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sequence number at t he r ecei vi ngend.RC: Ret r ansm ssi ons Count t o keept r ack of t he number of r et r ansm s-s i ons f or each t r ans m t t ed dat abl oc k.ESC: Er r or St atus Code t o i ndi cat egr oup ACK: An acknowl edgement f i el dused t o conc i se l y r epr esent agr oup of ACK/ NAK( s) .

var i ous e r r or c ondi t i ons .

The al gor i t h m of t h i s er r or - c or r ec t i onscheme i s di v i ded i nt o t hr eef unct i ona l components , namel y ,t r ansm t process , r ece i ve pr ocess andACK/ NAK pr ocess.A. t r ans m t P roces s

When a dat a bl ock ent er s node 1, i t i sassi gned a GN and st or ed i n t he TB.The hi ghest speed t r unk avai l abl e t henwi l l be sel ec t ed f or i t s t r ans m s s i on.I f al l t r unks ar e bus y , t hen whi cheverbecomes avai l abl e f i r s t wi l l bes el ec ted. Once a t runk i s s e l ec t ed,t hena) ass i gn an SN equal t o curr ent AQpoi nt er .b) put t he SN, TB addr ess of t hi s dat abl ock i n t he AQ of t he sel ectedt r unk .c ) t r a ns m t t he dat a bl oc k wi t h i t sGN, SN and i t s encoded CRC over t hes el ect ed t r unk t o the r ecei v i ngnode.d) r eset RC and i ncr ement t he AQpoi nter by one.

B. Recei ve Pr ocess[I] Recei ve wi t h no e r r o rwhen a dat a bl ock arr i ves at t her ecei v i ng node wi t h no er r or det ect ed:i t s SN mat ches t he number poi nt ed t oby t he RQ poi nt er ; and t he DB i s notf ul l , t hena) s t ore the dat a bl ock i n t he DB.b) put t he SN of t hi s dat a bl ock i nt he RQ; set t he ESC t o 0.c) put t he GN and t he DB address oft he dat a bl ock i n t he GN i ndext abl e accor di ng t o i t s a ss oc i at edGN; s e t i t s ESC t o 0.d) i ncrement t he RQ poi nter by one.e) prepare ' group ACK' and pi ggybacki t wi t h t he next out go i ng databl ock whenever poss i bl e.I f t he DB i s f u l l , no ACK i s sent backeven i f t he dat a b l ock i s r ecei vedc or r ec t l y.121 Recei ve wi t h er r orWhen t he dat a bl ock arr i ves at t her ecei vi ng node and has been det ect edt o be i n er r or , t hen

s t ore t he data b l ock i n t he DB.put t he SN of t hi s da ta bl ock i nt he RQ; set t he ESC t o 1.put t he GN and t he DB addr ess oft he dat a bl ock i n t he GN i ndext abl e accor di ng t o i t s as s oc i at edGN; s e t i t s ESC to 1.i ncr ement t he RQ poi nt er by one.pr epar e ' gr oup ACR' and pi ggybacki t wi t h t he next out goi ng databl ock whenever poss i bl e.[31 Recei ve dupl i cate bl ockI f t h e i nc om ng dat a bl oc k i s er r or -f r ee, but det ec t ed t o be a dupl i cat ebl ock, t hena) di scar d t hi s dat a boc .b ) prepar e ' group ACK f or thedupl i cat e and al l o ther bl ocksacknowl edged t oget her wi t h t hi sdupl i cat e bl ock.1 4 1 Mi s s i ns bl ock( s )I f t he i ncom ng da ta bl ock i s e r r o r -f r ee and no t a dupl i cat e b l ock , buti t s SN does not mat ch t he numberpoi nted t o by t he RQ poi nter , t hens t o re t hi s da ta bl ock i n t he DB.put t he SN i n the RQ.put t he GN and DB addr ess of t he

data bl ock i n t he GN i ndex t abl e :s et i t s ESC t o 0.s e t t he RQ, poi n te r t o SN+l .prepare group ACK' f or ther ece i ved data bl ock and a l lexpec t ed bl ocks not r ece i ved, i . e.f r om t he RQ number t o SN- 1 t o t hesendi ng node.C. ACK/ NAK pr ocesses111 Process i ng ACKWhen an ACK i s r ecei ved by t he sendi ngnode . al l co r r espondi ng ent r i esspeci f i ed by t he ACK ar e r emoved f r omt he AQ.121 Pr ocessi nq NAKWhen t he sendi ng node r ecei ves a NAK,i t checks t he cor r espondi ng RC. I ft he maxi mum number of r et r ansm ss i onsi s not exceeded, t hena) put t he TB address of t hi s dat abl ock and i t s GN i n t he AQ ent r ypoi nted t o by t he AQ poi nt er.b ) i ncr ement RC by one.c ) r et r ans m t t hi s dat a bl ock wi t h theor i gi nal GN and an SN whi ch i si ndi cated by t he cur r ent AQpoi nt e r .d) i ncr ement t he AQ poi nter by one.Ot her wi se , i f t he maxi mum number ofr et r ans m s s i on i s exceeded, t hena) r e t r ans m t t hi s data b l ock ands i gna l t he r ecei v i ng end t hat t hef or war d er r or - c or r ec t i on i st empor ar i l y suspended.b) c l ear the cont ent of t he AQ andr es ta r t t he AQ poi nt er f r om one.

