CONG NGHE VigN THONG
SO LUOC VE
TRUYEN THONG QUANG QUA KHONG GIAN
G I O I THIEU
Traydn thdng quang tdc do cao da va dang ddng vai tid het sire quan trgng trong mang vidn thdng.
Traydn thdng quang dugc chia thdnh 2 loai: tmyen thdng quang qua sgi quang va tmyen thdng quang qua khdng gian (FSO - Free Space Optics).
Cac he thdng tmyen thdng quang su dung sgi quang hidn nay ed kha nang traydn tai vdi dung lugng ldn, ket ndi nhieu ngudi diing vd cung cdp nhieu loai dich vu nhu thoai, fax, hinh anh, sd lieu.
Cimg cd khd nang tmyen ddn tdc do eao, nhung cac hd thdng FSO dd dang ldp dat, di ehuyen hoac thiet lap lai cdu hinh mang khi cdn. FSO ed dd an todn cao vi sii dung thdng tin tdm nhin thdng (line-of- sight) va tinh hudng cua biip sdng quang cao. Cae cdng nghd FSO xudt hidn ldn ddu tidn vao nhung nam 1960 tiong eae iing dung quan su. Ddn cudi nhung nam 1980, nhung san pham thuang mai da xudt hidn nhung khdng thanh cdng vi nhirng rao can edng nghd. Cu ly ngdn, dung lugng thdp, vdn de vd giu: thang hang giu:a bd phdt va bd thu cung nhu dnh hudng cua thdi tiet Id nhung ban che chinh vdo thdi gian dd.
ThS. Pham Thi Thuy Hien
TRUYEN THONG QUANG QUA KHONG G I A N
FSO (hay tmyen thdng quang vd tuyen) la edng nghd vien thdng sir dung su tmyen lan anh sang trong khdng gian de tmyen tin hieu giu:a hai diem [1]. Day la cdng nghd tmyen thdng bang rdng tdm nhin thdng, trong dd tin hidu quang, thay vi tmyen trong sgi quang, dugc phat di tiong mot blip sdng quang qua khdng gian. Mdt mang tmyen thdng quang vd tuydn bao gdm cdc bd thu-phat quang (gdm mdt khdi thu vd mot khdi phat) cung cap kha nang thdng tin hai chidu. Mdi khdi phat quang su dung mdt ngudn quang va mdt thdu kinh de phdt tin hidu quang qua khdng gian tdi khdi thu. Tai phia thu, mdt thdu kinh khde dugc su dung de thu tin hidu, thdu kinh nay dugc ndi vdi khdi thu ed do nhay cao qua mdt sgi quang. Mdt tuyen FSO bao gdm hai bd thu-phat duge dat trong tdm nhin thang. Thdng thudng, cdc bd thu phat dugc gan trdn ndc cdc tda nha hoae sau eac cira sd (Hinh 1). Cu ly boat ddng cua mdt tuyen FSO tir vai tram mdt tdi vai km.
Cac u'u diem cua FSO
Cdc uu diem cua FSO bao gdm:
Hmh 1. He thfing truydn th6ng quang qua khOng gian
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T6n xa
H^p thu
Suong mil jLrk% ^^ h6n loan
Hinh 2. Nhung thach thuc ddi v6i FSO
- Khdng ydu cdu cdp phdp phd tdn vd tuyen.
- Khdng bi arih hudng eua nhidu didn tir.
- De dang trien khai ldp dat.
- Khd nang an toan eao.
