Kiến thức về mạng máy tính: Phần 2

Chirong 4<br /> TANG MANG<br /> <br /> 4.1 CAC MO HINH DICH VU CUA TANG MANG<br /> <br /> Nhu da trinh bay trong chuang trudc, ting giao van cung cap djch<br /> vu tmyen thdng gitra hai tiln trinh dang chay tren hai may tinh khac nhau.<br /> De cung cap dugc djch vu nay, ting giao van phai sir dung djch vu cung cap<br /> dudng truyen gitra hai may tinh cua ting mang. Ndi cu thi ban, ting mang<br /> chuyen gdi tin (segment) cua ting giao van tir may tinh nay din may tinh<br /> khac. T?ii may tinh giri, tit ca cac segment cua ting giao van dugc chuyin<br /> xudng tang mang. Nhiem vu ciia ting mang la chuyin nhirng segment nay<br /> den may tinh dich va giri tdi thuc thi nao dd d ting giao van ben tren. Cdng<br /> viec chuyen segment tir ting giao van may tinh nguon din ting giao van<br /> may tinh dich ciia ting mang chinh la ndi dung cua chuang nay. Chiing ta se<br /> thay rang khdng gidng ting giao van, ting mang gom nhieu may tinh va cac<br /> router trung gian. Vi thi, giao thire ting mang la mdt trong nhirng giao thirc<br /> phirc tap nhat.<br /> <br /> 216<br /> <br /> E r d system H1<br /> Applcatbn;<br /> Transport i<br /> <br /> Ftouter<br /> <br /> Data link<br /> Physical<br /> Data link<br /> Ptiysical<br /> <br /> 1<br /> H'^itvX'rk<br /> <br /> /<br /> <br /> ,<br /> <br /> Metv>/ork 1<br /> Data link<br /> Physical I '""'^-^ Data link |<br /> <br /> Physical j<br /> <br /> Hinh 4.1 IVIgt mo hinh mang do-n gian<br /> <br /> Hinh 4.1 minh hoa mot mang don gian vdi hai may tinh (HI va H2)<br /> va mot so router tren dudng tmyln giii-a H1 va H2. Tang mang d may tinh<br /> giri thuc hien budc giri diu tien tren toan bd hanh trinh ciia gdi tin. Vi du<br /> nlu HI giri gdi tin din H2 thi ting mang tren HI se truyen gdi tin nay din<br /> router gin nhit: R2. Tai may tinh nhan (ching ban H2), tang mang se nhan<br /> gdi tin tir router gin nd nhit (trong trudng hgp nay la R2) va chuyen len cho<br /> ting giao van tai H2. Vai trd chinh ciia router la chuyen gdi tin tir mdt dau<br /> vao nao dd (input) tdi mot diu ra nao dd (output). Chu y rang cac tang tren<br /> cua tang mang (giao van, img dung) khdng boat ddng tai cac router trong<br /> Hinh 4.1 bdi vi router khdng cin thilt phai chay cac giao thiie d ting giao<br /> van hay ting img dung (ngoai trir cac muc dich kiem soat).<br /> Vai trd ciia ting mang don gian chl la chuyen gdi tin tir may tinh giri<br /> din may tinh nhan. Vi thi, ting mang cd ba chirc nang quan trong sau day:<br /> 217<br /> <br /> Xdc dinh duerng di (Path determination): Ting mang phai xac djnh<br /> cac router trung gian hay tuyIn dudng (path) ma gdi tin dugc truyin tir noi<br /> gii-i den noi nhan. Thuat toan xac djnh tuyIn dudng nhu vay goi la "thuat<br /> toan djnh tuyen" (routing algorithm). Thuat toan djnh tuyIn se quylt djnh<br /> dudng di ciia cac gdi tin tir may tinh nhan din may tinh giri (trong vi du la<br /> may tinh HI va may tinh H2). Trgng tam ciia chuong nay la cac thuat toan<br /> djnh tuyen. Trong phin 4.2, chiing ta se nghien cim ly thuylt ciia thuat toan<br /> djnh tuyen, tap trung vao hai kilu thuat toan: Link state va Distance vector.