Dịch vụ VOIP

7/30/2019 Dch v VOIP

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Dch v

VOIP


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Chng 1 Tng quan v VoIP

Chng 1 tng Quan V Voip

Dch v in thoi IP l dch v ng dng cao cp cho php truyn ti cc

cuc m thoi s dng h tng mng IP. Nguyn tc VoIP gm vic s ho tn

hiu ging ni, nn tn hiu s ho, chia tn hiu thnh cc gi v truyn nhng

gi s liu ny trn nn IP. n ni nhn, cc gi s liu c ghp li, gii m ra

tn hiu analog phc hi m thanh.

Trong dch v in thoi IP c th c s tham gia ca 3 loi i tng cung

cp dch v nh sau:

- Nh cung cp Internet ISP

- Nh cung cp dch v in thoi Internet ITSP

- Nh cung cp dch v trong mng chuyn mch knh

c th s dng c dch v in thoi IP, ngi s dng cn thng qua

mng Internet v cc chng trnh ng dng cho in thoi IP. Trong khi cc nh

cung cp dch v Internet cung cp s truy cp Internet cho khch hng ca h th

cc nh cung cp dch v in thoi ITSP cung cp dch v in thoi IP cho khch

hng bng cch s dng cc chng trnh ng dng dng cho in thoi IP. C th

ni rng dch v truy cp Internet cung cp bi cc ISP cha cung cp dch

v in thoi IP. Ng

i s dng cn phi truy nhp vo nh cung cp dch v inthoi IP khi s dng in thoi IP. H khng th gi hoc nhn cc cuc m thoi

thng qua dch v in thoi IP nu ch c truy nhp vo mng Internet. phc

v cho vic truyn thng gia nhng ngi s dng trn cc my tnh u cui ca

mng Internet, cc cng ty phn mm cung cp cc trng trnh ng dng dng

cho in thoi IP thc hin vai tr ca ITSP. i vi ngi s dng trn mng

chuyn mch knh, h s truy nhp vo ISP hoc ITSP thng qua cc im truy

nhp trong mng chuyn mch knh.

VoIP da trn s kt hp ca mng chuyn mch knh v chuyn mch gi lmng IP. Mi loi mng c nhng c im khc bit nhau. Trong mng chuyn

mch knh mt knh truyn dn dnh ring c thit lp gia hai thit b u

cui thng qua mt hay nhiu nt chuyn mch trung gian. Dng thng tin truyn

trn knh ny l dng bit truyn lin tc theo thi gian. Bng thng ca knh dnh

ring c m bo v c nh trong qu trnh lin lc (64Kbps i vi mng in

thoi PSTN), v tr thng tin l rt nh ch c thi gian truyn thng tin trn

knh. Khc vi mng chuyn mch knh, mng chuyn mch gi (Packet

Switching Network) s dng h thng l

u tr ri truyn ti cc nt mng. Thng

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tin c chia thnh cc gi, mi gi c thm 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 x l v lu tr trong mt thi gian nht nh ri mi c

truyn n nt tip theo sao cho vic s dng knh c hiu qu cao nht. Trongmng chuyn mch gi khng c knh dnh ring no c thit lp, bng thng

ca knh logic gia hai thit b u cui thng khng c nh, v tr thng tin

ln hn mng chuyn mch knh rt nhiu.

p dng VoIP c th khai thc tnh hiu qu ca cc mng truyn s liu,

khai thc tnh linh hot trong pht trin cc ng dng mi ca giao thc IP. Nhng

VoIP cng phc tp v i hi gii quyt nhiu vn .

1.1 Cu hnh ca mng in thoi IPTheo cc ngin cu ca ETSI, cu hnh chun ca mng in thoi IP c th

bao gm cc phn t sau:

- Thit b u cui kt ni vi mng IP

- Mng truy nhp IP

- Mng xng sng IP

- Gateway

- Gatekeeper

- Mng chuyn mch knh

- Thit b u cui kt ni vi mng chuyn mch knh

Trong cc kt ni khc nhau cu hnh mng c th thm hoc bt mt s phn

t trn.

Cu hnh chung ca mng in thoi IP gm cc phn t Gatekeeper,

Gateway, cc thit b u cui thoi v my tnh. Mi thit b u cui giao tip

vi mt Gatekeeper v giao tip ny ging vi giao tip gia thit b u cui v

Gateway. Mi Gatekeeper s chu trch nhim qun l mt vng, nhng cng c

th nhiu Gatekeeper chia nhau qun l mt vng trong trng hp mt vng c

nhiu Gatekeeper.

Trong vng qun l ca cc Gatekeeper, cc tn hiu bo hiu c th c

chuyn tip qua mt hoc nhiu Gatekeeper. Do cc Gatekeeper phi c kh

nng trao i cc thng tin vi nhau khi cuc gi lin quan n nhiu Gatekeeper.

Cu hnh ca mng in thoi IP c m t trong hnh 1.1.

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Gatekeeper

Gateway

Gateway

DNS Server

Mng chuyn

mch knh

IP NetworkPC

Telephone

Mng chuynmch knh

TelephonePC

Hnh 1.1 Cu hnh ca mng in thoi IP

Chc nng ca cc phn t trong mng nh sau:

1/Thit b u cui:

Thit b u cui l mt nt cui trong cu hnh ca mng in thoi IP. N

c th c kt ni vi mng IP s dng mt trong cc giao din truy nhp. Mt

thit b u cui c th cho php mt thu bao trong mng IP thc hin cuc gi

ti mt thu bao khc trong mng chuyn mch knh. Cc cuc gi s c

Gatekeeper m thit b u cui hoc thu bao ng k gim st.

Mt thit b u cui c th gm cc khi chc nng sau:

- Chc nng u cui: Thu v nhn cc bn tin;- Chc nng bo mt knh truyn ti: m bo tnh bo mt ca knh

truyn ti thng tin kt ni vi thit b u cui.

- Chc nng bo mt knh bo hiu: m bo tnh bo mt ca knh bo

hiu kt ni vi thit b u cui.

- Chc nng xc nhn: thit lp c im nhn dng khch hng, thit b

hoc phn t mng, thu nhp cc thng tin dng xc nh bn tin bo hiu hay

bn tin cha thng tin c truyn hoc nhn cha.

- Chc nng qun l: giao tip vi h thng qun l mng.

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- Chc nng ghi cc bn tin s dng: xc nh hoc ghi li cc thng tin v

s kin ( truy nhp, cnh bo ) v ti nguyn.

- Chc nng bo co cc bn tin s dng: bo co cc bn tin c s

dng ra thit b ngoi vi.

2/Mng truy nhp IP

Mng truy nhp IP cho php thit b u cui, Gateway, Gatekeeper truy nhp

vo mng IP thng qua c s h tng sn c. Sau y l mt vi loi giao din truy

nhp IP c s dng trong cu hnh chun ca mng in thoi IP:

- Truy nhp PSTN

- Truy nhp ISDN

- Truy nhp LAN- Truy nhp GSM

- Truy nhp DECT

y khng phi l tt c cc giao din truy nhp IP, mt vi loi khc ang

c nghin cu s dng cho mng in thoi IP. c im ca cc giao din

ny c th gy nh hng n cht lng v tnh bo mt ca cuc gi in thoi

IP.

3/GatekeeperGatekeeper l phn t ca mng chu trch nhim qun l vic ng k, chp

nhn v trng thi ca cc thit b u cui v Gateway. Gatekeeper c th tham

gia vo vic qun l vng, x l cuc gi v bo hiu cuc gi. N xc nh ng

dn truyn bo hiu cuc gi v ni dung i vi mi cuc gi. Gatekeeper c

th bao gm cc khi chc nng sau:

- Chc nng chuyn i a ch E.164 ( S E.164 l s in thoi tun th

theo cu trc v k hoch nh s c m t trong khuyn ngh E.164 ca Lin

minh vin thng quc t ITU) : chuyn i a ch E.164 sang a ch IP v ngc

li truyn cc bn tin, nhn v truyn a ch IP truyn cc bn tin, bao gm

c m la chn nh cung cp.

- Chc nng dch a ch knh thng tin: nhn v truyn a ch IP ca cc

knh truyn ti thng tin, bao gm c m la chn nh cung cp.

- Chc nng dch a ch knh: nhn v truyn a ch IP phc v cho bo

hiu, bao gm c m la chn nh cung cp.

- Chc nng giao tip gia cc Gatekeeper: thc hin trao i thng tin gia

cc Gatekeeper.

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- Chc nng ng k: cung cp cc thng tin cn ng k khi yu cu dch

v.

- Chc nng xc nhn: thit lp cc c im nhn dng ca khch hng,

thit b u cui hoc cc phn t mng.- Chc nng bo mt knh thng tin: m bo tnh bo mt ca knh bo

hiu kt ni Gatekeeper vi thit b u cui.

- Chc nng tnh cc: thu thp thng tin tnh cc.

- Chc nng iu chnh tc v gi cc: xc nh tc v gi cc.

- Chc nng qun l: giao tip vi h thng qun l mng.

- Chc nng ghi cc bn tin s dng: xc nh hoc ghi li cc thng tin v

s kin (truy nhp, cnh bo) v ti nguyn.

- Chc nng bo co cc bn tin s dng: bo co cc bn tin

c sdng ra thit b ngoi vi.

4/Gateway

Gateway l mt phn t khng nht thit phi c trong mt giao tip H.323.

N ng vai tr lm phn t cu ni v ch tham gia vo mt cuc gi khi c s

chuyn tip t mng H.323 ( v d nh mng LAN hay mng Internet) sang mng

phi H.323 ( v d mng chuyn mch knh hay PSTN). Mt Gateway c th kt

ni vt l vi mt hay nhiu mng IP hay vi mt hay nhiu mng chuyn mchknh. Mt Gateway c th bao gm: Gateway bo hiu, Gateway truyn ti knh

thoi, Gateway iu khin truyn ti knh thoi. Mt hay nhiu chc nng ny c

th thc hin trong mt Gatekeeper hay mt Gateway khc.

- Gateway bo hiu SGW: cung cp knh bo hiu gia mng IP v mng

chuyn mch knh. Gateway bo hiu l phn t trung gian chuyn i gia bo

hiu trong mng IP ( v d H.323) v bo hiu trong mng chuyn mch knh (v

d R2, CCS7). Gateway bo hiu c cc chc nng sau:

+ Chc nng kt cui cc giao thc iu khin cuc gi.+ Chc nng kt cui bo hiu t mng chuyn mch knh: phi hp hot

ng vi cc chc nng bo hiu ca Gateway iu khin truyn ti knh thoi.

+ Chc nng bo hiu: chuyn i bo hiu gia mng IP vi bo hiu

mng chuyn mch knh khi phi hp hot ng vi Gateway iu khin truyn

ti knh thoi.

+ Chc nng giao din mng chuyn mch gi: kt cui mng chuyn mch

gi.

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+ Chc nng bo mt knh bo hiu: m bo tnh bo mt ca knh bo

hiu kt ni vi thit b u cui.

+ Chc nng qun l: giao tip vi h thng qun l mng.

+ Chc nng ghi cc bn tin s dng: xc nh hoc ghi li cc thng tin vs kin (truy nhp, cnh bo) v ti nguyn.

+ Chc nng bo co cc bn tin s dng: bo co cc bn tin c s


- Gateway truyn ti knh thoi MGM: cung cp phng tin thc hin

chc nng chuyn i m ho. N s chuyn i gia cc m ho trong mng IP

vi cc m ho truyn trong mng chuyn mch knh. Gateway truyn ti knh

thoi bao gm cc khi chc nng sau:

+ Chc nng chuyn i a ch knh thng tin: cung cp a ch IP cho ccknh thng tin truyn v nhn.

+ Chc nng chuyn i lung: chuyn i gia cc lung thng tin gia

mng IP v mng chuyn mch knh bao gm vic chuyn m ho v trit

ting vng.