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The r ecei ved data bl ocks i n t hede l i ver y buf f er wi l l be r eassembl edaccor di ng t o t he GCF s er i al number anddel i ver ed t o t he des t i na t i oncomput ers . Al t hough consecut i ve dat abl ocks may be t r ansm t t ed vi adi f f e rent t r unks , we have a t wo-st agef l ow cont r o l mechani sm t o ensur e t hatt he dat a bl ocks wi l l be r ecei ved byt he des t i nat i on i n t he or der of theGCF ser i a l number . The f i r s t - s t agef l ow cont r ol mechani sm uses t he SN ofeach t r unk. The second- st age f l owcont r ol uses t he GCF ser i al number ofeach node.We use t he ' gr oup ACK' t o over come t hepr obl ems of asymmet r i c t r af f i c ar r i valr a tes , di f f er ent t r unk r a tes andmul t i pl e t r unks.EXAMPLEThe f ol l owi ng exampl e i s i nt ended t ohel p i l l us t r at e t he oper at i o n of t hi spr o t o col . W t h r ef er e nc e t o F i gur e 2 ,i t i s assumed t hat 13 dat a bl ocks wi t hGN sequenced f r om 11 t o 23 ar et r ans m t t ed f r o m node 1 t o node 2 vi at wo channel s wi t h di f f e rentt r ansm ss i on speeds. Fordemonst r at i on pur pose, snap shot s oft he cont ent s of AQs, RQs and t he GNI ndex Tabl e are t aken at t wo i nst ant sof t i me.

At t he f i r st acknowl edgement , node 2has r ecei ved 6 bl ocks wi t h GN 11 t o16. RQl , RQ2 and GN I ndex Tabl e ar eupdated t o i ndi cate t hat b l ocks wi t hGN 11 t hr ough 13 have been r ecei veder r or - f r ee and subsequent l y de l i ver ed.However , err or - f r ee bl ock wi t h GN 16was not del i vered si nce bl ock wi t h GN14 has been f ound m ssi ng and bl ockwi t h GN 15 has been det ect ed wi t her r or .A ' gr oup ACK' i s t hen prepar ed andt hen r et urned t o node 1 acknowl edai ngt he pr oper r ecei pt s of t he good bl ocks( i . e. bl ocks wi t h SN 1, 2, and 4 onchannel 1, and SN 1 on channel 2) andr equest i ng t he r et r ansm ss i on of t hem ssi ng bl ock wi t h SN 3 ( GN 14) f r o mchannel 1 and t he err or bl ock wi t h SN2 ( GN 15) f r omchannel 2.Upon r ecei pt of t he ' group ACK' atnode 1, ent r i es cor r espondi ng t o t heACKed good bl ocks ar e del et ed f r om AQ1and AQ2. To prepar e f or t her et r ans m s s i o ns , t he t r ans m t - pr o c es swi l l s el ec t an avai l abl e c hannel ,ass i gn a new SN, and i ncr ement RC byone, i n t he assoc i a te AQ ( i . e. AQ1) .Not e t hat t he or i g i nal GN r emai ns wi t ht he r et r ansm t t ed bl ock so that ther eassembl i ng of dat a bl ocks based onGN ser i al number can be proper l yper f or med usi ng t he GN I ndex Tabl e at

t he r ecei v i ng end. I n t hi s exampl e ,t he m ssi ng bl ock wi t h GN 14 i sass i gned SN 7 on channel 1 and t heerr or bl ock wi t h GN 15 i s assi gned SN4 on channel 2.Si m l ar l y, t he second acknowl edgementACKs t he good bl ocks and NAKs t heerr or b l ock wi t h SN 5 on channel 1.Bl ocks wi t h GN 14 t hrough 16 can nowbe del i vered; whi l e bl ocks wi t h GN 18t hr ough 22 wi l l be hel d unt i l t heerr or b l ock wi t h GN 17 i sr e t r ans m t t ed.3 PERF ORMANCE ANALYSI S3. 1 DELAY TI MEAverage del ay f r om node 1 t o node 2.

DR1 =( W1 + PT + TH) * ( l - &) +nZ [ ( i +l ) (W1+PT+TH)+i(W2+PT)1.Ei- 1- E)i =lAver age del ay f r om node 2 t o node 1.

