Ebook Công nghệ VoIP và các giải pháp nâng cao chất lượng dịch vụ: Phần 2

Nối tiếp nội dung của phần 1 cuốn sách "Công nghệ VoIP và các giải pháp nâng cao chất lượng dịch vụ", phần 2 giới thiệu tới người đọc các nội dung: Các giao thức điều khiển, QoS và các yếu tố chính ảnh hưởng đến chất lượng thoại IP, các giải pháp nâng cao chất lượng thoại. Mời các bạn cùng tham khảo.

c a sd goi dua tren tieu chuan la kha nang co cac tieu chuan md cho cac lap dieu khien cuoc goi N hung chuan m a nay duac cung cap bdi cac giao thuc n h u H.323, MGCP SIP va cac giao dien m d dugc phat trien rong khap qua c a sd ha tang dong goi Mot trong cac cong viec cua giao thuc dieu khien cuoc goi la chi cho cac dong RTP bat dau d dau va ket thuc d dau Giao thuc dieu khien cuoc goi hoan tat nhiem vu nay bang viec bien dich giua cac dia chi IP va cac sd dien thoai.

Hien nay chung ta chua the thay the hoan toan mang dien thoai chuyen mach cong cong (PST N ) bang cong nghe VoIP bai con nhieu dieu phirc tap ben trong cac giao thirc mdi chay tren nen tang ciia IP Tuy nhien, gan day cac giao thirc cho bao hieu cuoc goi va dieu khien thiet bi dang dugc chuan hoa chung ta dang gdn dat den mot mdi trudng co tinh lien ket hoat dong cao Giao thirc dieu khien truyen thong (M GC P) hien nay da trd thanh tieu chuan chinh thirc, trong khi do cac cai tien d u g c thira

C hut m g IV: Cac giao I hire dieu khien 139

nhan gan day trong phien han 4 cua H.323 da tao dieu kien thuan lgi khi ket hgp voi cac giao thirc khac de tao ra cac giai phap cho lie thong truyen thoai hoan chinh va dac tinh ket noi ngang cap cho cac m ang goi Giao thirc khai tao phic’n (SIP) dang dirge xem nhu giao thirc bao hieu chinh trong he thong chuyen mach mem (softsvvitch) Hinh 4.1 dudi day la mo ta chong giao thirc sir dung cho VoIP

Hinh 4 1 : Chong giao (hire VoIP

Giao thirc H.323 phien ban 1 va 2 ho trg H.245 tren nen TCP, Q.93 1 tren nen TC P va RAS tren nen UDP Cac phien ban

3 va 4 cua H.323 ho trg them H.245 va Q.931 tren nen UDP Giao thirc SIP ho trg ca TCP va UDP Trong mang NG N cac cuoc goi thoai deu la cac cuoc goi VoIP Chinh vi the phan tilp theo se d<3 cap den cac giao thirc bao hieu trong mang VoIP mot cach chi tiet han

140 Cong nghe VoIP vd cdc giai phap

Bang 4.1 Bo giao thuc 11.323

Bao hi£u c u q c gpi (Cail Signalling) H.225

Dieu khien phuang tien (Media

Control)

H.245

Bo codec am thanh (Audio Codecs) G 711, G.722, G.723.1 G 728 G 729

Bo codec vi-de-o (Video Codecs) H.261,H 263

Chia se du li$u (Data Sharing) T 120

T ru yln tai phuang tien (Media

Transport)

RTP/RTCP

Cac thanh phan co ban ciia he thong mang H.323 dugc nio

ta trong hinh 4.2 dudi day Bao gom cac dau cuoi, cong ket ndi thiet bi dieu khien cong ndi (gatekeeper) va khoi dieu khien da diem (MCU)

Chuang IV: Cdc giao I hue dieu khien 141

Dien thoai

Hinh 4.2: Cdc thanh phan mang H.323

4.2.1 Thiet bi dau cuoi

Dau cuoi cung cap thong tin diem - diem va da diem vai cac ddu cuoi khac Dau cuoi H.323 gom cac khoi nhu dieu khien

he thdng, khdi truyen tai p h u a n g tien ma hoa audio va giao dien voi mang IP Phan thiet bi tuy chon co the la ma hoa video va thiet bi truyen du lieu

4.2.2 Cong ket noi

Cong ket ndi cung cap giao dien giua hai mang khac nhau Cdng k i t ndi H.323 ket noi mang H.323 voi mang khong phai H.323 nhu PSTN Cong ket noi chuyen ddi giua audio (am thanh) video va cac dinh dang truyen du lieu cung n h u cac giao thuc va he thong thong tin Cong ket ndi chi can thiet khi

142 Cong nghe VoIP vd cdc giai phap

phai ket noi vai mang phi 11.323, do do khong can thiet khi ket noi giua hai thiet hi dau cuoi H.323

4.2.3 Thiet bj dieu khien cong noi

Thiet bi dieu khien cong noi la tuy chon, co the sir dung hoac khong Thiet bi dieu khien cong noi cung cap cac djch vu truac khi dien ra cuoc goi va dich vu dieu khien cuoc goi cho cac diem cuoi H.323 Tuy nhien, neu thiet bi dieu khien cong noi

co mat trong m ang no se co nhiem vu: bien dich dja chi, dieu khien chap nhan, dieu khien bang thong va quan ly viing

Cac chirc nang tuy chon ciia thiet bi dieu khien cong noi bao gom: bao hieu dieu khien cuoc goi, xac thuc cuoc goi, quan ly bang thong, quan ly cuoc goi

4.2.4 Khoi dieu khien da diem (MCU)

Khoi dieu khien da diem la diem cuoi (endpoint) ho trg hoi nghi giua 3 thanh vien hoac nhieu han MCU dien hinh bao gom

bo dieu khien da diem (M C) va mot hoac nhieu bo xir ly da diem (MP) MC xir ly dieu khien va bao hieu de ho trg hoi nghj trong khi MP nhan d u lieu audio, video va luong d u lieu, xir ly va

phan bo chung den cac diem cuoi thanh vien trong hoi nghi

da diem

Bo giao thuc H.323 bao gom nhieu giao thirc Bo giao thuc

ho trg chap nhan cuoc goi, thiet lap, trang thai, giai phong luong

p h u a n g tien va cac ban tin trong he thong H.323 Cac giao thirc dugc ho trg ca c a che truyen dua ra goi tin cay va khong tin cay qua mang IP nhu minh hoa trong hinh 4.3

C huang IV Cdc giao thuc dieu khien 143

Truyen tai TCP tin c$y Truyen tai UDP khong tin cay

H 245

Dieu khien cuoc goi

IP Lap vat ly va lien ket du lieu

Hinh 4.3: Cdc lap ciia bo giao thuc H.323

Bo giao thirc H.323 bao gom ba viing dieu khien:

- Bao hieu dang ky, thira nhan va trang thai (RAS): Bao hieu RAS cung cap dieu khien truac cuoc goi trong mang thiet

bi dieu khien cong noi H.323

- Bao hieu dieu khien cuoc goi (H.225): dua tren khuyen nghi H.225 ciia ITU-T ma no chi ra viec sir dung cac ban tin ban hieu Q.931 Kenh dieu khien cuoc goi tin cay TCP dugc tao ra tren mang IP vai ma cong la 1720

