Tổng quan về VoIP(VNPRO)

8/2/2019 Tng quan v VoIP(VNPRO)

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Tng quan v VoIP (Voice over IP)

Internet Voice, cng c bit nh thoi qua giao thc( Voice Over IP), l mt cng ngh m chophp to cuc gi dng kt ni bng thng rng thay v dng ng dy in thoi tng t(analog). Nhiu dch v dng Voice over IP c th ch cho php bn gi ngi khc dng cngloi dch v, tuy nhin cng c nhng dch v cho php gi nhng ngi khc dng s in thoinh s ni b,ng di, di ng, quc t. Trong khi cng c nhng dch v ch lm vic qua mytnh, hay loi in thoi qua IP(IP phone) c bit. Cng c vi dch v cho php dng in thoitruyn thng qua mt b iu hp (adaptor).

VoIP cho php thc hin cuc dng my tnh qua mng d liu nh Internet. VoIP chuyni tn hiu thoi t in thoi tng t analog vo tn hiu s (digital) trc khi truyn quaInternet, sau chuyn i ngc li u nhn. Khi to mt cuc gi VoIP dng in thoi

vi mt b iu hp, chng ta s nghe m mi gi, quay s s xy ra sau tin trnh ny. VoIP cth cng s cho php to mt cuc gi trc tip t my tnh dng loi in thoi tng ng haydng microphone.

VoIP cho php to cuc gi ng di qua mng d liu IP c sn thay v phi c truynqua mng PSTN ( public switched telephone network). Ngy nay nhiu cng ty thc hingii php VoIP ca h gim chi ph cho nhng cuc gi ng di gia nhiu chi nhnh xanhau.

Trc y, khi da vo giao tip thoi trn mng PSTN. Trong sut cuc gi gia hai a im,ng kt ni th c dnh ring cho bn thc hin cuc gi. Khng c thng tin khc c th

truyn qua ng truyn ny, cho d vn cn tha lng bng thng sn dng. Sau vi s xuthin ca mng giao tip d liu, nhiu cng ty u t cho mng giao tip d liu chia sthng tin vi nhau, trong khi thoi v fax vn tip tc s dng mng PSTN.Nhng ngy nay iu ny khng cn l vn na, vi s pht trin nhanh chng v c sdng rng ri ca IP, chng ta tin rt xa trong kh nng gim chi ph trong vic h tr truynthoi v d liu, Gii php tch hp thoi vo mng d liu, v cng hot ng bn cnh vi hthng PBX hin ti hay nhng thit b in thai khc, n gin cho vic m rng kh nngthoi cho nhng v tr xa. Traffic thoi thc cht s c mang t do (free) bn trn mng d


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liu thng qua c s h tng v thit b phn cng c sn

Mc d nhng khi nim v VoIP l n gin, Tuy nhin thc hin v ng dng VoIP l phctp. gi voice, thng tin phi c tch bit thnh nhng gi (packet) ging nh d liu. Gi lnhng phn thng tin c chia nh d dng cho vic gi gi, cng c th dng k thut nn gi

tit kim bng thng, thng qua nhng tin trnh codec (compressor/de-compressor).C rt nhiu loi giao thc dng thc hin dch v VoIP, nhng giao thc bo hiu (signaling)VoIP ph bin l SIP v H323. C SIP v H323 u cho php ngi dng thc hin cng cngvic: thit lp giao tip cho nhng ng dng a phung tin (multimedia) nh audio, video,nhng giao tip d liu khc. Nhng H323 ch yu c thit k cho nhng dch v a phungtin, trong khi SIP th ph hp cho nhng dch vu VoIP.

RTP (Real-time Transport Protocol) nh ngha nh dng chun ca gi tin cho vic phn phiaudio v video qua Internet.

VoIP lm vic nh th no?Khi ni vo ng nghe hay microphone, ging ni s to ra tn hiu in t, l nhng tn hiuanalog. Tn hiu analog c chuyn sang tn hiu s dng thut tan c bit chuyn i.Nhng thit b khc nhau c cch chuyn i khc nhau nh VoIP phone hay softphone, nu dngin thoi analog thng thng th cn mt Telephony Adapter (TA). Sau ging ni c sha s c ng vo gi tin v gi trn mng IP.

Trong sut tin trnh mt giao thc nh SIP hay H323 s c dng iu khin (control) cucgi nh l thit lp, quay s, ngt kt ni v RTP th c dng cho tnh nng m bo tincy v duy tr cht lng dch v trong qu trinh truyn.

S ha tn hiu Analog

Biu din tn hiu tng t(analog) thnh dng s (digital) l cng vic kh khn. V bn thndng m thanh nh ging ni con ngi dng analog do cn mt s lng ln cc gi trdigital biu din bin (amplitude), tn s(frequency) v pha (phase), chuyn i nhng gitr thnh dng s nh phn(zero v one) l rt kh khn. Cn thit cn c c ch dng thchin s chuyn i ny v kt qu ca s pht trin ny l s ra i ca nhng thit b c gi lcodec (coder-decoder) hay l thit b m v gii m.

Tn hiu n thoi analog (ging ni con ngi) c t vo u vo ca thit b codec v cchuyn i thnh chui s nh phn u ra. Sau qu trnh ny thc hin tr li bng cchchuyn chui s thnh dng analog u cui, vi cng qui trnh codec.

