IEEE 802.11 v chun ha mng LAN khng dy (WLAN) Phn ny cp chi tit, tm hiu su v cc kha cnh ca b tiu chun IEEE 802.11Mc ch thy c tnh cht v ng dng ca cc b tiu chun ny.

3.1. Gii thiu b tiu chun IEEE 802.11 IEEE 802.11 l mt tp cc chun ca t chc IEEE bao gm cc c t k thut lin quan n h thng mng khng dy. Mc ch ca chun IEEE 802.11 nh IEEE nh ngha l " cung cp kt ni khng dy ti cc thit b, hoc cc trm t ng m yu cu trin khai nhanh, v xch tay

hoc cm tay, hoc c gn ln cc phng tin chuyn ng bn trong mt vng".

Chun IEEE 802.11 m t mt giao tip "truyn qua khng kh" (over-the-air) s dng sng v tuyn truyn nhn tn hiu gia mt thit b khng dy v tng i hoc im truy cp (AP), hoc gia 2 hay nhiu thit b khng dy vi nhau (m hnh ad-hoc).

3.2. Kin trc ca chun IEEE 802.11 3.2.1. Cc thnh phn kin trc

Chun IEEE 802.11 nh ngha tng vt l v tng MAC cho mt mng WLAN. Chun ny nh ngha ba tng vt l khc nhau cho mng WLAN 802.11, mi tng hot ng mt di tn khc nhau v s dng cc tc 1 Mbps v 2 Mbps. Thnh t c bn ca kin trc 802.11 l t bo (cell) vi tn gi trong 802.11 l BSS (c gi l Tp hp dch v c bn, Basic Service Set).

Mi BSS thng gm mt vi my trm khng dy v mt trm c s trung tm c gi l AP (access point). Cc my trm (c th di ng hoc c nh) v trm trung tm lin lc vi nhau bng giao thc MAC IEEE 802.11 khng dy. C th kt ni nhiu trm AP vi nhau bng mng hu tuyn Ethernet hoc mt knh khng dy khc to mt h thng phn tn (DS distributed system). i vi cc giao thc tng cao hn, h thng phn tn ny nh l mt mng 802 n.

Cc my trm dng chun IEEE 802.11 c th nhm li vi nhau to thnh mt mng ad hoc - mng khng c iu khin trung tm v khng c kt ni vi "th gii bn ngoi". Trong trng hp ny, mng c hnh thnh tc thi khi mt s thit b di ng tnh c thy mnh ang gn nhau trong khi ang c nhu cu lin lc m khng tm thy mt c s h tng mng sn c ti ch (chng hn mt BBS 802.11 vi mt trm AP). Mt v d v mng ad hoc c hnh thnh l khi mt vi ngi mang my tnh xch tay gp nhau ti mt bn tu v mun trao i d liu m khng c mt trm AP gn . Tng t trong nh mng Ethernet hu tuyn 802.3, cc my trm trong mng WLAN 802.11 phi phi hp vi nhau khi dng chung mi trng truyn dn (tn s radio). Giao thc MAC c nhim v iu khin s phi hp ny. MAC IEEE 802.11 l giao thc CSMA/CA.

Ton b lin kt li mng LAN khng dy bao gm cc cell khc nhau, cc im truy cp v h phn phi tng ng, c xem xt thng qua m hnh OSI, nh mt mng n chun IEEE 802, v c gi l Tp hp dch v c m rng (ESS).

Hnh 3.1 m t mt chun mng LAN IEEE 802.11 tiu biu:

Hnh 3.1. Mng WLAN IEEE 802.11 tiu biu

Chun cng nh ngha khi nim Portal, l mt thit b lin kt gia mng LAN chun IEEE 802.11 v mng LAN chun IEEE 802 khc. Khi nim ny m t v l thuyt phn chc nng ca cu chuyn dch.

Mc d chun khng yu cu s ci t tiu biu tt yu phi c AP v Portal trn mt thc th vt l n.

3.2.2. Kin trc cc lp trong m hnh OSI ca chun IEEE 802.11

Chun IEEE 802.11 nh ngha lp vt l (PHY) v lp iu khin truy cp mi trng (MAC) cho cc mng WLAN. N nh ngha lp vt l hot ng tc d liu 1Mbps v 2 Mbps trong bng tn RF 2.4 GHz v trong hng ngoi (IR). N c m rng hai ln vo nm 1999 thnh chun IEEE 802.11a nh ngha lp vt l cho bng 5GHz tc 54 Mbps, v chun IEEE 802.11b nh ngha lp vt l cho bng tn 2.4 GHz tc 5.5 v 11 Mbps.

3.2.2.1. Lp vt l

Chun IEEE 802.11 quy nh cc lp vt l nh bng 3.1.