- Dieu k h iln va truyen tai thong tin M edia (H.245 va RTP/RTCP): H.245 xir ly cac ban tin dieu khien tir dau den cuoi giua cac diem cuoi H.323 Thu tuc giao thirc H.245 thiet lap kenh cho truyen audio, video dir lieu va thong tin kenh dieu khiln T ru y ln da p h u a n g tien trong H.323 dugc cung cap bai RTP va RTCP

4.2.5 Trinh tu thiet lap cuoc goi

N£u xem xet mot cach chi tiet thi cuoc goi giua hai dau cuoi H.323 dugc th ilt lap n h u hinh 4.4 duai day

144 Cong nglw VoIP va cdc giai phap

•Call Signaling Messages

Hinh 4.4: Thiet lap cuoc goi H.323

- T rudc het ca 2 phai da dugc dang ky tai thiet hi dieu khiencong noi

- Dau cuoi A gui yeu cau tdi thiet bi dieu khien cong ndi de nghi thiet lap cuoc goi

- Thiet bi dieu khien cong ndi gui cho dau cuoi A thong tin can thiet ve dau cuoi B

C huang IV Cdc giao I hire dieu khien 145

- Dau cuoi A giri ban tin SETUP tai dau cuoi B

- Dau cuoi B tra ldi bang ban tin Call Proceeding va dong thdi lien lac vdi thiet bi dieu khien cong noi de xac nhan quyen thiet lap cuoc goi

ma bao hieu cuoc goi khong duo c dinh tuyen tdi thiet bi dieu khien cong ndi H.323 hd tro nhieu kich ban thiet lap cuoc goi phirc tap khac

H.323 la he thong ghep lai duoc xay dung tir cac thiet bi tap trung thong m inh nhu: thiet bi dieu khien cong ndi MCU, cong ket ndi va diem cuoi Mac dii chuan H.323 trong phien ban gan day nhat co phdn hoan thien hon song van de van nay sinh nhu thdi gian thidt lap cuoc goi dai, qua nhieu chirc nang thiet bj dieu khidn cdng ndi phai thuc hien va kha nang m d rong khi sir dung ki£u bao hieu cuoc goi djnh tuyen qua thiet bi dieu khien cdng ndi (GKRCS)

Khi cdn sir dung cdng ket ndi dung luong ldn de ket ndi mang PSTN, ngudi ta se sir dung giao thirc dieu khien cdng don gian (SGCP: Simple Gateway Control Protocol) va Giao thuc didu khidn cdng p h u a n g tien (MGCP: M edia Gateway Control Protocol) dd thay thd giao thirc cho cdng ket ndi H.323 Cac he thdng didu khien cuoc goi nay co ve hieu qua hon dap img nhu cau cua cac nha cung cap cd ldn

146 Cdng nghe VoIP vd cdc giai phap

Cung vay, giao thuc khoi tao phien SIP (Session Initiation Protocol) co the giai quyet mot so ton tai cua H.323 va no kha nang sc thay the H.323

4.3 GIAO THU’C KHOI TAO PHIEN (SIP)

SIP la giao thuc dieu khien bao hieu thuoc lap ung dung, duoc phat trien nhu la mot chuan m a RFC 2543 cua ILTF Khac voi H.323, no dua tren nguon goc Web (HTTP) va co thiet ke kieu mo-dun, dem gian va de dang m a rong vai cac ung dung thoai SIP SIP la mot giao thuc bao hieu de thiet lap, duy tri va ket thuc cac phien da phuang tien nhu: thoai IP, hoi nghi va cac ung dung tuang tu khac lien quan den viec truyen thong tin da phuang tiyn

U u diem chinh cua SIP so vai cac giao thuc bao hieu khac

la cung cap mot su m em deo N o dirge thiet ke nhanh va dan gian Giao thuc SIP cung cap cac chuc n in g sau:

- Djnh vi nguai dung thong qua dia chi tu an g tu nhu email

- N ang luc nguai dung: cac tham so phien co the thuang lugng giua hai phia

- Lgi l'ch ngucri dung: Xac dinh dua tren kieu ben bi gpi muon tien hanh truyen thong

Giao thuc SIP bao gom hai thanh phan: dai ly tram cua ngudi su dung (user agent) va may chii mang (netw ork server) User agent la mot ling dung ket cuoi he thong ma no bao gom ca user agent client (UA C) khdi tao cuoc gQi va user agent server (UAS) no se tra ldi cuoc goi Kien true cua SIP cho phep thong tin ngang cap sir dung giao thuc may khach/may chii

Network servers bao gom bon kieu la: Proxy server (may chu liy quyen) Location server (may chu cuc bo; Redirect

C huang IV: Cdc giao thuc dieu khien 147

server (may chu c h u y ln tidp) va Register server (may chu dang ky)

ITU-T khong phai la to chuc tieu chuan duy nhat dua ra ke hoach thiet lap ket noi thoai IP va dong goi audio Hem han hat

ky mot to chuc quan ly cac chuan muc Internet, IETF cung co nhung yeu cau cua rieng minh doi voi nhung he thong VoIP va dugc goi la "Giao thuc dieu khien phien" - SIP

N hung nguai de xuat SIP cho rang H.323 dang xuat hien trong bao hieu A TM va ISDN, la khong thich hgp cho dieu khien he thong VoIP noi chung va trong thoai Internet noi rieng Yeu cau nay khang dinh la H.323 von di rat phuc tap ho trg cac chuc nang phan 1cm la khong can thiet cho thoai IP do do doi hoi chi phi cao va khong hieu qua Vi du H.323 xac djnh 3 phucmg phap khac nhau dc phoi hgp hoat dong giua H.225 va H.245, vai cac ket noi khac nhau, H.245 ngang qua ket noi H.225 va tien hanh ph u an g phap "ket noi nhanh" cua 2 giao thuc tich hgp Mac du hau het cac kha nang thuc hien chi ho trg cho cac ket noi nhanh, tinh tu an g thich H.323 lien quan den yeu cau ho trg cua

ca 3 p h u an g phap

Ddng thai, nhung n g u a i ung ho SIP cung cho rang H.323 khong co kha nang m d rong yeu cau ddi vdi giao thirc bao hieu cho cdng nghe chang han n h u VoIP, la nhung cdng nghe chac

ch in se phat triln va hd trg cac dich vu va dac tinh mdi Kinh nghiem trong sir dung cac giao thirc Internet mail (SM TP) da cung cap rdt nhieu triet ly phat trien SIP, trong do tap trung vao kha nang thich img ciia bao hieu trong tu an g lai

