I HC QUC GIA H NI TRNG I HC CNG NGH

-----YZ-----

Bi xun Hnh

NGHIN CU CNG NGH TRUYN THOI QUA INTERNET S DNG GIAO THC TCP/IP (VOIP)

KHO LUN TT NGHIP I HC H CHNH QUY

Ngnh: in T Vin Thng Cn b hng dn: TS. Ng Thi Tr

H ni - 2005

CHNG 1

GIAO THC TCP/IP

1.1. Mt s khi nim ca mng TCP/IP .................................................2

1.2. Cc thnh phn vt l ca mng TCP/IP .........................................3

1.2.1. Cc thit b tnh ton (Computing devices) ................................3

1.2.2. ng truyn ............................................................................3

1.2.3. Cc b giao tip mng (NIC-Network Interface Card) ...............3

1.2.4. B tp trung (Hub) ....................................................................3

1.2.5. B iu ch v gii iu ch (Modem).......................................4

1.2.6. B chn ng (Router) ............................................................4

1.2.7. Phn mm mng ........................................................................4

1.3. Cc giao thc trong TCP/IP..............................................................6

1.3.1. Giao thc dng thi gian thc RTP (Real Time Protocol) ..........9

1.3.2. Giao thc UDP (User Datagram Protocol) .................................11

1.3.3. Giao thc iu khin truyn tin (TCP) .......................................12

1.3.4. Giao thc IP (Internet Protocol) ................................................15

1.3.4.1. Khi qut v giao thc IP .............................................15

1.3.4.2. Tng giao din mng ....................................................15

1.3.4.3. Tng Internet ...............................................................16

1.3.4.4. Tng giao vn ..............................................................16

1.3.4.5. Tng ng dng .............................................................16

1.3.4.6. a ch IP.....................................................................18

1.4. Tiu chun H.323 ..............................................................................20

1.4.1. Phm vi ca H.323 ....................................................................21

1.4.2. Cc dch v H.323.....................................................................22

1.4.3. Cc kiu d liu c nh ngha trong H.323 ...........................22

1.4.4. Cc thnh phn trong H.323 ......................................................23

1.5. Qu trnh truyn d liu trong TCP/IP ............................................24

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CHNG 2 TNG QUAN V VOIP

2.1. Tng quan v VOIP...........................................................................27 2.1.1. Gii thiu chung v VOIP .........................................................27 2.1.2. Hot ng ca VOIP .................................................................28 2.1.3. So snh gia VOIP v mng chuyn mch cng cng (PSTN) ...32 2.2. Cc c trng ca VOIP ...................................................................34 2.3. Hn ch ca vic s dng in thoi VOIP ......................................36 2.4. ng dng ca VOIP ..........................................................................37 2.5. Cc thnh phn trong mng VOIP ...................................................39 2.5.1. Cc thit b u cui .................................................................39 2.5.2. Gateway....................................................................................40 2.5.3. Gatekeeper ................................................................................41 2.5.4. Khi iu khin v x l a im (MCU) ..................................42 2.5.5. Cc Proxy .................................................................................42

CHNG 3

BO HIU V X L CUC GI TRONG VOIP

3.1. Gii thiu chung ................................................................................43 3.2. nh tuyn knh iu khin v bo hiu cuc gi ............................44 3.2.1. nh tuyn knh iu khin cuc gi .........................................45 3.2.2. nh tuyn knh bo hiu cuc gi ............................................46 3.3. Cc th tc bo hiu .........................................................................47 3.3.1. Thit lp cuc gi (Giai on 1) ................................................48 3.3.2. Thit lp knh iu khin (Giai on 2).....................................51 3.3.3. Thit lp knh truyn thng o (Giai on 3).............................51 3.3.4. Tham s cuc gi ......................................................................52 3.3.4.1. Thay i rng bng tn ............................................52 3.3.4.2. Trng thi ....................................................................54 3.3.5. Kt thc cuc gi ......................................................................55

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CHNG 4 VN NN TN HIU V GIM THIU TR TRONG VOIP

4.1. Tng qut ..........................................................................................59 4.2. Cc k thut nn tn hiu trong VOIP ..............................................60 4.2.1. Nguyn l chung ca b m ho CELP......................................63 4.2.2. Nguyn l b m ho CS-CELP ................................................65 4.2.3. Nguyn l b gii m CS-CELP ................................................66 4.2.4. Chun nn G.729A....................................................................67 4.2.5. Chun nn G.729B ....................................................................68 4.2.6. Chun nn G.723.1....................................................................69 4.2.7. Chun nn GSM 06.10 (Global System for Mobile) ...................71 4.2.8. Kh ting vng .........................................................................72 4.3. Tr v vn gim thiu tr ........................................................73 4.4. Vn gim thiu ho ngun tr ......................................................75 4.4.1. Ti thiu ho ghi m bn truyn ................................................75 4.4.2. Ti thiu ho tr Modem ...........................................................76 4.4.3. Ti thiu ho b m Jitter ........................................................76 4.4.4. Tr u cui n u cui .........................................................77

CHNG 5

VN M BO CHT LNG DCH V. KH NNG P DNG VOIP VIT NAM V TRN TH GII

5.1. Gii thiu ...........................................................................................78 5.2. Cht lng dch v (QoS) ...................................................................79 5.3. Phn cp cht lng ............................................................................80 5.4. Vn m bo cht lng dch v .....................................................81 5.5. Trin khai VOIP Vit Nam...............................................................82 5.6. Trin khai VOIP trn th gii ..............................................................83 Kt lun

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Danh mc cc t vit tt

Vit tt Ting Anh Ting Vit

ACELP Algebraic Code Excite LinearPrediction

D on tuyn tnh kch thch is

ACD Automatic Call Distribution T ng phn phi cuc gi ACF Admission Confirmation Xc nhn yu cu truy nhp ACR Admission Control Routine Th tc iu khin thu np ADSL Asymmetric Digital Subcriber Line ng thu bao s khng i xngARJ Admission Reject T chi truy nhp ARQ Admission Request Yu cu truy nhp ASCCII

American Standard Code for Information Interchange

B k t ASCCII

ATM Asynchronous Transfer Mode Phng thc truyn dn khngng b

BCF Bandwidth Change Confirmation Cng nhn thay i rng bngtn

B-ISDN Broadband-Integrated Services Digital Network

Mng s a dch v bng rng

BRJ Bandwidth Change Reject T chi thay i rng bng tn BRQ Bandwidth Change Request Yu cu thay i bng tn

CIF Common Intermediate Format khun dng trung gian chung CRV Call Reference Value Gi tr tham chiu cuc gi DCF Disengage Confirmation Xc nhn gii phng DNS Domain Name Service Dch v tn min DRQ Disengage Request Yu cu gii phng DS0 Digital Signal Level 0 Tn hiu s cp 1 (64Kbps) DTMF Dial Tone Multi Frequency Quay s a tn EAN Enterprise Area Network Mng x nghip ECTF Enterprise Computer Telephony

Forum Din n my tnh truyn thng

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ETSI European Technical Standard Institute

Vin tiu chun k thut chu u

GT Global Transit Chuyn vng ton cu GW Gateway Tram trung chuyn HTTP Hypertext Transfer Protocol Giao thc truyn siu vn bn IETF Internet Engineering Task Force IP Internet Protocol Giao thc Internet IPX Internetwork Protocol Exchange Chuyn i giao thc InternetworkIRQ Information Request Yu cu thng tin IS Integrated Service Tch hp dch v ISDN Integrated Services Digital Nework Mng s a dch v ISP Internet Services Provider Cc nh cung cp dch v Internet ISUP ISDN User Part ISDN Pha ngi dng ITU-T International Telecommunication

Union - Telecommunication Standardization

Hip hi vin thng quc t

IVR Interactive Voice Response Tng tc thoi LAN Local Area Network Mng ni b LCF Location Confirmation Cng nhn cp pht LDAP Link Access Procedure on D

channel Th tc truy nhp kt ni knh D

LRJ Location Reject T chi cp pht LRQ Location Request Yu cu cp pht MC Multipoint Controller B iu khin a im MCS Multipoint Communications System H thng lin lc a im

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LI CM N

Trc ht em xin gi ti thy gio, Tin s Ng Thi Tr - Gim c Trung tm tin hc v o lng - i Truyn hnh Vit Nam - li cm n chn thnh v su sc trc tip hng dn , ch bo tn tnh trong sut qu trnh em lm lun vn.

Em cng xin chn thnh cm n cc thy c gio trong Trng i hc Cng Ngh - i hc Quc Gia H Ni ht lng dy bo, gip em trong nhng nm hc i Hc, gip em c nhng kin thc v kinh nghim qu bu trong cuc sng v tng lai.

Cui cng, em xin cm n nhng ngi thn trong gia nh v bn b gip , ng vin em hon thnh lun vn ny.

H ni, thng 05 nm 2005

Sinh vin

Bi xun Hnh

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LI NI U

Ngy nay, cc dch v vin thng rt phong ph v a dng, bao gm cc dch v truyn thng tn ti trong thi gian di v cc dch v mi c pht trin trong thi gian gn y. Tin ca n chnh l s pht trin ca cng ngh in t v cng ngh mng. C th hn, chng ta c th ni rng s a dng ca cc dch v vin thng bt ngun t s pht trin ca cc cng ngh lin quan nh truyn dn, x l tn hiu, chuyn mch v cng ngh phn mm

Mng in thoi truyn thng PSTN tr nn quen thuc v ngy cng c v tr quan trng trong i sng con ngi, nht l trong cuc sng hin i, khi m vn lin lc ca con ngi tr nn cn thit hn bao gi ht. Tuy nhin, mng vin thng cho n tn by gi vn hu ht s dng k thut chuyn mch knh, trong s cung cp cho mi cuc gi mt ng kt ni vt l 64kbps ring bit t ngi gi n ngi b gi. Theo th cht lng thoi qua phng thc ny rt tt v gn nh tr thnh mt tiu chun cho cc dch v thoi. Tuy nhin, 64kbps l mt dung lng kh ln truyn mt di tn gii hn di 3,4 KHz. Nht l trong thi i ngy nay khi m lng thng tin cn lun chuyn trn mng ngy cng nhiu.

Cng ngh VOIP (Voice Over Internet Protocol) l mt cng ngh truyn thoi hon ton khc vi phng thc truyn thng. VOIP thc hin vic ly mu tn hiu thoi, s ho v ng gi chng thnh cc gi tin v truyn qua Internet s dng cng ngh chuyn mch gi, cng vi cc cng ngh nn ting ni tc thp s gip vic khai thc mng vin thng mt cch hiu qu hn nhiu so vi phng php truyn thng v cho php c th m rng nhiu dch v tin tin trong tng lai m rt hn ch vi mng in thoi truyn thng.

Tuy nhin, VOIP cng t ra nhiu thch thc cho cc nh thit k h thng c th cung cp mt cht lng thoi c th chp nhn c hay thm ch tng ng vi in thoi truyn thng. Chng hn nh vn tr hay ting vng, nhng vn nh hng trc tip n cm gic m thanh. Trc

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iu kin , vic nghin cu tm hiu v VOIP l mt iu ht sc cn thit. Lun vn ny s cung cp mt ci nhn tng quan v VOIP, v khi nim, cu hnh, cc chun trong VOIP, nhng yu t nh hng trc tip n cht lng thoi.

VOIP l mt cng ngh tuy c nghin cu nhiu nhng ni chung n vn cn l mt cng ngh mi v cn nhiu iu cn nghin cu nn vic hiu mt cch chi tit v su hn khng th gi gn trong mt lun vn. Hn na, thi gian lm lun vn khng nhiu nn trong nghin cu v trnh by chc chn khng trnh khi sai st. Rt mong c s ng gp kin ca cc thy c gio, bn b, nhng ngi c cng quan tm v VOIP.

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CHNG 1 GIAO THC TCP/IP

Trong nhng nm gn y, lu lng thoi pht trin tng i n nh trong khi lu lng s liu pht trin ht sc mnh m. Mng thoi pht trin tt song n ch p ng c ng dng c bn l truyn thoi v mt s ng dng d liu dng mt lng bng thng nh tng ng. Trong khi cc mng d liu (Internet, PN, VPN...) p ng c nhiu ng dng s liu v thu ht c s lng ngi dng ln. iu ny c ngha l phn ln lu lng thoi s c truyn qua mng chuyn mch gi trong thi gian khng xa. in thoi qua Internet gy c s ch mnh m nht v c kh nng tr thnh nn tng cho mng thoi dng cng ngh chuyn mch gi. Mt khc, v phng din cng ngh, mng d liu khng ngng pht trin p ng nhu cu ngy cng cao ca ngi dng nh: lun phi pht trin cng ngh mi truyn d liu tc cao hn, dung lng ln hn ... Cn mng thoi th pht trin n nh, cch thit lp cuc gi khng khc nhau nhiu so vi cch y vi thp nin. T nhng phn tch ngi ta a ra cc gii php nhm hp nht lu lng thoi v d liu vo mt c s h tng mng n nht, mt trong cc gii php l cng ngh VOIP (Voice over Internet Protocol).Truyn thng thoi c tr gip bi cc giao thc Internet gi l VOIP. VOIP c bit li cun ngi dng bi chng c gi c tng i thp. Thc t, cht lng chuyn tip ng di in thoi qua IP ngy mt nng ln, hin nay tr thnh mt trong nhng bc tin quan trng hi t nhng cng ngh truyn thng d liu, video v m thanh (voice).

