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wWw.VietLion.Com G G I I Á Á O O T T R R Ì Ì N N H H M M N N G G M M Á Á Y Y T T Í Í N N H H Hà nội 11-2000

Giao trinh mang may tinh (Toan tap)

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Tìm hiểu mạng máy tính là đề tài dnah2 cho những ai yêu thích và đang muốn tìm tòi học về Mạng. (sưu tầm)

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  • 1. wWw.VietLion.Com GGIIOO TTRRNNHH MMNNGG MMYY TTNNHH H ni 11-2000
  • 2. wWw.VietLion.Com Chng 1 S lc lch s pht trin ca mng my tnh Vo gia nhng nm 50 khi nhng th h my tnh u tin c a vo hot ng thc t vi nhng bng n in t th chng c kch thc rt cng knh v tn nhiu nng lng. Hi vic nhp d liu vo cc my tnh c thng qua cc tm ba m ngi vit chng trnh c l sn. Mi tm ba tng ng vi mt dng lnh m mi mt ct ca n c cha tt c cc k t cn thit m ngi vit chng trnh phi c l vo k t mnh la chn. Cc tm ba c a vo mt "thit b" gi l thit b c ba m qua cc thng tin c a vo my tnh (hay cn gi l trung tm x l) v sau khi tnh ton kt qu s c a ra my in. Nh vy cc thit b c ba v my in c th hin nh cc thit b vo ra (I/O) i vi my tnh. Sau mt thi gian cc th h my mi c a vo hot ng trong mt my tnh trung tm c th c ni vi nhiu thit b vo ra (I/O) m qua n c th thc hin lin tc ht chng trnh ny n chng trnh khc. Cng vi s pht trin ca nhng ng dng trn my tnh cc phng php nng cao kh nng giao tip vi my tnh trung tm cng c u t nghin cu rt nhiu. Vo gia nhng nm 60 mt s nh ch to my tnh nghin cu thnh cng nhng thit b truy cp t xa ti my tnh ca h. Mt trong nhng phng php thm nhp t xa c thc hin bng vic ci t mt thit b u cui mt v tr cch xa trung tm tnh ton, thit b u cui ny c lin kt vi trung tm bng vic s dng ng dy in thoi v vi hai thit b x l tn hiu (thng gi l Modem) gn hai u v tn hiu c truyn thay v trc tip th thng qua dy in thoi. Hnh 1.1. M hnh truyn d liu t xa u tin Nhng dng u tin ca thit b u cui bao gm my c ba, my in, thit b x l tn hiu, cc thit b cm nhn. Vic lin kt t xa c th thc hin thng qua nhng vng khc nhau v l nhng dng u tin ca h thng mng. Trong lc a ra gii thiu nhng thit b u cui t xa, cc nh khoa hc trin khai mt lot nhng thit b iu khin, nhng thit b u cui c bit cho php ngi s dng nng cao c kh nng tng tc vi my tnh. Mt trong nhng sn phm quan trng l h thng thit b u cui 3270 ca IBM. H thng bao gm cc mn hnh, cc h thng iu khin, cc thit b truyn thng c lin kt vi cc trung tm tnh ton. H thng 3270 c gii thiu vo nm 1971 v c s dng dng m rng kh nng tnh ton ca trung tm my tnh ti cc vng xa. lm gim nhim v truyn thng ca my tnh trung tm v s lng cc lin kt gia my tnh trung tm vi cc thit b u cui, IBM v cc cng ty my tnh khc sn xut mt s cc thit b sau: Thit b kim sot truyn thng: c nhim v nhn cc bit tn hiu t cc knh truyn thng, gom chng li thnh cc byte d liu v chuyn nhm cc byte ti my tnh trung tm x l, thit b ny cng thc hin cng vic ngc li chuyn tn hiu tr li ca my tnh trung tm ti cc trm xa. Thit b trn cho php gim bt c thi gian x l trn my tnh trung tm v xy dng cc thit b logic c trng. Thit b kim sot nhiu u cui: cho php cng mt lc kim sot nhiu thit b u cui. My tnh trung tm ch cn lin kt vi mt thit b nh vy l c th phc v cho tt c cc thit b u cui
  • 3. wWw.VietLion.Com ang c gn vi thit b kim sot trn. iu ny c bit c ngha khi thit b kim sot nm cch xa my tnh v ch cn s dng mt ng in thoi l c th phc v cho nhiu thit b u cui. Hnh 1.2: M hnh trao i mng ca h thng 3270 Vo gia nhng nm 1970, cc thit b u cui s dng nhng phng php lin kt qua ng cp nm trong mt khu vc c ra i. Vi nhng u im t nng cao tc truyn d liu v qua kt hp c kh nng tnh ton ca cc my tnh li vi nhau. thc hin vic nng cao kh nng tnh ton vi nhiu my tnh cc nh sn xut bt u xy dng cc mng phc tp. Vo nhng nm 1980 cc h thng ng truyn tc cao c thit lp Bc M v Chu u v t cng xut hin cc nh cung cp cc dnh v truyn thng vi nhng ng truyn c tc cao hn nhiu ln so vi ng dy in thoi. Vi nhng chi ph thu bao chp nhn c, ngi ta c th s dng c cc ng truyn ny lin kt my tnh li vi nhau v bt u hnh thnh cc mng mt cch rng khp. y cc nh cung cp dch v xy dng nhng ng truyn d liu lin kt gia cc thnh ph v khu vc vi nhau v sau cung cp cc dch v truyn d liu cho nhng ngi xy dng mng. Ngi xy dng mng lc ny s khng cn xy dng li ng truyn ca mnh m ch cn s dng mt phn cc nng lc truyn thng ca cc nh cung cp. Vo nm 1974 cng ty IBM gii thiu mt lot cc thit b u cui c ch to cho lnh vc ngn hng v thng mi, thng qua cc dy cp mng cc thit b u cui c th truy cp cng mt lc vo mt my tnh dng chung. Vi vic lin kt cc my tnh nm trong mt khu vc nh nh mt ta nh hay l mt khu nh th tin chi ph cho cc thit b v phn mm l thp. T vic nghin cu kh nng s dng chung mi trng truyn thng v cc ti nguyn ca cc my tnh nhanh chng c u t. Vo nm 1977, cng ty Datapoint Corporation bt u bn h iu hnh mng ca mnh l "Attached Resource Computer Network" (hay gi tt l Arcnet) ra th trng. Mng Arcnet cho php lin kt cc my tnh v cc trm u cui li bng dy cp mng, qua tr thnh l h iu hnh mng cc b u tin. T n nay c rt nhiu cng ty a ra cc sn phm ca mnh, c bit khi cc my tnh c nhn c s dng mt cnh rng ri. Khi s lng my vi tnh trong mt vn phng hay c quan c tng ln nhanh chng th vic kt ni chng tr nn v cng cn thit v s mang li nhiu hiu qu cho ngi s dng. Ngy nay vi mt lng ln v thng tin, nhu cu x l thng tin ngy cng cao. Mng my tnh hin nay tr nn qu quen thuc i vi chng ta, trong mi lnh vc nh khoa hc, qun s, quc phng, thng mi, dch v, gio dc... Hin nay nhiu ni mng tr thnh mt nhu cu khng th thiu c. Ngi ta thy c vic kt ni cc my tnh thnh mng cho chng ta nhng kh nng mi to ln nh: S dng chung ti nguyn: Nhng ti nguyn ca mng (nh thit b, chng trnh, d liu) khi c tr thnh cc ti nguyn chung th mi thnh vin ca mng u c th tip cn c m khng quan tm ti nhng ti nguyn u.
  • 4. wWw.VietLion.Com Tng tin cy ca h thng: Ngi ta c th d dng bo tr my mc v lu tr (backup) cc d liu chung v khi c trc trc trong h thng th chng c th c khi phc nhanh chng. Trong trng hp c trc trc trn mt trm lm vic th ngi ta cng c th s dng nhng trm khc thay th. Nng cao cht lng v hiu qu khai thc thng tin: Khi thng tin c th c s dng chung th n mang li cho ngi s dng kh nng t chc li cc cng vic vi nhng thay i v cht nh: p ng nhng nhu cu ca h thng ng dng kinh doanh hin i. Cung cp s thng nht gia cc d liu. Tng cng nng lc x l nh kt hp cc b phn phn tn. Tng cng truy nhp ti cc dch v mng khc nhau ang c cung cp trn th gii. Vi nhu cu i hi ngy cng cao ca x hi nn vn k thut trong mng l mi quan tm hng u ca cc nh tin hc. V d nh lm th no truy xut thng tin mt cch nhanh chng v ti u nht, trong khi vic x l thng tin trn mng qu nhiu i khi c th lm tc nghn trn mng v gy ra mt thng tin mt cch ng tic. Hin nay vic lm sao c c mt h thng mng chy tht tt, tht an ton vi li ch kinh t cao ang rt c quan tm. Mt vn t ra c rt nhiu gii php v cng ngh, mt gii php c rt nhiu yu t cu thnh, trong mi yu t c nhiu cch la chn. Nh vy a ra mt gii php hon chnh, ph hp th phi tri qua mt qu trnh chn lc da trn nhng u im ca tng yu t, tng chi tit rt nh. gii quyt mt vn phi da trn nhng yu cu t ra v da trn cng ngh gii quyt. Nhng cng ngh cao nht cha chc l cng ngh tt nht, m cng ngh tt nht l cng ngh ph hp nht.
  • 5. wWw.VietLion.Com Chng 2 Nhng khi nim c bn ca mng my tnh Vi s pht trin ca khoa hc v k thut, hin nay cc mng my tnh pht trin mt cch nhanh chng v a dng c v quy m, h iu hnh v ng dng. Do vy vic nghin cu chng ngy cng tr nn phc tp. Tuy nhin cc mng my tnh cng c cng cc im chung thng qua chng ta c th nh gi v phn loi chng. I. nh ngha mng my tnh Mng my tnh l mt tp hp cc my tnh c ni vi nhau bi ng truyn theo mt cu trc no v thng qua cc my tnh trao i thng tin qua li cho nhau. ng truyn l h thng cc thit b truyn dn c dy hay khng dy dng chuyn cc tn hiu in t t my tnh ny n my tnh khc. Cc tn hiu in t biu th cc gi tr d liu di dng cc xung nh phn (on - off). Tt c cc tn hiu c truyn gia cc my tnh u thuc mt dng sng in t. Ty theo tn s ca sng in t c th dng cc ng truyn vt l khc nhau truyn cc tn hiu. y ng truyn c kt ni c th l dy cp ng trc, cp xon, cp quang, dy in thoi, sng v tuyn ... Cc ng truyn d liu to nn cu trc ca mng. Hai khi nim ng truyn v cu trc l nhng c trng c bn ca mng my tnh. Hnh 2.1: Mt m hnh lin kt cc my tnh trong mng Vi s trao i qua li gia my tnh ny vi my tnh khc phn bit mng my tnh vi cc h thng thu pht mt chiu nh truyn hnh, pht thng tin t v tinh xung cc trm thu th ng... v ti y ch c thng tin mt chiu t ni pht n ni thu m khng quan tm n c bao nhiu ni thu, c thu tt hay khng. c trng c bn ca ng truyn vt l l gii thng. Gii thng ca mt ng chuyn chnh l o phm vi tn s m n c th p ng c. Tc truyn d liu trn ng truyn cn c gi l thng
  • 6. wWw.VietLion.Com lng ca ng truyn - thng c tnh bng s lng bit c truyn i trong mt giy (Bps). Thng lng cn c o bng n v khc l Baud (ly t tn nh bc hc - Emile Baudot). Baud biu th s lng thay i tn hiu trong mt giy. y Baud v Bps khng phi bao gi cng ng nht. V d: nu trn ng dy c 8 mc tn hiu khc nhau th mi mc tn hiu tng ng vi 3 bit hay l 1 Baud tng ng vi 3 bit. Ch khi c 2 mc tn hiu trong mi mc tn hiu tng ng vi 1 bit th 1 Baud mi tng ng vi 1 bit. II. Phn loi mng my tnh Do hin nay mng my tnh c pht trin khp ni vi nhng ng dng ngy cng a dng cho nn vic phn loi mng my tnh l mt vic rt phc tp. Ngi ta c th chia cc mng my tnh theo khong cch a l ra lm hai loi: Mng din rng v Mng cc b. Mng cc b (Local Area Networks - LAN) l mng c thit lp lin kt cc my tnh trong mt khu vc nh trong mt to nh, mt khu nh. Mng din rng (Wide Area Networks - WAN) l mng c thit lp lin kt cc my tnh ca hai hay nhiu khu vc khc nhau nh gia cc thnh ph hay cc tnh. S phn bit trn ch c tnh cht c l, cc phn bit trn cng tr nn kh xc nh vi vic pht trin ca khoa hc v k thut cng nh cc phng tin truyn dn. Tuy nhin vi s phn bit trn phng din a l a ti vic phn bit trong nhiu c tnh khc nhau ca hai loi mng trn, vic nghin cu cc phn bit cho ta hiu r hn v cc loi mng. III. S phn bit gia mng cc b v mng din rng Mng cc b v mng din rng c th c phn bit bi: a phng hot ng, tc ng truyn v t l li trn ng truyn, ch qun ca mng, ng i ca thng tin trn mng, dng chuyn giao thng tin. a phng hot ng: Lin quan n khu vc a l th mng cc b s l mng lin kt cc my tnh nm trong mt khu vc nh. Khu vc c th bao gm mt ta nh hay l mt khu nh... iu hn ch bi khong cch ng dy cp c dng lin kt cc my tnh ca mng cc b (Hn ch cn l hn ch ca kh nng k thut ca ng truyn d liu). Ngc li mng din rng l mng c kh nng lin kt cc my tnh trong mt vng rng ln nh l mt thnh ph, mt min, mt t nc, mng din rng c xy dng ni hai hoc nhiu khu vc a l ring bit. Tc ng truyn v t l li trn ng truyn: Do cc ng cp ca mng cc b c xy dng trong mt khu vc nh cho nn n t b nh hng bi tc ng ca thin nhin (nh l sm chp, nh sng...). iu cho php mng cc b c th truyn d liu vi tc cao m ch chu mt t l li nh. Ngc li vi mng din rng do phi truyn nhng khong cch kh xa vi nhng ng truyn dn di c khi ln ti hng ngn km. Do vy mng din rng khng th truyn vi tc qu cao v khi t l li s tr nn kh chp nhn c. Mng cc b thng c tc truyn d liu t 4 n 16 Mbps v t ti 100 Mbps nu dng cp quang. Cn phn ln cc mng din rng cung cp ng truyn c tc thp hn nhiu nh T1 vi 1.544 Mbps hay E1 vi 2.048 Mbps.
