GT Mang May Tinh - TG Le Dinh Danh

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MC LC MC LC .......................................................................................................................1 CHNG 1. NHP MN MNG MY TNH ...........................................................5

1.1. M U..............................................................................................................5 1.2. CC KHI NIM C BN ...............................................................................5

1.2.1. Lch s pht trin .........................................................................................5 1.2.2. Cc yu t ca mng my tnh......................................................................7

1.2.2.1. ng truyn vt l ...............................................................................8 1.2.2.2. Kin trc mng my tnh .......................................................................9

1.2.3. Phn loi mng my tnh ............................................................................11 1.2.3.1. Theo khong cch a l......................................................................11 1.2.3.2. Da theo k thut chuyn mch ..........................................................11 1.2.3.3. Phn loi theo kin trc mng.............................................................14

1.3. KIN TRC PHN TNG V M HNH OSI..............................................14 1.3.1. Kin trc phn tng.....................................................................................14 1.3.2. Mt s khi nim c bn.............................................................................15 1.3.3. M hnh OSI ...............................................................................................16

1.3.3.1. Gii thiu .............................................................................................16 1.3.3.2. Chc nng cc tng trong m hnh OSI ..............................................17 1.3.3.3. Cc dch v v hm .............................................................................19

1.3.4. Cc m hnh chun ho khc ......................................................................22 1.3.4.1. M hnh TCP/IP...................................................................................22 1.3.4.2. M hnh SNA........................................................................................23

1.4. H IU HNH MNG ..................................................................................25 1.4.1. c im quy nh chc nng ca mt h iu hnh mng. ......................25 1.4.2. Cc tip cn thit k v ci t ...................................................................26 1.4.3. Cc kiu h iu hng mng .......................................................................27

1.4.3.1. Kiu ngang hng (peer-to-peer) ..........................................................28 1.4.3.2. Kiu h iu hnh mng c my ch (server based network).............28 1.4.3.3. M hnh khch/ch (client/server).......................................................29

1.4.4. Cc chc nng ca mt h iu hnh mng................................................31 1.5. KT NI LIN MNG.........................................................................................32

1.5.1. Cc tip cn.................................................................................................32 1.5.2. Giao din kt ni .........................................................................................33

1.6. CU HI V BI TP ....................................................................................33 CHNG 2. KIN TRC PHN TNG OSI ............................................................34

2.1. TNG VT L (PHYSICAL)..........................................................................34 2.1.1. Vai tr v chc nng ca tng vt l. .........................................................34


2.1.2. Cc chun cho giao din vt l ...................................................................35 2.2. TNG LIN KT D LIU (DATA LINK) ...................................................36

2.2.1. Vai tr v chc nng ca tng lin kt d liu ...........................................36 2.2.2. Cc giao thc ca tng lin kt d liu.......................................................37 2.2.3. Cc giao thc hng k t..........................................................................37 2.2.4. Cc giao thc hng bit..............................................................................41

2.3. TNG MNG (NETWORK)............................................................................43 2.3.1. Vai tr v chc nng ca tng mng...........................................................43 2.3.2. Cc k thut chn ng trong mng my tnh..........................................44

2.3.2.1. Tng quan ............................................................................................44 2.3.2.2. Cc gii thut tm ng ti u ..........................................................45

2.3.3. Tc nghn trong mng ................................................................................47 2.3.4. Giao thc X25 PLP.....................................................................................48 2.3.5. Cng ngh chuyn mch nhanh ..................................................................50

2.3.5.1. Mng chuyn mch khung Frame Relay (FR)..................................50 2.3.5.2. K thut ATM.......................................................................................51

2.3.6. Dch v OSI cho tng mng........................................................................52 2.4. TNG GIAO VN (TRANSPORTATION) ....................................................52

2.4.1. Vai tr v chc nng ca tng Giao vn.....................................................52 2.4.2. Giao thc chun cho tng Giao vn............................................................52 2.4.3. Dch v OSI cho tng Giao vn..................................................................53

2.5. TNG PHIN (SESSION) ................................................................................53 2.5.1. Vai tr v chc nng ca tng Phin ..........................................................53 2.5.2. Dch v OSI cho tng Phin .......................................................................54 2.5.3. Giao thc chun cho tng Phin .................................................................54

2.6. TNG TRNH DIN (PRESENTATION) .......................................................54 2.6.1. Vai tr v chc nng ca tng Trnh din...................................................54 2.6.2. Dch v OSI cho tng Trnh din................................................................54 2.6.3. Giao thc chun cho tng Trnh din..........................................................54

2.7. TNG NG DNG (APPLICATION) ............................................................55 2.7.1. Vai tr v chc nng ca tng ng dng ...................................................55 2.7.2. Chun ho tng ng dng ...........................................................................55

2.8. CU HI V BI TP ....................................................................................55 CHNG 3. MNG CC B MNG LAN ..........................................................56

3.1. C TRNG MNG CC B .......................................................................56 3.2. KIN TRC MNG CC B.........................................................................56

3.2.1. Topology .....................................................................................................56 3.2.1.1. Hnh sao (star).....................................................................................56 3.2.1.2. Hnh vng (ring) ..................................................................................57


3.2.1.3. Dng ng thng (Bus) .....................................................................57 3.3.2. ng truyn vt l ....................................................................................59

3.3. CC PHNG PHP TRUY NHP NG TRUYN VT L..............60 3.3.1. Gii thiu ....................................................................................................60 3.3.2. Phng php CSMA/CD ............................................................................61 3.3.3. Phng php Token Bus.............................................................................62 3.3.4. Phng php Token Ring ...........................................................................63 3.3.5. So snh cc phng php ...........................................................................64

3.4. PHN CNG V CC THIT B MNG .....................................................65 3.4.1. Thit b cu thnh mng my tnh ..............................................................65 3.4.2. Cc thit b ghp ni mng .........................................................................66

3.5. CC CHUN LAN ..........................................................................................67 3.5.1. Chun Ethernet............................................................................................67

3.5.1.1. 10BASE-5.............................................................................................68 3.5.1.2. 10BASE-2.............................................................................................69 3.5.1.3. 10BASE-T ...........................................................................................70

3.5.2. Token Ring..................................................................................................72 3.5.3. FDDI (Fiber Distributed Data Interface) ....................................................73

3.5. CU HI V BI TP ....................................................................................73 CHNG 4. NHNG VN C BN CA MNG MY TNH ......................74

4.1. KIM SOT LI ..............................................................................................74 4.1.1. Phng php pht hin li vi bt chn l ..................................................74 4.1.2. Phng php m sa sai Hamming ............................................................74 4.1.3. Phng php m d vng (CRC) ...............................................................75

4.2. IU KHIN LU LNG V IU KHIN TC NGHN .....................76 4.2.1. Cc khi nim ............................................................................................76 4.2.2. iu khin lu lng theo c ch ca s trt...........................................77 4.2.3. iu khin tc nghn ..................................................................................79

4.2.3.1. Hin tng tc nghn ..........................................................................79 4.2.3.2. Cc gii php iu khin tc nghn ....................................................80

4.3. AN TON THNG TIN TRN MNG ..........................................................81 4.3.1. Gii thiu.........................................................................................................81

4.3.2. Cc lp bo mt trong mng.......................................................................82 4.3.3. Bo v d liu bng mt m............................................................................83

4.3.3.1. Quy trnh mt m .................................................................................84 4.3.3.2. Phng php i ch ..........................................................................85 4.3.3.3. Phng php thay th..........................................................................86 4.3.3.4. Phng php s dng chun mt m (DES) .......................................87 4.3.3.4. Phng php s dng kha cng khai (Public key)............................89


4.3.3.5. So snh cc phng php mt m.......................................................93 4.5. nh gi hiu nng mng...................................................................................94

4.5.1. Khi nim hiu nng v cc o hiu nng mng ...................................94 4.5.2. Tm quan trng ca vic nh gi hiu nng mng my tnh ....................95 4.5.3. Cc phng php nh gi hiu nng mng...............................................95

4.6. CU HI V BI TP ....................................................................................97 CHNG 5. TCP/IP V INTERNET .........................................................................98

5.1. GII THIU CHUNG V INTERNET ............................................................98 5.1.1. Lch s pht trin ca mng Internet v b giao thc TCP/IP ...................98 5.1.2. S tng trng ca Internet ........................................................................99

5.2. KIN TRC MNG INTERNET...................................................................100 5.2.1. M hnh TCP/IP........................................................................................100 5.2.2. H giao thc TCP/IP.................................................................................102

5.3. GIAO THC TCP ...........................................................................................103 5.3.1. Gii thiu ..................................................................................................103 5.3.2. Cu trc gi s liu TCP...........................................................................103 5.3.3. Thit lp v kt thc kt ni TCP .............................................................105 5.3.3. iu khin lu lng trong TCP ..............................................................105

5.3.3.1. Khi ng chm.................................................................................105 5.3.3.2. Tnh thi gian kh hi mt cch thng minh ....................................107 5.3.3.3. Trnh tc nghn.................................................................................108

5.3.4. Giao thc UDP (User Datagram protocol) ...............................................111 5.4. GIAO THC LIN MNG IP........................................................................112

5.4.1. Gii thiu ..................................................................................................112 5.4.2. Cu trc gi s liu IP..............................................................................112 5.4.3. Cc lp a ch IP......................................................................................114 5.4.4. Cc bc thc hin ca giao thc IP ........................................................115

5.5. PHN CHIA MNG CON .............................................................................116 5.6. A CH IPV6 .................................................................................................117 5.7. INTRANET V INTERNET ..........................................................................117 5.8. MT S NG DNG TRN INTERNET ....................................................117 5.9. CU HI V BI TP ..................................................................................117

DANH MC TI LIU THAM KHO ....................................................................118


CHNG 1. NHP MN MNG MY TNH

1.1. M U

Mng my tnh pht sinh t nhu cu mun chia s, dng chung ti nguyn v cho php giao tip trc tuyn (online) cng nh cc ng dng a phng tin trn mng. Ti nguyn gm c ti nguyn phn mm (d liu, chng trnh ng dng, ...) v ti nguyn phn cng (my in, my qut, CD ROM,.). Giao tip trc tuyn bao gm gi v nhn thng ip, th in t. Cc ng dng a phng tin c th l pht thanh, truyn hnh, in thoi qua mng, hi tho trc tuyn, nghe nhc, xem phim trn mng.

Trc khi mng my tnh c s dng, ngi ta thng phi t trang b my in, my v v cc thit b ngoi vi khc cho ring mnh. c th dng chung my in th mi ngi phi thay phin nhau ngi trc my tnh c ni vi my in. Khi c ni mng th tt c mi ngi ngi ti cc v tr khc nhau u c quyn s dng my in .

S kt hp ca my tnh vi cc h thng truyn thng, c bit l vin thng, to ra cuc cch mng trong vn t chc khai thc v s dng h thng my tnh. M hnh tp trung da trn my tnh ln c thay th m hnh cc my tnh n l c kt ni li cng thc hin cng vic, hnh thnh mi trng lm vic nhiu ngi s dng phn tn, cho php nng cao hiu qu khai thc ti nguyn chung t nhng v tr a l khc nhau. Cc h thng nh th c gi l mng my tnh.

Mng my tnh ngy nay tr thnh mt lnh vc nghin cu pht trin v ng dng ct li ca Cng ngh thng tin. Cc lnh vc nghin cu pht trin v ng dng ca mng: kin trc mng, nguyn l thit k, ci t v cc ng dng trn mng.

