Ti liu ny c th dnh cho nhng bn cha c kin thc g v lnh vc mng khng dy. Lot bi vit c tham kho ch yu t sch Cisco Press Official Study Guide ca mn CCNP Switching. Phn lab cho cc vn l thuyt ny c th tham kho trong CCLabPro Switching hoc tp sch lab Wireless.

C bn v Wireless LAN

Gii thiu

Cc h thng mng switched Ethernet thng c dng trong cc mng doanh nghip ngy nay. Cc kt ni Ethernet thng c dng t thit b lp li (core layer device), xung n lp phn phi (distribution), xung dn n lp truy cp (access layer). Theo truyn thng, cc ngi dng u cui phi dng dy mng kt ni vo h thng mng campus. Tuy nhin, cng ngh mng khng dy cho php cc thit b lp access ca mng campus c th m rng n ngi dng cui m khng cn dng dy. Vi vic dng cc thit b mng khng dy, ngi dng cui c th tr nn c ng v c th di chuyn d dng m khng h b mt kt ni mng.

Bi vit ny s gii thiu mt ci nhn tng quan v cc cng ngh c dng trong mng khng dy WLAN. Khi hiu v quen thuc vi mt vi l thuyt c bn ca mng khng dy, bn s c kh nng hiu, thit k v dng cc thit b mng khng dy m rng h thng mng kt ni vi ngi dng.

C bn v mng khng dy

Bi vit ny s gii thiu mng khng dy ni b WLAN t gc nhn thc t. Ti liu trnh by da trn nhng kin thc bn c trong cc ch v mng chuyn mch LAN trong kha hc ccnp switching. Sau cng, mc tiu ca bi vit ny gip bn c kin thc v wireless c th tch hp cng ngh ny vo mng ca bn.

So snh mng c dy v mng khng dy

Mt mng khng dy c tch hp mt cch chnh xc vo mng switched c dy nh th no? Ngc li, chc nng switching s tch hp vo mng khng dy nh th no? Trc khi tr li cc cu hi ny, bn c th cn so snh hai cng ngh ny vi nhau.

mc c bn nht, mng c dy th s dng dy v mng khng dy s khng c dy. iu ny thot nghe c v khi hi, nhng tht ra n cho thy mt vi khc nhau c bn mc vt l m bi vit s cp n phn sau.

Mng Ethernet truyn thng c nh ngha bi cc chun IEEE 802.3. Mi kt ni Ethernet phi hot ng trong tnh trng c kim sot nghim ngt, c bit i vi nhng c ch lin quan n lp vt l. V d, cc c ch v trng thi kt ni, tc kt ni v ch duplex phi hot ng theo ng chun m t. Wireless LAN cng phi c yu cu tng t nhng li c nh ngha trong 802.11.

Nhng thit b Ethernet dng dy phi truyn v nhn cc Ethernet frame theo phng thc Carrier Sense Multiple Access/Collision Detect (CSMA/CD). Theo , trn mt phn on mng dng chung, khi cc my PC truyn thng theo ch half duplex, tng PC c th ni chuyn t do vi nhau trc, v sau b xung t hay cn gi l ng (collision) nu cc thit b khc cng ang ni chuyn. Ton b tin trnh pht hin xung t (collision) da trn vic cc kt ni c dy c mt chiu di ti a v c mt tr ti a khi mt frame i t mt u ca phn on mng ny n mt u kia ca phn on. Khi h tng mng l dng chung, bt k mt tn hiu in ny cng c truyn dn trn ng dy cng c th xung t vi mt tn hiu ca mt thit b khc. Khi hai hoc nhiu Ethernet frame chng lp ln ng truyn mt thi im no , collision xy ra. Collision s dn n cc li bit v mt frame (bit error).

Nhng kt ni Ethernet hot ng theo ch full duplex s khng gp phi vn collision hay cnh tranh nhau v bng thng. Mc d vy, cc kt ni ny vn phi tun th theo cng mt c t. V d, nhng Ethernet frame vn phi truyn v nhn trong mt khong thi gian trn mt kt ni full duplex. iu ny s p t chiu di ca on cp dng trong full duplex v half duplex phi l ging nhau.

Mc d cc mng WLAN cng da trn mt tp hp cc chun kht khe, chnh yu t phng tin truyn cng l mt thch thc. Ni chung, khi mt PC kt ni n mt mng c dy, PC s chia s kt ni mng vi mt s lng my bit trc cng kt ni vo mng c dy . Khi cng mt PC dng mt mng khng dy, n cng chia s tng t, nhng thng qua khng kh. Trong mng khng dy, h tng r rng l khng tn ti cc on dy cp mng hay cc cm mng. Tht ra cc ngi dng mng khng dy wireless khc cng ton quyn s dng cng khng gian truyn chung .

Mng wireless LAN sau tr thnh mt mng dng chung, trong c mt s lng my cnh tranh vi nhau dng khng kh, tc h tng mng mi thi im. Vn xung t (collision) l mt vn mun tha trong lnh vc khng dy bi v mi thit b khng dy u trong ch half-duplex.

Mng 802.11 lun lun hot ng ch half duplex bi v cc trm truyn v nhn s dng cng mt tn s. Ch c mt my truyn mt thi im, nu khng, s c collision xy ra. c th tr thnh full duplex, tt c cc my phi truyn trong mt tn s khc v s nhn trong mt tn s khc. Mc d iu ny nghe c v kh thi, chun 802.11 khng cho php hot ng ch full duplex.

Bi 2: Trnh nghn trong mng khng dy.