Chuong 1 Sua

7/28/2019 Chuong 1 Sua

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XY DNG H THNG VOIP DA TRN M NGUN M ASTERISK

Lun vn tt nghip Trang 1

CHNG I: TNG QUAN V CNG NGH VoIP

1.1Gii thiu v cng ngh VoIPVoIP (Voice over Internet Protocol) l phng thc cho php truyn m

thanh thi gian thc trn bng thng Internet v cc kt ni IP (Internet Protocol) sdng trong giao thc TCP/IP (Transmission Control Protocol/Internet Protocol).Phng thc ny da trn k thut chuyn mch gi, thay th cng ngh truynthoi c dng chuyn mch knh. Trong VoIP tn hiu m thanh (voice signal) sc chuyn i thnh cc gi tin (data packets) thng qua mi trng mngInternet trn giao thc TCP/IP, sau li c chuyn thnh tn hiu m thanh nthit b ngi nhn.

VoIP cho php to mt cuc gi trc tip t my tnh qua mng d liu

Gatekeeper thc hin cc cuc gi nh s ni b, ng di, di ng hay quc tm thoi s dng PC hay in thoi IP phone.

VoIP cng c th ghp nhiu knh thoi trn mt ng truyn tn hiu gipgim chi ph ng k so vi cch gi in thoi thng thng. Hin nay vi nhu cucng cao v vn v chi ph th gii php VoIP cho php trao i cuc gi ngdi qua mng IP c sn thay v phi truyn qua mng PSTN (Public SwitchTelephone Network) nhm gim chi ph cho cc cuc gi ng di gia nhiu chinhnh khc nhau.

Hnh 1.1: M hnh h thng VoIP

Trong VoIP u tin tn hiu thoi cng c s ha ly mu vi tn s8Khz sau s c lng t ha 8bit/mu nhng thay v truyn trn mng PSTNvi tc 64Khz qua cc mng chuyn mch nh trn in thoi thng thng th


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tn hiu s c nn xung tc thp, c ng gi v chuyn ln trn mng IP.Ti bn nhn th cc gi tn hiu ny s c gii nn thnh cc lung PCM (Pulse-code modulation) 64Kbps v rp theo ng th t bn tin ban u, gii m tn hiu

thoi truyn n thu bao b gi.

Hnh 1.2: Truyn thoi VoIP

thc hin vic ny, in thoi IP thng c tch hp sn cc nghi thcbo hiu chun nh SIP (Session Initiation Protocol) hay H.323, kt ni ti mttng i IP (IP PBX) ca doanh nghip hay nh cung cp dch v. in thoi IP c

th dng nh mt in thoi thng thng nhng thay v kt ni mng in thoiqua line PSTN th li kt ni trc tip vo mng LAN (Local Area Network) hocphn mm (softphone) ci t trn PC.

VoIP da trn s kt hp gia chuyn mch knh v chuyn mch gi trongmng IP. Trong mng chuyn mch knh th mt knh truyn dn dnh ring cthit lp gia hai u thit b u cui thng qua mt hay nhiu nt chuyn mchtrung gian. Dng thng tin truyn ln knh ny l dng bit truyn lin tc theo thigan. Bng thng ca knh dnh ring c m bo v c nh trong qu trnh linlc (64Kbps i vi mng PSTN), v tr thng tin l rt nh. Khc vi chuynmch knh th chuyn mch gi (Packet Switching Network) s dng h thng lutr ri truyn ti cc nt mng. Thng tin c chia thnh cc gi, mi gi cthm cc thng tin iu khin cn thit cho qu trnh truyn nh l a ch ni gi,a ch ni nhn Cc gi thng tin n nt mng c s l v lu tr trong mtthi gian nht nh ri mi c truyn n nt tip theo sao cho vic s dng knhc hiu qu cao nht. Trong mng chuyn mch gi khng c knh ring no c


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thit lp, bng thng ca knh logic gia hai thit b u cui thng khng cnh, v tr thng tin ln hn mng chuyn mch knh rt nhiu.1.2c im ca cng ngh VoIP

