Chủ đề: VoIP_Part_II_3: Hệ thống báo hiệu trong mạng thoại

3.Signaling system
Mo ta cac kenh bao hieu giua cac pbx; giua pbx va co; va bao hieu dac biet bang isdn

3.1.Channel associated signaling system: T1
Channel associated signaling la phuong thuc bao hieu thuong duoc su dung giua cac pbx voi nhau. Pbx va cac thiet bi cisco thuong su dung cac kenh E1 va T1 de van chuyen voice. Muc dich chinh cua t1 la viec chuyen thong tin tin hieu theo hai phuong thuc channel associated signaling va common channel signaling
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Dac tinh kenh bao hieu cua T1 co dang:
• Mot T1 frame dai 193 bit (8 bit cho moi 24 khe thoi gian) cong voi 1 bit framing. Kenh hoat dong voi chu ki 125 microsecond voi tan so lay mau 8000 mau tren s. do do toc do truyen se la (8 bit * 24 khe +1 framing bit * 8000 mau/s =1.544 Mpbs)
• T1 co hai kieu framing quan trong:
 Super frame (SF) hoac D4 bao gom 12 frame cho mot chuoi.
 ESF (extended SF): cung bao gom 1 bit framing di theo sau 192 bit.

Do moi kenh DS0 mang 64 kbps va chuan nen G.711 la 64 kbps nen se khong co cho de van chuyen tin hieu. De thuc hien voice; T1 su dung frame thu 6 de van chuyen tin hieu.
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Trong moi frame thu 6; cac bit LSB cho moi voice channel duoc su dung de bao hieu. Phuong thuc nay duoc goi la RBS (robbed-bit signaling). Khi SF trien khai theo phuong thuc nay; bit tin hieu duoc van chuyen tai frame thu 6 goi la A bit va frame thu 12 goi la B bit. Trong tin hieu dieu khien; A va B bit cung cap cac chi dan near va far-end off hook

Ngoai ra A va B bit con bieu dien cho cac trang thai bao hieu khac nhau hoac cac dac trung dieu khien khac nhu onhook; offhook; idle; busy; ringing; addressing….

ESF su dung cac bit RBS tai frame 6; 12; 18; 24 nen goi la bao hieu ABCD; cung ca them dieu khien va bao hieu

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Do viec bao hieu duoc thuc hien ben trong cac kenh DS0 nen con duoc tham chieu den nhu la bao hieu in-band (kenh bao hieu nam trong kenh du lieu) hay con goi la channel associated signaling

3.2.Channel associated signaling system: E1
Cach dong goi va bao hieu cua cac duong E1:
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Trong framing va signaling cua E1; 30 trong so 32 kenh dung cho viec truyen du lieu hoac voice. Thong tin framing su dung timeslot 1 trong khi kenh 16 (timeslot 17) duoc su dung cho bao hieu chung cho moi kenh con lai.
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Trong dinh dang E1 frame; 32 khe thoi gian se tao nen mot frame. Mot multiframe bao gom 16 frame E1. cac khe thoi gian duoc danh so tu 1 den 32; cac multiframe timeslot duoc cau hinh:
• Timeslot 1 chi mang thong tin framing
• Timeslot 17 trong frame dau tien cua 16-frame multiframe chi cho biet day la bat dau cua multiframe (ki hieu boi chu M)
• Cac timeslot con lai cua frame nay se mang tin hieu bao hieu

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Common channel signaling khac voi channel associated signaling o cho tat ca cac kenh deu su dung mot kenh bao hieu chung cho viec thiet lap cuoc goi. Su dung E1 la mot vi du: giao thuc bao hieu IDSN Q.931 co the duoc trien khai tai timeslot 17 de trao doi call-setup mess voi cac thiet bi telephony duoc gan voi no

Vi du ve common channel signaling signaling:
• Mot vai pbx vendor su dung common channel signaling cho t1 va e1 va thuc hien common channel signaling doc quyen giua cac pbx. (vi du cisco su dung T-COMMON CHANNEL SIGNALING)
• ISDN : su dung Q931 lam kenh bao hieu
• Q signaling
• Signaling system 7 (SS7): day la mot out-of-band network duoc thuc hien boi nhieu cong ty dien thoai va duoc su dung de bao hieu cung nhu cung cap cac dich vu khac

3.3.ISDN
Day la mot phuong thuc truy cap danh cho mot mang. ISDN co the duoc thuc hien theo hai cach : BRI va PRI; trong khi bri chi co hai kenh van chuyen du lieu thi pri ho tro toi 23 kenh (t1) va 30 kenh voi e1. moi cach thuc hien deu ho tro mot kenh D de van chuyen tin hieu common channel signaling

Kien truc mang ISDN:
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Kenh B mang thong tin nhu voice; data va video tai toc do 64 kbps DS0. ki hieu 2B+D
Kenh D mang tin hieu giua cac CPE va mang; su dung giao thuc Q931 hoac QSIG. Ki hieu : 23B+D hoac 30B+D

3.4.Giao thuc bao hieu lop 3 Q 931
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Giao thuc nay su dung mot tap chuan cac ban tin de giao tiep. Cac ban tin dung de:
• Thiet lap cuoc goi: thong tin bao gom: bao hieu; call proceeding; ket noi; ACK ket noi; progress; thiet lap va ACK thiet lap
• Call inform phase: du lieu duoc chuyen giua user va mang sau khi cuoc goi duoc thiet lap. Cho phep user co the dung hoac resume cuoc goi. Cac thong tin van chuyen trong phase nay bao gom: hold; hold ACK; hold reject; resume….
• Call clearing: huy mot cuoc goi. Thong tin bao gom: disconnect; release; reslease complete; restart…
• Cac ban tin khac: dam phan mang..

3.5.Giao thuc bao hieu so 7 SS7
Vi du su dung bao hieu so 7:
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Khi thuc hien chuc nang tai mang cua khach hang; ss7 co the duoc thuc hien nhu mot common channel signaling thong qua mang thoai.

Loi ich khi dung SS7:
• Kha nang giao dich giua cac pbx duoc mo rong ra toan mang
• Co mot kenh danh rieng cho van chuyen bao hieu


3.6.Bao hieu lien mang
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Vi du: FXS phone su dung FXS loop-start signaling de ket noi toi PBX. Khi user quay so 9 cho viec quay so ra ngoai; no mang theo tin hieu tren T1 su dung channel associated signaling. Sau khi cuoc goi den CO; bao hieu duoc mang theo kenh SS7 de toi ISDN switch. Sau do cuoc goi duoc chuyen sang Q931 toi ISDN phone tai ben duoc goi

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