  • 7. wWw.VietLion.Com ( y bps (Bit Per Second) l mt n v trong truyn thng tng ng vi 1 bit c truyn trong mt giy, v d nh tc ng truyn l 1 Mbps tc l c th truyn ti a 1 Megabit trong 1 giy trn ng truyn ). Thng thng trong mng cc b t l li trong truyn d liu vo khong 1/107 -108 cn trong mng din rng th t l vo khong 1/106 - 107 Ch qun v iu hnh ca mng: Do s phc tp trong vic xy dng, qun l, duy tr cc ng truyn dn nn khi xy dng mng din rng ngi ta thng s dng cc ng truyn c thu t cc cng ty vin thng hay cc nh cung cp dch v truyn s liu. Ty theo cu trc ca mng nhng ng truyn thuc c quan qun l khc nhau nh cc nh cung cp ng truyn ni ht, lin tnh, lin quc gia. Cc ng truyn phi tun th cc quy nh ca chnh ph cc khu vc c ng dy i qua nh: tc , vic m ha. Cn i vi mng cc b th cng vic n gin hn nhiu, khi mt c quan ci t mng cc b th ton b mng s thuc quyn qun l ca c quan . ng i ca thng tin trn mng: Trong mng cc b thng tin c i theo con ng xc nh bi cu trc ca mng. Khi ngi ta xc nh cu trc ca mng th thng tin s lun lun i theo cu trc xc nh . Cn vi mng din rng d liu cu trc c th phc tp hn nhiu do vic s dng cc dch v truyn d liu. Trong qu trnh hot ng cc im nt c th thay i ng i ca cc thng tin khi pht hin ra c trc trc trn ng truyn hay khi pht hin c qu nhiu thng tin cn truyn gia hai im nt no . Trn mng din rng thng tin c th c cc con ng i khc nhau, iu cho php c th s dng ti a cc nng lc ca ng truyn hay nng cao iu kin an ton trong truyn d liu. Dng chuyn giao thng tin: Phn ln cc mng din rng hin nay c pht trin cho vic truyn ng thi trn ng truyn nhiu dng thng tin khc nhau nh: video, ting ni, d liu... Trong khi cc mng cc b ch yu pht trin trong vic truyn d liu thng thng. iu ny c th gii thch do vic truyn cc dng thng tin nh video, ting ni trong mt khu vc nh t c quan tm hn nh khi truyn qua nhng khong cch ln. Cc h thng mng hin nay ngy cng phc tp v cht lng, a dng v chng loi v pht trin rt nhanh v cht. Trong s pht trin s lng nhng nh sn xut t phn mm, phn cng my tnh, cc sn phm vin thng cng tng nhanh vi nhiu sn phm a dng. Chnh v vy vai tr chun ha cng mang nhng ngha quan trng. Ti cc nc cc c quan chun quc gia a ra cc nhng chun v phn cng v cc quy nh v giao tip nhm gip cho cc nh sn xut c th lm ra cc sn phm c th kt ni vi cc sn phm do hng khc sn xut.
  • 8. wWw.VietLion.Com Chng 3 M hnh truyn thng I. S cn thit phi c m hnh truyn thng mt mng my tnh tr mt mi trng truyn d liu th n cn phi c nhng yu t sau: Mi my tnh cn phi c mt a ch phn bit trn mng. Vic chuyn d liu t my tnh ny n my tnh khc do mng thc hin thng qua nhng quy nh thng nht gi l giao thc ca mng. Khi cc my tnh trao i d liu vi nhau th mt qu trnh truyn giao d liu c thc hin hon chnh. V d nh thc hin vic truyn mt file gia mt my tnh vi mt my tnh khc cng c gn trn mt mng cc cng vic sau y phi c thc hin: My tnh cn truyn cn bit a ch ca my nhn. My tnh cn truyn phi xc nh c my tnh nhn san sng nhn thng tin Chng trnh gi file trn my truyn cn xc nh c rng chng trnh nhn file trn my nhn san sng tip nhn file. Nu cu trc file trn hai my khng ging nhau th mt my phi lm nhim v chuyn i file t dng ny sang dng kia. Khi truyn file my tnh truyn cn thng bo cho mng bit a ch ca my nhn cc thng tin c mng a ti ch. iu trn cho thy gia hai my tnh c mt s phi hp hot ng mc cao. By gi thay v chng ta xt c qu trnh trn nh l mt qu trnh chung th chng ta s chia qu trnh trn ra thnh mt s cng on v mi cng on con hot ng mt cch c lp vi nhau. y chng trnh truyn nhn file ca mi my tnh c chia thnh ba module l: Module truyn v nhn File, Module truyn thng v Module tip cn mng. Hai module tng ng s thc hin vic trao i vi nhau trong : Module truyn v nhn file cn c thc hin tt c cc nhim v trong cc ng dng truyn nhn file. V d: truyn nhn thng s v file, truyn nhn cc mu tin ca file, thc hin chuyn i file sang cc dng khc nhau nu cn. Module truyn v nhn file khng cn thit phi trc tip quan tm ti vic truyn d liu trn mng nh th no m nhim v c giao cho Module truyn thng. Module truyn thng quan tm ti vic cc my tnh ang hot ng v san sng trao i thng tin vi nhau. N cn kim sot cc d liu sao cho nhng d liu ny c th trao i mt cch chnh xc v an ton gia hai my tnh. iu c ngha l phi truyn file trn nguyn tc m bo an ton cho d liu, tuy nhin y c th c mt vi mc an ton khc nhau c dnh cho tng ng dng. y vic trao i d liu gia hai my tnh khng ph thuc vo bn cht ca mng ang lin kt chng. Nhng yu cu lin quan n mng c thc hin module th ba l module tip cn mng v nu mng thay i th ch c module tip cn mng b nh hng.
  • 9. wWw.VietLion.Com Module tip cn mng c xy dng lin quan n cc quy cch giao tip vi mng v ph thuc vo bn cht ca mng. N m bo vic truyn d liu t my tnh ny n my tnh khc trong mng. Nh vy thay v xt c qu trnh truyn file vi nhiu yu cu khc nhau nh mt tin trnh phc tp th chng ta c th xt qu trnh vi nhiu tin trnh con phn bit da trn vic trao i gia cc Module tng ng trong chng trnh truyn file. Cch ny cho php chng ta phn tch k qu trnh file v d dng trong vic vit chng trnh. Vic xt cc module mt cch c lp vi nhau nh vy cho php gim phc tp cho vic thit k v ci t. Phng php ny c s dng rng ri trong vic xy dng mng v cc chng trnh truyn thng v c gi l phng php phn tng (layer). Nguyn tc ca phng php phn tng l: Mi h thng thnh phn trong mng c xy dng nh mt cu trc nhiu tng v u c cu trc ging nhau nh: s lng tng v chc nng ca mi tng. Cc tng nm chng ln nhau, d liu c ch trao i trc tip gia hai tng k nhau t tng trn xung tng di v ngc li. Cng vi vic xc nh chc nng ca mi tng chng ta phi xc nh mi quan h gia hai tng k nhau. D liu c truyn i t tng cao nht ca h thng truyn ln lt n tng thp nht sau truyn qua ng ni vt l di dng cc bit ti tng thp nht ca h thng nhn, sau d liu c truyn ngc ln ln lt n tng cao nht ca h thng nhn. Ch c hai tng thp nht c lin kt vt l vi nhau cn cc tng trn cng th t ch c cc lin kt logic vi nhau. Lin kt logic ca mt tng c thc hin thng qua cc tng di v phi tun theo nhng quy nh cht ch, cc quy nh c gi giao thc ca tng. Hnh 3.1: M hnh phn tng gm N tng II. M hnh truyn thng n gin 3 tng Ni chung trong truyn thng c s tham gia ca cc thnh phn: cc chng trnh ng dng, cc chng trnh truyn thng, cc my tnh v cc mng. Cc chng trnh ng dng l cc chng trnh ca ngi s dng c thc hin trn my tnh v c th tham gia vo qu trnh trao i thng tin gia hai my tnh. Trn mt my tnh vi h iu hnh a nhim (nh Windows, UNIX) thng c thc hin ng thi nhiu ng dng
  • 10. wWw.VietLion.Com trong c nhng ng dng lin quan n mng v cc ng dng khc. Cc my tnh c ni vi mng v cc d liu c trao i thng qua mng t my tnh ny n my tnh khc. Vic gi d liu c thc hin gia mt ng dng vi mt ng dng khc trn hai my tnh khc nhau thng qua mng c thc hin nh sau: ng dng gi chuyn d liu cho chng trnh truyn thng trn my tnh ca n, chng trnh truyn thng s gi chng ti my tnh nhn. Chng trnh truyn thng trn my nhn s tip nhn d liu, kim tra n trc khi chuyn giao cho ng dng ang ch d liu. Vi m hnh truyn thng n gin ngi ta chia chng trnh truyn thng thnh ba tng khng ph thuc vo nhau l: tng ng dng, tng chuyn vn v tng tip cn mng. Tng tip cn mng lin quan ti vic trao i d liu gia my tnh v mng m n c ni vo. d liu n c ch my tnh gi cn phi chuyn a ch ca my tnh nhn cho mng v qua mng s chuyn cc thng tin ti ch. Ngoi ra my gi c th s dng mt s phc v khc nhau m mng cung cp nh gi u tin, tc cao. Trong tng ny c th c nhiu phn mm khc nhau c s dng ph thuc vo cc loi ca mng v d nh mng chuyn mch, mng chuyn mch gi, mng cc b. Tng truyn d liu thc hin qu trnh truyn thng khng lin quan ti mng v nm trn tng tip cn mng. Tng truyn d liu khng quan tm ti bn cht cc ng dng ang trao i d liu m quan tm ti lm sao cho cc d liu c trao i mt cch an ton. Tng truyn d liu m bo cc d liu n c ch v n theo ng th t m chng c x l. Trong tng truyn d liu ngi ta phi c nhng c ch nhm m bo s chnh xc v r rng cc c ch ny khng ph thuc vo bn cht ca tng ng dng v chng s phc v cho tt c cc ng dng. Tng ng dng s cha cc module phc v cho tt c nhng ng dng ca ngi s dng. Vi cc loi ng dng khc nhau (nh l truyn file, truyn th mc) cn cc module khc nhau. Hnh 3.2 M hnh truyn thng 3 tng Trong mt mng vi nhiu my tnh, mi my tnh mt hay nhiu ng dng thc hin ng thi (Ti y ta xt trn mt my tnh trong mt thi im c th chy nhiu ng dng v cc ng dng c th thc hin ng thi vic truyn d liu qua mng). Mt ng dng khi cn truyn d liu qua mng cho mt ng dng khc cn phi gi 1 module tng ng dng ca chng trnh truyn thng trn my ca mnh, ng thi ng dng kia cng s gi 1 module tng ng dng trn my ca n. Hai module ng dng s lin kt vi nhau nhm thc hin cc yu cu ca cc chng trnh ng dng. Cc ng dng s trao i vi nhau thng qua mng, tuy nhin trong 1 thi im trn mt my c th c nhiu ng dng cng hot ng v vic truyn thng c chnh xc th cc ng dng trn mt my cn phi c mt a ch ring bit. R rng cn c hai lp a ch:
  • 11. wWw.VietLion.Com Mi my tnh trn mng cn c mt a ch mng ca mnh, hai my tnh trong cng mt mng khng th c cng a ch, iu cho php mng c th truyn thng tin n tng my tnh mt cch chnh xc. Mi mt ng dng trn mt my tnh cn phi c a ch phn bit trong my tnh o. N cho php tng truyn d liu giao d liu cho ng ng dng ang cn. a ch c gi l im tip cn giao dch. iu cho thy mi mt ng dng s tip cn cc phc v ca tng truyn d liu mt cch c lp. Cc module cng mt tng trn hai my tnh khc nhau s trao i vi nhau mt cch cht ch theo cc qui tc xc nh trc c gi l giao thc. Mt giao thc c th hin mt cch chi tit bi cc chc nng cn phi thc hin nh cc gi tr kim tra li, vic nh dng cc d liu, cc quy trnh cn phi thc hin trao i thng tin. Hnh 3.3 V d m hnh truyn thng n gin Chng ta hy xt trong v d (nh hnh v trn): gi s c ng dng c im tip cn giao dch 1 trn my tnh A mun gi thng tin cho mt ng dng khc trn my tnh B c im tip cn giao dch 2. ng dng trn my tnh A chuyn cc thng tin xung tng truyn d liu ca A vi yu cu gi chng cho im tip cn giao dch 2 trn my tnh B. Tng truyn d liu my A s chuyn cc thng tin xung tng tip cn mng my A vi yu cu chuyn chng cho my tnh B (Ch rng mng khng cn bit a ch ca im tip cn giao dch m ch cn bit a ch ca my tnh B). thc hin qu trnh ny, cc thng tin kim sot cng s c truyn cng vi d liu. u tin khi ng dng 1 trn my A cn gi mt khi d liu n chuyn khi cho tng vn chuyn. Tng vn chuyn c th chia khi ra thnh nhiu khi nh ph thuc vo yu cu ca giao thc ca tng v ng gi chng thnh cc gi tin (packet). Mi mt gi tin s c b sung thm cc thng tin kim sot ca giao thc v c gi l phn u (Header) ca gi tin. Thng thng phn u ca gi tin cn c:
  • 12. wWw.VietLion.Com a ch ca im tip cn giao dch ni n ( y l 3): khi tng vn chuyn ca my B nhn c gi tin th n bit c ng dng no m n cn giao. S th t ca gi tin, khi tng vn chuyn chia mt khi d liu ra thnh nhiu gi tin th n cn phi nh s th t cc gi tin . Nu chng i n ch nu sai th t th tng vn chuyn ca my nhn c th pht hin v chnh li th t. Ngoi ra nu c li trn ng truyn th tng vn chuyn ca my nhn s pht hin ra v yu cu gi li mt cch chnh xc. M sa li: m bo cc d liu c nhn mt cch chnh xc th trn c s cc d liu ca gi tin tng vn chuyn s tnh ra mt gi tr theo mt cng thc c sn v gi n i trong phn u ca gi tin. Tng vn chuyn ni nhn thng qua gi tr xc nh c gi tin c b li trn ng truyn hay khng. Bc tip theo tng vn chuyn my A s chuyn tng gi tin v a ch ca my tnh ch ( y l B) xung tng tip cn mng vi yu cu chuyn chng i. thc hin c yu cu ny tng tip cn mng cng to cc gi tin ca mnh trc khi truyn qua mng. Ti y giao thc ca tng tip cn mng s thm cc thng tin iu khin vo phn u ca gi tin mng. Hnh 3.4: M hnh thit lp gi tin Trong phn u gi tin mng s bao gm a ch ca my tnh nhn, da trn a ch ny mng truyn gi tin ti ch. Ngoi ra c th c nhng thng s nh l mc u tin. Nh vy thng qua m hnh truyn thng n gin chng ta cng c th thy c phng thc hot ng ca cc my tnh trn mng, c th xy dng v thay i cc giao thc trong cng mt tng. III. Cc nhu cu v chun ha i vi mng Trong phn trn chng ta xem xt mt m hnh truyn thng n gin, trong thc t vic phn chia cc tng nh trong m hnh trn thc s cha . Trn th gii hin c mt s c quan nh chun, h a ra hng lot chun v mng tuy cc chun c tnh cht khuyn ngh ch khng bt buc nhng chng rt c cc c quan chun quc gia coi trng. Hai trong s cc c quan chun quc t l:
