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CHALLENGES INMOBILED Hoc NETWORKING

Self-Organization inMobile Ad Hoc Networks:The Approach of Terminodes

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Ljubica BlaZevit, Levente Buttyan, Srdjan tap kun, Silvia Giordano, Jean-Pierre Hubaux, andJean-Yves Le BoudecSwiss Federal Institute of Technology, Lausanne

ABSTRACTThe Terminodes project i s designing ii widc-

a r e a m o b i l e a d h o c ne t wo rk w l i i c l i i s meant t ol i e t i s c c l i n ;I p u h l i c e n v i r o n n i c n t : i n o u rappro; ic l i . t h e n e t w o r k i s r un b y u s e r s t h e m -sclvcs. W e give ;I glolxil descript ion o f tlic hiriltl-i n g blocks i i scd li!~he I i as ic operat ion o f t h enetwork; they ; i l l rely on v a r i o u s concepts o f sell ' -o rga 1 z;i t io 11 . li o i t ing ti s c s ;I c o iib i11it io 11o tgcograp iy-Ixisccl i 1o rn i a t oii a iid I cii I MA N ET-l ike protocols . Terminode pos i t ioning s ol i ta incc lhy either G P S or ;I rcI: it ivc pos i t i on ing mc thot l .M o h i cy iii:inagciiie n t iscs se I -organ zcd vi t ti ;Iregions. T c r m i n o d c s e m p lo y ;I f o i - m o i v i r t t i a lm o n e y cal led nuglets tis i i i i incent ive to col l ; iho-r;i c . La s y . w e tl i c i ss d i c c t ions lo ' irovicI ings o m e lcvcl of' w x r i t y .

INTRODUCTIONThe Terminodes pro jec t i s ;I long-term rcsciircliproject (2000-20 IO) a i m ed at study ing a n d pro-totyping large-scale self-org:inized mol i i l c ; i d ho cnetworks. In t h i s f ramework th e n o d e s arc c;iIIccII C I ' I I I it r odes ( tc r ii 1a I+ 11 d c ). TI1e p ro,jc c cl i I t -guishes itsclf from other rcsciircli pro, j cc ts in t h i s;irc;i i n s e v e r a l w a y s . F i r s t . i t ciicoii1p;isscs ;illlaye rs ii nd explores i11c rlaye r in C ract ons. fromth e fundamentals of the physical I:iycr tip to soft-w i r e ; i rc l i i tcc t t i rc ; i i icl appl icat ions. Second . i t i st r e e f'rom slio t - t c r m coni p a t ihi c o n t r a i11ssuch iis i n te ropera t ion wi t h IPv4 i i c t \ v ( i r ks (wl i ic l i\\ill ic stiidicd i n t l i c pro, icc. t . l io ivcvc~ ' ) .~ ; i h th u tn o t lc;ts[. i [ t r i c \ to c a p t i i r e tlic I i i i s i i i c b s : i n c ls o c i e t a l p o t c i t i;i g c i i c r a cd h y I ie (>;I i -; icI i g i nshift previously clcscrihcd.Sc l t -org; i i i i~ ; rr io i i s the k c y \ \~ o r d t tlic pro,Jcct[ I . S e I.-

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D

U Figure 1. Ho w AGPF wol-kswhen a terminodewith EU Is lzas s ome data to send to a terminodewith EUIIJ and there is no corinectivityalong theshortest linefioui S oD. S has a path toD givenby a list of geographical locations called anchoredpoitits { A P I ,A P 2 ) . First, eoclesic packet for-wurditig in th e direction ofAPI is used. Afiersome hops the pucket airives at terniiiiode Aw liicli j k l s h nt it is close toA P I . At A th e pocketi s forwnrderl by irsitig geodesic pucket forwardiiig~ O I I I C So B (i.e., close to A1'2)2),t starts .seridqyc o i i i e ~ 0 C it } i i i [ l . s / /rotU is TLR-retrcliublerrrid}br?vcrr(/.s lie p(ickc1 IOD IJ J / i i c w / i . s(!f TLR .

i l l tile dirL'ctiOr10fA1'2. SecOtld, whet^ th e JUCketIIW pucket l o l 4 ~ u l ~~ t . s. ust, Ivlrl~rr he pN"kcl

simply :I t r ip let o t gcogriiphic coorrlitiates (longi-t t i t le . a t t t ide . ;I t i t n d c ) o h I i nccl. f o r c s a m p1c,I>y mc;itis o f t h e G I 5 or the GI's-lrcc posit ion-ing m c t h o d t l i s c i i~sc r l ater.In ;I t c rn i inodc i i c t w o i - k . two important fac-to rs :iffcct t h e dcs iz i i 01 ' ;I s o l u t i o n I'or p a c k e trouting. 1-it-st.sca lab i l i t y i s t- cqu i i -ed . i n tcrnis o fh o t h nunihcr oi n o d e s ; i n c l geographical c( ivcr-a g e . Second, n e t w o r k i i oc l cs ai-c iisci- e q u i p -iii c i i I , :I nd t l i e r e l o e ii i a y h c ; iva i I ii Ill o ti I ys p o railica I y . T h i s s c co nd rcq 11 eme11 i m p o s c ss o n i c incentive f o r users t o c o o p e r a t e a n c l l o r -w ; t r d pi ickcts oti I>chalfof o t h e r s . A soliition isdiscusscc1 later.A s ;I c o n s c q i t c i i c c 01 ' tlie a h o v c r e q u i r e -i i icnts. o u r so lu t ion is clcsigned s u c h t h a t ;I t c r -minoclc relics only on itself ancl a small numbero f o t h e r t c rm i r ioc lcs t o r packet f o r w a r d i n g . I nxic l i t ion , we cope w i t h u n c e r t a i n t y i n th e nct-work by considering multipath routing a s ;I rule ,no t ;is a n c s c c p t i o n . W e use ;I combination o ftwo routing m e t h o d s : t c r m i n o d c l o c a l I-outing( T L I I ) ;Irid IC I n 1 ioclc ire ti10 t c 1'011 t i ng ( T I < I

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In a small wo rldgraph, roughlyspeaking, any

tw o vertices arelikely to be con-

nected through ashort sequence of

intermediatevertices This

means that anytw o terminodesare likely to be

connected wit h asmall number of

intermediatefriends I

aI

* .. * *: o ~ n re a I

ba .e.