Chng ta c th nhn nhn VOIP nh l kh nng thit lp cc cuc gi in thoi v gi nhng bn fax (facsimiles) qua mng d liu IP c s vi cht lng dch v thch hp v cc ph thp hn nhiu. Nhng nh sn xut thit b coi y l mt c hi i mi v b sung cn i vi nhng nh cung cp dch v Internet. Ngi s dng ang tm kim cc kiu ng dng mi ca s hp nht m thanh, d liu.Truyn m thanh thnh cng qua mng chuyn mch gi mang li mt c hi ln, tuy nhin vic thc thi VOIP cn nhiu yu t nh: cng ngh, c s h tng, phn mm, h thng... Trong cun ti liu nh ny ch yu cp ti cc vn tng quan ca cng ngh VOIP qua cc tiu chun hin ang c khuyn ngh s dng rng ri trn th gii.

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TCP/IP (Transmission Control Protocol/Internet Proctocol) l mt b giao thc cho php cc thit b khc nhau c th truyn thng c vi nhau trn mng my tnh. TCP/IP c pht trin nh mt d n nghin cu bt u vo nm 1969 ca ARPA (Advanced Research Project Agency) thuc B quc phng M nhm kt ni cc my tnh ca cc trung tm nghin cu ln trong ton lin bang, nh du s pht trin ca mng ARPA, tin thn ca mng thng tin ton cu Internet ngy nay. 1.1. MT S KHI NIM CA MNG TCP/IP.

Cc thnh phn logic ca mng TCP/IP c th rt ging nhau v mt cu trc cng nh mt cu to vt l, nhng chng li thc hin cc chc nng hon ton khc nhau. Cc trm ( host ).

Vi TCP/IP, thut ng trm ch bt k cc thit b tnh ton no kt ni vo h thng mng my tnh v truyn thng s dng h giao thc TCP/IP. Trm s dng cc chng trnh ng dng cho php chng c th truyn thng c vi nhau. Mt trm c th l mt my tnh ln (mainframe), my vi tnh (microcomputer) hay mt my vi tnh c nhn (personal computer) Mng (Network).

Cc host c kt ni vi nhau qua cc lin kt vt l to thnh mt mng, mng l tp hp ca hai hay nhiu my tnh kt ni vi nhau s dng mt cu hnh nht nh. Kin trc TCP/IP l c lp vi cc mng, ngha l n cho php cc mng bt k c th c kt ni thnh vin (internetwork) min sao chng s dng h giao thc TCP/IP. Cc b nh tuyn (Routers).

Router l mt thit b cung cp kh nng kt ni ca mt s lng ln cc mng ring bit to nn mng TCP/IP. Chc nng chnh ca router l nh tuyn cc dng lu lng khi chng n theo cc con ng kh dng ti thi im nh tuyn t mng vt l ny sang mng vt l khc. Cc n v d liu l cc gi tin trong TCP/IP, chng c lp vi nhau v c th c router nh tuyn theo cc con ng khc nhau trc khi chng n cng mt ch. Chc nng nh tuyn c th c thc hin bng phn mm nh tuyn c ci t trn mt host hay c th c hot ng nh mt thit b c bit cung cp kh nng nh tuyn, trong cch th hai thng c s dng trong cc mng TCP/IP ln.

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1.2. CC THNH PHN VT L CA MNG TCP/IP

Mng TCP/IP c xy dng trn c s cc mng vt l ring bit nhau c th cng hay khc cu hnh, min sao chng tun th mt cch nghim ngt cc nh ngha trong h giao thc TCP/IP. Cc thit b trong mng TCP/IP c phn thnh cc nhm sau: 1.2.1. Cc thit b tnh ton (Computing Devices)

Mng TCP/IP nhn chung, bao gm cc thit b tnh ton a mc ch, chng hn nh cc my tnh c nhn PC, cc trm lm vic (work station) c th cng loi hay khc loi. Cc thit b cho mc ch c bit, chng hn nh my in thng minh, c th c kt ni trc tip vo mng m thng thng c gn vo mt h thng my tnh ca mng. Cc thit b tnh ton c s dng cho ngi dng cui, trn chy cc chng trnh ng dng gip cho ngi s dng cui khai thc mng gi l host. 1.2.2. ng truyn

ng truyn c th l v tuyn hoc hu tuyn, bao gm cc phng tin truyn dn tn hiu nh: h thng cp (cp xon, cp ng trc, cp quang ), cc ng truyn v tuyn nh hng ngoi, sng viba, cc tuyn thng tin v tinh. 1.2.3. Cc b giao tip mng (NIC - Network Interface Card) i khi cn gi l card mng hay b thch ng mng (adapter) c cm vo mi trm ca mng. Nhim v ca NIC l thc hin cc chc nng phn cng ca thit b tnh ton. N lm nhim v ca tng vt l, to cc lin kt vt l truyn ti cc lung thng tin c yu cu t cc lp cao hn sau khi m ho thnh cc xung in. Mi NIC c mt a ch vt l duy nht trn mng, cc nh sn xut NIC c phn b cc dy a ch dnh cho mnh m bo khng c s trng lp a ch ca hai NIC bt k. Mt s thit b tnh ton c th NIC c tch hp, cn li hu ht u cn b sung NIC khi cn kt ni cc thit b vo mng. Card giao tip mng l loi thit b ph thng nht ni my tnh vi mng. Trong NIC c mt b thu pht vi mt s kiu u ni. 1.2.4. B tp trung (Hub)

Hub l b chia hay cng c th gi l b tp trung dng u mng. S dng cc Hub u ni cc trm ca mng qua mt im tp trung. Ngi ta chia lm cc loi Hub sau:

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Hub b ng (Passive Hub). Hub ny khng x l tn hiu m n ch n thun thc hin chc nng t hp tn hiu t mt s on cp mng. Khong cch ti a gia Hub vi my tnh phi nh hn 1/2 khong cch ti a cho php gia hai my tnh trong mng.

Hub ch ng (Active Hub) Loi Hub ny c th khuch i v x l cc tn hiu in t truyn gia cc thit b ca mng lm cc tn hiu ny tr nn mnh hn.

Hub thng minh (Intelligent Hub) Ging nh Hub ch ng nhng c thm cc chc nng qun tr Hub.

Hub chuyn mch (Switching Hub) Loi ny cho php chn ng rt nhanh, hin nay n ang dn thay th cho Brigde v Router.

1.2.5. B iu ch v gii iu ch (Modem) L thit b c chc nng chuyn i tn hiu s thnh tn hiu tng t v ngc li, dng kt ni cc my tnh qua ng in thoi. Modem cho php trao i th in t, truyn file, truyn fax Modem khng th dng kt ni cc mng xa vi nhau. Tuy nhin n c th kt hp vi Router kt ni mng thng qua mng in thoi chuyn mch cng cng. 1.2.6. B chn ng (Router) Router l mt thit b thng minh, n c th thc hin cc gii thut chn ng i ti u cho cc gi tin theo mt ch tiu no . Router cho php ni cc kiu mng khc nhau thnh mt lin mng. Mun vy cc Router phi c thit k sao cho c th lm vic c vi nhiu giao thc. Hin nay cc router ca cc hng ni ting nh CISCO, BAY NETWORKS. u c th lm vic c vi hu ht cc giao thc ph bin. 1.2.7. Phn mm mng Mt phn cn phi k n khi nghin cu mng TCP/IP l phn mm mng. NIC thc hin cc chc nng mc thp trong mng nhm cung cp mt giao din gia host v ton mng. Phn cn li, ngi s dng c th giao tip c vi mng cng nh giao tip c vi nhau, cn n cc phn mm mng. Cc phn mm mng l cu ni gia ngi dng v cc giao din vt l, vic thng dch cc lnh ca ngi s dng l trong sut i vi ngi s dng. Ngi s dng khai thc mng thng qua cc tp lnh hay cc tin ch mng v phn mm mng gip thng dch cc yu cu ca ngi s dng v

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lm vic vi cc tng thp hn. Trong khi cc thit b phn cng l c nh th phn mm mng c cc phin bn ci tin gip cho vic khai thc mng c hiu qu hn.

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1.3. CC GIAO THC TRONG TCP/IP. Kin trc mng TCP/IP nh ngha cc cp giao thc truyn thng tng ng vi cc tng cng lp ca cc h thng khc nhau.

Layer n

Giao thc tng n

Layer 3 Layer 3

Layer 2 Layer 2

Layer 1 Layer 1 Giao thc tng 1

Giao thc tng 2

Giao thc tng 3

Hnh 1.1. S nh ngha cc giao thc, giao din TCP/IP

Cc giao thc nh ngha cc dng d liu c trao i gia cc lp cng cp ca hai h thng khc nhau, ng thi quy nh quy tc d liu ny c truyn nh th no gia hai mng. Tp hp cc giao thc quan trng hot ng trong tng Internet v tng Transport cung cp cc dch v truyn thng c bn. Cc giao thc truyn thng c th cung cp cc dch v hai dng: kt ni c nh hng (connection-oriented) v kt ni khng nh hng (connectionless). Kt ni c nh hng (connection-oriented).

Giao thc kt ni c nh hng yu cu cc bn tham gia cuc truyn phi c mt kt ni logic gia cc thc th ng mc. Cc dch v c cung cp bi giao thc khng nh hng tng t nh cc dch v cung cp bi h thng in thoi truyn thng. Qu trnh truyn gm ba pha c nh ngha r rng:

Pha 1: Ci t kt ni (tng t nh vic quay s cuc gi) Pha 2: Truyn d liu (tng t nh vic m thoi)

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Pha 3: Gii phng kt ni (tng t nh ng tc t my, kt thc cuc gi)

Cc thng s ca kt ni phi c tho thun t u gia cc thc th tham gia cuc truyn qua th tc thit lp cuc gi, i khi cn gi l th tc bt tay (handshake). Vi kt ni c nh hng, truyn d liu lun lun lin quan n hai thc th truyn thng. Nu mt thc th mun truyn thng vi mt thc th th ba hay nhiu hn th n cn phi ci t cc kt ni ring bit n mi thc th v truyn d liu n mi thc th trong mt qu trnh ring. Hn na, ngi nhn ch c xc nh duy nht ti mt thi im.

Sau khi thit lp cuc truyn, dng d liu theo cng mt ng kt ni n ch. Cc thng tin cn c yu cu trong qu trnh truyn ch l cc thng tin nhn dng kt ni cng nh d liu c kt hp nh th no m khng phi l cc thng tin nhn dng bn nhn.

Kt ni c nh hng c s dng cung cp cc dch v yu cu tin cy cao v cc dch v truyn thng c tnh lin tc. Ngi gi c th yn tm rng cc gi tin c n ch mt cch chnh xc v theo ng th t. Nu c mt s c xy ra, ton b kt ni c th s phi c gii phng thc hin mt kt ni mi v cc thc th cn phi c thng bo v s kin ny. S gii phng kt ni c th do cc thc th tham gia cuc truyn, cng c th do bn thn giao thc, bi v bt k mt pha no trong giao thc cng c th xy ra li ti bt k thi im no. Kt ni khng nh hng (connectionless).

Giao thc kt ni khng nh hng hot ng nh hot ng chuyn th ca bu in, trong mi bc th c c gng chuyn n ch da vo cc thng tin trn b th. Vi kt ni khng nh hng, khng yu cu cc pha r rt nh trong lin kt c nh hng. Vic truyn thng tin c th ch thng qua mt phase duy nht. im khc bit ln nht ca phng thc kt ni khng nh hng v phng thc kt ni nh hng l cc gi tin c lu chuyn mt cch c lp v c th theo cc con ng rt khc nhau trc khi n ch.

Kt ni khng nh hng c bit c tc dng khi tng s d liu l nh v khng lin tc. Bi v khi thi gian cn thit lp knh truyn s ln v thi gian tr dnh ring cho knh truyn kh ln, thm ch tng thi gian ny c th ln hn c thi gian truyn d liu thc s.

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Mt u im ca lin kt khng nh hng l s dng cc nh dng a pht p (multicast) cho php truyn thng ti nhiu ngi cng mt lc. Tuy nhin, vi kt ni khng nh hng, khng c g m bo rng cc gi tin n ng ch theo ng th t gi, bi v cc gi tin c th chu du theo cc con ng khc nhau trc khi n ch v do chng c th gy nn hin tng Jitter trong VOIP s c nghin cu k hn phn sau ca lun vn. V vy, kt ni khng nh hng thng c s dng cung cp cc dch v t tin cy, nhng li gim thiu chi ph. Phng thc ny t ra hiu qu hn phng thc kt ni c nh hng trnh by trn.

Cu trc TCP/IP c xy dng da trn giao thc Internet, mt giao thc theo kiu khng nh hng, hot ng trn tng Internet trong cu hnh phn tng TCP/IP. Nhng dch v n cung cp c s dng hoc l vi giao thc UDP hoc l vi giao thc TCP. Vic la chn phng thc kt ni no ty thuc vo hon cnh thc t ca cc mng, cc dch v mng cn cung cp.

Hnh1.2. Cc tng ca b giao thc TCP/IP

Transport layer ( Hostlayer )

Internet layer (Getway layer) Network Interface layer

FPT TELNET SMTP DNS SNMP

TranssmissionControl Protoco

l

User Datagram RIP

Application layer

I nternet Protocol

ARP

ICMP

Ethernet Token Ring Token bus Fiber

Application layer

FTP TELNET SMTP DNS

Transsmission Control Protocol

User Datagram RIP

Internet Protocol

ARP

ICMP

Ethernet Token Ring Token bus Fiber

SNMP

Internet layer (Getway layer) Network Interface layer

Transport layer (Host layer)

FTP (File Transfer Protocol): giao thc truyn file, cho php ngi dng ly hoc gi file ti mt my khc.

Telnet: chng trnh m phng thit b u cui, cho php ngi dng login vo mt my ch t mt my tnh no trn mng.

SMTP (Simple Mail Transfer Protocol): mt giao thc th tn in t.

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DNS (Domain Name Server): dch v tn min, cho php nhn ra my tnh t mt tn min thay cho chui a ch Internet kh nh.

