1.Cu hnh chung mng frame relay

Hnh 1.1- mng frame relay C s to c mng Frame relay l cc thit b truy nhp mng FRAD (Frame Relay Access Device), cc thit b mng FRND (Frame Relay Network Device), ng ni gia cc thit b v mng trc Frame Relay. Hnh 1-1. Thit b FRAD c th l cc LAN bridge, LAN Router v.v... Thit b FRND c th l cc Tng i chuyn mch khung (Frame) hay tng i chuyn mch t bo (Cell Relay - chuyn ti tng hp cc t bo ca cc dch v khc nhau nh m thanh, truyn s liu, video v.v..., mi t bo di 53 byte, y l phng thc ca cng ngh ATM). ng kt ni gia cc thit b l giao din chung cho FRAD v FRND, giao thc ngi dng v mng hay gi F.R UNI (Frame Relay User Network Interface). Mng trc Frame Relay cng tng t nh cc mng vin thng khc c nhiu tng i kt ni vi nhau trn mng truyn dn, theo th tc ring ca mnh. Trong OSI 7 lp, lp 3 - lp network, Frame Relay khng dng th tc g c (Transparent). 2.Nguyn tc hot ng Ngi s dng gi mt Frame (khung) i vi giao thc LAP-D hay LAPF (Link Access Protocol D hay F), cha thng tin v ni n v thng tin ngi s dng, h thng s dng thng tin ny nh tuyn trn mng. Cng ngh Frame Relay c mt u im c trng rt ln l cho php ngi s dng dng tc cao hn mc h ng k trong mt khong thi gian nht nh , c ngha l Frame Relay khng c nh rng bng (Bandwith) cho tng cuc gi mt m phn phi bandwith mt cch linh hot iu m X25 v thu knh ring

khng c. V d ngi s dng k hp ng s dng vi tc 64 kb, khi h chuyn i mt lng thng tin qu ln, Frame Relay cho php truyn chng tc cao hn 64 kb. Hin tng ny c gi l "bng n" - Bursting. Thc t trn mng li rng ln c rt nhiu ngi s dng vi v s frame chuyn qua chuyn li, hn na Frame Relay khng s dng th tc sa li v iu hnh thng lng (Flow control) lp 3 (Network layer), nn cc Frame c li u b loi b th vn cc frame c chuyn i ng a ch, nguyn vn, nhanh chng v khng b tha b thiu l khng n gin. m bo c iu ny Frame relay s dng mt s nghi thc sau. (1) DLCI (Data Link Connection Identifier) - Nhn dng ng ni data. Cng nh X.25, trn mt ng ni vt l Frame Relay c th c rt nhiu cc ng ni o, mi mt i tc lin lc c phn mt ng ni o ring trnh b ln, c gi tt l DLCI. (2) CIR ( committed information rate ) - Tc cam kt. y l tc khch hng t mua v mng li phi cam kt thng xuyn t c tc ny. (3) CBIR ( Committed burst information rate ) - Tc cam kt khi bng n thng tin. Khi c lng tin truyn qu ln, mng li vn cho php khch hng truyn qu tc cam kt CIR ti tc CBIR trong mt khong thi gian (Tc) rt ngn vi ba giy mt t, iu ny tu thuc vo "nghn" ca mng cng nh CIR. (4) DE bit ( Discard Eligibility bit ) - Bit nh du Frame c kh nng b loi b khi xy ra tc nghn. (5) S dng LMI (Local Manegment Interface): thng bo trng thi nghn mng cho cc thit b u cui bit. LMI l chng trnh iu khin gim st on kt ni gia FRAD v FRND. 3. Mch o trong Frame Relay Cng ngh Frame relay v khi nim c bn l s dng mch o (Virtual Circuits)