C 4 bc lin quan n qu trnh s ha(digitizing) mt tn hiu tng t(analog):

Ly mu (Sampling)Lng t ha (Quantization)M ha (Encoding)


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Nn ging ni (Voice Compression)

Multiplexing: Ghp knh l qui trnh chuyn mt s tn hiu dng thi qua mt phng tintruyn dn.PAM(pulse-amplitude modulation)- iu ch bin xung

TDM(Time Division Multiplexing)-Ghp knh phn chia theo thi gian:Phn phi khong thigian xc nh vo mi knh, mi knh chim ng truyn cao tc trong sut mt khang thigian theo nh k.FDM(Frequency Division Multiplexing)-Ghp knh phn chia theo tn s: Mi knh c phnphi theo mt bng tn xc nh, thng thng c b rng 4Khz cho dch v thoi.PCM(Pulse code modulation)- iu ch theo m: l phng php thng dng nht chuyn icc tn hiu analog sang dng digital ( v ngc li) c th vn chuyn qua mt h thng truyndn s hay cc qu trnh x l s. S bin i ny bao gm 3 tin trnh chnh: ly mu, lng tho, m ho. Tin trnh ny hot ng nh sau:

Giai on u tin cu PCM l ly mu cc tn hiu nhp (tn hiu i vo thit b s ho), n to ra

mt tun t cc mu analog di dng chui PAM. Cc mu PAM c di bin ni tip nhau,sau phn chia di bin ny thnh mt s gii hn cc khong. Tt c cc mu vi cc bin no nu mu no ri vo mt khong c bit no th c gn cng mc gi tr cu khong. Cng vic ny c gi l lng t ho. Cui cng trong b m ho, ln ca cc muc lng t ho c biu din bi cc m nh phn

Ly mu (Sampling)

Tn hiu m thanh trn mng in thoi c ph nng lng t n 10Khz. Tuy nhin, hu htnng lng u tp trung phn thp hn trong di ny. Do tit kim bng thng trong cch thng truyn c ghp knh theo FDM v c TDM. Cc knh in thoi thng gii hn bngtn trong khong t 300 n 3400Hz. Tuy nhin trong thc t s c mt t nng lng nhiu cchuyn qua di dng cc tn s cao hn tn s hiu dng 3400Hz.

Do ph tn s c th c m rng n 4Khz, theo l thuyt Nyquist: khi mt tn hiu th cly mu ng thi mi khong nh k v c tc t nht bng hai ln ph tn s cao nht, sau nhng mu ny s mang thng tin cho php vic ti to li chnh xc tn hiu thit bnhn. Vi ph tn s cao nht cho thoi l 4000Hz hay 8000 mu c ly trong mt giy, khongcch gia mi mu l 125 micro giy.


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Lng t ho (Quantization)

Tin trnh k tip ca s ha tn hiu tun t l biu din gi tr chnh xc cho mi mu c ly.Mi mu c th c gn cho mt gi tr s, tng ng vi bin (theo chiu cao) ca mu.

Sau khi thc hin gii hn u tin i vi bin tng ng vi di mu, n lt mi mu sc so snh vi mt tp hp cc mc lng t v gn vo mt mc xp x vi n. Qui nh rngtt c cc mu trong cng khong gia hai mc lng t c xem c cng gi tr. Sau gi trgn c dng trong h thng truyn. S phc hi hnh dng tn hiu ban u i hi thc hintheo hng ngc li.

M ha (Encoding)


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Mi mc lng t c ch nh mt gi tr s 8 bit, kt hp 8 bit c 256 mc hay gi tr. Qui cbit u tin dng nh du gi tr m hoc dng cho mu. By bt cn li biu din cho ln;bit u tin ch na trn hay na di ca dy, bit th hai ch phn t trn hay di, bit th 3 chphn tm trn hay di v c th tip tc.

Ba bc tin trnh ny s lp li 8000 ln mi giy cho dch v knh in thoi. Dng bc th tl ty chn nn hay tit kim bng thng. Vi ty chn ny th mt knh c th mang nhiucuc gi dng thi.

Nn ging ni(Voice Compression)

Mc d k thut m ha PCM 64 Kps hin hnh l phng php c chun ha, nhng c viphng php m ha khc c s dng trong nhng ng dng c bit. Cc phng php nythc hin m ha ting ni vi tc nh hn tc ca PCM, nh tn dng c kh nngca h thng truyn dn s. Chc hn, cc m ha tc thp ny s b hn ch v cht lng, tbit l nhiu v mo tn s.

Mt s v d h thng m ha ting ni tc thp:

CVSD( Continuously variable slope delta modulaton) K thut ny l mt dn xut ca iu chdelta, trong mt bit n dng m ha mi mu PAM hoc ln hn hoc nh hn mu trc. V khng hn ch bi 8 bit, m ha c th hat ng tc khc nhau vo khang 20 Kps.

ADPCM( Adaptive differential PCM): K thut ny l mt dn xut ca PCM chun, s khcbit gia cc mu lin tip nhau c m ha, thay v tt c cc mu iu c m ha, ctruyn trn ng dy. CCITT c ngh mt chun ADPCM 32 Kps, 24 Kps, 16Kbs cho m hating ni.

Chun PCM th cng c bit nh chun ITU G.711Tc G.711: 64 Kps=(2*4 kHz)*8 bit/muTc G.726: 32 Kps=(2*4 kHz)*4 bit/muTc G.726: 24 Kps=(2*4 kHz)*3 bit/muTc G.726: 16 Kps=(2*4 kHz)*2 bit/mu

Packetizing voice


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Mi mt khi ging ni c s ho v c nn li, n phi c chia thnh nhng phn nh, t vo gi IP, VoIP th khng hiu qua cho nhng gi tin nh, trong khi nhng gi tin ln thto ra nhiu tr, do nh hng ca vi loi header m kch thc cu d liu thoi(voice data )cng s nh hng. V d header cu IP, UDP, RTP l 40 byte, nu gi tin voice cng ch khong

40 byte th hon ton khng hiu qu, kch thc gi tin ln nht c th trong mi trng Ethernetl 1500 byte, dng 40 byte cho header cn li 1460 byte c th s dng cho phn d liu thoi,tng ng vi 1460 mu(samples) khng c nn hay thi gian t phn d liu vo gitin. Nu gi b mt nhiu hay n ch khng ng th t s lm cho cuc thoi b ngt qung.