SNMP (Simple Network Management Protocol): giao thc qun tr mng, cung cp nhng cng c qun tr mng.

RIP (Routing Internet Protocol): Giao thc dn ng ng. ICMP (Internet Control Messege Protocol): nghi thc thng bo

li. UDP (User Datagram Protocol): Giao thc truyn khng kt ni,

cung cp dch v truyn khng tin cy nhng tit kim chi ph truyn. TCP (Transmission Control Protocol): Giao thc hng kt ni,

cung cp dch v truyn thng tin tng. IP (Internet Protocol): Giao thc Internet chuyn giao cc gi tin

qua cc my tnh n ch. ARP (Address Resolution Protocol): C ch chuyn a ch TCP/IP

thnh a ch vt l ca cc thit b mng.

1.3.1. Giao thc dng thi gian thc RTP (Real time protocol)

Ngoi cc giao thc c dng trong truyn ti d liu ni chung trn mng nh TCP, UDP, dng tn hiu thoi v video trong VOIP c mt s c trng khc vi dng d liu bnh thng. Do ko theo mt s giao thc c khuyn ngh dng trong VOIP cho mc ch truyn ti thoi v video cng nh ng dng a phng tin khc. Cc c im l:

S lin tc. Cc gi tin phi c sp xp theo ng th t khi chng n bn nhn, cc

gi n c th khng theo th t v nu gi tin b mt th bn nhn phi d tm hay b li s mt cc gi tin ny.

S ng b bn trong cc phng thc truyn thng. Cc khong lng trong ting ni c trit v nn li gim thiu bng thng cn thit, tuy nhin khi n bn nhn, thi gian gia cc khong lng ny phi c khi phc mt cch chnh xc.

S ng b gia cc phng thc truyn thng: C th tn hiu thoi s dng mt phng thc truyn thng trong khi tn hiu video li s dng mt phng thc truyn thng khc, cc tn hiu ting v hnh phi c ng b mt cch chnh xc, gi l s ng b ting - hnh.

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S nhn din phng thc truyn ti: Trong Internet, thng thng cn thay i s m ho cho phng thc truyn ti (payload) trn hnh trnh truyn hiu chnh thay i rng bng thng sn sng hoc kh nng cho ngi dng mi kt ni vo nhm. Mt vi c ch cn c s dng nhn din s m ho cho mi gi n.

Cc tnh cht ny c h tr bi cc phng thc truyn ti (transport protocols).

a pht p thn thin: (multicast friendly): RTP v RTCP l k thut cho a pht p, cung cp kh nng m rng cuc hi thoi nhiu bn. Trn thc t, chng c thit k c th hot ng trong c cc nhm a pht p nh, ph hp cho cc cuc in m ba bn. i vi cc nhm ln, chng s dng a pht p qung b (broadcasting).

c lp thit b: RTP cung cp cc dch v cn thit chung cho phng thc truyn thng thi gian thc ni chung nh thoi, video hay bt k mt b m ho, gii m c th no c s nh ngha cc phng thc m ho v gii m ring bng cc thng tin tiu v nh ngha.

Cc b trn v chuyn i: Cc b trn l thit b nm gi phng thc truyn thng t mt vi ngi s dng ring l, trn hoc ni chng vo cc dng phng thc truyn thng chung, chuyn i chng vo khun dng khc v gi n ra. Cc b chuyn i c ch cho s thu nh bng thng yu cu ca dng s liu t dng s liu chung trc khi gi vo tng kt ni bng thng hp hn m khng yu cu ngun pht RTP thu nh tc bit ca n. iu ny cho php cc bn nhn kt ni theo mt lin kt nhanh vn nhn c truyn thng cht lng cao. RTP h tr c cc b trn v c cc b chuyn i.

M ho thnh mt m: Cc dng phng thc truyn thng RTP c th m ho thnh mt m dng cc kho, vic m ho m bo cho vic thng tin trn mng c an ton hn.

Tiu ca gi RTP di 12 bytes (nh hnh di). Trng V ch r phin bn ca giao thc, c X bo hiu c trng ca tiu m rng gia tiu c

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nh v s liu i theo. Nu bit P l 1, phn s liu c m thm m bo lin kt y cho s m ho thnh mt m.

Hnh 1.3: Cu trc tiu c nh RTP

1.3.2. Giao thc UDP User Datagram Protocol.

UDP hot ng tng giao vn ca m hnh mng TCP/IP. N cho php chng trnh ng dng truy cp trc tip n gi tin ca dch v chuyn giao ging nh dch v m giao thc IP cung cp. N cho php ng dng trao i thng tin qua mng vi t thng tin iu khin nht. UDP l giao thc khng kt ni, km tin cy v n khng c c ch kim tra tnh ng n ca d liu truyn. Cu trc gi tin UDP

0 31 Source Port Destination Port Message length Checksum Data

V P X CSRC M payload type Sequnce number

Timestamp Synchronization source identifier (SSRC) Contributing source identifiers (CSRC) Header extension Payload ( audio, video ... )

Dest Add

Source Add

Type Field IP Header UDP data CRC

8 16 310

Hnh 1.4. Cu trc gi tin UDP

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Mi gi thng tin UDP gi l mt phn trong Datagram, c phn lm 2 phn header v data. Trong header cha thng tin v a ch cng ngun, a ch cng ch, di ca gi v checksum. Source Port: s hiu cng trm ngun gi tin. Destination Port: s hiu cng trm cui nhn tin. Length: tng di, tnh theo byte ca gi tin (bao gm c phn tiu

v phn d liu). Checksum: y l trng duy nht trong gi tin UDP cung cp cho mc

ch kim tra v sa li. Nu gi tr tnh checksum c tnh ton bi trm ch khng khp vi gi tr checksum trong gi tin th gi tin s b loi b. Thng thng trm ch thng s dng mt b nh m lu gi

hng i ca cc gi tin n cn x l. Mt server khi thng tin vi trm khch thng khng bit trc c bao nhiu gi tin s n trong mt khong thi gian. Nu nh s lng gi tin n qu kh nng ca hng i th cc gi tin n sau s b loi b m khng c x l. 1.3.3. Giao thc iu khin truyn tin (Transmission Control Protocol-TCP)

Mt s ng dng i hi giao thc giao vn cung cp dch v chuyn giao thng tin tin cy s dng TCP bi n cung cp dch v kim tra ng n v d liu c truyn thch hp. Mt s c im ca dch v tin cy m TCP cung cp l: Hng dng (Stream Orientation).

TCP coi d liu n gi i nh l mt dng byte. Do , TCP m bo s th t ca cc byte gi nhn. Trng Sequence number v Acknowledgment number trong header ca TCP gi du ca cc byte trong ton b dng d liu truyn. TCP chun khng bt h thng phi s dng mt s c bit no nh s dng byte, mi h thng t chn mt s m n s dng lm im bt u. Mi trm cui phi bit s th t m trm kia dng. Chng phi trao i mt segment ng b h thng s s c s dng (Synchronize sequenence number-SYN) trong qu trnh bt tay. Trng Sequence number trong SYN segment cha s bt u (ISN) ca dng s nh th t, s ny c nh ngha ngu nhin.

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Chuyn giao b m (Buffered Transfer). gim s lng truyn thng, giao thc c gng s dng s lng ti

thiu cc segment truyn ti thng tin. Do vy, giao thc s dng ti a di c th ca segment. Kt ni hai chiu (Full Duplex Connection).

TCP/IP cung cp kt ni cho c hai u ca lin kt. iu ny c ngha l tng pha di cung cp kh nng gi tr thng bo t trm ch tr li trm ngun. TCP l giao thc hng kt ni, n thit lp quan h logic gia 2 trm tham gia lin kt. Thng tin iu khin gi l handshake c trao i gia 2 trm thit lp hi thoi trc khi thng tin c truyn.

TCP cung cp dch v tin cy vi mt c ch gi l PAR Positve Acknowledgment with Retransmission. n gin l trm ngun tip tc gi thng tin i cho ti khi n nhn c thng bo d liu c nhn chnh xc ti trm ch. n v thng tin chuyn giao gia cc ng dng gi l segment, mi segment cha checksum m bo rng segment khng b ph hy trn ng truyn, nu segment khng b ph hy, trm nhn gi thng bo xc nhn li trm gi. Nu segment b ph hy, trm nhn hy b n v sau mt thi gian no , trm gi s gi li gi tin m n khng nhn c thng bo xc nhn.

Ti trm gi Thng tin trn mng Ti trm nhn

Hnh 1.5. Truyn thng s dng TCP

Receive ACK

Send packet 2

Receive ACK

Send packet 1Receive packet

Send ACK 1

Receive packet

Send ACK 2

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Gi tin TCP Source Port Destination Port Sequence Number Acknowledgment

Offset Reserved Flags Window Checksum Urgent Pointer Option Padding Data

Dest Add

Source Add Type field IP header TCP segment CRC

Hnh 1.6. Gi tin TCP cha trong mt Ethenet frame

Source Port (16 bit): cng ngun trm gi. Destination Port (16 bit): cng ch trm nhn. Sequence Number (32 bit): s th t ca gi tin. Acknowledgment Number (32 bit): s hiu ca Segment tip theo m trm

ngun ang ch nhn. Offset (32 bit): s lng cc t 32 bit trong TCP header. Reserved: dnh, khng s dng. Flags: c dn. Window (16bit): cp pht credit kim sot lung d liu. y chnh l

s lng cc byte d liu, bt u t byte c ch ra trong vng ACK Number m trm ngun sn sng nhn.

Checksum (16 bit): M kim tra li s dng CRC (Circle Redundant Check) cho ton b segment .

Urgent Pointer (16 bit): Con tr ny tr ti s hiu tun t ca byte i theo sau d liu khn, cho php bn nhn bit c di ca d liu khn, vng ny ch c hiu lc khi bit URG c lp.

Option ( di thay i): khai bo cc la chn ca TCP, trong c di ti a ca vng TCP data trong mt segment.

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Padding: phn chn thm vo header m bo header lun kt thc mt mc 32 bit, n bao gm ton s 0.

Data: cha d liu tng trn gi xung, c di ti a ngm nh l 536 byte, gi tr ny c th iu chnh bng khai bo trong phn Option.

Mt chng trnh ng dng trong mt host truy nhp vo cc dch v do TCP cung cp thng qua mt cng. Mt cng kt hp vi mt a ch IP to thnh mt socket duy nht trong lin mng. Dch v TCP c cung cp nh mt lin kt logic gia mt cp socket. Mt socket c th tham gia vo nhiu lin kt vi cc socket xa khc nhau. Trc khi truyn d liu, gia hai trm cn phi thit lp mt lin kt TCP gia chng v khi khng cn nhu cu truyn d liu th lin kt c gii phng. 1.3.4. Giao thc IP 1.3.4.1. Khi qut v giao thc IP

B giao thc TCP/IP l t hp ca nhiu giao thc cc tng khc nhau nhng thng thng m hnh phn lp trong cc h thng TCP/IP c xem l m hnh gin lc ca OSI gm 4 tng nh sau:

Application layer Presentation layer Session layer

Application layer

Transport layer Transport layer Network layer Internet layer Data link layer Physical layer

Network access layer

Hnh 1.7. Cu trc TCP/IP so snh vi m hnh OSI

1.3.4.2. Tng giao din mng ( Network Interface layer ). i khi tng ny cn c gi l tng mng ni ht ( Local Network Layer ). Tng giao din mng c mt tt c cc kt ni vt l gia cc thit b mng ( host, trm trung chuyn ). N bao gm cc kt ni vt l gia cc thit b, cc thnh phn phn cng ca c s h tng mng v c chc nng tng ng vi tng vt l v tng lin kt d liu trong m hnh OSI. Tng giao din mng cung cp cc phng tin kt ni vt l cp, card mng, giao thc

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kt ni, giao thc truy cp ng truyn. N cung cp cc dch v cho tng trn l tng Internet. 1.3.4.3. Tng Internet Tng ng vi tng mng trong m hnh OSI. N cung cp mt a ch logic cho giao din vt l mng. C th n gii quyt cc vn nh nh a ch, phn phi gi tin, nh tuyn Tng ny cho php kt ni mt cch mm do cc mng vt l khc nhau, chng hn mng Ethernet, Token Ring, Token Bus. Giao thc thc hin ca tng ny l giao thc IP (Internet Protocol). Ngoi ra, tng ny cn s dng cc b giao thc ARP (Address Resolution Protocol) thc hin nh x a ch vt l (MAC) cho tng di cung cp a ch IP (c cc host v cc trm trung chuyn). 1.3.4.4. Tng giao vn (Transport layer) Tng ng vi tng vn ti trong m hnh OSI, n thc hin kt ni gia hai my ch trn mng, iu khin lung v kim sot li. C hai giao thc c pht trin h tr tng ny, l: giao thc iu khin truyn dn (TCP) v giao thc UDP (User Datagram Protocol). Trong TCP c s dng cho cc ng dng yu cu tin cy v hiu qu cao. Ngoi ra, giao thc bn tin c ngi dng ICMP (Internet Control Messege protocol) cho php cc trm trung chuyn trao i thng tin qun l v iu khin bng cc bn tin. C ba giao thc ny c ng dng trong VOIP. TCP c s dng thit lp cuc gi vi yu cu tin cy cao. UDP c s dng truyn cc mu thoi vi yu cu tnh hiu qu cao v chi ph gim thiu. ICMP c cc thit b VOIP s dng thay i cc kt ni v gii quyt cc vn trong truyn thng.