+ Chc nng dch m ho: nh tuyn cc lung thng tin gia mng IP v

mng chuyn mch knh.

+ Chc nng giao din vi mng chuyn mch knh: kt cui v iu khin

cc knh mang thng tin t mng chuyn mch knh.+ Chc nng chuyn i knh thng tin gia mng IP v mng chuyn

mch knh: chuyn i gia knh mang thng tin thoi, Fax, d liu ca mng

chuyn mch knh v cc gi d liu trong mch IP. N cng thc hin cc chc

nng x l tn hiu thch hp nh: nn tn hiu thoi, trit ting vng, m ho,

chuyn i tn hiu Fax v iu tit tc modem tng t. Thm vo , n cn

thc hin vic chuyn i gia tn hiu m a tn DTMF trong mng chuyn mch

knh v cc tn hiu thch hp trong mng IP khi cc b m ho tn hiu thoi

khng m ho tn hiu m a tn DTMF. Chc nng chuyn i knh thng tingia mng IP v mng chuyn mch knh cng c th thu nhp thng tin v lu

lng gi v cht lng knh i vi mi cuc gi s dng trong vic bo co

chi tit v iu khin cuc gi.


+ Chc nng ghi cc bn tin s dng: xc nh hoc ghi li cc thng tin v




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- Gateway iu khin truyn ti knh thoi MGWC: ng vai tr phn t

kt ni gia Gateway bo hiu v Gatekeeper. N cung cp chc nng x l cuc

gi cho Gateway, iu khin Gateway truyn ti kn thoi, nhn thng tin bo hiu

ca mng chuyn mch knh t Gateway bo hiu v thng tin bo hiu ca mngIP t Gatekeeper. Gateway iu khin truyn ti knh thoi bao gm cc chc

nng sau:

+ Chc nng truyn v nhn cc bn tin.

+ Chc nng xc nhn: thit lp cc c im nhn dng ca ngi s dng,

thit b hoc cc phn t mng.

+ Chc nng iu khin cuc gi: lu gi cc trng thi cuc gi ca

Gateway. Chc nng ny bao gm tt c cc iu khin kt ni logic ca Gateway.

+ Chc nng bo hiu: chuyn i gia bo hiu mng IP v bo hiu mngchuyn mch knh trong qu trnh phi hp hot ng vi Gateway bo hiu.


Chc nng ghi cc bn tin s dng: xc nh hoc ghi li cc thng tin v




1.2 Cu trc kt niHnh 1.2 m t cc thnh phn c bn ca mng phc v cho dch v thoi

qua Internet.

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H.323 Gatekeeper

PPP Access Server

VoIP-H.323 Gateway

DNS Server

Mng chuyn

mch knh

IP Network

H.323 Terminal

Mng chuyn

mch knh

Telephone

HubRouter

PB X

H.323 Terminal

Telephone

Hnh 1.2 Cc phn t c bn ca mng in thoi IP

V c bn c th chia cu trc kt ni trong cc ng dng dch v thoi

Internet thnh ba loi:

- Kt ni PC-PC

- Kt ni PC-My thoi

- Kt ni My thoi-My thoi

1.2.1 Kt ni PC-PC

Khi thc hin kt ni PC vi PC v mt hnh thc c th chia lm hai loi:

- Kt ni thng qua mng LAN hoc mt mng IP.

-

Kt ni gia mt PC trong mng IP ny vi mt PC trong mng IP khc thngqua mng PSTN .

1.2.2 Kt ni PC-My thoi

i vi cc kt ni PC v my thoi, do c s chuyn tip t mng Internet

sang mng SCN nn bao gi cng c s tham gia ca Gateway.

Sau y l mt s tnh hung kt ni mt PC v mt my thoi:

- Mt mng LAN/Mt nh qun tr vng

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y l kt ni gia mt u cui IP v mt my in thoi. Trong mng

LAN c cu trc n gin nht gm mt Gateway, mt Gatekeeper v cc u

cui IP to thnh mt phn mng LAN .

Trong tr

ng hp ny cc u cui IP v Gateway mun hot ng u ngk vi Gatekeeper v mi bo hiu thc hin cuc gi u do Gatekeeper iu

khin.

- Hai mng LAN/Mt Gatekeeper/Mt nh qun tr vng.

Trong trng hp ny cc phn t H.323 nm trong hai mng LAN nhng

cuc gi ch do mt Gatekeeper gi vai tr lm nh qun tr vng iu khin . Cu

hnh ny thch hp cho vic xy dng mng ca mt cng ty.

- Hai mng LAN/Hai Gatekeeper/Mt nh qun tr vng.

Trong trng hp ny cc phn t H.323 nm trong hai mng LAN. V cim th n gn ging vi trng hp trn, nhng nh c Gatekeeper th hai nn

mi mng LAN c mt Gatekeeper iu khin. Nh phng thc iu khin s

mm do hn cho php nh qun tr vng iu khin lu lng trong cc mng

LAN v lu lng chuyn giao gia chng. Ton b bo hiu cuc gi do

Gatekeeper ni trc tip vi u cui IP ng vai tr lm nh qun tr vng iu

khin.

- Hai mng LAN/Hai nh qun tr vng/C kt ni trc tip vi nhau.

Trng hp ny thc hin kt ni c lin quan n hai mng LAN do hai nhqun tr mng khc nhau qun l . Trao i bn tin bo hiu cuc gi gia chng

thng qua knh bo hiu ni trc tip gia hai hai Gatekeeper.

- Hai mng LAN/Hai nh qun tr vng/Kt ni thng qua Gatekeeper trung gian

Trong trng hp kt ni c lin quan n hai mng LAN m cc Gatekeeper

ca chng khng c knh bo hiu ni trc tip vi nhau th thc hin cuc gi

chng phi thng qua mt hay nhiu Gatekeeper khc ng vai tr lm cu ni.

1.2.3 Kt ni My thoi-My thoiTrong kt ni gia hai my in thoi c thc hin thng qua mng IP

thay v c kt ni trong mng PSTN.

1.3 c im ca in thoi IPin thoi IP ra i nhm khai thc tnh hiu qu ca cc mng truyn s liu,

khai thc tnh linh hot trong pht trin cc ng dng mi ca giao thc IP v n

c p dng trn mt mng ton cu l mng Internet. Cc tin b ca cng ngh

mang n cho in thoi IP nhng u im sau:

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+ Gim chi ph cuc gi: u im ni bt nht ca in thoi IP so vi dch

v in thoi hin ti l kh nng cung cp nhng cuc gi ng di gi r vi

cht lng chp nhn c. Nu dch v in thoi IP c trin khai, chi ph cho

mt cuc gi

ng di s ch t

ng

ng vi chi ph truy nhp internet. Nguynnhn dn n chi ph thp nh vy l do tn hiu thoi c truyn ti trong mng

IP c kh nng s dng knh hiu qu cao. ng thi, k thut nn thoi tin tin

gim tc bt t 64 Kbps xung thp ti 8 Kbps (theo tiu chun nn thoi

G.729A ca ITU-T) kt hp vi tc x l nhanh ca cc b vi x l ngy nay

cho php vic truyn ting ni theo thi gian thc l c th thc hin c vi

lng ti nguyn bng thng thp hn nhiu so vi k thut c.

So snh mt cuc gi trong mng PSTN vi mt cuc gi qua mng IP, ta

thy:Chi ph phi tr cho cuc gi trong mng PSTN l chi ph phi b ra duy tr

cho mt knh 64kbps sut t u cui ny ti u cui kia thng qua mt h thng

cc tng i. Chi ph ny i vi cc cuc gi ng di (lin tnh, quc t) l kh

ln.

Trong trng hp cuc gi qua mng IP, ngi s dng t mng PSTN ch

phi duy tr knh 64kbps n Gateway ca nh cung cp dch v ti a phng.

Nh cung cp dch v in thoi IP s m nhn nhim v nn, ng gi tn hiu

thoi v gi chng i qua mng IP mt cch c hiu qu nht ti c Gatewayni ti mt mng in thoi khc c ngi lin lc u kia. Vic kt ni nh vy

lm gim ng k chi ph cuc gi do phn ln knh truyn 64Kbps c thay

th bng vic truyn thng tin qua mng d liu hiu qu cao.

+ Tch hp mng thoi, mng s liu v mng bo hiu: Trong in thoi

IP, tn hiu thoi, s liu v ngay c bo hiu u c th cng i trn cng mt

mng IP. iu ny s tit kim c chi ph u t xy dng nhng mng ring

r.

+ Kh nng m rng (Scalability): Nu nh cc h tng i thng l nhngh thng kn, rt kh thm vo nhng tnh nng th cc thit b trong mng

internet thng c kh nng thm vo nhng tnh nng mi. Chnh tnh mm do

mang li cho dch v in thoi IP kh nng m rng d dng hn so vi in

thoi truyn thng.

+ Khng cn thng tin iu khin thit lp knh truyn vt l: Gi

thng tin trong mng IP truyn n ch m khng cn mt s thit lp knh no.

Gi ch cn mang a ch ca ni nhn cui cng l thng tin c th n c

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ch. Do vy, vic iu khin cuc gi trong mng IP ch cn tp trung vo chc

nng cuc gi m khng phi tp trung vo chc nng thit lp knh.

+ Qun l bng thng: Trong in thoi chuyn mch knh, ti nguyn bng

thng cung cp cho mt cuc lin lc l c nh (mt knh 64Kbps) nh

ng trongin thoi IP vic phn chia ti nguyn cho cc cuc thoi linh hot hn nhiu. Khi

mt cuc lin lc din ra, nu lu lng ca mng thp, bng thng dnh cho lin

lc s cho cht lng thoi tt nht c th; nhng khi lu lng ca mng cao,

mng s hn ch bng thng ca tng cuc gi mc duy tr cht lng thoi chp

nhn c nhm phc v cng lc c nhiu ngi nht. im ny cng l mt

yu t lm tng hiu qu s dng ca in thoi IP. Vic qun l bng thng mt

cch tit kim nh vy cho php ngi ta ngh ti nhng dch v cao cp hn nh

truyn hnh hi ngh, iu m vi cng ngh chuyn mch c ngi ta khngthc hin v chi ph qu cao.

+ Nhiu tnh nng dch v: Tnh linh hot ca mng IP cho php to ra nhiu

tnh nng mi trong dch v thoi. V d cho bit thng tin v ngi gi ti hay

mt thu bao in thoi IP c th c nhiu s lin lc m ch cn mt thit b u

cui duy nht (V d nh mt thit b IP Phone c th c mt s in thoi dnh

cho cng vic, mt cho cc cuc gi ring t).

+ Kh nng multimedia: Trong mt cuc gi ngi s dng c th va ni

chuyn va s dng cc dch v khc nh truyn file, chia s d liu, hay xemhnh nh ca ngi ni chuyn bn kia.

in thoi IP cng c nhng hn ch:

+ 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 khnng 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 v/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.

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+ 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 cng 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 lctruy 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/hoc nhng tnh nng

vt tri hn so vi dch v in thoi hin ti.

1.4 Cc ng dng ca VoIP

1.4.1 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 khitiu 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.

1.4.2 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.

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14/74


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

ckh nng kim sot v iu khin cc cuc thoi thng qua mng Internet.

1.4.3 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 TechnologiesInternet PhoneJACK.

Thay v gi qua mng in thoi truyn thng, khch hng c th gi cho bn

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 n cng ty ca bn cng ging nh vic h gi

qua mng PSTN.

Bng vic s dng chng trnh chng hn Internet PhoneJACK, bn cng c

th x l cc cuc gi cng ging nh cc x l cc cuc gi khc. Bn c thnh 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 nhnhau.

1.4.4 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.

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15/74


1.4.5 Dch v fax qua IP

Nu bn gi nhiu fax t PC, c bit l gi ra nc ngoi th vic s dng

dch v Internet faxing s gip bn tit kim c tin v c knh thoi. Dch v

ny s chuyn trc tip t PC ca bn 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.