Thng thng, cn khong 10us n 30 us (trung bnh l 20us) t d liu thoi vo bn tronggi tin, v d phn d liu thoi(voice data) v kch thc 160 byte khng nn cn khong 20us t phn d liu thoi vo bn trong gi tin. S lng d liu thoi bn trong gi tin cn cnbng gia s hiu qu trong s dng bng thng v cht lng ca cuc thoi

S ng gi cho qu trnh vn chuyn gi thoi

Gii thiu mt s giao thc chnh:

H323: y l giao thc chun ca ITU. Giao thc ny ban u c pht trin cho h thng aphng tin(multimedia) phng tin trong mi trng khng hng kt ni (connectionless), nhLAN. H323 l chun nh ngha tt c tin trnh lin quan n vn ng b cho thoi,video, v truyn d liu. H323 nh ngha tn hiu cuc gi end-to-end.

MGCP: Chun oc pht trin iu khin cng ra vo (gateway) cho h thng PSTN. MGCPnh ngha giao thc, hay qui tc iu khin (control) cho cng ra vo ca VoIP m ktni vi thit b iu khin cuc gi (call-control) bn ngoi, thng c gi l tc nhngi(call agent), MGCP cung cp tnh nng bo hiu cho nhng thit b r tin v tr ra nh lcng ra vo, y l nhng thit b m khng c tnh nng to y tn hiu cho thoi nh lH323. Thc cht, bt k lc no khi xy ra s kin nh qu trnh nhc ng nghe (off hook) ticng thoi (voice port) ca cng ra vo, cng thoi s thng bo s kin n tc nhn gi. Sau tc nhn gi to tn hiu n thit b m cung cp dch v, nh l tn hiu mi quay s.

SIP: y l giao thc chi tit m xc nh nhng ch th v nhng p ng, thit lp hay ktthc cuc gi. SIP cng m t chi tit v cc tnh nng nh bo mt (sercurity), s y nhim(proxy), vn chuyn (transport). SIP nh ngha ra c ch bo hiu cuc gi end-to end giacc thit b. SIP l giao thc text-based m c nhiu tnh nng tng t HTTP, nh l cng mhnh yu cu (request) v p ng (response).

RTP: Giao thc chun ca IETF v media-stream. RTP mang d liu thoi qua mng. RTP cungcp s trnh t v thng s thi gian (time stamp) x l ng th t ca gi tin thoi.

RTCP: Cung cp tnh nng iu khin thng tin ngoi bng (out-of-band) cho mt lungRTP. Mi lung RTP c tng ng lung RTCP m thng bo nhng s liu thng k trn


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cuc gi. RTCP c dng cho tnh nng thng bo QoS.VoIP v m hnh OSI

tch hp thnh cng lu lng (traffic) thoi trong h thng hng kt ni nh PSTN vo hthng khng hng kt ni nh mng IP cn yu cu tnh nng ca rt nhiu c ch bo hiu. Do ngi dng c th to mng khng hng kt ni xut hin nh mng hung kt ni.

Nhng ng dng nh l Cisco IP Softphone v CiscoManager cung cp giao din cho ngi dng to ging ni, chuyn i v nn ging ni my tnh ca h trc khi c mang qua mng.

Codecs nh ngha ra tin trnh nn d liu thoi. Loi codec c th c chn bi cu hnhca ngi dng hay thng qua tnh nng tha thun ca thit b.Trong qu trnh thc hin h thng VoIP, thoi dng RTP bn trong UDP mang d liu thoiqua mng. V lp mng thoi s oc t trong header ca IP, m gi tin IP c th n chkhng ng th t v khng ng b, do nhng gi tin phi c sp xp v ng b li trockhi gi gi tin thoi n ngi dng. V UDP khng cung cp dch v nh l s trnh t (sequencenumber) v thng s v thi gian, tuy nhin RTP cung cp chc nng xp xp th t.

C rt nhiu phng thc bo hiu c dng trong h thng VoIP, nh l H323 v SIP th nhngha phng thc bo hiu cuc gi end-to-end, MGCP nh ngha phng thc tch bitchc nng bo hiu vi chc nng cuc gi. MGCP dng call agent i din cho tnh nng iukhin bo hiu cho nhng thit b u cui. Thit b iu khin trung tm ch tham gia vo qutrnh thit lp cuc gi. Nhng lung traffic thoi vn c gi trc tip t end-to-end.

RTP v RTCP

RTP cung cp chc nng mng vn chuyn end-to-end cho nhng ng dng truyn d lium yu cu thi gian thc (real-time) nh l m thanh v video. Nhng chc nng bao gmnhn din loi d liu, s trnh t, tham s thi gian v gim st tin trnh gi.


8/22

RTP l thnh phn quan trng ca VoIP bi v n cho php thit b ch sp xp v iu chnh lithi gian cho gi tin thoi trc khi c gi n ngi dng. Mt header RTP cha tham s thigian v s trnh t nhm cho thit b nhn lu vo b nh m, kh jitter v gc tr (lacency)bng cch ng b nhng gi tin pht li (playback) dng m thanh lin tc. RTP dng s trnht ch sp xp li th t gi tin. RTP khng yu cu s truyn li nu mt gi tin b mt.

V d: Nh nhng gi thoi khi c gi n ch, chng c th i trn nhng con ng khcnhau n ch, mi con ng c th khc nhau v khong cch, tc truyn, kt qu l gitin n khng ng th th khi chng n ch. Khi ngun to ra cuc gi, d liu thai s cng gi li, RTP s gn vo nhng gi tin vi tham s thi gian v s trnh t v gi i. dchdn, RTP s sp xp nhng gi tin v gi chng n b x l tn hiu s (digital signal processor-DSP) cng tc khi chng c gi i ngun gi.