Figure 2 . An Excinzple of th e sitnulation ureathat consists of three towns. Terniinodes stuywithin i r scope of the sunie town fo r a certuintiiiniber of niovenzeiits,and then choose ut run-doni mother town urea to which they niove.

A m ay hav e ;I good p a t h t o B because A c a nr c a c h B by app l y i ng T L R , or becausc A m a n -aged to ma in ta in one or several r out c vectors toB t h a t work w e l l . W h e n A wants to d iscover apath to dcs t ina t ion C. A inay re qu ire ass istance1'1-om ;I f r i e n d U . B may t ry to f i n d a p a t h to C( p e r h a p s wi t h t l i c he l p o f i t s ow n fr i ends) . I f Bf i nds the des i red path i t r e t u rns i t t o A. A payslor the service provided by B i n tiirglets.W e v i c w a t e r m i n o d e n c t w o r k a s a graph.with edges rcprcscnt ing the f r iends relat ion. W ecoiijccttii-e t h a t this graph I x i s the propert ies o f ;Is i i i i i l l w o r l d graph. Smal l w or l cl g r a p h s i irc verylarge graplis that tend to he spai-sc and clusteredancl 1i;ivc ;I s m a l l t l i an ic t c r 171. I n ;I smal l w orIcIgi-apli. roughly speaking. an y tw o v c r t i c c s a r cl i k c l y t o li e connec ted through ii short scqucncco f i n t c rn i ec l i u t c vcr t iccs. T h i s m e a n s t h a t a n ytw o tc rni inodcs i irc l ikc ly to he conncc tc t l wi t h ;Is n i i I n 1 m h e r 01' in tc r m c d at c friends.T c rni o d e ircnio c ro i ing i)r cons ta 11 I m o d -iliecl by path m aintenance.A te riii i nodc no rm a l l y a t t em p t s t o i n t i 1 a i nseveral p a t h s (described by ro ute vectors) to an ysingle clcstination of interest to it , a n d tiscs scv-era1 o f t l i c m i n para l le l . A terminode constant lym on i t o r s tlic cost of each p a th : t h e co s t is givenin t e r m s ot nuglc ts . packet l oss. and delay. Othe rfactors l ik e rohustncss. ztahil i ty. an d security ar c;11w e levan t to [ l i e c o s t o f ;I path. This al lows ;Itcrminoclc to imprclvc paths. and clclctc ohsolcteo r ni;i1 I ' u n c t o 11ng p a t lis. I n add i t i on . iniorm I-t i o n about tlic v a l u e o f t h e p; i ths i s t i s c d l o r ;It c r m i n o c l e t o dc c i c l c how to s p l i t t h e t r a f f i camong t h e m ul t i p l e p a th s t h a t csist t o the c lcs t i -nat ion . A t e rm i i i odc s e n d s i i io i -c d;it:i a l o n g th ep a t h s t h a t g i v e t h e l e a s t c onges t i on f c c d h ; ~ c kinl 'ormatic)n.

PERFORMANCE OF TERMINODE OUTINGW c c.:ii-ricd out s i i i i u l a t i o i i s ol' tlic TLR ; i n c l I'l jcc t ivc 1. l.l)j\ n1;tnagcnicrit i st o cl i x t r i hu tc t lie Ioc;i[ ion inf .oi -mat on ins ide t l i en c t w o r k i n ;I dynamic. sca lab le . scc i i r c . and t'airway (i.e.. withotit privi leging a n y n o d e o r region).A t t h e siiiiic t i m e . \\e ; i re no t c o n c e r n e d w i t hm a n a in i g t lie csac I oc i i t o n in n rm at on . AI Iw c r e q u i r e i s t h ; i t tlic L D A o f the des t i na t i on .l ea rned a t t l i c soiircc. i s ac c u ra te enou gh t h a tt he p i ckc cvcn ti i I i t r ivcs ;it sonic tc ri i i nodewhich f inds th e dcstin;ition inside i t s TLI

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of l o c a t i o n i n f o r m a t i o n . T h e m a i n f u n c t i o n s ofthis architecture are:Maintaining location informationDistr ibuting location information inside thenetworkThese func t ions a re per formed in a rel iable androbust way.In th i s a rch i tec ture , each node adver t i ses i t scurrent posit ion (LDA) to a geographical regioncalled the virtrin[ ho me region ( V H R ) . T h e V H Rh a s a f i xe d c e n t e r CVHRn d a v a r i a b l e r a d i u stha t ad apts to the dens i ty of the a rea conta in ingt h e V H R , i n o r d e r to maintain an approximatelycons tan t number of nodes inside the VHR.Th e fixed center is computed using som e pre-def ined publ ic ly known hash func t ion H . H is as t a ti c m a p p i n g b e t w e e n t h e s p a c e of E U I s{ E U I , } a n d t h e g e o g r a p h i c a l s p a c e o f a t e r m i -n o d e n e t w o r k s u c h t h a t H ( E U I A ) = C V H R o reach A E {EUZ!} .Termino de A d oes location update by sendingposit ion advertisements toward i ts VHR. All ter-minodes in A's VHR store the mapping betweenA's E U I a n d L D A . T h e y a c t a s a m a n a g e m e n ts y s t e m f o r A f o r t h e t i m e t h e y b e l o n g t o A 'sVH R; once they leave it , they lose this role.T h e r a d i us of t h e V H R is i n c r e a s e d o rdecreased if t h e n u m b e r of nodes in the VH R isi n a d e q u a t e . T h i s c a n h a p p e n b e c a u s e e i t h e rt h e r e ar c too n iany nodes in t h e V H R , a n d t h emanagement and re t r ieva l of the location infor-m a t i o n r e q u i r e s t o o m a n y c o m m u n i c a t i o n s , o rthere a r c i i o t e n o u g h n o d e s , a n d t h e s y s te m i sn o t robust .When t e rminode B wants to retr ieve the loca-t i o n o f A (i t must know A's EUI) . it sends a querytoward A's V H R ( w h ic h c a n h e c o m p u t e d by Bs ir icc ~ l i c i a h function is puhlicly known). On cethis request is received. the loc;ition information01 ' A is sent back hy the nodes o f t h e V H K o f A .1 his information is used to s ta r t comm unica t ionwith A. I h r i n g t h e communicatioii , A and B relyon a dircct system o f mobili ty tracking. Figure 3i l l u s tr a t e s t h e d i s t r i b u t i o n a n d r e t r i e v a l of th elocation informatim i n ;I tcrniinadc network.T h e r e a r c s c v er i il p o s s i b l e l o c a t i o n u p d a t eschemes that can he applied in 21 t c rminode ne t -work.The fo l lowing scheme s a re proposed lo r per -sona l communica t ions ne tworks (PCNs) 181 an dc a n b e u s e d i n ii t e r m i n o d e n e t w o r k. I n t h erinrcr.-hctsccl loca tion update scheme, each te rmi-node per iod ica l ly send s ii loca t ion u pdate to i t sV H R . In the cli\iii/ic~c-hii.seil update scheme, eachterminode tracks the dist irncc i t has nioved sincei t s l a s t u p d a t e a n d sends i t s l o c a t i o n u p d a t ewhenever t l ie distance exceeds ii certain thresh-old. I n th e p r c t l i c ~ i r ~ ,l i . s t t i~icc-hase t lcheme. tliet e r m i n o d e r e p o r t s t o its VHR both i t s loca t iona n d v e l o c i t y . B a s e d on t h i s i n f o r m a t i o n a n d ;Imobi l i ty pa t te rn . the loca t ion o f t h e t e r m i n o d ec an h c prcil ictcd. The tcrniinotlc checks i ts Ioc;i-t ion pcriodic:illy ;iiid send.; ;I location t~pda tc oits VH I

