Chủ đề: VoIP_Part_II_1: Cơ bản về công nghệ thoại analog

Trong chu de tiep theo minh se noi ve cac ket noi thoai su dung cong nghe analog va cong nghe so.

1.Analog voice basic
Noi dung
• Nhan ra cac thanh phan cua local loop conn
• Tin hieu duoc su dung trong local loop
• On hook; off hook va ringing signaling
• Pulse dialing va dual tone
• Call progress indicator
• Type of trunk
• Signaling used on trunk
• Wiring scheme
• Wink-start; im-start; delay-start
• Hai phuong thuc de giam echo

1.1.Local loop
Ket noi truyen thong
Ðính Kèm 193
local loop la mot cap day ket noi tu thue bao cho toi switch cua telco. Trong hai day nay, mot day la tip va day con lai la ring

trong hau het truong hop; day ring gan voi nguon (battery) con day tip noi voi dat. Khi de off hook (kenh may hoac nhac may): se cho phep mot dial tone toi duoc handset.

1.2.Local loop signaling
ba kieu bao hieu trong local loop:
• Supervisory
• Address
• Informational

Supervisory
Bao hieu onhook; off hook va ringing
• Onhook: ho mach: chi khi nao co chuong moi dong mach
• Off hook: dong mach; luong dien truyen den switch thong bao co mot ai do dang co thuc hien mot cuoc goi
• Ringing:

Address
Hai dang chuyen dia chi (so duoc goi): pulse hoac DTMF
Pulse dialing:
Ðính Kèm 194
de co the thuc hien cuoc goi bang dien thoai quay so; user can phai quay mot vong de gui so di. Cac so can duoc sinh ra tai cac rate cho truoc voi mot muc do chiu loi nao do. Moi pulse bao gom mot break va mot make. Break la khoang thoi gian mach doc mo con make la khoang thoi gian mach dong. O Mi: 60% break va 40 % make

Qua trinh dial pulse dien ra nhu sau:
• Khi mot user goi mot ai do bang cach quay mot so
• Khi dial duoc giai phong; vong quay tro lai trang thai cu
• Trong khi vong quay tro lai trang thai cu; mot cam-driven switch mo va dong cac ket noi toi telco. So luong cac vong dong va mo do se duoc bieu thi cho mot so

DTMF
Ðính Kèm 195

Information
Ðính Kèm 196
cac thong so bao hieu thong tin duoc su dung de thong bao cho user biet trang thai cuoc goi. Cac thong bao gom:
• Dial tone: bao hieu telco da san sang nhan so ma user se quay
• Busy tone: cuoc goi ko the thuc hien vi phone dang xa dang ban
• Ringback: telco thong bao rang dang thuc hien cuoc goi thay cho nguoi goi
• Congestion: thong bao mang dien thoai o xa dang chan cuoc goi
• Reorder tone: thong bao la moi cuoc goi local dang ban (thuong xay ra voi PBX noi bo chi co vai ba so ra ngoai)
• Receiver off hook: chi ra rang ben nhan dang o trang thai off hook voi mot thoi gian mo rong ma khong co mot cuoc goi nao (de kenh may qua lau)
• No such number
• Confirmation tone: telco dang hoan thanh cuoc goi


1.3.Trunk connection
Ðính Kèm 197
nhiem vu chinh cua cac duong trunk la cung cap ket noi giua cac switch. Cac switch can phai dinh tuyen cuoc goi toi dung trunk hoac telephone line. Mac du co the co nhieu thue bao cung chia se mot trunk nhung chi co mot user duoc su dung trunk tai mot thoi diem. Khi cuoc goi ket thuc, ho giai phong trunk va switch se dung no de phuc vu cuoc goi sau. Co the co vai kieu trunk giua hai switch:
• Private trunk line: (tie trunk): noi nhieu PBX trong cung mot cong ty. De ket noi thuong su dung cac loai giao dien dac biet goi la receive and transmit (E&M) int
• CO trunk: ket noi truc tiep giua PBX cua mot cong ty voi local CO cua nha cung cap dich vu. CO trunk thuong su dung cac ket noi FXO. Vi du: khi user nhan phim 9: cho phep ket noi di qua PBX va den CO trunk de co the truy cap vao mang PSTN (vi du nhu phai an so 054 o dau thi moi co the goi ra ngoai duoc). Co mot vai CO trunk dac biet duoc su dung cho telephony network. Mot DID (direct inward dialing) trunk: cho phetp nguoi goi o ngoai co the toi duoc dich o ben trong mot cong ty ma khong can phai ket noi qua operator
• Interoffice trunk: ket noi hai trunk cua CO trong cung mot telco

