VI di gii sir ne tA bit diu truyen eft lieu cho nut B, mOt lit sau nu tD ding quy& djnh truyen de lieu cho nut B, boi vi D Ichang nhfui they nut A dang hoot dOng khi D
f. Chi de 'thong cinh bin
Ben canh the dO citnh bao trong IEEE 802.15.4 can de suit giao thirc chi do kh8ng canh bao, mOt vii diem Iduic nhau ca ban giaa 2 the do nay la:
+ Trong the dO khong canh bao tram dieu hanh lchong giri khung cinh bao, str ving met al poi tin canh bao duqc thiet bj ton dung trong khoang then gian nay de Clang b0 vOi tram dieu hanh.
+ Mai goi giri fir thiet bi !thong sir dung khe CSMA — CA. do kh6ng co clang b0 trong thbi diem rut lui, them vao de thiet hi chi thi hanh duy nhat met lin them da
mai throng, neu kenh rei thl qua tranh thim nh#p thanh cong.
+ Tram dieu hanh pith boat dOng theo chu k5r nhung thiet bj thi co the hoot dOng theo 1#p lich ciut rieng no, no chi host dOng khi, co goi da lieu hok goi dieu
lchien can guff di hoc co da lieu duqc giri tai chinh no fir tram dieu lchien.
2.2 Giao that (tong bt) thal gian
Vert de theri gian nit quan trong trong nhieu img dung va giao thirc trong mang
cam hien. Cac nut co the do that gian bing cach ding cac xung dang ha ctic b0 lay tir
ck b0 dao dOng. Boi vi cac pha ngiu nhien lam djch chuytn va lam tr8i toc do da b° dao dOng, do v#y died gian cuc 130 ciut cac not se bit diu sai khk di lam cho mang mit
Dei Hnr Th5ng I.nng Al 777h -Rhi Tian Ting
di st deng be. Do vjy viec deng be thai gian c6 vai to rat quan trong trong host deng cua mang cam bien.
Deng be th&i gian la phucmg thirc cho phip cac thuc the rieng biet trong met nhOm deng be xung ding he cua chting hoac ding be vai that gian Wan cau phei hqp (UTC). Phan nay se giii thich tai sao an deng be that gian va dua ra met s8 giao thirc ding be Witte nhau.
Tpi sao can ding bP thei gian trong mpng cam Nin ?
❖ Mang cam bien can lien ket vat the giai thuc de biet khi nio met hien ttrqng
xity ra.
❖ Djch vu ca bin chinh cua mang cam bien la tich ho de lieu. Do de can deng be gifts cic nut de c6 the tich hqp da lieu truyen den Sink
❖ MOt vii giao thtic yeu eau ding be thai gian: quin 19 eau hinh mang.
❖ Cic nut cam bien thutmg nh6, gii thinh thip nen be dao gang thubmg khong
chinh xic, han naa chimg bj gioi han ve &Mg luqng nen thuang c6 the de sleep de Het ldem nang luqng.
Sau day, ta xit vi du don giin minh hoa sir can thiet cia de chinh xic ve th&i gian nhu sau (hlnh 2.9)
Hinh 2.9.Xdc clinh pie din cita Om thank a xa beri mat day sensor
Met sting am phit ra ttlr met nguim am 6. lchoing cich xa the gong den met day cic sensor vi ta c6 the Ink doin g6c tai trong tnrbng hqp nay. m8i mot sensor deu biet vi tri cult cltmg va luu lei thei gian den dm am thanh. Trong truang hqp cu the Mar hinh ye, g6c 0 c6 the duqc xic djnh khi d va x da Net, dimg cong thirc luqng giitc:
x=d * sinO do de 0 = arcsin(d/x).
Dal Mr Thing 1.nng A1777h Tign Time
Khoitng each gifta the sensor co the xic djnh tit cac vi tri da biet ctia cac sensor vl x c6 the thu duqc tit dO loch thbi gian t A gifts cac Ian cam biAn cfla sensor va von tac cita am thanh c 330m/ s , sir dung Ong dint :
x=c•At.