  • 13. wWw.VietLion.Com ISO (The International Standards Organization) - L t chc tiu chun quc t hot ng di s bo tr ca Lin hp Quc vi thnh vin l cc c quan chun quc gia vi s lng khong hn 100 thnh vin vi mc ch h tr s pht trin cc chun trn phm vi ton th gii. Mt trong nhng thnh tu ca ISO trong lnh vc truyn thng l m hnh h thng m (Open Systems Interconnection - gi tt l OSI). CCITT (Commit Consultatif International pour le Telegraphe et la Tlphone) - T chc t vn quc t v in tn v in thoi lm vic di s bo tr ca Lin Hip Quc c tr s chnh ti Geneva - Thy s. Cc thnh vin ch yu l cc c quan bu chnh vin thng cc quc gia. T chc ny c vai tr pht trin cc khuyn ngh trong cc lnh vc vin thng. IV. Mt s m hnh chun ha 1. M hnh OSI (Open Systems Interconnection) M hnh OSI l mt c s dnh cho vic chun ho cc h thng truyn thng, n c nghin cu v xy dng bi ISO. Vic nghin cu v m hnh OSI c bt u ti ISO vo nm 1971 vi mc tiu nhm ti vic ni kt cc sn phm ca cc hng sn xut khc nhau v phi hp cc hot ng chun ho trong cc lnh vc vin thng v h thng thng tin. Theo m hnh OSI chng trnh truyn thng c chia ra thnh 7 tng vi nhng chc nng phn bit cho tng tng. Hai tng ng mc khi lin kt vi nhau phi s dng mt giao thc chung. Trong m hnh OSI c hai loi giao thc chnh c p dng: giao thc c lin kt (connection - oriented) v giao thc khng lin kt (connectionless) Giao thc c lin kt: trc khi truyn d liu hai tng ng mc cn thit lp mt lin kt logic v cc gi tin c trao i thng qua lin kt ny, vic c lin kt logic s nng cao an ton trong truyn d liu. Giao thc khng lin kt: trc khi truyn d liu khng thit lp lin kt logic v mi gi tin c truyn c lp vi cc gi tin trc hoc sau n. Nhim v ca cc tng trong m hnh OSI: Tng ng dng (Application layer): tng ng dng quy nh giao din gia ngi s dng v mi trng OSI, n cung cp cc phng tin cho ngi s dng truy cp v s dng cc dch v c m hnh OSI. Tng trnh by (Presentation layer): tng trnh by chuyn i cc thng tin t c php ngi s dng sang c php truyn d liu, ngoi ra n c th nn d liu truyn v m ha chng trc khi truyn bo mt. Tng giao dch (Session layer): tng giao dch quy nh mt giao din ng dng cho tng vn chuyn s dng. N xc lp nh xa gia cc tn t a ch, to ra cc tip xc ban u gia cc my tnh khc nhau trn c s cc giao dch truyn thng. N t tn nht qun cho mi thnh phn mun i thoi ring vi nhau. Tng vn chuyn (Transport layer): tng vn chuyn xc nh a ch trn mng, cch thc chuyn giao gi tin trn c s trc tip gia hai u mt (end-to-end). bo m c vic truyn n nh trn mng tng vn chuyn thng nh s cc gi tin v m bo chng chuyn theo th t.
  • 14. wWw.VietLion.Com Hnh 3.5: M hnh 7 tng OSI Tng mng (Network layer): tng mng c nhim v xc nh vic chuyn hng, vch ng cc gi tin trong mng, cc gi tin ny c th phi i qua nhiu chng trc khi n c ch cui cng. Tng lin kt d liu (Data link layer): tng lin kt d liu c nhim v xc nh c ch truy nhp thng tin trn mng, cc dng thc chung trong cc gi tin, ng cc gi tin... Tng vt l (Phisical layer): tng vt l cung cp phng thc truy cp vo ng truyn vt l truyn cc dng Bit khng cu trc, ngoi ra n cung cp cc chun v in, dy cp, u ni, k thut ni mch in, in p, tc cp truyn dn, giao din ni kt v cc mc ni kt.. 2. M hnh SNA (Systems Netword Architecture) Thng 9/1973, Hng IBM gii thiu mt kin trc mng my tnh SNA (System Network Architecture). n nm 1977 c 300 trm SNA c ci t. Cui nm 1978, s lng tng ln n 1250, ri c theo cho n nay c 20.000 trm SNA ang c hot ng. Qua con s ny chng ta c th hnh dung c mc quan trng v tm nh hng ca SNA trn ton th gii. Cn lu rng SNA khng l mt chun quc t chnh thc nh OSI nhng do vai tr to ln ca hng IBM trn th trng CNTT nn SNA tr thnh mt loi chun thc t v kh ph bin. SNA l mt c t gm rt nhiu ti liu m t kin trc ca mng x l d liu phn tn. N nh ngha cc quy tc v cc giao thc cho s tng tc gia cc thnh phn (my tnh, trm cui, phn mm) trong mng. SNA c t chc xung quanh khi nim min (domain). Mt SNA domain l mt im iu khin cc dch v h thng (Systems Services control point - SSCP) v n s iu khin tt c cc ti nguyn , Cc ti nguyn y c th l cc n v vt l, cc n v logic, cc lin kt d liu v cc thit b. C th v SSCP nh l "tri tim v khi c" ca SNA. N iu khin SNA domain bng cch gi cc lnh ti mt n v vt l, n v vt l ny sau khi nhn c lnh s qun l tt c cc ti nguyn trc tip vi n. n v vt l thc s l mt "i tc" ca SSCP v cha mt tp con cc kh nng ca SSCP. Cc n v vt l m nhim vic qun l ca mi nt SNA. SNA phn bit gia cc nt min con (Subarea node) v cc nt ngoi vi (peripheral node). Mt nt min con c th dn ng cho d liu ca ngi s dng qua ton b mng. N dng a ch mng v mt s hiu ng (router suember) xc nh ng truyn i ti nt k tip trong mng.
  • 15. wWw.VietLion.Com Mt nt ngoi vi c tnh cc b hn. N khng dn ng gia cc nt min con. Cc nt c ni v iu khin theo giao thc SDLC (Synchronous Data Link Control). Mi nt ngoi vi ch lin lc c vi nt min con m n ni vo. Mng SNA da trn c ch phn tng, trc y th 2 h thng ngang hng khng c trao i trc tip. Sau ny pht trin thnh SNA m rng: Lc ny hai tng ngang hng nhau c th trao i trc tip. Vi 6 tng c tn gi v chc nng tt nh sau: Tng qun tr chc nng SNA (SNA Function Manegement) Tng ny tht ra c th chia tng ny lm hai tng nh sau: Tng dch v giao tc (Transaction) cung cp cc dch v ng dng n ngi dng mt mng SNA. Nhng dch v nh : DIA cung cp cc ti liu phn b gia cc h thng vn phng, SNA DS (vn phng dch v phn phi) cho vic truyn thng bt ng b gia cc ng dng phn tn v h thng vn phng. Tng dch v giao tc cng cung cp cc dch v v cu hnh, cc dch v qun l iu khin cc hot ng mng. Tng dch v trnh din (Presentation Services): tng ny th lin quan vi s hin th cc ng dng, ngi s dng u cui v cc d liu h thng. Tng ny cng nh ngha cc giao thc cho vic truyn thng gia cc chng trnh v iu khin truyn thng mc hi thoi. Tng kim sot lung d liu (Data flow control) tng ny cung cp cc dch v iu khinlung lu thng cho cc phin t logic ny n n v logic khc (LU - LU). N thc hin iu ny bng cch gn cc s trnh t, cc yu cu v p ng, thc hin cc giao thc yu cu v p ng giao dch v hp tc gia cc giao dch gi v nhn. Ni chung n ym tr phng thc khai thc hai chiu ng thi (Full duplex). Tng kim sot truyn (Transmission control): Tng ny cung cp cc iu khin c bn ca cc phn ti nguyn truyn trong mng, bng cch xc nh s trnh t nhn c, v qun l vic theo di mc phin. Tng ny cng h tr cho vic m ha d liu v cung cp h thng h tr cho cc nt ngoi vi. Tng kim sot ng dn (Path control): Tng ny cung cp cc giao thc tm ng cho mt gi tin qua mng SNA v kt ni vi cc mng SNA khc, ng thi n cng kim sot cc ng truyn ny. Tng kim sot lin kt d liu (Data Link Control): Tng ny cung cp cc giao thc cho vic truyn cc gi tin thng qua ng truyn vt l gia hai node v cng cung cp cc iu khin lu thng v phc hi li, cc h tr cho tng ny l cc giao thc SDLC, System/370, X25, IEEE 802.2 v 802.5. Tng kim sot vt l (Physical control): Tng ny cung cp mt giao din vt l cho bt c mi trng truyn thng no m gn vi n. Tng no nh ngha cc c trng ca tn hiu cn thit lp, duy tr v kt thc cc ng ni vt l cho vic h tr kt ni.
  • 16. wWw.VietLion.Com Hnh 3.6: Tng ng cc tng cc kin trc SNI v OSI
  • 17. wWw.VietLion.Com Chng 4 M hnh kt ni cc h thng m Open Systems Interconection Vic nghin cu v OSI c bt u ti ISO vo nm 1971 vi cc mc tiu nhm ni kt cc sn phm ca cc hng sn xut khc. u im chnh ca OSI l ch n ha hn gii php cho vn truyn thng gia cc my tnh khng ging nhau. Hai h thng, d c khc nhau u c th truyn thng vi nhau mt cc hiu qu nu chng m bo nhng iu kin chung sau y: Chng ci t cng mt tp cc chc nng truyn thng. Cc chc nng c t chc thnh cng mt tp cc tng. cc tng ng mc phi cung cp cc chc nng nh nhau. Cc tng ng mc khi trao i vi nhau s dng chung mt giao thc M hnh OSI tch cc mt khc nhau ca mt mng my tnh thnh by tng theo m hnh phn tng. M hnh OSI l mt khung m cc tiu chun lp mng khc nhau c th khp vo. M hnh OSI nh r cc mt no ca hot ng ca mng c th nhm n bi cc tiu chun mng khc nhau. V vy, theo mt ngha no , m hnh OSI l mt loi tiu chun ca cc chun. I. Nguyn tc s dng khi nh ngha cc tng h thng m: Sau y l cc nguyn tc m ISO quy nh dng trong qu trnh xy dng m hnh OSI Khng nh ngha qu nhiu tng vic xc nh v ghp ni cc tng khng qu phc tp. To cc ranh gii cc tng sao cho vic gii thch cc phc v v s cc tng tc qua li hai tng l nh nht. To cc tng ring bit cho cc chc nng khc bit nhau hon ton v k thut s dng hoc qu trnh thc hin. Cc chc nng ging nhau c t trong cng mt tng. La chn ranh gii cc tng ti cc im m nhng th nghim trong qu kh thnh cng. Cc chc nng c xc nh sao cho chng c th d dng xc nh li, v cc nghi thc ca chng c th thay i trn mi hng. To ranh gii cc tng m cn c nhng mc tru tng khc nhau trong vic s dng s liu. Cho php thay i cc chc nng hoc giao thc trong tng khng nh hng n cc tng khc. To cc ranh gii gia mi tng vi tng trn v di n. II. Cc giao thc trong m hnh OSI
  • 18. wWw.VietLion.Com Trong m hnh OSI c hai loi giao thc chnh c p dng: giao thc c lin kt (connection - oriented) v giao thc khng lin kt (connectionless). Giao thc c lin kt: trc khi truyn d liu hai tng ng mc cn thit lp mt lin kt logic v cc gi tin c trao i thng qua lin kt ny, vic c lin kt logic s nng cao an ton trong truyn d liu. Giao thc khng lin kt: trc khi truyn d liu khng thit lp lin kt logic v mi gi tin c truyn c lp vi cc gi tin trc hoc sau n. Nh vy vi giao thc c lin kt, qu trnh truyn thng phi gm 3 giai on phn bit: Thit lp lin kt (logic): hai thc th ng mc hai h thng thng lng vi nhau v tp cc tham s s s dng trong giai on sau (truyn d liu). Truyn d liu: d liu c truyn vi cc c ch kim sot v qun l km theo (nh kim sot li, kim sot lung d liu, ct/hp d liu...) tng cng tin cy v hiu qu ca vic truyn d liu. Hy b lin kt (logic): gii phng ti nguyn h thng c cp pht cho lin kt dng cho lin kt khc. i vi giao thc khng lin kt th ch c duy nht mt giai on truyn d liu m thi. Gi tin ca giao thc: Gi tin (Packet) c hiu nh l mt n v thng tin dng trong vic lin lc, chuyn giao d liu trong mng my tnh. Nhng thng ip (message) trao i gia cc my tnh trong mng, c to dng thnh cc gi tin my ngun. V nhng gi tin ny khi ch s c kt hp li thnh thng ip ban u. Mt gi tin c th cha ng cc yu cu phc v, cc thng tin iu khin v d liu. Hnh 4.1: Phng thc xc lp cc gi tin trong m hnh OSI Trn quan im m hnh mng phn tng tng mi tng ch thc hin mt chc nng l nhn d liu t tng bn trn chuyn giao xung cho tng bn di v ngc li. Chc nng ny thc cht l gn thm v g b phn u (header) i vi cc gi tin trc khi chuyn n i. Ni cch khc, tng gi tin bao gm phn u (header) v phn d liu. Khi i n mt tng mi gi tin s c ng thm mt phn u khc v c xem nh l gi tin ca tng mi, cng vic trn tip din cho ti khi gi tin c truyn ln ng dy mng n bn nhn.