1.4.6 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 goi t ng (ACD). Mt u im ca thoi IP l kh nng kt hp c thoi v

d liu trn cng mt knh.

1.5 Nhn xtTrong chng ny ta mi ch trnh by v m hnh in thoi IP m cha i

su vo cc vn cn gii quyt trong cng ngh ny nh bo hiu, x l tn hiu

thoi v vn m bo cht lng dch v. Trong cc chng sau ta s ln lt

gii quyt vn ny.

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16/74

Chng2 Chun H.323

Chng 2 Chun H.323

u nm 1996 mt nhm cc cng ty ln (Microsoft, Intel...) t chc hi

ngh Voice over IP nhm thng nht tiu chun cho cc sn phm ca cc nh

cung cp. n thng 5/1996, ITU-T ph chun c t H.323. Chun H.323 cung

cp nn tng k thut cho truyn thoi, hnh nh v s liu mt cch ng thi qua

cc mng IP, bao gm c Internet. Tun theo chun H.323, cc sn phm v cc

ng dng a phng tin t nhiu hng khc nhau c th hot ng cng vi nhau,

cho php ngi dng c th thng tin qua li m khng phi quan tm ti vn

tng thch.

H.323 cng ng thi gii quyt cc ng dng ct li ca in thoi IP thng

qua vic nh ngha tiu chun v tr cho cc tn hiu m thanh, nh ngha mc

u tin trong vic chuyn ti cc tn hiu yu cu thi gian thc trong truyn thng

Internet. (H.324 nh ngha vic truyn ti cc tn hiu m thanh, hnh nh v d

liu qua mng in thoi truyn thng, trong khi H.320 nh ngha tiu chun

cho truyn ti cc tn hiu m thanh, hnh nh v d liu qua mng t hp a dch

v ISDN).

n nay H.323 pht trin thng qua hai phin bn. Phin bn th nht

(Version 1)

c thng qua vo nm 1996 v phin bn th hai (Version 2)

cthng qua vo thng mt nm 1998. ng dng ca chun ny rt rng bao gm c

cc thit b hot ng c lp (stand-alone) cng nh nhng ng dng truyn

thng nhng trong mi trng my tnh c nhn, c th p dng cho m thoi

im-im cng nh cho truyng thng hi ngh. H.323 cn bao gm c chc

nng iu khin cuc gi, qun l thng tin a phng tin v qun l bng thng

ng thi cn cung cp giao din gia mng LAN v cc mng khc.

2.1 Chng giao thc H.323Khuyn ngh ca ITU-T v chun H.323 a ra cu trc giao thc cho cc

ng dng H.323 bao gm cc khuyn ngh trong hnh 2.1.

H.245: khuyn ngh v bo hiu iu khin truyn thng multimedia.

H.225.0: ng gi v ng b cc dng thng tin a phng tin (thoi,

truyn hnh, s liu). Khuyn ngh ny bao gm giao thc RTP/RTCP v cc th

tc iu khin cuc gi Q.931 (DSS 1).

Cc chun nn tn hiu thoi: G.711 (PCM 64 kbps), G.722, G.723, G.728,

G.729.

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17/74

Chng2 Chun H.323

Cc chun nn tn hiu video: H.261, H.263

T.120: Cc chun cho cc ng dng chia s s liu.

Knh

S liu

Knh

Video

LAN (Ethernet, Token Ring,...)

IP

TCP UDP

RTP

AudiocodecG.711G.722G.723G.728G.729

Videocodec

H.261H.263RT

CP

(Knhiuk

hinA/V)

RA

S

H.225.0

(Q.931)

(Knhiukhin

cucgi)

H.2

45

(Knhiukhin

truynthng)

Data

application

T.120

Knh

AudioCc knh iu khin

Hnh 2.1 Chng giao thc H.323.

2.2 Cc thnh phn trong h thng H.323Cu trc ca mt h thng H.323 v vic thng tin gia h thng H.323 vi

cc mng khc c ch ra trn Hnh 2.2.

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18/74

Chng2 Chun H.323

Cc dng thng tin trong h thng H.323 c chia thnh cc loi sau:

- Audio (thoi): l tn hiu thoi c s ho v m ho. gim tc

trung bnh ca tn hi thoi, c ch pht hin tch cc thoi c th c s dng.

Tn hiu thoi c i km vi tn hiu iu khin thoi.

- Video (hnh nh): l tn hiu hnh nh ng cng

c s ho v m ho.Tn hiu video cng i km vi tn hiu iu khin video.

- S liu: bao gm tn hiu fax, ti liu vn bn, nh tnh, file, ...

- 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 iu khin cuc gi (Call control signals): c s dng cho cc

chc nng iu khin cuc gi nh l thit lp cuc gi, kt thc cuc gi, ...

(1) : Mt gateway c th cung cp mt hay nhiu kt ni ti GSTN, N-ISDN v B-ISDN

Hnh 2.2 : Cu trc h thng H.323

(

H.323

Terminal

H.323

MCU

Mng chuyn mch gi

H.323

Gatekeeper

H.323

Terminal

H.323

Terminal

GSTN N-ISD

H.323

Gateway

B-ISD

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19/74

Chng2 Chun H.323

- Tn hiu knh RAS: c s dng thc hin cc chc nng: ng k

tham gia vo mt vng H.323, kt np/tho g mt im cui (endpoint) 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.

V mt logic, h thng H.323 bao gm cc thnh phn:

- Thit b u cui H.323 (H.323 Terminal): 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 chuyn mch knh khc (PSTN/ISDN)

- Gatekeeper: L mt thnh phn khng bt buc. N thc hin cc chc

nng qun l hot ng ca h thng. Khi c mt gatekeeper trong h thng, mithnh phn trong h thng phi thc hin th tc ng k vi gatekeeper. Tt c

cc im cui H.323 (terminal, gateway, MCU) ng k vi gatekeeper to

thnh mt vng H.323 (H.323 zone) do gatekeeper qun l (Hnh 2.3).

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.

H.323 Zone

Terminal GatewayGatekeeper

Terminal Terminal Router Router Router

Terminal Terminal

Hnh 2.3 Vng H.323 (H.323 Zone)

2.2.1 Thit b u cui H.323

Hnh 2.3 miu t cc thnh phn chc nng ca mt thit b u cui H.323.

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20/74

Chng2 Chun H.323

GiaodinvimngL

AN

(LANInterface)

Chc nng iu khin h thng

(System Control)

RAS Control H.225.0

Call Control H .225.0

H.245 Control

Trchiuthu

(ReceivePathDelay)

Lp

nggidliuMultimedia

,chunH.225.0

(H.225.0Layer)

Audio Codec

G.711, G.722,

G.723, G.728,

G.729(G.711:

Bt bu c

Camera/

dis la

Micro/

S eaker

ng dng

s liu

Giao din

iu

khin h

thng cho

ngi s

dng

Cc chc nng

Hnh 2.3 Thit b u cui H.323 (H.323 Terminal)

- Cc phn giao tip vi ngi s dng.

- Cc b codec (Audio v video).

- Phn trao i d liu t xa (telematic).

- Lp (layer) ng gi (chun H.225.0 cho vic ng gi multimedia).

- Phn chc nng iu khin h thng

- V giao din giao tip vi mng LAN.

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Chng2 Chun H.323

Tt c cc thit b u cui H.323 u phi c mt n v iu khin h thng,

lp ng gi H.225.0, giao din mng v b codec thoi. B codec cho tn hiu

video v cc ng dng d liu ca ngi s dng l tu chn (c th c hoc

khng).

- Giao din vi mng LAN (LAN Interface):

Giao din vi mng LAN phi cung cp cc dch v sau cho lp trn (lp ng

gi d liu multimedia H.225.0):

Dch v thng tin tin cy u cui n u cui (v d nh TCP hay SPX).

Dch v ny phc v cho knh iu khin H.245 v knh d liu.

Dch v truyn thng tin khng tin cy u cui n u cui (v d nh UDP

hay IPX). Dch v ny phc v cho cc knh Audio, cc knh Video, v knh iukhin RAS.

Cc dch v ny c th l song cng hay bn song cng, thng tin unicast hay

multicast tu thuc vo ng dng, kh nng ca thit b u cui v cu hnh ca

mng LAN.

- B codec video (Video codec):

B video codec l thnh phn tu chn, cung cp cho thit b u cui kh

nng truyn video.

- B codec thoi (audio codec):Tt c cc thit b u cui H.323 u phi c thnh phn ny. N m nhn

chc nng m ho v gii m tn hiu thoi. Chc nng m/gii m dng thoi

PCM 64kbps lut A v lut (theo khuyn ngh G.711) l bt buc. Ngoi ra b

codec c th c thm chc nng m/gii m thoi theo cc thut ton khc gm:

CS-ACELP (khuyn ngh G.729 v G.729A), ADPCM (khuyn ngh G.723), LD-

CEPT (G.728), m ho bng rng (G.722).

Vi cc b codec thoi c nhiu kh nng m ho, thut ton c s dng

cho m/gii m thoi s c m phn gia cc terminal tham gia cuc m thoi(qu trnh ny c gi l trao i kh nng). Trong trng hp ny terminal phi

c kh nng hot ng khng i xng (v d nh m ho tn hiu pht s dng

theo khuyn ngh G.711 (PCM64), gii m tn hiu thu c theo G.728 (LD-

CEPT)).

Thit b u cui Terminal c th gi i nhiu knh thoi cng mt lc tu

thuc vo ng dng.

Cc gi thoi phi c gi ln tng giao vn (transport layer) mt cc nh

k theo nhng khong thi gian c xc nh bi chc nng codec no ang c

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Chng2 Chun H.323

s dng (khong thi gian ca khung tn hiu thoi). S phn phi gi thoi ln lp

trn (lp giao vn) khng c mun hn 5ms sau khi kt thc khong thi gian

ca khung thoi trc .

Thit b u cui H.323 c th thu mt vi knh thoi (m thoi hi ngh).Trong trng hp ny, terminal cn thc hin chc nng trn cc knh thoi li

thnh mt knh hn hp a n ngi s dng (Audio Mixing). S lng cc

knh thoi b hn ch cn c vo ti nguyn sn c ca mng.

- Tr chiu thu:

Chc nng tr chiu thu bao gm vic thm vo dng thng tin thi gian thc

mt tr m bo duy tr s ng b v b jitter ca cc gi n. tr

thm vo phi tnh n thi gian tr do x l tn hiu khi thu. Dng tn hiu chiu

pht khng

c lm tr.- Knh s liu (Data Channel):

Knh d liu trong thit b u cui H.323 l khng bt buc. Knh d liu c

th l n hng hay hai hng tu thuc vo tng ng dng. Nn tng ca ng

truyn s liu trong thit b u cui H.323 l chun T.120. Trong lun n phn

ny cng khng c m t chi tit.

- Chc nng iu khin truyn thng multimedia (chun H.245):

Chc nng iu khin truyn thng s dng knh iu khin truyn thng

H.245 truyn ti cc thng ip iu khin hot ng truyn thng u cui tiu cui bao gm:

+ Trao i kh nng (Capabilities Exchange).

+ ng m cc knh logic cho tn hiu media (tn hiu thi gian thc)

- Chc nng bo hiu RAS (Registration - Admission - Status):

Chc nng bo hiu RAS s dng cc thng ip H.225.0 thc hin cc th

tc iu khin gia termnal v gatekeeper, bao gm:

+ Khm ph gatekeeper.

+ ng k (registration) tham gia vo vng H.323.+ nh v im cui.

+ iu khin kt np, tho g (Admission/Desengage).

+ Thay i bng thng s dng(bandwidth changes).

+ Thng bo trng thi (status).

- Chc nng bo hiu cuc gi:

Chc nng bo hiu cuc gi s dng bo hiu cuc gi H.225.0 (Q.931)

thit lp kt ni gia cc im cui H.323.