1.3.4.5. Tng ng dng (Application layer) Tng ng vi lp phin, lp trnh din v lp ng dng trong m hnh OSI, ch c mt cc host v h tr qu trnh x l hay ng dng t User to Host v Host to User. N bao gm cc giao thc truy nhp t xa (Telnet), truyn file (ITP), dch v World Wide Web, Th in t (SMTP) Giao thc IP hot ng trong tng Internet ca m hnh phn lp TCP/IP. N cung cp cc dch v trao i d liu t mt trm ngun n mt trm ch. Cc trm ngun v ch c th cng mt mng hay khc mng vt l. IP hot ng trong mi trm hoc trong cc b nh tuyn dc theo tuyn ng ca gi tin di chuyn t trm ngun n trm ch. n v d liu m

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mt tin trnh ng dng gi t lp giao vn xung c gi l mt gi tin IP, i khi cn c gi n gin l mt gi tin (packet). Chc nng chnh ca IP l t chc cc gi tin thnh cc nh dng d liu v chuyn n trm ch. Khng c c cu kim tra lung thng tin trong giao thc IP, n l mt giao thc theo kiu khng lin kt. Do , giao thc IP cng gy ra cc vn nh mt gi, lp gi hay cc kiu gi n khng theo th t Khun dng gi tin IP.

Vers Hlen Service type Total length

Identification Flag Fragment offset Time to live Protocol Header checksum

Source IP address Destination IP address IP option Padding IP datagram data ( Max 65535 byte )

Destination Addres

Source Address

Type field IP data TCP seg

CRC

Hnh 1.8. Gi thng tin IP trong mt frame Ethernet

Time to live: di 8 bit, dng xc nh xem datagram ny c b truyn lp li hay khng (thng thng n c gn l 1, nu b lp li n s b gn l 0.

Protocol: di 8 bit, trng ny cho bit lp giao thc cao hn no s c s dng (UDP hay l TCP).

CRC: trng ny c dng kim tra s ton vn ca header. IP option: cha cc thng tin nh d ng, bo mt, xc nhn thi gian. Padding: trng in thm cc s 0 m bo header kt thc ti mt a

ch bi ca 32. Source IP address, Destination IP address: cha a ch ca station gi v

a ch ca station ch.

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Gi tin IP cha mt trng checksum kim tra tnh ton vn ca IP header, nu IP header li, gi tin IP b loi b v giao thc lp trn s truyn mt gi tin khc.

Tng IP lm cng vic dn ng cc gi tin qua mng Internet t my tnh ny ti my tnh khc, qua cc mng khc nhau cho ti khi n n c trm ch hoc b li. Vic truyn gi tin qua cc mng khc nhau c thc hin thng qua mt thit b kt ni gia hai mng gi l gateway. Khi mt thng tin truyn qua cc mng khc nhau, n c th b chia ra thnh nhiu gi nh hn. Thng tin truyn c th qu ln c th truyn trn mt gi tin trn mt mng khc. Vn ny ch gp phi khi gateway c ni gia cc mng vt l khc nhau. Mi kiu mng c mt di ti a gi tin c th truyn (Maximum Transmission Unit - MCU). Nu thng tin nhn c t mng ny di hn MCU ca mng kia, n cn phi c chia nh ra thnh nhiu mnh truyn. 1.3.4.6. a ch IP a ch IP nhm xc nh duy nht mi trm lm vic trn mng TCP/IP. Do vic nh a ch cn m bo khng trng lp ngay c khi c mt s lng ln cc trm kt ni vo mng. a ch trn TCP/IP c phn bit thnh 3 loi: a ch vt l (a ch MAC).

a ch ny c di 48 bit, n c xc nh bi cc thit b giao din mng (NIC). Mi NIC khi sn xut c mt a ch vt l v khi n c s dng kt ni cc trm vo mng th trm s mang a ch vt l ca NIC . Tn Internet (Internet Address).

mi trm khi kt ni vo mng u c cung cp mt tn duy nht. y l a ch 32 bit v thng c biu din bi cc nhm 8 bit, to thnh 4 nhm. Tn trm (host name).

Vic biu din cc a ch Internet d sao vn gy cho ngi s dng kh nh, thay v mi trm c mt a ch Internet th mi trm s c xc nh bng cc tn, c biu din bng cc ch ci v c phn cp mt cch d nh. Chng hn nhu tn: l mt tn trm, trong ta bit c trm ny u, thuc t chc hay quc gia no (vit nam-vn).

Trn thc t, vic s dng tn trm c s dng rng ri. B giao thc gip phn gii t a ch IP sang a ch vt l v ngc li l ARP/RARP

27

(Address Resolution Protocol/Reversed Address Resolution Protocol) v dch v DNS (Domain Name Service) c s dng nh x t a ch IP sang tn host name v ngc li. Cc giao thc ny hot ng theo c ch Server-Client, khi c yu cu chng gi i mt bn tin yu cu Server phn gii a ch tr li bng mt bn tin trong c cha kt qu. Phn lp a ch IP.

Mng Internet dng h thng a ch IP (32 bit) nh v cc my tnh lin kt vi n. C hai cch nh a ch, ph thuc vo cch lin kt ca tng my.

Nu cc my tnh c kt ni trc tip vi mng Internet th trung tm thng tin Internet (Network Information Centre-NIC) s cp cho cc my tnh mt a ch IP.

Nu cc my tnh khng kt ni trc tip vi mng Internet thng qua mt mng cc b th ngi qun tr mng s cp cho cc my tnh mt a ch IP. H thng a ch ny c thit k mm do qua mt s phn lp, c 5 lp a ch IP l: A, B, C, D, E. S khc nhau c bn gia cc lp a ch ny l kh nng t chc cc cu trc con ca n.

0 Netid Hostid

10 Netid Hostid

110 Netid Hostid

1110 Multicast address

11110 Reverved for future use

Class A

Class E

Class D

Class C

Class B

Hnh 1.9. Cu trc phn lp a ch IP a ch lp A

Lp A s dng byte u tin ca 4 byte nh a ch mng. Nh hnh trn, n c nhn ra bi bit u tin trong byte u tin ca a ch c tr gi 0. Ba byte cn li c s dng nh a ch my trong mng.

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C 126 a ch lp A vi s my tnh trong mng l 2563 2 = 16.777.214 my cho mi a ch lp A. a ch lp A thng c cp cho nhng t chc c s lng my tnh ln. Nguyn nhn ch c 126 network trong khi dng 8 bit v bit u tin mang gi tr 0 dng nh ngha lp A. Do vy cn li 7 bit nh t 0 127, tuy nhin ngi ta khng s dng mt a ch cha ton cc con s 1 hoc 0 nn ch cn li 126 mng lp A c s dng. Gi tr byte u tin ca lp A s lun nm trong khong t 1 ti 126, mi mt byte trong 3 byte cn li s c gi tr trong khong 1 n 254.

a ch lp A c dng: Network.host.host.host a ch lp B

Mt a ch lp B c nhn ra bi 2 bit u tin ca byte th nht mang gi tr 10. Lp B s dng 2 byte u tin ca 4 byte nh a ch mng v 2 byte cui nh a ch my trong mng.

C 64*256 2 = 16.128 a ch mng lp B vi 65.534 my cho mi a ch lp B.

a ch lp B c dng: Network.network.host.host Byte u tin ca mt a ch lp B nm trong khong 128 ti 191.

a ch lp C Mt s t chc c quy m nh c th xin cp pht a ch lp C. Mt a

ch lp C c nhn ra vi 3 bit u mng gi tr 110. Mng lp C s dng 3 byte u nh a ch mng v 1 byte cui nh a ch my trong mng. C 2.097.150 a ch lp C, mi a ch lp C c 254 my.

a ch lp C c dng: Network.network.network.host a ch lp C c nhn ra vi byte u tin trong khong 192 ti 223. 1.4. TIU CHUN H.323 a ra nn tng cho vic truyn tn hiu thoi, video, s liu qua mng chuyn mch gi da trn giao thc IP, t chc vin thng quc t ITU-T a ra khuyn ngh v chng giao thc H.323. Bng cch tun theo chun H.323, cc sn phm phn mm v phn cng ca cc nh sn xut khc nhau c th giao tip vi nhau qua mt c s h tng mng chung m khng cn quan tm v s tng hp.

Chng giao thc H.323 ln u tin c cng nhn vo nm 1996, v th h th 2 c cng nhn vo nm 1998. Chun H.323 h tr cho c nhng thit b ng ring r v nhng thit b c gn vo cng ngh my tnh c

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nhn cng nh cc cuc m thoi hi ngh im - im v a im. H.323 bao gm cc chun cho php truyn thng qua mi trng mng LAN (Local NetWork) v mng chuyn mch gi vi s m bo v cht lng dch v (QoS - Quality of Service) ng thi kt hp c vi cc mng khc. Do H.323 l khuyn ngh quan trng cho vic thc hin truyn thng a phng tin (Multimedia), n bao gm cc giao thc v bo hiu H.225, giao thc iu khin H.245, giao thc cho php truyn cc ng dng thi gian thc RTP, cc chun m ho v nn tn hiu nh cc chun G.x.

1.4.1.Phm vi ca H.323

H.323 nh ngha ch yu cho hot ng dng cho u cui H.323 ngang hng. Khuyn ngh cng nh ngha thm mt s phn t H.323 logic. Cc phn t ny bao gm cc Gatekeeper iu khin, gim st v bin i a ch. B iu khin a im MU (Multipoint Controller) v b x l a im MCU (Multipoint Controll Unit) cho hi thoi nhiu bn. Chng khuyn ngh H.323 v phm vi ca n gm:

Audio CodecG.711 G.723 G 729

Video CodecH.261 H.263

RTP

Data interfaceT.120

System Control

H.245 Control

Q.931 Call Setup

RAS Control H.225

ng dng thoi

ng dng video

ng dng s liu

Giao din ngi s dng iu

khin h thng

LAN Interface

Hnh 1.10: Phm vi ng dng ca H.323

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Knh H.245: iu khin truyn thng.

Knh H.225: iu khin cuc gi.

Khuyn ngh H.245: M t c php v nh ngha cc bn tin, tm lc nhng th tc iu khin c chc nng thit lp v gim st qu trnh lin lc a phng tin (data, video, audio) gia hai u cui. Cc bn tin ca H.245 thc hin kim sot hot ng ca khi H.323 bao gm kh nng trao i, ng m knh logic, yu cu ch u tin, iu khin lung, ra lnh, ch th.

Khuyn ngh H.225: M t phng thc kt hp gia thoi, d liu, video v tn hiu iu khin, phng thc m ho v ng gi cho qu trnh truyn ti thng tin gia hai thit b H.323 trn mng truyn s liu gi. H.225.0 cng m t giao thc v nh dng bn tin cho tng i Gateway. Khuyn ngh ny bao gm giao thc RTP/RTCP.

Cc chun nn tn hiu thoi: G.711, G.723, G.728. Cc chun m ho v nn tn hiu video: H.261, H.263 Cc chun cho cc ng dng chia s s liu: T.120

1.4.2. Cc dch v H.323

H thng H.323 gp chung mt s chun, chng hon ton cho php thnh lp v iu khin cc cuc gi im - im (point to point) cng nh cc cuc hi thoi a im. Cc my tnh c nhn v cc thit b khc c th chia s hot ng thc hin trn m thanh, hnh nh, s liu qua tt c cc cu hnh mng. S phi hp nhp nhng vi cc h thng trn cc mng chuyn mch knh c h tr thng qua cc Gateway. H.323 cung cp mt m hnh truyn thng a dng, cht ch vi cc iu khin linh hot v cc kt ni c thit lp gia cc phn.

1.4.3. Cc kiu d liu c nh ngha trong H.323

Cc kiu d liu c nh ngha trong H.323 c chia thnh cc loi sau:

Thoi ( Audio ): l tn hiu thoi c s ho v m ho. gim tc trung bnh ca tn hiu thoi, cc b ly mu tn hiu thoi ch hot ng khi c tn hiu m thanh thc s. Cc khong lng c trit v nn li. Tn hiu thoi i km vi tn hiu iu khin thoi.

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Hnh nh ng (Video): l cc tn hiu hnh nh ng, n i km vi tn hiu iu khin Video.

S liu (Data): bao gm cc tn hiu FAX, ti liu vn bn, nh tnh, file ...

Tn hiu iu khin cuc gi ( Call Control Signals ): c s dng cho cc chc nng iu khin cuc gi nh l thit lp cuc gi, gim st cuc gi, kt thc cuc gi ...

Tn hiu iu khin truyn thng (Communication Control Signals): l cc thng tin iu khin trao i gia cc thnh phn chc nng trong h thng thc hin iu khin truyn thng gia chng nh: trao i kh nng, ng m cc knh logic, cc thng ip iu khin lung v cc chc nng khc.

Tn hiu knh RAS: c s dng thc hin cc chc nng nh vic ng k tham gia vo mt vng H.323, kt np, tho g mt im cui khi vng, thay i bng thng v cc chc nng khc lin quan n chc nng qun l hot ng ca cc im cui trong mt vng H.323.

1.4.4. Cc thnh phn trong H.323

V mt logic, h thng H.323 c cu trc nh hnh:

Gatekeeper

H.323 Over ISDN

PSTN

Telephone H.323 Over Port

Gateway MCU

H.323DNS

Hnh 1.11. Cc thnh phn ca h giao thc H.323

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Thit b u cui H.323: l mt trm cui trong mng LAN, m nhn vic cung cp truyn thng hai chiu theo thi gian thc.

H.323 Gateway: cung cp kh nng truyn thng gia h thng H.323 v cc h thng khng phi H.323 khc, chng hn nh h thng chuyn mch in thoi cng cng (PSTN-Public Switching Network).

Gatekeeper: l mt thnh phn bt buc. N thc hin cc chc nng qun l hot ng ca h thng. Khi c mt Gatekeeper trong h thng, mi thnh phn trong h thng phi thc hin ng k vi Gatekeeper. Tt c cc im u cui H.323 ( Terminal, gateway, MCU ... ) ng k vi Gatekeeper to thnh mt vng H.323 do Gatekeeper qun l.

n v iu khin lin kt a im MCU (Multipoint Control Unit): thc hin chc nng to kt ni a im h tr cc ng dng truyn thng nhiu bn. Thnh phn ny cng l tu chn.