  • 19. wWw.VietLion.Com Ti bn nhn cc gi tin c g b phn u trn tng tng tng ng v y cng l nguyn l ca bt c m hnh phn tng no. Ch : Trong m hnh OSI phn kim li ca gi tin tng lin kt d liu t cui gi tin III. Cc chc nng ch yu ca cc tng ca m hnh OSI. Tng 1: Vt l (Physical) Tng vt l (Physical layer) l tng di cng ca m hnh OSI l. N m t cc c trng vt l ca mng: Cc loi cp c dng ni cc thit b, cc loi u ni c dng , cc dy cp c th di bao nhiu v.v... Mt khc cc tng vt l cung cp cc c trng in ca cc tn hiu c dng khi chuyn d liu trn cp t mt my ny n mt my khc ca mng, k thut ni mch in, tc cp truyn dn. Tng vt l khng qui nh mt ngha no cho cc tn hiu ngoi cc gi tr nh phn 0 v 1. cc tng cao hn ca m hnh OSI ngha ca cc bit c truyn tng vt l s c xc nh. V d: Tiu chun Ethernet cho cp xon i 10 baseT nh r cc c trng in ca cp xon i, kch thc v dng ca cc u ni, di ti a ca cp. Khc vi cc tng khc, tng vt l l khng c gi tin ring v do vy khng c phn u (header) cha thng tin iu khin, d liu c truyn i theo dng bit. Mt giao thc tng vt l tn ti gia cc tng vt l quy nh v phng thc truyn (ng b, phi ng b), tc truyn. Cc giao thc c xy dng cho tng vt l c phn chia thnh phn chia thnh hai loi giao thc s dng phng thc truyn thng d b (asynchronous) v phng thc truyn thng ng b (synchronous). Phng thc truyn d b: khng c mt tn hiu quy nh cho s ng b gia cc bit gia my gi v my nhn, trong qu trnh gi tn hiu my gi s dng cc bit c bit START v STOP c dng tch cc xu bit biu din cc k t trong dng d liu cn truyn i. N cho php mt k t c truyn i bt k lc no m khng cn quan tm n cc tn hiu ng b trc . Phng thc truyn ng b: s dng phng thc truyn cn c ng b gia my gi v my nhn, n chn cc k t c bit nh SYN (Synchronization), EOT (End Of Transmission) hay n gin hn, mt ci "c " (flag) gia cc d liu ca my gi bo hiu cho my nhn bit c d liu ang n hoc n. Tng 2: Lin kt d liu (Data link) Tng lin kt d liu (data link layer) l tng m ngha c gn cho cc bt c truyn trn mng. Tng lin kt d liu phi quy nh c cc dng thc, kch thc, a ch my gi v nhn ca mi gi tin c gi i. N phi xc nh c ch truy nhp thng tin trn mng v phng tin gi mi gi tin sao cho n c a n cho ngi nhn nh. Tng lin kt d liu c hai phng thc lin kt da trn cch kt ni cc my tnh, l phng thc "mt im - mt im" v phng thc "mt im - nhiu im". Vi phng thc "mt im - mt im" cc
  • 20. wWw.VietLion.Com ng truyn ring bit c thit lp ni cc cp my tnh li vi nhau. Phng thc "mt im - nhiu im " tt c cc my phn chia chung mt ng truyn vt l. Hnh 4.2: Cc ng truyn kt ni kiu "mt im - mt im" v "mt im - nhiu im". Tng lin kt d liu cng cung cp cch pht hin v sa li c bn m bo cho d liu nhn c ging hon ton vi d liu gi i. Nu mt gi tin c li khng sa c, tng lin kt d liu phi ch ra c cch thng bo cho ni gi bit gi tin c li n gi li. Cc giao thc tng lin kt d liu chia lm 2 loi chnh l cc giao thc hng k t v cc giao thc hng bit. Cc giao thc hng k t c xy dng da trn cc k t c bit ca mt b m chun no (nh ASCII hay EBCDIC), trong khi cc giao thc hng bit li dng cc cu trc nh phn (xu bit) xy dng cc phn t ca giao thc (n v d liu, cc th tc.) v khi nhn, d liu s c tip nhn ln lt tng bit mt. Tng 3: Mng (Network) Tng mng (network layer) nhm n vic kt ni cc mng vi nhau bng cch tm ng (routing) cho cc gi tin t mt mng ny n mt mng khc. N xc nh vic chuyn hng, vch ng cc gi tin trong mng, cc gi ny c th phi i qua nhiu chng trc khi n c ch cui cng. N lun tm cc tuyn truyn thng khng tc nghn a cc gi tin n ch. Tng mng cung cc cc phng tin truyn cc gi tin qua mng, thm ch qua mt mng ca mng (network of network). Bi vy n cn phi p ng vi nhiu kiu mng v nhiu kiu dch v cung cp bi cc mng khc nhau. hai chc nng ch yu ca tng mng l chn ng (routing) v chuyn tip (relaying). Tng mng l quan trng nht khi lin kt hai loi mng khc nhau nh mng Ethernet vi mng Token Ring khi phi dng mt b tm ng (quy nh bi tng mng) chuyn cc gi tin t mng ny sang mng khc v ngc li. i vi mt mng chuyn mch gi (packet - switched network) - gm tp hp cc nt chuyn mch gi ni vi nhau bi cc lin kt d liu. Cc gi d liu c truyn t mt h thng m ti mt h thng m khc trn mng phi c chuyn qua mt chui cc nt. Mi nt nhn gi d liu t mt ng vo (incoming link) ri chuyn tip n ti mt ng ra (outgoing link) hng n ch ca d liu. Nh vy mi nt trung gian n phi thc hin cc chc nng chn ng v chuyn tip. Vic chn ng l s la chn mt con ng truyn mt n v d liu (mt gi tin chng hn) t trm ngun ti trm ch ca n. Mt k thut chn ng phi thc hin hai chc nng chnh sau y: Quyt nh chn ng ti u da trn cc thng tin c v mng ti thi im thng qua nhng tiu chun ti u nht nh.
  • 21. wWw.VietLion.Com Cp nht cc thng tin v mng, tc l thng tin dng cho vic chn ng, trn mng lun c s thay i thng xuyn nn vic cp nht l vic cn thit. Hnh 4. 3: M hnh chuyn vn cc gi tin trong mng chuyn mch gi Ngi ta c hai phng thc p ng cho vic chn ng l phng thc x l tp trung v x l ti ch. Phng thc chn ng x l tp trung c c trng bi s tn ti ca mt (hoc vi) trung tm iu khin mng, chng thc hin vic lp ra cc bng ng i ti tng thi im cho cc nt v sau gi cc bng chn ng ti tng nt dc theo con ng c chn . Thng tin tng th ca mng cn dng cho vic chn ng ch cn cp nhp v c ct gi ti trung tm iu khin mng. Phng thc chn ng x l ti ch c c trng bi vic chn ng c thc hin ti mi nt ca mng. Trong tng thi im, mi nt phi duy tr cc thng tin ca mng v t xy dng bng chn ng cho mnh. Nh vy cc thng tin tng th ca mng cn dng cho vic chn ng cn cp nhp v c ct gi ti mi nt. Thng thng cc thng tin c o lng v s dng cho vic chn ng bao gm: Trng thi ca ng truyn. Thi gian tr khi truyn trn mi ng dn. Mc lu thng trn mi ng. Cc ti nguyn kh dng ca mng. Khi c s thay i trn mng (v d thay i v cu trc ca mng do s c ti mt vi nt, phc hi ca mt nt mng, ni thm mt nt mi... hoc thay i v mc lu thng) cc thng tin trn cn c cp nht vo cc c s d liu v trng thi ca mng. Hin nay khi nhu cu truyn thng a phng tin (tch hp d liu vn bn, ho, hnh nh, m thanh) ngy cng pht trin i hi cc cng ngh truyn dn tc cao nn vic pht trin cc h thng chn ng tc cao ang rt c quan tm. Tng 4: Vn chuyn (Transport)
  • 22. wWw.VietLion.Com Tng vn chuyn cung cp cc chc nng cn thit gia tng mng v cc tng trn. n l tng cao nht c lin quan n cc giao thc trao i d liu gia cc h thng m. N cng cc tng di cung cp cho ngi s dng cc phc v vn chuyn. Tng vn chuyn (transport layer) l tng c s m mt my tnh ca mng chia s thng tin vi mt my khc. Tng vn chuyn ng nht mi trm bng mt a ch duy nht v qun l s kt ni gia cc trm. Tng vn chuyn cng chia cc gi tin ln thnh cc gi tin nh hn trc khi gi i. Thng thng tng vn chuyn nh s cc gi tin v m bo chng chuyn theo ng th t. Tng vn chuyn l tng cui cng chu trch nhim v mc an ton trong truyn d liu nn giao thc tng vn chuyn ph thuc rt nhiu vo bn cht ca tng mng. Ngi ta chia giao thc tng mng thnh cc loi sau: Mng loi A: C t sut li v s c c bo hiu chp nhn c (tc l cht lng chp nhn c). Cc gi tin c gi thit l khng b mt. Tng vn chuyn khng cn cung cp cc dch v phc hi hoc sp xp th t li. Mng loi B: C t sut li chp nhn c nhng t sut s c c bo hiu li khng chp nhn c. Tng giao vn phi c kh nng phc hi li khi xy ra s c. Mng loi C: C t sut li khng chp nhn c (khng tin cy) hay l giao thc khng lin kt. Tng giao vn phi c kh nng phc hi li khi xy ra li v sp xp li th t cc gi tin. Trn c s loi giao thc tng mng chng ta c 5 lp giao thc tng vn chuyn l: Giao thc lp 0 (Simple Class - lp n gin): cung cp cc kh nng rt n gin thit lp lin kt, truyn d liu v hy b lin kt trn mng "c lin kt" loi A. N c kh nng pht hin v bo hiu cc li nhng khng c kh nng phc hi. Giao thc lp 1 (Basic Error Recovery Class - Lp phc hi li c bn) dng vi cc loi mng B, y cc gi tin (TPDU) c nh s. Ngoi ra giao thc cn c kh nng bo nhn cho ni gi v truyn d liu khn. So vi giao thc lp 0 giao thc lp 1 c thm kh nng phc hi li. Giao thc lp 2 (Multiplexing Class - lp dn knh) l mt ci tin ca lp 0 cho php dn mt s lin kt chuyn vn vo mt lin kt mng duy nht, ng thi c th kim sot lung d liu trnh tc nghn. Giao thc lp 2 khng c kh nng pht hin v phc hi li. Do vy n cn t trn mt tng mng loi A. Giao thc lp 3 (Error Recovery and Multiplexing Class - lp phc hi li c bn v dn knh) l s m rng giao thc lp 2 vi kh nng pht hin v phc hi li, n cn t trn mt tng mng loi B. Giao thc lp 4 (Error Detection and Recovery Class - Lp pht hin v phc hi li) l lp c hu ht cc chc nng ca cc lp trc v cn b sung thm mt s kh nng khc kim sot vic truyn d liu. Tng 5: Giao dch (Session) Tng giao dch (session layer) thit lp "cc giao dch" gia cc trm trn mng, n t tn nht qun cho mi thnh phn mun i thoi vi nhau v lp nh xa gia cc tn vi a ch ca chng. Mt giao dch phi c
  • 23. wWw.VietLion.Com thit lp trc khi d liu c truyn trn mng, tng giao dch m bo cho cc giao dch c thit lp v duy tr theo ng qui nh. Tng giao dch cn cung cp cho ngi s dng cc chc nng cn thit qun tr cc giao dnh ng dng ca h, c th l: iu phi vic trao i d liu gia cc ng dng bng cch thit lp v gii phng (mt cch lgic) cc phin (hay cn gi l cc hi thoi - dialogues) Cung cp cc im ng b kim sot vic trao i d liu. p t cc qui tc cho cc tng tc gia cc ng dng ca ngi s dng. Cung cp c ch "ly lt" (nm quyn) trong qu trnh trao i d liu. Trong trng hp mng l hai chiu lun phin th ny sinh vn : hai ngi s dng lun phin phi "ly lt" truyn d liu. Tng giao dch duy tr tng tc lun phin bng cch bo cho mi ngi s dng khi n lt h c truyn d liu. Vn ng b ha trong tng giao dch cng c thc hin nh c ch kim tra/phc hi, dch v ny cho php ngi s dng xc nh cc im ng b ha trong dng d liu ang chuyn vn v khi cn thit c th khi phc vic hi thoi bt u t mt trong cc im mt thi im ch c mt ngi s dng quyn c bit c gi cc dch v nht nh ca tng giao dch, vic phn b cc quyn ny thng qua trao i th bi (token). V d: Ai c c token s c quyn truyn d liu, v khi ngi gi token trao token cho ngi khc thi cng c ngha trao quyn truyn d liu cho ngi . Tng giao dch c cc hm c bn sau: Give Token cho php ngi s dng chuyn mt token cho mt ngi s dng khc ca mt lin kt giao dch. Please Token cho php mt ngi s dng cha c token c th yu cu token . Give Control dng chuyn tt c cc token t mt ngi s dng sang mt ngi s dng khc. Tng 6: Trnh by (Presentation) Trong giao tip gia cc ng dng thng qua mng vi cng mt d liu c th c nhiu cch biu din khc nhau. Thng thng dng biu din dng bi ng dng ngun v dng biu din dng bi ng dng ch c th khc nhau do cc ng dng c chy trn cc h thng hon ton khc nhau (nh h my Intel v h my Motorola). Tng trnh by (Presentation layer) phi chu trch nhim chuyn i d liu gi i trn mng t mt loi biu din ny sang mt loi khc. t c iu n cung cp mt dng biu din chung dng truyn thng v cho php chuyn i t dng biu din cc b sang biu din chung v ngc li. Tng trnh by cng c th c dng k thut m ha xo trn cc d liu trc khi c truyn i v gii m u n bo mt. Ngoi ra tng biu din cng c th dng cc k thut nn sao cho ch cn mt t byte d liu th hin thng tin khi n c truyn trn mng, u nhn, tng trnh by bung tr li c d liu ban u.