00

0

0

00 0 0

Figure 3. An example of the distribution and retrieval of location informa-tion. (1 ) Terminode D periodically sends its posi tion (LDAD) to its VHR.Whe n terminode S wants to communicate with D, it j r s t comp utes the centerof the VH R of D as H(EUID)and th en sends a request toward the regionaround this point. Whe n this request is received by th e terminodes in the V H RofD, ( 3 ) LDADis sent back to S.

I n o r d e r t o h a v e r o b u s t a n d s e l f - o p e r a t i n gm a n a g e m e n t , t e r m i n o d e s m u s t c o o p e r a t e t os t o r e l o c a ti o n i n f o r m a t i o n a n d s e r v e a s t e m p o -r a r y d i s tr i b u t e d m e m o r y f o r o t h e r t e r m i n o d e s .This may be st imulated by paying nuglets to tliet e r m i n o d e s i n t h e V H R . T h e a d v a n t a g e of thisa p p r o a c h i s t h a t i t h a s a r e a s o n a b l e a m o u n t o fcomm unica t ions to d i s t r ibu te and re t r ieve loca-t i o n i n f o r m a t i o n , a n d n o c e n t r a l o r s t a t i c a l l ydefined servers are necessary.I n t h e p r o p o s e d s o l u t i o n for niohil i ty man-agement , t e rminode B tha t needs a loca t ion ofclose terminode A may contact ;I potential ly dis-tan t VtHR of A . O n e p o s s i bl e s o l u t i o n to th i s

problem is to organize the V HR-ba sed mobil itym a n a g e m e n t i n a hierarch ica l way in order t oscale better in a large network. In this solution.every terminode would havc several geograpli i-cally distr ibuted VHRs that contain i ts location.For re t r iev ing loca t ion informat ion of anotherterminode, a terminode would contact t l ie corre-s p o n d i n g V H R c l o s e st t o i t. R e f e r e n c e 191 pre-sen ts how to geographica l ly d i s t r ibu te loca t ionservers in a large network.GPS-FREE OSITIONIN G

Terminode positioning is used by every terminodeto obtain its position. The position o f ;I terminodecan tie obtained by means o f th e G P S posit ioningm e t h o d . H o w e v e r. t h e r e a r e s i t u a t i o n s w h e r eGPS is iiot available. This happens. notably whenth e GPS signal is too weak (e.g.. indoor) . when i tis jammed, o r when 21 GPS receiver m u s t be avoid-ed for cost or integriition rcasc)ns.I n t h i s s e c t i o n w e h r i c fl y d e s c r i b e t h e S e l fPosit ion i ng Algorithm (S PA ) f o r posit c) n ng ofte rminodes i n ;I t c rminodc netw)r!i t h i c t i s GPS-l're c . SPA provides p o s I i o n i n fo ni a1 ion t U tc r -ni i odes in th e sceiia r os w he c a n in f r istr uci 11 -cdocs no t csist ;incl GI's ciiniioi OL'uwd. A c ~ m -plctc description o f SI'A is given in [ I O ! .

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Figure 4. PA uses the distances between the terminodes and builds theNCS. Lines in Fig. 4a represent the distunces terminodes can obtain by apply-ing the TO A method. This is done by rneasuriiig signal propagation timebetween the tw o neighboring terminodes. Figure 4b shows the NCS. Th e valuenext to each terminode represents its position within the NCS.S P A u s e s d i s t an c e m e a s u r e m e n t s b e t w e e nt e r m i n o d e s to build the network coordinate sys-t e m (NCS) (F ig . 4). T h e t i m e of a r r i v a l ( T O A )[ 11 1 m c t h o d i s u s e d to o b t a i n t h e d i s t a n c eb e t w e e n t w o t e r m i n o d e s . D e s p i t e t h e d i s t a n c em e a s u r e m e n t e r r o r s a n d t h e m o t io n of t e rn i i -nodes, S PA provides sufficient location infornia-t i o n a n d a c c u r a c y to s u s t a i n b a s i c n e t w o r kfunctions. For t h e s a k e o f s impl ic i ty we presentS P A in t w o d i m e n s i o n s , b u t i t c a n e a s i l y b ee x t e n d e d to p r o v i d e p o s i ti o n i n f o r m a t i o n i nthrcc dimensions.S P A i s p e r f o r m e d a t e a c h t c r m i n o d c a n dconsists of the following steps:A t c r m i n o d c n i c a s u r c s d i s t a n c e s to i t siic ighbors a n d sc nds this in form i t ion t o 211 1its neighbors.A t c rminod c bui lds i ts loco/ coortliiirilr . S J J S -i c i i i (LCS) and computcs thc p o s i t i o n s of i t sneighbors i n LCS.A tcrminodc compu tes t l ic density factor ofits / I - h o p neighborhood (i .c . . tlic number o ft e rminodes i i i the i i -hop nc ig l iho i -hood) .