148 Cdng nghe VoIP vd cac giai phap

SIP dua tren y tucrng va cau true cua HTTP ( Hypertext Transfer Protocol), giao thuc trao doi thong tin cua World Wide Web No dugc dinh nghTa nhu mot giao thuc Client-Server, trong do cac yeu cau dugc ben goi (client) dua ra va ben bi gpi (server) tra ldi SIP su dung mot so kieu ban tin va cac trudng

mao dau cua HTTP, xac dinh noi dung luong thong tin theo mao dau thuc the (md ta noi dung - kieu loai) va cho phep xac nhan cac p h u a n g phap su dung giong nhau dugc sir dung tren Web.SIP dinh nghTa cac ban tin INVITE va AC K giong nhu ban tin Setup va Connect trong H.225, trong do ca hai deu dinh nghTa qua trinh m d 1 kenh dang tin cay ma thong qua do cuoc gpi co the di qua Tuy nhien khac vdi H.225, do tin cay cua kenh nay khong phu thuoc vao TCP Cac nha de xuat SIP kien nghi rang viec tich hgp do tin cay vao lap ung dung nay cho phep ket hop mot cach chat che cac gia tri dieu chinh de ung dung, co the tdi

uu hoa VoIP h an la phu thuoc vao nhung gia tri "muc dich chung chung" cua TCP

Cuoi cung, SIP du a vao giao thuc mo ta phien (SDP) mot tieu chuan khac cua IETF, de thuc hien su sap xep tuan g tu theo

c a cau chuyen ddi dung lugng cua H.245 SDP dugc dung de nhan dang m a tong dai trong nhung cuoc gpi sir dung mot mo ta nguyen ban dan SDP cung d u ac sir dung de chuyen cac phan tir thong tin ciia giao thirc bao hieu thdi gian thuc RTSP de sap xep cac tham so hoi nghi da diem va djnh nghTa khuon dang chung cho nhieu loai thong tin khi dupe chuyen trong SIP

Cuoc tranh luan lieu chuan H.323/H.225 hay SIP phii hop cho thoai Internet hay khong thi chi mang tinh chat hoc thuat Ca hai cach deu co nhung thuan lgi va bat lgi Trong khi H.323 co thuan lgi r k ldn trong thj trudng (da xuat hien trudc SIP) thi SIP

( huang IV Cue giao thirc dieu khien 149

co lgi the ciia IETF la mot trong so cac to chirc tieu chuan hoat dong tich cuc Voi muc dich nghien ciru cac giao thirc va giai phap cho van de VoIP, tot nhat la luu y den ca FI.323 va SIP.Theo dinh nghTa ciia IETF, SIP la “giao thirc bao hieu lop img dung mo ta viec khai tao, thay doi va huy cac phien ket noi tuang tac da p h u an g tien giua nhung ngudi sir dung” SIP co the sir dung cho rat nhieu djch vu khac nhau trong mang IP nhu dich

vu thoai, hoi nghj thoai, email, day hoc tu xa, quang ba (M PEG, MP3 ), truy nhap HTML, XML, hoi nghi truyen hinh

4.3.1 Phan mem chuyen mach cuoc gpi SIP

Phan mem chuyen cuoc goi SIP co cau true n h u sau:

Cac thanh phan cua SIP

Hinh 4.5: Cau trite SIP

150 Cong nghe VoIP vd cdc gidi phap

Trong hinh tren, dai ly ngudi dung ( User Agent) la thiet bi dSu cuoi trong mang SIP, co the la mot may dien thoai SIP, c6

t h i la may tinh chay phan mem dau cuoi SIP

May chu uy quyen (Proxy Server) la phan mem trung gian hoat dong vira nhu may chu va vira nhu may tram de thuc hien cac yeu cau thay mat cac dau cuoi khac Tat ca cac veu cau dirge xir ly tai cho boi may chu uy quyen neu co the, hoac dugc chuyen cho cac may chu khac Trong trudng hop may chu uy quyen khong true tiep dap ung cac yeu cau nay thi may chu uy quyen se thuc hien khau chuyen doi hoac dich sang khuon dang thich hop truac khi chuyen di

M ay chu dinh vi (Location Server) la phan mem dinh vi thue bao, cung cap thong tin ve nhung vj tri co the cua phia bj goi cho cac phan mem may chu uy quyen va may chu chuyen doi dja chi

M ay chu chuyen doi dja chi (Redirect Server) la phan mem nhan yeu cau SIP va chuyen doi dja chi SIP sang mot so dja chi

khac va gui lai cho dau cuoi Khong giong n h u may chu uy quyen, m ay chu chuyen doi dja chi khong bao g ia hoat dong nhu mot dau cuoi, tuc la khong gui di bat cu yeu cau nao May chu chuyen doi dja chi (Redirect Server) cung khong nhan hoac huy cuoc gpi

M ay chu dang ky (Registrar Server) la phan m em nhan cac yeu cau dang ky Trong nhieu trudng hap may chu dang ky dam nhiem luon mot sd chuc nang r n ninh n h u xac nhan ngudi sir dung Thong thudng may chu dang ky dupe cai dat cung vai may chii uy quyen va may chu hay dja chi hoac cung cap djch

vu djnh vj thue bao Mdi lan d£u cuoi dupe bat len (thi du may

C hu</ng IV: Cdc giao thuc dieu khien 151

dien thoai hoac phan mcm SIP) thi dau cudi lai dang ky voi may chu Neu dau cuoi can thong bao cho may chu ve dja diem cua minh thi ban tin R EG ISTER cung duoc gui di Noi chung cac dau cuoi deu thuc hien viec dang ky lai mot cach djnh ky

4.3.2 Cac ban tin SIP, mao dau va danh so

D uai day la cac ban tin cua SIP:

- INVITE: Bat dau thiet lap cuoc goi bang cach giri ban tin mdi dau cuoi khac tham gia

- ACK: Ban tin nay khang djnh may tram da nhan dugc ban tin tra ldi ban tin INVITE

- BYE: Bat dau ket thuc cuoc goi

- CANCEL: Huy yeu cau dang nam trong hang dgi

- REGISTER: Dau cuoi SIP sir dung ban tin nay de dang ky vdi may chu dang ky

- OPTIONS: Sir dung de xac djnh nang luc cua may chii

- INFO: Sir dung de tai cac thong tin nhu am bao DTMF Giao thirc SIP co n h ilu diem trung hgp vdi giao thirc HTTP.Cac ban tin tra ldi cac ban tin SIP neu tren gom co:

- lxx - Cac ban tin chung

- 2xx - Thanh cdng

- 3xx - C huyen dja chi

- 4xx - Yeu cSu khong dugc dap ung

- 5xx - Su co ciia m ay chii

- 6xx - Su co toan mang

152 Cong nghe VoIP vd cdc g ia i phap

Cac ban tin SIP deu co khuon dang text, tu an g tu nhu HTTP Mao dau cua ban tin SIP cung tu a n g tu nhu HTTP va SIP cung ho trg M IM E (mot so chuan ve email) Sau day la thi

du ve mao dau cua ban tin SIP:

4.3.3 Thiet lap va huy cuoc goi SIP

Trong mang SIP qua trinh thiet lap va huy mot phien ket noi

th u an g gom co 6 budc nhu sau:

C h u a n g I V C u e g ia o thirc dieu khien 153

a pE

INVITE 302 (Dich chuyen tarn thdi)

— ACK

-180 (rung chuong)