  • 24. wWw.VietLion.Com Tng 7: ng dng (Application) Tng ng dng (Application layer) l tng cao nht ca m hnh OSI, n xc nh giao din gia ngi s dng v mi trng OSI v gii quyt cc k thut m cc chng trnh ng dng dng giao tip vi mng. cung cp phng tin truy nhp mi trng OSI cho cc tin trnh ng dng, Ngi ta thit lp cc thc th ng dng (AE), cc thc th ng dng s gi n cc phn t dch v ng dng (Application Service Element - vit tt l ASE) ca chng. Mi thc th ng dng c th gm mt hoc nhiu cc phn t dch v ng dng. Cc phn t dch v ng dng c phi hp trong mi trng ca thc th ng dng thng qua cc lin kt (association) gi l i tng lin kt n (Single Association Object - vit tt l SAO). SAO iu khin vic truyn thng trong sut vng i ca lin kt cho php tun t ha cc s kin n t cc ASE thnh t ca n.
  • 25. wWw.VietLion.Com Chng 5 Cc c tnh k thut ca mng cc b Trn thc t mng cc b l mt h thng truyn d liu gia cc my tnh vi mt khong cch tng i hp, iu cho php c nhng la chn a dng v thit b . Tuy nhin nhng la chn a dng ny li b hn ch bi cc c tnh k thut ca mng cc b, l tp hp cc quy tc chun c quy c m tt c cc thc th tham gia truyn thng trn mng phi tun theo m bo cho mng hot ng tt. Cc c tnh chnh ca mng cc b m chng ta ni ti sau y l: Cu trc ca mng (hay topology ca mng m qua th hin cch ni cc mng my tnh vi nhau ra sao). Cc nghi thc truyn d liu trn mng (cc th tc hng dn trm lm vic lm th no v lc no c th thm nhp vo ng dy cp gi cc gi thng tin ). Cc loi ng truyn v cc chun ca chng . Cc phng thc tn hiu I. Cu trc ca mng (Topology) Hnh trng ca mng cc b th hin qua cu trc hay hnh dng hnh hc cu cc ng dy cp mng dng lin kt cc my tnh thuc mng vi nhau. Cc mng cc b thng hot ng da trn cu trc nh san lin kt cc my tnh v cc thit b c lin quan. Trc ht chng ta xem xt hai phng thc ni mng ch yu c s dng trong vic lin kt cc my tnh l "mt im - mt im" v "mt im - nhiu im ". Vi phng thc "mt im - mt im" cc ng truyn ring bit c thit lp ni cc cp my tnh li vi nhau. Mi my tnh c th truyn v nhn trc tip d liu hoc c th lm trung gian nh lu tr nhng d liu m n nhn c ri sau chuyn tip d liu i cho mt my khc d liu t ti ch. Theo phng thc "mt im - nhiu im " tt c cc trm phn chia chung mt ng truyn vt l. D liu c gi i t mt my tnh s c th c tip nhn bi tt c cc my tnh cn li, bi vy cn ch ra i ch ch ca d liu mi my tnh cn c vo kim tra xem d liu c phi dnh cho mnh khng nu ng th nhn cn nu khng th b qua. Hnh 5.1: Cc phng thc lin kt mng Ty theo cu trc ca mi mng chng s thuc vo mt trong hai phng thc ni mng v mi phng thc ni mng s c nhng yu cu khc nhau v phn cng v phn mm.
  • 26. wWw.VietLion.Com II. Nhng cu trc chnh ca mng cc b 1. Dng ng thng (Bus) Trong dng ng thng cc my tnh u c ni vo mt ng dy truyn chnh (bus). ng truyn chnh ny c gii hn hai u bi mt loi u ni c bit gi l terminator (dng nhn bit l u cui kt thc ng truyn ti y). Mi trm c ni vo bus qua mt u ni ch T (T_connector) hoc mt b thu pht (transceiver). Khi mt trm truyn d liu, tn hiu c truyn trn c hai chiu ca ng truyn theo tng gi mt, mi gi u phi mang a ch trm ch. Cc trm khi thy d liu i qua nhn ly, kim tra, nu ng vi a ch ca mnh th n nhn ly cn nu khng phi th b qua. Sau y l vi thng s k thut ca topology bus. Theo chun IEEE 802.3 (cho mng cc b) vi cch t tn qui c theo thng s: tc truyn tnh hiu (1,10 hoc 100 Mb/s); BASE (nu l Baseband) hoc BROAD (nu l Broadband). 10BASE5: Dng cp ng trc ng knh ln (10mm) vi tr khng 50 Ohm, tc 10 Mb/s, phm vi tn hiu 500m/segment, c ti a 100 trm, khong cch gia 2 tranceiver ti thiu 2,5m (Phng n ny cn gi l Thick Ethernet hay Thicknet) 10BASE2: tng t nh Thicknet nhng dng cp ng trc nh (RG 58A), c th chy vi khong cch 185m, s trm ti a trong 1 segment l 30, khong cch gia hai my ti thiu l 0,5m. Dng kt ni ny c u im l t tn dy cp, tc truyn d liu cao tuy nhin nu lu lng truyn tng cao th d gy ch tc v nu c trc trc trn hnh lang chnh th kh pht hin ra. Hin nay cc mng s dng hnh dng ng thng l mng Ethernet v G-net. 2. Dng vng trn (Ring) Cc my tnh c lin kt vi nhau thnh mt vng trn theo phng thc "mt im - mt im ", qua mi mt trm c th nhn v truyn d liu theo vng mt chiu v d liu c truyn theo tng gi mt. Mi gi d liu u c mang a ch trm ch, mi trm khi nhn c mt gi d liu n kim tra nu ng vi a ch ca mnh th n nhn ly cn nu khng phi th n s pht li cho trm k tip, c nh vy gi d liu i c n ch. Vi dng kt ni ny c u im l khng tn nhiu dy cp, tc truyn d liu cao, khng gy ch tc tuy nhin cc giao thc truyn d liu phc tp v nu c trc trc trn mt trm th cng nh hng n ton mng. Hin nay cc mng s dng hnh dng vng trn l mng Tocken ring ca IBM. 3. Dng hnh sao (Star) dng hnh sao, tt c cc trm c ni vo mt thit b trung tm c nhim v nhn tn hiu t cc trm v chuyn tn hiu n trm ch vi phng thc kt ni l phng thc "mt im - mt im ". Thit b trung tm hot ng ging nh mt tng i cho php thc hin vic nhn v truyn d liu t trm ny ti cc trm khc. Ty theo yu cu truyn thng trong mng , thit b trung tm c th l mt b chuyn mch (switch), mt b chn ng (router) hoc n gin l mt b phn knh (Hub). C nhiu cng ra v mi cng ni vi mt my. Theo chun IEEE 802.3 m hnh dng Star thng dng: 10BASE-T: dng cp UTP, tc 10 Mb/s, khong cch t thit b trung tm ti trm ti a l 100m.
  • 27. wWw.VietLion.Com 100BASE-T tng t nh 10BASE-T nhng tc cao hn 100 Mb/s. u v khuyt im u im: Vi dng kt ni ny c u im l khng ng hay ch tc trn ng truyn, lp t n gin, d dng cu hnh li (thm, bt trm). Nu c trc trc trn mt trm th cng khng gy nh hng n ton mng qua d dng kim sot v khc phc s c. Nhc im: di ng truyn ni mt trm vi thit b trung tm b hn ch (trong vng 100 m vi cng ngh hin i) tn ng dy cp nhiu, tc truyn d liu khng cao. Hin nay cc mng s dng hnh dng hnh sao l mng STARLAN ca AT&T v S-NET ca Novell. Hnh 5.2 : Cc loi cu trc chnh ca mng cc b. ng thng Vng Trn Hnh sao ng dng Tt cho trng hp mng nh v mng c giao thng thp v lu lng d liu thp Tt cho trng hp mng c s trm t hot ng vi tc cao,khng cch nhau xa lm hoc mng c lu lng d liu phn b khng u. hin nay mng sao l cch tt nht cho trng hp phi tch hp d liu v tn hiuting.Cc mng n thoi cng cng c cu trc ny phc tp Tng i khng phc tp i hi thit b tng i phc tp .Mt khc vic a thng ip i trn tuyn l n gin, v ch c 1 con ng, trm pht ch cn bit a ch ca trm nhn , cc thng tin dn ng khc th khng cn thit Mng sao c xem l kh phc tp . Cc trm c ni vi thit b trung tm v ln lt hot ng nh thit b trung tm hoc ni c ti cc dy dn truyn t xa Hiu sut Rt tt di ti thp c th gim hiu sut rt mau khi ti tng C hiu qu trong trng hp lng lu thng cao v kh n nh nh s tng chm thi gian tr v s xung cp so vi cc mng khc Tt cho trng hp ti va tuy nhin kch thc v kh nng , suy ra hiu sut ca mng ph thuc trc tip vo
  • 28. wWw.VietLion.Com sc mnh ca thit b trung tm. Tng ph Tng i thp c bit do nhiu thit b pht trin ha chnh v bn sm phm th trng .S d tha knh truyn c khuyn gim bt nguy c xut hin s c trn mng Phi d tr gp i ngun lc hoc phi c 1 phng thc thay th khi 1 nt khng hot ng nu vn mun mng hot ng bnh thng Tng ph rt cao khi lm nhim v ca thit b trung tm, thit b trung tm khng c dng vo vic khc .S lng dy ring cng nhiu. Nguy c Mt trm b hng khng nh hng n c mng. Tuy nhin mng s c nguy c b tn hi khi s c trn ng dy dn chnh hoc c vn vi tuyn. Vn trn rt kh xc nh c li rt d sa cha Mt trm b hng c th nh hng n c h thng v cc trm phc thuc vo nhau. Tm 1 repeater hng rt kh ,v li vic sa cha thng hay dng mu mo xc nh im hng trn mng c a bn rng rt kh tin cy ca h thng ph thuc vo thit b trung tm, .nu b hng th mng ngng hot ng S ngng hot ng ti thit b trung tm thng khng nh hdng n ton b h thng . Kh nng m rng Vic thm v nh hnh li mng ny rt d.Tuy nhin vic kt ni gia cc my tnh v thit b ca cc hng khc nhau kh c th v chng phi c th nhn cng a ch v d liu Tng i d thm v bt cc trm lm vic m khng phi ni kt nhiu cho mi thay i Gi thnh cho vic thay i tng i thp Kh nng m rng hn ch, a s cc thit b trung tm ch chu ng ni 1 s nht nh lin kt. S hn ch v tc truyn d liu v bng tn thng c i hi mi ngi s dng. Cc hn ch ny gip cho cc chc nng x l trung tm khng b qu ti bi tc thu np ti ti cng truyn v gi thnh mi cng truyn ca thit b trung tm thp . Hnh 6.4 : Bng so snh tnh nng gia cc cu trc ca mng LAN III. Phng thc truyn tn hiu Thng thng c hai phng thc truyn tn hiu trong mng cc b l dng bng tn c s (baseband) v bng tn rng (broadband). S khc nhau ch yu gia hai phng thc truyn tn hiu ny l bng tng c s ch chp nhn mt knh d liu duy nht trong khi bng rng c th chp nhn ng thi hai hoc nhiu knh truyn thng cng phn chia gii thng ca ng truyn.
  • 29. wWw.VietLion.Com Hu ht cc mng cc b s dng phng thc bng tn c s. Vi phng thc truyn tn hiu ny ny tn hiu c th c truyn i di c hai dng: tng t (analog) hoc s (digital). Phng thc truyn bng tn rng chia gii thng (tn s) ca ng truyn thnh nhiu gii tn con trong mi di tn con cung cp mt knh truyn d liu tch bit nh s dng mt cp modem c bit gi l b gii / iu bin RF cai qun vic bin i cc tn hiu s thnh tn hiu tng t c tn s v tuyn (RF) bng k thut ghp knh. IV. Cc giao thc truy cp ng truyn trn mng LAN truyn c d liu trn mng ngi ta phi c cc th tc nhm hng dn cc my tnh ca mng lm th no v lc no c th thm nhp vo ng dy cp gi cc gi d kin. V d nh i vi cc dng bus v ring th ch c mt ng truyn duy nht ni cc trm vi nhau, cho nn cn phi c cc quy tc chung cho tt c cc trm ni vo mng m bo rng ng truyn c truy nhp v s dng mt cch hp l. C nhiu giao thc khc nhau truy nhp ng truyn vt l nhng phn thnh hai loi: cc giao thc truy nhp ngu nhin v cc giao thc truy nhp c iu khin. 1. Giao thc chuyn mch (yu cu v chp nhn) Giao thc chuyn mch l loi giao thc hot ng theo cch thc sau: mt my tnh ca mng khi cn c th pht tn hiu thm nhp vo mng, nu vo lc ny ng cp khng bn th mch iu khin s cho trm ny thm nhp vo ng cp cn nu ng cp ang bn, ngha l ang c giao lu gia cc trm khc, th vic thm nhp s b t chi. 2. Giao thc ng dy a truy cp vi cm nhn va chm (Carrier Sense Multiple Access with Collision Detection hay CSMA/CD ) Giao thc ng dy a truy cp cho php nhiu trm thm nhp cng mt lc vo mng, giao thc ny thng dng trong s mng dng ng thng. Mi trm u c th c truy nhp vo ng dy chung mt cch ngu nhin v do vy c th dn n xung t (hai hoc nhiu trm ng thi cng truyn d liu). Cc trm phi kim tra ng truyn gi d liu i qua c phi ca n hay khng. Khi mt trm mun truyn d liu n phi kim tra ng truyn xem c rnh hay khng gi gi d liu ca, nu ng truyn ang bn trm phi ch i ch c truyn khi thy ng truyn rnh. Nu cng mt lc c hai trm cng s dng ng truyn th giao thc phi pht hin iu ny v cc trm phi ngng thm nhp, ch i ln sau cc thi gian ngu nhin khc nhau. Khi ng cp ang bn trm phi ch i theo mt trong ba phng thc sau: Trm tm ch i mt thi gian ngu nhin no ri li bt u kim tra ng truyn. Trm tip tc kim tra ng truyn n khi ng truyn rnh th truyn d liu i. Trm tip tc kim tra ng truynn khi ng truyn rnh th truyn d liu i vi xc sut p xc nh trc (0 < p < 1). Ti y phng thc 1 c hiu qu trong vic trnh xung t v hai trm cn truyn khi thy ng truyn bn s cng rt lui v ch i trong cc thi gian ngu nhin khc nhau. Ngc li phng thc 2 c gng gim thi gian trng ca ng truyn bng cc cho php trm c th truyn ngay sau khi mt cuc truyn kt thc song nu lc c thm mt trm khc ang i th kh nng xy ra xung t l rt cao. Phng thc 3 vi gi tr p phi la chn hp l c th ti thiu ha c kh nng xung t ln thi gian trng ca ng truyn.