Gim chi ph cuc gi: y l u im ni bt ca VoIP so vi dch v inthoi ng di thng thng l kh nng cung cp cuc gi ng di gi rvi cht lng cuc gi chp nhn c. Chi ph cuc gi ng di c tnh bngchi ph truy cp mng Internet. Mt gi cc chung c thc hin trn mngInternet do s tit kim chi ph ng k cho cc dch v thoi v Fax. S chia schi ph thit b v thao tc gia nhng ngi s dng thoi v d liu cng tngcng hiu qu qua vic s dng mng. Cng vi k thut nn thoi tin tin lgim tc truyn d liu t 64Kbps xung 8Kbps mt knh thoi 64Kbps lcny c th phc v ng thi 8 knh thoi c lp. Nh vy vic s dng ti u

bng thng chnh l l do ln nht gip cho chi ph thc hin cuc gi thoi VoIPthp.Tch hp nhiu dch v: do vic thit k c s h tng tch hp nn c kh

nng h tr tt c cc hnh thc thng tin cho php chun ha tt hn v gim thius thit b. Cc tn hiu bo hiu, thoi v c s liu u chia s cng mng IP. Tchhp a dch v s tit kim chi ph u t nhn lc, chi ph xy dng cc mngring l.

Tnh thng nht: Vic s dng thng nht giao thc TCP/IP cho tt c ccng dng ha hn gim bt phc tp v tng cng tnh mm do. Cc ng dnglin quan nh dch v danh b v dch v an ninh mng c th c chia s d dng

hn.Kh nng m rng v nng cao ng dng: thoi v fax ch l cc ng dng

khi u cho VoIP, cc li ch trong thi gian di hn c mong i t cc ngdng a phng tin (multimedia) v a dch v. Tnh linh hot ca mng IP chophp to ra nhiu tnh nng mi trong dch v thoi. ng thi tnh mm do cnto kh nng m rng mng v cc dch v.

Qun l bng thng: Trong PSTN, bng thng cung cp cho mt cuc gi lc nh. Trong VoIP, bng thng c cung cp mt cch lnh hot v mm dohn nhiu. 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 bu cui m ch c cc chun ty vo bng thng c th ca mnh, bn thn cc ucui c th t iu chnh h s nn v do iu chnh c cht lng cuc gi.

Cho php ngi dng chuyn tip vng d dng: Cc cuc gi c th cchuyn hng bt k u trn th gii nh vo cc c im ca giao thc SIP.


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Kim sot tt hn nh bo co y : Cc thit lp VoIP lu tr thng tincuc gi n v gi i trong mt c s d liu trn my ch ca bn, qua chophp bo co y v chi ph gi v lu lng gi.

Bn cnh nhng c im ni bt , cng ngh thoi VoIP cng tn ng 1s hn ch sau cn c khc phc a vo khai thc ti u: Tnh bo mt: VoIP c xy dng trn nn tng Internet rng khp v

hn hp vn khng an ton trong c nhiu loi my tnh khc nhau cng ccdch v khc nhau s dng chung mt c s h tng, do s dn n kh nng ccthng tin c th b nh cp khi cc gi tin b thu lm hoc nh tuyn sai a chmt cch c khi chng truyn trn mng.

K thut phc tp: truyn tn hiu theo thi gian thc trn mng chuynmch 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 phi c mt k thut nn tn hiu t c nhng yucu kht khe: t s nn ln ( gim c tc bit xung), c kh nng suy onv to li thng tin ca cc gi b tht lcTc x l ca cc b Codec phi nhanh khng lm cuc thoi b gin on. ng thi c s h tng ca mngcng cn c nng cp ln cc cng ngh mi nh Frame Relay, ATM c tc cao hn v phi c c ch thc hin chc nng QoS (Quality of Service).