- Lp ng gi thng tin (H.225.0 layer):

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23/74

Chng2 Chun H.323

Cc knh logic mang thng tin thoi, video, s liu hay thng tin iu khin

c thit lp theo cc th tc iu khin m t trong khuyn ngh H.245. Cc

knh logic hu ht l n hng v c lp trn mi hng truyn. Mt vi knh

lgic nh

knh s liu c th l hai h

ng v lin quan n th tc m knh haihng ca H.245. Mt s lng bt k cc knh logic c th c s dng

truyn ngoi tr knh iu khin H.245 (ch c mt knh cho mi cuc gi).

Ngoi ra cc im cui H.323 cn s dng thm hai knh cho bo hiu cuc gi

v cc chc nng lin quan n gatekeeper (RAS).

a. S knh logic (Logical Channel Number - LCN):

Mi mt knh logic c ch ra bi mt s knh logic (LCN) trong khong t

0 cho n 65535 nhm mc ch ph hp vi knh logic tng ng trong kt ni

tng giao vn. S knh logic

c bn pht chn mt cch tu tin ngoi tr knhlogic 0 c dnh ring cho knh iu khin h.245.

b. Gii hn tc bit ca knh logic:

Bng thng ca mt knh logic phi c gii hn bi mt gi tr cn trn suy

ra t kh nng pht ti thiu v kh nng thu ca thit b u cui. Da trn gii

hn ny, mt thit b u cui phi m knh logic vi tc gii hn knh thp

hn hoc bng cn trn v bn pht c th pht bt c dng thng tin no c tc

khng qu tc gii hn ca knh.

Tc gii hn knh ch ra tc ca dng d liu mang thng tin ni dungca knh m khng bao gm cc phn mo u giao thc.

Khi thit b u cui khng c thng tin no gi i trong mt knh th thit

b u cui khng cn phi gi i cc thng tin lp vo duy tr tc ca knh.

2.2.2 H.323 gateway

Gateway mang cc tnh nng phc v cho hot ng tng tc ca cc thit b

trong h thng vi cc thit b trong mng chuyn mch knh nh PSTN, ISDN,...

Thit b cng H.323 c b tr nm gia cc thnh phn trong h thng H.323 vicc thit b nm trong cc h thng khc (cc mng chuyn mch knh SCN). N

phi cung cp tnh nng chuyn i khun dng d liu truyn v chuyn i th

tc mt cch thch hp gia mng LAN cc loi mng m gateway kt ni ti, c

th:

- Thc hin chuyn i khun dng d liu thoi, video, s liu nu cn.

- Thc hin chc nng thit lp cuc gi, hu cuc gi i vi c hai pha

mng LAN v mng chuyn mch knh (SCN - Switched Circuit Network).

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25/74

Chng2 Chun H.323

- iu khin kt np (Admission Control): iu khin vic cho php hot

ng ca cc im cui.

- iu khin bng thng (Bandwidth Control): iu khin cp hoc t chi

cp mt phn bng thng cho cc cuc gi ca cc thit b trong h thng.- Qun l vng (Zone Management): Thc hin cc chc nng trn vi cc

im cui H.323 ng k vi gatekeeper (mt vng H.323).

Ngoi ra, GateKeeper c th cung cp cc chc nng tu chn sau:

- Bo hiu iu khin cuc gi (Call Control Signalling): Gatekeeper c th

nhn v x l bo hiu cuc gi iu khin hot ng ca cc thit b u cui

hoc nh hng cc thit b u cui ni trc tip vi nhau qua knh bo hiu

cuc gi (Call Signalling Channel). Trong tr

ng hp th hai, Gatekeeper trnhc vic phi x l cc thng ip iu khin.

- iu khin cho php cuc gi (Call Authorization): Gatekeeper c th t

chi thc hin cuc gi t mt thit b u cui ny ti mt thit b u cui khc.

L do ca vic ny c th l s gii hn truy nhp n mt thit b u cui hay

gateway hoc l gii hn truy nhp trong mt khong thi gian.

- Qun l bng thng (Bandwidth Management): Chc nng ny cho php

gatekeeper iu khin lng bng thng cp cho mt cuc gi ca mt im cui

trong h thng. Vic iu khin ny c th thc hin ngay trong khi cuc gi angtin hnh. Chc nng ny bao gm c chc nng iu khin vic cung cp bng

thng cho cc cuc gi.

- Qun l cuc gi (Call Management): Gatekeeper c th duy tr mt danh

sch ca cc cuc gi ang c tin hnh, nh bit c thit b no ang bn

hoc cung cp thng tin cho chc nng qun l bng thng.

- Tnh cc (Billing): Mi cuc gi trong h thng c mt gatekeeper u

phi thng qua s qun l ca gatekeeper, do vy s rt thun tin nu nh

gatekeeper m nhn chc nng tnh cc dch v.

2.2.4 n v iu khin lin kt a im MCU

2.2.4.1 c im

- MCU h tr vic thc hin cc cuc m thoi hi ngh gia nhiu thit

b u cui. Trong chun H.323, MCU bt buc phi c mt b iu khin a

im MC (Multipoint Controller) v c hoc khng mt vi MP (Multipoint

Processor).

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26/74

Chng2 Chun H.323

- MC v MP l cc phn ca MCU nhng chng c th khng tn ti trong

mt thit b c lp m c phn tn trong cc thit b khc. V d nh: mt

gateway c th c th mang trong n mt MC v mt vi MP thc hin kt ni

ti nhiu thit b u cui; mt thit b u cui c th mang mt b MC c ththc hin cng mt lc nhiu cuc gi.

- MC iu khin vic lin kt gia nhiu im cui trong h thng bao

gm:

- X l vic m phn gia cc thit b u cui quyt nh mt kh

nng x l dng d liu media chung gia cc thit b u cui.

- Quyt nh dng d liu no s l dng d liu multicast.

- MC khng x l trc tip mt dng d liu media no. Vic x l cc

dng d liu s do cc MP m nhim. MP s thc hin vic trn, chuyn mch,x l cho tng dng d liu thi gian thc trong cuc hi ngh.

2.2.4.2 Hi ngh nhiu bn

Vic truyn thng tin trong mng IP tn ti di ba hnh thc: Unicast,

multicast v broadcast.

- Unicast: vi unicast, thit b u cui phi thc hin vic truyn gi d

liu ti tng ch kt ni vi n.

- Multicast: Truyn thng multicast gi mt gi d liu ti mt nhm ccch trong mng m khng phi truyn lp li gi d liu .

- Broadcast: truyn thng broadcast gn ging truyn thng multicast

nhng gi d liu c truyn ti mi im cui trong mng.

Unicast v broadcast s dng mng khng hiu qu do cc gi phi truyn lp

li hoc phi truyn i khp mng. Truyn d liu multicast s dng bng thng

ca mng hiu qu hn do cc trm trong nhm truyn ch c mt dng d liu

duy nht.

Trong h thng H.323 cuc hi ngh nhiu bn c th c ba loi cu hnh hi

ngh sau:

- Cu hnh tp trung (Centralized Multipoint Conference).

- Cu hnh phn tn (Decentralized Multipoint Conference).

- Cu hnh lai (Hybrid Multipoint Conferrence).

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27/74

Chng2 Chun H.323

2.3 B giao thc RTP/RTCPTn hiu thoi sau khi nn xung tc thp c ng gi li truyn i

trong mng chuyn mch gi. C nhiu cch thc ng gi tn hiu thoi truyn

trong mng IP. Mt trong nhng cch thc c p dng nhiu nht l b giaothc RTP/RTCP nh tnh linh hot v kh nng gim st trng thi dng thng tin

mt cch hiu qu ca n.

2.3.1 Vai tr ca RTP/RTCP

Giao thc RTP (Realtime Transport Protocol) cung cp cc chc nng giao

vn ph hp cho cc ng dng truyn d liu mang c tnh thi gian thc nh l

thoi v truyn hnh tng tc. Nhng dch v ca RTP bao gm trng ch th loi

ti trng (payload identification), nh s th t cc gi, in tem thi gian (phc

v cho c ch ng b khi pht li tn hiu bn thu)...

Thng thng cc ng dng chy giao thc RTP bn trn giao thc UDP

s dng cc dch v ghp knh (multiplexing) v kim tra tng (checksum) ca

dch v ny; c hai giao thc RTP v UDP to nn mt phn chc nng ca giao

thc tng giao vn. Tuy nhin RTP cng c th c s dng vi nhng giao thc

khc ca tng mng v tng giao vn bn di min l cc giao thc ny cung cp

c cc dch v m RTP i hi. Giao thc RTP h tr vic truyn d liu ti

nhiu ch s dng phn b d liu multicast nu nh kh nng nay c tng

mng hot ng bn di n cung cp.

Mt iu cn lu l bn thn RTP khng cung cp mt c ch no m bo

vic phn pht kp thi d liu ti cc trm m n da trn cc dch v ca tng

thp hn thc hin iu ny. RTP cng khng m bo vic truyn cc gi theo

ng th t. Tuy nhin s th t trong RTP header cho php bn thu xy dng li

th t ng ca cc gi bn pht.

i cng vi RTP l giao thc RTCP (Realtime Transport Control Protocol) c

cc dch v gim st cht lng dch v v thu thp cc thng tin v nhng ngi

tham gia vo phin truyn RTP ang tin hnh.

Giao thc RTP c c tnh cho cha hon thin. N ch cung cp cc dch

v ph thng nht cho hu ht cc ng dng truyn thng hi ngh a phng tin.

Mi mt ng dng c th u c th thm vo RTP cc dch v mi cho ph hp

vi cc yu cu ca n. Cc kh nng m rng thm vo cho RTP c m t

trong mt profile i km. Ngoi ra, profile cn ch ra cc m tng ng s dng

trong trng PT (Payload type) ca phn tiu RTP ng vi cc loi ti trng

(payload) mang trong gi.

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Mt vi ng dng c th nghim cng nh thng mi c trin khai.

Nhng ng dng ny bao gm cc ng dng truyn thoi, video v chun on

tnh trng mng (nh l gim st lu lng). Tuy nhin, mng Internet ngy nay

vn ch

a th h tr

c y yu cu ca cc dch v thi gian thc. Cc dchv s dng RTP i hi bng thng cao (nh l truyn audio) c th l gim

nghim trng cht lng ca cc dch v khc trong mng, Nh vy nhng ngi

trin khai phi ch n gii hn bng thng s dng ca ng dng trong mng.

2.3.2 Cc ng dng s dng RTP

2.3.2.1 Hi ngh m thoi n gin

Cc ng dng hi ngh m thoi n gin ch bao gm vic truyn thoi

trong h thng. Tn hiu thoi ca nhng bn tham gia c chia thnh nhngon nh, mi phn c thm vo phn tiu ca giao thc RTP. Tiu RTP

mang thng tin ch ra cch m ho tn hiu thoi (nh l PCM, ADPCM, hay

LPC...). Cn c vo thng tin ny, cc bn thu s thc hin gii m cho ng.

Mng Internet cng nh cc mng gi khc u c kh nng xy ra mt gi v

sai lch v th t cc gi. gii quyt vn ny, phn tiu RTP mang thng

tin nh thi v s th t cc gi, cho php bn thu khi phc nh thi vi ngun

pht. S khi phc nh thi c tin hnh c lp vi tng ngun pht trong hi

ngh. S th t gi c th c s dng c tnh s gi b mt trong khi truyn.Cc gi thoi RTP c truyn i theo cc dch v ca giao thc UDP c th

n ch nhanh nht c th.

gim st s ngi tham gia vo hi ngh v cht lng thoi h nhn c

ti mi thi im, mi mt trm trong hi ngh gi i mt cch nh k mt gi

thng tin RR (Reception report) ca giao thc RTCP ch ra cht lng thu ca

tng trm. Da vo thng tin ny m cc thnh phn trong hi ngh c th tho

thun vi nhau v phng php m ho thch hp v vic iu chnh bng thng.