A terminode with the highest density factorf o r m s a locatioi i rejkrence group ( i . e . , theterminodes in i ts ni-hop neighborhood) an dc o m p u t e s t h e c e n t e r a n d d i r e c t i o n of t h eNCS.Al l t e r m i n o d e s c o m p u t e t h e i r p o s it i o n i nth e NCS.B e l o w , w e d e f i n e t h e o p e r a t i o n of S P A i nmore detail .LOCALCOORDINATEYSTEM

E v e ry t e r m i n o d e i d e f i n e s i t s LCS ( L C S i ) bychoos ing two ne ighbor te rminodes p a n d 4 sucht h a t t h e d i s t a n c e d,,, b e t w e e n p a n d q is knownan d l a r g e r t h a n z e r o , a n d t h e t e r m i n o d e s i , 12,a n d q d o n o t l ay on t h e s a m e l in e . T e r m i n o d e ibecomes the cen ter of L C S , with the coordinates(0,O). The d i rec t ion of LCS ; is defincd such thatt e r m i n o d e p l ay s o n t h e p o s i t i v e x axis of t h ec o o r d i n a t e s y s te m a n d t e r m i n o d e (1 has a pos i -tive y component .E v e r y t e r m i n o d e i c o m p u t e s p o s i t i o n s of it sneighbors in its LCS, . I f t e rminode j is the neigh-bor of i , p , a n d q ( i . e .. t h e d i s t a n c e s [/,., d(/,,d,,,are known to i) , i ts posit ion is then com pute d hyt r ia ngu ii t io 11 (;in i n e r sec io ii of t h I -ec c i rc les )[I 1. T h i s is i l lus t ra tee l i n F i g . 5 . I f k is n o t 21n e i g h b o r o f p a n d q . o h t a i n s k ' s p o s i t i o n i nLC S, by choosing two ncighbors of k whose posi-t ions in LCS, ;it-c iilrcatly obtained.T h e c h o ic e o f p :ind (/ can he opt imized wi threspec t to the des i red coverage and accui-acy o fthe algori t h i n .

NETWORKOORDINATEYSTEMAfte - h i ild ing t lie r LCSs. t e rm n o d e s c o m p i t cthcir tlcnsity factors. 7'11~crminoclc w i th t h e higli-es t density factor \vi11 "slave" th e otlicr tcrminocles: i n c l it s i i i - h c i p i ieiglihorli~iocliccomcs ; (octiriorrl . l q i , ~ l , ~ ~ c . r , i ~ o ~ ~ ~ lLIIC) I t tilt 11ct\\#oI-k. i-ilc~ ~ 1 1 t; i n c l r l i r cc t ion of the NC S ; I I -C 111ci1 o m p ~ ~ f c d1sth c g c o n i c t r i c c c n t c r 01 th e ~c r n i i n o c l c sn LRGarid t h e average of the clii-cc1ionhol. tl icir c o o r d i -nate systems. respectively. Tliis inloi-mat ion is then1iro1xigated to tlic tei-iiiiiiorlcs i i i t h e nc t \vo Ik . ; i n dthey comp utc tlicir p o h i l i o n in NCS.

PERFORMANCE OF THESELFPOSITIONINGLGORITHM

AND LIMITATIONSW e h a v e e v a l i ~ i i c c lS P A . ;I l i d dcta I c d p e i - f o r -ni;incc r e s u l t s c ; i n l i e l o u n c l ii i I ( l ] . i i iclwork ottcrmiiiocics is i~cp i -csc i i ter li y :I gi; iph C;,,. w11c1-evertices cori-cspi i id I O c r n i i n o t l c ~ i i id ;in e d g eex is ts lict\vc.cn;I p i i i - of tcriiiincirlc~ t 1I1cyar c iiigr i p i C ; $. \v 11c I'C vc r t i c s e o 1-cs po 11d t c I t c I'minoclcs a n d ;i n cdgc exis t s I i c t w c c n ;I I x i i i - 01 't c r i i i iiodcs i I he y can p i - ~ i p i i g : ~c t lie i ii lo r i i1: i -t ion ; i l x i u t thc NCS t o c;icIi orher. W e o h s c i - \ .ctlic celgc cciniicc1ivity c i f tlicsc graphs. k i n g th eiiii 11111 1 m 11Im l i c i - of edges w l i ~ s ccnio\J;11 tr011i;I gi-apli m a k e ;I $i-;i1ili d i ~ c o i i n c c t e t l . 'lic edgeconnectivity (it'(;:, i s liere rcteri-cd io ;IS tlic c o i l -nectivi~y 1 tlic t c r m i n ~ i t l c ictnci1-k. and tlic cclgccciniiccti\. it! o f (;c;,3,, is I c f c i - ~ . c d o ;IS t he c o n -ncctivit!, 01' S P A .

C'lcsarly. the coiincctivir!; o f SI';\ \vi11 a l \v : iyslie stii:illcr. ci r cqti:tl ic ) 11i; i I 01. I I I C ~ c i - i i i i i i odcIL'I

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w o r k . I n t h e i d e a l c a s e , t h e i n f o r m a t i o n a b o u tt h e N C S w o u ld b e p r o p a g a t e d b e t w e e n e a c hp a i r of n o d e s i n e a c h o t h e r ' s p o w e r r a n g e , a n dthus the n ode connec t iv ity and SPA connectivitywould be equa l . T he connec t iv i ty of SP A t h e r e -f o r e s h o w s t h e s e n s i t i v i ty of SPA t o t h e g i v enconnectivity of the ne twork .F i g u r e 6 i l lus t ra tes the ne twork connec t iv i tyand the connectivity of SPA fo r a randomly gen-era ted ne twork of te rminodes . In th i s example ,for the cho sen powe r range network connectivityi s c l e a r ly h i g h e r t h a n SPA c o n n e c t iv i t y . T h ee x a m p l e also i n d i c at e s a n d o u r r e s u lt s i n [ lo]further show that not very high network connec-t iv i ty ensures tha t the SPA connectivity remainslarger than zero.T h e a c c u r a cy of t h e p o s i t i o n s o b t a i n e d b ySPA s t rongly depend s on the d is tance measure-ments and on t h e s p e e d of movement of termin-o d e s . H i g h e r s p e e d s of t e r m i n o d e s i n t r o d u c ea d d i t i o n a l e r r o r s , a n d t h e p o s i t i o n s of t e r m i n -o d e s n e e d to be f requent ly recompu ted . Inaccu-r a t e p o s i t i o n s of t e r m i n o d e s h a v e n e g a t i v ei m p a c t o n t h e p e r f o r m a n c e of t h e p a c k e t f o r -warding algorithm in a terminode network.INCENTIVE FOR COOPERATION