RTCP(Real-Time Transport Control Protocol)

RTCP gim st cht lng ca qu trnh phn phi d liu v cung cp tin trnh iu khin

thng tin. RTCP cung cp thng tin phn hi da theo iu kin ca mng:RTCP cung cp c ch cho nhng thit b lin quan trong phin (session) RTP trao i thng tinv gim st v iu khin phin. RTCP gim st cht lng ca cc yu t nh l m gi (packetcount), mt gi, tr, jitter. RTCP truyn gi bng 1% bng thng ca phin, nhng mt tc xc nh trong t nht mi 5 giy.Tham s thi gian Network Time Protocol(NTP) da vo cc xung c ng b. Tham s thigian RTP tng ng th c to ngu nhin v da vo tin trnh ly my gi d liu. C haiNTP v RTP th c t trong gi RTCP bi ngi gi d liu.V d ng dng RTCPTrong sut mi cuc gi RTP, nhng gi thng bo th c to t nht mi 5 giy. Trong iukin mng c cht lng km, mt cuc gi c th b ngng kt ni do lng ln gi b mt. Khixem xt nhng gi tin qua h htng phn tch gi, ngi qun tr c th kim tra thng tin trongheader ca RTCP m bao gm s lng gi mt, jitter.

Gim kch thc header vi CRTP


9/22

Tng ng vi nhiu s lng giao thc cn thit vn chuyn thoi qua mt mng IP, thheader ca gi tin c th ln. Dng CRTP (Compression RTP) trn nhng lin kt link-by-link tit kim bng thng.

Dng CRTP nn header IP/UDP/RTP t 40 byte xung cn 2 byte vi khng c checksum v t40 byte xung 4 byte nu c checksum. Nn header RTP th rt c ch trong trng hp kchc d liu ca RTP l nh, v d nh d liu m thanh s c nn trong khong 20 v 50byte.

CRTP lm vic da vo gi thit l hu ht tt c cc trng trong header IP/UDP/RTP lkhng thay i hay s thay i c th nhn bit c. Nhng trng khng thay i bao gm ach ngun-ch ca IP v s cng ngun-ch ca UDP, cng nh nhng trng khc trong tt c 3header.

V d v CRTP: Trong mi trng thoi da vo gi tin th khi ging ni ly mu c ng

khung (framing) mi 20 ms, phn d liu 20 byte c to. Khng c CRTP, kch thuc gi baogm nhng thnh phn sau:IP header (20 byte)UDP header (8 byte)RTP header (12 byte)D liu (20 byte)Header l hai ln kch thc ca d liu: IP/UDP/RTP (20+8+12=40 byte) trong khi phn d liul 20 byte. Khi to nhng gi tin mi 20 ms trn lin kt c tc chm, phn header chim phnln bng thng.

Lu khi dng CRTP nn header

Nn cu hnh RTCP trn nhng cng giao tip c nhng iu kin sau;Trn nhng lin kt c bng tn hpLin kt tc thp (


10/22

Nn hot ng trn nhng lin kt link-by-link, kch hot nn trn c hai u cui ca lin ktserial c bng thng thp c th tit kim rt nhiu bng thng nu nh c s lng ln trafficRTP trn lin kt serial . Tuy nhin, nn c th yu cu phi x l nhiu s gy ra tnh trng tiuhao ti nguyn sn dng trn cc thit b.

Calculating Bandwidth RequirementCodec Bandwidth

Mt trong s nhng yu t quan trng ngi qun tr mng cn nhc trong qu trnh ln khoch xy dng nng lc mng voice thch hp. Nhng ngi qun tr mng phi hiu lulng bng thng c s dng cho mi cuc gi VoIP. Khi hiu r v lu lng bngthng s dng cho mng VoIP th ngi qun tr mng c th a ra nhng cng c tnh tonnhm lp ra nhng k hoch pht trin nng lc mng Voice hiu qu.Di y l danh sch nhng Codec v lng bng thng c s dng lin quan:

G711: C ch iu ch m xung(PCM) G.711 s dng lng bng thng nhiu nht khi so snhvi cc c ch m (coding) khc. C ch ny ly mu vi 8000 mu mt giy, vi mi mu cchiu di 8 bit, tng cng l 64.000 bps

G726: C ch m ADPCM(adaptive differential PCM) dng lng bng thng t hn. Trong khivi PCM ly mu vi chiu di 8 bit, th ADPCM dng 4,3 hay 2 bit cho mi mu, do kt qutng bng thng c dng trong s 32.000, 24.000, hay 16.000 bpsG.728: C ch m G728 LDCELP(low-delay code excited linear prediction) nn nhng mu PCMdng k thut codebook. Lng bng thng s dng tng cng khong 16.000bpsG729: C ch m G729 v G728A (CS-ACELP) cng nn nhng mu PCM bng cch dng kthut codebook tin tin. Lng bng thng s dng tng cng l 8000bpsG723: C ch m G723 v G729A (MPMLQ) dng thut tan look-ahead. Kt qu ca thut tonny l lng bng thng s dng nm trong khong 5300-6300bps.

Ngi qun tr nn cn bng gia cht lng ca cuc gi vi chi ph s dng bng thng

trong mng khi chn loi codec. Vi loi codec chim lng bng thng ln hn th chi ph chomi cuc gi cng s cao hn.