2.3.2.2 Hi ngh in thoi truyn hnh

Nu c hai dng tn hiu thoi v truyn hnh u c s dng trong hi ngh

th ng vi mi dng s c mt phin RTP (RTP session) c lp. Mi mt phin

RTP s ng vi mt cng (port number) cho thu pht cc gi RTP v mt cng thu

pht cc gi RTCP. Cc phin RTP s c ng b vi nhau cho hnh nh v

m thanh ngi dng nhn c n khp.

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L do b tr cc dng thng tin thoi v truyn hnh thnh nhng phin

RTP tch bit l cho cc thit b u cui ch c kh nng thoi cng c th

tham gia vo cuc hi ngh truyn hnh m khng cn c bt k thit b h tr no.

2.3.2.3 Translator v Mixer

Cc ng dng miu t phn trn u c im chung l bn thu v bn pht

u s dng chung mt phng php m ho thoi. Trong trng hp mt ngi

dng c ng kt ni tc thp tham gia vo mt hi ngh gm cc thnh vin

c ng kt ni tc cao th tt c nhng ngi tham gia u buc phi s dng

kt ni tc thp cho ph hp vi thnh vin mi tham gia. iu ny r rng l

khng hiu qu. khc phc, mt translator hoc mt mixer c t gia hai

vng tc ng truyn cao v thp chuyn i cch m ho thch hp giahai vng. im khc bit gia translator v mixer l mixer trn cc dng tn hiu

a n n thnh mt dng d liu duy nht trong khi translator khng thc hin

vic trn d liu.

2.3.3 Khun dng gi RTP

Tiu giao thc RTP bao gm mt phn tiu c nh thng c mi gi

RTP v mt phn tiu m rng phc v cho cc mc ch nht nh.

2.3.3.1 Phn tiu c nh

Tiu c nh c miu t trong hnh 2.4.

0 2 3 4 8 9 16 31

V=2 P X CC M PT sequence number

timestamp

synchronization source identifier (SSRC)

contributing source list (CSRC)

......

Hnh 2.4 Tiu c nh gi RTP.

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12 octets (byte) u tin ca phn tiu c trong mi gi RTP cn cc octets cn

li thng c mixer thm vo trong gi khi gi c mixer chuyn tip n

ch.

- Version(V): 2 bit.Trng ny ch ra version ca RTP. Gi tr ca trng ny l 2.

- Padding (P): 1 bit.

Nu bit padding c lp, gi d liu s c mt vi octets thm vo cui gi

d liu. Octets cui cng ca phn thm vo ny s ch kch thc ca phn thm

vo ny (tnh theo byte). Nhng octets ny khng phi l thng tin. Chng c

thm vo p ng cc yu cu sau:

Phc v cho mt vi thut ton m ho thng tin cn kch thc ca gi c

nh.Dng cch ly cc gi RTP trong trng hp nhiu gi thng tin c mang

trong cng mt n v d liu ca giao thc tng di.

- Extension (X): 1 bit.

Nu nh bit X c lp, theo sau phn tiu c nh s l mt tiu m

rng.

- Marker (M): 1 bit.

Tu tng trng hp c th m bt ny mang nhng ngha khc nhau ngha

ca n c ch ra trong mt profile i km.- Payload Type (PT): 7 bits.

Trng ny ch ra loi ti trng mang trong gi. Cc m s dng trong trng

ny ng vi cc loi ti trng c quy nh trong mt profile i km.

- Sequence Number: 16 bits.

Mang s th t ca gi RTP. S th t ny c tng ln mt sau mi gi RTP

c gi i. Trng ny c th c s dng bn thu pht hin c s mt gi

v khi phc li trnh t ng ca cc gi. Gi tr khi u ca trng ny l ngu

nhin.- Timestamp (tem thi gian): 32 bits.

Tem thi gian phn nh thi im ly mu ca octets u tin trong gi RTP.

Thi im ny phi c ly t mt ng h tng u n v tuyn tnh theo thi

gian cho php vic ng b v tnh ton jitter. Bc tng ca ng h ny

phi nh t c chnh xc ng b mong mun khi pht li v chnh

xc ca vic tnh ton jitter. Tn s ng h ny l khng c nh, tu thuc vo

loi khun dng ca ti trng. Gi tr khi u ca tem thi gian cng c chn

mt cch ngu nhin. Mt vi gi RTP c th mang cng mt gi tr tem thi gian

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Chng2 Chun H.323

nu nh chng c pht i cng mt lc v mt logic (v d nh cc gi ca cng

mt khung hnh video). Trong trng hp cc gi d liu c pht ra sau nhng

khong thi gian bng nhau (tn hiu m ho thoi tc c nh, fixed-rate

audio) th tem thi gian

c tng mt cch u n. Trong tr

ng hp khc gi trtem thi gian s tng khng u.

- S nhn dng ngun ng b SSRC (Synchronization Source Identifier):

32 bits.

SSCR ch ra ngun ng b ca gi RTP, s ny c chn mt cch ngu

nhin. Trong mt phin RTP c th c nhiu hn mt ngun ng b. Mi mt

ngun pht ra mt dng cc gi RTP. Bn thu nhm cc gi ca cng mt ngun

ng b li vi nhau pht li tn hiu thi gian thc. Ngun ng b c th l

ngun pht cc gi RTP pht ra t mt micro, camera hay mt RTP mixer.- Cc s nhn dng ngun ng gp (CSRC list - Contributing Source list):

c t 0 n 15 mc mi mc 32 bt.

Cc s nhn dng ngun ng gp trong phn tiu ch ra nhng ngun

ng gp thng tin v phn ti trng ca gi. Cc s nhn dng ny c Mixer

chn vo tiu ca gi v n ch mang nhiu ngha trong trng hp dng cc

gi thng tin l dng tng hp to thnh t vic trn nhiu dng thng tin ti

mixer. Trng ny gip cho bn thu nhn bit c gi thng tin ny mang thng

tin ca nhng ngi no trong mt cuc hi ngh.S lng cc s nhn dng ngun ng gp c gi trong trng CC ca

phn tiu . S lng ti a ca cc s nhn dng ny l 15. Nu c nhiu hn 15

ngun ng gp thng tin vo trong gi th ch c 15 s nhn dng c lit k

vo danh sch.

Mixer chn cc s nhn dng ny vo gi nh s nhn dng SSRC ca cc

ngun ng gp.

2.2.3.2 Phn tiu m rngC ch m rng ca RTP cho php nhng ng dng ring l ca giao thc

RTP thc hin c vi nhng chc nng mi i hi nhng thng tin thm vo

phn tiu ca gi. C ch ny c thit k mt vi ng dng c th b qua

phn tiu m rng ny (m vn khng nh hng ti s hot ng) trong khi

mt s ng dng khc li c th s dng c phn .

Cu trc ca phn tiu m rng nhhnh 2.5:

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0 2 3 4 8 9 16 31

defined by profile length

header extension

...

Hnh 2.5 Tiu m rng ca gi RTP.

Nu nh bit X trong phn tiu c nh c t bng 1 th theo sau phn

tiu c nh l phn tiu m rng c chiu di thay i.

- 16 bit u tin trong phn tiu c s dng vi mc ch ring cho

tng ng dng c nh ngha bi profile. Thng n c s dng phn bit

cc loi tiu m rng.

- Length: 16 bits. Mang gi chiu di ca phn tiu m rng tnh theo

n v l 32 bits. Gi tr ny khng bao gm 32 bit u tin ca phn tiu m

rng.

2.3.4 Giao thc iu khin RTCP

Giao thc RTCP da trn vic truyn u n cc gi iu khin ti tt c cc

ngi tham gia vo phin truyn. N s dng c ch phn phi gi d liu trong

mng ging nh giao thc RTP, tc l cng s dng cc dch v ca giao thc

UDP qua mt cng UDP c lp vi vic truyn cc gi RTP.

2.3.4.1 Cc loi gi iu khin RTCP

Giao thc RTCP bao gm cc loi gi sau:

- SR (Sender Report): Mang thng tin thng k v vic truyn v nhn

thng tin t nhng ngi tham gia ang trong trng thi tch cc gi.

- RR (Receiver Report): Mang thng tin thng k v vic nhn thng tin t

nhng ngi tham gia khng trng thi tch cc gi.

- SDES (Source Description items): mang thng tin miu t ngun pht gi

RTP.

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- BYE: ch th s kt thc tham gia vo phin truyn.

- APP: Mang cc chc nng c th ca ng dng.

Gi tr ca tr

ng PT (Packet Type) ng vi mi loi gi

c lit k trongbng sau:

Loi gi SR RR SDES BYE APP

PT (Decimal) 200 201 202 203 204

Mi gi thng tin RTCP bt u bng mt phn tiu c nh ging nh gi

RTP thng tin. Theo sau l cc cu trc c chiu di c th thay i theo loi

gi nhng lun bng s nguyn ln 32 bits. Trong phn tiu c nh c mttrng ch th di. iu ny gip cho cc gi thng tin RTCP c th gp li vi

nhau thnh mt hp gi (compound packet) d truyn xung lp di m khng

phi chn thm vo cc bit cch ly. S lng cc gi trong hp gi khng quy nh

c th m tu thuc vo chiu di n v d liu lp di.

Mi gi RTCP u phi c truyn trong hp gi d cho trong hp gi ch c

mt gi duy nht. Khun dng ca hp gi c xut nh sau:

Tip u m ho (Encription Prefix): (32 bit) 32 bit u tin c dnh nu

v ch nu hp gi RTCP cn c m ho. Gi tr mang trong phn ny cn ch trnh trng vi 32 bit u tin trong gi RTP.

Gi u tin trong hp gi lun lun l gi RR hoc SR. Trong trng hp

khng thu, khng nhn thng tin hay trong hp gi c mt gi BYE th mt gi

RR rng dn u trong hp gi.

Trong trng hp s lng cc ngun c thng k vt qu 31 (khng va

trong mt gi SR hoc RR) th nhng gi RR thm vo s theo sau gi thng k

u tin. Vic bao gm gi thng k (RR hoc SR) trong mi hp gi nhm thng

tin thng xuyn v cht lng thu ca nhng ngi tham gia. Vic gi hp gi ic tin hnh mt cch u n v thng xuyn theo kh nng cho php ca

bng thng.

Trong mi hp gi cng bao gm gi SDES nhm thng bo v ngun pht tn

hiu.

Cc gi BYE v APP c th c th t bt k trong hp gi tr gi BYE phi

nm cui cng.

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2.3.4.2 Khong thi gian gia hai ln pht hp

gi RTCPCc hp gi ca RTCP c pht i mt mt cch u n sau nhng khong

thi gian bng nhau thng xuyn thng bo v trng thi cc im cui tham

gia. Vn l tc pht cc hp gi ny phi m bo khng chim ht lu

lng thng tin dnh cho cc thng tin khc.

Trong mt phin truyn, lu lng tng cng cc i ca tt c cc loi thng

tin truyn trn mng c gi l bng thng ca phin (session bandwidth). Lu

lng ny c chia cho cc bn tham gia vo cuc hi ngh. Lu lng ny c

mng dnh sn v khng cho php vt qu khng nh hng n cc dch vkhc ca mng. Trong mi phn bng thng ca phin c chia cho cc bn tham

gia phn lu lng dnh cho cc gi RTCP ch c php chim mt phn nh v

bit l 5% khng nh hng n chc nng chnh ca giao thc l truyn cc

dng d liu media.

2.3.4.3 Khun dng gi SR

Khun dng gi SR (Sender Report) c miu t trong hnh 2.6.