I n a t e rminode ne twork , (111 networking servicesa r e p r o v i d e d b y t h c t e r m i n o d e s t h e m s e l v e s .However. service provision is not in the interestof t e r m i n o d e s ( o r . more precisely, their users) ,b e c a u s c i t c o n s u m e s e n e r g y ( r e d u c e s b a t t e r yl i fe time) wi thout any d i rec t advantages . Th cre-f o r e , t c r n i i n o d c s ( t h c i r u s e r s ) m a y t e n d t o b eselfish: they use scrviccs provided by others, butd o not want to provide se rv ices to t h e c o m n i u -n i i y . ' Clc;trly. s c lf 'i s h i c r m i n o d e s m a y h i n d e rt li e f ti nct io n i ng o 1 t I1c n e t w o r k c o n i pl e e ly .T h e re lo re . ii sr i ii ti 1alion nicc ha 11 sm is rc q u i rc dt h a t c n c o u r ; ~ s c s sers t o p r o v i d e s e r v i ce s t oeach o ther .So far. wireless mobile :id hoc networks havehe e n e nvisio 11ed 111;I 11 f o r mi i t i ry a p p i cii t io n sand c r i s i s s i tua t ions (e .g . . i n rcsciie operations).I n hotl i c a s e s . t h e u s e r s of t h e n e t w o r k ; i r eas s u nied t o b e Io ng to t hc s a me ad m n i st r a io na u t h o r i t y (e.g.. t o the s a m e p l a t o o n or rescueteam) , and thus na tura l ly mot iva ted to c o o p e r -a te . I n terminode nctworks this assumption doesnot hold, because each user can be considered tobe ;I distinct adminis tratio n autho rity with selfishobjectives.O u r a p p r o a c h t o s ol v e th i s pr o b l e m a n ds t i m u l a te t e r m i n o d e s t o c o o p e r e i s b a s e d onthe introduction o f :I virtual currency. which wec a ll n u g l et s . a n d m e c h a n i s m s f o rc h a rg ng irew ard ng se rvicr usageiprov is ion. W eass u in c t 11 11t t tie t e r in i n o d e h a r d w a r e com swith an init ial s tock 0 1 ntiglcts . which h a v e no. . . .I..__D.. .#.-_.* -I--. _."' V t , t / . \ . s / / t l r t ' I l / ( / l / s1V.s I l ~ l i ~ l ,i l l1 . o l l l r o l l l i ' l ' t 1 1 / 1, ; t l t ' l - / l / / -/ l r o ( / l / \ ~ l l ~ l l i l l ~ l o r .12 / l / t l i ~ r s l ( / / / ( lI r t r 1 / - L y / l ( / r /.\c1:\ / . S l / t ~ l l \ ~

l ' t l i t / / , ~ l l l l l l ~ l ~ l ~ , \ l/ t I ~ lt ' . < l ~ r i r t ' t ' , s( 1 t l ' i ' l ' lI l O t I i ' rlt l t l . \t di / i l / t l / l i ~ / ~ l dt ~ / ~ l l i l t l l l i l l ~ . \I ~ i l i / t l o i l l / ; i z l 1 > 1 , 1/ i / i~ -io t o i l t i I I I I : ~ , ,i . t i l C , .

t i o ( l i , . \ . /hi;\. t i t t ~ t ~ p t ' rid i 1 1 ( 1 1 . t l i ~ 1 - t ,r l r t l v+.irl.c,. mrd[ / I ) IO / / l t / l ' i ' I / ? ( ' l ' t ' t / l / t R ' l / / 1 ' 1 . 1 ' / i l / ' k t / i ~ i l ' / i ' i / ~ ~ ~ ~1 1 1 d .ski//.< 0(1(1 / I / \ . C ' t 1 1 1 r i t1 t 1 1 I I ~ ~ I I I I ~ ~ I ~ I I I I I . \ .i t i i i ~ c \ ~ c ~ , :'t111 11tri.c~

I I

Ip Figure 6.A n example of ( a ) network connectivity and (h ) SP A connectivityfo r a network of 400 terminodes with a power range of I1 0 m arid a node den-sity of 400 terminodeslkm2.

m o n e t a r y v a l u e a n d c a n o n l y b e u s e d w i t h int e r m i n o d e n e t w o r k s . T h e n a t u r a l i d e a i s t h a tte rminodes mus t pay for a se rv ice to t h o s e t e r -minodes tha t p rovided the se rv ice . This makesn u g l e t s i n d i s p e n s a b l e f o r u s i n g t h e n e t w o r k ;t h u s , e a c h ( r a t i o n a l ) u s e r i s i n t e r e s t e d i ni n c r e a s i n g h e r s t o c k of nt ig lct s . Th e exc lus iveway to achieve this is to provide scrviccs to th ecommuni ty . I n th e sequel we il lustrate this ideaon a spec i f ic example : charg ingi rcwi i rd i iig thepacket forwarding service.Packe t forward ing can be paid by e i ther theor ig ina tor or the des t ina t ion of t h e p a c k e t . W eca l l the approach in which t h e o r i g i n a t o r p a y st h e pncke t purse m o d e l . [ i i th i s model . the se r -vice charge i s distr ibuted among t l ic torwardingt e r m i n o d e s in the fo l lowing way. W hcn sendingt lic packe t , the or ig in a tor loads i t w i t h ;I n u n -bcr of nugle t s suf f ic ien t to reach tlic dcstir i i i-t ion . Each forward ing te rminode acqui res sonicnugle t s f rom the packe t and t h u s increases \ l i es t o c k of i t s n u g l e t s . Th e e x a c t n u m b e r ofn u g l e t s ta k e n f r o m t h e p a c k e t n i ay d e p e n d onmany th ings , inc luding t h e cost of f o r w a r d i n g( in te rms of battery p o w e r ) , the ctirrcnt batter!!s ta tus of the forward ing te rminode . and i ts cur -r e n t n u m b e r of n u g l e t s . I f 21 p a c k e t d o c s n o thave enough nugle t s to be forwarded. i t is dis-c a r d e d . F i g u r e 7 i l lu s t r a t e s th e p a c k e t p u r s emodel .T h e m a i n a d v a n t a g e o f t h i s m o d e l . b e s i d e ss t imula ting coopera t io n am ong the te rm inodcs .is t h i i t i t niay also deter users from sending t isc-less data and overloadins t l ic network. The maindisadvantage is t h a t it might be diff icult to esti-n ia tc the number of nuglets rcquired t o reach ;Igiven destination.This lat ter problem is overcome in the p i c k e irrutle n ~ o d e lwhere th e destinii t ion pays t o r t h ep a c k e t f o r w a r d i n g . I n this m o d e l t h e p a c k e tdoes n o t carry nuglets. but is traded t u r nuglctsby forw:irding tcrni i i iodcs . Each I'urw;irding tcr-minode " b u y s " i t f r o m t l i c p r e v i o u s o n e f o rsonic nugIets.' ; ind " ~ ( c l I s " t to t h e nes t one (01-to th e d e s t i n a t i o n ) l o r n i o r c n t i g l c t s r h a n i tpaid. In this \ \ay . c:ich I'or\\arding n[>cie iiicre;is-