Vn ting vng: Nu nh trong mng thoi, tr thp nn ting vngkhng gy nh hng nhiu th trong mng IP, do tr ln nn ting vng nh hngnhiu n cht lng thoi. V thi gian x l lu nn ting vng nh hng ln n

cht lng cuc gi. Ngoi ra VoIP c th gp nhng vn nh khng th s dng dch v

khi cp in, khng th kt ni n cc dch v khn cp nh: cp cu, bo chy 1.3Cc thnh phn trong mng VoIP

Mng VoIP phi c kh nng thc hin cc chc nng m mng in thoicng cng thc hin, ngoi ra phi thc hin chc nng ca mt gateway gia mngIP v mng PSTN.

Cc thnh phn ct li ca mng VoIP bao gm: Gateway, VoIP server, IPnetwork, End User Equipments


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XY DNG H THN

Lun vn tt nghip

H

VoIP Server:mt cho cc cuc gi VoSIP c gi l SIP sermng in thoi IP, n cmt s dch v quan trbng thng, chuyn il mt vng bao gm cVoIP server cng qunserver.

IP network: lnht l mng Internet. Cvo mng IP thng qua c

Gateway: l tph bin l kt ni mnganalog sang digital v ngtrc tip vo ng truybn thc hin cuc g

C 3 loi gateway l Gateway truy Gateway iu Gateway bo

G VOIP DA TRN M NGUN M

h 1.3: Cc thnh phn trong mng VoIP

l cc my ch trung tm c chc nng IP. Trong mng H.323 c gi l Gatekeeer. Ta c th xem VoIP server nh l b nung cp chc nng qun l cuc gi mt cg khc nh l: nhn dng cc u cui v ga ch, ng k hay tnh cc Mi VoIc thit b u cui ng k, nhng cl mt vng trong trng hp mt vng

cc loi mng d liu s dng giao thc To php thit b u cui, Gateway, VoIP s

s h tng sn c.it b c chc nng kt ni hai mng khIP v mng PSTN. L thnh phn gip chuc li. thc hin cuc gi ta cn kt nn Internet, ng k ti khon VoIP v si.

:n ti knh thoikhin truyn ti knh thoiiu

TERISK

Trang 5

h tuyn v boer, trong mngo ca h thngch tp trung v

ateway, qun lserver s qung c th nhiuc nhiu VoIP

P/IP, ph binerver truy nhp

ng ging nhau,yn i tn hiui VoIP gatewaydng in thoi


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Hnh 1.4: VoIP Gateway tch hp sn SIP hoc H.323

End User Equipment: Softphone v my tnh c nhn (PC): bao gm 1 headphone, 1 phn

mm v 1 kt ni Gatekeeper. Cc phn mm min ph ph bin nh Skype, Ekiga,GnomeMeeting, Microsoft Netmeeting, SIPSet,

in thoi Analog vi IP adapter: s dng dch v VoIP th inthoi Analog phi gn vi 1 IP adapter c th kt ni vi VoIP server.

IP phone: l cc in thoi dng ring cho mng VoIP

Hnh 1.5: Thit b u cui IP Phone

1.4 PBX v IP PBX1.4.1 PBX & IP PBX

PBX (Private Branch Exchange) l mt tng i nhnh ring, l mt mngin thoi ring c s dng trong phm vi mt cng ty. Nhng ngi s dng hthng in thoi PBX dng chung mt s ng kt ni vi PSTN truyn thng thc hin cc cuc gi ra bn ngoi.

C 4 ty chn v h thng in thoi PBX khc nhau:

PBX PBX Thu/o IP PBX IP PBX Thu/o

Xu hng hin nay, Tng i in thoi IP-PBX l mt thnh phn trong giiphp/h thng IP Telephony, IP-Centrex. trin khai IP Telephony bn cn c


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tng i IP, in thoi analog/IP, in thoi softphone, Voice Gateway cc loi,ng truyn (leased line, Gatekeeper, LAN, WAN,...). Mt s tnh nng c tchhp trn tng i IP hoc c th nm trn cc server khc nhau. V d mt tng i

IP c th bao gm SIP/H.323 server, SIP/H.323 proxy, IVR server, Recordingserver, Gateway.