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0 2 3 8 16 31

V=2 P RC PT = 200 length

SSRC ca ngun gi gi SR

NTP timestamp (32 bits gi)

NTP timestamp (32 bits tr)

RTP timestamp

S lng gi pht i ca ngun gi gi SR

S lng octets pht i ca ngun gi gi SR

SSRC_1 (SSRC ca ngun ng b th nht)

fraction lost cumulative number of packets lost

extended highest sequence number received

interarrival jitter

last SR (LSR)

delay since last SR (DLSR)

SSRC_2 (SSRC ca ngun ng b th hai)

...

profile-specific extension

Hnh 2.6 Khun dng gi SR

Gi SR bao gm 3 phn bt buc:

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1/Phn tiu di 8 octets

ngha ca cc trng nh sau:

- Version (V) v Padding (P):

Mang ngha ging nh trong tiu ca gi RTP.- Reception Report Count (RC): 5 bits.

S lng ca cc khi bo co tin cha trong gi. Nu trng ny mang gi tr

0 th y l gi SR rng.

- Packet Type (PT): 8 bits:

Ch th loi gi. Vi gi SR gi tr ny bng 200 (thp phn).

- Length: 16 bits.

Chiu di ca gi RTCP tr i 1 (tnh theo n v 32 bits). Chiu di ny bao

gm phn tiu v phn padding thm vo cui gi.- SSRC: 32 bits

Ch th ngun ng b cho ni pht ra gi SR ny.

2/Phn thng tin bn gi

Phn thng tin bn gi di 20 octets v c trong mi gi SR. Cc trng c

ngha nh sau:

- NTP timestamp (tem thi gian NTP): 64 bits.

Ch ra thi gian tuyt i khi gi bo co c gi i. Tem thi gian ny ckhun dng thi gian theo giao thc NTP (Network Time Protocol): Thi gian tnh

theo giy vi mc l 0h UTC ngy 1-1-1900; phn nguyn ca gi tr thi gian l

32 bit u tin; 32 bits cn li biu din phn thp phn.

- RTP timestamp (tem thi gian RTP): 32 bits.

Gi tr ca trng ny tng ng vi gi tr ca trng NTP timestamp trn

nhng c tnh theo n v ca nhn thi gian RTP trong gi d liu RTP v vi

cng mt lch ngu nhin ca nhn thi gian RTP trong gi d liu RTP.

- S lng gi pht i ca ngun gi gi SR (Senders packet count): 32bits.

S lng tng cng ca cc gi d liu RTP c truyn t ngun gi gi SR

k t khi bt u vic truyn thng tin cho ti thi im gi SR c to ra.

Trng ny c xo v khng trong trng hp ngun gi i s nhn dng

SSRC ca n. Trng ny c th c s dng c tnh tc d liu ti trng

trung bnh.

- S lng octets c ngun gi gi SR gi i (Sender octets count): 32

bit.

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S lng tng cng ca cc octets phn payload c truyn i trong cc gi

RTP bi ngun gi gi SR k t khi bt u vic truyn cho n thi im gi SR

ny c to ra.

3/Cc khi bo co thu (Reception Report blocks)

Phn ny bao gm cc khi thng tin bo co v vic thu cc gi t cc trm

trong phin truyn. S lng cc bo co c th l 0 trong trng hp gi bo co

rng. Mi khi bo co thng k v vic nhn cc gi RTP ca mt ngun ng

b, bao gm:

- S nhn dng ngun (SSRC_n): 32 bits.

- T l mt gi (fraction lost): 8 bits.

T l mt gi thng tin tnh t lc gi gi SR hoc RR trc . T l mt gic tnh bng cch em chia gi tr ca trng cho 256.

- S lng gi mt tng cng (cumulative number of packets lost): 24 bits.

Tng s gi mt k t lc bt u nhn. S gi mt bao gm c nhng gi n

ch qu mun.

ALSR

S

R

DLS

A - LSR

tr kh hi = A - LSR - DLSR

Hnh 2.7 Xc nh tr kh hi.

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- S th t cao nht nhn c: 32 bits.

16 bit tr mang s th t cao nht nhn c ng vi gi tr khi u l ngu

nhin. 16 bits gi mang s th t cao nht tng ng vi gi tr khi u bng 0.

- Jitter khi n ch: 32 bits.Mang gi tr c tnh jitter ca cc gi khi n ch. c tnh theo n v

ca trng timestamp v c biu din di dng s nguyn khng du. Jitter

c tnh l gi tr lm trn ca chnh lch khong cch v thi gian gia hai

gi bn thu v bn pht.

- Tem thi gian ca gi SR trc (LSR): 32 bits.

Mang gi tr tem thi gian thu gn ca gi SR trc . Nu trc khng

c gi SR no th trng ny bng 0.

- tr tnh t gi SR tr

c (DLSR): 32 bits. tr (tnh theo n v 1/65536 giy) gia thi im nhn gi SR trc t

ngun SSRC_n v thi im gi gi RR cha thng tin bo co cht lng nhn

tn hiu ca ngun n.

Hai trng LSR v DLSR ca khi bo co th r c s dng xc nh

tr kh hi gia hai ngun r v ngun n l ngun gi gi SR. Hnh sau minh ho

vic xc nh tr kh hi gia hai ngun n v r. Thi im A ngun n nhn

c gi RR t ngun r c ghi li v tr i gi tr ca trng LSR ca khi bo

co r ra c tr tng cng. Gi tr thu c li c tr i trng DLSR cakhi r tm ra tr kh hi ca gi thng tin gia n v r.

2.3.4.4 Khun dng gi RR

Gi RR (Receiver Reprort) c khun dng ging nh gi SR ngoi tr trng

PT mang gi tr bng 201 v khng mang phn thng tin v ngun gi. Khun

dng gi RR c miu t trong hnh 2.8.

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0 2 3 8 16 31

V=2 P RC PT = 201 length

SSRC ca ngun gi gi RR

SSRC_1 (SSRC ca ngun ng b th nht)

fraction lost cumulative number of packets lost

extended highest sequence number received

interarrival jitter

last SR (LSR)

delay since last SR (DLSR)

SSRC_2 (SSRC ca ngun ng b th hai)

...

profile-specific extension

Hnh 2.8 Khun dng gi RR

2.3.4.5 Khun dng gi SDES

Gi SDES (System Description).

Gi SDES c khun dng nh trong hnh 2.9 bao gm mt phn tiu v cc

on thng tin m t ngun.

1/Phn tiu

- Cc trng V (version), P (padding), length, PT (packet type) mang

ngha ging nh ca gi SR, PT bng 202.

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- SC (Source count): 5 bits.

S lng ca cc on thng tin m t ngun.

0 2 3 8 16 31V=2 P SC PT = 202 length

SSRC/CSRC_1

SDES cc mc m t ngun

...

SSRC/CSRC_2

...

Hnh 2.9 Khun dng gi SDES

2/Phn miu t ngun

Mi on thng tin miu t ngun bao gm mt cp s nhn dng ngun

SSRC/CSRC theo sau l cc mc miu t (SDES Items). Cc mc miu t c

cu trc chung nhhnh 2.10.

0 8 16 31

Item length Thng tin m t ngun

Hnh 2.10 Mc miu t

- Item (8 bits).

Ch th loi mc m t. Gi tr ca trng ny tng ng vi cc loi mcmiu t sau:

CNAME (Canonical Name) (item = 1): Phn thng tin m t mang s nhn

dng tng giao vn c nh i vi mt ngun RTP.

NAME (item = 2): phn thng tin m t mang tn m t ngun.

EMAIL (item = 3): Thng tin m t l a ch Email ca ngun.

PHONE (item = 4): Thng tin m t l s in thoi ca ngun.

LOC (item = 5): Thng tin m t l a ch ca ngun.

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TOOL (item = 6): Thng tin m t l tn ca ng dng to ra dng thng tin

media.

NOTE (item = 7): Cc ch thch v ngun.

PRIV (item = 8): Dnh cho cc thng tin khc.

2.3.4.6 Khun dng gi BYE

Gi BYE c s dng thng bo mt hay mt vi ngun s ri khi phin

truyn. Trng thng tin v l do ri khi phin l tu chn (c th c hoc

khng).

0 2 3 8 16 31

V=2 P SC PT = 203 length

SSRC/CSRC

...

Length reson for leaving (opt)

Hnh 2.11 Khun dng gi BYE

2.3.4.7 Khun dng gi APP

Khun dng gi APP c miu t trong hnh 2.12. Gi ny c s dng

dnh cho cc chc nng c th ca tng ng dng.

0 2 3 8 16 31

V=2 P SC PT = 204 length

Name (ASCII)

D liu ca ng dng

Hnh 2.12 Khun dng gi APP

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2.4 Bo hiu v x l cuc gi

2.4.1 Chuyn i a ch

2.4.1.1 a ch mng

Mi mt thit b H.323 c gn t nht mt a ch mng nhn dng. Mtvi thit b H.323 c th cng chia s mt a ch mng, cc mng khc nhau th

khun dng a ch mng cng khc nhau. Trong cng mt cuc gi, im cui c

th s dng cc a ch mng khc nhau trn cc knh khc nhau.

2.4.1.2 nh danh im truy nhp dch v giao

vn TSAP

i vi mi a ch mng, mi thit b H.323 c th c vi im truy nhp

dch v lp giao vn TSAP (Transport layer Service Access Point). Cc TSAP ny

cho php dn mt vi knh c cng chung a ch mng vi nhau. Cc im cui

c mt TSAP mc nh l TSAP knh bo hiu cuc gi. TSAP knh iu khin

RAS l TSAP mc nh ca Gatekeeper. Cc im cui v thit b H.323 s dng

nh danh TSAP ng i vi knh iu khin H.245, knh Audio, Video v Data.

Gatekeeper s nh danh TSAP ng i vi cc knh bo hiu cuc gi. Trong

qu trnh ng k im cui, cc knh RAS v bo hiu c th c nh tuyn li

ti TSAP ng.

2.4.1.3 a ch th

Mt im cui c th c lin kt ti mt hoc nhiu a ch th (alias

address). Mt a ch th c th i din cho im cui hoc phin hi ngh m

im cui ch tr. Cc a ch th cung cp mt phng php nh a ch khc

cho im cui. Trong mt vng, cc a ch th l duy nht. Gatekeeper, MC v

MP khng c a ch nh danh. Khi h thng khng c Gatekeeper, th im cui

pha ch gi s nh a ch im cui b gi bng cch s dng a ch lp giaovn" knh bo hiu cuc gi ca im cui b gi. Khi c Gatekeeper trong h

thng, im cui ch gi c th nh a ch im cui b gi thng qua "a ch

lp giao vn knh bo hiu cuc gi ca n hoc a ch th. Mt im cui c

th c nhiu hn mt a ch th c truyn ti cng "a ch lp giao vn.

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Chng2 Chun H.323

2.4.2 Cc knh iu khin

2.4.2.1 Knh RAS

Knh RAS dng truyn ti cc bn tin s dng trong qu trnh ng k

im cui v tm kim Gatekeeper m lin kt mt a ch nh danh ca imcui vi a ch lp giao vn knh bo hiu cuc gi ca n. Knh RAS l knh

khng tin cy, v th trong khuyn ngh H.225 khuyn ngh thi gian gii hn

nh trc v s ln gi yu cu cho mt vi loi bn tin. Khi mt im cui hoc

Gatekeeper khng tr li yu cu trong khong thi gian nh trc, th c th s

dng bn tin RIP (Request In Progress) ch ra rng n ang x l yu cu. Khi

nhn c bn tin RIP, im cui hoc Gatekeeper s xo thi gian gii hn nh

trc v b m s ln gi li.

1/ Tm kim Gatekeeper

im cui s tm kim Gatekeeper m n ng k, vic tm kim ny c th

c thc hin bng th cng hoc t ng.

Vic tm kim th cng da vo cc phng php khng thuc phm vi ca

khuyn ngh ny xc nh Gatekeeper lin kt vi im cui. im cui c

ci t theo "a chlp giao vn ca Gatekeeper lin kt vi im cui .

