n ,2002.[1 0 ] H ab i t R esea rch . h t t p : / / w w w . h a b i t r e s e a r c h . c o m / .2002.[11 ] Z . H ass . O n th e R e lay in g C ap a b i l i t y o f t h eR e c o n f i g u r a b l e W i r e l e s s N e t w o r k. I n P r o c . IE E E7th Vehi cular Technology Con feren ce, Vol 2, pp.1148-1152, May 1997.

106


12/12

[12] W. Hei nze lma n, A. Chand_rakasan, andH. Balakr ishnan. Energy-Eff icient Communicat ionProtocol for Wireless Microsensor Networks. InHa w a i i In t e r n a t i o n a l C o n f e r e n c e o n Sy s t e mSciences , Jan. 2000.[13] J. Hill, R. Szewczyk, A. Woo, S. Hollar, D. E.Culler , and K. S . J . P is ter . System ArchitectureDirections for Networked Sensors. In Proc .Archi tec tural Suppor t for Programming Languagesand Opera t ing Sys tems, pages 93-104, 2000.[14] C. Intanagonwiwat, R. Govindran, and D. Estr in.Directed Dif fusion: A Scalable and RobustCommunicat ion Paradigm for Sensor Networks . InP r o c . S i x t h A C M / IE E E In t ' l C o n f . o n M o b i leC o mput i n g a n d N e t w o r k i n g , Aug. 1999.[15] D. Johnson and D. Maltz . Dynamic SourceRouting in Ad-Hoc Wireless Networks . InT. Imiel inski and H. Ko r th, edi tors , . MobileComput ing, pages 153-181. Kluwer Academ icPublishers, 1996.[16] P . Karn. MACA - A New Channel Access Methodfor Packet Radio. In P r o c . 9 t h C o mput e rN e t w o r k i n g C o n f e r e n c e , 1990.[17] S. Lindsey, C. Raghavendra, and K. Sivalingam.Data Gather ing in Sensor Networks us ing theEnergy*Delay Metr ic . In Proc . Paral le l and

Dis t r ibuted Process ing Sympos ium, Apr. 2001.[18] LINX Technologies, Inc. Sc series transceivermodule design guide.ht tp: / /www.linxtechnologies .com/, Apr . 2001.[19] Lotek Corp. ht tp: / /www.lotek.com. 2002.[20] S. Madden, R. Szewczyk, M. J. Franklin, andD. Culler . Suppor t ing Aggregate Quer ies OverAd-Hoc W ireless Sensor Networks . In Proc . 4 thIE E E W o r k s ho p o n M o b i l e C o mput i n g Sy s t e msand Appl ica t ions , June 2002.[21] A. Mainwaring, J. Polastre, R. Szewczyk, andD. Culler. Wireless Sensor Networks for HabitatMonitoring. In In te l Research, IR B- TR- 02- 006,June 2002.[22] Maxstream. 24xstream wireless modem datasheet .h t tp : / /w w w .maxs t r eam.ne t / , Ju ly 2002.[23] S. Meguerdichian, F. Koushanfar, M. Potkonjak,and M. Sr ivastava. Coverage Problems in WirelessAd-Hoc Sensor Networks . In P r o c . IE E E In f o c o m2001, Vol 3, pp. 1380-1387, Apr. 2001.[24] S. Meguerdichian, F. Koushanfar, G. Qu, andM. Potkonjak. Exposure In Wireless Ad HocSensor Networks. In Proc . MobiC om '01, pp.139-150, July 2001.[25] Mpala Research Centre.h t tp : / /w w w .nasm.edu/ceps /mpa la . 2002 .[26] S . Mur thy and J . Garcia-Luna-Aceves . AnEff icient Routing Protocol for Wireless Networks .A C M M o bi l e N e t w o r k s a n d A ppl i c a t i o n s Jo ur n a l ,Spec ia l Issue on Rout ing in Mobi leC o m m u n i c a t i o n N e t w o r k s, Oct. 1996.[27] NASA Satelf i te Tracking of Thre aten ed Species .

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