Cac thach thu'c doi vdi he thong FSO
Gidi ban co ban eua FSO do mdi tradng tmyen dan gay ra [2]. Ngoai vide tuyet vd mua ed thd lam cdn tid dudng tmyen quang, FSO chiu anh hudng manh bdi suong mil vd su hdn loan cua khdng khi. Nhung thaeh thiie ehinh trong vide thidt kd cde hd thdng FSO nhu hinh 2.
a) Suong mii
Suong mu la mdt thach thire ehinh. Suong mu la hoi nude duge tap hgp tir nhung 0gt nudc nhd cd dudng kinh vdi tram micro mdt nhung ed thd ldm thay ddi dae tinh tmyen lan cua dnh sang hoac ngan can hoan toan su tmyen lan eua dnh sdng
thdng qua su ket hgp cua cdc hidn tugng hap thu, tdn xa vd phdn xa. Didu nay cd thd ddn den su suy giam mat dd cdng sudt cua biip sdng phdt. giam eu ly boat ddng eua tuydn FSO.
b) Sunhdpnhay
Su nhdp nhdy la su bidn ddi vd khdng gian cua cudng dd sdng gay ra bdi su h6n loan khdng khi.
Gid vd su thay ddi nhidt dd tao ra nhirng tiii khi cd m$t dd thay ddi nhanh ddn tdi su thay doi nhanh ehi sd chidt xudt, dd chinh la nguydn nhdn gay ra su hdn loan. Cdc tiii khi ndy ddng vai tid nhu nhung tiidu kinh ed dac tinh thay ddi theo thoi gian vd ldm ty Id ldi bit cua cae hd thdng FSO tang manh, dac bidt la khi cd dnh sdng mat tidi.
e) Su trdi blip
Su trdi bup xay ra khi ludng gid hdn loan (gid xoay) ldn ban dudng Mnh cua bup sdng quang gdy ra su dich chuydn chdm nhung dang ke cua bup
TAP CHI CNTT&TT KY 1 (10.2009)
Hinh 3: Ki^n true mgng luCii Hinh 4: Ki6n true mang di^m — da di^m
- ^ •' S tit ^^^ *^<
Hinh 5: Kien true diem — diem Hinh 6: Kien true vong voi phan nhanh
sdng quang. Su tidi bup ciing cd the la kdt qud cua cdc boat ddng dia chdn gay ra su dicb chuyen tuong ddi giu:a vi tri cua laser phat va bd thu quang.
d) Giu thing hudng phat-thu khi tda nha dao ddng
Giu: thdng hudng giu:a khdi phdt vd khdi thu Id rdt quan tigng nhdm ddm bao su thanh cdng cua vide tmyen tin hidu. Day thue su la vdn dd phire tap khi sir dung blip sdng hep phan tan gde va tdm nhin (FOV). Su dan nhidt cua cae phdn khung tda nha hoae nhung tran ddng ddt yeu cd the gay ra su lech hudng. Trong khi su dan nhidt cd dae tinh ehu ky theo ngay hoac miia thi ddng ddt lai khdng the du dodn duge. Mdt nguydn nhan gay ra su lech hudng nu:a Id gid, dac bidt khi eae thiet bi thu phat dugc dat trdn ede tda nhd cao. Su dao ddng cua tda nhd la mdt qud trinh ngdu nhidn lam dnh hudng den hidu
nang cua he thdng va gay ra ldi.
e) Sir an toan cho mit
Vdi su gia tang cua cac he thdng tmyen thdng quang vd tuydn sir dung eae biip laser hudng ve cdc vimg dan eu mat do cao, su an todn cho mdt la vdn de ddng dugc quan tam. Nhiing he thdng FSO nay phdi an toan ddi vdi mdt, cd nghia Id ehimg phdi khdng gay nguy hiem cho nhung ngudi vd tinh gap phdi cac bup sdng quang. Ydu cdu nay rd rang se tao ra gidi ban trdn cho cudng dd biip sdng phat cua laser.
CAC K I E N T R U C M A N G FSO
Cdc he thdng FSO cd the duge thiet ke vd lam vide trong bdt ky cdu hinh mang nao, bao gdm cau hinh ludi, diem - diem, diem - da diem, vdng. Vide nay giiip eho cac nha cung cdp mang khu vuc dd
thi de dang xay dung va md rdng mang tde dd cao tdi cdc khach hdng.