I

- 200 (O K -ACK- RTP MEDIA PATH

) -I

- B Y E

-200 (OK)

Oai ly ngudi stf dung

— INVITE

180 (rung chuong)

•200 (OK) — -ACK-

•BYE-200 (OK)

Hinh 4.6: Thiet lap va huy cuoc gpi SIP

1 Dang ky, khdi tao va dinh vj dau cuoi

2 Xac dinh p h u o n g tien cua cuoc goi tuc la mo ta phien ma dau cuoi dugc mdi tham du

3 Xac dinh m ong muon cua dau cuoi bi goi, tra ldi hay khong Phia bj goi phai giri ban tin xac nhan chap thuan cuoc goi hoac tir choi

154 Cong nghe VoIP va cdc g ia i phap

4 Thiet lap cuoc goi

5 Thay doi hay dieu khien cuoc gpi (thi du nhu chuyencuoc gpi)

6 Huy cuoc gpi

Qua trinh thiet lap va huy cuoc gpi SIP co ban dupe mo ta troim hinh 4.6 sau

4.3.4 Tinh nang cua SIP

Giao thuc SIP dupe thiet k<3 vdi n h ung tieu chi sau:

- l ich hop vdi cac giao thuc da co cua IETF

- Don gian va co kha nang m d rong

- Hd tro tdi da su di dong cua dau cuoi

- De dang tao tinh nang mdi cho djch vu

a) Tich hap vai cdc giao thirc c1d co ciia IETF

Cac giao thirc khac ciia IETF co the sir dung de xay dimg nhung irng dung SIP SIP co the hoat dong ciing vdi nhieu giao thirc nhu:

- RSV P (Resource Reservation Protocol): Giao thirc danh trudc tai nguyen mang

- RTP (Real-tim e transport Protocol): Giao thirc truyen tai thdi gian thuc

- RTSP (Real Time Streaming Protocol): G iao thuc tao luong thdi gian thuc

- SAP (Session Advertisement Protocol): G iao thuc thong bao trong phien ket ndi

( huang IV; Cdc giao I hire dieu khien 155

- SDP (Session Description Protocol): Giao tlu'rc mo ta phien ket noi da p h u an g tien

- M IM E (Multipurpose Internet Mail Extension - M d rong thir tin Internet da muc dich): Giao thirc thu dien tu

- H TTP (Hypertext Transfer Protocol): Giao thuc truyen sieu van ban

- COPS (Com m on Open Policy Service): Dich vu chinh sach m d chung

- OSP (Open Settlement Protocol): Giao thirc thoa thuan md

h) Dan gian vd co kha ndng m a rong

SIP co rat it ban tin, khong co cac chirc nang thira nhung SIP co the sir dung dc thiet lap nhung phien ket noi phirc tap nhu hoi nghi Don gian, gon nhe, dua tren khuon dang van ban SIP

la giao thirc ra ddi sau va da khac phuc dugc diem yeu cua nhieu giao thirc trudc day

Cac phan m em ciia may chii uy quyen, may chu dang ky may chii chuyen ddi dia chi may chu dinh vi co the chay tren cac may chii khac nhau va viec cai dat them may chu hoan toan khong anh h u dng den cac may chu da co Chinh vi the he thong chuyln mach SIP co the de dang nang cap

c) Ho tra loi da su di dong cua ddu cuoi

Do cd may chii uy quyen may chii dang ky va may chii

c h u y ln ddi dja chi he thong luon nam dugc dja diem chinh xac ciia thue bao Thi du thue bao vdi dja chi ptit@vnpt.com.vn cd thd

156 Cdng nghe VoIP vii ciic g ia i phap

nhan dugc cuoc ggi thoai hay thong diep a bat cu dia diem nao qua bat cu dau cuoi nao nhu may tinh de ban, may xach tay, ditjn thoai SIP Vcri SIP rat nhieu djch vu di dong inoi dugc ho trg

d) De dang tao tinh ndng mdi cho dich vu vd dich vu mdi

La giao thuc khoi tao phien trong mang chuyen mach goi SIP cho phep tao ra nhung tinh nang moi hay djch vu mai mot cach nhanh chong Ngon n g u xir ly cuoc ggi (Call Processing Language) va Giao dien cong ket noi chung (Com m on Gateway Interface) la mot so cong cu de thuc hien dieu nay SIP ho tra cac djch vu thoai n h u c h a cuoc goi, chuyen tiep cuoc goi, khoa cuoc goi (call waiting, call forwarding, call blocking ), ho trg thong diep thong nhat

4.3.5 So sanh H.323 va SIP

G iua H.323 va SIP co nhieu diem tu a n g dong Ca hai deu cho phep dieu khien, thiet lap va huy cuoc goi Ca H.323 va SIP deu ho trg tat ca cac djch vu can thiet, tuy nhien co mot so diem khac biet giua hai chuan nay Do la:

- H.323 ho trg hoi nghi da p h u an g tien rat phirc tap Hoi

nghj H.323 ve nguyen tac co the cho phep cac thanh vien sir dung nhung djch vu n h u bang thong bao, trao doi d u lieu, hoac hoi nghj video

- SIP ho trg SIP-CGI (SIP - Common G atew ay Interface)

- SIP ho trg dieu khien cuoc goi tir mot dau cuoi th u 3 Hien nay H.323 dang dugc nang cap de ho trg chirc nang nay

C huang IV : Cdc giao thuc dieu khien 157

Bang 4.2: So sanh H.323 vd SIP

va Web Cu phap va ban tin tuong tu1 nhu HTTP

Co so la mang thoai Giao thuc bao hieu tuan theo chuan ISDN Q.SIG

SIP

Dau cuoi thong minh H.323

Cac may chu loi May chu uy quyen SIP,

may chu chuyen doi dia chi, may chu djnh vj

va may chu dang ky.

Dieu khien cong noi H.323

Tinh hinh hien nay Giai doan thi> nghiem

kha n3ng cung hoat dong cua thiet bi cua c^c nha cung cap khac nhau da k it thuc SIP nhanh chong tro nen pho b iin

Da duo c su dung rong rai

T r i th iit lap cuoc

goi

1,5 RTT (round-trip time, tire chu ky gui ban tin va nhan ban tin tra loi hay xac nhan)

Phien ban 1 va 2: may chu phai giam sat trong suot thoi gian cuoc goi

va phai giu trang thai

k it noi TCP -> han c h i kha nang md rang va giam do tin cay

Bao hi§u quang ba

(cho ca nh6m .)

158 Cong nghe VoIP va cuc g ia i phap

Ch£t luong dich vy St> dyng cac giao thirc

khac nhir RSVP, OPS, OSP de dam bao chat luong dich vu

Thidt bi dieu khi4n c6ng noi dieu khi£n bang thdng H 323 khuyen nghj dung RSVP d4 li/u trO tai nguyen mang.

d3ng ky, c6 xac nhan dau cuoi va ma hoa

Chi dang ky khi trong mang co thi£t b| di&u khien cong noi, xac nhan va ma h6a theo chu£n H.235

Dinh vi dau cuoi va

dinh tuyen cuoc goi

Diing SIP URL de danh dia chi Djnh tuyen nho sir dung may chu thay dia chi

va may chu dinh vi.