Tc ng kch thc mu v kch thc gi i vi bng thng


11/22

Kch thc mu th c th thay i v s nh hng n lng bng thng tng cng c dng,mi mu c ly t codes DSP th c nh ngha nh digital(s) v c ng gi voPDU(protocol data unit). Cisco dng DSP to ra nhng mu da vo vic s ha vi gi tr10ms m thanh. Mc nh cc thit b Voice ca Cisco ng gi 20ms m thanh vo mi PDU btchp loi codec c s dng. Khi nhiu mu c ng gi vo tng PDU th lng bng thngtng cng s gim, tuy nhin ng gi nhiu mu vo tng PDU s gy ra nhiu tr.

V d tnh ton lng byte c ng gi:Dng cng thc n gin c th xem xt s lng byte c ng gi trong mt PDU da vobng thng codec v kch thc ca mi mu l 20ms:Byte_per_Sample=(Kch thc mu* bng thng ca codec)Nu dng G711, ta c kt qu:Byte_per_Sample=(0.20*64000)/8Byte_per_Sample=160

nh hng ca hearder lp 2Nhng yu t khc nh hng n bng thng l header giao thc lp 2 m c dng manggi Voice. Bn thn ca gi voice mang header IP/UDP/RTP l 40 byte, ph thuc vo giaothc lp 2 c dng, th kch c ca gi tin c th gia tng ng k. Kch thc header lp 2ln th s yu cu nhiu bng thng hn mang gi tin. Di y l kch c header ca vi laigiao thc lp 2:Ethernet: Kch thc header ca ethernet l 18byte trong , 6byte dng cho a ch MAC ngun,6 byte dng cho a ch MAC ch, 2 byte trng type dng xc nh lai giao thc lp 3, v 4byte dng cho trng kim tra li (CRC)Multilink Point-to-Point Protocol(MLP): Tng cng l 6 byte, 1 byte cho bit chui bit bt uca frame, 1byte cho trng a ch, 2 byte cho trng type, 2 byte trng CRC.


12/22

FRF.12: Tng cng 6 byte; 2 byte s dung cho trng a ch DLCI, 2 byte cho FRF.12 v 2 bytecho CRC.

nh hng Sercurity v Tunneling

S ng gi tun theo mt s c ch bo mt v tnh nng tunneling s cng lm tng kchthc gi tin Voice, v cng nn c xem xt trong qu trnh tnh ton bng thng. Khi dngVPN, IP Security(IPSec) s thm t 50 n 57 byte, y l mt kch thc ng k khi so snh vivi kch thc nh ca gi tin Voice. L2TP/GRE thm 24 byte. Khi dng MLP, 6 byte s cthm vo mi gi, Multiprotocol Label Switching (MPLS) thm nhn(label) 4 byte cho mi gitin. Tt c nhng giao thc tunneling v sercurity nn c cn nhc khi ln k hoch cho yu cuv bng thng.V du: VPN thm IP header mi, l 20 byte, cng vi header VPN khong 20 ti 60 byte, phthuc vo loi VPN c dng.

Vy kch thc gi tin s l 100 ti 160 byte

tnh ton tng bng thng cho mt gi 160 byte vi kiu m l G729, kt qu tnh tan nhsau:160 byte*8=1280 bitTng bng thng=1280bit/20ms=64000 bps

Nhng loi ng gi c bitC rt nhiu loi ng gi cn c xem xt mang gi tin Voice trn mi trng IP. lnhng loi ng gi c bit khi s dng nhng loi giao thc nh l X25, Ipv6 over Ipv4, Layer


13/22

2 Forwarding(L2F), v nhng loi giao thc khc cng nn c cn nhc trong qu trnh tnhtan bng thng tng cng.Tnh ton tng bng thng cho mt cuc gi trn mi trng IPVic chn la loi codec, loi giao thc lp 2, kch thc mu v c ch nn RTP s c nhng nhhng nht nh i vi bng thng tan phn. thc hin s tnh ton cn phi xem xt n

nhng yu t gp phn to ra s tng ng:Nhiu bng thng yu cu cho codec=nhiu bng thng tc yu cuKch thc header ca giao thc lp 2 ln= nhiu bng thng c yu cuKch thc mu(sample) ln= t bng thng c yu cu.Nn RTP=gim ng k lng bng thng c yu cu.

V d tnh tan lng bng thng :Bng thng tng cng=([Kch thc header lp 2+Kch thc header IP_UDP_RTP+ Kch thcSample]/Kch thc sample)*Tc CodecGi s dng codec G.729, vi kch thc sample l 20 ms, dng Frame Relay khng c CRTP:Bng thng tng cng=([6+40+20]/20)*8000Bng thng tng cng =26.400bps

nh hng ca VAD(Voice Active Detection) n bng thng


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Trung bnh trong tng s 24 cuc gi ng thi hay nhiu hn c th cha n 35% thigian khng c thoi (silence). Vi h thng thoi truyn thng, tt c nhng cuc gi dng cnh lng bng thng l 64kbs bt chp cho d l c thoi c truyn i hay khng. Trong mngVoIP ca Cisco, tt c cuc ni chuyn v khong thi gian im lng th c ng vo gi tin.VAD loi b tt c cc gi khng thoi. Thay v phi gi nhng gi tin khng thoi, gateway caVoIP chn lung d liu vi nhng gi tin khng c khong lng, do hiu qu hn trong vics dung bng thng.

Do VAD s tit kim khong 35% bng thng hn khi so vi lng bng thng cn thit cho24 cuc gi ng thi.V d: Khi dng G711 vi kch thc phn d liu l 160 byte, bng thng c yu cu l82.000 bps, vi vic kch hat VAD, c th gim bng thng cn s dng xung 53.560 bps. iuny c c bng cch gim i 35% bng thng.

Thc hin h tr thoi trong mng Enterprise

Khi m nhiu Enterprise c nhu cu tch hp nhng ng dng voice, video, audio, data vo

cng mt c s h tng chung cn c gi l mng hi t (converged network) hay l mngth h k tip.