Kien true mang lu'di
Mdt mang ludi bao gdm mdt chudi cac nut mang dugc kdt ndi vdi nhau vdi mire dd du thira
nhdt dinh eho vide du phdng (Hinh 3). Trong nhung mang kidu nay, moi niit mang ddu duoc kdt ndi vdi cdc nut khac mdt each trac tidp hoac thdng qua mdt sd Chang. Mire du thira tiong mang quyet dinh mire ket ndi tiong mang. Do dd, sd lugng niit mang cdng nhieu thi mirc du thira ket ndi cang ldn, kha nang dam bao lidn lac cang eao. Kidn tnic mang ludi cd dd tm eay eao, dd them nut mang nhung bi gidi ban ve khodng cdch.
Kien true mang diem - da diem
Trong kien tnic mang diem - da didm (Hinh 4), mdt nut mang ddng vai tid nhu mdt nut khdi ddu vd nlueu tuyen quang vd myen duge bdt ngudn tir nut ndy. Phuang phap hidu qua nhdt la ndi mdi tuyen FSO tdi mdt thidt bi ldp 2 hoac ldp 3 dat trong mdt phdng cua tda nhd. Sau dd cde tuydn nay dugc ndi bdng sgi quang tdi bd chuydn mach hoac dinh tuydn dat d vi tri bdt ky tiong tda nhd, cd the Id tdng trdn eimg hay d mdt phdng khde. Kien tnic diem - da diem cung cdp ket ndi vdi ehi phi re hon vd thuan tidn eho vide thdm nut mang vdi ehi phi vd bang thdng thdp hon kien tnic diem - diem.
Kien true mang nhieu tuyen diem - diem
Kien tiiic mang nhidu tuydn diem - diem (Hinh 5) thieh hop eho nhQng tradng hgp can tao ra mdt myen md rdng vugt qua gidi ban eu ly hoae bi bdt
budc phdi nit ngdn cu ly tuydn quang do vdn dd thdi tidt. Day la kidu kdt ndi gidnh ridng vdi khd nang cung cdp bang tan rdng hon.
Kien true mang vong
Trong kien tiiie mang vdng (Hinh 6), ede niit mang ndi vdi nhau thdng qua cae tuydn quang vd tuyen tgo thanh vdng khdp kin vd ed thd phan nhanh thanh cac tuydn didm - didm. Kien tnic mang vdng cd dd tin eay cao hon kidn true didm - diem va diem - da didm. Ddng thdi, nd lai khdng phire tap nhu kidn tnic mang ludi.
KET N O I TOC DO CAO G I Q A CAC TOA
NHA V a i FSO
Hidn nay, cac doanh nghidp dang gap phai vdn dd qud tai luu lugng mang tai cac ket ndi giiia ede tda nhd. Vdi cdc doanh nghiep su dung cdc mang ndi bd dua trdn tieu chudn Gigabit Ethernet, cdc ket ndi 2.048 (hoae 1.544) Mbit/s giira ede tda nha se ldm ban che luu lugng kdt ndi. Trong khi dd, cdc doanh nghidp vdi ydu cdu sd lidu ldn mong mudn traydn ddn dung lugng eao giira eac tru sd doanh nghidp ma khdng su dung cdc kdt ndi sgi quang ehi phi cao. Vide ldp dat sgi quang cCuig phirc tap va tdn thdi gian hon. Ngodi ra, vide xin cdp phdp, van de an ninh, ddo ranh, dat cap va ydu cdu ve mdi tradng ciing Id cdc vdn de tid ngai. Dd loai bd cdc van de tia ngai tidn vd tang luu lugng ket ndi, eac doanh nghidp ed cdc tda nhd ndm tiong tdm nhin thing ehuyen sang su dung eac gidi phap FSO. Tdm lai, cdc giai phap FSO cho phdp loai bd:
3 0 I TAP CHI CNTT&TT KY 1 (10.2009)
- Tdc nghen ve luu lugng.