<br /> Chiing ta se thay do phirc tap ciia thuat toan djnh tuyIn phu thude vao sd<br /> lugng router tren dudng truyin. Dieu nay dan din djnh tuyIn phan cip, mot<br /> chu de dugc trinh bay trong phin 4.3.<br /> Chuyen mach (Switching): Khi gdi tin din diu vao, router phai<br /> quyet djnh giri gdi tin din diu ra thich hgp nao. Vi du, gdi tin tir may HI<br /> den router Rl se phai dugc chuyin din router kl tilp tren dudng tdi H2.<br /> Trong phan 4.6 chiing ta se nghien cim boat ddng cua router va qua trinh<br /> chuyen mdt gdi tin tir dau vao din diu ra trong mot router dien ra nhu thi nao.<br /> Thiet lap ducfttg truyen (Call setup): Trong phin trudc trinh bay vl<br /> TCP, chiing ta thiy rang hai thuc thi truyin thdng phai ed mdt giai doan<br /> "bat tay" trudc khi trao ddi dtr lieu thuc sir. Dilu nay cho phep ben giii va<br /> ben nhan thiet lap cae thdng tin trang thai cin thilt (vi du, sd thir tu khdi<br /> diu, do Idn cira sd). Vdi mdt so kiln true mang khac (vi du ATM) ddi hdi<br /> cac router tren tuyIn dudng tir ngudn din dich phai "bit tay" nhau trudc khi<br /> bat diu truyen dir lieu thuc su. Trong ting mang, qua trinh nay dugc ggi la<br /> thiet lap dudng truyin (call setup). Chiing ta sc thiy ting mang trong kiln<br /> true Internet khdng ddi hdi cdng viec nay.<br /> Tuy nhien, trudc khi di sau vao chi tilt cac khai niem va triln khai<br /> ciia tang mang, chung ta se xem xet tong quat nhimg kilu djch vu khac nhau<br /> ciia tang mang.<br /> <br /> 4.1.1 Mo hinh dich vu mang<br /> Khi tang giao van d thilt bj giri chuyin cac gdi tin xudng ting mang,<br /> lieu tang giao van cd thi tin cay ting mang chuyin gdi tin nay din dich<br /> khdng? Lieu khi giri nhilu gdi tin, chiing cd dugc chuyin din tang giao van<br /> ciia thiet bj nhan theo dung thir tu khdng ? Van toe giri cae gdi tin cd dugc<br /> xac djnh trudc khdng? Tang mang cd phan hdi lai cac thdng tin lien quan<br /> den tac nghen khdng? Md hinh djch vu mang se tra Idi nhiing cau hdi nay va<br /> nhieu van de khac.<br /> 218<br /> <br /> Chuyin mach gdi (datagram) va chuyen mach ao (virtual circuit)<br /> Cd Ic dilm triru tugng quan trong nhit ma tang mang che dau cac<br /> ling tren la viec cd su dung m.ach ao (Virtual Circuit - VC) hay khdng. Ve<br /> khia canh nao dd mach ao tuang tu mang dien thoai truyen thdng (mac dii<br /> mang dien thoai sir dung mach thuc). Cd ba giai doan trong chuyen mach ao:<br /> Thih lap mach do: trong ca giai doan thilt lap, phia giri thdng bao<br /> dja chi nhan vdi ting mang, yeu ciu ting mang thilt lap VC. Tang mang xac<br /> djnh tuyIn dudng giira ben gui va ben nhan, tire la chudi cac cung dudng<br /> (hay dudng kit noi - link) cung nhu cac thilt bj chuyin mach (switch - mit<br /> trung gian) ma tit ca cac gdi dtr lieu se di qua. Giai doan nay yeu cau viec<br /> cap nhat bang djnh tuyIn va du trir tai nguyen trong moi thilt bj chuyen mach.