1.2. CC KHI NIM C BN

1.2.1. Lch s pht trin

Cui nhng nm 60 xut hin cc mng x l gm cc trm cui (terminal) th ng c ni vo mt my x l trung tm. My tnh trung tm hu nh m nhim tt c mi vic t x l thng tin, qun l cc th tc truyn d liu, qun l s ng b ca cc trm cui, qun l cc hng i, x l cc ngt t cc trm cui,... M hnh ny bc l cc yu im nh: tn qu nhiu vt liu (ng truyn) ni cc trm vi trung tm, my tnh trung tm phi lm vic qu nhiu dn n qu ti.

gim nh nhim v ca my tnh trung tm ngi ta gom cc trm cui vo b gi l b tp trung (hoc b dn knh) trc khi chuyn v trung tm. Cc b ny c chc nng tp trung cc tn hiu do trm cui gi n vo trn cng mt ng truyn. S khc nhau gia hai thit b ny th hin ch:

B dn knh (multiplexor): c kh nng truyn song song cc thng tin do trm cui gi v trung tm.

B tp trung (concentrator): khng c kh nng ny, phi dng b m lu tr tm thi d liu


Trong h thng ny, mi s lin lc gia cc trm cui vi nhau phi i qua my tnh trung tm, khng c ni trc tip vi nhau h thng trn khng c gi l mng my tnh m ch c gi l mng x l (hnh 1.1).

T cui nhng nm 70, cc my tnh c ni trc tip vi nhau to thnh

mng my tnh nhm phn tn ti ca h thng v tng tin cy.

B tp trung

B tin x l B tp trung/ b dn knh

My tnh trung tm

PC PC

PC

PC

PC

PC

PC

PC

PC

Hnh 1.1. Mng x l vi cc b tin x l

B tin x l B tin x l

B tin x

PC

PC

PC

PC

PC

PC PC

Hnh 1.2. Mng my tnh- ni trc tip cc b tin x l

PC

B tp trung

PC PC PC


Cng nhng nm 70 xut hin khi nim mng truyn thng (communication network), trong cc thnh phn chnh ca n l cc nt mng (Node), c gi l b chuyn mch (switching unit) dng hng thng tin ti ch.

Cc nt mng c ni vi nhau bng ng truyn gi l khung ca mng. Cc my tnh x l thng tin ca ngi s dng (host) hoc cc trm cui (terminal) c ni trc tip vo cc nt mng khi cn th trao i thng tin qua mng. Bn thn cc nt mng thng cng l my tnh nn c th ng thi ng c vai tr my ca ngi s dng. V vy chng ta khng phn bit khi nim mng my tnh v mng truyn thng. (Xem hnh 1.3).

Cc my tnh c kt ni thnh mng nhm t cc mc ch sau:

- Chia s cc ti nguyn c gi tr cao (thit b, chng trnh, d liu,..) khng ph thuc vo khong cch a l ca ti nguyn v ngi s dng.

- Tng tin cy ca h thng: do c kh nng thay th khi xy ra s c i vi mt my tnh no .

1.2.2. Cc yu t ca mng my tnh

Mng my tnh c th c nh ngha: mng my tnh l tp hp cc my tnh c ni vi nhau bi cc ng truyn vt l theo mt kin trc no . Nh vy c hai khi nim m chng ta cn phi lm r, l ng truyn vt l v kin trc ca mt mng my tnh.

T

H

H

T

H

T T

T

Nt mng

Hnh 1.3. Mt mng truyn thng

Ch thch: T (Terminal): thit b u cui H (Host): my ch, my trm


1.2.2.1. ng truyn vt l

ng truyn vt l dng chuyn cc tn hiu gia cc my tnh. Cc tn hiu biu th cc gi tr d liu di dng cc xung nh phn (on - off). Tt c cc tn hiu u thuc dng sng in t (tri t tn s sng radio, sng ngn, tia hng ngoi). ng vi mi loi tn s ca sng in t c cc ng truyn vt l khc nhau truyn tn hiu.

Hin nay c hai loi ng truyn: + ng truyn hu tuyn: cp ng trc, cp i dy xon (c bc kim, khng bc

kim), cp si quang. + ng truyn v tuyn: radio, sng cc ngn, tia hng ngoi. Cp ng trc dng truyn cc tn hiu s trong mng cc b hoc lm mng

in thoi ng di. Cu to gm c mt si kim loi trung tm c bc bi mt lp cch in v mt li kim loi chng nhiu. ngoi cng l v bc cch in. Si kim loi trung tm v li kim loi lm thnh hai si dn in ng trc

C hai loi cp ng trc khc nhau vi nhng ch nh khc nhau v k thut

v thit b ghp ni i km: cp ng trc mng (gi thnh r, dng ph bin), cp ng trc bo (t hn, c kh nng chng nhiu tt hn, thng c dung lin kt mng trong mi trng cng nghip). Cp i dy xon: c s dng rng ri trong cc mng in thoi c th ko

di hng cy s m khng cn b khuych i. Cu to gm nhiu si kim loi cch in vi nhau. Cc si ny tng i mt xon li vi nhau nhm hn ch nhiu in t. C hai loi cp xon i c s dng hin nay: cp c bc kim loi (STP), cp khng bc kim loi (UTP). Cp si quang: l cp truyn dn sng nh sng, c cu trc tng t nh cp

ng trc vi cht liu l thu tinh. Tc l gm mt dy dn trung tm (mt hoc mt b si thu tinh hoc plastic c th truyn dn tn hiu quang) c bc mt lp o c tc dng phn x cc tn hiu tr li gim s mt mt tn hiu. C hai loi cp si quang l: single-mode (ch c mt ng dn quang duy nht), multi-mode (c nhiu ng dn quang). Cp si quang c suy hao tn hiu thp, khng b nh hng ca nhiu in t v cc hiu ng in khc, khng b pht hin v thu trm, an ton thng tin trn mng c bo m. Tuy nhin cp si quang kh lp t, gi thnh cao. Sng cc ngn thng c dng truyn gia cc trm mt t v cc v tinh.

Chng truyn cc tn hiu qung b t mt trm pht ti nhiu trm thu.

Hnh 1.4. Cp ng trc


Sng hng ngoi: Mi trng truyn dn sng hng ngoi l mt mi trng nh hng, trong din hp v vy n ch thch hp cho mt mng din hp bn knh t 0.5m n 20 m, vi cc thit b t b di chuyn. Tc truyn d liu xung quanh 10Mbps Sng radio: mi trng truyn dn sng radio l mt mi trng nh hng

trong mng din rng vi bn knh 30 km. Tc truyn d liu hng chc Mbps. Lin quan n ng truyn vt l chng ta c cc khi nim sau:

- Bng thng (cn gi l di thng - bandwidth): Bng thng l mt khi nim cc k quan trng trong cc h thng truyn thng. Hai phng php xem xt bng thng c tm quan trng trong nghin cu cc mng l bng thng tng t (analog) v bng thng s (digital). Bng thng tng t l o phm vi tn s m ng truyn c th p ng c trong mt h thng in t dng k thut tng t. n v o lng cho bng thng tng t l Hz, hay s chu k trn giy. V d, bng thng ca cp in thoi l 400-4000Hz, c ngha l n c th truyn cc tn hiu vi cc tn s nm trong phm vi t 400 n 4000Hz. Bng thng s o lng lng thng tin ti a t ni ny n ni khc trong mt thi gian cho trc. n v c bn o lng bng thng s l bt/giy (bps) v cc bi ca n l Kilbit/giy (kbps), Megabit/giy (Mbps), Gigabit/giy (Gbps), Terabit/giy (Tbps).. Bng thng ca cp truyn ph thuc vo di cp. Cp cng di th bng thng cng gim. Do vy khi thit k mng phi ch r di chy cp ti a, bi v ngoi gii hn d th cht lng truyn tn hiu khng cn c bo m.

- Thng lng (throughput): thng lng l lng thng tin thc s c truyn qua trong mt n v thi gian. Cng nh bng thng, n v ca thng lng l bps v cc bi ca n: Kbps, Mbps, Gbps, Gbps, Tbps. Trong mt mng LAN bng thng c th cho php 100Mbps, nhng iu ny khng c ngha l mi ngi dng trn mng u c th di chuyn thc s 100 Megabit d liu trong mt giy. iu ny ch ng trong nhng iu kin v cng l tng. Do nhiu l do, thng lng thng nh hn rt nhiu so vi bng thng s ti a ca mi trng mng.

- Hiu sut s dng ng truyn (utilization): i lng ny c trng cho hiu sut phc v ca ng truyn trong mng. N c o bng t l % gia thng lng v bng thng ca ng truyn.

- tr (delay): tr l thi gian cn thit truyn mt gi tin t ngun n ch. tr thng c o bng miligiy (ms), giy (s). tr ph thuc vo bng thng ca mng. Bng thng cng ln th tr cng nh.

- suy hao l o s yu i ca tn hiu trn ng truyn. N cng ph thuc vo di cp. Cn nhiu t gy ra bi tin n in t bn ngoi lm nh hng n tn hiu trn ng truyn.

1.2.2.2. Kin trc mng my tnh

Kin trc mng my tnh (network architecture) th hin cch ni cc my tnh vi nhau ra sao v tp hp cc quy tc, quy c m tt c cc thc th tham gia truyn thng trn mng phi tun theo m bo cho mng hot ng tt.


Cch ni cc my tnh c gi l hnh trng (topolopy) ca mng hay ni cho gn l topo mng. Cn tp hp cc quy tc, quy c truyn thng c gi l giao thc (protocol) ca mng. Topo v giao thc l hai khi nim rt c bn ca mng my tnh, v th chng s c trnh by c th hn trong nhng phn sau:

y Topo mng C hai kiu kt ni mng ch yu l im - im (point-to-point) v qung b

(broadcast hay point-to-multipoint). Theo kiu kt ni im - im, cc ng truyn ni tng cp nt vi nhau v

mi nt u c trch nhim lu tr tm thi sau chuyn tip d liu i ti ch. Do cch lm vic nh th nn mng kiu ny cn c gi l mng lu v chuyn tip (store and forward). Ni chung cc mng din rng u s dng nguyn tc ny. Hnh 2 cho mt s dng topo ca mng loi ny.

Theo kiu qung b, tt c cc nt mng dng chung mt ng truyn vt l. D liu gi i t mt nt mng c th c tt c cc nt mng cn li tip nhn ch cn ch ra a ch ch ca d liu mi nt kim tra xem c phi l gi cho mnh hay khng. Hnh 3 cho mt s dng topo ca mng loi ny.

Trong cc topo dng vng hoc dng tuyn tnh cn c mt c ch trng ti

gii quyt xung t khi nhiu nt mun truyn tin cng mt lc. Vic cp pht ng truyn c th l ng hoc tnh. Cp pht tnh thng dung c ch quay vng phn chia ng truyn theo cc khong thi gian nh trc. Cp pht ng l cp pht theo yu cu hn ch thi gian cht v ch ca ng truyn.

Hnh 1.5. Mt s topo mng im-im

Hnh 1.6. Mt s topo mng qung b


y Giao thc mng Vic trao i thng tin cho d l n gin nht, cng u phi tun theo nhng

quy tc nht nh. Hai ngi ni chuyn vi nhau mun cho cuc ni chuyn c kt qu th t nht c hai cng phi ngm nh tun theo quy tc: khi ngi ny ni th ngi kia phi nghe v ngc li. Vic truyn tn hiu trn mng cng vy, cn phi c nhng quy tc, quy c v nhiu mt:

+ Khun dng ca d liu: c php v ng ngha + Th tc gi v nhn d liu + Kim sot cht lng truyn + X l cc li, s c

Tp hp tt c cc quy tc, quy c trn gi l giao thc mng. Yu cu v x l v trao i thng tin ca ngi s dng ngy cng cao th giao thc mng cng phc tp. Cc mng c th c giao thc khc nhau tu thuc vo s la chn ca nh thit k.