Virtual Vircuits (VCs): Mch o l ng lin lc chuyn dng imim gia hai u cui trong mng chuyn mch gi hoc cell-relay. N cung cp lin kt hng kt ni tm thi hoc chuyn dng thng qua mt mng dng b nh tuyn (router) hoc chuyn mch. Mch o c th l mch o thng xuyn PVC (Permanent Virtual Circuit) hay cn gi l mch o c nh v mch o chuyn mch SVC (Switched Virtual Circuits) hay cn gi l mch o khng thng xuyn : - PVCs : Kt ni gia cc trm cui (endpoint) hay cc vng (site) c nh ngha trc, thng b ng b ng thng tin nh v c bo m. Trong cc dch v chuyn mch nh Frame relay, khch hng c th tha thun trc cc u cui ca P C vi nh cung cp. i vi mng ni b, ngi qun l to trc cc P C nh hng ng truyn thng qua cc phn ring bit trn mng. Mt PVC c th c thm khi c yu cu nh cn mt site mi, thm b ng thng, nh tuyn li, hoc khi chng trnh ng dng cn mt port tn ti ni chuyn vi port khc. i vi P C mch o c xc nh, duy tr bi cc h thng v cc nt trn mch, nhng tuyn ng tht s qua mng chuyn gi c th thay i ph thuc vo vic nh tuyn qua cc kt ni b ngt hay bn rn. Quan trng l cc gi c truyn ti theo th t trn mt tuyn c bit v ti ni cng theo th t. - SVCs : S C (Switched irtual Circuit mch o chuyn mch). Mt kt ni tm thi theo yu cu gia cc trm cui, ging nh mt ln gi in thoi l kt ni tm thi. Cc kt ni ch ko di chng no cn thit v c tt khi hon tt. Nh cung cp c th khch hng xc nh S C hoc thit t mt s S C tin nh m khch hng thng yu cu nht. S C c thit lp kt ni nhanh chng v b ng thng c chia s da trn ngi s dng.

S C khng c thi hnh bi cc nh truyn thng hay c i l ca Frame relay. 4. Cu trc khung frame relay

Hnh 4-1 khung frame relay Cu trc khung ca Frame Relay (hnh 4-1) hon ton tng t nh X25 ch khc l khung ny c trng a ch A di hn (2 byte) v khng c trng lnh C v Frame relay khng c th tc hi p. Tuy nhin trn thc t khng c mt cuc ni no hon ho ti mc tuyt i, thu pht khng c mt li nh, v vy vn phi cn ti trng FCS phn tch c cc Frame c li cng nh theo di c s th t ca chng. Cu trc ca mt khung c cc phn sau: + 1 byte dnh cho c F (flag) dn u. + 2 byte a ch A (adress) bit khung chuyn ti u . + Trng I (Information)dnh cho d liu thng tin c nhiu byte . + 2 byte cho vic kim tra khung - FCS (Frame Check Sequence) phn tch v bit c cc gi thiu, , ng, sai trn c s tr li cho pha pht bit. + 1 byte c F kt thc. Nh vy cu trc khung ca Frame Relay v gi X25 c bn ging nhau u c c i trc m ng v kt thc bo v cho d liu thng tin i gia. Chi tit ca mt khung (hnh 4-2)

Hnh 4-2 Chi tit cu trc khung frame relay 4.1 Byte th nht v byte cui cng: Flag - c lun c gi tr 01111110. Th hin theo m Hexal l 7E. 4.2 Hai byte tip dnh cho a ch ( Header ca Frame Relay) Trong .

4-3 Cu trc header frame relay Byte th 2 bao gm: * Bit 0 - EA: Extended Address. Khi khch hng dng nhiu cn m rng thm a ch c ngha l tng s DLCI th dng bit m rng a ch EA. Bnh thng nh hnh v 4 gii thiu y th gi tr EA ca byte 2 l 0 v EA ca byte 3 l 1. Nu m rng nh hnh 4-3 th EA s l 0, 0, 1 theo th t trn xung. * Bit 1 C/R Command/ respond. Bit ny dng hi v p, nhng mng Frame Relay khng dng m ch dnh cho cc thit b u cui (FRAD)

s dng mi khi cn trao i thng tin cho nhau, Bit C/R do FRAD t gi tr v c gi nguyn khi truyn qua mng. * T bit 2 n bit 7 DLCI byte th 2 c 6 bit v byte th 3 c 4 bit tng cng 10 bit nhn dng ng ni data ni cch khc l a ch ni nhn, 10 bit c th nhn dng ti 1024 a ch. Khi cc ng kt ni o DLCI pht trin thm chng ta c th dng 3 byte a ch nh hnh 4-3, lc ny s c 16 bit a ch tng ng 65536 a ch. Byte th 3. + Bit 1 - bit EA ( trnh by trong phn trn) + Bit 2 - bit DE. Bt nh du cc Frame m mng li, thit b c quyn loi b n nu nh nghn ca mng cao. Mng li hoc FRAD s t bit DE = 1 cho cc Frame pht i vi tc cao hn tc khch hng ng k (CIR) m mng phi cam kt m bo. Tuy nhin cc khung Frame ny vn c chuyn i bnh thng ti ngi nhn nu nghn mng thp, nhng nu nghn mng cao th nhng Frame c DE = 1 ny s b loi b trc tin. Bnh thng bit DE = 0. (Hnh v 4-4).