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.@ Figure 7 .In the packet purse model, the packet is loaded with nuglets by th eoriginator. Forwarding terminodes take some nuglets out of th e packet as IIreward for ihe pac ket forwarding service h e y provide. I n th e f i g r o q th e 1 i r i 1 7 1 -bers beside ih e terminodes represent their current stock of'nirg1et.r 11). Whensericlirig a packer to G, A loads 30 rtuglets in ii ( 6 ) . Foiwartlirig terinirlot1c.s Carid E theti take 5 (c ) and 12 (cl) nuglels ou1 o f t h e pricket, xspec t iwly .

e s i t s n u m b e r of nug le t s , and th e t o t a l cost off o r w a r d i n g ii packet is covered by tlic dcs t ina -tion of the packet .Besides relieving the originator o f est imatingthe number of nuglets that would be required todeliver a packet, another advantage of this modelis that it is better suited for multicast coninitin-cii io n s. A d is i d va n t age is t h i t t h is itp p ro ichdoes no t deter users from flooding the network.Clear ly , the models descr ibed a b o v e must bcsecu red and p ro tec ted aga ins t var ious a t t a c k s .The basic problems to be solvcd are related t unuglct forgery, protect ion oc th e integrity u f thepac k c t purse ( in the packe t pu r se m o d e l ) , a n dthe fa ir exchange o f packc ts for nuglcts (i n t h ep a c k e t t r ade model). Pro tcc t ion m a y h e hascclon t Ii e ii p p i a ion of ;I tUmpe I--es s I 11 11ii I c Iw;irc module i n each t c r m i n o d e . w h i c h c a n h cused fo r t l ie management of nuglcts and crypto-g raph ic cod ing o f messages [ 121. The cha l l cngcis to f ind a t rade-off between t h e rohus tncss oftlie solution a n c l its efficiency: foi-\vartling a sin-g le packc t should no t r equ i re complex c r y p t o -g r ii p h i c p r o I c o s a n d 11 e a v y e o ii pu t ii t io i :i Ieffort. hecause the cost of these m a y w e l l cscecdtlic v a l u e of thc service.I n orde r to sec tlic cffccts o f tlic introductionof t h e charging models on t h c pcrf 'ormancc o fthe network, we conductcd appropriate >i i i iu Ia-t i o n s . W e c o n i p a r c d t h e t l i rotrgl ipt i t o f t h eScodcsic packet 1'orw;ircling algori thm dcsc r ihc t lin ; i n ea r l i e r s e c t i o n to t l ic tlii-oiigliptit t h e n et -w o r k ; ich icvcs wlicn t h i s algorithm is cstcntlcdwith t h e packet ptirsc modcl. W c siiiitilatctl tw oversions o ! tlic p;ickct purse model: OIIC in wliiclie;ich fo i -wa d i ig t e rn i i ioclc ;icq U i es ;I c ( n s t a inum hc r o f i i t iglcts. fisccl hy llw ori$iiiitoi.. l'roiiit l i c p : i c k c ~ p u r s e ( I ' I ' M \\, i! l i i x c c l p e r - h o pcliiirgcs). a i i c l ; i i i o thc r iii \ v l i i c l i t l i c Iliiliilici. 01 .nuglcts chargcd hy tlic l 'or\ \~ : i i~di i igL . r i i i i i i o c k , 5 i\t l c t c r i n i i i c ~ l o p li y I i o p u z i i i s : iuc, t iol)i i i : pi- i i ic i -

p l e s ( PPM w i t h a u c t i o n s ) . F i g u r e 8 shows theresults we obtained for various initial battery lev-els. As expected, the throughput decreases whenth e PPMs arc used, since in this case packets canbe dropped by forwarding terminodes d ue to thelow value of tlic p a c k e t p u r s e or the f ixed per-hop charge. However, except for very low batteryIcvels, the difference among tlic pe r fo rmance ofth e three algorithms is n c ~ t ignificant (Icss than 5percent) . For fur ther deta i ls on tlie simulations.the interested rcader is referred to [ 131.SECURITY

Securi ty in networks ( in cluding wireless mobilead hoc networks) is concerned with confidential-ity and integrity of information, a s well as legiti-mate use and avai labi l i ty of services . Althoughthe securi ty concerns remain the same, there arece r t a in l imi ta t ions in wire le s s mob i l e ne tworkstha t a r e no t typically found in fixed networks,an d make security problems more complex:Mobile ad hoc networks do not have central-ized monitoring and/or management points.Bccausc of node mobil i ty , t l ie topology of

t h e network changcs dynamically: the nodesarc often spmdica l ly connec ted .T h e noc l c s communici i tc vi21 wireless links.wliicli c;in he eavesdropped easily.The nodes ar c o f t e n h a t t c r ~ - p o w c r c d ~here-fo re , coiiiiiiti i i icat i o n ii nd coni pu t i ng costsshould be rcduccd.1-lccausc o f their pliysiciil c1i;iractcristics. t l icn o d e s h a v e I ii i t ccl c o m p ut ing po\vc -( c c .-ta in lv less t1i;in nonmobile dev i c es ) .The node5 c ' t i i i he p l i y ~ i c ; i l l y ;iptiii-cd ; i n c lc o n i 17 ani isccl. ti i i i k c i 1 l ' i x c d i ic twor h s.wlicre c r i t i ca l d e v i ce s (e.g.. r o u t c r ~ );in h elocked i i i "s;ifc roo i i i s , ' .A I ' u i i c I a m c n t a I tool I O ;icliisec ti i i y 0b.j cc t vc s i c i - y l ~og ri i pl i y. cl-y[J ogi~il-p Ii y i s i ii d i p c i s a Ii C t o e