1.2.3. Phn loi mng my tnh

C nhiu cch phn loi mng my tnh tu thuc vo yu t chnh c chn lm ch tiu phn loi: khong cch a l, k thut chuyn mch, kin trc ca mng.

1.2.3.1. Theo khong cch a l

Nu ly khong cch a l lm yu t chnh phn loi th mng my tnh c phn thnh 4 loi: mng cc b, mng th, mng din rng, mng ton cu.

- Mng cc b (Local Area Networks - LAN): ci t trong phm vi tng i hp (v d nh trong mt ta nh, mt c quan, mt trng hc,..), khong cch ln nht gia cc my tnh ni mng l vi chc km tr li.

- Mng th (Metropolitan Area Networks - MAN): ci t trong phm vi mt th, mt trung tm kinh t x hi, c bn knh nh hn 100 km.

- Mng din rng (Wide Area Networks - WAN): phm vi ca mng c th vt qua bin gii quc gia v thm ch c lc a.

- Mng ton cu (Global Area Networks - GAN): phm vi rng khp ton cu. Mng Internet l mt v d cho loi ny.

Chng ta cng cn lu rng: khong cch a l c dng lm mc ch mang tnh tng i. Cng vi s pht trin ca cc cng ngh truyn dn v qun tr mng th nhng ranh gii ngy cng m nht i.

1.2.3.2. Da theo k thut chuyn mch

Nu ly k thut chuyn mch lm yu t chnh phn ploi th ta c 3 loi: mng chuyn mch knh, mng chuyn mch thng bo v mng chuyn mch gi.


a. Mng chuyn mch knh

Khi c hai thc th cn trao i thng tin vi nhau th gia chng s thit lp mt knh c nh v c duy tr cho n khi mt trong hai bn ngt lin lc. Cc d liu ch c truyn theo con ng c nh .

Nhc im:

+ Tn thi gian thit lp knh c nh gia hai thc th + Hiu sut s dng ng truyn thp v s c lc knh b b khng do c hai bn u ht thng tin cn truyn trong khi cc thc th khc khng c php s dng knh truyn ny.

b. Mng chuyn mch thng bo

Thng bo (message) l mt n v thng tin ca ngi s dng c khun dng c qui nh trc. Mi thng bo u c cha vng thng tin iu khin trong ch nh r ch n ca thng bo. Cn c vo thng tin ny m mi nt trung gian c th chuyn thng bo ti nt k tip theo ng dn ti ch ca n.

Mi nt cn phi lu tr tm thi c thng tin iu khin trn thng bo sau chuyn tip thng bo i. Tu thuc vo iu kin ca mng, cc thng bo khc nhau c th truyn theo ng truyn khc nhau.

Hnh 1.7. Mng chuyn mch knh

Hnh 1.8. Mng chuyn mch thng bo


u im so vi mng chuyn mch knh: - Hiu sut s dng ng truyn cao v khng b chim dng c quyn m c

phn chia gia nhiu thc th. - Mi nt mng c th lu tr thng bo cho ti khi knh truyn ri mi gi thng

bo i, v vy gim c tnh trng tc nghn mch. - C th iu khin vic truyn tin bng cch sp xp u tin cho cc thng bo. - C th tng hiu sut s dng di thng bng cch gn a ch qung b gi

thng bo ng thi ti nhiu ch. Nhc im:

- Khng hn ch kch thc ca cc thng bo, dn n ph tn lu tr tm thi cao v nh hng ti thi gian p (respone time) v cht lng truyn tin.

- Thch hp cho cc dch v th tn in t hn l cc p dng c tnh thi gian thc v tn ti tr do lu tr v x l thng tin iu khin ti mi nt.

c. Mng chuyn mch gi

Mi thng bo c chia lm nhiu phn nh hn c gi l cc gi tin c khun dng quy nh trc. Mi gi tin cng cha cc thng tin iu khin, trong c a ch ngun (ngi gi) v ch (ngi nhn) ca gi tin. Cc gi tin ca mt thng bo c th i qua mng ti ch bng nhiu con ng khc nhau. bn nhn, th t nhn c c th khng ng th t c gi i.

So snh mng chuyn mch thng bo v mng chuyn mch gi:

Ging nhau: phng php ging nhau Khc nhau: Cc gi tin c gii hn kch thc ti a sao cho cc nt mng c

th x l ton b gi tin trong b nh m khng cn phi lu tr tm thi trn a. V th mng chuyn mch gi truyn cc gi tin qua mng nhanh chng v hiu qu hn so vi mng chuyn mch thng bo. Nhng vn kh khn ca mng loi ny l vic tp hp cc gi tin to li thng bo ban u ca ngi s dng, c bit trong trng hp cc gi c truyn theo nhiu ng khc nhau. Cn phi ci t c ch nh du gi tin v phc hi gi tin b tht lc hoc truyn b li cho cc nt mng.

Do c u im mm do v hiu sut cao hn nn hin nay mng chuyn mch gi c s dng ph bin hn cc mng chuyn mch thng bo. Vic tch hp c hai

Hnh 1.9. Mng chuyn mch gi


k thut chuyn mch (knh v gi) trong mt mng thng nht (c gi l mng dch v tch hp s- Intergrated Services Digital Networks, vit tt l ISDN) ang l mt xu hng pht trin ca mng ngy nay.

1.2.3.3. Phn loi theo kin trc mng

Ngi ta cn phn loi mng theo kin trc mng (topo v giao thc s dng). Cc mng thng hay c nhc n nh: mng SNA ca IBM, mng ISO, mng TCP/IP.

1.3. KIN TRC PHN TNG V M HNH OSI

1.3.1. Kin trc phn tng

gim phc tp ca vic thit k v ci t mng, hu ht cc my tnh u c phn tch thit k theo quan im phn tng. Mi h thng thnh phn ca mng c xem nh mt cu trc a tng, trong mi tng c xy dng trn tng trc n. S lng cc tng cng nh tn v chc nng ca mi tng tu thuc vo nh thit k. Trong hu ht cc mng, mc ch ca mi tng l cung cp mt s dch v nht nh cho tng cao hn. Mi tng khi s dng khng cn quan tm n cc thao tc chi tit m cc dch v phi thc hin.

Nguyn tc ca kin trc mng phn tng: - Mi h thng trong mt mng u c cu trc tng nh nhau (s lng tng,

chc nng ca mi tng).

Giao thc tng N

Giao thc tng i + 1

Giao thc tng i

Giao thc tng i -1

Giao thc tng 1

Tng N

Tng i +1

Tng i

Tng i -1

Tng 1

Tng N

Tng i +1

Tng i

Tng i -1

Tng 1

ng truyn vt l

Hnh 1.10. Minh ho kin trc phn tng tng qut

H thng A H thng B


- D liu khng c truyn trc tip t tng i ca h thng ny sang tng th i ca h thng kia (ngoi tr i vi tng thp nht). Bn gi d liu cng vi cc thng tin iu khin chuyn n tng ngay di n v c th cho n tng thp nht. Bn di tng ny l ng truyn vt l, y s truyn tin mi thc s din ra. i vi bn nhn th cc thng tin c chuyn t tng di ln trn cho ti tng i ca h thng nhn.

- Gia hai h thng kt ni ch tng thp nht mi c lin kt vt l cn tng cao hn ch l lin kt logic hay lin kt o c a vo hnh thc ha cc hot ng ca mng, thun tin cho vic thit k v ci t cc phn mm truyn thng.

Cc vn cn phi gii quyt khi thit k cc tng - C ch ni, tch: mi mt tng cn c mt c ch thit lp kt ni, v c mt

c ch kt thc kt ni khi m s kt ni l khng cn thit na. - Cc quy tc truyn d liu: Trong cc h thng khc nhau d liu c th truyn

theo mt s cch khc nhau: + Truyn mt hng (simplex) + Truyn hai hng ng thi (full-duplex) + Truyn theo c hai hng lun phin (half-duplex)

- Kim sot li: ng truyn vt l ni chung l khng hon ho, cn phi tho thun dng mt loi m pht hin, kim tra li v sa li. Pha nhn phi c kh nng thng bo cho bn gi bit cc gi tin no thu ng, gi tin no pht li.

- di bn tin: Khng phi mi qu trnh u chp nhn di gi tin l tu , cn phi c c ch chia bn tin thnh cc gi tin nh.

- Th t cc gi tin: Cc knh truyn c th gi khng ng th t cc gi tin, do cn c c ch bn thu ghp ng th t ban u.

- Tc pht v thu d liu: Bn pht c tc cao c th lm lt bn thu c tc thp. Cn phi c c ch bn thu bo cho bn pht bit tnh trng iu khin lu lng hp l.

1.3.2. Mt s khi nim c bn

Tng (layer) - Mi qu trnh trao i thng tin gia hai i tng u thc hin qua nhiu bc,

cc bc ny c lp tng i vi nhau. Thng tin c trao i gia hai i tng A, B qua 3 bc:

- Pht tin: Thng tin chuyn t tng cao tng thp - Nhn tin: Thng tin chuyn t tng thp tng cao - Qu trnh trao i thng tin trc tip qua ng truyn vt l (thc hin tng

cui cng)


Giao din, dch v, n v d liu - Mi quan h gia hai tng k nhau gi l giao din - Mi quan h gia hai tng ng mc ca hai h thng khc nhau gi l giao thc - Thc th (entity): l thnh phn tch cc trong mi tng, n c th l mt tin

trnh trong h a x l hay l mt trnh con cc thc th trong cng 1 tng cc h thng khc nhau (gi l thc th ngang hng hay thc th ng mc). Mi thc th c th truyn thng ln tng trn hoc tng di n thng qua mt giao din (interface). Giao din gm mt hoc nhiu im truy nhp dch v (Service Access Point - SAP). Ti cc im truy nhp dch v tng trn ch c th s dng dch v do tng di cung cp. Thc th c chia lm hai loi: thc th cung cp dch v v s dng dch v: + Thc th cung cp dch v (service provide): l cc thc th tng N cung cp

dch v cho tng N +1. + Thc th s dng dch v (service user): l cc thc th tng N s dng

dch v do tng N - 1 cung cp. - n v d liu s dng giao thc (Protocol Data Unit - PDU) - n v d liu dch v (Service Data Unit - SDU) - Thng tin iu khin (Protocol Control Information - PCI)

Mt n v d liu m 1 thc th tng N ca h thng A gi sang thc th tng N mt h thng B khng bng ng truyn trc tip m phi truyn xung di truyn bng tng thp nht thng qua ng truyn vt l.

+ D liu tng N-1 nhn c do tng N truyn xung gi l SDU. + Phn thng tin iu khin ca mi tng gi l PCI. + tng N-1 phn thng tin iu khin PCI thm vo u ca SDU to thnh

PDU. Nu SDU qu di th ct nh thnh nhiu on, mi on b sung phn PCI, to thnh nhiu PDU.

Bn h thng nhn trnh t din ra theo chiu ngc li. Qua mi tng PCI tng ng s c phn tch v ct b khi PDU trc khi gi ln tng trn.

1.3.3. M hnh OSI

1.3.3.1. Gii thiu

Khi thit k cc nh thit k t do la chn kin trc mng ring ca mnh. T dn n tnh trng khng tng thch gia cc mng: phng php truy nhp ng truyn khc nhau, s dng h giao thc khc nhau,... S khng tng thch lm cho ngi s dng cc mng khc nhau khng th trao i thng tin vi nhau c. S thc bch ca khch hng khin cho cc nh sn xut v nhng nh nghin cu, thng qua t chc chun ho quc t v quc gia tm ra mt gii php chung dn n s hi t ca cc sn phm mng. Trn c s nhng nh thit k v cc nghin cu ly lm khung chun cho sn phm ca mnh.