DR2 =( W2 + PT + T L ) - ( l - t )nZ [ ( i +l ) W2+PT+TL) +i ( W1+PT) 1 E ~ I- )i =lWher en = number of r e tr ansm ss i onsW = dat a bl ock average wai t i ngt i me at node i sec

pi = l oadi ng f ac to r at node i .PAi =bl ock ar r i val r a te at node i .E = dat a bl ock er r o r r at e.PT = one way pr opagat i on del ay.TH = 2[ ( LV + HD) / ( CH + CL)]TL = ( LV + HD) / CLLV = aver age dat a bl ock l engt h.HD = l engt h of over head i n each

2= Pi / [ ( l - pi ) PAi ]

dat a bl ock .3. 2 QUEUE AND BUFFER MANAGEMENTLet AQ1, AQ2, and AQ3 t o r epr esent t hemaxi mum AQ l engt h of t he hi gh- speedt r unk , t he l ow- speed t r unk f r om node 1t o node 2, and t he l ow- speed t r unkf r om node 2 t o node 1, r e s pec t i vel y.Al so , l et LP1, LP2, and LP3 r epr esentt he l oop l engt h of t he hi gh- speedt r unk, t he l ow- speed t r unk f r om node 1t o node 2, and t he l ow- speed t r unkf r om node 2 t o node 1, r es pec t i v el y .Then,LP1 =l +( W1 + W2 +2PT +TL ) * CH/ ( LV+HD)

LP2 = 2 + ( W1 + W2 + 2PT) / TLLP3 =l+[W1 + W2 +2PT +( LV+HD) / CH] / TLAQi = m n( Ni , LPi )

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Ni = maxi mum number of out st andi ngBFTi = t he s i ze of t r ans m t buf f er atbl oc ks .node i .BFTl = AQl + AQ2 P1/ ( 1- P1BFT2 = AQ, P2/ ( l - P2)

Def i ne RQ1, RQ2, and RQ, t o r epres entt he maxi mum RQ l ength of t he hi gh-speed t r unk and t he l ow- speed t r unkat node 2, and t he l ow- speed t r unk atnode 1.RQi = m n I Ni , ( n' LPi )BFDi = t he s i ze of t he del i ver ybuf f er at node i .BFD2=m n(N, [ a+E. n- ( LP1 LP2 - 1 ) J )

BFDl = m nl N, [ a + E. n. (LP, - l ) ] )a = t he buf f er r equi r ed due t ochannel l oadi ng bet ween t her ecei v i ng node and t hedest i nat i on comput er .4. NUMERI CAL EXAMPL ESI n t he f o l l owi ng exampl es, we assumea. CH = 2. 048 MBPS, CL = 224 KBPSb. dat a bl ock l ength = 4800 bi t s

( i nc l udi ng t he over head)c. ACK l ength = 192 bi t sd. one- way propagat i on del ay,e. N = 65535PT = 1. 25 sec

Usi ng t hese assumpt i ons and t hepr evi ous ana l yses , we obt a i nLP1 = AQ, = 1100 ( n=2 and ~=10 - ~LP2 = LP3 = AQ2 = AQ, 120

Fi g. 3 shows t he aver age del ay DR1 asa f unct i on of t he normal i zed l oad pwi t h di f f er e nt bl oc k er r or r at es .Fi g. 4 shows t he aver aue si ze of t het r a ns m t buf f er v s. p. I n4t hi s f i gur e ,we as sume n=2 and = 10 . We al s ost udy t he ef f ect on BFT and BFD byvar yi ng t he val ues of n and E. Wef i nd t hat BFT i s not s ens i t i ve t o nwhen i s smal l and t hat BFD i s verys ens i t i ve t o E.5 CONCLUSI ONS AND DI SCUSSI ONSI n t hi s paper, a modi f i ed S- ARQ schemei s proposed as an er r or cor r ect i ons cheme f or t he GCF upgr adecommuni cat i ons syst em The oper at i onof t hi s scheme was pr esent ed andi l l us t r ated by exampl es. An anal ys i sof queue and buf f er r equi r ement s f ort hi s scheme was al so hi ghl i ghted.F i nal l y , numer i cal exampl es were thenprovi ded t o demonst r at e t heper f ormance of t he proposed al gor i t hmTo m ni m ze del ay, we woul d need t ot urn t he er r or - cor rec t i on pr oces s of fwhen t he pr e- establ i shed thr eshol d f orr et r ans m s s i ons i s ex ceeded. Thi swi l l al s o reduce the s i ze of t hedel i v er y - b uf f er r e qui r e d at t her ece i v i ng node .REFERENCES[l] Sof t ware Def i n i t i on Document ,Mark I VA Net work Consi derat i onPro j ec t , Er ro r Cor r ec t i on andSwi t chi ng Ass embl y, GMH- 5208-OP- A, p. 52 , J P L, J ul y 1982.

NND D

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FIGUR 1

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TO CHANNEL 1

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f nd ACKNOWLEDGEMENT 1stACNOWLEDGEMENT ToCHANNELINSTANT AT 1st ACKNOWLEDGEMENTSN TBADDRESS RC SN TBADDRESS RC

GN 14 GN 104 GN165 G N 1 7

GN19 (TRANSMITTER SIDE)

GN INDEXGN DB ADDRESS ESC

ADDRESS3 MISSING4

(RECEIVERSIDE) I l ADDRESS IINSTANT AT 2 nd ACKNOWLEDGEMENT

GN 22

(TRANSMITTER SIDE)

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ETRANSMITED BLOCK RRORED DURING TRANSM ISSION MISSING DURING TRANSMISSION.--- .FIGURE

3.53009

EmNwtN

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o 0 0.5 .oT H R O U G H P U T , pFIGURE 4

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