Cho d = lm ve At = 0.001s thi 0 ;--; 0336 (radians). Neu dang ha cira cac sensor chinh xic den 5001.ts , sv sai loch ye thbi gian thvc c6 the 6 Ichoing gifta 500 va 1500 ps , va vi the gia tri cfra g6c 0 c6 the thay en tong lchoing 0 0.166 va 0 ez 0.518 . Vi the, met sai so nh6 trong khi clang bo then gian c6 the clan ton dO loch clang 16 khi trerc doan.
CAn phai chti y ring thtri gian dimg trong mang cam bien phii 11 theri gian tv nhien (physical time), da la hai mit cam bien phii c6 sr cam bien nhu nhau ye is va is cfra mOt mit cam bien cing gin \red Is trong thin gian thvc (real time) hok thbi gian town cau phai hop (coordinated universal time — UTC) tang tat. Then gian to nhien phai duqc phin biet v6i Ichai niem ve thbi gian logic (logical time) la thbi gian ma cho phep quyet djnh viec sip xip cac sum kien trong ho thong phan ha tilumg !cluing can thiat phai chi ra bat ki sv lien quan nao den then gian thvc.
2.2.1 Dow ha Sc mit cam blin va sr chthh sac
Hiu het cac thiet bj clang ha cita cac nut cam bien va may tinh diu c6 cau to giOng nhau. Mai nut c6 mOt b0 dao dung 6 mOt tan so xac djnh va met may dem xung dao dOng. Phan mem ctia cac nut chi truy nhklp tai gia trj cua b0 dem nay vi th&i gian gifta hai lin tang nay quytt djnh each giiti quyet van de then gian: cac sv kien xay ra gitta hai lin tang nay !thong the duqc ph$n bi'e't tir cac nhin thbi gian ctia chimg. BO dao deng thutmg c6 dO tr8i, doll sv djch nglu nhien so v6i tan so ten danh nghia, hay can gqi la do loch clang ha. Dieu nay phu thuOc vio sv khong tong suat cita tinh the, hay cac dieu kien min twang nhu ap sat, nhiet de... do vey viec trier khai mpg cam bien ten thvc to khic nhieu so yeti trong phOng thi nghiem. DO loch clang ha duqc do bring ppm (parts per million), n6 dua ra con s6 ve s6 dao dOng them vio hok so dao dOng N mit ma clang ha tao ra tong luqng thbi gian din cho 1 trieu dao dOng a toe dO danh
Tan s6 dao dOng thay dui theo then gian. C6 2 kik thay
• Thay dai ngan hen: do thay &Si nhiet de, do thay dai tong din 1p nguen cung cap, 1p suit Ichong khi...
❖ Thay dui din han: do sv lao h6a cita cac bO dao dOng
Dal 14nr Thing I.nng Al 7776 -Rhi Tan Ting
Ngu&i ta thuang gia dinh tan só cac be dao ding la on dinh vira phai trong pham vi tir vii phitt den vii chuc phut. Dieu nay cling n6i len rang cac tilt* toil .' (long 1,0 thin gian phai ding bo vii phid mOt lan de theo kip su thay dOi dm tan s6. Vi the giao thirc tang thtri gian la nit can thiet.
MOt dieu can quan tam nfta la bao lau thl giao thirc dong be thtri gian chay mOt Ian? Gia sir met nut chi dieu chinh de dich pha i 0 va thc do troi cua dao dOng la x ppm co dinh, de chinh xac you cau la 8 s, thl sau khoang thoi gian khoang s can thiet phai thing bij lai. V6i x = 20ppm, dO chinh xac la is thi sau 50s phai ding 1,0 lai. Cac mo hinh hien 41 ngay nay deu c6 ging ink Itrqng chinh xac khong chi N i ma con Oi de keo dai chu Id truOrc khi phiti ding be lai. MOt Ian nUa phai nhan manh ring quit trinh ding be met lan khong c6 hieu qua vi toc do troi thay clOi, va thutrng thi ta gioi han duqc tOc dO ten len nhat pi > 0 ma thee man
1 d
H