Hnh 1.6: H thng PBX

1.4.2 Cu hnh v s h thng tng i IP PBXTng i IP-PBX chia lm 02 nhm:Nhm tng i ng gi theo mt s phn cng quy nh v thng c

license km theo. Cc loi tng i ny do cc hng ln cung cp nh Avaya,

Siemens, Alcatel-Lucent, Cisco, Panasonic, Ericson, Nortel, LG,...Nhm ph bin th hai theo xu hng cng ngh m ngun m vn hnh

da trn cc tng i IP dng softswitch - phn mm c ci t ln ServerPC/Server Destop, s dng cc card Asterisk hoc Voice Gateway kt ni viPSTN qua cc port FXO/FXS, E1/T1/J1, BRI,...u cui s dng l cc in thoiAnalog/IP, phn mm softphone.

H thng in thoi VoIP/ h thng tng i IP PBX bao gm mt hocnhiu in thoi chun SIP/ in thoi VoIP, mt my ch IP PBX v c th tychn bao gm mt VoIP gateway. My ch IP PBX tng t nh mt my ch

proxy, cc my khch SIP c th l in thoi dng phn mm hay phn cng, ngk vi my ch IP PBX v khi mun thc hin cuc gi th s yu cu my IP PBXthit lp kt ni. My IP PBX c mt danh mc tt c mi in thoi / ngi dngv a ch SIP tng ng ca h v do vy c kh nng kt ni tt c cc cuc gitrong mng hay dn hng cuc gi t bn ngoi thng qua VoIP gateway hay mtnh cung cp dch v VoIP.


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Hnh 1.7: H thng IP PBX

1.5 Nguyn l hot ng ca VoIPNguyn l hot ng ca VoIP l chuyn tn hiu thoi analog thnh tn hiu

s (digital) ri ng gi thnh cc gi tin truyn i trn nn mng. Khi ti chcc gi tin ny c sp xp theo th t ban u, gii m v khi phc thnh tnhiu thoi ban u. Cc qu trnh din ra nh sau:

S ha tn hiu analog: m thanh ca ging ni khi qua microphone sto ra tn hiu thoi l nhng tn hiu tng t (analog). truyn nhng m thanhny qua my tnh ca ngi nhn th cc sng m thanh ny phi c s ha vmy tnh ch x l dng tn hiu s 01010... v vy tn hiu ny s c s ha

chuyn thnh tn hiu s (digital) thng qua cc thit b VoIP nh IP phone hayVoIP gateway, softphone. Sau tn hiu s s c ng gi v chuyn ln trnmng IP. Vic s ha tn hiu analog c thc hin qua 4 bc: Ly mu(Sampling), lng t ha (Quantization), m ha (Encoding), nn ging ni (VoiceCompression).

Chuyn cc gi tin tn hiu thoi c s ha trn thnh cc gi tin IP,khi ti ch cc gi tin ny s c sp xp theo th t nh tn hiu ban u, giim v phc hi thnh tn hiu thoi ban u. Qu trnh ny din ra ngc li viqu trnh trn.

1.6 Cc phng thc thc hin cuc gi trong VoIP1.6.1 M hnh truyn thoi PC PC

L phng thc u tin cho h thng truyn thoi qua mng IP. Thoi PC-PC c thc hin trn c s s dng phn mm dng ring cho vic truyn thoigia cc PC. Vi 1 knh truyn mng Internet c sn. Mi my tnh cn trang b 1


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headphone, microphone, soundcard v c kt ni trc tip vi mng Internetthng qua modem hoc card mng.

Hnh 1.8: M hnh thoi PC to PC

1.6.2 M hnh truyn thoi PC to PhoneL m hnh c ci tin hn so vi m hnh PC PC. M hnh ny cho php

ngi s dng my tnh thc hin cuc gi n mng PSTN thng thng v ngcli trn ng mng lin kt IP. thc hin cuc gi thng thng qua mng nh

trn th h thng phi trang b cc Gateway (VoIP Gateway). Cht lng cuc giph thuc vo kt ni Gatekeeper v dch v nh cung cp.