1.5. QU TRNH TRUYN D LIU TRONG TCP/IP. Gi s qu trnh truyn d liu gia hai trm t trm A sang trm

B, qu trnh truyn d liu xy ra nh sau:

Identical Frame

Identical datagram

Identical packet

Identical message

Application

Transport

Internet

Network i f

Transport

Internet

Network i f

Physical Net

Application

Hnh 1.12. Truyn gi tin trong mng TCP/IP

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- u tin, d liu c x l v gi t tng ng dng, tng ny c nhim v t chc d liu theo khun dng v trt t nht nh tng Application my B c th hiu c. Tng Application gi d liu xung tng di theo dng byte ni byte. Cng vi d liu, tng ny cng gi xung cc thng tin iu khin khc gip xc nh a ch ngun v a c ch ca d liu.

- Khi xung n tng Transport (tng TCP), dng d liu s c ng gi thnh cc gi c kch thc khng nht thit bng nhau nhng phi nh hn 64kb theo nh dng ca gi tin TCP, sau d liu c gi xung tng IP.

- Xung n tng IP, gi li c chia nh thnh cc gi tin cho ph hp bi v khng phi mng no cng cho php cc gi tin c ln bt k. Gi tin sau c thm vo phn tiu IP theo nh dng gi IP. D liu sau c chuyn xung tng Datalink. - Khi xung n tng Datalink, gi tin s c gn thm mt tiu v chuyn ti cc giao din vt l i vo mng, lc ny cc cu trc gi tin c gi l cc khung (frame).

Trong khi lu chuyn trn mng, cc gi tin c dn ng bi cc b nh tuyn. Router thc ra l mt module mng ch c hai tng l Network v Datalink. Cc frame ti Router s c tng Datalink lc b header m tng ny thm vo l chuyn ln tng IP. Tng IP sau s da vo thng tin trong phn tiu m quyt nh tuyn ng tip theo cho gi IP. Sau gi IP ny s li c chuyn xung tng di i vo mng. Qu trnh c th tip tc cho n khi d liu n ti my B.

- Khi ti my B, cc gi d liu li c x l theo mt quy trnh ngc li, cc gi tin ln lt c lc b tiu cui cng chuyn ln tng ng dng ca my B di dng mt dng byte ni byte.

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Data

TCP header Data

IP header TCP header Data

Ethernet header

IP header TCP header Data Ethernet trailer Network

Internet

Transport

Application

Hnh 1.13. To khung trong TCP/IP

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CHNG 2

TNG QUAN V VOIP

2.1. TNG QUAN V VOIP.

2.1.1 Gii thiu chung v VOIP

Ngy nay cc dch v vin thng rt phong ph v a dng bao gm cc dch v truyn thng tn ti trong thi gian di v cc dch v mi c pht trin trong thi gian gn y. Tin ca n chnh l s pht trin ca cng ngh in t v cng ngh mng. C th hn, chng ta c th ni rng s a dng ca cc dch v vin thng bt ngun t s pht trin ca cc cng ngh lin quan nh truyn dn, x l tn hiu, chuyn mch v cng ngh phn mm.

Internet ang i tin phong trong vic m ra cc hnh thc mi cho vic lin lc ca con ngi nh e-mail, ni chuyn ti ch (live chat) v news-group. Tuy vy Internet cng c th c s dng cho cc hnh thc lin lc c in nh ngi dng c th gi in thoi bng Internet. Khi gi in thoi qua Internet th ting ni ca ngi gi v ting ni ca ngi nhn s c chia nh thnh cc gi tin. Cc gi tin ny c chuyn i qua mng bng cch s dng giao thc TCP/IP ca Internet. y chnh l cng ngh VOIP (Voice Over Internet Protocol). VOIP l mt c hi tt thit k cc h thng truyn thng Multimedia ton cu c th thay th mng vin thng hin nay v mt chc nng m khng lm tr ngi n di sn cng ngh c tn ti t lu i.

Mt u im quan trng ca VOIP l cc dch v phi thoi, chng hn cc ng dng lu tr hay cc ng dng truyn d liu c th chia s c s h tng vi h thng VOIP. Tht vy, cng mt c s u cui c th s dng cho c truyn thoi v truyn d liu dng cng ngh chuyn mch gi. Cng ngh chuyn mch gi xem cc gi tin ca cc mu thoi cng nh cc gi tin d liu nh nhng n v d liu bnh thgn v truyn chng qua mng Internet, vic cn li cu cc u cui l x l cc lung d liu n ng vi chc nng ca chng. Hn na iu ny gip cho vic trin khai cc dch v gia tng trong thoi mt cch d dng, chng hn ngoi thoi truyn thoi truyn thng,

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h thng c th truyn c hnh nh cho cc cuc hi thoi video, truyn hnh Internet iu ny gip cho vic trin khai VOIP ni ring v cc dch v gia tng trong tng lai ni chung m khng cn u t hn cc ng dng c sn.

VOIP l g? u nm 1995 cng ty VOCALTEC a ra th trng sn phm phn

mm thc hin cuc thoi qua internet u tin trn th gii. Sau c nhiu cng ty tham gia vo lnh vc ny. Thng 3 nm 1996, VOLCALTEC kt hp vi DIALOGIC tung ra th trng sn phm kt ni mng PSTN v Internet. Hip hi cc nh sn xut thoi qua mng my tnh sm ra i v thc hin chun ho dch v thoi qua mng internet. Vic truyn thoi qua internet gy c ch ln trong nhng nm qua v dn c ng dng rng ri trong thc t.

VoIP l cng ngh truyn ti cc cuc lin lc thoi bng cch s dng giao thc Internet (Internet protocol - IP). in thoi truyn thng c thc hin da trn cng ngh chuyn mch knh m cho php gii quyt vn thi gian thc. Giao thc IP da trn cng ngh chuyn mch gi m trc y ch c dng truyn d liu hoc cc ng dng mng internet. Do vic truyn thoi da trn giao thc IP l gii php truyn thoi da trn cng ngh chuyn mch gi, iu ny mang li nhiu li ch cho ngi s dng cng nh cho cc nh cung cp dch v. thc hin VoIP ngi ta phi s dng cc k thut cho php thc hin cc cuc gi vi thi gian thc, l cc giao thc Real Time Stream Protocol (RTSP), Secsion initiation Protocol (SIP).

2.1.2 Hot ng ca VOIP

VOIP c th c thc hin bng hai cch:

Cch th nht: L s dng my tnh cng vi phn cng v phn mm c bit gi

in.

Cch th hai: Ngi ta thng gi l in thoi Internet (Internet Telephony) th ngi

gi in thoi bng in thoi thng thng, ngoi tr vic cuc in m ny c truyn qua Internet ch khng phi l qua dch v in thoi thng

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thng. iu ny s gip gim chi ph ng k, c bit l vi cc cuc in thoi ng di.

Tnh cch mng ca vic s dng PC gi in thoi qua Internet chnh l gi c. Nu hai ngi s dng u c my tnh kt ni vo Internet th thc t h khng phi tr tin cho cuc gi m ch phi tr tin cho dch v truy cp Internet nh khi ngi dng s dng cc dch v khc ca Internet nh tm kim trn trang Web hay s dng email, cuc gi c th thc hin n bt k ni no trn th gii m ch cn truy cp Internet. Tuy nhin, ch c th thc hin c cuc gi t hai ngi u c PC, do vy ngi s dng vn cha th thay th in thoi truyn thng bng dch v ny. Hn na, hin nay cc chng trnh cho php hai ngi dng ni chuyn thng qua PC thng khng h tr nhau, do thng yu cu hai ngi s dng cng mt chng trnh.

C rt nhiu phn mm phc v cho vic gi in thoi qua Internet. Tuy vy khng c chng trnh phn mm no cho php dng in thoi gi. Thay vo ngi dng s ni vo micro c gn vi my tnh v nghe qua loa v card m thanh. Tuy mi phn mm ca cc hng khc nhau c cch thc hot ng khc nhau, nhng nhn chung chng u c mt cch chung chng ta thc hin cuc gi. thc hin cuc gi, ta phi bit a ch Internet ca ngi b gi. Thng thng c cung cp di dng cc tn gi hay cc bit danh (nick name), sau khi kt ni c vi ngi b gi, cuc gi c bt u.

Mt chng trnh min ph thng dng cho php cc ngi s dng c th ni chuyn qua Internet bng PC l chc nng Voice Chat ca phn mm Yahoo Messenger.

Nu ni chuyn qua Internet m s dng in thoi bnh thng th cch thc hot ng s khc i. Ngi dng s thc hin cuc gi nh bnh thng. Tuy vy, sau khi ngi gi ni, ting ni s c s ho v chuyn thnh cc gi tin IP. Sau chng c chuyn i s dng cng mt cng ngh TCP/IP ging nh l chuyn cc gi tin bnh thng trn Internet. u kia ca ng dy, d liu ting ni c s ho li c chuyn i thnh tn hiu tng t v c pht ra loa ca ngi nhn.

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Gi in thoi qua Internet dng PC. Mc d mi phn mn c mt giao din ring, c cc chc nng cng nh

cch thc gi khc nhau, nhng phn ny s tp chung gii thiu mt cch tng qut nht cch thc ca cc cuc gi qua Internet, xem xt cch cc cuc gi din ra v so snh v cch th hai l s dng in thoi gi qua Internet.

1. Bc u tin l ngi s dng phi kt ni vo Internet nh mt nh cung cp dch v kt ni Internet, sau s dng mt chng trnh gi in qua Internet, khi c kt ni vi mt danh sch cc ngi c th nhn cuc gi, l cc ngi cng ang kt ni vo mng Internet (online) v s dng cng mt phn mm hi thoi.

2. Ngi gi chn mt ngi ang online bng vic la chn tn (thng l bit danh-nick name) ca ngi trong danh sch. Khi , tn ca ngi c chn s c phn gii a ch Internet ca ngi s dng dch v DNS (Domain Name Service).

3. Khi ngi gi bt u thc hin cuc gi, DNS phi thc hin phn gii a ch t tn ca ngi gi v ngi b gi ra a ch IP, sau cuc gi s c thit lp, my tnh ca ngi gi s bo cho ngi nhn bit l c ngi cn lin lc. Nu ngi b gi chp nhn thc hin cuc gi qua my tnh ca mnh th hai ngi c th bt u ni chuyn qua micro v nghe qua loa hay headphone.

4. Khi ngi dng ni vo microphone, phn mm s chuyn ging ni thnh cc tp tin cha d liu nh phn my tnh c th c c. N cng nn d liu ting tp nh hn v c th truyn qua Internet nhanh hn v thc t cc tp tin m thanh c dung lng rt ln.

5. Phn mm cng thc hin gim st v iu khin cuc gi. Nu l ni vi Internet tc cao th n s to tp tin m thanh mang cht lng tt hn, nu l ni kt vi Internet tc thp th tp tin m thanh s c cht lng km hn v c th s phi mt nhiu thi gian truyn cc tp tin qua mng Internet.

6. Phn mm s chia cc tp tin mang ging ni thnh cc gi tin gi qua Internet s dng giao thc TCP/IP. Qu trnh ny kh phc tp v bao gm nhiu cng on.

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7. Cc gi tin c gi ti ngi nhn, y phn mm my tnh ca ngi nhn s thc hin theo chiu ngc li, cc gi tin s c gin nn v c pht ra loa ca ngi nhn. i khi cc gi tin c gi ti c trc trc tc l mt vi gi tin c th b tht lc, n tr hay trng lp, khi phn mm phi quyt nh xy dng li cc gi tin cn c vo cc gi tin trc m bo cht lng m thanh c th c khi phc mt cch chnh xc nht c th. Khi cc gi tin b mt n tr th phn mm phi quyt nh b cc gi tin ny i.

Cch gi in thoi qua Internet s dng in thoi truyn thng Khi ngi dng s dng my in thoi thng thng thc hin cuc

gi th c trong cu hnh mng v phng thc hot ng cng phi khc i v c mt phn cuc gi s i qua mng PSTN nn cn c mt s thit b khc lm cu ni gia mng PSTN v mng Internet. Cu hnh v chc nng ca chng s c nghin cu k phn sau. Qu trnh thc hin mt cuc gi:

1. Khi cuc gi c thc hin, u tin tn hiu s i qua mng in thoi chuyn mch cng cng (PSTN-Public Switching Telephone Network) ging nh cc cuc in thoi bnh thng. n y cha c s khc nhau no gia chng.

2. Tn hiu sau c gi n mt cng thoi IP (IP Voice Gateway) c bit cng ny l mt my ch cng vi phn mm ca my ch. N c th c t ti chi nhnh ca cng ty in thoi hoc cng c th c t ti mt a im khc. Cng ni s chuyn i tn hiu ting ni thnh tn hiu s ri nn n li. Nu khng nn, tp tin ca cuc hi thoi c th rt ln c th truyn kp thi qua mng Internet.

3. Cng ni s chia nh tn hiu ting ni c s ho v nn thnh cc gi tin IP. Cc gi tin ny sau cng c ng gi theo cc khun dng nht nh v sau c gi i qua Internet ging nh cc gi tin IP khc bng cc giao thc TCP/IP.

4. Cc gi tin c chuyn ti cng thoi gn u kia nht ca cuc hi thoi. Cng ni ting ni IP ti ni nhn s thu cc gi tin, gii nn, chuyn chng tr li dng ban u v sau gi chng qua mng PSTN n ngi u kia ca cuc ni chuyn.