  • 30. wWw.VietLion.Com Khi lu lng cc gi d liu cn di chuyn trn mng qu cao, th vic ng c th xy ra vi s lng ln c gy tc nghn ng truyn dn n lm chm tc truyn tin ca h thng. 3. Giao thc dng th bi vng (Token ring) y l giao thc truy nhp c iu khin ch yu dng k thut chuyn th bi (token) cp pht quyn truy nhp ng truyn tc l quyn c truyn d liu i. Th bi ay l mt n v d liu c bit, c kch thc v ni dung (gm cc thng tin iu khin) c quy nh ring cho mi giao thc. Theo giao thc dng th bi vng trong ng cp lin tc c mt th bi chy quanh trong mng Th bi l mt n v d liu c bit trong c mt bit biu din trng thi s dng ca n (bn hoc ri). Mt trm mun truyn d liu th phi i n khi nhn c mt th bi rnh. Khi trm s i bit trng thi ca th bi thnh bn, np gi d liu c km theo a ch ni nhn vo th bi v truyn i theo chiu ca vng. V th bi chy vng quang trong mng kn v ch c mt th nn vic ng d liu khng th xy ra, do vy hiu sut truyn d liu ca mng khng thay i. Trong cc giao thc ny cn gii quyt hai vn c th dn n ph v h thng. Mt l vic mt th bi lm cho trn vng khng cn th bi lu chuyn na. Hai l mt th bi bn lu chuyn khng dng trn vng. 4. Giao thc dung th bi cho dng ng thng (Token bus) y l giao thc truy nhp c iu khin trong cp pht quyn truy nhp ng truyn cho cc trm ang c nhu cu truyn d liu, mt th bi c lu chuyn trn mt vng logic thit lp bi cc trm . Khi mt trm c th bi th n c quyn s dng ng truyn trong mt thi gian xc nh trc. Khi ht d liu hoc ht thi on cho php, trm chuyn th bi n trm tip theo trong vng logic. Nh vy trong mng phi thit lp c vng logic (hay cn gi l vng o) bao gm cc trm ang hot ng ni trong mng c xc nh v tr theo mt chui th t m trm cui cng ca chui s tip lin sau bi trm u tin. Mi trm c bit a ch ca cc trm k trc v sau n trong th t ca cc trm trn vng logic c th c lp vi th t vt l. Cng vi vic thit lp vng th giao thc phi lun lun theo di s thay i theo trng thi thc t ca mng. V. ng cp truyn mng ng cp truyn mng l c s h tng ca mt h thng mng, nn n rt quan trng v nh hng rt nhiu n kh nng hot ng ca mng. Hin nay ngi ta thng dng 3 loi dy cp l cp xon cp, cp ng trc v cp quang. 1. Cp xon cp y l loi cp gm hai ng dy dn ng c xon vo nhau nhm lm gim nhiu in t gy ra bi mi trng xung quanh v gia chng vi nhau. Hin nay c hai loi cp xon l cp c bc kim loi ( STP - Shield Twisted Pair) v cp khng bc kim loi (UTP -Unshield Twisted Pair). Cp c bc kim loi (STP): Lp bc bn ngoi c tc dng chng nhiu in t, c loi c mt i giy xon vo nhau v c loi c nhiu i giy xon vi nhau.
  • 31. wWw.VietLion.Com Cp khng bc kim loi (UTP): Tnh tng t nh STP nhng km hn v kh nng chng nhiu v suy hao v khng c v bc. STP v UTP c cc loi (Category - Cat) thng dng: Loi 1 & 2 (Cat 1 & Cat 2): Thng dng cho truyn thoi v nhng ng truyn tc thp (nh hn 4Mb/s). Loi 3 (Cat 3): tc truyn d liu khong 16 Mb/s , n l chun cho hu ht cc mng in thoi. Loi 4 (Cat 4): Thch hp cho ng truyn 20Mb/s. Loi 5 (Cat 5): Thch hp cho ng truyn 100Mb/s. Loi 6 (Cat 6): Thch hp cho ng truyn 300Mb/s. y l loi cp r, d ci t tuy nhin n d b nh hng ca mi trng. 2. Cp ng trc Cp ng trc c hai ng dy dn v chng c cng mt trc chung, mt dy dn trung tm (thng l dy ng cng) ng dy cn li to thnh ng ng bao xung quanh dy dn trung tm (dy dn ny c th l dy bn kim loi v v n c chc nng chng nhiu nn cn gi l lp bc kim). Gia hai dy dn trn c mt lp cch ly, v bn ngoi cng l lp v plastic bo v cp. Cc loi cp Dy xon cp Cp ng trc mng Cp ng trc dy Cp quang Chi tit Bng ng, c 4 v 25 cp dy (loi 3, 4, 5) Bng ng, 2 dy, ng knh 5mm Bng ng, 2 dy, ng knh 10mm Thy tinh, 2 si Loi kt ni RJ-25 hoc 50-pin telco BNC N-series ST Chiu di on ti a 100m 185m 500m 1000m S u ni ti a trn 1 on 2 30 100 2 Chy 10 Mbit/s c c c c Chy 100 Mbit/s c Khng Khng c Chng nhiu Tt Tt Rt tt Hon ton Bo mt Trung bnh Trung bnh Trung bnh Hon ton tin cy Tt Trung bnh Tt Tt
  • 32. wWw.VietLion.Com Lp t D dng Trung bnh Kh Kh Khc phc li Tt D D Tt Qun l D dng Kh Kh Trung bnh Chi ph cho 1 trm Rt thp Thp Trung bnh Cao ng dng tt nht H thng Workgroup ng backbone ng backbone trong t mng ng backbone di trong t mng hoc cc ta nh Hnh 5.3: Tnh nng k thut ca mt s loi cp mng Cp ng trc c suy hao t hn so vi cc loi cp ng khc (v d nh cp xon i) do t b nh hng ca mi trng. Cc mng cc b s dng cp ng trc c th c kch thc trong phm vi vi ngn mt, cp ng trc c s dng nhiu trong cc mng dng ng thng. Hai loi cp thng c s dng l cp ng trc mng v cp ng trc dy trong ng knh cp ng trc mng l 0,25 inch, cp ng trc dy l 0,5 inch. C hai loi cp u lm vic cng tc nhng cp ng trc mng c hao suy tn hiu ln hn Hin nay c cp ng trc sau: RG -58,50 ohm: dng cho mng Thin Ethernet RG -59,75 ohm: dng cho truyn hnh cp RG -62,93 ohm: dng cho mng ARCnet Cc mng cc b thng s dng cp ng trc c di thng t 2,5 - 10 Mb/s, cp ng trc c suy hao t hn so vi cc loi cp ng khc v n c lp v bc bn ngoi, di thng thng ca mt on cp ni trong mng l 200m, thng s dng cho dng Bus. 3. Cp si quang (Fiber - Optic Cable) Cp si quang bao gm mt dy dn trung tm (l mt hoc mt b si thy tinh c th truyn dn tn hiu quang) c bc mt lp v bc c tc dng phn x cc tn hiu tr li gim s mt mt tn hiu. Bn ngoi cng l lp v plastic bo v cp. Nh vy cp si quang khng truyn dn cc tn hiu in m ch truyn cc tn hiu quang (cc tn hiu d liu phi c chuyn i thnh cc tn hiu quang v khi nhn chng s li c chuyn i tr li thnh tn hiu in). Cp quang c ng knh t 8.3 - 100 micron, Do ng knh li si thu tinh c kch thc rt nh nn rt kh khn cho vic u ni, n cn cng ngh c bit vi k thut cao i hi chi ph cao. Di thng ca cp quang c th ln ti hng Gbps v cho php khong cch i cp kh xa do suy hao tn hiu trn cp rt thp. Ngoi ra, v cp si quang khng dng tn hiu in t truyn d liu nn n hon ton khng b nh hng ca nhiu in t v tn hiu truyn khng th b pht hin v thu trm bi cc thit b in t ca ngi khc.
  • 33. wWw.VietLion.Com Ch tr nhc im kh lp t v gi thnh cn cao , nhn chung cp quang thch hp cho mi mng hin nay v sau ny. 4. Cc yu cu cho mt h thng cp An ton, thm m: tt c cc dy mng phi c bao bc cn thn, cch xa cc ngun in, cc my c kh nng pht sng trnh trng hp b nhiu. Cc u ni phi m bo cht lng, trnh tnh trng h thng mng b chp chn. ng chun: h thng cp phi thc hin ng chun, m bo cho kh nng nng cp sau ny cng nh d dng cho vic kt ni cc thit b khc nhau ca cc nh sn xut khc nhau. Tiu chun quc t dng cho cc h thng mng hin nay l EIA/TIA 568B. Tit kim v "linh hot" (flexible): h thng cp phi c thit k sao cho kinh t nht, d dng trong vic di chuyn cc trm lm vic v c kh nng m rng sau ny.
  • 34. wWw.VietLion.Com Chng 6 Cc thit b lin kt mng I. Repeater (B tip sc) Repeater l loi thit b phn cng n gin nht trong cc thit b lin kt mng, n c hot ng trong tng vt l ca m hnh h thng m OSI. Repeater dng ni 2 mng ging nhau hoc cc phn mt mng cng c mt nghi thc v mt cu hnh. Khi Repeater nhn c mt tn hiu t mt pha ca mng th n s pht tip vo pha kia ca mng. Hnh 6.1: M hnh lin kt mng ca Repeater. Repeater khng c x l tn hiu m n ch loi b cc tn hiu mo, nhiu, khuch i tn hiu b suy hao (v c pht vi khong cch xa) v khi phc li tn hiu ban u. Vic s dng Repeater lm tng thm chiu di ca mng. Hnh 6.2: Hot ng ca b tip sc trong m hnh OSI Hin nay c hai loi Repeater ang c s dng l Repeater in v Repeater in quang. Repeater in ni vi ng dy in c hai pha ca n, n nhn tn hiu in t mt pha v pht li v pha kia. Khi mt mng s dng Repeater in ni cc phn ca mng li th c th lm tng khong cch ca mng, nhng khong cch lun b hn ch bi mt khong cch ti a do tr ca tn hiu. V d vi mng s dng cp ng trc 50 th khong cch ti a l 2.8 km, khong cch khng th ko thm cho d s dng thm Repeater.
  • 35. wWw.VietLion.Com Repeater in quang lin kt vi mt u cp quang v mt u l cp in, n chuyn mt tn hiu in t cp in ra tn hiu quang pht trn cp quang v ngc li. Vic s dng Repeater in quang cng lm tng thm chiu di ca mng. Vic s dng Repeater khng thay i ni dung cc tn hin i qua nn n ch c dng ni hai mng c cng giao thc truyn thng (nh hai mng Ethernet hay hai mng Token ring) nhng khng th ni hai mng c giao thc truyn thng khc nhau (nh mt mng Ethernet v mt mng Token ring). Thm na Repeater khng lm thay i khi lng chuyn vn trn mng nn vic s dng khng tnh ton n trn mng ln s hn ch hiu nng ca mng. Khi la chn s dng Repeater cn ch la chn loi c tc chuyn vn ph hp vi tc ca mng. II. Bridge (Cu ni) Bridge l mt thit b c x l dng ni hai mng ging nhau hoc khc nhau, n c th c dng vi cc mng c cc giao thc khc nhau. Cu ni hot ng trn tng lin kt d liu nn khng nh b tip sc phi pht li tt c nhng g n nhn c th cu ni c c cc gi tin ca tng lin kt d liu trong m hnh OSI v x l chng trc khi quyt nh c chuyn i hay khng. Khi nhn c cc gi tin Bridge chn lc v ch chuyn nhng gi tin m n thy cn thit. iu ny lm cho Bridge tr nn c ch khi ni mt vi mng vi nhau v cho php n hot ng mt cch mm do. thc hin c iu ny trong Bridge mi u kt ni c mt bng cc a ch cc trm c kt ni vo pha , khi hot ng cu ni xem xt mi gi tin n nhn c bng cch c a ch ca ni gi v nhn v da trn bng a ch pha nhn c gi tin n quyt nh gi gi tin hay khng v b xung bng a ch. Hnh 6.3: Hot ng ca Bridge Khi c a ch ni gi Bridge kim tra xem trong bng a ch ca phn mng nhn c gi tin c a ch hay khng, nu khng c th Bridge t ng b xung bng a ch (c ch c gi l t hc ca cu ni). Khi c a ch ni nhn Bridge kim tra xem trong bng a ch ca phn mng nhn c gi tin c a ch hay khng, nu c th Bridge s cho rng l gi tin ni b thuc phn mng m gi tin n nn khng chuyn gi tin i, nu ngc li th Bridge mi chuyn sang pha bn kia. y chng ta thy mt trm khng cn thit chuyn thng tin trn ton mng m ch trn phn mng c trm nhn m thi.
  • 36. wWw.VietLion.Com Hnh 6.4: Hot ng ca Bridge trong m hnh OSI nh gi mt Bridge ngi ta a ra hai khi nim : Lc v chuyn vn. Qu trnh x l mi gi tin c gi l qu trnh lc trong tc lc th hin trc tip kh nng hot ng ca Bridge. Tc chuyn vn c th hin s gi tin/giy trong th hin kh nng ca Bridge chuyn cc gi tin t mng ny sang mng khc. Hin nay c hai loi Bridge ang c s dng l Bridge vn chuyn v Bridge bin dch. Bridge vn chuyn dng ni hai mng cc b cng s dng mt giao thc truyn thng ca tng lin kt d liu, tuy nhin mi mng c th s dng loi dy ni khc nhau. Bridge vn chuyn khng c kh nng thay i cu trc cc gi tin m n nhn c m ch quan tm ti vic xem xt v chuyn vn gi tin i. Bridge bin dch dng ni hai mng cc b c giao thc khc nhau n c kh nng chuyn mt gi tin thuc mng ny sang gi tin thuc mng kia trc khi chuyn qua V d : Bridge bin dch ni mt mng Ethernet v mt mng Token ring. Khi Cu ni thc hin nh mt nt token ring trn mng Token ring v mt nt Enthernet trn mng Ethernet. Cu ni c th chuyn mt gi tin theo chun ang s dng trn mng Enthernet sang chun ang s dng trn mng Token ring. Tuy nhin ch y cu ni khng th chia mt gi tin ra lm nhiu gi tin cho nn phi hn ch kch thc ti a cc gi tin ph hp vi c hai mng. V d nh kch thc ti a ca gi tin trn mng Ethernet l 1500 bytes v trn mng Token ring l 6000 bytes do vy nu mt trm trn mng token ring gi mt gi tin cho trm trn mng Ethernet vi kch thc ln hn 1500 bytes th khi qua cu ni s lng byte d s b cht b.