hnh 4-4 : Minh ho cho bit DE ->Bc: (Committed Burst Size): L s lng d liu data ti a mng li chp nhn truyn i trong cc khong thi gian Tc . ->Tc: (Committed Rate Measurement Interval): Tc = Bc/CIR l khong thi gian m FRAD cho php gi Bc v thm ch c Be.

->Be: (Exess Burst Size): L s lng d liu data ti a m mng khng m bo truyn tt nhng vn truyn th xem. + Bit 3 - Bit BECN v Bit 4 - Bit FECN. Hai bit ny do mng li t cho tng cuc ni mt (Tng DLCI) bo cho cc FRAD bit iu hnh thng lng. Khi b nghn cc bit ny c t = 1 theo 4 trng hp sau y trn c s ca

cc trng hp FECN v BECN * DLCI (Data Link Connection Identifier): Trn mt ng vt l frame relay c th c rt nhiu ng ni o, mi mt i tc c phn cho mt ng ni o ring trnh b ln c gi l DLCI hay cn c gi l logical port. C n c vo DLCI mng frame relay chuyn tip lu lng cho my u cui ngi dng UNI (user NetWork Interface) xa. Qu trnh nh x v nh tuyn c honh thnh nh cc bng khc nhau gi l bng chuyn mch, bng nh tuyn, bnh nh x hoc bng kt ni xuyn. S dng DLCI trong mng: Trong hnh di: SWA chp nhn cc frame t port A c cha DLCI 1 va DLCI 2 trong header ca frame. Khi truy xut ti bng nh tuyn v tm thy c cha DLCI 1 nn chuyn n port B v DLCI1 ny c nh x thnh DLCI 21. frame c cha DLCI2 s c chuyn n port C v c nh x thnh DLCI 45. Cc frame ny s c chuyn n SW B v SW C. qua kt ni xuyn SW B v SW C s thc hin tng t SW A v phn pht n cc UNI t xa n thit b ngi dng cui cng, m trong v d ny l cc router.

4.3 Trng thng tin I Trng thng tin ca mt Frame c th thay i di nhng u cha hai loi thng tin chnh l thng tin d liu ca ngi dng (Application Data hay User Data ) v thng tin v giao thc tng lp s dng PCI (Protocol Control Information) thng bo cho lp tng ng ca bn nhn bit. Hnh 4-5

4-5 Trng thng tin I trong cu trc khung . 4.4 Hai Byte kim tra khung - FCS (Frame Check Sequence). Hai byte 16 bit kim tra khung (FCS) i st vi trng thng tin phn user data thc cht l kt qu ca kim tra d theo chu k - CRC (Cyclic Redundacy Check). CRC ni chung l mt gi tr c tnh ton theo mt phng php ring ph thuc vo tng s byte ca mt khi d liu (Block of data), gi tr ny s c bn pht gi sang bn pha thu, bn thu cng m li v so snh vi gi tr bn pht gi sang, nu hai gi tr nh nhau c ngha l d liu truyn i tt, nu khc nhau l c li. i vi Frame relay CRC kim tra t bit th nht ca trng a ch cho ti bit cui cng trng thng tin. FCS c FRAD pht m v FRAD u thu m li (Cc FRND cng m) nh hnh 2-10. Pht hin FCS sai u th Frame b hu ti .

4-6 Kim tra li cc khung gi i b ng FCS.

5. Multicasting K thut frame relay cung cp mt c trng ty chn gi l multicasting y l cng ngh semi-broacast m ngi dng c th gi hoc nhn mt lung ti nhiu ngi dng khc. Hot ng da vo mch o do vy kt ni phi c ci t trc khi lung multicasting c th c chuyn. User ch cn gi 1 bng copy ca frame vi gi tr DLCI dnh ring trong header. Mng s t ng sao cc frame ny ln v phn pht bng sao ny n cc user khc cn nhn lu lng. Hnh 5-1 trnh by nhng hot ng cho dch v multicasting mt chiu. Nguyn tc dch v ny nh sau: + Lung chuyn ti ch mt hng + Trm ch (l) c quy nh nh nhng mch o, my ch a lung phi nh x a lng DLCI (Mdlci) t trm gc ti mt DLCI quy c. + Lung chuyn ti t l (leaves) ti gc (root) khng c cung cp a lung hai chiu, cc dch v n lung ring bit.