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5. Tn hiu thoi s truyn qua mng PSTN ging nh cc cuc gi in thoi khc. Ngi nhn cuc in m s c bo bng chung in thoi nh bnh thng. Khi ngi nhn ni qua in thoi, tn hiu thoi s c gi n cng thoi IP gn nht thng qua mng PSTN. Sau tn hiu thoi s c x l nh u kia ca cuc gi.

Nh vy, cc cuc gi in qua Internet dng in thoi thng thng th gn nh trong sut vi ngi s dng. Phn thay i cu trc mng, thit lp dch v l do nh cung cp dch v (chng hn nh dch v gi 171 ca Tng cng ty Vin thng Qun i Viettel) thc hin. Thng thng ngi s dng phi quay mt m s c bit no trc cch gi thng thng (171 hay 178). M s c bit ny s dn ng cuc gi n vi dch v ca nh cung cp. Hn na, ch mt phn t n hiu ca cuc gi i qua mng Internet, phn cn li t in thoi ca ngi s dng n cng thoi IP gn nht phi c truyn qua mng PSTN, do gi ca cuc gi gim i ng k nhng khng th r nh cch thc gi bng PC.

Bn cnh hai cch gi trn, mt s nh cung cp dch v cn cho php ngi s dng gi n bt k my in thoi no (c nh, di ng .) bt k u trn th gii thng qua trang Web ca Cng ty, cch thc hot ng rt n gin, ngi gi ch cn ng k mt ti khon v nh cung cp dch v s cung cp cc th np tin (prepaid card) i vi hnh thc tr trc hay thanh ton qua th tn dng vi hnh thc tr tin sau. Nu l tr trc, ngi gi np tin cho ti khon ca mnh nh cch s dng cho in thoi di ng tr tin trc, sau tu vo thi gian s dng m ti khon ca ngi gi s b tr tin dn. Gi cc cho cc cuc gi quc t tu vo cc m s ca nc gi n nhng thng r hn nhiu so vi hnh thc in thoi truyn thng.

Hin nay dch v ny hot ng rt si ng, c rt nhiu nh cung cp vi gi thnh rt r. Di gc k thut th y l mt thnh tu ca s kt hp cng ngh thng tin v mng Vin thng.

2.1.3. So snh gia VOIP v mng chuyn mch in thoi cng cng (PSTN)

VOIP c mt s kha cnh khc vi mng chuyn mch in thoi (PSTN-Private Switching Telephone Network) c v thut ng, cu trc v cc giao thc. S khc nhau ny nh hng n thit k ca cc dch v VOIP. V

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c bn, VOIP da vo m hnh t u cui n u cui (end to end) cho s truyn cc mu thoi. Cc giao thc bo hiu gia cc h thng u cui bao hm trong cuc gi . Cc b nh tuyn xem cc gi tin bo hiu ny nh nhng gi tin ca cc mu thoi thng thng cng nh bt k gi s liu no khc. Tuy nhin, VOIP c th li dng cc b nh tuyn bo hiu hoc Proxy h tr chc nng nh xc nh v tr ngi s dng, trong trng hp ny cc Proxy ch dng cho nh tuyn ca cc thng ip bo hiu khi ng. Cc gi tin tip theo c th trao i trc tip ti cc thit b.

C s h tng trong PSTN c s dng khng hiu qu, khi mt cuc gi c yu cu th h thng phi thc hin kt ni mt ng truyn vt l thc s thng qua cc knh trung k. Knh ny s b cuc gi duy nht chim dng t khi bt u cuc gi cho n khi kt thc cuc gi. Thc t cuc gi in thoi l khng lin tc, ngha l c cc khong lng (chim kh nhiu thi gian ca cuc gi) gia cc li thoi, trong khong thi gian knh truyn thc s b lng ph. S lng ph ny l kh ln i vi cc cuc gi ng di v ti nguyn b chim dng ln. Trong khi , VOIP da trn cng ngh chuyn mch gi, khng c s thit lp knh ring, ng truyn ch thc s b chim dng khi c d liu truyn, cc khong lng gia cc gi tin c th s dng tuyn truyn thng cho vic truyn gi ca cc cuc thoi khc.

Bn thn Internet l a dch v v cc dch v l c lp. N cung cp cc cng c truyn ti cc gi tin end-to-end cho cc dch v c trin khai ti cc h thng u cui qua cc giao thc v phn mm tng cao hn. iu ny lm cho n dn u trong kh nng mm do v m rng. Trong Internet, giao thc RSVP c s dng gi trc ti nguyn cho mt kt ni da vo nhu cu bng thng cn thit cuc truyn din ra dc theo cc Router trn tuyn. VOIP c lp vi s gi trc ti nguyn ny, n c th din ra trc hay sau khi c dng s liu thc t c truyn. Do , VOIP c th c dng cho mt cuc gi m khng cn gi trc ti nguyn trong mng khi kh nng ca mng l va .

Kh nng bo hiu trong mng PSTN ca cc thit b u cui l c hn, cc dch v gia tng trong mng PSTN l khng th hoc rt hn ch nh: nhn din cc im u cui, nhn din dch v, nhn din ai tr tin cuc gi v nh cung cp truyn ti. PSTN cng tri buc ngun gc cuc gi vi vic thanh ton cc ph. Ngi khi ng cuc gi bt buc l ngi tr tin cc cho

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cuc gi, ngoi tr c s tho thun nhn cng cho vic ny, chnh l vic ng k qua nhn vin khai thc v cuc gi ngi b gi tr tin. Trong khi , VOIP da trn nn tng Internet c th cung cp cc dch v ny mt cch d dng, thun tin v t ng.

VOIP cung cp dch v, chng hn nh phn bit chung theo nhm gi n, nhn din ngi gi n, nhn din u cui mt cch phong ph. Hn na, do tnh m, a dch v ca Internet cng cho php pht trin trong tng lai cc dch v m rng ca dch v thoi truyn thng s c cung cp bi cc nh cung cp khc nhau. iu ny to nn s thun tin cho u vo cng nh s cnh tranh cao hn trn th trng ca cc dch v Internet nh l VOIP.

2.2. CC C TRNG CA VOIP

Cht lng dch v c th iu chnh c. Cht lng ca VOIP ph thuc vo nhiu yu t, quan trng nht l bng

thng. Do khng c s bt buc no v mt thng lng gia cc thit b u cui m ch c cc chun tu vo bng thng c th ca mnh, bn thn cc u cui c th t iu chnh h s nn v do iu chnh c cht lng cuc gi.

S an ton. VOIP c xy dng trn nn tng Internet vn khng an ton, do s

dn n kh nng cc thng tin c th b nh cp khi cc gi tin b thu lm hoc nh tuyn sai a ch mt cch c khi chng truyn trn mng. Cc giao thc SIP (Session Ineitiation Protocol giao thc khi u phin) c th thnh mt m v xc nhn cc thng ip bo hiu u cui. RTP (Real Time Protocol) h tr m thnh mt m ca phng thc truyn thng trn ton tuyn c m ho thnh mt m m bo truyn thng an ton.

Nhn din ngi s dng. Cc dch v in thoi truyn thng n gin v ISSDN (Intergrated

Services Digital Network - mng s dch v tch hp) a ra kh nng nhn din ngi s dng cn hn ch. Giao thc trao i thi gian thc RTP dng cho VOIP h tr d dng ch ra ngi ni trong c a pht p (multicast) v cu hnh ghp ni khc, thm ch c th cung cp nhiu thng tin hn v ngi gi.

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Giao din ngi s dng. Cc h thng VOIP c nhiu kh nng giao tip ngi - my phong ph

hn. Giao din cung cp bi VOIP thn thin hn, c th thay i v thm nhiu tnh nng tr gip hn.

Lin kt thoi-my tnh. VOIP thc hin cc cuc gi t in thoi - my tnh hay my tnh - my

tnh mt cch d dng, hin nay c nhiu sn phm h tr cc kt ni ny.

c trng c mt nhiu ni. VOIP khng chu nh hng bi c s h tng mng c nh, mng Internet

c s dng rng ri s thc y s c mt ca cc thit b VOIP nhiu ni bng nhiu phng thc kt ni.

a phng tin. VOIP khng ch n thun l thoi, cc dch v khc cng c th i km

vi VOIP nh mt phn ca cuc gi, chng hn nh hi ngh video, cc ng dng a phng tin.

S dng bng thng hiu qu hn. VOIP khng yu cu cc knh truyn thng dnh ring cho n trong sut

cuc m thoi m trn thc t thi gian hi thoi thc s ch chim 40% ton b cuc hi thoi. Do cc gi tin c th tn dng mng mt cch tuyt i n ch bng cc con ng khc nhau. Sau knh truyn c th c s dng bi mt cuc gi khc cng lc .

S tch bit thoi v iu khin lung. Trong in thoi truyn thng, lung bo hiu dc tch di v truyn ti

trn mt mng tch bit, phi duyt qua tt c cc chuyn mch trung gian thit lp mch. Trong khi , gi gi trong Internet khng yu cu thit lp cuc gi. iu khin trong VOIP c thc hin ngay trn bn thn cuc gi. Ngha l n khng yu cu mt h thng bo hiu v iu khin ring bit.

Li ch cho nh truyn ti. Trong VOIP, cc b m ho ch thc s hot ng khi c lung tn hiu, do

cc khong lng trong cuc hi thoi c th c nn v trit m bo thng tin truyn trn mng l t nht, s dng trit hiu sut ca knh truyn

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vt l. PSTN thng trit khong lng qua cc lin kt ton cu, VOIP thc hin trit khong lng ti cc u cui thu nh ti a bng thng cn thit. Cc mng chuyn mch gi cng thch hp hn cho vic hp knh. Tt nhin s nn tn hiu thoi nhm t c tc bt thp, ko theo cht lng thoi b gim i. Ngy nay c cc b m ho tc bt thp m li m bo cht lng rt tt nh s pht trin ca cc b vi x l tc cc cao.

2.3. HN CH CA VIC S DNG IN THOI VOIP

K thut phc tp: Truyn tn hiu theo thi gian thc trn mng chuyn mch gi l rt kh

thc hin do mt gi trong mng l khng th trnh c v tr khng c nh ca cc gi thng tin khi truyn trn mng. c c mt dch v thoi chp nhn c, cn thit phi c mt k thut nn tn hiu t c nhng yu cu kht khe: t s nn ln ( gim c tc bit xung), c kh nng suy on v to li thng tin ca cc gi b tht lc... Tc x l ca cc b Codec (Coder and Decoder) phi nhanh khng lm cuc m thoi b gin on. ng thi c s h tng ca mng cng cn c nng cp ln cc cng ngh mi nh Frame Relay, ATM,... c tc cao hn hoc phi c mt c ch thc hin chc nng QoS (Quality of Service). Tt c cc iu ny lm cho k thut thc hin in thoi IP tr nn phc tp v khng th thc hin c trong nhng nm trc y Vn bo mt (security):

Mng Internet l mt mng c tnh rng khp v hn hp (hetorogenous network). Trong c rt nhiu loi my tnh khc nhau, cc dch v khc nhau cng s dng chung mt c s h tng. Do vy khng c g m bo rng thng tin lin quan n c nhn cng nh s lin lc truy nhp s dng dch v ca ngi dng c gi b mt.

Nh vy, in thoi IP chng t n l mt loi hnh dch v mi rt c tim nng. Trong tng lai, in thoi IP s cung cp cc dch v hin c ca in thoi trong mng PSTN v cc dch v mi ca ring n nhm em li li ch cho ng o ngi dng. Tuy nhin, in thoi IP vi t cch l mt dch v s khng tr nn hp dn hn PSTN ch v n chy trn mng IP. Khch hng ch chp nhn loi dch v ny nu nh n a ra c mt chi ph thp v nhng tnh nng vt tri hn so vi dch v in thoi hin ti.

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2.4. NG DNG CA VOIP Dch v thoi qua Internet:

in thoi Internet khng cn ch l cng ngh cho gii s dng my tnh m cho c ngi s dng in thoi quay vo gateway. Dch v ny c mt s nh khai thc ln cung cp v cht lng thoi khng thua km cht lng ca mng thoi thng thng, c bit l trn cc tuyn quc t. Mc d vn cn mt s vn v s tng thch ca cc gateway, cc vn ny s sm c gii quyt khi tiu chun H.323 ca ITU c s dng rng ri.

Sut t khi cc my tnh bt u kt ni vi nhau, vn cc mng tch hp lun l mi quan tm ca mi ngi. Mng my tnh pht trin bn cnh mng in thoi. Cc mng my tnh v mng in thoi song song tn ti ngay trong cng mt c cu, gia cc c cu khc nhau v trong mng rng WAN. Cng ngh thoi IP khng ngay lp tc e do n mng in thoi ton cu m n s dn thay th thoi chuyn mch knh truyn thng. Sau y l mt vi ng dng tiu biu ca dch v thoi Internet. Thoi thng minh.

H thng in thoi ngy cng tr nn hu hiu: r, ph bin, d s dng, c ng. Tuy nhin n ch c 12 phm iu khin. Trong nhng nm gn y, ngi ta c gng to ra thoi thng minh, u tin l cc thoi bn, sau l n cc server. Nhng mi c gng u tht bi do tn ti cc h thng c sn.

Internet s thay i iu ny. K t khi Internet ph khp ton cu, n c s dng tng thm tnh thng minh cho mng in thoi ton cu. Gia mng my tnh v mng in thoi tn ti mt mi lin h. Internet cung cp cch gim st v iu khin cc cuc thoi mt cch tin li hn. Chng ta c th thy c kh nng kim sot v iu khin cc cuc thoi thng qua mng Internet.

Dch v tnh cc cho b gi. Thoi qua Internet gip nh khai thc c kh nng cung cp dch v tnh

cc cho b gi n cc khch hng nc ngoi cng ging nh khch hng trong nc. thc hin c iu ny, khch hng ch cn PC vi h iu hnh Windows9x, a ch kt ni Internet (tc 28,8Kbps hoc nhanh hn), v chng trnh phn mm chuyn i, chng hn nh Quicknet's Technologies Internet PhoneJACK.