Hnh 1.9: M hnh truyn thoi PC-Phone.

1.6.3 M hnh truyn thoi Phone Phone.L m hnh VoIP s dng ph bin nht hin nay c ci tin t m hnhPC to Phone s dng mng Internet lm phng tin lin kt vi cc mng PSTN.

M hnh ny c thit lp c ch 2 chiu cung cp cc c ch chuyn i giaothc truyn thoi cng nh bo hiu gia mng thoi PSTN v mng thoi qua IP.Trong m hnh ny th ngi s dng c cp mt m s c bit gi l gi trcng kt ni gia mng PSTN v mng IP. Gi tr ny do nh mng cung cp. Qutrnh chuyn i giao thc gia hai mng PSTN v IP c thc hin qua VoIPGateway.

Hnh 1.10: M hnh truyn thoi Phone to Phone


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1.7Cc giao thc ca VoIPVoIP cn 2 loi giao thc: Signaling protocol v Media Protocol Signaling Protocol: iu khin vic ci t cuc gi. Cc loi signaling

protocol bao gm: H.323, SIP. MGCP, Megaco/H.238 v cc loi giao thc dngring nh UNISTIM, SCCP, Skype, CorNet-IP,..

Media Protocol: iu khin vic truyn ti voice data qua mi trngmng IP. Cc loi Media Protocol nh RTP (Real Time Protocol), RTCP (RTPcontrol Protocol), SRTP (Secure Real Time Transport Protocol) v SRTCP.

Signaling Protocol nm tng TCP v cn tin cy cao, trong khi MediaProtocol nm trong tng UDP. C rt nhiu loi giao thc dng thc hin dch vVoIP, nhng giao thc bo hiu VoIP ph bin nh SIP v H.323. C SIP v H.323u cho ngi dng thc hin cng cng vic thit lp giao tip cho nhng ng

dng a phng tin (multimedia) nh video, audio v nhng giao tip d liukhc. H.323 th ch yu c thit k ng dng cho cc dch v a phng tintrong khi SIP th ph hp cho nhng dch v VoIP.

1.8Cc loi in thoi SIP/in thoi VoIPH thng in thoi VoIP i hi phi s dng cc in thoi SIP/in thoi

VoIP. in thoi Sip c vi phin bn/loi khc nhau: Softphone SIP/VoIP in thoi SIP dng phn mm

Hnh 1.11: Soft phone IP Phone 3CX v X-Lite


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in thoi VoIP qua USB

Hnh 1.12: USB Phone in thoi VoIP:

Hnh 1.13: in thoi SIP dng phn cng (IP phone) S dng in thoi analog vi b chuyn i ATA: Nu ngi s dng

mun dng chic in thoi hin ti vi h thng in thoi VoIP, th c th sdng b chuyn i ATA.

Hnh 1.14: B chuyn i ATA cho php kt ni in thoi analog vi VoIP

1.9Cc tnh nng ng dng v dch ca tng i IP PBX/VoIP

Kt ni ti cc cuc gi ni b v mng in thoi c nh: PC to PC, PCto Phone/Telephone, Phone/telephone to Phone/Telephone. Conference call m thoi nhiu ngi Cc dch v t ng:

o Call Forwarding (chuyn tip cuc gi).o Cance Call Forwarding


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o Tranfero Pickupo Auto Attendant (IVR) - tng tc thoio Phn phi cuc gi t ng ACD - Automated Call Distributiono Call Parko Voice mailo Voicemail transfer: Tnh nng cho php bn chuyn cuc gi vo hp

th thoi khi bn khng rnh nghe.o Voicemail dial: Nu bn khng mun in thoi ca ngi nhn

chung (trnh lm phin khng cn thit), bn c th ni trc tip vo Voice mailca ngi nhn. Ngi nhn sau s nghe li thng tin ca bn t Voice mail.