V l do , nm 1977, T chc tiu chun ho quc t (International Organization for Standardization - ISO) lp ra mt tiu ban nhm a ra mt khung chun nh th. Kt qu l vo nm 1984 ISO xy dng m hnh 7 tng gi l m hnh tham chiu cho vic ni kt cc h thng m (Reference Model for Open Systems Interconnection - OSI Reference Model) gi tt l m hnh OSI. M hnh ny c dng lm c s ni kt cc h thng m phc v cho cc ng dng phn tn Mi h thng tun theo m hnh tham chiu OSI u c th truyn thng tin vi nhau.

1.3.3.2. Chc nng cc tng trong m hnh OSI a. Tng vt l (Physical)

- Tng vt l lin quan n truyn dng cc bit gia cc my vi nhau bng ng truyn vt l. Tng ny lin kt cc giao din hm c, quang v in vi cp. Ngoi ra n cng chuyn ti nhng tn hiu truyn d liu do cc tng trn to ra.

- Vic thit k phi bo m nu bn pht gi bt 1 th bn thu cng phi nhn bt 1 ch khng phi bt 0

- Tng ny phi quy nh r mc in p biu din d liu 1 v 0 l bao nhiu von trong vng bao nhiu giy

- Chiu truyn tin l 1 hay 2 chiu, cch thc kt ni v hu b kt ni - nh ngha cch kt ni cp vi card mng: b ni c bao nhiu chn, chc nng

ca mi chn Tm li: Thit k tng vt l phi gii quyt cc vn ghp ni c, in, to ra cc

hm, th tc truy nhp ng truyn, ng truyn cc bt.

NG DNG 7

TRNH DIN 6

PHIN 5

GIAO VN 4

MNG 3

LIN KT D LIU 2

VT L 1

7 APPLICATION

6 PRESENTATION

5 SESSION

4 TRANSPORT

3 NETWORK

2 DATA LINK

1 PHYSICAL

Giao thc tng 7

Giao thc tng 6

Giao thc tng 5

Giao thc tng 4

Giao thc tng 3

Giao thc tng 2

Giao thc tng 1

ng truyn vt l

Hnh 1.11. M hnh OSI 7 tng

H thng A H thng B

L nh Danh - Gio trnh Mng my tnh


b. Tng lin kt d liu (data link)

- Cung cp phng tin truyn thng tin qua lin kt vt l m bo tin cy: gi cc khi d liu vi c ch ng b ho, kim sot li v kim sot lung d liu cn thit

- Cc bc tng lin kt d liu thc hin: + Chia nh thnh cc khi d liu frame (vi trm bytes), ghi thm vo u v

cui ca cc frame nhng nhm bt c bit lm ranh gii gia cc frame + Trn cc ng truyn vt l lun c li nn tng ny phi gii quyt vn

sa li (do bn tin b hng, mt v truyn li) + Gi cho s ng b tc gia bn pht v bn thu Tm li: tng lin kt d liu chu trch nhim chuyn khung d liu khng li t

my tnh ny sang my tnh khc thng qua tng vt l. Tng ny cho php tng mng truyn d liu gn nh khng phm li qua lin kt mng

c. Tng mng (Network)

- Lp a ch cc thng ip, din dch a ch v tn logic thnh a ch vt l - Kim sot v iu khin ng truyn: nh r cc b tin c truyn i theo con ng no t ngun ti ch. Cc con ng c th l c nh i vi nhng mng t thay i, cng c th l ng ngha l cc con ng ch c xc nh trc khi bt u cuc ni chuyn. Cc con ng c th thay i tu theo trng thi ti tc thi.

- Qun l lu lng trn mng: chuyn i gi, nh tuyn, kim sot s tc nghn d liu (nu c nhiu gi tin cng c gi i trn ng truyn th c th xy ra tc nghn )

- Kim sot lung d liu v ct hp d liu (nu cn) d. Tng giao vn (Transport)

- Thc hin vic truyn d liu gia hai u nt (end - to - end). - Thc hin kim sot li, kim sot lung d liu t my my. m bo gi tin

truyn khng phm li, theo ng trnh t, khng b mt mt hay sao chp. - Thc hin vic ghp knh, phn knh ct hp d liu (nu cn). ng gi thng ip, chia thng ip di thnh nhiu gi tin v gp cc gi nh thnh mt b.

- Tng ny to ra mt kt ni cho mi yu cu ca tng trn n. Khi c nhiu yu cu t tng trn vi thng lng cao th n c th to ra nhiu kt ni v cng mt lc c th gi i nhiu b tin trn ng truyn.

e. Tng phin (Session)

- Cung cp phng tin truyn thng gia cc ng dng: cho php ngi s dng trn cc my khc nhau c th thit lp, duy tr, hu b v ng b ho cc phin truyn thng gia h vi nhau.

- Nhim v chnh:



+ Qun l th bi i vi nhng nghi thc: hai bn kt ni truyn thng tin khng ng thi thc hin mt s thao tc. gii quyt vn ny tng phin cung cp 1 th bi, th bi c th c trao i v ch bn no gi th bi mi c th thc hin mt s thao tc quan trng

+ Vn ng b: khi cn truyn i nhng tp tin di tng ny chn thm cc im kim tra (check point) vo lung d liu. Nu pht hin thy li th ch c d liu sau im kim tra cui cng mi phi truyn li

f. Tng trnh din (Presentation)

- Quyt nh dng thc trao i d liu gia cc my tnh mng. Ngi ta c th gi y l b dch mng. bn gi, tng ny chuyn i c php d liu t dng thc do tng ng dng gi xung sang dng thc trung gian m ng dng no cng c th nhn bit. bn nhn, tng ny chuyn cc dng thc trung gian thnh dng thc thch hp cho tng ng dng ca my nhn.

- Tng trnh din chu trch nhim chuyn i giao thc, bin dch d liu, m ho d liu, thay i hay chuyn i k t v m rng lnh ho.

- Nn d liu nhm lm gim bt s bt cn truyn - tng ny c b i hng hot ng i hng cc hot ng nhp/xut gi n cc ti nguyn trn my phc v

g. Tng ng dng (Application)

- Cung cp cc phng tin ngi s dng c th truy nhp c vo mi trng OSI, ng thi cung cp cc dch v thng tin phn tn.

- Tng ny ng vai tr nh ca s dnh cho hot ng x l cc trnh ng dng nhm truy nhp cc dch v mng. N biu din nhng dch v h tr trc tip cc ng dng ngi dng, chng hn nh phn mm chuyn tin, truy nhp c s d liu v email.

- X l truy nhp mng chung, kim sot li v phc hi li.

1.3.3.3. Cc dch v v hm

Dch v l mt dy, mt tp cc thao tc s cp hay l cc hm nguyn thy m mt tng cung cp cho tng trn n. Dch v lin quan n 2 tng k nhau

a. Dch v nh hng lin kt v dch v khng lin kt

mi tng trong m hnh OSI c hai loi dch v: dch v nh hng lin kt (connection - oriented service) v dch v khng nh hng lin kt (connectionless service)

- Dch v nh hng lin kt: l dch v theo m hnh in thoi, trc khi truyn d liu cn thit lp mt lin kt logic gia cc thc th ng mc

- Dch v khng lin kt: khng cn phi thit lp lin kt logic v mt n v d liu c truyn l c lp vi cc n v d liu trc hoc sau n. Loi dch v ny theo m hnh bu in: mi bn tin hay mi bc th cn c mt a ch c th bn nhn



Trong phng php lin kt qu trnh truyn thng gm c 3 giai on: - Thit lp lin kt (logic): hai thc th ng mc hai h thng s thng lng vi

nhau v tp cc tham s s s dng trong giai on truyn sau (th hin bng hm CONNECT).

- Truyn d liu: d liu c truyn vi cc c ch kim sot v qun l km theo (kim sot li, kim sot lung d liu, ct/hp d liu,...) tng tin cy v hiu qu ca vic truyn d liu (hm DATA).

- Hu b lin kt (logic): gii phng cc ti nguyn h thng c cp pht cho lin kt dng cho cc lin kt khc (hm DISCONNECT). Trong mi loi dch v c c trng bng cht lng dch v. C dch v i hi

bn nhn tin gi thng bo xc nhn khi tin cy c bo m. C nhng ng dng khng chp nhn s chm tr do phi xc nhn s truyn tin

(VD h thng truyn tin). Nhng c nhiu ng dng nh th tn in t ngi gi ch cn c mt dch v vi tin cy cao, chp nhn s chm tr.

b. Cc hm nguyn thu ca dch v

Mt dch v gm 1 s thao tc s cp hay cc hm nguyn thu. Mt thc th cung cp dch v cho mt thc th tng trn n thng qua vic gi cc hm nguyn thu. Cc hm nguyn thu ch r chc nng cn phi thc hin v dng chuyn d liu vo thng tin iu khin. C 4 hm nguyn thu c dng xc nh tng tc gia cc tng k nhau (hnh 1.12).

- Request (yu cu): ngi s dng dch v dng gi chc nng hoc yu cu thc th khc thc hin mt cng vic no .

- Indication (ch bo): ngi cung cp dch v dung gi mt chc nng no , ch bo mt chc nng c gi mt im truy nhp dch v.

- Response (tr li): ngi s dng dch v dng hon tt mt chc nng c gi t trc bi mt hm nguyn thu Indication im truy nhp dch v .

- Confirm (xc nhn): ngi cung cp dch v dng hon tt mt chc nng c gi t trc bi mt hm nguyn thu Response ti im truy nhp dch v.

Quy trnh thc hin mt giao tc gia hai h thng A v B : - Tng N+1 ca A gi xung tng N k di n mt hm Request - Tng N ca A cu to mt n v d liu gi yu cu sang tng N ca B theo

giao thc tng N xc nh - Nhn c yu cu, tng N ca B ch bo ln tng N+1 k trn n bng hm

Indication - Tng N ca B tr li bng hm Response gi xung tng N k di n - Tng N ca B cu to mt n v d liu gi tr li v tng N ca A theo

giao thc tng N xc nh - Nhn c tr li, tng N ca A xc nhn vi tng N+1 k trn n bng hm

Confirm, kt thc mt giao tc gia hai h thng.



Cc thao tc s cp ni chung l c tham s (VD Connect.Request). Cc tham s gm:

- a ch my gi - a ch my nhn - Loi dch v - Kch thc cc i ca bn tin Nu thc th b gi khng chp nhn kch thc cc i m bn tin a ra n c th

yu cu kch thc mi trong thao tc ca hm Response. Cc chi tit ca qu trnh tho thun l mt phn ca nghi thc. Cc dch v c th xc nhn hoc khng xc nhn.

- Cc dch v xc nhn c th c cc hm nguyn thu: Request, Indication, Response, Confirm

- Dch v khng xc nhn c hai hm nguyn thu: Resquest, Indication Trong thc t loi dch v connect lun lun l c xc nhn, cn cc loi dch v

DATA l khng xc nhn hoc c xc nhn.

STT Hm nguyn thu ngha

1. CONNECT.Request Yu cu thit lp lin kt

2. CONNECT.Indication Bo cho thc th b gi

3. CONNECT.Response ng hay khng ng

4. CONNECT.Confirm Xc nhn vi bn gi vic kt ni c c chp nhn hay khng

5. DATA.Request Bn gi yu cu truyn d liu

6. DATA.Indication Bo cho bn nhn bit l d liu n

7. DISCONNECT.Request Yu cu hu b lin kt

8. DISCONNECT.Indication Bo cho bn nhn

(N+1) Layer (N+1) Layer

System A System B

(N) Layer (N) Layer

Confirm Request

SAP

Reponse Indication

Interface

SAP

t

Protocol (N)

Sevice user

Hnh 1.12. S hot ng ca cc hm nguyn thu



V d: 1. CONNECT.Request Bn quay s in thoi ca c Lan 2. CONNECT.Indication Chung reo 3. CONNECT.Response C Lan nhc my 4. CONNECT.Confirm Chung ngng reo 5. DATA.Request Bn ni chuyn vi c Lan 6. DATA.Indication C Lan nghe thy bn ni 7. DATA.Response C tr li bn 8. DATA.Confirm Bn nghe thy cu tr li 9. DISCONNECT.Request Bn cp my 10. DISCONNECT.Indication C Lan nghe thy bn cp my.

