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VoIP
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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.
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 2
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
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 3
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
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 4
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
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 5
(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.
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 6
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
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 7
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.
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 8
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.
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 9
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.
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 10
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.
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 11
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.
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 12
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
Lm quen vi VoIP CTT602 H thng Vin thng
B mn MMT&VT | Khoa CNTT | H KHTN TP HCM | Thng 7/2013 Trang 13
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.
Lm quen vi VoIP CTT602 H thng Vin thng
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Hnh 7 : Nguyn nhn xy ra trt