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Thay v gi qua mng in thoi truyn thng, khch hng c th gi qua Internet bng vic s dng chng trnh phn mm chng hn nh Internet Phone ca Vocaltec hoc Netmeeting ca Microsoft. Vi cc chng trnh phn mm ny, khch hng c th gi cng ging nh vic h gi qua mng PSTN.

Bng vic s dng chng trnh chng hn Internet PhoneJACK. Ngi gi c th nh tuyn cc cuc gi ny ti cc nh vn hnh, ti cc dch v t ng tr li, ti cc ACD. Trong thc t, h thng in thoi qua Internet v h thng in thoi truyn thng l hon ton nh nhau. Dch v Callback Web

"WorldWide Web" lm cuc cch mng trong cch giao dch vi khch hng ca cc doanh nghip. Vi tt c cc tim nng ca web, in thoi vn l mt phng tin kinh doanh quan trng trong nhiu nc. in thoi web hay "bm s" (click to dial) cho php cc nh doanh nghip c th a thm cc phm bm ln trang web kt ni ti h thng in thoi ca h. Dch v bm s l cch d nht v an ton nht a thm cc knh trc tip t trang web ca bn vo h thng in thoi.

Dch v Fax qua IP Nu gi nhiu fax t PC, c bit l gi ra nc ngoi th vic s dng

dch v Internet faxing s gip tit kim c tin v c knh thoi. Dch v ny s chuyn trc tip t PC qua kt ni Internet.

Khi s dng dch v thoi v fax qua Internet, c hai vn c bn: - Nhng ngi s dng dch v thoi qua Internet cn c chng trnh

phn mm chng hn Quicknet's Internet PhoneJACK. Cu hnh ny cung cp cho ngi s dng kh nng s dng thoi qua Internet thay cho s dng in thoi bn truyn thng.

- Kt ni mt gateway thoi qua Internet vi h thng in thoi hin hnh. Cu hnh ny cung cp dch v thoi qua Internet ging nh vic m rng h thng in thoi hin hnh. Dch v Call center

Gateway call center vi cng ngh thoi qua Internet cho php cc nh kim duyt trang Web vi cc PC trang b multimedia kt ni c vi b phn phi cc cuc gi t ng (ACD). Mt u im ca thoi IP l kh nng kt hp c thoi v d liu trn cng mt knh.

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2.5. CC THNH PHN TRONG MNG VOIP.

2.5.1. Cc thit b u cui

Cc phn t u cui trong h thng VOIP c th c chia lm hai loi: cc phn t nm trong phm vi khuyn ngh H.323 v cc phn t nm ngoi phm vi H.323

Cc phn t nm ngoi phm vi H.323 Thit b vo/ra video: bao gm camera, mn hnh v cc thit b

iu khin x l nn tn hiu video v thc hin phn chia cc khung hnh.

Thit b vo/ra Audio: bao gm micro, loa, my in thoi, cc thit b trn ghp knh Audio v thit b kh ting vng.

Cc ng dng ngi dng: S dng giao din T.120 hoc cc dch v d liu khc trn cc knh ring dnh cho s liu.

Giao din mng LAN: cung cp giao din vi cc mng LAN khc, cc thit b ny h tr bo hiu v mc tn hiu tu theo cc chun quc gia v quc t.

Cc giao tip iu khin h thng cho ngi s dng: cung cp giao din cho ngi s dng iu khin h thng v khai thc dch v c cung cp.

Cc phn t nm trong khuyn ngh H.323 B m ho v gii m video: c nhim v m ho video ni

ngun gi i v gii m tn hiu video nhn c ni ch hin th trn mn hnh. B m ho v gii m video l phn t tu chn trong thit b tu chn trong thit b u cui H.323. Mt thit b H.323 khi c chc nng giao tip video th n s c b m ho v gii m video. Thng qua giao thc iu khin H.245, mt thit b u cui H.323 c th cng lc truyn i nhiu hn mt knh video, n cng c th nhn cng mt lc nhiu knh video. Trong trng hp ny, thit b u cui phi c chc nng ca b trn hoc b chuyn mch tn hiu video.

B m ho v gii m Audio: cc thit b H.323 bt buc phi c b m ho v gii m audio theo chun G.711, kh nng truyn v

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nhn tn hiu m ho PCM theo lut A hay lut . Cc thit b u cui cng c th c kh nng m ho v gii m tn hiu audio theo cc chun nn G.722, G.728, G.729, MPEG Audio v G.723. Khi m ho v gii m tn hiu Audio ca thit b u cui H.323 c kh nng hot ng mt cch khng i xng trn cng mt knh ting, ngha l n c th gi tn hiu m ho theo G.711 v nhn tn hiu m ho theo chun G.728 nu nh n c kh nng m ho v gii m vi c hai dng tn hiu ny. Thit b u cui H.323 c th c kh nng truyn cng nh nhn nhiu knh Audio cng lc, c s dng khi hi thoi a im. Lc n phi c kh nng trn m thanh, n s s dng kh nng x l ng thi ca H.245 xc nh c bao nhiu lung tn hiu m thanh c th c gii m ng thi.

B m nhn tn hiu: tt c cc gi tin khi nhn c u c lu vo b nh m nhn iu khin tr trn ng nhn tn hiu. N thc hin chc nng cng thm tr vo cc gi tin tu theo tr ca chng t c s ng b ho v b Jitter ca cc gi tin n hay iu khin s ng b ca cc lung tn hiu vi nhau.

Khi iu khin h thng: mi thit b u cui H.323 u c mt khi iu khin h thng. Khi iu khin h thng c 3 chc nng iu khin tn ti c lp vi nhau, l:

iu khin theo H.245 ( iu khin truyn thng ) Chc nng bo hiu RAS. Chc nng iu khin H.225.0 (iu khin cuc gi).

2.5.2. Gateway

Gateway l cu ni gia cc mng H.323 v phn cn li ca th gii. S khc nhau ny c v mt phng tin truyn ti, v giao thc hot ng ca cc thit b hay phng thc m ho c s dng. Do , mt Gateway nh x tt c cc giao thc iu khin cuc gi khng theo H.225.0. Gateway cn thc hin nhim v gim st cuc gi vo/ra, qun l trng thi cuc gi cng nh vic kt thc cuc gi cng nh vic kt thc cuc gi nh th no.

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Hai thit b H.323 cng nm trong mt mng LAN c th kt ni trc tip vi nhau m khng cn n Gateway. Nhng khi hai thit b H.323 mun kt ni vi nhau thng qua mng chuyn mch cng cng PSTN, chng cn c kt ni thng qua hai Gateway hai u ca cc im cui.

2.5.3. Gatekeeper

Mt Gatekeeper qun l tt c hot ng ca c mt vng gm cc thit b u cui, trm trung chuyn, khi iu khin a im MCU. Theo ITU-T, Gatekeeper l mt thc th ty chn trong mi trng H323. Tuy vy trn thc tin th n l mt thc th cn thit ca mng. Khi c mt trong mng th chc nng ca Gatekeeper c chia thnh hai nhm: nhm chc nng bt buc v nhm chc nng khng bt buc.

Cc chc nng bt buc ca Gatekeeper: - Chc nng bin dch a ch (Address Translation): cung cp kh nng

chuyn i t mt a ch hnh thc (dng tn gi) ca cc thit b u cui v Gateway sang a ch truyn ti trong mng (a ch IP). Chuyn i ny da trn bng i chiu a ch c cp nht thng xuyn bng bn tin ng k dch v ca cc u cui.

- iu khin truy nhp: Gatekeeper s chp nhn mt truy nhp mng LAN bng cch s dng cc bn tin H.225.0 l ARQ/ACF/ARJ (AdmissionRequest, Admission Confirm,Admission Reject). Vic iu khin ny da trn rng bng tn v ng k dch v hoc cc thng s khc do nh sn xut quy nh. y cng c th l mt th tc rng c ngha l chp nhn mi yu cu truy nhp ca cc thit b u cui.

- iu khin rng bng tn: Gatekeeper h tr vic trao i cc bn tin H.225.0 l BRQ/BCF/BRJ (Bandwith Request/Bandwith Confirm/Bandwith Reject) iu khin rng bng tn ca mt cuc gi. y cng c th l mt th tc rng c ngha l n chp nhn mi yu cu v s thay i rng bng tn.

- iu khin vng: y ch vng c trng cho tp hp tt c cc phn t H.323 gm thit b u cui, Gateway, MCU c ng k hot ng vi Gatekeeper.

Cc chc nng khc ca Gatekeeper: - iu khin bo hiu cuc gi. - Hn ch truy nhp.

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- Gim st rng bng tn. - Gim st cuc gi. Nh cc dch v ca Gatekeeper cho php thc hin cc chc nng

nh tnh cc cho bng thng chim dng, tng thch hot ng gia cc thit b quay s PBX v cc thit b u cui IP-base, cc c im nh tuyn v phn phi gi t ng cho cc trung tm cuc gi a phng tin. 2.5.4. Khi iu khin v a x l a im (MCU Multipoint Control Unit)

Mt MCU cung cp kh nng iu khin v qun l cc cuc hi thoi a phng tin nhiu thnh phn tham gia. N phi hp vi tt c cc phng tin ca cc thnh vin tham gia v c th cung cp cc tnh nng nh trn m thanh, la chn hnh nh cho cc im cui khng th thc hin iu ny mt cch cc b ging nh chuyn i m ca cc dng phng thc truyn thng khc nhau ni gia cc thit b u cui khng tng thch khc. Hn na, mt MCU c th cung cp tnh nng iu khin vi vai tr ca ngi iu khin v kh nng phn cng hi ngh trong cc cuc hi thoi a im, n cng qun l vic vo/ra ca cc thnh vin tham gia hi thoi. Trong mng in thoi, mt vi tng i ring (PBX - Private Branch Exchange) h tr cc chc nng cu ni m thanh tng t nh mt MCU. MCU bt buc phi c mt b iu khin a im MC v c hoc khng c mt b x l a im MP. MC v MP l cc thnh phn ca MCU nhng c th chng khng tn ti trong mt thit b c lp m c phn tn trong cc thit b khc. V d mt Gateway c th mang trong n mt MC v mt vi MP thc hin kt ni n nhiu thit b cui, mt thit b cui c th mang mt hay mt vi MC c th thc hin nhiu cuc gi cng lc. 2.5.5. Cc Proxy

Mt Proxy H.323 l thnh phn th nm c th c k n. N c th trong sut vi hot ng ca giao thc H.323. N khng bao ph r rng trong khuyn ngh ca ITU-T, n thc hin vic iu khin lm sao cc cuc hi ngh cc cuc hi thoi H.323 c th i qua tng la (firewall). Mt Proxy H.323 ng vai tr tng t nh cc Proxy khc, chng thc hin vic kim sot lung d liu v ch cho cc dng d liu hp php i qua firewall. N cng thc hin vic xem xt ch ra ngi s dng no c th khi to v nhn cc cuc gi H.323, nhng ch n no thch hp v mt ngi dng ring bit no c php dng video ...

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CH NG 3 B O HI U V X L CU C G I TRONG VOIP

3.1. GII THIU CHUNG. IP l mt cng ngh kt ni theo kiu khng nh hng. Vic truyn thng qua mng IP khng yu cu cc knh bo hiu vt l ring bit, thm ch c th khng yu cu c tn hiu thit lp kt ni. IP c thit k li b cc nt mng tham gia vo trong cuc truyn. Chng hn trong dch v th in t email, khng cn c mt thit lp no gia cc im u cui trong mng IP, hay nh ng dng duyt Web, c mt kt ni c thit lp t cc im u v im cui trong mng, tuy nhin n khng yu cu cc router dc ng truyn phi tham gia vo cuc ni. Kt ni ch tn ti nh l trng thi bo hiu ti hai im u cui. Cc dch v thoi c mt s im khc bit so vi cc dch v thng thng, l:

Dng tn hiu c s lin h cht ch vi nhau. Chng yu cu mt cp cht lng (QoS) dch v nht nh. Trong mt s trng hp, ngi vn hnh h thng cn tnh cc theo s

cuc gi m khng theo thi gian ca cuc gi. Ngi s dng cui c th c mt a ch Internet ng, iu ny l do

ngi s dng cui c th di ng, khi di chuyn vo cc vng ca cc subnet khc, hay do trn thc t mt s nh cung cp dch v Internet (ISP) s cung cp cho ngi s dng cc a ch khc nhau.

Cc im khc bit ny gy ra nhiu vn cn gii quyt trong vic ng b ngi s dng trong cc ng dng thoi, trong yu cu c hai pha u phi c nhn dng khi cuc truyn din ra, n khc vi cc dch v khc, chng hn nh email, trong khng yu cu ngi nhn v ngi gi phi ng thi hin din khi gi.

Mt giao thc bo hiu VOIP cn phi m bo y cc chc nng sau:

Chuyn i a ch: Thng thng ngi s dng ch c tn hay a ch email ca ngi nh hi thoi, do t cc tn , giao thc phi c nhim v chuyn i sang thnh a ch IP c th thit lp hi thoi ti. S chuyn i ny i khi khng n gin ch l tra bng nh danh

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trong my ch v cc thng tin v ngi s dng lun thay i theo ngy, theo gi. Do bng chuyn i ny phi lun c cp nht thng xuyn.

c trng dn xp: Cho php mt nhm h thng u cui c th tho thun trn phng thc truyn thng no trao i v cc tham s ca n c lin quan, chng hn nh l cc thng s xc nh phng thc m ho. iu ny l cn thit v cc b m ho v kiu ca phng thc truyn thng khng cn thit ging nhau i vi cc bn tham gia hi thoi. Hn na, mt phn mm m ho/gii m c th h tr nhiu phng thc truyn thng cng nh cc phng php m ho khc nhau. c trng dn xp cng bao hm trong vic qun l cc thnh vin tham gia hi thoi, mt ngi tham gia hi thoi c th mi hay ngt kt ni vi mt ngi khc cng tham gia hi thoi.

c trng thay i: Trong cuc hi thoi, cc thnh vin tham gia hi thoi c th yu cu cc thnh vin khc tng hay gim cc chc nng do tng hay gim cc thnh vin tham gia hi thoi. Do cc giao thc bo hiu cng phi c kh nng thay i cc cu hnh ca cc phin hi thoi ang din ra.