- Ydu cdu xin phdp vd cdp gidy phdp.
Vide dao ranh, edng va dat cap.
- Van de lidn quan tdi hgp ddng thud (eho thud) tda nhd.
- Tdn thdi gian ldp dat.
- Chi phi eao.
HE T H O N G FSO V A V A N D E A N N I N H M A N G
Mac du FSO la cdng nghd khdng day nhung nd khdng phat qudng ba tdi bdt ky ngudi nao va tat ca mgi ngudi. FSO phdt bup sdng anh sdng hep, tan sd rdt cao tdi mdt noi xdc dinh. Do dd rdt khd eho mdt ca nhan nao dd ed the thu trdm thdng tin ma khdng bi phat hidn. Cde he thdng FSO thudng dugc ldp dat cang eao cang tdt de cae phuong tidn giao thdng qua lai khdng lam dnh hudng tdi bup sdng quang.
Mdt con chim cd the ldm gian doan vide tmyen thdng tin, nhung dieu dd ehi xay ra trong chdc ldt va he thdng se nhanh chdng phuc hdi. Trong khi dd, vide thu tidm thdng tin ddi hdi phdi dat thiet bi tren dudng di eua bup sdng tiong khoang thdi gian dai.
Phai thira nhan rdng vide dat thiet bi thu tidm thdng tin glu:a hai khdi FSO Id rdt khd khan do kich thudc bup quang rat hep, khd xdc dinh dugc vi tri biip, blip lai thudng dugc dat d rat cao va khdng gdn bat cir thir gi. Vide phdt hidn thidt bi thu tidm
Id hoan toan cd kha nang thuc hien duge do biip quang trong tam nhin thing, cd the su dung cac may quay phim de gidm sat vide ldp dat thiet bi thu trdm va dudng tmyen eua bup sdng quang de phat hidn bdt cir boat ddng kha nghi nao.
CAC U'NG D U N G C U A FSO
Vd mat cdng nghd, ung dung it thach thirc nhat la su dung FSO lam dudng tmydn sd lieu ket ndi cac tda nha dd thi (kdt ndi giua cac mang LAN). Trong img dung nay, cu ly tuyen FSO tir vai tram met cho tdi vai km, vide trien khai FSO don gian va tdn it chi phi ldp dat ban bdt ky loai cap nao. FSO cd the su dung lam dudng tmyen dan tde do cao ndi ngudi diing Internet vdi nha cung cdp hoac cac mang khae. Nd cung cd the dugc su dung lam he thdng mang vdng dd thi de cung cdp cac ket ndi tde dd cao eho cae doanh nghidp. FSO ed the dugc dimg de mang luu lugng cua mang di ddng tir anterma tdi cdc thiet bi khac cua mang. FSO tham ehi ed the dimg de tmyen sd lidu giua mot tdu vu tm d xa va mdt tram d gdn trai ddt.
KET L U A N
Tuong lai ngay cang ddi hdi phdi ed cac gidi phdp tmyen ddn tdc do cao de ddp img ydu cdu eua cac doanh nghiep, td chirc va ca nhan. Cac gidi phdp cung cdn phdi cd chi phi hidu qua, trien khai nhanh, tmydn dan thdng tin mdt each an toan va tin cay.
FSO ed the dap img cac ydu cdu ndy va se dugc sir dung ngay cang nhieu trong tuong lai.
Tai lieu t h a m khao
[1] D. KEDAR and S. ARNON, Urban optical wireless communication networks: the main challenges and possible solutions, IEEE Commun. IVIag., vol. 42, no. 5, IVIay 2004.
[2] R. R. INIGUEZ, S. M. IDRUS, and Z. SUN, Optical Wireless Communications: IR for wireless connectivity, CRC Press, 2008.
