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Lm quen vi VoIP CTT602 H thng Vin thng

B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 1

LM QUEN VI VoIP

1. Mc tiu : Bi hng dn gip sinh vin c th:

Nm r c cng ngh VoIP v so snh vi mt s cng ngh khc Nm r cc giao thc trn mng VoIP v mt s khi nim lin quan Bit c mt s thit b VoIP v cc trng hp s dng

2. Ni dung

2.1. PBX - Private Branch Exchange

PBX - l h thng tng i ni b c t ti nh thu bao. PBX vi mc tiu chia s

nhiu thu bao ni b gi ra th gii bn ngoi thng qua mt vi ng trung k, ni

cch khc PBX l h thng trung chuyn gia cc ng dy in thoi bn ngoi ca

cng ty in thoi ti my in thoi ni b trong tng i PBX. V th nn s lng my

in thoi ni b lun nhiu hn s ng dy ni n PBX t bn ngoi.

PBX thc hin chuyn mch cuc gi cc my in thoi ni b vi nhau v vi cc my

in thoi bn ngoi thng qua ng trung k. ng thi thc hin chuyn mch cc

cuc gi in thoi t bn ngoi vo cc my in thoi ni b.



Ngoi vic chuyn mch cuc gi PBX cung cp nhiu tnh nng s dng cho nhiu mc

ch khc nhau ca khch hng m bn thn cc ng dy in thoi t cng ty in

thoi kt ni n khng th thc hin c nh l cc tnh nng tng tc thoi (IVR),

Voicemail, phn phi cuc gi t ng (ACD)

Hin nay vi vic pht trin mnh m ca cng ngh VoIP, chng ta cn c thm thut

ng IP PBX. y l h thng chuyn mch PBX vi cng ngh VoIP.

2.2. VOIP - Voice over Internet Protocol

VoIP l mt cng ngh cho php truyn thoi s dng giao thc mng IP, trn c s h tng sn c ca mng Internet. VoIP c th va thc hin mi loi cuc gi nh trn mng in thoi knh truyn thng (PSTN) ng thi truyn d liu trn c s mng truyn d liu.

Cc cuc gi trong VoIP da trn c s s dng kt hp c chuyn mch knh v chuyn mch gi. Trong mi loi chuyn mch u c u, nhc im ring ca n v s dng u im ca mi loi chuyn mch trn th VoIP kt hp s dng c hai loi chuyn mch knh v chuyn mch gi.

2.3. PSTN - Public Switched Telephone Network

PSTN l mng chuyn mch in thoi cng cng hay ni cch khc l mng kt ni tt c cc h thng tng i chuyn mch-mch.

S khc bit gia mng PSTN vi mng Internet l v kha cnh chuyn thoi. Mng PSTN mun thc hin cuc gi gia hai thu bao th h thng phi dnh ring mt knh



truyn 64kbps chuyn ti tn hiu thoi trn , cn cuc gi in thoi trn mng Internet th tn hiu thoi c ng gi v chuyn i trn cng knh truyn vi nhiu dch v khc. V l cht lng cuc gi trn mng PSTN bao gi cng tt hn trn mng Internet nhng i li chi ph li t hn rt nhiu.

PSTN c pht trin trn chuNn ITU cn mng Internet c pht trin trn chuNn IETF c hai mng trn u s dng a ch nh tuyn cuc gi, PSTN s dng cc con s in thoi chuyn mch cuc gi gia cc tng i in thoi trong khi trn mng Internet, a ch IP s c s dng nh tuyn cc gi thoi.

2.4. TDM - Time Division Multiplexing

L k thut ghp knh phn chia thi gian, nhiu tn hiu c th truyn ng thi trn mt ng truyn, TDM c s dng chuyn thoi trong h thng mng PSTN . C hai chuNn ghp knh TDM c bn l E1 vi 30 knh thoi trn mt khung tc 2Mbps v T1 vi 24 knh thoi tc 1.5Mbps.

Hnh 1 : Minh ha ghp knh theo thi gian



2.5. Cc Hnh Thc Bo Hiu Giao Tip TDM

a. FXO v FXS

FXO l mt thit b mch in c kt ni vi ng line in thoi thng thng, ng dy ny chnh l ng dy in thoi c nh ca chng ta ang s dng hin nay. FXO l thit b ng vai tr nh thit b MODEM 56kbps hay my Fax dng kt ni vi ng dy in thoi.

FXS l thit b mch in giao tip vi my in thoi analog thng thng ging thit b ATA.

Card TDM s dng trong h thng Asterisk thng tch hp thit b FXO v thit b FXS (ging b ATA). C hai giao din hot ng song cng ( ngha l qu trnh truyn thng c truyn v nhn ng thi trong c hai hng).