1.3.4. Cc m hnh chun ho khc

1.3.4.1. M hnh TCP/IP

Mc d m hnh tham chiu OSI c chp nhn rng ri khp ni, nhng chun m v k thut mang tnh lch s ca Internet li l TCP/IP (Transmission Control Protocol/Internet Protocol). M hnh tham chiu TCP/IP v chng giao thc TCP/IP to kh nng truyn d liu gia hai my tnh t bt k ni no trn th gii, vi tc gn bng tc nh sng. M hnh TCP/IP c tm quan trng trong lch s, gn ging nh cc chun cho php in thoi, nng lng in, ng st, truyn hnh v cng ngh bng hnh pht trin cc thnh.

Hnh 1.13 trnh by hai m hnh tham chiu TCP/IP v OSI tin so snh.

Cc tng ca m hnh tham chiu TCP/IP B quc phng M gi tt l DoD (Department of Defense) to ra m hnh tham

chiu TCP/IP v mun mt mng c th tn ti trong bt c iu kin no, ngay c khi c chin tranh ht nhn. DoD mun cc gi d liu xuyn sut mng vo mi lc, di bt c iu kin no, t bt c mt im n mt im khc. y l mt bi ton thit k cc k kh khn m t lm ny sinh ra m hnh TCP/IP, v vy tr thnh chun Internet pht trin.

Hnh 1.13. Tng ng cc tng cc kin trc TCP/IP v OSI



Tng ng dng (Application) Cc nh thit k TCP/IP cm thy rng cc giao thc mc cao nn bao gm cc

tng trnh by v tng phin. n gin, h to ra mt tng ng dng kim sot cc giao thc mc cao, cc vn ca tng trnh by, m ho v iu khin hi thoi. TCP/IP tp hp tt c cc vn lin quan n ng dng vo trong mt tng, v m bo d liu c ng gi mt cch thch hp cho tng k tip. Tng vn chuyn (Transportation)

Tng vn chuyn cp n cc vn cht lng dch v nh tin cy, iu khin lung v sa li. Mt trong cc giao thc ca n la TCP, TCP cung cp cc phng thc linh hot v hiu qu thc hin cc hot ng truyn d liu tin cy, hiu xut cao v t li. TCP l giao thc c to cu ni (connection-oriented). N tin hnh hi thoi gia ngun v ch trong khi bc thng tin tng ng dng thnh cc n v gi l segment.To cu ni khng c ngha l tn ti mt mch thc s gia hai my tnh, thay v vy n c ngha l cc segment ca tng 4 di chuyn ti v lui gia hai host cng nhn kt ni tn ti mt cch lun l trong mt khong thi gian no . iu ny coi nh chuyn mch gi (packet switching). Tng Internet

Mc tiu ca tng Internet l truyn cc gi tin bt ngun t bt k mng no trn lin mng v n c ch trong iu kin c lp vi ng dn v cc mng m chng tri qua. Giao thc c trng khng ch tng ny c gi l IP. Cng vic xc nh ng dn tt nht v hot ng chuyn mch gi din ra ti tng ny.

Tng truy xut mng (Host to network)

Tn ca tng ny c ngha kh rng v c phn hi ri rm. N cng c gi l tng host-to-network. N l tng lin quan n tt c cc vn m mt gi IP yu cu to mt lin kt vt l thc s, v sau to mt lin kt vt l khc. N bao gm cc chi tit k thut LAN v WAN, v tt c cc chi tit trong tng lin kt d liu cng nh tng vt l ca m hnh OSI.

M hnh TCP/IP hng n ti a linh hot ti tng ng dng cho ngi pht trin phn mm. Tng vn chuyn lin quan n hai giao thc TCP v UDP (User Datagram Protocol). Tng cui cng, tng truy xut mng lin kt n cc k thut LAN hay WAN ang c dng.

Trong m hnh TCP/IP khng cn quan tm n ng dng no yu cu cc dch v mng, v khng cn quan tm n giao thc vn chuyn no ang c dng, ch c mt giao thc mng IP. y l mt quyt nh thit k c cn nhc k. IP phc v nh mt giao thc a nng cho php bt k my tnh no, bt c u, truyn d liu vo bt c thi im no.

1.3.4.2. M hnh SNA 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.

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:

Hnh 1.14. Tng ng cc tng cc kin trc SNA v OSI



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 phin v hp tc gia cc phin 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.

1.4. H IU HNH MNG

1.4.1. c im quy nh chc nng ca mt h iu hnh mng. Mi trng mng c nhng c im ring, khc vi mi trng ch dng my tnh c

nhn (PC), th hin cc c trng sau: Trc ht l mi trng nhiu ngi dng. c im ny dn n cc nhu cu lin lc

gia nhng ngi s dng, nhu cu bo v d liu v ni chung l bo v tnh ring t ca ngi s dng.

Mng cn l mi trng a nhim, c nhiu cng vic thc hin trn mng. c im ny s pht sinh cc nhu nhu cu chia s ti nguyn, nhu cu lin lc gia cc tin trnh nh trao i d liu, ng b ho.



L mi trng phn tn, ti nguyn (thng tin, thit b) nm cc v tr khc nhau, ch kt ni thng qua cc ng truyn vt l. iu ny pht sinh cc nhu cu chia s ti nguyn trn ton mng nhng s phn tn cn c trong sut i n khng gy kh khn cho ngi s dng.

C nhiu quan nim cng nh cc gii php mng khc nhau. iu ny sinh nhu cu giao tip gia cc mng khc nhau.

Lm vic trn mi trng mng chc chn s phc tp hn mi trng my n l. V th rt cn c cc tin ch gip cho vic s dng v qun tr mng d dng v hiu qu.

Tt c cc nhu cu trn phi c tnh ti trong h iu hnh mng.

1.4.2. Cc tip cn thit k v ci t thit k v ci t mt h iu hnh mng c hai cch tip cn khc nhau: (1) Tn trng tnh c lp ca cc h iu hnh cc b c trn cc my tnh ca

mng. Khi h iu hnh mng c ci t nh mt tp cc chng trnh tin ch chy trn cc my khc nhau ca mng. Gii php ny tuy khng c p nhng d ci t v khng v hiu ha c cc phn mm c.

(2) B qua cc h iu hnh c trn cc my v ci t mi hon ton mt h iu hnh thun nht trn ton mng, gi l h iu hnh phn tn. Gii php ny p hn v phng din h thng so vi gii php trn, nhng b li phc tp trong cng vic th ln hn rt nhiu. Mt khc, vic tn trng tnh c lp v chp nhn s tn ti ca cc sn phm h thng c l mt im hp dn ca cc tip cn th nht. Bi vy ty theo iu kin c th m ta p dng gii php no cho ph hp. Sau y ta xem xt c th hn v tng gii php ni trn

H iu hnh theo gii php (1)

T tng ch o ca gii php ny l cung cp cho mi ngi t tng ch o ca gii php ny l cung cp cho mi ngi s dng mi tin trnh ng nht m ta gi l Agent lm nhim v cung cp mt giao din ng nht v tt c cc h thng cc b c Agent qun l mt c s d liu cha cc thng tin v cc h thng cc b v chng trnh d liu ca ngi s dng trong trng hp n gin nht Agent ch hot ng nh mt b x l lnh, dch cc lnh ca ngi s dng thnh ngn ng lnh ca h thng cc b ri gi chng thc hin trc khi mi chng trnh thc hin, Agent phi m bo rng tt c cc tp cn thit s dng. Vic ci t mng nh vy s chng li hai cng vic chnh: thit k ngn ng lnh ca mng v ci t Agent.

Cch tip nhn ny n gin v khng gy nh hng n h thng cc b c sn. Thm ch cc h thng cc b khng cn thit n s tn ti ca mng. Nhng gii php ny ch c th kh thi khi m tt c cc tp tin cn thit u bit trc Agent c th gi chng ti mt h thng cc b trc khi chng trnh bt u hot ng. Ngoi ra rt kh thc hin cc tng tc vo ra m chng trnh li khng bit ti s tn ti ca mng. Mt gii php tng qut hn nhm b tin trnh ang chy li bng cch tm tt tt c cc li gi h thng System Call ca n chng c th thc hin trong bi cnh ca h thng qun l tp ca mng (NetWork file System).

H iu hnh theo gii php (2)



Trong trng hp ny ngi ta gi l h iu hnh phn tn v c th c thit k mt trong hai m hnh: M hnh tin trnh hoc m hnh i tng.

Trong m hnh tin trnh mi ti nguyn (tp, a, thit b ngoi vi, ) c qun l theo mt tin trnh no v h iu hnh mng iu khin s tng tc gia cc tin trnh . Cc dch v ca h iu hnh mng tp trung truyn thng nh qun l tp, ln lch cho b x l, iu khin terminal,... c qun l bi cc Server c bit c kh nng tip nhn cc yu cu thc hin dch v tng ng trong nhiu trng hp cc Server c th chy nh tin trnh ca ngi s dng thng thng.

Trong m hnh i tng, th gii bao gm cc i tng khc nhau, mi i tng c mt kiu (type), mt biu din, v mt tp cc thao tc c th thc hin trn n. thc hin mt thao tc trn mt i tng, chng hn c mt tp tin trn mt tin trnh ngi s dng phi c giy php i vi i tng. Nhim v c bn ca h iu hnh y l qun l cc giy php v cp pht cc giy php cho cc tin trnh thc hin cho cc thao tc cn thit. Trong mt h tp trung, bn thn h iu hnh nm gi cc giy php bn trong ngn nga nhng ngi s dng c gi mo chng. Trong mt h phn tn cc giy php c lun chuyn theo mt cch no mi tin trnh u c c hi nhn c giy php v sao cho ngi s dng khng th t to ra c chng.

Vic thit k h iu hnh phn tn theo mi hnh i tng l mt hng i rt trin vng v tn ti nhiu vn cn gii quyt trn vn hn. Cn i vi tin trnh th chng ta c th thy r nhim v then cht chnh l xy dng c ch lin lc gia cc tin trnh (Interprocess Communication - IPC). lm iu ngi ta s dng mt trong hai cch: dng li gi hm (Function/procedure Calls) hoc chuyn thng bo (message passing).

Khi cc li gi hm hoc th tc c dng lm c ch IPC, h thng y bao gm tp v cc hm (hoc th tc) c vit tt theo ngn ng no . M ca cc hm no c phn tn cho cc b vi x l. thc hin vic truyn thng gia cc my, mt hm trn my ny c th gi mt hm trn my khc. Ng ngha ca cc li gi hm y cng ging nh i vi cc li gi hm thng thng: hm gi b treo cho n khi hm gi c kt thc, tham s c truyn t hm gi cho n hm c gi, cn kt qu c chuyn theo chiu ngc li. Cch tip cn ny dn n mt h iu hnh c vit nh mt chng trnh ln, u im l cht ch v nht qun, tuy nhin thiu mm do.

Nu dng phng php chuyn thng bo ca c ch IPC th cc tin trnh s lin tc vi nhau bng cch chuyn thng bo. M ca cc tin trnh c tch bit v c th vit bng cc ngn ng khc. Cch tip cn ny i hi nhiu vn hn cch tip cn gi hm, chng hn vn a ch ha thit lp cc lin kt o, ct, hp thng bo, kim sot lung d liu truyn thng bo (broad casting).

