Din n Sinh vin Bch Khoa H Ni > Ging ng Bch Khoa > VIN IN T VIN THNG > H thng Vin thng > Aloha protocol

Xem phin bn y : Aloha protocolxkens Mnh mi tham gia Forum..cho gp bi n(mi dch cho bi tp ln) cho vui nha!!8-> Giao thc ALOHA

PDA

10-05-2007, 10:56 PM

Mt tp cc tiu chun trao i thng tin gia hai h thng my tnh hoc hai thit b my tnh vi nhau c gi l giao thc (Protocol).C c giao thc cn c gi l nghi thc hoc nh c ca mng my tnh. nh gi kh nng ca mt mng c phn chia bi cc trm nh th no. H s ny c quyt nh ch yu bi medium access (hiu qu s dng mi trng truy xut) ca giao thc, mi trng ny dng tuyn tnh hoc vng.. Giao thc Aloha l giao thc OSI lp 2 cho mng LAN vi Broadcast topology V c bn bc u phng thc ny l: * Nu c d liu,th gi d liu * Nu tin xung t vi ng truyn khc th s gi li sau Pure Aloha c thng lng cc i khong 18.4%.iu ny c ngha l khong 81.6% tng bng thng b b ph mt do xung t gi.Tnh ton thng lng c bn da trn gi thit rng ton b qu trnh ti sau mt im Phn Phi vi s trung bnh ti ca 2G ti trn mi 2X giy.Bi vy tham s Lam-a ti im Phn Phi l 2G.nh t ti G=0.5 dn n kt qu l thng lng cc i 0.184 (tc l 18.4%) Mt bc pht trin mi ca giao thc Aloha l Slotted Aloha,n ri rc ha khe thi gian v tng thng lng cc i ln ti 36.8%. Mt trm ch c th gi bt u ca khe thi gian,v do xung t c gim thiu. Trong trng hp s trung bnh ca ton b lng n l G ti trn 2X giy,n y tham s Lam-a ti G. Thng lng t cc i cho G=1. * * * * C th lm gim xc sut xung t khung To cc khe thi gian rng X giy C c trm ch c php g khung khi bt u mt khe thi gian Backoff time l s nguyn ln khe thi gian

C n ch rng cc c tnh ca Aloha vn khng c nhiu s khc bit so vi Wi-Fi,cn xung t h thng vn cn nhiu bt cp trong cc h thng ny.V d nh 802.11b vi khong 2-4Mb/s thng lng thc,tri vi l thuyt l 11Mbit/s thng lng cc i. Lch s Norm Abramson l mt gio s khoa hc ti Stanford,v ng thi cng l mt ngi ham m lt sng . Sau khi n Hawaii vo nm 1969, Abramson c li ngh c lm vic ti i Hc Hawaii.Nm 1970 Abramson bt u lm vic ti mt trm h thng giao tip d liu v tuyn c ti tr bi Larry Robert ni c nhim v lin lc cc o Hawai li vi nhau. Mng chuyn mch gi khng dy u tin trn th gii . Abramson sau c gng c IMP(Interface Message Processor) t Roberts v kt ni ALOHAnet vi ARPANET t ti o chnh vo nm 1972.y l ln u tin mt h thng khc c kt ni vi ARPAnet. Mt s bin th ca giao thc Aloha(nh Slotted Aloha) cn xut hin sau giao thc giao din v tuyn cc mng khng dy thng dng nh ARDIS,Mobitex,C DPD,GSM Aloha Mode * X: Transmission time X=L/R * S:Throughput s Frame truyn thnh cng trong X giy * G:Load s yu cu truyn trung bnh trong X giy * P success: xc sut truyn khung thnh cng * S=GP success * Bt u truyn khung trong X giy s c kh nng b xung t,ngc li s c kh nng thnh cng nu truyn sau 2X giy