Hnh 2 : Cc kiu kt ni FXO v FXS



(a) My in thoi vai tr FXO kt ni vi FXS (PSTN)

(b) PBX kt ni vi FXO v FXS

(c) ATA ng vai tr nh FXS kt ni vi my in thoi vai tr FXO.

b. Bo hiu analog gia u cui v tng i

Khi chng ta nhc my in thoi gi th nghe tn hiu mi quay s, khi cuc gi u bn kia b bn th chng ta nghe tn hiu bn (busy tone) cc loi tn hiu nh vy gi l cc tn hiu bo hiu analog gm tn hiu mi quay s, tn hiu bn, rung chung, trng thi nhc my, gc my. Cc loi tn hiu trn c trao i gia thit b FXO v FXS. C nhiu phng thc bo hiu khc nhau ng vi tng ni s dng, v th ti ni kt ni vi ng dy in thoi cn xem xt h ang s dng phng thc bo hiu g, t chng ta khai thc loi tn hiu bo hiu cho thch hp, cc phng thc bo hiu nh

Loop Start, Ground Start. Mt minh ha cho vic s dng sai phng thc bo hiu l khi chng ta khai bo phng thc bo hiu gia h thng Asterisk v ng dy in thoi khc nhau th dn n Asterisk s khng nhn bit c tn hiu gc my, iu ny s lm cho Asterisk khng bao gi gii ta c cuc gi thc hin cuc gi mi.

c. Bo hiu gia cc tng i

SS7 l h thng bo hiu s 7 c pht trin bi AT&T v ITU l h thng bo hiu chuyn cc cuc gi gia cc tng i trong mng PSTN . Trong h thng bo hiu s 7 tn hiu chuyn ti trn ng trung k kt ni gia hai tng i gm c hai mch ring, mt cho thoi v mt cho bo hiu, nh vy thoi v bo hiu c th chuyn trn hai knh vt l khc nhau.



2.6. Giao Thc Trn Mng in Thoi VoIP

a. SIP - Session Initation Protocol

SIP l giao thc Internet dnh cho bo hiu VoIP c pht trin bi IETF cung cp vi chc nng ging h thng bo hiu s 7 nhng da trn nn IP. SIP l mt giao thc bo hiu lp ng dng s dng port 5060 truyn thng. SIP c th c truyn vi giao thc lp vn chuyn l UDP hoc TCP. Asterisk hin ti khng c dng TCP truyn ti cc bn tin SIP nhng m cc phin bn tng lai c th h tr n. SIP c dng thit lp, chnh sa, kt thc cc phin truyn thng nh l cc cuc gi in thoi Internet. SIP khng truyn thng a phng tin gia cc im u cui. Giao thc SIP thc hin chc nng thit lp v bo hiu cuc gi, khi ni n bo hiu cuc gi l ch nh cc tn hiu bo hiu nh: tn hiu mi quay s, tn hiu bn

b. RTP v NAT

Cc cuc gi trn Internet vi giao thc SIP c chia thnh cc gi thoi v c chuyn qua giao thc RTP. RTP l giao thc khng ch thc hin chuyn cc gi thoi qua Internet m cn c c video na. Mt cuc gi thng thng c hai hng thng tin l nhn v pht, RTP lm vic chuyn cc gi d liu thoi cng trn hai hng. N AT l mt tr ngi ln trong giao thc RTP. Trong giao thc RTP nh chng ta bit l thoi c chuyn trn hai ng khc nhau l ng pht v ng thu, vn N AT nm ch tn hiu thoi t bn ngoi vo bn trong qua giao thc RTP khng thc hin c cn chiu ngc li th thc hin tt.

c. IAX - Inter Asterisk Exchange

IAX l giao thc bo hiu VoIP c pht trin bi tc gi ca phn mm Asterisk



khc phc nhng hn ch trong giao thc SIP v H.323. Khng ging nh giao thc SIP chuyn ti thoi v bo hiu trn hai knh khc nhau (out of band), IAX chuyn ti thoi v bo hiu trn cng mt knh (in band). IAX gii quyt c vn N AT cp trn phn giao thc SIP. Mc khc IAX l giao thc ti u trong vic s dng bng thng, cho php nhiu gi d liu thoi trn cng mt IP header, c ch chuyn ti nhiu cuc gi trn cng mt gi IP c gi l trung k (trunk).

Cc kt ni IAX gia cc my ch h tr chuyn mch vi nhiu thng tin lm sao nh tuyn cuc gi c chia s hiu qu gia cc my ch Asterisk. IAX h tr mt s lng ln cc kiu m ho, bt k m ho no c h tr trong Asterisk u c th c s dng cho cc knh IAX.