1.4.3. Cc kiu h iu hng mng Trn mng cc b c hai kiu h iu hnh mng: kiu ngang hng v kiu da trn

am ch:



1.4.3.1. Kiu ngang hng (peer-to-peer) Mi trm u c quyn bnh ng nh nhau v u c th cung cp ti nguyn cho

cc trm khc. Cc ti nguyn cung cp c c th l tp (tng ng vi thit b l a), my in. Ni chung trong cc mng ngang hng khng c vic bin mt my tnh thnh mt trm lm vic ca mt my tnh khc. Trong mng ngang hng, thng thng cc my s dng chung mt h iu hnh.

Win 3.1, Win 95, NT Workstation, AppleShare, Lanstic v Novell Lite l cc h iu hnh mng ngang hng .

Cc c im ca mng ngang hng:

- Thch hp vi cc mng cc b quy m nh, n l, cc giao thc ring l, mc thp v gi thnh r.

- Cc mng ngang hng c thit k ch yu cho cc mng ni b va v nh v s h tr tt cc mng dng mt nn v mt giao thc. Cc mng trn nhiu nn, nhiu giao thc s thch hp hn vi h iu hnh c my ch dch v.

- Yu cu chia s file v my in mt cch hn ch cn n gii php ngang hng.

- Ngi dng c php chia s file v ti nguyn nm trn my ca h v truy nhp n cc ti nguyn c chia s trn my ngi khc, nhng khng c ngun qun l tp trung.

- V mng ngang hng khng cn my c th lm my ch. Chng thng l mt phn ca h iu hnh nn hay l phn b sung cho h iu hnh v thng r hn so vi cc h iu hnh da trn my ch.

- Trong mt mng ngang hng, tt c cc my tnh c coi l bnh ng, bi v chng c cng kh nng s dng cc ti nguyn c sn trn mng.

Nhng thun li:

- Chi ph ban u t - khng cn my ch chuyn dng.

- Ci t - Mt h iu hnh c sn (v d Win 95) c th ch cn cu hnh li hot ng ngang hng.

Nhng bt li:

- Khng qun l tp trung c

- Bo mt km

- C th tn rt nhiu thi gian bo tr

1.4.3.2. Kiu h iu hnh mng c my ch (server based network) Trong h iu hnh kiu ny, c mt s my c vai tr cung cp dch v cho my

khc gi l my ch (ng hn phi gi l my cung cp dch v m khi th phi xem l my t).



Cc dch v c nhiu loi, t dch v tp (cho php s dng tp trn my ch) , dch v in (do mt my ch iu khin nhng my in chung ca mng) ti cc dch v nh th tn, WEB, DNS ...

Trong mng c my ch, h iu hnh trn my ch v my trm c th khc nhau. Ngay trong trng hp my ch v my trm s dng cng mt h iu hnh th chc nng ca bn trn my ch cng c th khc vi chc nng ci t trn my trm.

Sau y l mt s h iu hnh c dng my ch: Novell Netware 4.1 Microsoft NT V4.0, Server, OS/2 LAN Server v Banyan Vines V6.0.

c im ca cc h iu hnh c my ch:

- H iu hnh cho cc mng an ton, hiu sut cao, chy trn nhiu nn khc nhau (k c phn cng, h iu hnh v giao thc mng)

- Mt my ch l mt my tnh trong mng c chia s bi nhiu ngi dng, nh cc my dch v file, my dch v in, my dch v truyn tin. Ni cch khc, n c thit k cung cp mt dch v c th - khc vi cc h my tnh nhiu ngi dng, tp trung v a mc ch - mc d my dch v file kt hp vi cc h thng nh h iu hnh mng Novell's NetWare 3.xx hay 4.xx thng hot ng theo cch .

- Kim sot quyn s dng trn tan mng ti my ch.

- Cung cp cc dch v th mc trn tan mng.

- Cc gii php da trn my ch c coi l s qun tr mng tp trung v thng l my qun l mng ni b chuyn dng.

- Bn thn my ch c th ch l my ch chuyn dng nh Novell Netware 4.1, my ny khng th hot ng nh mt my trm. Cng c nhng h iu hnh m my ch NT cng c th c s dng nh mt my trm.

1.4.3.3. M hnh khch/ch (client/server) u thp nin 60, vic s dng my tnh thc hin theo m hnh tp trung. Cc

trm thc s ch lm vic giao tip cn vic x l thc s tin hnh mt my tnh no . Nh vy vi m hnh ny hon ton khng c x l cng tc. Mt pht trin tip theo l m hnh x l ch t (master/slaver) vi vic mt my x l v chuyn giao mt s cng vic cho cc my cp thp hn, hon ton khng c vic my cp thp hn lin lc hoc giao vic theo chiu ngc li. Nh vy qu trnh cng tc ch l mt chiu.

Mt bc t ph trong m hnh tnh ton cng tc l m hnh chia s thit b (shared device) theo mt my c th cho my khc s dng thit b ca mnh (ch yu l a v my in). H iu hnh mng theo kiu ngang hng hay c s dng my ch dch v u c th dng cho m hnh ny. Tuy nhin ch mc ny thi th chnh CPU cha b chia s ngha l cha c s phn tn trong x l m ch yu l phn tn thng tin. Ngay c vic s dng my in t xa cng khng mang ngha ca x l phn tn v thc cht ch l gi ni dung in ti hng i ca mt my in do mt my tnh no qun l m thi. My ch cung cp dch v in khng to ra gi tr mi cho cng vic ca my u thc dch v in.



Trong nhng nm gn y xut hin m hnh khch ch trong mt s my ch ng vai tr cung ng dch v theo yu cu ca cc my trm. My trm trong m hnh ny gi l my khch (client) l ni gi cc yu cu x l v my ch. My ch (server) x l v gi kt qu v my khch. My khch c th tip tc x l cc kt qu ny phc v cho cng vic. Nh vy my khch chu trch nhim ch yu v giao din v ch m nhn mt phn x l. Trong m hnh khch/ch x l thc s phn tn.

Ta ni n m hnh khch ch ch khng ni n h iu hnh khch ch v trn thc t m hnh khch ch yu cu phi c mt h iu hnh da trn my ch d my ch ny trong mng cc b hay my ch cung cp dch v t mt mng khc. Hu ht cc ng dng trn Internet l ng dng khch ch s dng t xa.

Lu rng cc tin trnh khch v ch i khi c th thc hin trn cng mt my tnh

- Client process v server process c th hot ng trn cng mt b x l, trn cc b s l khc nhau cng mt my (cc b x l song song), hoc trn cc b x l khc nhau trn cc my khc nhau (x l phn tn).

- Mt iu quan trng cn nhn thy l c h iu hnh ngang hng v h iu hnh da trn my ch u c th tha mn m hnh khch/ch. Trn thc t, hu ht cc h iu hnh hin i u cung cp t nht mt vi chc nng khch-ch.

H iu hnh khch/ch

Cc h iu hnh cho cu trc khch/ch bao gm: Sun Solaris NFS, UnixWare NFS, Novell Netware v Windows NT Server.

- H iu hnh khch/ch cho php mng tp trung cc chc nng v cc ng dng ti mt hay nhiu my dch v file chuyn dng. Theo cch ny, chng c th hot ng nh trng hp c bit ca h iu hnh da trn my ch.

- Cc my dch v file tr thnh trung tm c h thng, cung cp s truy cp ti cc ti nguyn v cung cp s bo mt. Cc my trm ring l (my khch) c truy nhp ti cc ti nguyn c sn trn my dch v file.

- OS cung cp c ch tch hp tt c cc b phn ca mng v cho php nhiu ngi dng ng thi chia s cng mt ti nguyn bt k v tr vt l

- Cc h iu hnh ngang hng cng c th hot ng nh h iu hnh khch/ch nh vi Unix/NFS v Windows 95.

Cc im thun li ca mt mng khch/ch:

- Cho php c iu khin tp trung v khng tp trung: Cc ti nguyn v bo mt d liu c th c iu khin qua mt my ch chuyn dng hay ri rc trn tan mng.

- Chng qu ti mng

- Cho php s dng cc my, cc mng chy trn cc nn khc nhau

- m bo ton vn d liu

- Gim chi ph pht trin h thng



1.4.4. Cc chc nng ca mt h iu hnh mng Sau y l cc chc nng c th m mt h iu hnh mng.

Cung cp phng tin lin lc gia cc tin trnh, gia nhng ngi s dng v gia cc ti nguyn ni chung ca ton mng. C th k dn cc kha cnh sau:

+ Chuyn d liu gia cc tin trnh

+ ng b ho cc tin trnh

+ Cung cp phng tin lin lc gia ngi s dng. mc thp c th l to, lu chuyn v hin th cc thng bo nng trc tuyn, mc cao c th l nhn tin (paging) hoc th tn in t (Email)

H tr cho cc h iu hnh ca my trm - cho php truy nhp ti my ch t cc my trm. Cc h iu hnh mng hin i u cung cp cc h tr cho cc h iu hnh khc nhau chy trn cc my trm khch. Sau y l mt s v d minh ho vn ny:

Cc h iu hnh UNIX cung cp cc chng trnh chy trn DOS c tn l NFS (Network File System) khi ng trn DOS cc my PC c th s dng h thng tp ca cc my ch UNIX.

Mt s h iu hnh nh Windows NT v Windows 95 cung cp h tr cho cc dch v th mc Novell (NDS) cho php chng truy nhp trc tip ti ti nguyn trn my ch Novell Netware.

Dch v nh tuyn v cng ni - cho php truyn thng gia cc giao thc mng khc nhau. V d mt my chy trn Novell NetWare vi giao thc IPX/SPX khng th chy trc tip cc ng dng trn TCP/IP nh mt s cc ng dng Internet. Tuy vy nu c cc modun chuyn i giao thc bin cc gi tin IPX/SPX thnh gi tin TCP/IP khi cn gi t mng Netware ra ngoi v ngc li th mt my chy Netware c th giao tip c vi Internet. Kin trc ca Netware c ODI (Open Datalink Interface ) l phn chuyn i v chng (bao gi) cc giao thc khc nhau.

Dch v danh mc v tn. (Name /Directory Services) + c th khai thc tt ti nguyn trn mng, NSD cn nhn thy mt cch d dng

cc tn ti nguyn (thit b, tp) ca ton mng mt cch tng th. V th mt dch v cung cp danh mc ti nguyn l v cng quan trng.

+ ng nhin vic NSD nhn thy cc ti nguyn no cn ph thuc vo thm quyn ca ngi . Mi khi vo mng, khi NSD c mng nhn din, h c th nhn thy nhng ti nguyn c php s dng.

+ Trong NOVELL dch v chnh l NDS (Netware Directory Services). Trong Windows NT hay Windows95 chnh l chc nng browser m ta thy c ci t trong explorer. Trong UNIX vi lnh mount ta c th kt ni tn ti nguyn ca mt h thng con vo h thng ti nguyn chung.

Bo mt Chc nng ny m bo vic kim sot cc quyn truy cp mng, quyn s dng ti nguyn ca mng. Cc phng php c p dng bao gm :

Dng cc dch v a iu khin bo mt:



+ Chia a cng ca my ch thnh cc phn c gi l volume hay partition sau gn volume c php cho ngi dng

+ nh cc thm quyn trn tp v th mc. C nhiu loi thm quyn. It nht th cc thm quyn c c, c ghi v c thc hin c p dng cho a s cc h iu hnh mng. Mt s h iu hnh quy nh thm quyn kh chi tit nh quyn c xo, quyn c sao chp, quyn xem th mc, quyn to th mc. Cc quyn ny li c xem xt cho n tng nhm i tng nh c nhn, nhm l vic hay tt c mi ngui.