Chng ny s cp n cc th tc bo hiu gia cc u cui VOIP trong cc trng hp vi s c mt v khng c mt ca Gatekeeper. y l ni dung phn bo hiu trong khuyn ngh H.245 ca ITU.

3.2. NH TUYN KNH IU KHIN V BO HIU CUC GI. C 3 knh bo hiu tn ti c lp nhau lin quan n bo hiu v x l cuc gi: knh iu khin H.245, knh bo hiu cuc gi v knh bo hiu RAS. Trong mng khng c Gatekeeper, cc bn tin bo hiu cuc gi c truyn trc tip gia hai u cui ch gi v b gi bng cch truyn bo hiu a ch trc tip. Trong cu hnh mng ny th thu bao ch gi phi bit c a ch bo hiu ca thu bao b gi trong mng v v vy c th giao tip mt cch trc tip. Nu trong mng c Gatekeeper th trao i bo hiu gia thu bao ch gi v Gatekeeper c thit lp bng cch s dng knh RAS ca Gatekeeper truyn a ch. Sau khi trao i bn tin bo hiu c thit lp, khi

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Gatekeeper mi c xc nh truyn cc bn tin trc tip gia hai u cui hay nh tuyn chng qua Gatekeeper. 3.2.1. nh tuyn knh iu khin cuc gi. Khi cc bn tin bo hiu cuc gi c Gatekeeper nh tuyn th sau knh iu khin H.245 s c nh tuyn theo 2 cch: Knh iu khin H.245 c thit lp mt cch trc tip gia cc thit b

u cui, khi ch cho php kt ni trc tip hai thit b u cui. Knh iu khin H.245 c thit lp t thit b u cui ny ti thit b

u cui khc thng qua Gatekeeper. Khi cho php Gatekeeper nh tuyn li knh iu khin H.245 ti mt MC khi thc hin dch v hi ngh.

Cc phng thc kt ni c trnh by nh hnh sau:

: Knh bo hiu RAS

u cui 1 u cui 2

4 5 6 7 1 2 3 8

9

: Knh iu khin H.245

: Knh bo hiu cuc gi

Gatekeeper

1: ARQ 6: ACF/ARJ 2: ACF/ARJ 7: Connect 3: Setup 8: Connect 4: Setup 9: Knh H.245 5: ARQ

Hnh 3.1. Kt ni trc tip hai thit b u cui

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Gatekeeper

1: ARQ 6: ACF/ARJ : Knh bo hiu cuc gi 2: ACF/ARJ 7: Connect : Knh iu khin H.245 3: Setup 8: Connect : Knh bo hiu RAS 4: Setup 9: Knh 5: ARQ 10: Knh

Hnh 3.2. Gatekeeper nh tuyn knh iu khin H.245 3.2.2. nh tuyn knh bo hiu cuc gi Cc bn tin bo hiu cuc gi c th truyn theo mt trong hai phng thc v vic chn cc phng thc bo hiu no l do Gatekeeper quyt nh. Th nht: cc bn tin bo hiu cuc gi c truyn t thu bao ny qua

thu bo kia thng qua Gatekeeper gia hai thit b u cui nh hnh di:

Hnh 3.3. Bn tin bo hiu cuc gi c nh tuyn qua Gatekeeper

u cui 1 u cui 2

1 2 3 8 9 4 5 6 7 10

Gatekeeper

u cui 1 u cui 2

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Th hai: cc bn tin bo hiu cuc gi c truyn trc tip gia hai

thit b u cui

Gatekeeper

1: ARQ 4: Setup : Knh bo hiu cuc gi

u cui 1 u cui 2 3

6

1 2 4

5

2: ACF/ARJ 5: ARQ : Knh bo hiu RAS 3: Setup 6: ACF/ARJ Hnh 3.4. Bn tin bo hiu cuc gi c truyn trc tip gia 2 u cui C hai phng thc ny u cng s dng cc kt ni ging nhau vi cng mc ch, dng bn tin c s dng cng ging nhau, cc bn tin thit lp bo hiu c trao i trn knh RAS ca Gatekeeper, sau ti trao i bn tin bo hiu cuc gi trn knh bo hiu cuc gi. Sau cng mi thit lp cuc gi trn knh iu khin H.245. Trong phng thc Gatekeeper nh tuyn cc bn tin th n c th ng knh bo hiu cuc gi khi vic thit lp cuc gi hon thnh hoc vn duy tr knh ny h tr cc dch v b xung. Ch c Gatekeeper mi c th ng knh bo hiu cuc gi, nhng khi Gatekeeper tham gia vo cuc gi th cc knh ny khng c php ng. 3.3. CC TH TC BO HIU. Ngi ta chia mt cuc gi lm 5 giai on chnh, gm:

- Giai on 1: thit lp cuc gi (call setup) - Giai on 2: thit lp knh iu khin (control setup) - Giai on 3: thit lp knh hi thoi o (vitual conference channel setup)

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- Giai on 4: dch v ( services ) - Giai on 5: kt thc cuc gi ( caall release )

Cc giai on c tin hnh c th nh sau: 3.3.1. Thit lp cuc gi ( Giai on 1 )

Mt cuc gi ch lin quan n Gateway khi cuc gi c s chuyn tip t mng PSTN sang mng LAN v ngc li. V vy v c bn c th phn bit cuc gi qua Gateway thnh hai loi:

- Cuc gi t mt thu bao in thoi vo mng LAN.

- Cuc gi t mt thu bao trong mng LAN ra mt thu bao trong mng thoi.

Trong giai on ny, cc phn t trao i vi nhau cc bn tin c nh ngha trong khuyn ngh H.225 theo mt trong cc th tc sau: C hai thit b u cui u khng k (Cuc gi c bn).

Khi c hai thit b u cui khng ng k vi Gatekeeper th chng s trao i trc tip cc bn tin vi nhau.

u cui 1

Setup (1)

Call proceeding (2)

Alert (3)

Connect (4)

u cui 2

Hnh 3.5. Cuc gi c bn khng c Gatekeeper C hai thu bao cng ng k vi mt Gatekeeper Tnh hung ny c hai trng hp xy ra

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1. C hai thu bao u cui u ng k ti mt Gatekeeper v Gatekeeper chn phng thc truyn bo hiu trc tip gia hai thu bao.

u tin, thu bao ch gi trao i vi Gatekeeper cp bn tin ARQ (1)/ACF(2) thit lp bo hiu. Trong bn tin ACF do Gatekeeper tr li cho thu bao ch gi c cha a ch knh bo hiu ca thu bao gi. Sau thu bao ch gi s cn c vo a ch ny gi bn tin setup (3) ti thu bao b gi. Nu thu bao b gi chp nhn yu cu, n s trao i cp bn tin ARQ (5)/ACF (6) vi Gatekeeper. Nu thu bao b gi nhn c ARJ (6), n s gi bn tin Release Complete ti thu bao ch gi.

Knh bo hiu RAS

Knh bo hiu cuc gi

Connect

Alerting

ACF/ARJ

ARQ Call proceeding

Setup

ACF/ARJ

ARQ

u cui 2 Gatekeeper1u cui 1

Hnh 3.6. Hai thu bao cng ng k vi mt Gatekeeper, truyn bo hiu trc tip

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2. C hai thu bao u cui u ng k vi mt Gatekeeper v bo hiu cuc gi c nh tuyn qua Gatekeeper nh hnh di:

ACF/ARJ

Setup

Call proceeding ARQ

ACF/ARJ

Alerting

Connect

Call proceedingCall proceeding

Setup

Connect

Alerting

Knh bo hiu RAS

Knh bo hiu cuc gi

ARQ

u cui 2 Gatekeeper 1u cui 1

Hnh 3.7. Hai thu bao cng ng k vi mt Getekeeper, bo hiu qua Gatekeeper

Ch c thu bao ch gi c ng k vi Gatekeeper 1. Ch c thu bao ch gi ng k, bo hiu truyn trc tip

2. Ch c thu bao ch gi, bo hiu qua Gatekeeper

Ch c thu bao b gi ng k vi Gatekeeper 1. Ch c thu bao b gi ng k, bo hiu truyn trc tip

2. Ch c thu bao b gi ng k, nh tuyn qua Gatekeeper

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Trng hp hai thu bao ng k vi hai Gatekeeper khc nhau Tnh hung ny c 4 trng hp xy ra:

1. C hai Gatekeeper u chn cch bo hiu trc tip gia hai thu bao.

2. Gatekeeper 1 pha ch gi truyn bn tin theo phng thc truyn trc tip cn Gatekeeper 2 pha b gi nh tuyn bo hiu cuc gi qua n.

3. Gatekeeper 1 pha ch gi nh tuyn bo hiu qua n cn Gatekeeper 2 pha b gi chn phng thc truyn bo hiu trc tip.

4. C hai Gatekeeper ny u chn phng thc nh tuyn bo hiu cuc gi cho chng.

3.3.2. Thit lp knh iu khin (Giai on 2)

Trong giai on 1 sau khi trao i cc bn tin thit lp cuc gi, cc u cui s thit lp knh iu khin H.245, knh iu khin ny c th do thu bao b gi thit lp c sau khi n nhn c bn tin Setup hoc do thu bao ch gi thit lp khi n nhn c bn tin Alerting hoc Call Proceeding. Trong trng hp khng nhn c bn tin Connect hoc mt u cui gi Release Complete, th knh iu khin H.245 s b ng. Chi tit v chc nng v hot ng ca knh iu khin H.245 c trnh by trong mc trn.

3.3.3. Thit lp knh truyn thng o (Giai on 3)

Sau kh trao i kh nng: tc nhn ti a, phng thc m ho v xc nh master - slave trong giao tip giai on 2, th tc iu khin knh H.245 s thc hin vic m knh logic truyn thng tin. Cc knh ny l knh H.225. Sau khi m knh logic truyn l m thanh v hnh nh th mi u cui truyn i mt bn tin H.225 Maximum Skew Indication xc nh thng s truyn.

Thay i ch hot ng: Trong giai on ny cc thit b u cui c th thc hin th tc thay i cu trc knh, thay i kh nng v ch truyn cng nh nhn.

Thay i cc lung tn hiu Video: Vic s dng ch th Video IndicateReady To ACtive c nh ngha trong khuyn ngh H.245 l khng bt buc, nhng khi s dng th th tc ca n nh sau: u tin thu bao ch gi s khng c php truyn video cho n khi thu bao b gi.

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Phn phi cc a ch lung d liu: Trong ch mt a ch, mt u cui s chim mt knh logic ti MCU hoc mt u cui khc. a ch ca cc knh cha trong bn tin Open Logical Channel v Open Logical Channel Ack. Trong ch a ch nhm, a ch nhm s c xc nh bi MC v c truyn ti cc u cui ca bn tin Communicasion Mode Comand. Mt u cui s bo cho MC vic m mt knh logic vi a ch nhm thng qua bn tin Open Logical Channel v MC s truyn bn tin ti tt c cc u cui trong nhm.

3.3.4. Tham s cuc gi (Giai on 4)

3.3.4.1. Thay i rng bng tn:

rng bng tn ca mt cuc gi c Gatekeeper thit lp trong khong thi gian thit lp trao i. Mt u cui phi chc chn rng tng tt c c truyn, nhn m thanh v hnh nh u phi nm trong rng bngGatec ththay rGate

th

c lung

tn thit lp. Ti mi thi im trong khi hi thoi, u cui hoc keeper u c th yu cu tng hoc gim rng bng tn. Mt u cui thay i tc truyn trn mt knh logic m khng yu cu Gatekeeper

i rng bng tn nu nh tng tc truyn v nhn khng vt qu ng bng tn hin ti. Trong trng hp ngc li th u cui phi yu cu keeper m n ng k thay i rng bng tn.

Th tc thay i rng bng tn thay i thng s bn truyn c hin trn hnh sau:

u cui 1 u cui 2

BCF/BRJ

BRQ

OpenLogicalChACK

OpenLogicalChannel

CloseLogicalChannelBCF/BRJ

BRQ

Gatekeeper2 Gatekeeper1

Hnh 3.8. Yu cu rng bng tn thay i thng s truyn

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Khi u cui 1 mun tng tc truyn trn knh logic trc ht n phi xc nh xem c th vt qu rng bng tn ca cuc gi hin ti khng. Nu c th th n s gi bn tin BRQ ti Gatekeeper 1. Khi nhn c bn tin BCF c ngha l c rng bng tn cho yu cu, u cui 1 s gi bn tin CloseLogicalChannel ng knh logic hin ti. Sau , n s m li knh logic bng cch gi bn tin OpenLogicalChannel c cha gi tr tc mi ti u cui 2. Ti u cui 2, trc ht n phi xc nh xem gi tr c vt qu rng bng tn ca knh hay khng. Nu chp nhn gi tr ny th n s trao i bn tin bng tn ca knh hay khng. Nu chp nhn gi tr ny th n s trao i rng bng tn BRD/BCF vi Gatekeeper 2. Nu rng bng tn cho yu cu thay i th u cui 2 s tr li u cui 1 bng bn tin OpenLogicalChannelACK, trong trng hp ngc li n s t chi bng bn tin OpenLogicalChann