Mt mng hi t, cho php s kt hp thoi, d liu v nhng s truyn bo hiu khc vo mtgiao tp mng n, tc cao, v phc tp hn i vi mng truyn thng hot ng c lp nhthoi v d liu tch bit. Bi v nhng kh khn ny, vic thit k, thc hin, v kim tra trn cs h tng ny, yu cu cn c s quan tm nghim tc ngi dng cui c th chp nhn c


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cht lng ca dch v.

Qu trnh thit k v thc hin c th chia lm 10 bc, bt u vi s nh ngha r rng canhng i tng trong mng hi t v nh ngha nhng ng dng m mng ny h tr. Trongsut qu trnh cn yu cu c ghi nhn li ca h thng. K hach ci t v kim tra cng bao

gm k hoch kim tra tnh tng thch ca nhiu gia nhng thnh phn ca nhiu hng. Giaion ci t nn c th nghim trong phng lab hay trn nhng phn an mng m c kimtra k lng cho s vn chuyn tin cy cho tt c cc ng dng:thoi, video, fax v d liu qua cs h tng mng hi t. Mi mt hot ng ca chng trnh th nghim c hon thnh , hthng phn cp sau c th c kim tra, trin khai, v ghi nhn li.

Hu ht nhng ngi dng cui u quen thuc vi nhng cuc gi qua PSTN v nhn thy rngtin trnh thit lp v kt thc cuc gi l tin cy v nhanh chng. Do mng hi t cng s phip ng nhng yu cu ti thiu . Tuy nhin thc t th c rt nhiu yu t c th nh hng

n cht lng ca dch v, bao gm bng thng ca nhng knh giao tip, yu t v tica mng, ting vng (echo) bn trong mng, v tr (delay).

S kh khn trong la chn v thc hin mng VoIP

Nhng kh khn xut pht t s khc nhau gia mng thoi v d liu, thoi l mng hngkt ni, trong khi mng d liu l khng hng kt ni, do c hai mng yu cu cch tip cnlung thng tin cng rt khc nhau. V d, mng thoi th c thit k vi 99.999% tin cy.Trong khi phn ln cc mng d liu th tin cy khng cao.

S khc nhau cn l loi dch v vn chuyn thng tin. Giao tip ca con ngi th rt nhy cmvi tr, cng vi nhng s thay i trong tr hay cn gi l jitter. Bn trong mng thoi,mch giao tip th c fixed, bng thng oc thit lp v nhng ti nguyn khc th c dnhring cho trong sut cuc gi, nhm gim ti thiu tr v jitter. Ngoi ra, bt k thng tin b mtc th c khi phc bi nhng ngi dng cui bng cch yu cu ngi tham gia lp li cmt cui. Tri li, mng chuyn gi (packet-switched) truyn thng khng thit lp ng i end-to-end c nh m thng qua tin trnh nh tuyn (routing) trn tng gi. tr v jitter l mt vn khng ln, v nhng thng ip (message) ln hu nh s c chia thnh nhiu gi tin nhbn thit b truyn, v sau cng s oc sp xp li bn thit b nhn. Hai tin trnh l vai trquan trng trong kin trc mng chuyn gi. Tuy nhin, nu mt trong s nhng gi trong trnh tny b mt, th thng ip ban u khng th ti to li v chuyn ln lp ng dng, ni mt cchkhc, thng ip ban u c th ti to li cho d c mt vi gi tin b tr nhng s khng th tito nu mt trong s gi tin ban u b mt.

Th hai, phi c s la chn khi quyt nh loi kin trc oc trin khai, c hai lai chnh vvi dng lai khc. Kin trc u tin da vo h thng chuyn mch thoi c bit nh l tngi chuyn mch hay PBX. Kin trc ny xut pht t kin trc thoi ban u ca n nh l mng


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PBX m kt ni nhng ngi dng cui bn trong cng cng ty, hay s lin kt gia nhiu v trgia nhng chi nhnh vi li tr s chnh. Khi s quan tm n tch hp giao tip thoi v d liutrong nhng t chc kinh doanh pht trin, nhng giao din PBX, nh l nhng kt ni h tr faxv d liu th c thm vo.S pht trin hp l k tip l thm nhng giao din (interface) h tr nhng mng khc nh l

ISDN, Ethernet v IP. Cui cng, kin trc ni b th oc chuyn t mng chuyn mch sangmng chuyn gi, m cho php nhng chc nng x l gi tin nh l gateway and gatekeeper cngtn ti vi nhng giao din station v trunk. V khi nhiu giao din th tng thch vi IP , thutng IP PBX ra i.

IP PBX-based Converged Network

Nhng la chn khc l kin trc da vo b nh tuyn m gim nh s u t cho nhng doanhnghip m xy dng mng d liu khng tp trung (distributed). Rt t s cu hnh li th cyu cu nng cp nhng b nh tuyn vi kh nng VoIP, nh l tnh nng qun l giao thng(traffic) v cht lng dch v (QoS), chc nng truy cp v bo mt v nhng yu cu x l VoIPkhc. Chc nng qun l v bo mt thng c thc hin trn mt vng, trong khi x l thoith c thc hin cc b. B nh tuyn th c t nhng ni c mt traffic cao, ytraffic c kt hp vn chuyn ti nhng v tr khc. Nu nh dng ca thoi v video dngdigital n c th c s ha v vn chuyn qua mng d liu vi email, trang webNhng b xl ng dng a phng tin(multimedia) nh l gatekeeper v gateway c th kt ni vo mngny nh bt k thit b h tr IP khc.