Dinh vi dau cudi si> dung E 164 hoac t6n ao H.323 va phi/cmg phap anh xa dia chi neu trong mang co thi£t bj di£u khien cong n6i Chirc nang dinh tuy§n do thiSt

bi dieu khi£n cong noi dam nhiem

Tinh nang thoai Ho tro cac tinh nang

cua cuoc goi co ban

Ho tro cac tinh nang cua cuoc gpi co ban Hoi nghi Hoi nghi co so, quan ly

phan tan

Duoc thi6t k6 nham ho tro rat nhi^u tinh nang h6i nghi, ke c i thoai video va dur Ii6u quan

ly tap trung nen MC c6 the tac nghen

Tao tinh nang va

dich vu moi

De dang, sir dung SIP- CGI va CPL

H 450 1

Tich hop vdi web Rat tot hd tro nhan

chudt de quay s6 (click-to-dial)

K6m

II

Chuong IV : Cue giao I hue dieu khien 159 4.4 GIAO THtTC MGCP

G iao thirc dieu khien cong noi ph uan g tien (M G C P) la giao thirc dung dc dieu khien cac cong noi thoai tir cac thiet bj dieu khien cuoc goi Giao thirc nay la chirc nang ciia bo dieu khien cong noi p h u a n g tien (M GC) hoac dai ly cuoc goi (CA)

Cong noi phirong tien (MG)

Cong noi phUdng tien (MG)

Hinh -I. 7; Cong ket ndi p h u a n g tien va bo dieu khien cdng ndi

phirong tien

Quan he giua MG va MGC (hay CA) dugc mo ta tren hinh 4.7 M GC thuc hien bao hieu cuoc goi, dieu khien MG MGC va MG trao d6i lenh vai nhau thong qua MGCP

4.4.1 Thiet lap cuoc gpi

Hinh 4.8 mo ta viec thiet lap va giai phong cuoc goi M GCP thonc th u a n g giua 2 thue bao PSTN ket noi vai cac tong dai sir dung C C S 7 de bao hieu

160 Cong nghe VoIP vd cdc gidi phap

Hinh 4.8: Thiet lap vd gidi phong cuoc goi M GCP

- Thue bao PSTN ben A (chu goi) nhac may, quay so roi giri cac chir so dia chi thue bao bi goi nay toi tong dai (ben chii gpi - ben A)

- Tong dai ben A sau khi nhan xong dja chi thue bao bi goi (ben B), tao ra ban tin ISUP IAM (Initial Address M essage) de

Chuang IV : Cac giao thuc dieu khien 161

giri bao hieu cho cong noi p h u a n g tien/thiet bj dieu khien cong noi p h u a n g ti$n (M G /M G C ) de chinh thuc dat van de ket noi voi thue bao bj goi (1)

- M G C gui cho M G ben A ban tin M G C P CRCX -

“CreateconnectionM (2)

- MG ben A tra lai bSng ban Resp - “Response” (3)

- MGC gui cho tong dai ben B (bi goi) ban tin I SUP IAM (4)

- MGC gui tiep ban tin M G CP CRCX cho M G ben B (5)

- MG ben B tra lai bang ban tin Resp (6)

- MGC giri cho M G ben A ban tin M G C P M DCX (7)

- MG ben A tra lai bang ban Resp (8)

- MGC giri cho MG ben B ban tin ISUP COT (Continuity message) de bao cao ket qua kiem tra tinh lien tuc (dugc yeu cau trong ban tin 1AM) tren mot mach (kenh B dugc chon) (9)

- Tong dai ben B giri cho MGC ban tin ISUP ACM (hoan tat viec nhan dia chi) dong thai rung chuong cho thue bao B (10)

- MGC chuyen cho tong dai ben A ban tin ISUP ACM nay (11)

- Sau khi thue bao B nhac may, tong dai ben B giri cho MGC ban tin ISUP ANM (AN sw er Message) thong bao viec nhac may nay (12)

- Ban tin ANM nay lai dugc MGC chuyen cho tong dai ben

A va hai ben thue bao A va B co the bat dau dam thoai (13)

D u ai day la thu tuc giai phong ket noi M GCP giua 2 thue bao A va B khi mot trong hai ben gia sir ben B dat may

- Tcing dai ben B giri ban tin ISUP REL cho MGC yeu cku

giai phong ket noi (1)

- MGC sau do giri ban tin ISUP REL nay cho tong dai ben A (2)

162 Cdng nghe VoIP va cdc gidi phup

- M GC giri ban tin M G CP DLCX “ Deleteconnection”cho

MG ben B de thong bao yeu cau M G giai phong ket noi (3)

- M G ben B giri tra lai ban tin M G CP Resp (4)

- MGC cung dong thai giri ban tin M GCP DLCX cho MG ben A de thong bao yeu cau M G giai phong ket noi (5)

- M G ben A cung giri tra lai ban tin M GCP R esp (6)

- Tong dai ben A giri cho MGC ban tin ISUP RLC thong bao hoan tat viec giai phong ket noi (7)

- MGC sau do cung giri cho tong dai ben B ban tin ISUP RLC thong bao hoan tat viec giai phong ket noi Ket noi ket thuc (8)

Chuang I V Cdc giao I hue dieu khien 163

4.4.3 So sanh gifra MGCP, SIP va H.323

M G C P khac vai SIP va H.323 a mot so diem nhu sau:

- M G C P la giao thuc kieu chii/tcr, trong khi SIP va H.323 la ngang hang (peer-to-peer)

- M GCP dugc sir dung giua M G va MGC

MGCP ra dai nham tach biet cac chuc nang bao hieu va chuc nang thiet lap d u a n g truyen M GC (hay CA, chuyen mr.ch mem ) sau khi nhan dugc yeu cau thiet lap cuoc goi SIP hoac H.323 se dung giao thuc M G C P de dieu khien cong ket noi thiet lap phien ket noi giua 2 dau cuoi

Trong hinh 4.9 ta thay chuc nang bao hieu da du gc tach biet

va do MGC dam nhiem Chung ta xem xet viec thiet lap cuoc goi trong hai truang hgp trong hinh 4.10 du ai day

Cong ket noi H.323

Bo dieu khien cdng ket ndi phUdng tien

Hinh 4.9: Cdng ket ndi H.323 vd MGC-1!-MG

164 Cdng nghe VoIP vd cdc giai phap

5 RTP / RTCP Cong ket noi

Oai ly cuoc gpi thiet bj dieu khien cdng ndi phuong tien

_ RTP/RTCP Cong ndi A Cong ndi B

Tren c a s a MGCP mot so giao thiic mai duac phat trien do la

- M E G A C O do IETF phat trien

- H.248 do ITU phat trien

Cac giao thuc nay deu la giao thuc dieu khien cong noi

p h u an g tien (M edia Gateway)