ngo_bakabon

10-05-2007, 11:28 PM

B xung n: Vi mc ch ban u l to ra mt mng thng tin lin lc v tuyn gia cc khu vc khc nhau ca i hc Hawai nm trn cc o khc nhau, v th mng ALOHA ra i. L mt trong cc phng thc truyn tin s dng chung knh truyn dn, y l knh v tuyn; v s dng knh v tuyn nn mt lc ch c mt trm gi v mt trm nhn. C th c nhiu trm trn cng mt mng. Khi 2 trm cng gi th s gy ra xung t v buc phi gi li gi tin, tuy nhin y p dng thut ton Back-off tc l sau mt khong thi gian c chn da trn thut ton cng tn th mi gi li, i vi Slotted ALOHA th Back-off l mt s nguyn ln ca Time-slot, tuy nhin khng qu 15. ALOHA trong th ng ca Hawai c ngha l "xin cho", hiu ngha y va l mng chuyn mch gi v tuyn u tin trn th gii va ging nh khi ta gi in, ta ni "alo". Mt phng thc khc c nguyn tc kh ging ALOHA, tuy nhin a ra nhiu gii php gim, trnh v khc phc li hn l C SMA (C arrier Sensor Media Access - a truy cp cm ng (theo di) sng mang), tc l n s theo di knh truyn chung, nu pht hin c trm khc ang gi thng tin th n s khng gi tin,...

Binhjuventus

11-05-2007, 02:21 AM

Nn c thm v xc sut, phn b Poisson c s dng tnh lu lng trong mng. C i ny nu ai i su vo mng s thy c s dng trong l thuyt hng i nh gi lu lng ca mng cng nh cht lng ca mng. C th th t k r v k nghin cu lm. Trong quang hc lng t, ngi ta cng s dng phn b Poisson (Nghin cu trong thng tin quang). Bon, t cng ch thm vo thng tin nho nh mi ngi bit khi nghin cu v Giao thc Aloha... BJ

Atox

11-05-2007, 04:16 PM

h h Aloha . ci ni hnh nh dng trong mn truyn s liu. Nh hi xa dch ti liu mn ny c thy n. Nh l trong MPLS c xi n nh gi cht lng dch v. Bc Bnh lm vi bi v aloha i cho anh em m tm mt :-" X pam cht :biggrin:

ngo_bakabon

11-05-2007, 09:34 PM

h h Aloha . ci ni hnh nh dng trong mn truyn s liu. Nh hi xa dch ti liu mn ny c thy n. Nh l trong MPLS c xi n nh gi cht lng dch v. Bc Bnh lm vi bi v aloha i cho anh em m tm mt :-" X pam cht :biggrin: h, by gi lm g cn mn truyn s liu, n nm trong h thng vin thng roi. Hiu xut truyn tun theo phn b poison, m a phn cc tin trnh ngu nhin (thc t) u tun theo Poison nn ni chung l rt rt nn hc cn thn Poison

Binhjuventus

12-05-2007, 05:35 AM

Hiu xut truyn tun theo phn b poison, m a phn cc tin trnh ngu nhin (thc t) u tun theo Poison nn ni chung l rt rt nn hc cn thn Poison C i ny th tu em , Nu c iu kin bn em nn xem li l thuyt xc sut v cc phn b c bn. Trong l thuyt mng thng tin. Phn b Poisson thch hp vi vic m hnh ho mt mng c v cng nhiu nt mng nhng c nhn mi nt mng li ng gp khng ng k vo ti ca ton mng. Lc c th dng phn b Poisson m hnh ho lng gi tin ti nt mng tnh theo mi slot. Bon, l vn v Phn b Poisson dng trong nh gi kh nng nghn mng vi Giao thc Aloha. Ngoi ra, phi k n Phn b Gauss (Hc trong thng tin th hu ht nhiu cng c m hnh ho theo phn b ny, da vo lut gii hn trung tm), khi nghin cu hin tng Fading, ngi ta s dng phn b rayleigh. Tu vo loi Fading m s dng c phn b Rice. c thm v phn b C HI-Square hiu thm v c hai phn b ny (Php cng bnh phng cc bin ngu nhin). Trong l thuyt hng i, ngi ta s dng phn b Poisson. Ni chung, l thuyt xc sut c c ng dng nhiu trong k thut thng tin. Tu tng lnh vc, tu iu kin