  • 37. wWw.VietLion.Com Hnh 6.5: V d v Bridge bin dch Ngi ta s dng Bridge trong cc trng hp sau : M rng mng hin ti khi t ti khong cch ti a do Bridge sau khi s l gi tin pht li gi tin trn phn mng cn li nn tn hiu tt hn b tip sc. Gim bt tc nghn mng khi c qu nhiu trm bng cch s dng Bridge, khi chng ta chia mng ra thnh nhiu phn bng cc Bridge, cc gi tin trong ni b tng phn mng s khng c php qua phn mng khc. ni cc mng c giao thc khc nhau. Mt vi Bridge cn c kh nng la chn i tng vn chuyn. N c th ch chuyn vn nhng gi tin ca nhng a ch xc nh. V d : cho php gi tin ca my A, B qua Bridge 1, gi tin ca my C, D qua Bridge 2. Hnh 6.6 : Lin kt mng vi 2 Bridge
  • 38. wWw.VietLion.Com Mt s Bridge c ch to thnh mt b ring bit, ch cn ni dy v bt. Cc Bridge khc ch to nh card chuyn dng cm vo my tnh, khi trn my tnh s s dng phn mm Bridge. Vic kt hp phn mm vi phn cng cho php uyn chuyn hn trong hot ng ca Bridge. III. Router (B tm ng) Router l mt thit b hot ng trn tng mng, n c th tm c ng i tt nht cho cc gi tin qua nhiu kt ni i t trm gi thuc mng u n trm nhn thuc mng cui. Router c th c s dng trong vic ni nhiu mng vi nhau v cho php cc gi tin c th i theo nhiu ng khc nhau ti ch. Hnh 6.7: Hot ng ca Router. Khc vi Bridge hot ng trn tng lin kt d liu nn Bridge phi x l mi gi tin trn ng truyn th Router c a ch ring bit v n ch tip nhn v x l cc gi tin gi n n m thi. Khi mt trm mun gi gi tin qua Router th n phi gi gi tin vi a ch trc tip ca Router (Trong gi tin phi cha cc thng tin khc v ch n) v khi gi tin n Router th Router mi x l v gi tip. Khi x l mt gi tin Router phi tm c ng i ca gi tin qua mng. lm c iu Router phi tm c ng i tt nht trong mng da trn cc thng tin n c v mng, thng thng trn mi Router c mt bng ch ng (Router table). Da trn d liu v Router gn v cc mng trong lin mng, Router tnh c bng ch ng (Router table) ti u da trn mt thut ton xc nh trc. Ngi ta phn chia Router thnh hai loi l Router c ph thuc giao thc (The protocol dependent routers) v Router khng ph thuc vo giao thc (The protocol independent router) da vo phng thc x l cc gi tin khi qua Router.
  • 39. wWw.VietLion.Com Router c ph thuc giao thc: Ch thc hin vic tm ng v truyn gi tin t mng ny sang mng khc ch khng chuyn i phng cch ng gi ca gi tin cho nn c hai mng phi dng chung mt giao thc truyn thng. Router khng ph thuc vo giao thc: c th lin kt cc mng dng giao thc truyn thng khc nhau v c th chuyn i gi tin ca giao thc ny sang gi tin ca giao thc kia, Router cng chp nhn kch thc cc gi tin khc nhau (Router c th chia nh mt gi tin ln thnh nhiu gi tin nh trc truyn trn mng). Hnh 6.8: Hot ng ca Router trong m hnh OSI ngn chn vic mt mt s liu Router cn nhn bit c ng no c th chuyn vn v ngng chuyn vn khi ng b tc. Cc l do s dng Router : Router c cc phn mm lc u vit hn l Bridge do cc gi tin mun i qua Router cn phi gi trc tip n n nn gim c s lng gi tin qua n. Router thng c s dng trong khi ni cc mng thng qua cc ng dy thu bao t tin do n khng truyn d ln ng truyn. Router c th dng trong mt lin mng c nhiu vng, mi vng c giao thc ring bit. Router c th xc nh c ng i an ton v tt nht trong mng nn an ton ca thng tin c m bo hn. Trong mt mng phc hp khi cc gi tin lun chuyn cc ng c th gy nn tnh trng tc nghn ca mng th cc Router c th c ci t cc phng thc nhm trnh c tc nghn.
  • 40. wWw.VietLion.Com Hnh 6.9: V d v bng ch ng (Routing table) ca Router. Cc phng thc hot ng ca Router l phng thc m mt Router c th ni vi cc Router khc qua chia s thng tin v mng hin co. Cc chng trnh chy trn Router lun xy dng bng ch ng qua vic trao i cc thng tin vi cc Router khc. Phng thc vc t khong cch : mi Router lun lun truyn i thng tin v bng ch ng ca mnh trn mng, thng qua cc Router khc s cp nht ln bng ch ng ca mnh. Phng thc trng thi tnh : Router ch truyn cc thng bo khi c pht hin c s thay i trong mng vch khi cc Routerkhc cp nht li bng ch ng, thng tin truyn i khi thng l thng tin v ng truyn. Mt s giao thc hot ng chnh ca Router RIP(Routing Information Protocol) c pht trin bi Xerox Network system v s dng SPX/IPX v TCP/IP. RIP hot ng theo phng thc vc t khong cch. NLSP (Netware Link Service Protocol) c pht trin bi Novell dng thay th RIP hot ng theo phng thc vct khong cch, mi Router c bit cu trc ca mng v vic truyn cc bng ch ng gim i.. OSPF (Open Shortest Path First) l mt phn ca TCP/IP vi phng thc trng thi tnh, trong c xt ti u tin, gi ng truyn, mt truyn thng... OSPF-IS (Open System Interconnection Intermediate System to Intermediate System) l mt phn ca TCP/IP vi phng thc trng thi tnh, trong c xt ti u tin, gi ng truyn, mt truyn thng...
  • 41. wWw.VietLion.Com IV. Gateway (cng ni) Gateway dng kt ni cc mng khng thun nht chng hn nh cc mng cc b v cc mng my tnh ln (Mainframe), do cc mng hon ton khng thun nht nn vic chuyn i thc hin trn c 7 tng ca h thng m OSI. Thng c s dng ni cc mng LAN vo my tnh ln. Gateway c cc giao thc xc nh trc thng l nhiu giao thc, mt Gateway a giao thc thng c ch to nh cc Card c cha cc b x l ring v ci t trn cc my tnh hoc thit b chuyn bit. Hnh 6.10: Hot ng ca Gateway trong m hnh OSI Hot ng ca Gateway thng thng phc tp hn l Router nn thng sut ca n thng chm hn v thng khng dng ni mng LAN -LAN. V. Hub (B tp trung) Hub thng c dng ni mng, thng qua nhng u cm ca n ngi ta lin kt vi cc my tnh di dng hnh sao. Ngi ta phn bit cc Hub thnh 3 loi nh sau sau : Hub b ng (Passive Hub) : Hub b ng khng cha cc linh kin in t v cng khng x l cc tn hiu d liu, n c chc nng duy nht l t hp cc tn hiu t mt s on cp mng. Khong cch gia mt my tnh v Hub khng th ln hn mt na khong cch ti a cho php gia 2 my tnh trn mng (v d khong cch ti a cho php gia 2 my tnh ca mng l 200m th khong cch ti a gia mt my tnh v hub l 100m). Cc mng ARCnet thng dng Hub b ng. Hub ch ng (Active Hub) : Hub ch ng c cc linh kin in t c th khuych i v x l cc tn hiu in t truyn gia cc thit b ca mng. Qa trnh x l tn hiu c gi l ti sinh tn hiu, n lm cho tn hiu tr nn tt hn, t nhy cm vi li do vy khong cch gia cc thit b c th tng ln. Tuy nhin nhng u im cng ko theo gi thnh ca Hub ch ng cao hn nhiu so vi Hub b ng. Cc mng Token ring c xu hng dng Hub ch ng. Hub thng minh (Intelligent Hub): cng l Hub ch ng nhng c thm cc chc nng mi so vi loi trc, n c th c b vi x l ca mnh v b nh m qua n khng ch cho php iu khin hot ng thng qua cc chng trnh qun tr mng m n c th hot ng nh b tm ng hay mt
  • 42. wWw.VietLion.Com cu ni. N c th cho php tm ng cho gi tin rt nhanh trn cc cng ca n, thay v pht li gi tin trn mi cng th n c th chuyn mch pht trn mt cng c th ni ti trm ch.
  • 43. wWw.VietLion.Com Chng 7 Giao thc TCP/IP Giao thc TCP/IP c pht trin t mng ARPANET v Internet v c dng nh giao thc mng v vn chuyn trn mng Internet. TCP (Transmission Control Protocol) l giao thc thuc tng vn chuyn v IP (Internet Protocol) l giao thc thuc tng mng ca m hnh OSI. H giao thc TCP/IP hin nay l giao thc c s dng rng ri nht lin kt cc my tnh v cc mng. Hin nay cc my tnh ca hu ht cc mng c th s dng giao thc TCP/IP lin kt vi nhau thng qua nhiu h thng mng vi k thut khc nhau. Giao thc TCP/IP thc cht l mt h giao thc cho php cc h thng mng cng lm vic vi nhau thng qua vic cung cp phng tin truyn thng lin mng. I. Giao thc IP 1. Tng qut Nhim v chnh ca giao thc IP l cung cp kh nng kt ni cc mng con thnh lin kt mng truyn d liu, vai tr ca IP l vai tr ca giao thc tng mng trong m hnh OSI. Giao thc IP l mt giao thc kiu khng lin kt (connectionlees) c ngha l khng cn c giai on thit lp lin kt trc khi truyn d liu. S a ch ha nh danh cc trm (host) trong lin mng c gi l a ch IP 32 bits (32 bit IP address). Mi giao din trong 1 my c h tr giao thc IP u phi c gn 1 a ch IP (mt my tnh c th gn vi nhiu mng do vy c th c nhiu a ch IP). a ch IP gm 2 phn: a ch mng (netid) v a ch my (hostid). Mi a ch IP c di 32 bits c tch thnh 4 vng (mi vng 1 byte), c th biu th di dng thp phn, bt phn, thp lc phn hay nh phn. Cch vit ph bin nht l dng k php thp phn c du chm (dotted decimal notation) tch cc vng. Mc ch ca a ch IP l nh danh duy nht cho mt my tnh bt k trn lin mng. Do t chc v ln ca cc mng con (subnet) ca lin mng c th khc nhau, ngi ta chia cc a ch IP thnh 5 lp, k hiu l A, B, C, D v E. Trong lp A, B, C cha a ch c th gn c. Lp D dnh ring cho lp k thut multicasting. Lp E c dnh nhng ng dng trong tng lai. Netid trong a ch mng dng nhn dng tng mng ring bit. Cc mng lin kt phi c a ch mng (netid) ring cho mi mng. y cc bit u tin ca byte u tin c dng nh danh lp a ch (0 - lp A, 10 - lp B, 110 - lp C, 1110 - lp D v 11110 - lp E). y ta xt cu trc ca cc lp a ch c th gn c l lp A, lp B, lp C Cu trc ca cc a ch IP nh sau: Mng lp A: a ch mng (netid) l 1 Byte v a ch host (hostid) l 3 byte. Mng lp B: a ch mng (netid) l 2 Byte v a ch host (hostid) l 2 byte. Mng lp C: a ch mng (netid) l 3 Byte v a ch host (hostid) l 1 byte. Lp A cho php nh danh ti 126 mng, vi ti a 16 triu host trn mi mng. Lp ny c dng cho cc mng c s trm cc ln.