o User permission to long/international call: Bn c th cp quyn bngmt m hoc theo s ni b.

o Hin th tn v s ID ngi gi, cho php ngi dng nhanh chngxc nh c ngi gi.

o PIN Sets: t password (PIN) cho cc cuc gi ra PSTN. Chn IDngi gi ra ngoi vng.

o Powerful Reporting: Thng k/bo co theo nhiu ch tiu:Accountcode (qun l ngi dng theo Account Code), Destination, channel,duration, Xem cc so snh Traffic Monthly, Traffic Daily, thng qua cc mhnh ha.Music on Hold: Nhc khi ch my, chuyn my, Parking, Cho phpcustomize cc file m thanh hay up cc file.Tao nhc chung ring bit i vi mt

nhm hay mt c nhno C th fax trong v ngoi h thng doanh nghip o Bo cuc gi u tin VoIP.o Min ph cc cuc gi n thu bao khc trong vng h thng

1.10 Nhng li ch ca h thng in thoi IP PBX VoIP D ci t v thit lp cu hnh hn nhiu so vi h thng in thoi

thng thng: Mt chng trnh phn mm chy trn my tnh c th tn dng khnng x l u vit ca my tnh v giao din ngi dng v cc tnh nng ca

Windows. Bt c ngi no c hiu bit v my tnh v Windows u c th ci tcu hnh h thng PBX thay v nh nhn vin ci t. D qun l hn nh giao din cu hnh trn web: Thng thng, giao

din ca mt h thng in thoi kh s dng v c thit k ch nhng ngici t h thng mi c th s dng hiu qu c. Mt h thng in thoi VoIP


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c giao din cu hnh da trn web, cho php ngi dng bo tr v hiu chnh hthng in thoi ca mnh mt cch d dng.

Gim chi ph cuc gi: Ngi dng c th tit kim c chi ph bngcch s dng mt nh cung cp dch v VoIP i vi cuc gi ng di hay cucgi quc t. Kt ni in thoi d dng gia cc vn phng/chi nhnh v gi minph.

Khng cn i dy in thoi ring s dng mng my tnh: H thngin thoi VoIP cho php ngi dng kt ni in thoi phn cng trc tip vocng mng tiu chun ca my tnh (m n c th chia s vi my tnh bn cnh).in thoi dng phn mm c th c ci t thng ln my tnh. iu ny cngha l ngi dng khng cn phi lp t v duy tr mt mng dy ring cho hthng in thoi m linh ng hn trong vic m rng v thm ngi dng.

Khng b kha bi nh sn xut s dng in thoi chun: Cc hthng in thoi VoIP l tiu chun m - tt c mi h thng in thoi VoIP u s

dng giao thc SIP. iu ny ngha l ngi dng c th s dng hu nh bt cin thoi VoIP chun SIP VoIP hay phn cng VoIP gateway no. Ngc li, hthng in thoi thng thng hay i hi in thoi phi d dng cc tnh nngcao cp v cc m un m rng thm tnh nng.

M rng c: Cc h thng thng thng hay pht trin ln thm nh:thm ng in thoi hay thm s my nhnh thng i hi nng cp nhng phncng t tin. Trong mt s trng hp, bn cn mt h thng in thoi hon tonmi. H thng in thoi VoIP th khng nh vy: mt chic my tnh bnh thng

c th qun l mt lng ln cc ng in thoi v cc s my nhnh ch cnthm cc in thoi mi vo trong mng m rng.

Dch v khch hng v hiu nng tt hn: V cc cuc gi l da trnmy tnh, nhng nh pht trin s cm thy d dng hn nhiu trong vic tch hpchng vi cc ng dng thng mi. V d: cuc gi n c th t ng m h skhch hng ca ngi gi ra, qua ci thin ng k dch v khch hng v gipgim chi ph bng cch gim thi gian phc v dnh cho mi khch hng. Cc cucgi i c th thc hin trc tip t Outlook, qua ngi dng khng cn phi gs in thoi vo.

in thoi da trn phn mm d s dng hn: Cc tnh nng tin tinca h thng in thoi nh hi m thng kh s dng hn trn cc my inthoi thng thng. in thoi SIP da trn phn mm th khng nh vy tt cmi tnh nng u c th c thc hin d dng t mt giao din ngi dng vicc ca s thn thin.


