Chủ đề: VoIP_Part_I_2: Mạng thoại truyền tải sử dụng công nghệ chuyển mạch gói

2.Packetized telephony network
Trong phan nay minh se mo ta hai phuong thuc dieu khien cuoc goi duoc su dung trong voice va data network. Bao gom:
• Nam loi ich cua PTK so voi circuit-switched network
• Mo ta ba ki thuat dieu khien cuoc goi su dung ptk
• So sanh gw function va xu li bao hieu trong mo hinh tap trung; phan tan
• Bay thanh phan cua voice network
• Truyen thong thoi gian thuc: voip; best-effort

2.1.Loi ich cua PTN
de tiet kiem chi phi goi dien thoai duong dai: hoi tu mang voice va data. Loi ich cua ptn so voi mang thoai truyen thong bao gom:
• Su dung hieu qua bang thong va thiet bi: mang thoai truyen thong su dung mot kenh 64 kbps cho moi cuoc goi voice. Ptn share bang thong giua cac logical connection va offload traffic volume tu voice switch
• Giam thieu chi phi cho mang thoai: mot luong lon cac thiet bi can cho ket noi cac kenh 64 k trong mot link toc do lon qua mang. Ptn don kenh voice theo data nen giam tien thue kenh
• Tich hop voice va data: data netwok hoat dong phan tach voi voice network bi gio tro thanh thanh phan mang luu luong chinh.
• Tang tien tu dich vu moi: gia tang cac dich vu: broadcast audio; unified mess…
• Tang tinh sang tao cho dich vu: unnified com su dung ha tang ip cho phep lien ket mot loat cac phuong thuc lien lac doc lap lai voi nhau nhu: fax; voice mail; email….
• Truy cap thiet bi lien lac moi: cho phep truy cap den cac thiet bi ma ha tang TDM ko co. vi du: pc; handheld device….
• Cung cap dich vu linh dong

Tuy co nhieu loi the nhung can quan tam toi cac van de sau truoc khi chuyen tu mot mang thoai truyen thong sang mang thoai IP:
• Hieu qua dau tu
• Cac nhan vien phu trach voice va data kho trao doi
• Cac thanh phan voice hien gio van con dung duoc
• Chi phi trien khai


2.2.Dieu khien cuoc goi
cho phep thiet lap; duy tri va huy mot voice flow thong qua mang. (chac la do cac chu voice gw thuc hien)

mac du cac giao thuc khac nhau co dinh huong dieu khien cuoc goi theo cac cach khac nhau nhung heu het chung deu cung cap nhung dich vu chung. Cac thanh phan co ban cua mot dieu khien cuoc goi bao gom:
• Call setup: kiem tra cau hinh call-routing de xac dinh dich cua mot cuoc goi. Cau hinh nay con chi ra bang thong yeu cau cho cuoc goi. Khi bang thong yeu cau da biet call admission control (CAC) xac dinh xem co du bang thong de phuc vu cuoc goi hay khong. Neu co call setup se phat ra mot setup mess gui toi dich. Neu khong se thong bao cho ben goi biet la tin hieu dang ban. Co rat nhieu giao thuc dieu khien cuoc goi nhu H323; MGCP (media gw control protocol) va SIP (session initiation protocol) deu dinh ra mot tap cac mess de trao doi trong qua trinh nay.
• Call maintanace: kiem tra packet count; packet loss va do tre jitter hoac delay khi cuoc goi da duoc thiet lap. Thong tin se duoc chuyen cho voice-enable device de xac dinh xem chat luong cuoc goi co tot khong; neu khong cuoc goi se bi huy
• Call teardown: thong bao cho gw giai phong tai nguyen de su dung cho cuoc goi tiep theo

Distributed call control

call control duoc quan li boi rat nhieu cac thanh phan trong mang. Dieu khien cuoc goi phan tan chi co the thuc hien duoc khi voice gw duoc cau hinh ho troc dieu khien cuoc goi truc tiep. Dieu khien cuoc goi phan tan cho phep cac gw thuc hien cac chuc nang sau:
• Nhan dien req cho dich vu
• Xu li so duoc goi
• Dinh tuyen cuoc goi
• Giam sat cuoc goi
• Huy cuoc goi

Centralized call control

dieu khien cuoc goi tap trung cho phep mot thiet bi ngoai call agent quan li bao hieu va xu li cuoc goi; chuyen cho gw chuyen tin hieu audio thanh cac goi tin voice sau khi cuoc goi duoc thiet lap. Call agent co trach nhiem cho toan bo viec bao hieu sau do huong dan gw gui cac tin hieu dac biet tai cac thoi diem dac biet

khi mot cuoc goi duoc thiet lap:
• Voice path se chay truc tiep giua hai gw khong chay giua hai agent
• Khi nao co huy cuoc goi; call agent se bao hieu cho gw giai phong tai nguyen cho cuoc goi khac

Su dung dieu khien cuoc goi tap trung co cac loi ich:
• Tap trung hoa cau hinh cho cac call adminition control.
• Call agent la cac thiet bi co kha nang hieu va tham gia trong hoat dong cua call control. Chuc nang nay cho phep user khong mua them cac router co chuc nang voice-gw day du (tuy nhien cac call agent nay cung chang re ti neo)
• MGCP la mot trong nhung mo hinh quan li tap trung dieu khien cuoc goi

2.3.Packet telephony component

cac thanh phan co ban cua mot mang voice bao gom:
• Ip phone: cung cap ip voice cho desk
• Gatekeeper: cung cap CAC; dieu khien bang thong va quan li; chuyen doi dia chi
• Gateway: cung cap chuyen doi giua mang voip va non-voip (pstn). No cung cung cap truy cap vat li cho cac thiet bi local analog va digital voice nhu telephone; fax; key se hay pbx
• MCU (multipoint control unit): cung cap cac ket noi realtime cho cac ben tham gia trong nhieu vi tri de tham gia cung mot cuoc hoi thao video conference
• Call agent: cung cap call control cho ip phone; cac; dieu khien bang thong; quan li va chuyen doi dia chi (the thi sinh ra thang gatekeeper de lam gi nhe)
• App server: cung cap cac dich vu nhu voice mail; unified mess va cisco callmanager attendant console
• Videoconference station: cung cap truy cap cho end-user trong hoi thoai truyen hinh

2.4.Best-effort delivery of real time data
voice va data co the chia se cung mot moi truong truyen dan tuy nhien dac tinh cua du lieu lai khac nhau rat nhieu: voice la du lieu kieu real time con data thi lai la kieu luu luong best-effort. Cach khac phuc la co giai phap QoS end-to-end