+ Thm quyn vo mng hay thc hin mt s dch v c nhn din qua tn ngui s dng v mt khu.

+ M ho cc gi tin trn mng.

+ Mt s h iu hnh cn cho php m ho phn cng kim sot vic s dng thit b.

Cung cp phng tin chia s ti nguyn. Nhng ti nguyn trn mng c th cho php nhiu ngi uc s dng. ng k nht l a (thc cht l tp v th mc) v my in (thc cht l my tnh qun l hng i ca my in). h iu hnh M phi c cc cng c cho php to ra cc ti nguyn c th chia s uc. Cc ti nguyn chia s c phi l cc ti nguyn c lp vi mi ng dng. Chnh v vy n phi c cung cp cc trnh iu khin (driver) ph hp vi mng. My in, modem .... l cc ti nguyn nh vy. Trn mng cng cn c cc cng c can thip vo hot ng ca cc ti nguyn mng v d: nh ch mt tin trnh truy nhp mng t xa, thay i th t hng trn my in mng...

To tnh trong sut ngi s dng khng nhn thy kh khn trong khi s dng cc ti nguyn mng cng nh ti nguyn ti ch. Chnh dch v th mc v tn ni trn l mt v d v chc nng ny. Trong Windows 95/NT ngi ta c th duyt th mc trn ton mng khng c g khc vi vic duyt th mc trong a cc b

Sao lu d phng - i vi bt k h thng no, chy trn mi trng no, vn sao lu d phng cng quan trng c th hi phc thng tin ca h thng sau mt s c gy mt d liu. Tuy nhin trong mi trng mng th vic sao lu c th thc hin c vic sao lu mt cch t ng qua mng. Chnh v th cc cc h iu hnh mng u cung cp cng c sao lu nh mt chc nng c bn. C nhiu phng php sao lu. Trn Novell cho php soi gng (mirroring) cc a m ta c th t trong khi ci t h thng. Novell c c mt dch v tn l SMS (Storage Management Services) cung cp cc cng c sao chp, hi phc khng ch d liu ca NSD m c d liu ca h thng v d NDS. NT c chc nng replicate khng nhng i vi a m cn mc th mc v nh k. iu rt cn thit khng ch trn mng cc b m ngay c trn mng rng.

1.5. KT NI LIN MNG

1.5.1. Cc tip cn Lin mng (Internetwork) l mt tp hp ca nhiu mng ring l c ni kt li

bi cc thit b ni mng trung gian v chng vn hnh nh ch l mt mng ln. kt



ni cc mng ang tn ti li vi nhau, ngi ta thng xut pht t mt trong hai quan im sau:

1) Xem mi nt ca mng con nh l mt h thng m 2) Xem mi mng con nh l mt h thng m.

Quan im 1 cho php mi nt ca mng con c truyn thng trc tip vi mt nt ca mt mng con bt k khc. Nh vy ton b cc mng con cng s l nt mt mng ln hn v tun th mt kin trc chung.

Trong khi vi quan im 2, hai nt thuc hai mng con khc nhau khng th bt tay trc tip vi nhau c m phi thng qua mt phn t trung gia, gi l giao din kt ni t gia hai mng con . C ngha l cng hnh thnh mt mng mng ln hn gm cc giao din kt ni v cc my tnh (host) c ni vi nhau bi cc mng con .

Tng ng vi hai quan im c hai chin lc kt ni mng khc nhau. Mt chin lc (tng ng vi quan im 1) tm cch xy dng cc chun chung cho cc mng (cc chun ca ISO, CCITT theo quan im ny). Mt chin lc khc (tngn vi quan im 2) c gng xy dng cc giao din kt ni tn trng tnh c lp ca cc cc mng hin c. R rng s hi t v mt chun chung l mt iu l tng, nhng r rng l khng th ngay tc khc loi b hng vn mng ang tn ti trn th gii c, m phi tm cch tn dng chng. Trn th trng hin nay c rt nhiu cc sn phm giao din cho php chuyn i gia cc mng khc nhau, l mt minh chng sng ng cho sc sng ca quan im 2.

1.5.2. Giao din kt ni Kt ni lin mng c th c thc hin nhng tng khc nhau, ty thuc vo

mc ch m ta dng cc thit b kt ni khc nhau. Bng di y lit k mt s thit b kt ni tng ng vi cc tng khc nhau:

1.6. CU HI V BI TP

(ang tip tc b sung)



CHNG 2. KIN TRC PHN TNG OSI

2.1. TNG VT L (PHYSICAL)

2.1.1. Vai tr v chc nng ca tng vt l.

Theo nh ngha ca ISO, tng vt l cung cp cc phng tin in, c, chc nng, th tc kch hot, duy tr v nh ch cc lin kt vt l gia cc h thng.

Trong , thuc tnh in lin quan n s biu din cc bt (cc mc tn hiu) v tc truyn cc bit, thuc tnh c lin quan n cc tnh cht vt l ca giao din vt l vi mt ng truyn (kch thc, cu hnh). Thuc tnh chc nng ch ra cc chc nng c thc hin bi cc phn t ca giao din vt l gi mt h thng v ng truyn vt l, v thuc tnh th tc lin quan n cc giao thc iu khin vic truyn cc xu boit qua ng truyn vt l.

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.

Trong hnh 2.1 a), A v B l hai h thng m c ni vi nhau bng mt on cp ng trc v mt on cp quang. Modem C chuyn i tn hiu t tn hiu s sang tn hiu tng t truyn trn cp ng, v modem D li chuyn i tn hiu t tn hiu tng t sang tn hiu s. Transducer E chuyn i t xung in thnh xung nh sng truyn qua cc quang. Cui cng Transducer F chuyn i thnh xung in i vo B.

Hnh 2.1 b) l mi trng logic ca tng vt l. y, mt thc th vt l l mt cu trc logic giao din vi ng truyn vt l. Cc thc th c mt trong cc h

H thng A C D E F H thng B

Cp ng trc Cp quang

a) Mi trng thc

Thc th tng vt

l

Thc th tng vt

l Thc th tng vt

l

Giao thc tng vt l Giao thc tng vt l

ng truyn vt l ng truyn vt l

SAP cho tng vt l SAP cho tng vt l

b) Mi trng logic

Hnh 2.1 Quan h ca tng vt l vi mi trng thc



thng A, B v cng c th c thc th vtt l giao din gia D v E. Thc th trung gian ny l mt b chuyn tip hot ng tng vt l giao din vi cc ng truyn vy l khc nhau.

Mt giao thc tng vt l gia cc thc th vt l quy nh pgng thc truyn (ng b, d b) v tc truyn,.. iu mong mun l giao thc pi c lp ti a vi ng truyn vt l cho mt h thng c th giao din vi nhiu ng truyn khc nhau. Do vy, cc chun vt l s phi bao gm khng ch cc thc th m cn c c t ca giao di vi n truyn. Di y ta s xem st cc chun .

2.1.2. Cc chun cho giao din vt l

Trc khi vo phn ny chng ta hy lm quen vi hai thut ng mi, l thit b cui d liu (Data Terminal Equipment DTE) v thit b cui knh d liu (Data Circuit Terminal Equipment DCE).

DTE l mt thut ng chung ch cc my ca ngi s dng cui (end-user), c th l my tnh hoc mt trm cui (terrminal). Tt c cc ng dng ca ngi dng u nm DTE. Mc ch ca vic ni mng chnh l ni cc DTE li vi nhau chia s ti nguyn, lu tr thng tin chung v trao i d liu.

DCE l thut ng chung ch cc thit b lm nhim v kt ni cc DTE vi ng truyn. N c th l mt Modem, Transducer, Multiplexing. DCE c th c ci t ngay bn trong DTE hoc ng ring nh mt thit b c lp. Chc nng ch yu ca n l chuyn i tn hiu biu din d liu ca ngi dng thnh tn hiu chp nhn c bi ng truyn v ngc li.

Trong hnh 2.1 trn, cc h thng m A, B chnh l cc DTE, cn cc Modem C, D v Tranducer E, F ng vai tr l cc DCE.

a s cc trng hp kt ni mng my tnh s dng cng mt kiu giao din vt l thun tin cho vic truyn thng trc tip gia cc sn phm khc loi, khi phi thc hin vic chuyn i rc ri. Cc c t v hot ng ca cc DTE v DCE c a ra bi nhiu t chc chun ha nh CCITT, EIA v IEEE. ISO cng cng b cc c t v cc u ni c hc kt ni gia cc DCE v DTE.

Vic truyn d liu ch yu c thc hin thng qua mng in thoi, bi th cc t chc trn a ra nhiu khuyn ngh v vn ny. Cc khuyn ngh loi V v loi X ca CCITT l mt v d in hnh. Chng l cc c t tng vt l c s dng ph bin nht trn th gii, c bit l Ty u. Bn cnh cc chun thuc h RS- (nay i thnh EIA-) ca EIA cng c s dng rt ph bin, c bit l Bc M. Di y ta s xem xt mt s chun thng dng nht.

V24/RS-232-C: L hai h chun tng ng ca CCITT v EIA nhm nh ngha giao din vt l

gia DTE v DCE (gia my tnh v Modem chng hn). V phng din c, cc sn phm ny s dng cc u ni 25 chn (25- pin connector). V in, cc chun ny quy nh cc tn hiu s nh phn 0 v 1 tng ng vi cc th hiu nh hn -3V v ln hn +3V. Tc tn hiu khng vt qu 20 Kbps vi khong cch ti a l 15m.

Trong trng hp c bit, khi khong cch gia cc thit b qu gn n mc cho php hai DTE c th truyn trc tip tn hiu vi nhau, lc cc mch RS-232-C vn c th c dng nhng khng cn c mt DCE na.



T nm 1987, RS-232-C c sa v dt tn li l EIA-232-D.

RS-449/422-A/423-A: Nhc im chnh ca V24/RS-232-C l s hn ch v tc i v khong cch.

ci thin yu im , EIA a ra mootj tp cc chun mi thay th, l RS-449, RS-422-A v RS-423-A. Mc d chun RS-232-C vn c s dng nhiu nht cho giao din DET/DCE, nhng cc chun mi ni trn cng ang ngy cng c s dng nhiu hn. RS-449 nh ngha cc dc trng c, chc nng, cn RS-422-A v RS-4232-A nh ngha cc c trng v in ca chun mi.

RS-449 tng t nh RS-232-C v c th lin tc vi chun c,. V phng din chc nng, RS-449 gi li ton b cc mch ca RS-232-C (tr mch AA), v thm vo 10 mch mi, trong c cc mch quan trng l: IS, NS, SF, LL, RL, TM.

V phng din c, RS-449 dng u ni 37-chn cho giao din c bn v dng mt u ni 9 chn ring bit cho knh ph. Song trong nhiu trng hp, ch c mt s chn c s dng.

V phng din th tc, RS-449 tng t nh RS-232-C. Mi mch c chc nng ring v vic truyn tin da trn cc cp tc ng-phn ng. V d DTE thc hin Request to Send th sau n s ik DCE tr li vi Clear to Send.

Ci tin ch yu ca RS-449 so vi RS-232-C l cc dc trng v in, v cc chun RS-422-A, RS-423-A nh ngha cc c trng . Trog khi RS-232-C c thit k thi i ca cc linh kin in t ri rc th cc chun mi c tip nhn cc u vit ca cng ngh mc t hp (IC). RS-423-A s dng phng thc truyn thng khng cn bng, t tc 3Kbps khong cch 1000m v 300 Kbps khong cch 10m. Ttrong khi , RS-422-A s dng phng thc truyn thng cn bng, t tc 100Kbps khong cch 1200m v ti 10 Mbps khong cch 12m.