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Router-based Converged NetworkThit k v thc hin mng VoIP

Mng hi t th h tr cho c thai v d liu, tin trnh thit k nn cn nhc cho c hai yu cu. Mi mng c nhng kh khn duy nht. Tuy nhin c 10 nguyn nn c xem xt trong ttc cc trng hp:


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nh danh c nhng i tng ca mng hi t, bao gm nhng ng dng c h tr, nhl hi tho qua video, Web site h tr thoi. nh ngha mc ch ngn gn ca nhng i tng cho php nhng ngi tham gia hiu r c nhng kh khn pha trc. Thm na, ghi nhnnhng yu t ph thuc m nh hng n nhng i tng ny.

Hiu c nhng mi trng hot ng ca thoi v d liu. Bao gm nhng yu t v titrn nhng khu vc mng LAN v WAN, cng vi nhng yu cu v d an trc lng bngcho ng dng mi, cn nhc nhng c im ca cc cuc gi hin ti, d tnh c trc khnng b tc nghn trong thi im no .

Chun b ti liu v nhng yu cu ca h thng (System Requiment Document) m m tnhng chc nng mng phi cung cp v bao gm nhng i tng thit k thch hp, nh l tr end-to-end, tnh tin cy, tnh d tha. Xem xt liu mt thnh phn kin trc nh l H323


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hay SIP th thch hp cho nhng ng dng mng ny. Cng cn nhc dn nhng s thay i cnthit i vi k hach quay s (dial plan), lin kt WAN, IP subnets, hay nhng vn m c thtc ng n ngi dng cui.

Cn nhc nhng h thng ph thuc m s h tr cho cho mng ny, nh l qun l mng,

phn tch mng, bo mt, tr gip. Xc nhn rng bt k nhng h thng h tr , nh l b phntch giao thc, giao tip qun l mng th cng c h tr cho mi trng mi. Chnh sa SystemRequiments System khi cn thit.

Tham kho c hai nh cung cp hin ti v sp ti c c nhng h tr cho yu cu hthng, cng tham kho kin ca nhng ngi c kinh nghim trong cng lnh vc. Ghi nhnli thng tin t kt qu ca qu trnh tham kho.

Trin khai k hoch ci t bao gm kim tra s tng tc gia cc thnh phn khc nhau.Cng chc rng nhng giao thc bo hiu gia nhng mng c th tng thch c vi nhau.

Chn nh cung cp thit b thng qua vic sp xp nhng bui gii thiu, v tip xc vinhng nh cung cp c kh nng, nh gi nhng xut .Trin khai k hoch ci t v k hach lm vic vi nhng nh cung cp c chn, v nhng tchc m tham gia vo d n. Kim tra nhng hng i quan trng trong k hoch trin khai, nhl mua thit b, thit lp mch giao tip, t c s tha thun gia cc bn tham gia cho nhngvn quan trng trong k hoch ny.

Ci t h thng mi th nghim trc phng lab, kim tra s hot ng v ng dngtrc khi c trin khai rng ri. giai an ny rt hu ch dng nhng cng c phn tchgiao thc nh Sniffer Portable analyzer nhm document qu trnh giao dch thnh cng nh thitlp cuc gi. Ti liu ny c hnh thnh t qu trnh gim st, s rt c ch nu c nhng vn pht sinh trong qu trnh trin khai. N cng rt c ch xc nh rng tt c dialing plan v si ca ac ch mng c chuyn i thnh cng ti mi trng mi vi nhng nhm nh trckhi c thc hin mi trng ln hn.

Khi i n giai an ci t, kim tra tt c nhng thnh phn mng, bao gm s kt hphot ng gia nhng h thng khng cng nh cung cp v nhng h thng ph thuc, nh b xl th thoi. Hun luyn ngi dng v ngi qun l v document nhng th tc m nhng cnhn cn bit i vi nhng tc ng n h thng mi. Han thnh bn v y ca mngv nhng ti liu cn thit khc.

nh gi cht lng dch v ca mng VoIP

S khc nhau c xem xt t phn u gia mng thoi v mng d liu l tin cy, ymng thoi th c thit k cho tin cy rt cao, trong khi mng d liu hot ng theo dch vbest-efforts. Nhng nu c k hoch nng cp mng thoi hin ti vi nn tng thoi trn nn IP(VoIP), th khng th mong i ngi dng chp nhn c s suy gim cht lng, c bitnh l cht lng dch v (Qos).C rt nhiu yu t nh hng n Qos bao gm nh s mt gi, tr, lng bng thng sndng, nhng giao thc c s dng trong kt ni WAN, s xut hin ca tr c th do nguyn


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nhn nh tr khng khng khp trong PSTN, kh khong lng (silence suppression) tn dngti u s thun li trong qu trnh giao tip v nhng iu khc.

Sources of Delays within the VoIP NetworkTuy nhin, c l yu t quan trong nht nh hng Qos l s lung ca tr ca qu trnh truynend-to-end. ITU-T khuyn ngh G.114, tr t a trong mt hng truyn c chp nhn l0.15 giy, mc d thi gian di hn nm trong khong 0.2 giy, c th c ngi dng chpnhn. Nu tr vt mc ng k th cuc thoi s kh khn hn. Trong phn thit k c thxem xt nhng yu t pht sinh tr sau:

Thut tan m v gii m tn hiu, thng vo khong 0.016-0.0375giy im bt u ndim kt thc.Ph thuc vo bng thng, ti, v tc truyn nhng knh LAN v WAN, c th cng tora tr do framing v queuing nm trong khong 0.005-0.025 giy. tr Routing, queuing, propagation qua WAN ph thuc vo mi trng truyn v khongcch, thng t 0.01-0.04 giy.Jitter, hay s thay i tc n ca gi trong lung audio v video. Thit b nhn s lu tr tmthi nhng gi n trc v i nhng gi n sau, sau mi mang ln lp ng dng, thng t0.02-0.04 giy hoc hn.