FX trunk (foreign exchange trunk): bao gom hai giao dien
• FXO: ket noi truc tiep toi thiet bi cua CO; duoc su dung ket noi mo rong voi mot vi tri khac. FXO ket noi mot PBX toi mot switch cua CO. Vi du: neu mot sep cua mot cong ty co PBX muon cai mot cai dien thoai o nha ong ta thay vi cai tai cong so. Khi do FXO se duoc su dung
• FXS: ket noi truc tiep toi thiet bi cua user (station equipment); duoc su dung de cung cap dich vu local

FX trunk la cac giao dien duoc ket noi toi cac switch nham ho tro ket noi toi hoac CO hoac local. Cac Office equipment bao gom cac switch khac (de mo rong ket noi). Station equipment bao gom telephone; fax; modem

1.4.Trunk signaling
cac kieu signal tren trunk:
• Loop start
• Ground start
• E&M Wink start
• E&M immediate start
• E&M delay start

Day la cac chuan bao hieu giua line va trunk cua mot mang thoai cung nhu la bao hieu chuan giua mot dien thoai va cong ty dien thoai. Cac kieu bao hieu nay la khac nhau voi cac kieu giao dien khac nhau nhu FXS; FXO hay E&M

Loop-start signaling
Ðính Kèm 198
(bao hieu supervisory : on hook va off hook)

loop start signaling cho phep mot user hoac mot telco co the chiem duoc mot line hoac mot trunk khi mot thue bao dang khoi tao mot cuoc goi. Bao hieu nay thuong duoc su dung tren local loop hon thay vi trunk (ket noi giua cac PBX voi nhau)

Mot ket noi co ba trang thai:
• Idle (on hook)
• Telephone seizure (off hook)
• CO seizure (ringing)

Tien trinh xu li loop start:
• Khi line o trong trang thai idle hay on hook; phone hoac PBX se mo hai day loop. Khi do CO hay FXS se de ring o nguon con tip noi dat
• Neu user nhac may hay chuyen sang trang thai off hook (mach se dong: line seizure): CO hoac FXS (cong tren PBX) se phat hien ra dong dien va tra lai dial tone
• Khi CO hoac FXS phat hien ra co mot cuoc goi den; ap cho ring mot dien the xoay chieu len pin 48 vol VDC lam chuong do va thong bao cho nguoi nhan cuoc goi. Khi phone hoac PBX tra loi cuoc goi; dong mach va CO hoac FXS se tat dien the chuong
Loop start la giai phap toi cho cac duong trunk co dung luong cuoc goi can thuc hien lon. Boi vi no dan den hien tuong glare hoac hien tuong chiem duong truyen o ca hai phia (co cuoc goi den dong thoi ben nhan cung nhac may de thuc hien cuoc goi)

Ground-start signaling
Ðính Kèm 199

khac phuc loi glare cua loop start bang cach cung cap dong dien de phat hien duoc tai ca hai phia: khi do se su dung req hoac confirm tai hai dau cuoi cua moi giao dien.

1.5.Line quality

echo la hien tuong phan xa am thanh do co su chenh lech ve tro khang tre n duong day.

Mac du local loop chi cho hai day nhung khi toi switch; ket noi se duoc thay doi thanh bon day converter. Trunk giua cac switch sau do chuyen tin hieu qua mang

Ðính Kèm 200

Mot mang thoai co the bao gom hai kieu echo: echo am thanh va echo dien. Echo am thanh thuong xay ra voi speakerphone khi voice nhan duoc tai mot speaker kich thich microphone va tin hieu quay nguoc tro lai nguoi noi. Echo dien xay ra khi khong co su thong nhat ve dien tren duong truyen.

Co hai thanh phan cua echo can quan tam: do lon cua am thanh doi lai hoac tre. Khi mot user co kinh nghiem voi delay; cuoc goi co the bi anh huong khi giong cua nguoi goi va nguoi duoc goi se chong len nhau

Khac phuc echo: thong qua cac bo loc