<br /> Truyen dit lieu: Sau khi thilt lap, dir lieu ed thi dugc chuyen trong VC.<br /> Gidi phong mach do: Giai doan nay bit diu khi phia giri (hoac phia<br /> nhan) bao cho ting mang yeu ciu ddng VC. Ting mang se thdng bao cho<br /> thilt bj diu cuoi ben kia cung nhu cac thilt bj chuyin mach tren VC de cap<br /> nhat lai cac bang djnh tuyIn, giai phdng tai nguyen.<br /> Cd sir khac biet tuy nhd - nhung quan trgng giGa thilt lap VC d tang<br /> mang va thilt lap kit ndi d ting giao van (giai doan bit tay 3 budc ciia<br /> TCP). Thilt lap kit ndi d tang giao van chi lien quan din cac thiet bj dau<br /> cudi d tren hai diu mut. Hai thilt bj ddng y thilt lap kit noi va thoa thuan<br /> cac thdng sd ciia kit ndi (vi du so thir tu khdi tao, do Idn cira so kilm soat<br /> luu lugng). Hai thiet bj diu cuoi nay se nhan bilt dugc ve sir kit noi d ting<br /> giao van, nhung cac thiet bj chuyin mach d giira thi khdng. Trai lai trong<br /> ting mang ciia mang chuyin mach ao, tit ca cac thilt bj chuyen mach gi&a<br /> hai thilt bj dau cudi diu tham. gia vao qua trinh thilt lap mach ao, va do dd<br /> diu nhan bilt dugc tit ca cac VC di qua.<br /> Thdng diep trao doi gitra cac thilt bj diu cudi yeu cau khdi tao hay<br /> kit thuc mach ao hay thdng diep trao ddi gitra cac thilt bj chuyen mach yeu<br /> ciu thilt lap VC (dl cap nhat bang chuyin mach) dugc ggi la thong diep<br /> bao hieu (signaling message). Giao thire dugc su dung dl trao doi nhimg<br /> thdng diep nay la giao thuc bao hieu (signaling protocol). Qua trinh thiet<br /> lap VC dugc minh boa trong Hinh 4.2. ATM, Frame Relay va X.25 la ba<br /> kiln tnic mang sir dung chuyen mach ao.<br /> Trong mang chuyin mach gdi, khi muon giri gdi tin, thiet bj dau<br /> cudi dat vao gdi tin dja chi thilt bj nhan va sau dd chuyin gdi tin<br /> 219<br /> <br /> vao mang. Nhu minh boa trong Hinh 4.3, khdng cd giai doan thilt lap VC<br /> nao. Nhung thiet bi trung chuyin trong mang chuyin mach gdi (dugc goi la<br /> bd djnh tuyen - router tren Intemet) khdng duy tri bit ky trang thai nao vl<br /> VC vi khdng cd VC. Thilt bj trung chuyin se djnh tuyIn gdi tin din dich<br /> bang each xac djnh dja chi dich, tim kilm tren bang djnh tuyIn va chuyin<br /> tiep gdi tin theo hudng den dich (giong viec chuyin thu binh thudng trong<br /> he thdng buu dien). Vi bang djnh tuyln cd thi dugc cap nhat lien tuc, nen<br /> cac gdi tin dugc giri tir thilt bj diu cuoi nay din thilt bj diu cudi khac cd thi<br /> di theo nhieu tuyen dudng khac nhau va din dich khdng theo thir tu. Mang<br /> Intemet cdng cdng ngay nay sir dung djch vu chuyin mach gdi.<br /> <br /> i. hitiate caB<br /> <br /> 2. Incoming call<br /> Hinh 4.2 Mo hinh djch vy c h u y i n mgch ao<br /> <br /> Application<br /> Transport<br /> Network<br /> Data fink 1. Send data<br /> Physical<br /> <br /> -^^^•*v J<br /> <br /> Af^lication<br /> Trai sport<br /> Network<br /> 2. Receive data ^<br /> Data link<br /> <br /> Hinh 4.3 Mo hinh chuyin m^ch goi<br /> <br /> 220<br /> <br />