Chun Tn s v tuyn (RF) Hng ngoi (IR) C ch Tc d liu cc i (Mbps)

IEEE 802.11 2.4 GHz DSSS 2

IEEE 802.11 2.4 GHz FHSS 2

IEEE 802.11 850 - 950 nm IR 2

IEEE 802.11a 5 GHz OFDM 54

IEEE 802.11b 2.4 GHz DSSS 11

Bng 3.1. So snh cc lp vt l ca chun IEEE 802.11. H thng tri ph nhy tn FHSS 2.4 GHz v h thng IR ca chun IEEE 802.11 t khi c s dng. Lp vt l OFDM 5 GHz c phm vi hn ch (xp x 15m) nn n t c s dng. a s cc sn phm hin ti thc hin cng ngh tri ph chui trc tip (DSSS) theo chun IEEE 802.11b tc d liu ln trn ti 11 Mbps do li th kh nng thc hin v gi thnh ca n.

Mc ch ca cng ngh tri ph l tng thm thng lng v tin cy ca truyn dn bng cch s dng nhiu di tn. DSSS hot ng bng cch chuyn i mi bit truyn thnh mt chui "chip" m thc cht l mt chui s 1 v 0. Sau chip ny c gi song song qua mt di tn rng. V s dng nhiu di tn, nn n tng cng tin cy truyn dn khi c giao thoa. V mi bit c biu din bi mt chui chip, nn nu phn no ca chui chip b mt v giao thoa, th

gn nh phn chip nhn c s vn phn bit bit gc.

3.2.2.2. iu khin truy cp mi trng (Lp MAC)

Hnh 3.2. Lp MAC

Trong khi lp vt l chun IEEE 802.11 khc vi chun IEEE 802.3 Ethernet, th ch tiu k thut ca MAC tng t nh ch tiu k thut ca MAC Ethernet chun IEEE 802.3 cng vi iu khin lin kt Logic (LLC) chun IEEE 802.2, n lm cho khng gian a ch MAC chun IEEE 802.11 thch hp vi khng gian a ch MAC ca cc giao thc IEEE 802. Trong khi MAC Ethernet chun IEEE 802.3 thc cht l CSMA/CD - a truy cp nhy sng mang pht hin xung t, th MAC chun IEEE 802.11 l CSMA/CA - a truy cp nhy sng mang trnh xung t. S khc nhau ny l do khng c phng cch thit thc truyn v nhn cng lc trn mi trng khng dy (mi trng WM). CSMA/CA c gng trnh cc va chm trn mi trng WM bng cch t mt khong thi gian thng tin trong mi khung MAC, cc trm thu xc nh thi gian cn li ca khung trn mi trng WM. Nu khong thi gian ca khung MAC trc ht v mt kim tra nhanh trn mi trng WM ch ra rng n khng bn, th trm truyn c php truyn. Bng cch ny, n cho php ni gi truyn bt k lc no m mi trng khng bn.

Ngoi cc tnh nng chun c thc hin bi cc lp MAC, lp MAC chun IEEE 802.11 cn thc hin chc nng khc lin quan n cc giao thc lp trn, nh Phn on, Pht li gi d liu, v Cc ghi nhn.

Lp MAC nh ngha hai phng php truy cp khc nhau, Hm phi hp phn tn v Hm phi hp im.

3.2.3. Phng php truy cp c bn: CSMA/CA

y l mt c ch truy cp c bn, c gi Hm phi hp phn tn, v c bn l a truy cp cm bin sng mang vi c ch trnh xung t (CSMA/CA). Cc giao thc CSMA c bit trong cng nghip, m ph bin nht l Ethernet, l giao thc CSMA/CD (CD ngha l pht hin xung t).

Giao thc CSMA lm vic nh sau: Mt trm truyn i cc cm bin mi trng, nu mi trng bn (v d, c mt trm khc ang pht), th trm s tr hon truyn mt lc sau, nu mi trng t do th trm c cho php truyn.

Loi giao thc ny rt c hiu qu khi mi trng khng ti nhiu, do n cho php cc trm truyn vi t tr hon, nhng thng xy ra trng hp cc trm pht cng lc (c xung t), gy ra do cc trm nhn thy mi trng t do v quyt nh truyn ngay lp tc.

Cc tnh trng xung t ny phi c xc nh, v vy lp MAC phi t truyn li gi m khng cn n cc lp trn, iu ny s gy ra tr ng k. Trong trng hp mng Ethernet, s xung t ny c on nhn bi cc trm pht i ti quyt nh pht li da vo gii thut exponential random backoff.

Cc c ch d tm xung t ny ph hp vi mng LAN ni dy, nhng chng khng c s dng trong mi trng mng LAN khng dy, v hai l do chnh:

1. Vic thc hin c ch d tm xung t yu cu s thi hnh ton song cng, kh nng pht v nhn ng thi, n s lm tng thm chi ph mt cch ng k.