Ni chung, l thuyt xc sut c c ng dng nhiu trong k thut thng tin. Tu tng lnh vc, tu iu kin nghin cu m h hay s dng cc phn b no m t qu trnh ngu nhin. Thn, BJ

Binhjuventus

12-05-2007, 05:36 AM

h h Aloha . ci ni hnh nh dng trong mn truyn s liu. Nh hi xa dch ti liu mn ny c thy n. Nh l trong MPLS c xi n nh gi cht lng dch v. Bc Bnh lm vi bi v aloha i cho anh em m tm mt :-" X pam cht :biggrin: Tic l anh k i su vo lnh vc ny, c ni cng ch l nhng ci rt chung chung, ai cng c th c v hiu c :D Hy vng c ai chuyn lm v lnh vc ny ni s sinh ng hn ;)

xkens

14-05-2007, 11:14 PM

Hix,ton cc cao th thui khng !! ....C ho mnh sa ci ngo_bakabon C SMA (C arrier Sensor Media Access - a truy cp cm ng (theo di) sng mang).............L Multiple c!

ngo_bakabon

16-05-2007, 11:53 PM

Hix,ton cc cao th thui khng !! ....C ho mnh sa ci ngo_bakabon C SMA (C arrier Sensor Media Access - a truy cp cm ng (theo di) sng mang).............L Multiple c! , ng roi, ch hiu ngh th no m g th @Binh: cm n bc

Binhjuventus @Ngo: Khng c g, din n trao i m, rt vui c trao i vi cc bn!

17-05-2007, 03:33 AM

sofpast

17-05-2007, 02:14 PM

Hix,ton cc cao th thui khng !! ....C ho mnh sa ci ngo_bakabon C SMA (C arrier Sensor Media Access - a truy cp cm ng (theo di) sng mang).............L Multiple c! h th vic s dng hai t ny khc nhau nh no nu dng Media thay v Multple? Nu i th c c khng? C i C SMA ny c hai loi l C SMA/C D v C SMA/C A/ Bn phn tch s khc nhau lun th nh?

dtvt1985

17-05-2007, 05:43 PM

C SMA:ci tin so vi Aloha ch bt u truyn khi khng thy node no ang truyn.Mi cng ngh C SMA u phi lng nghe ng truyn trc khi truyn. C SMA/C D:C i tin so vi C SMA ch bit dng vic truyn khi c xung t xy ra. C SMA/C A:C i tin so vi C SMA/C D ch khi thy ng truyn khng bn vn khng truyn ngay m i 1 khong thi gian ngu nhin ri kim tra li ng truyn.Nu tip tc thy khng bn th mi truyn.ng dng dng trong wireless! H H:-t :))

sofpast

17-05-2007, 06:11 PM

C SMA:ci tin so vi Aloha ch bt u truyn khi khng thy node no ang truyn.Mi cng ngh C SMA u phi lng nghe ng truyn trc khi truyn. C SMA/C D:C i tin so vi C SMA ch bit dng vic truyn khi c xung t xy ra. C SMA/C A:C i tin so vi C SMA/C D ch khi thy ng truyn khng bn vn khng truyn ngay m i 1 khong thi gian ngu nhin ri kim tra li ng truyn.Nu tip tc thy khng bn th mi truyn.ng dng dng trong wireless! H H:-t :)) h h hi bn lun nh:

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N lng nghe bng cch no? C SMA lng nghe bao nhiu lu th truyn? Bn mi th i mi h? C SMA/ C D: dng truyn khi c collision ri c re-send k? C SMA/ C A: C tht l ch c C A l k se- send ngay k? C n C D th k tht h?

- He trong wireless dng C A th okie ri nhng m n dng cch no cm nhn v truyn nh? h h cn vic ni v bn cht khc bit gia C D v C A th t thy th ny: - C D: chp nhn collide ri re - send - C A: avoidance nh. Ngay t objective ban u ca chng difference ri m.

xkens @Ngo: Khng c g, din n trao i m, rt vui c trao i vi cc bn! ng vy...dich vy thui,ch thc ra mnh cng chng hiu lm..:-??

17-05-2007, 10:42 PM

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