Trnh nghn trong mng khng dy WLAN

Khi hai hoc nhiu trm khng dy cng truyn mt thi im, tn hiu tr thnh b nhiu. My trm bn pha nhn ch c th nhn kt qu nh nhng d liu rc, nhiu hay b li. Tht ra, khng c mt cch thc r rng xc nh l xung t collision xy ra. Ngay c vi my truyn ang gy ra xung t cng khng nhn ra, v lc phn nhn ca n phi tt i. c mt c ch phn hi hiu qu, trong mng khng dy, bt c khi no mt trm truyn i mt frame, bn trm nhn phi gi mt frame ACK xc nhn l frame c nhn chnh xc, khng b li.

Cc frame ACK hot ng nh mt cng c c bn pht hin xung t, tuy nhin, cng c ny khng gip ngn nga xung t xy ra. Chun 802.11 dng mt phng php gi l Carrier Sense Multiple Access Collision Avoidance (CSMA/CA). Ch rng mng c dy 802.3 pht hin (detect) xung t, trong khi 802.11 c gng trnh (avoid) xung t.

Trnh nghn hot ng bng cch yu cu tt c cc my trm lng nghe trc khi n truyn i mt frame. Khi mt my trm c mt frame cn phi truyn, mt trong hai trng thi sau c th xy ra:

- Khng c thit b no khc ang truyn: lc ny my trm c th truyn frame i ngay lp tc. Bn my nhn d kin phi gi mt frame ACK xc nhn rng frame ban u n ng v khng b ng . - C mt thit b khc ang truyn mt frame: lc ny my ca ta phi ch cho n khi no frame ang truyn l hon tt, sau n phi ch mt khong thi gian ngu nghin trc khi c th truyn frame ca chnh n.

Cc frame wireless c th thay i v kch thc. Khi mt frame c truyn, lm th no cc my khc bit l frame c truyn hon tt v ng truyn (sng v tuyn) l rnh cho cc my khc s dng? R rng, cc my trm ch c th lng nghe trong yn lng, nhng nu lm th th khng phi lun lun l hiu qu. Cc my trm khng dy khc c th cng lng nghe v cng c th truyn cng mt thi im. Chun 802.11 yu cu tt c cc my trm phi ch mt khong thi gian. Khong thi gian ny c gi l khong thi gian gia cc frame DCF (DCF interframe space). Sau khong thi gian ny, cc my trm mi c th truyn.

Bn my truyn c th ch ra mt khong thi gian d kin gi i ht mt frame bng cch ch ra trong mt trng ca frame 802.11. Khong thi gian ny cha s timeslot (thng tnh bng n v microseconds) cn thit truyn frame. Cc my trm khc phi xem gi tr cha trong header ny v phi ch khong thi gian trc khi truyn cho chnh n.

Bi v tt c cc frame phi ch cng mt khong thi gian ch ra trong frame, tt c cc my c th s quyt nh cng truyn khi khong thi gian tri qua. iu ny c th dn n hin tng xung t, chnh l mt hin tng cn trnh.

Bn cnh thng s thi gian nu trn, cc trm khng dy cng phi trin khai mt b nh thi ngu nhin. Trc khi truyn mt frame, my tnh phi chn mt s ngu nhin time slot phi ch. Con s ny s nm trong khong t zero n kch thc ti a ca s cnh tranh. tng c bn ca cch lm ny l khi mt my mun truyn, mi my s ch mt khong thi gian ngu nhin, gim s trm c gng truyn ng thi cng lc.

Ton b tin trnh ny c gi l chc nng phi hp phn phi. Chc nng ny c m t trong hnh di y. Ba ngi dng wireless c cng mt frame phi truyn cc khong thi gian khc nhau. Mt chui cc s kin sau s xy ra:

1. Ngi dng A lng nghe v xc nh rng khng c ngi dng no khc ang truyn. Ngi dng A truyn frame ca n, ng thi qung b khong thi gian truyn frame. 2. Ngi dng B cng c frame truyn. Anh ta phi ch cho n khi no frame ca ngi dng A l hon tt, sau , phi ch ht khong thi gian DIFS (thi gian phi hp phn phi) hon tt. 3. Ngi dng B phi ch mt khong thi gian ngu nhin trc khi c gng truyn. 4. Khi ngi dng B ang ch, ngi dng C c frame phi truyn. Anh ta lng nghe v pht hin rng khng c ai ang truyn. Ngi dng C phi ch mt khong thi gian ngu nhin. Khong thi gian ny l ngn hn khong thi gian ngu nhin ca ngi dng B. 5. Ngi dng C truyn frame v qung b khong thi gian truyn. 6. Ngi dng B phi ch khong thi gian truyn frame ca ngi dng C cng vi khong thi gian gia cc frame DIFS trc khi c gng truyn li mt ln na.