5-1 Frame relay multicasting (dch v mt chiu) a lung 2 chiu l mt hot ng kp (hnh 5-2). Lung chuyn ti trong c hai iu khin t gc ti cc l v t cc l ti gc, phn trn cng ca hnh 5-2 trnh by hot ng ny, tt c cc trm u c kh n ng hai chiu. Multicasting server c mt bng nh x thng dch trong c hai iu khin. Phn cui ca hnh 5-2 trnh by ty chn cui cng cho a lung gi l dch v a lung N chiu. N l dch v a lung kp trong tt c nhng my gi v nhn chuyn ti ti mt m khc ti thi im bt k, trong hot ng ny cc gc v l khng tn ti, cc trm c coi l nh nhau.

hnh 5-2 Frame relay a lung (hai chiu) 6. C ch bo hiu trong Frame Relay 1. Thit lp cuc gi:

Frame relay dng chun ISDN Q391 c t cc thng ip cho iu khin kt ni frame.

+ Message ci t (setup message) : c gi bi ngi khi u Frame relay n mng Frame relay thit lp 1 cuc gi Frame relay. Trn 1 vng (Site) khc ca mng, n c gi bi cc node mng n cc user c gi bt u thit lp Frame relay ti cc vng t xa. Message ci t c cha 1 con s ca cc trng dng din t cc kiu ca message, kiu ca cc kh n ng m s c thit lp vi cuc gi, vi cc DLCI tng ng, s sut tr hon i qua end-to-end, cc thng s ca hot ng chnh, v tnh n ng ca mng OSI c cung cp, ang gi v gi cc phn ca a ch, v 1 s tnh n ng khc. + Nu message ci t c chp nhn bi user c gi, user s tr v 1 message kt ni (connect message), message c chuyn n node cc b v

chuyn qua mng n ni bt u cuc gi Frame relay, n cha cc s nhn bit khng ging nhau trong message, cng nh cc tham s phn pht s tr hon lu lng, cc DLCI. + Message xc nhn kt ni (Connect acknowledge) c gi bi mng n user Frame relay c gi, thng bo user r ng cuc gi n ni. N cng c th c gi trong lc ang gi user Frame relay pha trn lc ang nhn 1 message kt ni. + Message cnh bo (alerting message) : c gi bi user gi n mng v chuyn v pha user user ang gi ra du r ng cuc gi n user khi xng. 2. Xa cuc gi: C 3 cuc gi dng xo cc cuc gi Frame relay : + Message tt kt ni (Disconnect message) dng yu cu mng xa b cuc gi Frame relay. + Message gii thot (release message) : c gi bi user hay mng cho bit r ng kt xy ra v nu DLCI c dng th n phi c gii thot vic dng li pha bn kia. + Message gii thot hon tt (release complete message) : dng xa cuc gi v thit lp knh n c th ti dng. Trong lc cc hot ng chm dt kt ni, cc tham kho cuc gi s c gii thot. 3.Cc message iu khin kt ni khc : C 2 message dng iu khin kt ni khc + Message trng thi : cn c 1 message trng thi yu cu (status enquiry message) dng tr li n cc message trng thi + Message yu cu trng thi (status enquiry message) : c gi bi user hoc mng thu ht nhng thng tin v hot ng v cc th tc khc nhau ang xy ra trong mng Frame relay. Message trng thi v message yu cu trng thi cng c dng tng thut cc li chc chn v chun on cc message. 4.Cc nh dng message ca h thng bo hiu s 1 (DSS1) :

Cc trng c yu cu (Requiredfields): - Trng phn bit giao thc (Protocol discriminator) : chim octet u tin, mc ch phn bit cc message iu khin cuc gi UNI t cc message khc. Gi tr ca n theo chun ITU-T v ANSI l 00001000. - Trng tham chiu cuc gi (Call reference). Trng ny n m octet th 3 ca message, mc ch nhn bit mi cuc gi duy nht ti cc interface ca FR. - Trng kiu Message: chim 7 bits ca octet th 4 dng nhn bit cc kiu message khi n bt u gi qua interface. - Cc trng khc: mt s message c bn trong khi thc hin c din t nh l mt yu t thng tin, n c dng thng qua trng others. Cc yu t thng tin c s dng bao gm:

Cause: pht ra cc message, cha thng tin chn on. Bearer capability: dng nhn bit mt bearer service ITU-T I.122. N dng bi mng v ch nhn bit 1 dch v mng Channel identification: nhn bit cc kiu ca knh dng vi interface DLCI: Data link connection identifier dng nhn bit cc DLCI gn hay yu cu li cho kt ni ny. Progress indicator: din t cc hot ng xy ra trong kt ni, n cha trng location v c trng m t tin trnh.

Network specific facilities: dng ra du nu bt k cc kh n ng ca lp mng no cn n. N cng ra du cc kh n ng nyl user ring c hay khng, hoc c yu cu bi mng trong nc hoc quc t. Call state: kim tra trng thi cuc gi Display: dng cho user hin th data nh l cc k t ASCII End to end transit delay: yu cu cho bit tr thi gian hon ti a ng vi mt cuc gi b hon. Packet binary parameters: dng cho m hnh kt ni OSI kt hp vi cuc gi ny. Link core parameters: dng chp nhn cc thng s ct li ca lp lin kt FR.

4 yu t thng tin : calling party number, calling party subaddress, called party number, called party subaddress dng nhn bit nhng cuc gi v ang gi Connected number: nhn bit cc thng s kt ni Connected subaddress: dng nhn cc a ch con ca cc cuc gi

Transit netword selection: cho php user ch nh mng truy cp Repeat indicator: ra hiu cho php cc yu t thng tin c lp 7. Kim sot tc ngn trong Frame Relay 7.1 Cc trng hp xy ra tc ngn Mng Frame relay l 1 mng ca mng chuyn gi, mt trong nhng vn ch yu ca vic thit k mng Frame relay l kim sot tc nghn. V c bn, mng Frame relay l mng ca cc hng i, ti mi b x l cc frame, c mt hng i cc frame.v vy c 2 l do chnh dn n tc nghn: 1.do 1 thit b mng nhn qu nhiu frame,xy n hin tng trn b nh m. 2.do ngun pht frame vt qu tc truyn CIR(tc cam kt) trong khi c nhiu kt ni. 7.2 Qun l tc ngn V c bn, mng Frame relay l mng ca cc hng i, ti mi b x l cc frame, c mt hng i cc frame. Ti y, nu tc cc frame n vt qu tc m cc frame c th chuyn i th kch thc hng i t ng nhanh khng c gii hn (ngha l hng i s b trn). Ngay c khi cc frame n qu chm so vi cc frame c th chuyn i th hng i cng t ng nhanh khng km tc nh khi tc n xp x tc ng truyn. iu ny s dn n tc nghn n ha-tc nghn thp.Nu nh vic x l khng th lm gim tc nghn th s dn n tc nghn nghim trng. lu lng truyn tip tc t ng nn hng i ti cc node x l frame t ng nhanh. Hu qu l t tc nghn n ho mng Frame relay chuyn sang tc nghn nghim trng (serious congestion). Nguyn do l hng i ti cc node c gii hn, khi n b trn th b x l s hy frame, do ti ngun truyn s truyn li frame hu. iu ny lm tnh trng tc nghn tr nn xu hn v c qu nhiu frame truyn li, lu lng vn t ng v vng m tr nn bo ho, trong khi h thng vn tip tc truyn v nhn cc frame c cng nh frame mi, ngay c khi cc frame truyn thnh cng vn c truyn li v thi gian chp nhn l qu lu nn ngi gi cho r ng frame b hu nn truyn li. V d: mt mng Frame Relay c 20 thu bao vi cc ng 256 kbps v mt v n phng chnh c b ng thng mc T11,5Mbps. Nu c 20 site gi cc frame lin tc v v n phng chnh cng mt thi im, ta s c khong 20*256=5Mbps d liu cn i ra khi ng T1 , lm cho hng i ca tng i FRSwitch t ng nhanh.