Phng php tm kim Gatekeeper t ng cho php lin kt im cui -

Gatekeeper thay i theo thi gian, im cui c th khng bit Gatekeeper no lca n hoc c th cn nhn dng Gatekeeper khc nu li xy ra. Vic tm

kim t ng ch n chi ph qun tr thp hn trong cu hnh cc im cui

ring l, hn na n cn cho php thay th Gatekeeper m khng phi ci t li

cc im cui lin kt vi n.

2/ ng k im cui

ng k im cui l qu trnh im cui lin kt vo vng dch v v thng

bo cho Gatekeeper a ch nh danh cng nh a ch lp giao vn ca n. Sau

khi tm (t ng) c Gatekeeper, tt c cc im cui s ng k vi Gatekeeper

ny. Vic ng k ny phi c thc hin trc khi mt vi cuc gi no bt

u, v c th xy ra theo chu k khi cn thit. Mt Gateway hoc MCU c th

ng k theo mt hoc nhiu a ch lp giao vn. Vic ng k theo nhiu a ch

lp giao vn s lm cho vic nh tuyn cc cuc gi ti cc cng nh trc n

gin hn. im cui s gi yu cu ng k RRQ(Registration Request) ti

Gatekeeper, RRQ ny c gi ti a ch truyn knh RAS ca Gatekeeper.

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Chng2 Chun H.323

RCF

im cui khi ngYu cu khn n k (URQ)

Gatekeeper khi ngYu cu khn n k (URQ)

RRQ

UCF/URJ

URQ

UCF

URQ

im Gatekeeper

Hnh 2.13 Qu trnh ng k Gatekeeper

Sau khi tm c Gatekeeper, im cui s c c a ch mng ca

Gatekeeper ny v s dng b nhn dng TSAP knh RAS in hnh. Nu chp

nhn s ng k ca im cui, Gatekeeper s tr li li bng xc nhn ng k

RCF (Registration Confirmation), ngc li n s tr li bng tn hiu t chi RRJ

(Registration Reject). Hnh 2.13 minh ha qu trnh im cui ch ng k

Gatekeeper n l.

Mt im cui c th hy b vic ng k Gatekeeper ca n bng vic gi

bn tin Yu cu khng ng k URQ (Unregister Request) ti Gatekeeper can. Sau khi nhn c URQ, Gatekeeper s gi tr li bn tin UCF (Unregister

Confirmation). Lc ny im cui c php thay i a ch nh danh lin kt

vi a ch lp giao vn ca n. Trng hp im cui cha ng k vi

Gatekeeper trc , n s gi bn tin URJ ti im cui. Gatekeeper cng c th

hy b vic ng k ca im cui bng vic gi bn tin Yu cu khng ng

k URQ (Unregister Request) ti im cui, im cui s tr li bn tin UCF

(Unregister Confirmation). Khi cn thc hin mt vi cuc gi no , im cui

phi ng k li vi Gatekeeper trc hoc Gatekeeper mi.

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Chng2 Chun H.323

3/ nh v im cui

im cui hoc Gatekeeper c a ch nh danh ca mt im cui v mun

lin lc vi n, th c th dng bn tin Yu cu nh v LRQ. Bn tin LRQ nys c gi ti b nhn dng TSAP knh RAS ca Gatekeeper nh trc, hoc c

th gi bn tin GRQ qung b ti a ch qung b in hnh ca Gatekeeper.

Gatekeeper tng ng s gi tr li bn tin LCF cha thng tin cn thit ca im

cui hoc Gatekeeper ca im cui. Thng tin ny bao gm a ch knh bo hiu

cuc gi v knh RAS.

4/ M thng bo truy nhp

M thng bo truy nhp l mt xu c kim tra bn tin ci t v ccbn tin RAS. C hai li ch khi dng m truy nhp.

Th nht, chng cung cp kh nng bo mt a ch lp giao vn v a ch

nh danh ca im cui i vi ch gi. Khi tm im cui, ngi dng ch cn

gi m thng bo truy nhp cho pha ch gi. Gatekeeper bit im cui tng ng

vi m truy nhp thng bo thng qua qu trnh ng k, v th thng qua

Gatekeeper, nhng cuc gi s dng m thng bo truy nhp c th nh tuyn ti

im cui b gi.

Li ch th hai ca vic s dng m thng bo truy nhp l khng nh chcchn cc cuc gi c nh tuyn chnh xc thng qua cc thit b H.323. M

truy nhp do Gatekeeper tr li s c dng cc bn tin ci t gi bi im

cui. M thng bo truy nhp ny c th c Gateway s dng khng nh

rng im cui c php s dng ti nguyn ca Gateway.

2.4.2.2 Knh bo hiu

C 3 knh bo hiu tn ti c lp vi nhau lin quan n bo hiu v x l

cuc gi l: knh iu khin H.245, knh bo hiu cuc gi v knh bo hiuRAS. 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, thu bao ch gi phi bit a ch bo hiu ca

thu bao b gi trong mng.

Nu trong mng c gatekeeper, 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 thit lp c vic trao i bn tin bo hiu, th gatekeeper mi

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Chng2 Chun H.323

xc nh truyn cc bn tin trc tip gia hai u cui hay nh tuyn chng qua

gatekeeper.

Cc bn tin bo hiu cuc gi c th c truyn theo 1 trong 2 phng thc

v vic la chn gia cc ph

ng thc ny do Gatekeeper quyt nh:Th nht l cc bn tin bo hiu ca cuc gi c truyn t thu bao n ti

thu bao kia thng qua Gatekeeper gia hai thit b u cui (hnh 2.14).

1 2 3 4 5 6 78

u cui 1 u cui 2

Gatekeeper

Knh bo hiu RAS

Knh bo hiu cuc gi

1 ARQ

2 ACF/ARJ3 Set-up4 Set-up5 ARQ6 ACF/ARJ7 Connect8 Connect

Hnh 2. 14 Bn tin bo hiu ca cuc gi

c nh tuyn qua Gatekeeper

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

thit b u cui (hnh 2.15).

1 2 4 5

6

3u cui1 u cui 2

Gatekeeper

Knh bo hiu RAS

Knh bo hiu cuc gi

1 ARQ2 ACF/ARJ

3 Set-up4 ARQ5 ACF/ARJ6 Connect

Hnh 2. 15 Bn tin bo hiu c truyn trc tip gia cc thit b u cui

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C hai phng thc ny u 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 hiucuc gi trn knh bo hiu cuc gi. Sau mi ti thit lp 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 Gateway tham gia vo cuc gi th cc knh ny khng c

php ng.

2.4.2.3 Knh iu khin

1/ nh tuyn knh iu khin

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 th hin trn hnh 2.16 v 2.17.

1 2 3 4 5 6 78

9

Knh bo hiu RAS

Knh bo hiu cuc gi

Knh iu khin H.245

1 ARQ2 ACF/ARJ3 Set-up4 Set-up5 ARQ6 ACF/ARJ7 Connect8 Connect

9 H.245 Channel

u cui 1 u cui 2

Gatekeeper

Hnh 2. 16 Knh iu khin H.245 kt ni trc tip hai thit b u cui

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1 2 3 4 5 6 78 910

1 ARQ2 ACF/ARJ3 Set-up4 Set-up5 ARQ6 ACF/ARJ7 Connect8 Connect9 H.245 Channel

10 H.245 Channel

Knh bo hiu RAS

Knh bo hiu cuc gi

Knh iu khin H.245

u cui 1 u cui 2

Gatekeeper

Hnh 2. 17 Gatekeeper nh tuyn knh iu khin H.245

Knh iu khin H.245 c thit lp mt cch trc tip gia cc thit b u

cui, (hnh 2.16). Khi ch cho php kt ni trc tip 2 thit b u cui.

Knh iu khin H.245 c thit lp t thit b u cui ny ti thit b u

cui kia thng qua Gatekeeper (hnh 2.17). Khi cho php Gatekeeper nh

tuyn li knh iu khin H.245 ti mt MC khi thc hin dch v hi ngh.

2/ Gi tr tham chiu cuc gi CRV

Tt cc cc bn tin RAS v bo hiu cuc gi u cha gi tr tham chiuCRV (Call Reference Value). Cc gi tr CRV cho knh bo hiu v knh RAS l

c lp nhau. i vi knh bo hiu, CRV c s dng kt ni cc bn tin bo

hiu vi nhau, CRV c s dng trong tt c cc bn tin bo hiu gia cc thit b

H.323 lin quan ti cng cuc gi. knh RAS, CRV dng lin kt cc bn tin

knh RAS, gi tr CRV ny c s dng trong tt c cc bn tin RAS gia cc

thit b H.323 lin quan ti cng cuc gi.

3/ nh danh cuc ginh danh cuc gi (Call ID) l gi tr khc 0, c to bi thit b cui ch

gi v chuyn sang dng cc bn tin H.245. CAll ID dng lin kt cc bn tin

bo hiu v RAS lin quan ti cng cuc gi vi nhau. Tt c cc bn tin tham gia

qu trnh iu khin mt cuc gi th c chung mt Call ID.

2.4.3 Cc th tc bo hiu

Ngi ta chia mt cuc gi lm 5 giai on gm:

- Giai on 1: thit lp cuc gi.

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Chng2 Chun H.323

- Giai on 2: thit lp knh iu khin .

- Giai on 3: thit lp knh thoi o.

- Giai on 4: dch v.

- Giai on 5: kt thc cuc gi.

2.4.3.1 Bc 1 - Thit lp cuc gi

Vic thit lp cuc gi s dng cc bn tin c nh ngha trong khuyn ngh

H.225.0. C th xy ra 6 trng hp, l :

- Cuc gi c bn - C hai thit b u cui u khng ng k.

- C hai thu bao u ng k ti mt Gatekeeper.

- Ch c thu bao ch gi c ng k vi Gatekeeper.

- Ch c thu bao b gi c ng k vi Gatekeeper.- Hai thu bao ng k vi hai Gatekeeper khc nhau.

- Thit lp cuc gi qua Gateway.

Trong hu ht giao thc/bo hiu phc v cc ng dng thi gian thc, yu

cu v ngng thi gian x l cho php (Tout - Time Out) ca tng tn hiu v ca

c qu trnh bo hiu l bt buc. phng thc bo hiu trc tip, qu trnh bo

hiu din ra nhanh hn dn n xc xut thi gian x l bo hiu vt qu Tout t,

lm cho t l li cuc gi gim, hn na vic bo hiu trc tip gip cho qu trnh

ng b mng chnh xc. Tuy nhin, phng thc ny, yu cu cc u cuitham gia vo cuc gi phi c s tnh tng thch v bo hiu. phng thc bo

hiu gin tip thng qua Gatekeeper, qu trnh bo hiu din ra chm hn dn n

xc xut thi gian x l bo hiu vt qu Tout ln hn, v v th t l li cuc gi

cng nhiu hn. V phi thng qua (cc) Gatekeeper nn cu trc mng s phc

tp, vn t chc v ng b mng cn phi quan tm hn. phng thc ny, v

bo hiu thng qua Gatekeeper trung gian, v th vn tng thch bo hiu ch

lin quan n u cui v Gatekeeper, lm tng kh nng la chn u cui cho

ngi dng.Di y l chi tit cc th tc thit lp cuc gi, mt s trng hp s dng

bo hiu trc tip gia cc u cui, cc trng hp cn li s dng bo hiu gin

tip qua Gatekeeper.

1/ Cuc gi c bn - C hai thit b u cui u khng ng k

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Chng2 Chun H.323

u c u i 1

S e t - u p ( 1 )

C o n n e c t (4 )

C a l l p ro c e e d i n g ( 2 )

A l e r ti n g ( 3 )

K n h b o h i u c u c g i

u c u i 2

Hnh 2. 18: Cuc gi c bn khng c Gatekeeper

Khi c hai thit b u cui u khng ng k vi Gatekeeper, th chng strao i trc tip cc bn tin vi nhau nh hnh 2.18. Khi ch gi s gi bn tin

thit lp cuc gi ti lp TSAP trn knh bo hiu bit trc a ch ca thu

bao b gi.