C huang IV: Cdc giao thuc dieu khien 165

Bang 4.3: So sanh H.323 vd MGCP

1 Thue bao nhac m£y va quay so

2 Cong ket noi ph^n tich djnh tuyen

cuqc gpi

3 Hai cong ket noi trao doi thong tin

4 C6ng ket noi bj gpi do chuong 6 so

may bj gpi

5 Hai cong ket noi thiet Igp phien ket noi

RTP/RTCP

1 Thue bao nhac m^y va quay so

2 Cong ket n6i thong bao cho MGC

3 MGC phan tich so, djnh tuyen va gi>i lenh cho cong ket noi bj goi de

do chuong a so m3y bj gpi

4 MGC gi>i l#nh cho 2 cong ket noi de thiet Igp phien ket noi RTP/RTCP

4.5 GIAO T H U t M EGACO/H.248

Giao thirc M EG A C O /H 248 dira tren mo hinh chu/ta va la chuan quoc te cho viec dieu khien cong ket noi trong mang phan tan va la chuan m o dirge phat trien ket hgp giua IETF va ITU

M EGACO/H.248 tuy con don gian nhung hieu qua va rat linh hoat trong viec m a rong, cho phep xay dimg phan chia cac chuc ndng cong ket noi ben duoi lop dieu khien cuoc goi (nhu SIP,

H 323 ) N o nit linh hoat cho viec phat trien phan Ion cac dich

vu voi cac yeu cSu chdt lugng, gia ca khac nhau cung nhu ho trg

va phat trien cac m ang san co

Giao thuc M E G A C O phat trien tir giao thirc dieu khien cdng da p h u a n g tien (M G C P- M edia Gateway Control Protocol)

va dugc dinh nghTa boi RFC 3015 Giao thuc M EG A C O tach di£u k h iln bao hieu tir cong noi p h u ang tien (MG) giua hai mang khac nhau va dua chung vao bo dieu khien cong n6i

p h u an g tien (M G C) M EG A C O /H.248 chi ra giao thirc thong tin giua M GC va M G va khong dia chi thong tin giua cac k h cu6i

166 Cong nghe VoIP vd cdc giai phap

Trong mang the he sau N G N, mot trong cac giao thirc cung

co the dirge sir dung de dieu khien viec thuc hien cac cuoc noi thoai do la M EG ACO /H.248 N gudi ta cung ggi no la giao thirc dieu khien cong noi p h u an g tien tu a n g tu n h u MGCP Giao thirc nay dugc IETF phat trien va hoan thien, hien tai S G I 6 cua ITU

da chap nhan giao thirc nay duai ten 11.248

4.5.1 Chirc nang cua M EGACO

Theo ITU-T S G I 6 va IETF WG, M EG A C O la giao thircdieu khien giua thiet bi dieu khien cong noi p huang tien MGC

va cong noi p h u an g tien MG Giao thirc nay co chuc nang:

- Dieu khien cac dang thiet bi ket cuoi

- Ho trg kha nang dan xep cuoc goi

- Cac p h u an g an cuoc ggi da n gu ai sir dung

- Chat lugng dich vu va ho trg cho do luu lugng

- Bao loi trong giao thirc, cuoc goi, dung lugng va loi mang

G iao thirc M E G A C O co the dugc thiet ke theo ca dang ngang hang hay chii/ta, cac giao thirc nay co the cung ton tai trong mot mang M E G A C O hoan toan tu a n g thich vcri cac cach tiep can theo kieu ngang hang n h u SIP, H323, va no cung co

n hung uu diem trong cach tiep can dieu khien cong ket noi theo kieu chii/ta n h u M GCP

Mo hinh giao thirc M EGACO/H.248 co cau true lenh kha

( huang IV; Cdc giao thuc dieu khien 167

M EGACO co t h i dugc su dung tren rdt n h ilu phuomg tien mang khac nhau, bao g6m ca mang dua tren IP va mang dua tren ATM.Giao thuc chudn M EG A C O khong bi rang buoc vai bat cu cach tiep can dieu khien cuoc goi m uc ngang hang cu the nao Kien true cua M E G A C O dua tren lop dieu khien cong noi phuang tien M GC, lap cong noi p h u an g tien MG va chinh giao thuc M EGACO

Lap MGC bao gom tat ca kha nang dieu khien cuoc goi va cac dich vu nh u chuyen tiep cuoc goi, truyen, hoi nghi, giu cuoc goi Lap nay cung cung cap bat cu giao thuc m uc ngang hang nao de tu a n g tac vai cac M GC khac hoac cac thuc the ngang cap L ap nay quan ly tat ca cac tu a n g tac dac tinh va quan ly cac tuang tac vai cac thu tuc bao hieu n hu bao hieu so 7

Lap cong noi p h u a n g tien cung cap cac ket noi p h u a n g tien

tu mang chuyen mach goi tai mang chuyen mach goi, tu a n g tac vai nhung ket noi p h u a n g tien nay, dieu khien cac thiet bi cong ket noi L a p nay dugc xem n h u la lap a m uc cuoc goi, dong vai tro “ta ” trong kien true chu/ta

Do giao thuc M E G A C O hoan toan doc lap vai cac giao thuc dieu khien cuoc goi a m uc ngang hang khac nhu SIP, H.323, cac

he thong khac nhau co t h i dugc su dung a muc dieu khien cuoc goi vai chi phi toi th ilu a m uc dieu khien cong noi

4.5.2 Mo ta giao thirc M EGACO

M E G A C O su dung mo hinh tai nguyen don gian hieu qua

d l m o ta cac thuc the lo-gic trong cong noi phuang tien dugc thilt bi d ilu k h iln cong noi p h u an g tien Khai niem c a ban dua tren hai thuat n g u Ket cuoi (Termination) va N g u canh

168 Cdng nghe VoIP va cdc gidi phap (Context). Termination xac djnh cac luong hoSc cac phuang ti?n tai nguyen, cung cap cac tin hieu va phat di cac sir ki?n, thupc tinh, thong ke Tat ca cac tin hieu, sir kien, thupc tinh, thong ke dugc djnh nghTa trong cac goi tin giao thurc.

Cau true lenh trong M E G A C O kha d an gian nhung hieu qua

va mem deo Trong M E G A C O chi co 7 cau lenh tat ca: Add, Subtract, Modify, Move, Notify, AuditValue, ServiceChange. Tat

ca cac cau lenh tren deu hoat dong a Term ination theo m<?t

ph u an g thurc nhat quan

Cac cau lenh giura M G C va M G co the de dang nhom voi nhau thanh mot phien truyen (transaction) sur dung cac qui tic mem deo, d an gian Do vay dung lugng mao dau ban tin co the giam mot cach dang ke

Cac cau lenh sir dung bo mo ta (descriptor) trong cach tiep can cua M E G A C O de nhom cac thanh phan d u lieu lien quan Dieu nay lam tang tinh m em deo va kha nang phan tich du lieu Chi co nhung bo mo ta can cho nhung hoat dong mcri dugc giri theo ciing cac cau lenh, do do giam dung lugng mao dau

Tat ca cac tin hieu va sir kien du g c xem la xay ra tai Term ination va chung cung cap c a che de tu a n g tac vcri cac thuc the phia xa dai dien boi Term ination do Cac tin hieu va su kien dugc dinh nghTa trong goi tin Tin hieu bao gom am phat di, cac