7.3 Kim sot tc ngn C 2 phng php ch yu kim sot tc nghn trong frame relay: 7.3.1 FECN (Forward Explicit Congestion Notification) and BECN (Backward Explicit Congestion Notification )

Bit FECN c t thng bo cho h thng nhn r ng frame c nh du gp phi tc nghn.Tc nghn c bt u ti ni thun chiu vi frame nhn. Chin thut ny phn ng mt cch chm chp cc thng bo tc nghn vi hai l do : th nht, h thng khng phn ng mt cch tc thi mt bit FECN ring bit m i cho n khi x l trung bnh ca h thng trn mt interval bo cho bit c tc nghn. Th hai, h thng khng gim tc thi dng truyn ra m ch bo hiu dng truyn n. Khi xy ra tc nghn ti 1 thi im th bit FECN c gn b ng 1 (c gn bi tng i FR nhng khng th c gn bi bt k router no bi v router khng cn truyn tn hiu nghn) v duy tr cho n khi kch c trung bnh gim xung nh hn ngng cho trc.n thng bo cho bn nhn c tc nghn, bn nhn gi tin gim v ng thi thng bo cho bn pht gim pht.

Bit BECN c t thng bo cho h thng nhn r ng cc frame m ntruyn trn kt ni ny c th gp phi tc nghn. Tc nghn c bt u ti ni ngc chiu vi frame nhn. Chin thut ny phn ng mt cch nhanh chng cc thng bo tc nghn vi hai l do : th nht, h thng phn ng mt cch tc thi vi bit BECN n.Th hai, h thng gim tc thi dng ra hn l bo hiu gim dng n. Khi xy ra tc nghn ti 1 im th bits BECN c gn gi tr b ng 1(c gn bi tng i FR v router),mc nh khi truyn frame th gi tr ca bits BECN=0. Thit b DTE lun kim tra xem cn frame c bits FECN b ng 1hay khng.nu cn tn ti th pha pht (DTE) s tip tc gim n 0.875 gi tr thng lng trc ca n n khi khng cn nhn c frame relay c FECN=1 na,khi bn pht s t ng tc pht tin thm 1/16(0.0625) thng lng ca n,cho n ngng cho php ca nh cung cp dch v(CIR). Hot ng c bn ca FECN v BECN:

Ti bt k im trong mt router hoc bn trong m my Frame Relay thit b c th thit lp cc bit FECN, c ngha l khung(Frame Relay switch) ny tri qua tnh trng tc nghn. Ni cch khc, tc nghn tn ti hng v pha trc ca khung . Trong hnh ta c bc 1, router s gi mt khung, vi FECN = 0. Qu trnh tc nghn xy ra v cc b chuyn mch frame relay thng bo FECN = 1 bc 2.ng thi Frame Relay switch c th thit lp cc bit BECN trong khung tip theo gi ngc v R1 trn C (bc 3),nh m bo cho r1 bit c tc nghn xy ra trn VC m n truyn d liu lm R1 gim tc pht gi tin. 7.3.2 Loi b iu kin(DE bits):

Khi tc nghn nghim trng xy ra th bin php gim tc nghn l loi b frame da vo bits DE. V l m ni nu chuyn cc Frame vt qu tc cam kt, th nhng Frame s b loi b v bit DE c s dng. Tuy nhin c th chuyn cc frame i vi tc ln hn CIR hay thm ch hn c CBIR tu thuc vo trng thi ca mng Frame relay lc c nghn t hay nhiu (Thc cht ca kh nng ny l mn rng bng "Bandwith" ca nhng ngi s dng khc khi h cha dng n). Nu nghn ca mng cng nhiu (khi nhiu ngi cng lm vic) th kh nng ri ro b loi b ca cc Frame cng ln. Khi Frame b loi b, thit b u cui phi pht li Bist DE c quy nh: gi tr ca bits DE c xc nh nh hnh v,nu lung d liu nh hn gi tr Bc th bits DE c gn gi tr 0(gi tr mc nh) cn nu n m gia Bc v Bc + Be th gi tr bits l 1.v thng c nh du bi router(ngi dng) v ngi dng c th bit lu lng no quan trng hn lu lng no. Ti mt thi im,khi xy qua tc nghn nghim trng th thit b c quyn (router,switch..) s loi b cc gi tin c bits DE=1 gim tc nghn.Sau qu trnh tc nghn bn thu s yu cu pht li gi tin .

S gi tr bits DE