2.7. Thit B VoIP

a. IP Phone

y l thit b phn cng kt ni vi mng VoIP ging nh my in thoi bn thng thng nhng dnh cho VoIP, cn phi thc hin cu hnh trc khi s dng.



Hnh 3 : My in Thoi VoIP

b. Softphone

L mt phn mm c ci trn my tnh thc hin tt c cc chc nng ging nh thit b in thoi VoIP, cn lu khi s dng softphone l my tnh phi c card m thanh, headphone v firewall khng b kha.



Hnh 4 : in thoi Softphone

c. Card giao tip vi PSTN

Mun cho php cc my in thoi ni b trong h thng Asterisk kt ni v thc hin cuc gi vi mng PSTN chng ta cn phi c thit b phn cng tng thch. Thit b phn cng s dng cho h thng Asterisk do chnh tc gi lp cng ty Digium phn phi, y cng chnh l tng ln trong vic phn phi phn mm Asterisk l h thng ngun m s dng min ph.

Thit b phn cng thng k hiu bt u bng cm t TDMxyB trong x l s lng port FXS, y l s lng port FXO gi tr ti a ca x v y l 4, v d card TDM22B c ngha l c 2 fort FXS v 2 port FXO.



Hnh 5 : Card TDM22B gm 4 port 2 FXS v 2 FXO

d. ATA - Analog Telephone Adapter

ATA l thit b kt ni vi in thoi Analog thng thng n mng VoIP, mt thit b ATA gm c hai loi port: RJ-11 kt ni vi my in thoi analog thng thng cn RJ-45 kt ni vi mng VoIP.

ATA thc s l thit b FXS chuyn i tn hiu Analog sang tn hiu s s dng cho mng VoIP, tn dng my in thoi Analog nn trang b thit b ATA thay v phi trang b in thoi IP.

Thit b ATA s dng vi giao thc IAX c Digium phn phi l thit b ATA c s dng rng ri vi Asterisk c tn gi l IAXy.



Hnh 6 : Thit b ATA

2.8. Codecs

Thut ton codecs l mt tp cc quy lut c s dng chuyn i cc tn hiu thoi dng Analog sang tn hiu s v ngc li.

C nhiu thut ton codec thc hin chuyn i tn hiu analog sang tn hiu s dng nh phn (0,1) nh G711, GSM, G729ng vi mi thut ton c nhng u im ring, c bit l vic ti u s dng bng thng trn ng truyn, mc tiu cui cng cc thut ton a ra phi m bo cht lng cuc gi v tit kim bng thng nht. G729 l thut ton codec tt nht hin nay trong h thng VoIP. Vi h thng Asterisk s dng thut ton ny cn phi mua bn quyn s dng v y l codec khng min ph s dng.



2.9. QoS - Quality of Service

Cc yu t nh hng n cht lng ca dch v, i vi VoIP l cc yu nh hng n cht lng tn hiu thoi.

a. tr

Tr c nh ngha l khong thi gian tnh t lc tn hiu thoi i t ming ngi ni ti tai ngi nghe. Tr l yu t khng th trnh khi, tr i vi mng in thoi truyn thng (mng PSTN ) khong t 50ms n 70ms, theo khin ngh ca ITU-T m bo cht lng cho h thng VoIP th tr t u pht ti u thu hoc ngc li khng c vt qu 150ms, khi m tr vt qu 400ms n 500ms th khng th chp nhn c.

Cc nguyn nhn gy ra tr bao gm : Tr trn mng li, tr trn b m ha gii m, tr trong qu trnh ng gi, tr b m, tr x l ting ni

b. trt (Jitter)

Nguyn nhn gy ra trt l gi tn hiu thoi trong qu trnh truyn t ngun n ch s gp phi nhng iu kin khc nhau trn mng li do thng n ch vi nhng khong tr khc nhau. y l tnh khng ng nht ca tr. Mt nguyn nhn khc ca trt i vi ng dng PC-to-PC l do vic s dng h iu hnh phi thi gian thc, cc gi tin thoi phi i h iu hnh x l trong nhng khong thi gian khc nhau khi m h iu hnh ang phi x l i vi cc ng dng khc.

loi b tnh khng ng nht ca tr, ngi ta thit k cc b m tr (Jitter Buffer) ti ni n ca cc gi tin, cc gi tin n vi cc khong cch khc nhau v thi gian c lu ti b m ri pht khi b m vi khong tr nh nhau. Khi b m c kch thc cng ln th kh nng loi b tnh khng ng nht cng ln, tuy nhin vic s



dng b m s gy ra hin tng tr trong ng dng thoi, do cn c bi ton cn i gia tr v trt.

Ngoi 2 nguyn nhn trn cn mt s nguyn nhn na nh hng n cht lng thoi nh mt gi, gii hn bng thng, ting vng.



Hnh 7 : Nguyn nhn xy ra trt