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O n e p o s si b le a p p r o a c h to s o l v i n g t h e k e ym a n a g e m e n t p r o b l e m m a y b c ba s e d o n t h er e p l a c e m e n t of c e r t i f i c a t i o n a u t h o r i t i e s w i thcommuni t ies of users ( a PGP-like solution [141)a n d t h e d i s t r i b u t i o n of the cer t i f ica te d i rec toryf u n c t i on a m o n g t h e t e r m i n o d e s ( a V H R - l i k es o l u t i o n ). A n o t h e r a p p r o a c h m a y be to a d o p t asys e ni t ha t in1 11 ic t Iy g u a ra ii te c s t he au t h en ic -t y of public keys. such as identi ty-based systems1151 a n d t h o s e u s i n g i m p l i c i t l y c e r t i f i e d k e y s[ 161; however, this kind of solution s t i l l requiresa t rus ted au thor i ty , a t l eas t a t sys tem in i tia l iza-t ion. We are investigating a way to provide pub-li c ke y cryptograp hy in a to ta l ly se l f -organizedmanner .CON L S oN

W e present a g loba l descr ip t ion of some aspec tsof the Te rmin odes pro jec t. Mo re spec i fica lly , i tcan be described at three levels of abstraction.At the mos t concre te leve l , t e rminod es a re at e c h n ic a l c h a l l e n g e i n t h e g e n e r a l d o m a i n ofm o b i l e a d h o c n c t w o r k s . W e p r e s e n t a g l o b a ldcs gn o f ;I te r ni i node ne t w ) r k . Sc I - o rg ;In zcdr o u ti n g, I o ca ion ii i ii n iigc ni e n ,G PS - c ss p o s -t i o n i n g , ancl niiglets a r e o u r m a i n ingred icn ts .The comhina io n 0 1 a pi ih ic cnvironti icn t withtl ic absence o f ;in o p e r a t o r p w x :I nuiiihcr o fchallenges; we h w e showii how o u r glohal designaddresses them h y focusing on self-org~inizalion.A rciiiark:thlc feature ot' t e rminodes is thc pres-ciicc o f nuglcrs: they ar c iiscd iis the basis f o rincit ing users t o c o o p e r a t e . ;ind ;IS ;I congestionC O n t rol niech ii n ism for d iscovc r y ;In d r o i t i ngnicclianisnis. W e have focused i n this art icle ontlic most has ic c len ic i i t s o f ;I t c r n i i n o d c iiil 'ra-sti-ucttirc, n i i i i n l y t h o a c rcliitcd to t h e n e t w o r klaycr . roughly speaking . There ;ire many o t h e ri n i p o i - t i t i i t t o p i c s i n the terminode p ro jec t tha t\v c d o no t :rddrcss i n t h i s ; i r t i c l c . One i s t h eorg:\ 11za io n o f softw a re :ind conipo n c ii s. Se f -organization c ~ i ic \s icwed ;is ;I f o r m of coiiipc-t i t on. and ii t c I-m n o d e c o m p o n e i m a y Ixcoiiieohsolctc simply I~cc; iusc i iorc e f f i c i en t o n e s existw h i c h ai-c more ;ibtc to g c i the m o a t utility outo f t h e network . Thus. ;ill of o u r c o n i I x ) i i c n t s a rcmodif'icd ;IS t h e y hcconic ohsolcte. Second, cr -iii notlc ;Ipp ica io i s ii rc ;I wa y to c o n ii ti c t h ep a t h o p e n e d ti p Iy I n t e r n e t p e e r - t o - p e e r c o n i -p U i ng ;i p p ica t io 11s s tic h ;t s G n i tc I :\: ne two kc r a w i n g. se ;LC f o i i fo ii i 11t ion re sou rcc s. ;I li dIiighwiiy traffic jnm i i lcr ts a rc just ;I fcw csan i -plcs. A ke y t'c;ltiii.c uf al l o u r desigil i s sclf-orga-i i i z a t i o n . W h i l e thi5 teiids t o ni;ikc t h e des ignconsi clc i.ahl!; nio 1-c coni pl ex. wc 1x1hciicfi ts i n terms o f rohustncss a n d case of tisch;ivc t h c potcnti ; i l t o c h a n g e i h c w a y i i i which\\.e t h i n k of nctu'orks tind inlorination systems.A t 1 lie i i i t ci-nictli it c lc\~cl.ci-mi nodes ;Ire ;i nintcl lcc~ u;iI t i i i iulus: i n o t l i c i - \vorclx. they a r c ;I\ \ ; I ! to I u c I c.i-ciitivity i n orde r to i t lcntify iicwIcsc:ii.ch clia I C n s c s . An c s a r i i p le ci f :I q i i c h t io i:tIi-c:id!~gciici-;irccl i h ' . l - l c i \ ~ d< i vc d e f i n e ;I f{ir-nxtl iiioticl 1~ Liir cxcli;ingc'!.'

; \ I llic h i g h e s t Ic\,cl CIL ' ; i l ) h t r : i c t i o i i . i c rmi i i -ode' :ire ;I s l ) c i ~ l ~ i l / ~ ~ l ) l i t i L . ~ i lsioii. in \v l i ich tlicLI I i ini ; I i ii 01. h c .-()g;i n c'cc o i > id c rc d . I n l C c d . w c I ) C , I i

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REFERENCES( 1 1 J.-P. Hubaux et al.. "Toward Self-organized Mobile Ad-

Hoc Networks, The Terminodes Proiect." IEEE CommunMag, Jan 2001(21 J P Macker, V D Park, an d M S Corson, "Mobile andWireless internet Services: Putting the Pieces Together."E Commun Mag. . this issue.[3 ] S . Giordano. "Mobile Ad-hoc Networks," Handboo k ofWireless Network an d Mobil e Computing. I Stojmen-ovic. Ed., Wiley, to appear.

141 http.//www.t erminodes org[SI M. R Perlman and Z. J Haas. "Determining the OptimalConfiguration fo r the Zone Routing Protocol." IEEE/SAC, vol. 17 . no. 8, Aug. 1999161 L. Elazevic, S Giordano, and 1. Y . Le Boudec, "SelfOrganized Terminode Rout ing." Tech rep.DSC/2000/040, Swiss Federal I nsti tut e of Technology,Lausanne, Dec. 2000.