2/ C hai thu bao u ng k ti mt Gatekeeper

Tnh hung ny c 2 trng hp xy ra l bo hiu trc tip (c trnh by

di y) v bo hiu gin tip thng qua Gatekeeper.

C hai thu bao u cui u ng k ti mt Gatekeeper v Gatekeeper chnphng thc truyn bo hiu trc tip gia 2 thu bao (hnh 2.19). u tin, thu

bao ch gi trao i vi Gatekeeper thng qua 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 b gi. Sau thu bao ch gi s cn

c vo a ch ny gi bn tin Set-up (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) th n s gi bn tin Release Complete ti

thu bao ch gi.

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Chng2 Chun H.323

u cui 1 u cui 2

ARQ (1)

ACF/ARJ (2)

Set-up (3)

Call proceeding (4)

Gatekeeper 1

Alerting (7)

Connect (8)

ARQ (5)

ACF/ARJ (6)

Knh bo hiu RAS

Knh bo hiu cuc gi

Hnh 2. 19 : Hai thu bao u ng k vi mt Gatekeeper - bo hiu trc tip

3/ Ch c thu bao ch gi c ng k vi Gatekeeper

Khi cc bn tin bo hiu cuc gi do Gatekeeper nh tuyn, th th tc thit

lp cuc gi c th hin trn hnh 2.20. Trong trng hp ny cc th t bn tinca th tc ging ht trng hp trn, ch khc duy nht mt im l tt c cc

bn tin bo hiu gi t thu bao ny ti thu bao kia u thng qua phn t trung

gian l Gatekeeper 1.

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Chng2 Chun H.323

Gatekeeper 1 u cui 2

ARQ (1)

ACF (2)

Set-up (3)

Call Proceeding (5)

Alerting (6)

Connect (8)

Setup (4)

Call Proceeding (5)

Alerting (6)

Connect (7)

Knh bo hiu RASKnh bo hiu cuc gi

u cui 1

Hnh 2. 20 Ch c thu bao ch gi ng k - Gatekeeper nh tuyn

bo hiu

4/ Ch c thu bao b gi c ng k vi Gatekeeper

Trng hp bo hiu do Gatekeeper nh tuyn, th tc bo hiu c th hin

trn hnh 2.21. u tin, thu bao ch gi s gi bn tin Set-up (1) trn knh bo

hiu bit trc a ch ti thu bao b gi. Nu thu bao b gi chp nhn cuc

gi n s trao i bn tin ARQ (3)/ARJ (4) vi Gatekeeper. Trong bn tin ARJ mGatekeeper tr li cho thu bao b gi cha m yu cu nh tuyn cuc gi qua

Gatekeeper (routeCallToGatekeeper). Khi , thu bao b gi s gi bn tin

Facility (5) c cha a ch knh bo hiu ca Gatekeeper ti thu bao ch gi. Sau

, thu bao ch gi gi bn tin Release Complete (6) ti thu bao ch gi v cn

c vo a ch knh bo hiu thu bao ch gi s gi bn tin Set-up (7) ti

Gatekeeper, Gatekeeper gi bn tin Set-up (8) ti thu bao b gi. Sau , thu bao

b gi s trao i bn tin ARQ (9)/ACF (10) vi Gatekeeper, thu bao b gi gi

bn tin Connect (12) c cha a ch knh iu khin H.245 ti Gatekeeper.Gatekeeper s gi bn tin Connect (13) c cha a ch knh iu khin H.245 ca

thu bao b gi.

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Chng2 Chun H.323

T1524080-96

Gatekeeper 2 im cui 2

Setup (7)

Call Proceeding (2)

Alerting (11)

Connect (13)

Setup (8)

Call Proceeding (2)

ARQ (9)

ACF/ARJ (10)

Alerting (11)

Connect (12)

Cc bn tin RAS

Cc bn tin bo hiu

im cui 1

Setup (1)

Call Proceeding (2)

Facility (5)

Release Complete (6)

ACF/ARJ (4)

ARQ (3)

Hnh 2. 21 Ch c thu bao b gi ng k - Gatekeeper nh tuyn bohiu

5/ 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

nh tuyn bo hiu trc tip gia hai thu bao, (2) Gatekeeper 1 pha ch gi

truyn bo hiu theo phng thc 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,v (4) hai TB ng k vi 2 Gatekeeper v c hai Gatekeeper ny u chn phng

thc nh tuyn bo hiu cuc gi qua chng. Di y l chi tit v trng hp

(4).

Hai TB ng k vi 2 Gatekeeper v c hai Gatekeeper ny u chn phng

thc nh tuyn bo hiu cuc gi qua chng. Th tc bo hiu ca trng hp ny

c th hin trn hnh 2.22.

u tin TB ch gi trao i ARQ (1)/ACF (2) vi Gatekeeper 1, trong bn

tin ACF c cha a ch knh bo hiu ca Gatekeeper 1. Cn c vo a ch ny

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Chng2 Chun H.323

TB ch gi gi bn tin Set-up (3) ti Gatekeeper 1. Gatekeeper 1 s gi bn tin Set-

up(4) ti a knh bo hiu ca TB b gi, nu chp nhn TB b gi s trao i

ARQ (6)/ARJ(7) vi Gatekeeper 2, Trong bn tin ARJ(7) m Gatekeeper 2 tr li

cho TB b gi cha a ch knh bo hiu ca n v m ch th bo hiu nhtuyn cuc gi qua

u cui 1 Gatekeeper 1 Gatekeeper 2 u cui 2

ARQ (6)

ARJ (7)

Facility (8)

Set-up (4)

Call Proceeding (5)

Set-up (10)

Call Proceeding (5)

Alerting (14)

Connect (16)

Set-up (11)

Call Proceeding (5)

ARQ (12)

ACF/ARJ (13)

Alerting (14)

Connect (15)

Knh bo hiu RAS

Knh bo hiu cuc gi

ARQ (1)

ACF (2)

Set-up (3)

Call Proceeding (5)

Alerting (14)

Connect (17)

Release Complete (9)

Hnh 2. 22 Hai thu bao u ng k - nh tuyn qua hai Gatekeeper

Gatekeeper 2 (routeCallToGatekeeper). TB b gi tr li Gatekeeper 1 bn

tin Facility (8) cha a ch knh bo hiu ca Gatekeeper 2. Tip Gatekeeper 1

gi bn tin Release Complete ti TB b gi v gi bn tin Setup (10) ti a ch

knh bo hiu ca Gatekeeper 2 v Gatekeeper 2 gi Setup (11) ti TB b gi. TB

b gi trao i ARQ (12)/ACF (13) vi Gatekeeper 2 v tr li Gatekeeper 2 bng

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Chng2 Chun H.323

bn tin Connect (15) cha a ch knh iu khin H.245 ca n s dng bo

hiu H.245. Gatekeeper 2 gi Connect (16) ti Gatekeeper 1, bn tin ny cha a

ch knh iu khin H.245 ca TB b gi hoc a ch knh iu khin H.245 ca

Gatekeeper 2 tu thuc vo Gatekeeper 2 c chn nh tuyn knh iu khinH.245 hay khng. Sau Gatekeeper 1 gi Connect(17) ti TB ch gi, bn tin

ny cha a ch knh iu khin m Gatekeeper 1 nhn c t Gatekeeper 2

hoc l a ch knh iu khin H.245 ca Gatekeeper 1 nu n chn nh tuyn

knh iu khin H.245.

6/ Thit lp cuc gi qua Gateway

Nh trnh by trong mc 2.2.2, mt cuc gi ch lin quan n Gateway

khi cuc gi c s chuyn tip t mng PSTN sang mng LAN hoc ngc li.V vy v c bn c th phn bit cuc gi qua Gateway thnh 2 loi: cuc gi t

mt thu bao in thoi vo mng LAN v cuc gi t mt thu bao trong mng

LAN ra mt thu bao trong mng thoi.

2.4.3.2 Bc 2 - Thit lp knh iu khin

Khi kt thc giai on 1 tc l c ch gi ln b gi hon thnh vic trao

i cc bn tin thit lp cuc gi, th cc u cui s thit lp knh iu khin

H.245. Bn tin u tin c trao i gia cc u cui l terminalCapabilitySet cc bn thng bo cho nhau kh nng lm vic ca mnh. Mi mt thit b u

cui u c c tnh ring ni ln kh nng ch m ho, truyn, nhn v gii

m cc tn hiu a dch v. Knh iu khin ny c th do thu bao b gi thit lp

sau khi n nhn c bn tin Set-up 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 c gii phng.

2.4.3.3 Bc 3 - Thit lp knh truyn thng

Sau khi trao i kh nng (tc nhn ti a, phng thc m ho..) v xc

nh quan h master-slave trong giao tip giai on 2, th tc iu khin knh

H.245 s thc hin vic m knh logic truyn s liu. Cc knh ny l knh

H.225. Sau khi m knh logic truyn tn hiu l m thanh v hnh nh th mi

u cui truyn tn hiu s truyn i mt bn tin h2250MaximumSkewIndication

xc nh thng s truyn.

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1/ 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 (Ch truyn v

nhn l thng bo v ghi nhn ca cc u cui xc nh kh nng lm vicgia chng).

2/ Trao i cc lung tn hiu video

Vic s dng ch th videoIndicateReadyToActive 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 ch th sn sng truyn video. Thu bao ch gi s truyn bn tinvideoIndicateReadyToActive sau khi kt thc qu trnh trao i kh nng, nhng

n s khng truyn tn hiu video cho n khi nhn c bn tin

videoIndicateReadyToActive hoc nhn c lung tn hiu video n t pha

thu bao b gi.

3/ Phn phi cc a ch lung d liu

Trong ch mt a ch, mt u cui s m mt knh logic ti MCU hoc

mt u cui khc. a ch ca cc knh cha trong bn tin openLogicalChannelv openLogicalChannelAck.

Trong ch a ch nhm, a ch nhm s c xc nh bi MC v c

truyn ti cc u cui trong bn tin communicationModeCommand. Mt u

cui s bo cho MC vic m mt knh logic vi a ch nhm thng qua bn tin

openLogicalChannel v MC s truyn bn tin ti tt c cc u cui trong

nhm.

2.4.3.4 Bc 4 - Dch v cuc gi

C mt s dch v cuc gi c thc hin trn mng H.323 nh : thay i

rng bng tn, gim st trng thi hot ng, hi ngh c bit, cc dch v b

xung. y xin c trnh by hai loi dch v l thay i rng bng tn v

gim st trng thi hot ng.

1/ Thay i rng bng tn

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Chng2 Chun H.323

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 lung

truyn, nhn m thanh v hnh nh u phi nm trong rng bng tn thit

lp. Ti mi thi im trong khi hi thoi, u cui hoc Gatekeeper u c th

yu cu tng hoc gim rng bng tn. Mt u cui c th thay i tc

truyn trn mt knh logic m khng yu cu Gatekeeper thay i rng bng

tn nu nh tng tc truyn v nhn khng vt qu rng bng tn hin ti.

Trong trng hp ngc li th u cui phi yu cu Gatekeeper m n ng k

thay i rng bng tn.

u cui 1 u cui 2

BRQ (1)

BCF/BRJ (2)

Gatekeeper 1

CloseLogicalChannel (3)

BRQ (5)

BCF/BRJ (6)

Gatekeeper 2

OpenLogicalChAck (7)

OpenLogicalChannel (4)

Ch : Gatekeeper 1 v Gatekeeper 2 c th l mt Gatekeeper

Hnh 2. 21 Yu cu thay i rng bng tn - Thay i thng s truyn

Th tc thay i rng bng tn - thay i thn

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