Nhiu yu t nh l tr lan truyn trn mi trng WAN, th khng th khc phc v hanthin c. Ngoi ra, nh l chn loi m ha (encoder), l ty chn m c th nh hng nhiu sut mng. Nhiu thut ton m ha th c thit k, trong c mt vi loi l chun. Nhnchung vi nhng loi m khc nhau th c hoc khng cc thut ton nn, nn gip tit kim bngthng nhng c tr cao v cht lng thp. Mt s loi m ph bin nh:

G.711 dng k thut m PCM vi tc truyn l 64Kbps vi tr do m khng ng k


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G.723.1 dng hai k thut m ACELP 6.3 Kbps v MPMLQ 5.3 Kbps vi tr m 0.035giyG.729 dng k thuyt m CSACELP 8 Kbps vi tr m 0.015 giy.Do ngi qun tr cn phi cn bng gia bng thng v tr v phi cn bng gia mc chiph cao ca bng thng vi cht lng thp ca Qos t vic tng tr. S cn bng ny i khi

c c t vic dng loi m nh l G.711 cho nhng giao tip ni b vi lng bng thng lnv thng khng cn chi ph. i vi nhng cuc gi qua WAN, bng thng t v t tin th cth dng loi m nh l G.729.

Kt lun:Vic chn kin trc mng da trn PBX hay router cn tham kho kinh nghim ca nhng nhcung cp, nu nh khng chc ty chn no tt hn, hy xem xt tin trnh gii quyt vn catng nh cung cp hay t chc.Chn giao thc H323 hay SIP cho bo hiu cng nn tham kho kin ca nhng t chc hay c

nhn c kinh nghim, v v c hai u c nhng thun li v nhc im nn vic chn la laigiao thc s dng cng cn c vo s trin khai v pht trin ca t chc.Cn thn vi cn bng gia chi ph v Qos, bi v ngi dng quen vi cht lng thoi cacuc gi qua mng PSTN, do o trong qu trnh thit kt cn bt u vi i tng Qos trn mngth nghim v xem bao nhiu bng thng th cn m bo yu cu .Chn loi m (codec) cng c th nh hng n cht lng cuc gi. Vi d, m tc thp smang cht lng thoi tt cho nhng khu vc ni b. Tri li m tc cao s lm gim chtlng cuc gi, ph hp cho nhng cuc gi nhng khu vc xa nhau.

S khc nhau gia VoIP v IP telephony?

Mc d nhiu ngi dng hai thut ng VoIP v IP telephony thay th qua li, hai thut ngny l khc nhau. Tht ra, IP Telephony ph thuc vo VoIP cho cc chc nng hot ngca n. Bn cnh vn ng ngha, VoIP l cng ngh cho php IP telephony thay th mngPSTN truyn thng (Public Switched Telephone Network) bng mt gii php hon ton datrn my tnh (computer-based solution).

Cng ngh VoIP cho php truyn d liu voice trn mt mng d liu. VoIP bao gm vic chuyni cc tn hiu thoi dng analog sang dng s, truyn cc d liu dng s ny trn mt mng vchuyn i d liu s ny ngc v dng thoi tng t ban u. Nh vy, gii php VoIP thchin mt cch hiu qu chc nng ca mt h thng thoi analog truyn thng.

Mt in thoi bnh thng c th c dng a cc d liu thoi ca ngi gi vo mt hthng VoIP. Mt lot cc chuyn i t analog-sang-digital v cc tin trnh khc trn my tnhthay i tn hiu thoi sang dng data, sau c truyn trn mng cc b LAN hoc mng dinrng WAN hoc trn Internet.


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Mt chui cc tin trnh trn my tnh sau s chuyn i dng d liu ny ngc v dng tnhiu thoi ban u, dng tn hiu ny sau chuyn thnh m thanh bn in thoi ca nginhn.

Mt th h in thoi mi, c gi l IP phones, c trang b bn trong n mt my tnh nh,

thc hin chc nng chuyn i t tng t-s v t s-tng t. Cc in thoi ny c th cmtrc tip vo mt LAN hay WAN.

Vi VoIP, mt tng i ni b PBX vn c th c dng phn phi cuc gi trong ton doanhnghip. Cuc gi thoi c th c thc hin thng qua mt mng data hay thng qua cc nganalog n ngi nhn. Nu cuc gi c thc hin bn trong doanh nghip trn mt mng data,nhu cu cho mt ng truyn d liu l khng cn thit v gi tin c th truyn trn LAN nh lgi tin data.

IP telephony s dng cc kh nng ca VoIP v bao gm mt my c cc phn mm thay thhon ton h tng ca mng PSTN. IP telephony loi b nhu cu dng tng i PBX v thm chthay th in thoi bnh thng bng cc softphones cho php mt PC dng nh mt u cui.Vic thay th vn cabling v cc chc nng kim sot trong mng PSTN trc y s cn thmcc cng ngh b sung. V d, mt gii php da trn phn mm c gi l softswitch haycall agent thay th cc tng i PSTN kiu c trc y.

Nu dng softswitch, cc server s thay th PBX trong cc h thng IP telephony. Vi cc tng ic, mt cuc gi yu cu thit lp mt ng truyn ring bit trong sut mt cuc gi. Tuy nhin,nu dng softswitch, ngay khi hai thit b u cui thit lp kt ni, softswitch s sn sng cho

mt kt ni mi. Hn na, cc tn hiu thoi c truyn trn mng d liu ging nh cc d liukhc, lm cho cc mang d liu c th thc hin nhiu tc v, bao gm truyn thoi, d liu vvideo.

Mc d gii php IP telephony c th bao gm nhiu c im phc tp, gii php ny c th baogm cc c im tng t nh cng ngh c, chng hn nh hi thoi, chuyn cuc gi trong khivn duy tr chi ph thp do c truyn trn mng d liu bng thng cao.

Tổng quan về VoIP(VNPRO)

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