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Nhiu tnh nng c bao gm trong phin bn tiu chun: V h thngin thoi VoIP l da trn phn mm, cc nh pht trin c th ci thin cc tnhnng mt cch d dng hn. V vy, hu ht cc h thng in thoi VoIP u c

nhiu tnh nng bao gm tr li t ng, th thoi, xp hng cuc giNhng lachn ny thng t tin trong h thng thng thng.

Tit kim nhn lc trc tng i nh c ch t ng tr li IVR, phnphi cc cuc gi t ng (ACD), Voice mail

Tn dng c h tng sn c (Internet, mng LAN) Chuyn tip vng d dng.

1.11 Cc nhn t nh hng n cht lng VoIPi hi c bn nht ca h thng VoIP l phi c cht lng dch v thoi

tng ng vi cht lng thoi trong mng PSTN. C 3 nhn t quyt nh chtlng thoi Delay: l mt nhn t nh hng nhiu n cht lng thoi, thi gian tr

ln th cht lng cuc gi b nh hng ln. Hai vn gy ra bi s tr u cuitrong mt mng thoi l ting vang v hin tng chng ting. Ting vang tr thnhvn khi tr vt qu 50 ms. y l mt vn nh hng cht lng ng k,nn cc h thng VoIP phi kim sot v cung cp cc phng tin loi b tingvang. Hin tng chng ting (ging ngi ny gi ln ging ngi kia) tr nnng k nu tr mt chiu(one-way delay) ln hn 250 ms. C 4 loi tr thng xyra trong mng in thoi l: tr do x l, tr do lan truyn, tr do hng i v tr do

ni tip. S bin thin tr(Jitter trt): Jitter l s bin thin thi gian tr

gy nn bi s tr ng truyn khc nhau trn mng lm cho gi tin n chkhng ng thi im. Loi b jitter i hi thu thp cc gi v gi chng lu cho php cc gi chm nht n c pht li (play) ng th t, lm cho s trtng ln.

Mt gi tin: Trn mng s liu th vic mt gi tin c kh nng xy racao. Vic mt gi tin ny khng nh hng n cht lng dch v ca mng sliu. Tuy nhin, i vi tn hiu thoi trong mng IP th vic mt gi gy nh hng

nhiu n ti cht lng cuc gi v mng IP khng th cung cp mt s bo mrng cc gi tin s c chuyn ti ch ht. Cc gi tin s b loi b khi qu ti vtrong thi gian tc nghn. i vi gi tin th c th truyn li nhng i vi tn hiuthoi th vic truyn li gi tin TCP thng khng ph hp. ng thi, do c thca tn hiu thoi lin quan ti thi gian thc hin nn hin tng mt gi tin thoigy ra cc s c nghim trng trong qu trnh khi phc ting ni.


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Hnh 1.15: Hin tng mt gi tin khi truyn qua mng IP

Vi vic s dng giao thc RTP vn chuyn v gim st lung thng tinth hin tng mt gi tin c pht hin kip thi. Ta c th xc nh s lng gitin b mt. Ti mi bn tham gia thoi c th tnh tng i chnh xc t l gi tin bmt c gi t mt ngun. Thng thng t l ny vo khong 5-10%. T l nyc trao i qua trng fraction lost trong cc bn tin thng k c gi mt cchnh k. Trn thc t, mi gi tin thoi ch khong vi chc byte nn ta vn c cch b khi phc tn hiu m khng cn s dng c ch pht li. Cc cch tip

cn c s dng b li cc gi mt l thm vo cuc ni chuyn bng cch phtli gi cui cng, v gi i thng tin d.

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Chuong 1 Sua