Chuang IV: Cdc giao thuc dieu khien 169

M EG A C O /H 248 nhSm muc dich c h u y ln cong k i t noi H.323 vao trong M GC va MG Do do, de M SF sir dung giao thuc M EG A C O /H 248, can xac djnh su chuyen doi giua MGC

va MG va khoi chuc nang M SF khac Sir chuyen doi nay cho phep xac djnh dung vj tri giao thuc M EG ACO /H 248

Q oS V A C A C Y E U T O C H IN H A N H HlTONG

D E N C H A T L lT O N G T H O A I IP

Chtfdng V

5.1 CHAT L l T g f N G DICH V y IP

5.1.1 The nao la chat ltrong dich vu• D • •

Chat lugng dich vu - QoS (Quality o f Service) dugc xem

n h u la lap djch vu - CoS (Class o f Service) va loai djch vu - ToS

( Type o f Service). Muc tieu c a ban cua CoS va ToS la dat dugc bang thong va thai gian tre can thiet cho mot ung dung dac biet nao do CoS cho phep ng u ai quan trj co the nhom cac luong goi khac nhau, moi luong co tre va cac nhu cau bang thong rieng

T ru a n g ToS trong tieu de cua goi IP cho phep CoS ho trg

Noi mot cach d an gian nhat, chung ta co the coi chat lugng djch vu - QoS (Quality o f Service) n hu nhung cach thuc de co dugc n h ung djch vu cung cap d u lieu on djnh va hieu qua hay kha nang cua mang dam bao va duy tri cac muc thuc hien nhat djnh cho moi ung dung theo cac yeu cau da dugc chi ro cua moi

n g u ai su dung

QoS cho chung ta biet kha nang cua mot phan tu mang (co the la mot ung dung, la mot tram (host) hay mot bo djnh tuyen (router) de bao dam nhung yeu cau nao do ve luu lugng ve kha nang cung cap djch v u

C huang V: QoS vd cdc yeu to chinh anh huang. 171

Nha cung cap dich vu mang dam bao QoS cung cap cho nguai su dung va thuc hien cac bien phap de duy tri muc QoS khi dieu kien mang bi thay doi vi cac nguyen nhan nhu: nghen, hong hoc thiet bi hay loi lien k et QoS can dugc cung cap cho moi ung dung de n gu ai su dung co the chay ung dung do va muc QoS ma ung dung doi hoi chi co the dugc xac dinh boi nguai su dung, bai vi chi co n g u ai su dung m ai co the biet dugc chinh xac img dung ciia minh can gi de hoat dong tot N guai sir dung tu dong co the biet du g c mang can cung cap nhung gi can thiet cho irng dung ciia minh, giong nhu cac yeu cau cho cac ung dung phan mem ciia may tinh ca nhan (vi du: cau hinh may 128

V6 quan niem thi QoS la mot tai nguyen ciia mang, hay dung h a n la mot nhom tai nguyen mang, vi du nhu bang thong

va tri Q uan niem QoS la tai nguyen mang nhu dugc chi ra a hinh 5.1 QoS a "ben trong" mang va mang se cung dip QoS thich hgp cho cac img dung khi can thiet

172 Cong nghe VoIP vd cac giai phap

QoS trong mang, cac Cfng

dung yeu cau mang cung c5p

(phirong thirc cua PSTN)

Khong co QoS trong mang, tat ca cac img dung phai ti/ lam lay (phi/dng thifc cua IP/lntemet)

Hinh 5.1: X ay dim g mang cd QoS hay ho sung QoS vao mang

Cac nha cung cap dich vu mang dura ra thong tin dac ta ve gia tri thirc te cua cac thong so QoS theo mot trong hai cach sau

V ai moi trudng kenh ao cd dinh (PVC- Permanent Virtual Circuit), cac gia tri cua cac tham so QoS co the chi dcrn gian dugc ghi tren mot mau giay va trao cho dai dien cua nha cung

cap dich vu mang Fax, th u dien tu va tham chi ca goi dien de cung cap cung mot thong tin ve yeu cau cua khach hang, mien la khach hang va nha cung cap dich vu thong nhat dugc vai nhau

ve cach thuc sir dung de trao doi thong tin ve QoS QoS co hieu lire tren PVC khi PVC san sang V ai moi trudng kenh ao chuyen mach (SVC: Switched Virtual Circuit), cac gia trj cua thong so QoS dugc gui cho nha cung cap dich vu trong ban tin bao hieu thiet lap cuoc ggi, no la mot phan cua giao thuc bao hieu dugc sir dung de cung cap dich vu chuyen mach tren mang Ca hai

p h u an g phap deu co the dugc su dung trong moi cach ket hgp

C huang V QoS va cac yeu to chinh anh huang 173

tren mang Phirong phap PVC cho phep QoS dugc cung cap ranh rang hem, trong khi phuomg phap SVC doi hoi QoS tren mot ket noi cho tru ac du g c thiet lap lien tuc

Neu mot mang dugc toi uu hoan toan cho mot loai dich vu, thi nguai sir dung it phai xac dinh chi tiet cac thong so QoS Vi

du, vai mang PSTN, du gc toi uu cho thoai, khong can phai xac dinh bang thong hay tre can cho mot cuoc goi Tat ca cac cuoc gpi deu dugc dam bao QoS nh u da dugc quy dinh trong cac chuan lien quan cho dien thoai Neu mo-dem dugc sir dung de truyen dir lieu tren ket noi thoai thi cac thong so dugc cung cap bcri PSTN van khong the bi vi pham PSTN thich hgp cho thoai

do ban chat cua no, tuy nhien no lai khong hoan toan phii hgp cho nhieu img diing ngay nay, dac biet doi vai khia canh bang thong Tuy nhien, co dinh tat ca cac thong so QoS trong mang PSTN rat phii hgp cho thiet lap cuoc goi va dinh tuyen kha dan gian, hieu qua va nhanh

Thong thudng, co nhieu mire QoS khac nhau cung giong nhu la co nhieu img dung vay Cac ung dung lai thay doi rat ldn tham chi ca vdi n h u ng yeu cau dan gian ve bang thong Thoai so

co th i yeu cau bat ky tdc do nao trong khoang tir 8 den 64 kbit/s Video thi cd t h i chap nhan dugc d toe do 1,5 Mbit/s, nhung se t6t han n h ilu n l u la 6 Mbit/s Truy nhap Web va truyen tap tin sir dung bang thong cang nhieu cang tot trong pham vi co the, nhung lai khong can lien tuc Tuy nhien, bang thong tren PSTN

va ciia m ang dir lieu nhan dugc tir cac d udng thue rieng dua tren PSTN lai chi phuc vu gidi han tai toe do 64 kbit/s Mot ung dung