171 D. 1 Watts, "Small Worlds, The Dynamics of NetworksBetween Order and Randomness," Princeton Uni vPress, 1999.

(81 V W. S. Wong. V. C. M. Leung. "Location Managementfor Next-Generation Personal Communications Net -works," IEEE Net wor k, Sept./Oct. 2000.

(91 1. Li et al.. "A Scalable Location Service for GeographicAd Hoc Routing." Mobicom '00, oston, MA, 2000[ l o ] S . Capkun. M. Hamdi. and J . P. Hubaux, "GPS-free

Positioning in Mobil e Ad-Hoc Networks," Proc 3 4t hHICSS. Jan. 2001.I11 M. . Silventoin en and T. Rantalainen. "Mob ile St ationEmergency Locati ng in GSM." l E E E lnt 'f , Conf Pers.Wireless Commun., 199G.I 21 L. Buttyan and 1. P Hubaux. "Enforcing Service Avail-abi lit y in Mob ile Ad-ho c WANs," Proc Is t 1EEE:ACMWksp Mobi le Ad H o c Net an d Comp. (MobiHOC).Aug. 2000, pp 87-96.131 L Buttyan and J. P. Hubaux. "Nuglets. A Virtual Currencyto Stimulate Cooperation In Self-Organized Mobile Ad-hocNetworks." Tech. rep. DSV2OOl/OOlISwiss Federal Insti-tute o f Technology, Lausanne. Jan 2001(141 P. Zini niermann. T I w Of f ioa lPGP User's Goide. MITPress. 1995.

I 1 51 A Shamir, "ldcn tity-based Cryptosysteiiis and Signa-tu re Schemes." Advances 111 Cryptology - CRYPT0 '84.Springer-Verlag, 1985. pp 47-53[IGI M Girciui t. "Self-Certifi ed Public Keys," Advances 411C!yptology EUROCRYPT '97, Springer-Verlag. 1991.1pp 490-97

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At the highestlevel of

abstraction.terminodes are asocietallpoliticalvision, in whichthe u lt imate aim

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considered.

BIOGRAPHIES~ U K A BLAZEVIA I jub ica.b lazev ic@epf l .ch ) received her B.S.degree in 1993 f rom the Faculty of Electrical Engineering,Belgrade, Yugoslavia. From 1993 to 1996 she worked as aR&D engineer at the Institute M ihajl o Pupin in Belgrade.Currently she i s a Ph.0. student i n the Institute for Com-puter Communicat ions and Appl icat ions ( ICA) at theDepartment of Communication Systems of the Swiss Feder-al Institute of Technology, Lausanne (EPFL). Her researchinterests include routing in large mobile ad hoc networksand scalable multicast routing.LEVENTE UWAN levente.buttyan@epfl.ch) is a research andteachin g assistant and a Ph.D. stu den t in ICA at theDepartm ent o f Com municat ion Systems of EPFL. His cur-rent research interests include security issues in electroniccommerce and in self-organized ad hoc networ ks. Hereceived his M.Sc. degree i n compu ter science fromBudapest University o f Technology and Economics in 1995.SROJAN ~ A P K U Nsrdan.capkun@epfl.ch) received his B.Sc. inelectrical engineeringkomputer science from the Universityof Split, Croatia, in 1998. In 1999 he joined the doctoralschool i n communi cation systems at the Department ofCom muni cati on Systems, EPFL. In September 2000 hejoined ICA, where he is working towa rd his Ph.D. HIS cur-rent research interests include security and positi oningissues in mob ile ad hoc networks.SICVIA GIOROANOsilvia.giordano@epfI.ch) received her Ph.D.at the beg innin g of 1999 from ICA at EPFL. She is currentlyworking as seniodfir st assistant at ICA. Since 1999 she hasbeen an Editor of I Communications Magazine. She is

currently guest edit or of a Special Issue on Mobile Ad HocNetworks that will appear on MONET, as well as a SpecialIssue on M obile Ad Hoc Networks that will appear on clus-ter computing. Her current research interests include trafficcontrol and mobile ad hoc WANs.JEAN-PIERREUBAUXjean-pierre.hubaux@epfl.ch) joined EPFLas an associate professor in 1990; he was promot ed to fu llprofessor in 1996. He is co-founder and co-dlrector o f ICA.His current research is focused on mobile networking with aspecial interest in self-org anized mobile ad hoc network sand therefore in "terminodes." At the beginning of his activ-ity at EPFL, he defined the first curriculum in communicationsystems. In October 19 99 he became the first chair o f th enewly created Communication Systems Department. Beforejoining EPFL. he spent 10 years in France wi th Alcatel, wherehe was involved in R&D activities, m ostly in the area ofswitching systems architecture and software.JEAN-YVESE BOUDECj ean -yves. leboudec@epf l .ch ) i s fullprofessor at EPFL. He grad uate d fro m Ecole NormaleSuperieure de Saint-Cloud, Paris, where he obtained theAgregation in mathematics (rank 4) in 1980. He receivedhis doctorate in 1984 f rom the Univers ity of Rennes,France, and became an assistant professor at INSNIRISA,Rennes. In 1987 he joined Bell Northern Research, Ottawa,Canada, as a member of scientific staff in the Network andProduct Traffic Design Department. In 1988 he joined theIBM Zurich Research Laboratory, Rueshchlikon, Switzerland,where he was manager of the Customer Premises NetworkDepartment. In 1994 he formed the Laboratoire de Reseauxde Communication at EPFL. which in autumn 1997 becamepart of ICA. His interests are in the architecture and Derfor-mance of communication systems

ERRATAI n he a r t i c l e by L h y d Wood e / al. e n t i t l e d "Effects on TCP ofouting Strategies in S a t c l l i t e Cons te l la t ions , " which a p p e a r e d i ne March 2001 issuc o f I E E E Cot?znzutzicalions M u g m i n e ( p g .'2), p o r t i o n s of Figs. 3a n d 5 were reproduced incorrectly. Those)rtions are reproduced here as they should have appeared.-____

0.600

FTP transfer over Spaceway NGSO using TCP SACKAmount of file transferred as seen by application (K) x l o 34.03. 83. 63.43. 23. 02 .82.62. 42 22.01.81. 61. 41. 21 .o0.80.60.40 .2 I

2 0 0 400 6 0 0 8 0 0 1000T ime (5)(b)

I Progress of FTP transfer be twee n terminal s atQuito and Tokyo us ing Spaceway NGSO

FTP transfer over Teledesic using TCP SACKA mo u n t of f i le transferred as seen by applica tion (K ) x 1O3