  • 44. wWw.VietLion.Com Lp B cho php nh danh ti 16384 mng, vi ti a 65534 host trn mi mng. Lp C cho php nh danh ti 2 triu mng, vi ti a 254 host trn mi mng. Lp ny c dng cho cc mng c t trm. Hnh 7.1: Cu trc cc lp a ch IP Mt s a ch c tnh cht c bit: Mt a ch c hostid = 0 c dng hng ti mng nh danh bi vng netid. Ngc li, mt a ch c vng hostid gm ton s 1 c dng hng ti tt c cc host ni vo mng netid, v nu vng netid cng gm ton s 1 th n hng ti tt c cc host trong lin mng Hnh 7.2: V d cu trc cc lp a ch IP Cn lu rng cc a ch IP c dng nh danh cc host v mng tng mng ca m hnh OSI, v chng khng phi l cc a ch vt l (hay a ch MAC) ca cc trm trn mt mng cc b (Ethernet, Token Ring.). Trong nhiu trng hp, mt mng c th c chia thnh nhiu mng con (subnet), lc c th a thm cc vng subnetid nh danh cc mng con. Vng subnetid c ly t vng hostid, c th i vi lp A, B, C nh v d sau: Hnh 7.3: V d a ch khi b sung vng subnetid
  • 45. wWw.VietLion.Com n v d liu dng trong IP c gi l gi tin (datagram), c khun dng Hnh 7.4: Dng thc ca gi tin IP ngha ca thng s nh sau: VER (4 bits): ch version hin hnh ca giao thc IP hin c ci t, Vic c ch s version cho php c cc trao i gia cc h thng s dng version c v h thng s dng version mi. IHL (4 bits): ch di phn u (Internet header Length) ca gi tin datagram, tnh theo n v t ( 32 bits). Trng ny bt but phi c v phn u IP c th c di thay i ty . di ti thiu l 5 t (20 bytes), di ti a l 15 t hay l 60 bytes. Type of service (8 bits): c t cc tham s v dch v nhm thng bo cho mng bit dch v no m gi tin mun c s dng, chng hn u tin, thi hn chm tr, nng sut truyn v tin cy. Hnh sau cho bit ngh ca trng 8 bits ny. Precedence (3 bit): ch th v quyn u tin gi datagram, n c gi tr t 0 (gi tin bnh thng) n 7 (gi tin kim sot mng). D (Delay) (1 bit): ch tr yu cu trong D = 0 gi tin c tr bnh thng D = 1 gi tin tr thp T (Throughput) (1 bit): ch thng lng yu cu s dng truyn gi tin vi la chn truyn trn ng thng sut thp hay ng thng sut cao. T = 0 thng lng bnh thng v T = 1 thng lng cao R (Reliability) (1 bit): ch tin cy yu cu R = 0 tin cy bnh thng R = 1 tin cy cao
  • 46. wWw.VietLion.Com Total Length (16 bits): ch di ton b gi tin, k c phn u tnh theo n v byte vi chiu di ti a l 65535 bytes. Hin nay gii hn trn l rt ln nhng trong tng lai vi nhng mng Gigabit th cc gi tin c kch thc ln l cn thit. Identification (16 bits): cng vi cc tham s khc (nh Source Address v Destination Address) tham s ny dng nh danh duy nht cho mt datagram trong khong thi gian n vn cn trn lin mng. Flags (3 bits): lin quan n s phn on (fragment) cc datagram, Cc gi tin khi i trn ng i c th b phn thnh nhiu gi tin nh, trong trng hp b phn on th trng Flags c dng iu khin phn on v ti lp ghp b d liu. Ty theo gi tr ca Flags s c ngha l gi tin s khng phn on, c th phn on hay l gi tin phn on cui cng. Trng Fragment Offset cho bit v tr d liu thuc phn on tng ng vi on bt u ca gi d liu gc. ngha c th ca trng Flags l: bit 0: reserved - cha s dng, lun ly gi tr 0. bit 1: (DF) = 0 (May Fragment) = 1 (Don't Fragment) bit 2: (MF) = 0 (Last Fragment) = 1 (More Fragments) Fragment Offset (13 bits): ch v tr ca on (fragment) trong datagram tnh theo n v 8 bytes, c ngha l phn d liu mi gi tin (tr gi tin cui cng) phi cha mt vng d liu c di l bi s ca 8 bytes. iu ny c ngha l phi nhn gi tr ca Fragment offset vi 8 tnh ra lch byte. Time to Live (8 bits): qui nh thi gian tn ti (tnh bng giy) ca gi tin trong mng trnh tnh trng mt gi tin b qun trn mng. Thi gian ny c cho bi trm gi v c gim i (thng qui c l 1 n v) khi datagram i qua mi router ca lin mng. Thi lng ny gim xung ti mi router vi mc ch gii hn thi gian tn ti ca cc gi tin v kt thc nhng ln lp li v hn trn mng. Sau y l 1 s iu cn lu v trng Time To Live: Nt trung gian ca mng khng c gi 1 gi tin m trng ny c gi tr= 0. Mt giao thc c th n nh Time To Live thc hin cuc ra tm ti nguyn trn mng trong phm vi m rng. Mt gi tr c nh ti thiu phi ln cho mng hot ng tt. Protocol (8 bits): ch giao thc tng trn k tip s nhn vng d liu trm ch (hin ti thng l TCP hoc UDP c ci t trn IP). V d: TCP c gi tr trng Protocol l 6, UDP c gi tr trng Protocol l 17 Header Checksum (16 bits): M kim sot li ca header gi tin IP. Source Address (32 bits): a ch ca my ngun. Destination Address (32 bits): a ch ca my ch
  • 47. wWw.VietLion.Com Options ( di thay i): khai bo cc la chn do ngi gi yu cu (tu theo tng chng trnh). Padding ( di thay i): Vng m, c dng m bo cho phn header lun kt thc mt mc 32 bits. Data ( di thay i): Trn mt mng cc b nh vy, hai trm ch c th lin lc vi nhau nu chng bit a ch vt l ca nhau. Nh vy vn t ra l phi thc hin nh x gia a ch IP (32 bits) v a ch vt l (48 bits) ca mt trm. 2. Cc giao thc trong mng IP mng vi giao thc IP hot ng c tt ngi ta cn mt s giao thc b sung, cc giao thc ny u khng phi l b phn ca giao thc IP v giao thc IP s dng n chng khi cn. Giao thc ARP (Address Resolution Protocol): y cn lu rng cc a ch IP c dng nh danh cc host v mng tng mng ca m hnh OSI, v chng khng phi l cc a ch vt l (hay a ch MAC) ca cc trm trn mt mng cc b (Ethernet, Token Ring.). Trn mt mng cc b hai trm ch c th lin lc vi nhau nu chng bit a ch vt l ca nhau. Nh vy vn t ra l phi tm c nh x gia a ch IP (32 bits) v a ch vt l ca mt trm. Giao thc ARP c xy dng tm a ch vt l t a ch IP khi cn thit. Giao thc RARP (Reverse Address Resolution Protocol): L giao thc ngc vi giao thc ARP. Giao thc RARP c dng tm a ch IP t a ch vt l. Giao thc ICMP (Internet Control Message Protocol): Giao thc ny thc hin truyn cc thng bo iu khin (bo co v cc tnh trng cc li trn mng.) gia cc gateway hoc mt nt ca lin mng. Tnh trng li c th l: mt gi tin IP khng th ti ch ca n, hoc mt router khng b nh m lu v chuyn mt gi tin IP, Mt thng bo ICMP c to v chuyn cho IP. IP s "bc" (encapsulate) thng bo vi mt IP header v truyn n cho router hoc trm ch. 3. Cc bc hot ng ca giao thc IP Khi giao thc IP c khi ng n tr thnh mt thc th tn ti trong my tnh v bt u thc hin nhng chc nng ca mnh, lc thc th IP l cu thnh ca tng mng, nhn yu cu t cc tng trn n v gi yu cu xung cc tng di n. i vi thc th IP my ngun, khi nhn c mt yu cu gi t tng trn, n thc hin cc bc sau y: To mt IP datagram da trn tham s nhn c. Tnh checksum v ghp vo header ca gi tin. Ra quyt nh chn ng: hoc l trm ch nm trn cng mng hoc mt gateway s c chn cho chng tip theo. Chuyn gi tin xung tng di truyn qua mng. i vi router, khi nhn c mt gi tin i qua, n thc hin cc ng tc sau:
  • 48. wWw.VietLion.Com 1) Tnh chesksum, nu sai th loi b gi tin. 2) Gim gi tr tham s Time - to Live. nu thi gian ht th loi b gi tin. 3) Ra quyt nh chn ng. 4) Phn on gi tin, nu cn. 5) Kin to li IP header, bao gm gi tr mi ca cc vng Time - to -Live, Fragmentation v Checksum. 6) Chuyn datagram xung tng di chuyn qua mng. Cui cng khi mt datagram nhn bi mt thc th IP trm ch, n s thc hin bi cc cng vic sau: 1) Tnh checksum. Nu sai th loi b gi tin. 2) Tp hp cc on ca gi tin (nu c phn on) 3) Chuyn d liu v cc tham s iu khin ln tng trn. II. Giao thc iu khin truyn d liu TCP TCP l mt giao thc "c lin kt" (connection - oriented), ngha l cn phi thit lp lin kt gia hai thc th TCP trc khi chng trao i d liu vi nhau. Mt tin trnh ng dng trong mt my tnh truy nhp vo cc dch v ca giao thc TCP thng qua mt cng (port) ca TCP. S hiu cng TCP c th hin bi 2 bytes. Hnh 7.5: Cng truy nhp dch v TCP Mt cng TCP kt hp vi a ch IP to thnh mt u ni TCP/IP (socket) duy nht trong lin mng. Dch v TCP c cung cp nh mt lin kt logic gia mt cp u ni TCP/IP. Mt u ni TCP/IP c th tham gia nhiu lin kt vi cc u ni TCP/IP xa khc nhau. Trc khi truyn d liu gia 2 trm cn phi thit lp mt lin kt TCP gia chng v khi khng cn nhu cu truyn d liu th lin kt s c gii phng.
  • 49. wWw.VietLion.Com Cc thc th ca tng trn s dng giao thc TCP thng qua cc hm gi (function calls) trong c cc hm yu cu yu cu, tr li. Trong mi hm cn c cc tham s dnh cho vic trao i d liu. Cc bc thc hin thit lp mt lin kt TCP/IP: Thit lp mt lin kt mi c th c m theo mt trong 2 phng thc: ch ng (active) hoc b ng (passive). Phng thc b ng, ngi s dng yu cu TCP ch i mt yu cu lin kt gi n t xa thng qua mt u ni TCP/IP (ti ch). Ngi s dng dng hm passive Open c khai bo cng TCP v cc thng s khc (mc u tin, mc an ton) Vi phng thc ch ng, ngi s dng yu cu TCP m mt lin kt vi mt mt u ni TCP/IP xa. Lin kt s c xc lp nu c mt hm Passive Open tng ng c thc hin ti u ni TCP/IP xa . Bng lit k mt vi cng TCP ph bin. S hiu cng M t 0 Reserved 5 Remote job entry 7 Echo 9 Discard 11 Systat 13 Daytime 15 Nestat 17 Quotd (quote odd day 20 ftp-data 21 ftp (control) 23 Telnet 25 SMTP 37 Time 53 Name Server 102 ISO - TSAP 103 X.400
  • 50. wWw.VietLion.Com 104 X.400 Sending 111 Sun RPC 139 Net BIOS Session source 160 - 223 Reserved Khi ngi s dng gi i mt yu cu m lin kt s c nhn hai thng s tr li t TCP. Thng s Open ID c TCP tr li ngay lp tc gn cho mt lin kt cc b (local connection name) cho lin kt c yu cu. Thng s ny v sau c dng tham chiu ti lin kt . (Trong trng hp nu TCP khng th thit lp c lin kt yu cu th n phi gi tham s Open Failure thng bo.) Khi TCP thit lp c lin kt yu cu n gi tham s Open Sucsess c dng thng bo lin kt c thit lp thnh cng. Thng bo ny dc chuyn n trong c hai trng hp b ng v ch ng. Sau khi mt lin kt c m, vic truyn d liu trn lin kt c th c thc hin. Cc bc thc hin khi truyn v nhn d liu: Sau khi xc lp c lin kt ngi s dng gi v nhn d liu. Vic gi v nhn d liu thng qua cc hm Send v receive. Hm Send: D liu c gi xung TCP theo cc khi (block). Khi nhn c mt khi d liu, TCP s lu tr trong b m (buffer). Nu c PUSH c dng th ton b d liu trong b m c gi, k c khi d liu mi n s c gi i. Ngc li c PUSH khng c dng th d liu c gi li trong b m v s gi i khi c c hi thch hp (chng hn ch thm d liu na gi i vi hiu qu hn). Hm reveive: trm ch d liu s c TCP lu trong b m gn vi mi lin kt. Nu d liu c nh du vi mt c PUSH th ton b d liu trong b m (k c cc d liu c lu t trc) s c chuyn ln cho ngi s dng. Cn nu d liu n khng c nh du vi c PUSH th TCP ch ti khi thch hp mi chuyn d liu vi mc tiu tng hiu qu h thng. Ni chung vic nhn v giao d liu cho ngi s dng ch ca TCP ph thuc vo vic ci t c th. Trng hp cn chuyn gp d liu cho ngi s dng th c th dng c URGENT v nh du d liu bng bit URG bo cho ngi s dng cn phi s l khn cp d liu . Cc bc thc hin khi ng mt lin kt: Vic ng mt lin kt khi khng cn thit c thc hin theo mt trong hai cch: dng hm Close hoc dng hm Abort. Hm Close: yu cu ng lin kt mt cch bnh thng. C ngha l vic truyn d liu trn lin kt hon tt. Khi nhn c mt hm Close TCP s truyn i tt c d liu cn trong b m thng bo rng n ng lin kt. Lu rng khi mt ngi s dng gi i mt hm Close th n vn phi tip tc nhn d liu n trn lin kt cho n khi TCP bo cho pha bn kia bit v vic ng lin kt v chuyn giao ht tt c d liu cho ngi s dng ca mnh.
  • 51. wWw.VietLion.Com Hm Abort: Ngi s dng c th ng mt lin kt bt v s khng chp nhn d liu qua lin kt na. Do vy d liu c th b mt i khi ang c truyn i. TCP bo cho TCP xa bit rng lin kt c hy b v TCP xa s thng bo cho ngi s dng ca mnh. Mt s hm khc ca TCP: Hm Status: cho php ngi s dng yu cu cho bit trng thi ca mt lin kt c th, khi TCP cung cp thng tin cho ngi s dng. Hm Error: thng bo cho ngi s dng TCP v cc yu cu dch v bt hp l lin quan n mt lin kt c tn cho trc hoc v cc li lin quan n mi trng. n v d liu s dng trong TCP c gi l segment (on d liu), c cc tham s vi ngha nh sau: Hnh 7.5: Dng thc ca segment TCP Source Por (16 bits): S hiu cng TCP ca trm ngun. Destination Port (16 bit): S hiu cng TCP ca trm ch. Sequence Number (32 bit): s hiu ca byte u tin ca segment tr khi bit SYN c thit lp. Ny bit SYN c thit lp th Sequence Number l s hiu tun t khi u (ISN) v byte d liu u tin l ISN+1. Acknowledgment Number (32 bit): s hiu ca segment tip theo m trm ngun ang ch nhn. Ngm bo nhn tt (cc) segment m trm ch gi cho trm ngun. Data offset (4 bit): s lng bi ca 32 bit (32 bit words) trong TCP header (tham s ny ch ra v tr bt u ca ngun d liu). Reserved (6 bit): dnh dng trong tng lai Control bit (cc bit iu khin): URG: Vng con tr khn (Ucgent Poiter) c hiu lc.
  • 52. wWw.VietLion.Com ACK: Vng bo nhn (ACK number) c hiu lc. PSH: Chc nng PUSH. RST: Khi ng li (reset) lin kt. SYN: ng b ha s hiu tun t (sequence number). FIN: Khng cn d liu t trm ngun. Window (16 bit): cp pht credit kim sot ngun d liu (c ch ca s). y chnh l s lng cc byte d liu, bt u t byte c ch ra trong vng ACK number, m trm ngun san sng nhn. Checksum (16 bit): m kim sot li cho ton b segment (header + data) Urgemt Poiter (16 bit): con tr ny tr ti s hiu tun t ca byte i theo sau d liu khn. Vng ny ch c hiu lc khi bit URG c thit lp. Options ( di thay i): khai bo cc option ca TCP, trong c di ti a ca vng TCP data trong mt segment. Paddinh ( di thay i): phn chn thm vo header m bo phn header lun kt thc mt mc 32 bit. Phn thm ny gm ton s 0. TCP data ( di thay i): cha d liu ca tng trn, c di ti a ngm nh l 536 byte. Gi tr ny c th iu chnh bng cch khai bo trong vng options. III. Giao thc UDP (User Datagram Protocol) UDP (User Datagram Protocol) l giao thc theo phng