174 Cong nghe Vo IT va cac gidi phup

can 100 kbit/s thi hoac la co gang hoat dong a 64 kbit/s, ho3c la

bo phi 28 kbit/s neu sir dung 128 kbit/s Day la mat han che ciia cac mang chuyen mach kenh "tat ca bang thong tat ca thdi gian".Mang chuyen mach goi co the chia bang thong ra thanh nhieu phan rat thich hop cho cac irng dung d u lieu bung n6, nhung do khong phai la tat ca Mot mang can phai cd kha n5ng cung cap QoS yeu cau cho moi irng dung, khong can biet bang thong can thiet la co djnh hay khong Kha nang ve phia mang cap cho cac img dung cac dam bao QoS, vi du n h u la dam bao bang thong, du o c xem la phan cap QoS ciia mang

Phan cap la mot khia canh quan trong ciia QoS Phan cSp xac dinh cac thong sd QoS tot den mire nao ma ngudi sir dung

cd the dinh ro cho mot irng dung cu the Neu mang cung cap QoS khong du do min, thi no cd the gidi han ngudi sir dung truy nhap vao mang Lay mot vi du don gian, xet mot nha cung cap dich vu m ang thiet lap nhieu loai lap dich vu cho cac ung dung ciia ngudi sir dung Co nhieu luc ldp djch vu dugc dung dong nghia vdi QoS, nhung d day sir dung thuat n g u ldp djch vu vdi nghTa la mot tap hgp ciia cac gia trj thong sd QoS cd kha n&ng cung cap cho mot ldp, hay loai img dung nao do

N ha cung cap djch vu cd the dua ra mot ldp djch vu thoai tren mot m ang goi ma no dam bao bang thong 64 kbit/s giua cac dau cuoi la tre 100 ms vdi bien dong tre nho hem 10 ms Nhung neu mot img dung thoai mdi chi yeu cau 8 kbit/s thdi thi sao? Hay tham chi chi 4 kbit/s Bdi vi ngudi sir dung dugc dam bao 6

64 kbit/s, nen lugng bang thong nay noi chung phai du g c chia ra

C huang V: QoS vd cdc yeu to chinh dnh huang 175

tir toan bo bang thong tren mang Theo do, ngirai sir dung tra cho phan bang thong khong sir dung va nha cung cap dich vu du tru bang thong de co the dung de cung cap cho nhung nguai sir dung khac

Phan cap QoS t6t han se cho phep nguai sir dung thoai, tham chi trong ciing lap dich vu, xac djnh bang thong ho yeu cau chinh xac han Sir chinh xac nay m uon dat dugc thi phai tra gia bang do phirc tap ciia mang, day la ly do chinh cho viec giai han cac gia trj thong so QoS va dat ra cac lap djch vu trong giai doan dau

5.1.2 Tai sao chung ta can den chat luong djch vu cho mang IP

N h u chiing ta da biet, giao thirc c a s a ciia Internet: giao thirc

IP cung nhu cau true ciia ban than Internet deu dua tren sir trao doi cac goi d u lieu, cac goi d u lieu nay, vai phan dja chi nguon

va dich luu trong tieu de (header), co the tu luu chuyen qua cac

bo djnh tuyen lien ket giua cac m ang con ma khong can tai su trg giup cua phia giri hay nhan chiing

Tuy nhien, viec d a n gian hoa hoat dong ciia mang cung co cai gia ciia no Bai giao thirc IP rat d an gian, nen no khong co kha nang cung c5p n h ilu djch vu IP co the cung cap dja chi dich cho cac goi tin du lieu cung n h u co the phan doan cac goi tin de rdi rap lai chiing sau do n h ung IP khong the cung cap mot c a

c h i tru y ln d u lieu dam bao tin cay Cac bo djnh tuyen co quyen hiiy bo cac goi tin IP ma khong can thong bao gi cho phia giri cung n h u phia nhan Tat nhien la IP se dua vao cac giao thirc truy I n thong a tang tren no, vi du TCP de co the quan ly cac goi tin Tuy nhien cac giao thirc nay cung chi bao dam rang cac goi

176 Cong nghe VoIP va cdc gidi phdp

tin se dugc gui di ch u khong the dam bao ve tinh thoi gian thyc cua cac goi tin den hay ve thong lugng d u lieu N h u chung ta da biet, giao thuc IP chi cung cap dugc cai ggi la djch vy "best effort” (co gang toi da), ma thyc chat la djch vu truyen goi tin nhung khong quan tarn den khi nao goi tin se den dich cung nhu

co bao nhieu goi tin co the den dich

Dieu gi se xay ra neu mang khong thanh cong trong vi^cdam bao va duy tri QoS chinh xac cho mot ung dung cho truac?Dieu nay tuy thuoc vao su thoa thuan giua ngudi sir dung vd nhacung cap dich vu trong tru a n g hgp dich vu dugc quan ly boi hgp

dong, hay la giua nha cung cap dich vu va bo phan dieu chinhtrong tru an g hgp dich vu bang gia Dam bao chdt lugng cung laphan quan trong ciia hgp dong cho cac dich vu mang giua khachhang (dai dien cho n g u ai sir dung) va nha cung cap djch vy.Thong th u an g thi khach hang phai tra tien hang thang cho cacdich vu Dam bao chat lugng co the thiet lap mot hinh thirc boi

th u an g n h u giam b at gia tien djch vu mot thang neu nha cungcap dich vu khong cung cap dung chat lugng yeu cau trongthang do Trong nhung tru an g hgp nghiem trong neu van de vemang xay ra trong toan bo thang do, khach hang co the nhandugc djch vu mien phi

Dam bao chat lugng mang trong mot moi truang djch vu, hgp dong thu an g dugc the hien theo hinh thirc thoa thuan mire djch vu (SLA: Service Level Agreement) dugc thiet lap giua khach hang va nha cung cap djch vu SLA co the la mpt phan cua hgp dong djch vu hay la mot tieu chuan doc lap hoan toan SLA

( huang V: QoS va cac yeu to chinh anh huang I l l

dira ra cac yeu cau cua khach hang va cac hinh thuc bdi thucrng doi vai nha cung cap trong trudng hop xay ra su co SLA cung cung cap mot p h u an g phap thuan tien cho khach hang de so sanh cac dich vu do cac nha cung cap dich vu khac nhau dua ra

"co gang toi da", do do "khong tin cay" khi no tien tdi dam bao

ve QoS Tuy nhien, neu tat ca cac nha cung cap dich vu Internet (Internet Service Provider - ISP) muon quan tarn tdi QoS, thi van khong de gi them QoS vao mot mang IP tai ldp IP

Plurang phap tot nhat de cac nha cung cap dich vu Internet

co th i dat tdi dam bao QoS hay thoa thuan ket noi m uc dich vu (SLA) giua khach hang va ISP la vdi dich vu mang IP duac quan ly. Thuat n gu duac quan ly a day la bat cu cai gi ma nha cung cdp dich vu quan ly thay mat cho khach hang Vay thi cai

gi dang thirc su du g c quan ly tren mang IP? Do la QoS ma mang

IP cung cdp Dieu nay dugc thuc hien bang each cach ly cac bo djnh tuy In , cac lien ket su dung de cung cap dich vu IP cho mot khach hang cu the va su dung cac tai nguyen nay tren mot

n ln tang danh rieng mot phan phuc vu cho minh khach hang do Trong v a i trudng hgp cac bo dinh tuyen va cac lien k i t vdn