Ngoi cc chun trn EIA cn hpt trin cc chun khc nh EIA-530 thay th cho EIA-232 trong trng hp cc giao i hi tc cao hn 20Kbps, hau EIA-366 nh ngha giao din cho cc thit b t ng, mt modem v mt DTE.

2.2. TNG LIN KT D LIU (DATA LINK)

2.2.1. Vai tr v chc nng ca tng lin kt d liu Tng lin kt d liu cung cp cc phng tin truyn thng tin qua lin kt vt

l m bo tin cy thng qua cc c ch ng b ha, kim sat li v kim sot lung d liu.

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 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.



2.2.2. Cc giao thc ca tng lin kt d liu Cng ging nh tng Vt l, c rt nhiu giao thc c xy dng cho tng ny, gi

chung l cc giao thc lin kt d liu (Data Link Protcol- DLP). C DLP c phn chia thnh hai loi: ng b v d b. Trong , loi ng b li c chia thnh 2 nhm l hng k t v hng bit (xem s hnh 2.2). DLP d b:

Cc DLP d b s dng phng thc truyn d b, tc l khng cn c s ng b lin tc gia ngi gi v ngi nhn, N cho php mt n v d liu c truyn i bt k lc no m khng cn quan tm n cc tn hiu ng b trc . giao thc loi ny, cc bt c bit START v STOP c dng tch cc xu bit biu din cc k t trong dng d liu c truyn i. Cc giao thc loi ny thng c dng trong cc my in bo hoc cc my tnh trm cui tc thp. Phn ln cc my PC s dng phng thc truyn d b v tnh n gin ca n. DLP ng b:

Phng thc truyn thng ng b s dng cc k t c bit SYN, EOT hay n gin l cc c (flag) gia c kd liu cangi dng bo cho ngi nhn bit rng d liu ang n hay n.

Cc giao thc tng lin kt d liu ng b gm 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.

Di y chng ta s xem xt k hn hai loi giao thc ny.

2.2.3. Cc giao thc hng k t Cc giao thc loi ny xut hin t nhng nm 60 v n nay n vn c s dng.

Chng c dng cho c hai phng thc truyn 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 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.

Giao thc lin kt d liu

Giao thc ng b Giao thc d b

Giao thc hng k t Giao thc hng bit

Hnh 2.2. Phn loi cc giao thc lin kt d liu



Cc giao thc loi ny c th p ng cho cc phng thc khai thc ng truyn

khc nhau: mt chiu (simplex), hai chiu lun phin (half-duplex) v hai chiu ng thi (full-duplex).

i vi phng thc mt chiu, giao thc hng k t c dng rng ri nht l giao thc truyn tp Kermit do i hc Columbia (M) ch tc. Kermit c nhiu phin bn cho php truyn tp gia 2 my PC, hoc mt PC v mt my ch (file server) hoc mt my tnh ln (maiframe).

i vi phng thc hai chiu lun phin, giao thc hng k t ni ting nht l giao thc BSC (Binary Synchronous Control) hay cn gi l Bisync- mt sn phm ca IBM. Giao thc ny c ly Iso ly lm c s xy dng giao thc hng k t chun quc t vi tn gi l Basic Mode. Bi vy ta s trnh by chi tit v giao thc ny.

C rt t giao thc hng k t cho phng thc hai chiu ng thi. V d cho loi ny l giao thc gia cc nt chuyn mch (IMP Interface Message Protcol) trong mng ARPANET ca b quc phng M.

Giao thc BSC/Basic mode H giao thc ny p dng cho trng hp im-im, hoc im-nhiu im v hai

chiu lun phin; s dng cc k t c bit ca b m EBCDIC (i vi BSC) v ASCII (i vi Basic Mode).

Cc k t c bit gm: SOH (Start Of Header): ch bt u ca phn header STX (Start Of Text): ch phn bt u ca phn d liu (vn bn) ETX (End Of Text): ch phn kt thc ca phn d liu EOT (End Of Transmission): ch s kt thc ca mt hoc nhiu n v d liu v

gii phng lin kt). ETB (End Of Transmission Block): ch s kt thc ca mt khi d liu trong

trng hp d liu c chia thnh nhiu khi. ENQ (Enquiry): yu cu phc p t mt trm xa. DLE (Data Link Escape): thay i ngha ca cc k t iu khin khc ACK (Acknowledge): bo cho ngi gi bit nhn tt d liu NAK (Negative Acknowledge): bo cho ngi gi bit nhn khng tt d liu SYN (Synchronous Idle): k t ng b, dng duy tr s ng b gia ngi gi

v ngi nhn.

Hnh 2.3. Cc ng truyn kt ni kiu "mt im - mt im" v "mt im - nhiu im".



n v d liu (frame) ca n c khun dng nh sau: SOH Header STX Text ETX BCC

Trong BCC(block Check Character): l 8 bit kim tra li theo kiu bit chn l theo khi cho cc k t thuc vng Text (trong trng hp Basic Mode), hoc 16 bit kim tra li theo phng php CRC-16 cho vng Text (trong trng hp BSC). Cc phng php kim tra li s c cp trong chng 4. Kch thc vng Text c gii hn m bo c kim sot li khi truyn. Trong trng hp d liu ln th c th chia thnh nhiu khi nh (block). Gi s Text c chia lm 3 khi, khi khun dng cc khi d liu nh sau:

Khi 1: SOH Id Header STX Text1 ETB BCC

Khi 2: SOH Id STX Text2 ETB BCC

Khi 3: SOH Id STX Text3 ETB BCC

Cc th tc chnh ca BSC/Basic Mode: Mi truyn tin: Gi s trm A mun mi trm B truyn tin, A s gi lnh sau ti B:

EOT B ENQ Trong : B l a ch ca trm c mi truyn tin, EOT chuyn lin kt sang trng thi iu khin. Khi B nhn c lnh ny, c th xy ra hai trng hp,:

- Nu c tin truyn th trm B s cu to mt n v d liu v gi cho A. - Nu khng c tin gi, B s gi EOT tr li. pha A, sau khi gi lnh i qu mt thi gian xc nh trc m khng nhn c

tr li ca B, hoc l nhn c tr li sai th A s chuyn sang trng thi phc hi. Trng thai ny s c ni n ngay sau y. Mi nhn tin: Gi s trm A mun mi trm B nhn tin, A s gi lnh tng t nh trn ti B:

EOT B ENQ y EOT c th vng mt.

Khi B nhn c lnh ny, nu B sn sng nhn tin th trm B s gi ACK v A, ngc li n s gi NAK.

pha A, sau khi gi lnh i qu mt thi gian xc nh trc m khng nhn c tr li ca B, hoc l nhn c tr li sai th A s chuyn sang trng thi phc hi. Yu cu tr li:

Khi mt trm cn trm khc tr li mt yu cu no gi i trc, n ch cn gi lnh ENQ cho trm kia.



Ngng truyn tin (tm thi): gi lnh EOT Gii phng lin kt: gi lnh DLE EOT Tring thi phc hi: Khi mt trm no i vo trm thi phc h n s thc in

mt trong cc hnh ng sau: Lp li lnh gi n ln (n l mt soos nguyn chn trc) Gi yu cu tr li n ln Kt thc truyn bng cch gi lnh EOT

thy r hn phng thc trao i thng tin ca giao thc BSC/Basic Mode ta dng s minh ha hnh ... di y, trong c hai trng hp: thng thng v hi thoi.

Trm A Trm B

ENQ

ACK

STX...ETX BCC

ACK

STX...ETX BCC

ACK

EOT

ENQ

ACK

STX...ETX BCC

ACK

EOT

Trm A Trm B

ENQ

ACK

STX...ETX BCC

STX...ETX BCC

ACK

STX...ETX BCC

EOT

Hnh 2.4. S minh ha hot ng ca giao thc BSC/Basic Mode

(dng thng thng) (dng hi thoi)



2.2.4. Cc giao thc hng bit Giao thc HDLC

HDLC h tr 3 ch trao i s liu

NRM (Normal Response Mode) = ch tr li bnh thng: c s dng cu hnh khng cn i, S ch pht khi c yu cu ca P.

ARM (Asynchronous Response Mode) = ch tr li khng ng b: c s dng cu hnh khng cn i, cho php S pht khng cn nhn c yu cu ca P.

ABM (Asynchronous Balanced Mode) = ch tr li khng ng b cu hnh cn i; hu nh ch c s dng trong mng kt ni point-to-point + full-duplex. Hai thit b trao i vi nhau l bnh ng v chc nng (P v S)

Khun dng gi s liu (HDLC Frame Format)

Flag - trng ng b = 7EH = 0111.1110

Address - trng a ch, cha /c thit b ch

+ Group address

+ Broadcast address



Control - trng iu khin: kt ni, truyn v kt thc kt ni

Gi s liu I-Frame:

N(S), N(R) c s dng iu khin lu lng thu/pht. Ngoi ra N(S), N(R) cn xc nh ln ca ca s c s dng trao i s liu bng HDLC.

P/F= Poll/Final

P/F = 1 = P: yu cu S phi thc hin lnh v tr li kt qu thc hin; S bo co thc hin lnh

P/F = 0 = F: Ht thng tin cn gi

Gi iu khin S-Frame:

bit P/F ging nh trn

S = 00: RR (Receive Ready) - sn sng nhn, nhn ti gi tin th N(R)-1

S = 01: REJ (Reject) - yu cu pht li t N(R)

S = 10: RNR(Receive Not Ready) - cha sn sng, nhn ti N(R)-1

S = 11: SREJ (Selative REJ) - yu cu pht li c chn lc, ch ring N(R)

Gi iu khin U-Frame: Bo ni/tch h thng

SARM (1 1 1 1 P 0 0 0): yu cu ni c phn bit Master/Slave, tuy vy Slave c th hi.

SNRM (1 1 0 0 P 0 0 1): yu cu ni mode bnh thng, c Master/Slave, Slave khng c hi, ch c php tr li.

SABM (1 1 1 1 P 1 0 0): khng phn bit my chnh, my ph, c hai my coi nh nhau; nu P=1 th yu cu tr li.

DISC (1 1 0 0 P 0 1 0): yu cu tch h thng, nu tr li UA tc l ng . UA (1 1 0 0 F 1 1 0): thng bo tr li. (Control frame cng c th b mt, ging nh cc frame s liu, v th cng cn bin nhn (ACK). Frame c bit dnh cho mc ch ny l UA).

Nguyn tc hot ng ca HDLC

Qun tr thit lp v gii phng kt ni (V(x) = seq. #):

a) NRM multidrop link, truyn 1 hng

A: SNRM(B,P=1) (Polling B station)

B: UA(B,F=1)

A: DISC(B,P=1)

B: UA(B,F=1)

b) ABM point-to-point link, truyn 2 hng

A: SABM(B,P=1)

B: UA(B,F=1)



B: DISC(A,P=1)

A: UA(A,F=1)

2.3. TNG MNG (NETWORK)

2.3.1. Vai tr v chc nng ca tng mng

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.

Hnh 2.5. Lu thi gian thc hin giao thc HDLC



Tng mng cung cp 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.

Ngoi 2 chc nng quan trng ni trn, tng mng cng thc hin mt s chc nng khc, l: thit lp, duy tr v gii phng cc lin kt logic (cho tng mng), kim sot li, kim sot lung d liu, dn/tch knh, ct/hp d liu,..

2.3.2. Cc k thut chn ng trong mng my tnh

2.3.2.1. Tng quan

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.

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 2.6. M hnh chuyn vn cc gi tin trong mng chuyn mch



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 phn tn 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

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GT Mang May Tinh - TG Le Dinh Danh