2. Trn mi trng khng dy chng ta khng th gi thit tt c cc trm nghe thy c nhau (y l s gi thit c s ca s d tm xung t), v vic mt trm nhn thy mi trng t do v sn sng truyn khng tht s c ngha rng mi trng l t do quanh vng my thu.

vt qua cc kh khn ny, chun IEEE 802.11 s dng mt c ch trnh xung t vi mt s Ghi nhn tnh tch cc (Positive Acknowledge) nh sau:

Mt trm mun truyn cm bin mi trng, nu mi trng bn th n tr hon. Nu mi trng rnh vi thi gian c ch r (gi l DIFS, Distributed Inter Frame Space, Khng gian khung Inter phn tn), th trm c php truyn, trm thu s kim tra m CRC ca gi nhn c v gi mt gi chng thc (ACK). Chng thc nhn c s ch cho my pht bit khng c s xung t no xut hin. Nu my pht khng nhn chng thc th n s truyn li on cho n

khi n c tha nhn hoc khng c php truyn sau mt s ln pht li cho trc.

Cm bin sng mang o (Virtual Carrier Sense) gim bt xc sut kh nng hai trm xung t nhau v chng khng th nghe thy nhau, chun nh ngha mt c ch Cm bin sng mang o:

Mt trm mun truyn mt gi, trc ht n s truyn mt gi iu khin ngn gi l RTS (Request To Send) gm ngun, ch n, v khong thi gian giao dch sau (v.d. gi v ACK tng ng), trm ch s p ng (nu mi trng t do) bng mt gi iu khin p li gi l CTS (Clear To Send) gm cng thng tin khong thi gian.

Tt c cc trm nhn RTS v/hoc CTS, s thit lp ch bo Virtual Carrier Sense ca n (gi l NAV, Network Allocation Vector, Vect nh v mng) cho khong thi gian cho trc, v s s dng thng tin ny cng vi Cm bin sng mang vt l (Physical Carrier Sense) khi cm bin mi trng.

C ch ny gim bt xc sut xung t v vng my thu do mt trm n t my pht, lm ngn khong thi gian truyn RTS, v trm s nghe thy CTS v d tr mi trng khi bn cho n khi kt thc giao dch. Thng tin khong thi gian v RTS cng bo v vng my pht khi cc xung t trong thi gian ACK (bi cc trm nm ngoi phm vi trm nhn bit).

Cn ch thng tin khong thi gian ACK v cc khung RTS v CTS l cc khung ngn. N cng lm gim bt no ca cc xung t, v chng c nhn dng nhanh hn khi n c nhn dng nu ton b gi c truyn, (iu ny ng nu gi ln hn RTS mt cch ng k, nh vy l chun cho php k c cc gi ngn s c truyn m khng c giao dch RTS/CTS), v iu ny c iu khin bi mt tham s gi l ngng RTS.

Cc s sau cho thy mt giao dch gia hai trm A v B, v s thit lp NAV ca cc trm gn chng:

Hnh 3.3. Giao dch gia hai trm A v B, v s thit lp NAV

Trng thi NAV c kt hp vi cm bin sng mang vt l cho bit trng thi bn ca mi trng.

3.2.4. Cc chng thc mc MAC

Lp MAC thc hin d tm xung t bng cch ch i s tip nhn ca mt ghi nhn ti bt k on c truyn no (Ngoi l cc gi m c hn mt ni n, nh Qung b, cha c tha nhn).

3.2.5. Phn on v Ti hp

Cc giao thc mng LAN tiu biu s dng cc gi vi vi hng trm byte (v d, gi Ethernet di nht di trn 1518 byte) trn mt mi trng mng LAN khng dy. L do cc gi di c a chung s dng cc gi nh l:

V t l li bit BER ca thng tin v tuyn cao hn, xc sut mt gi b h tng thm theo kch thc gi. Trong trng hp b hng (v xung t hoc nhiu), gi nh nht vi t mo u hn gy ra s pht li gi. Trn mt h thng FHSS, mi trng c ngt nh k mi khi nhy tn (trong trng hp ny l mi 20 mili - giy), nh vy nh hn gi, nh hn c hi truyn b hon li sau thi gian ngng truyn.

Mc khc, n khng c gii thiu nh l mt giao thc mng LAN mi v n khng th gii quyt cc gi 1518 byte c s dng trn mng Ethernet, nh vy IEEE quyt nh gii quyt vn bng cch thm mt c ch phn on/ti hp n gin ti lp MAC.

C ch l mt gii thut Send - and - Wait n, trong trm pht khng cho php truyn mt

on mi cho n khi xy ra mt trong cc tnh hung sau y:

1. Nhn mt ACK cho on.

2. Quyt nh rng on cng c truyn li nhiu ln v th vo ton b khung.

Cn phi nh rng chun cho php trm c truyn ch mt a ch khc gia cc pht li ca mt on cho, iu ny c bit hu ch khi mt AP c vi gi ni bt vi cc ch n khc nhau v mt trong s chng khng tr li.

S sau biu din mt khung (MSDU) c chia thnh vi on (MPDUs):

Hnh 3.4. Khung MSDU

3.2.6. Cc khng gian khung Inter (Inter Frame Space)

Chun nh ngha 4 kiu khng gian khung Inter, c s dng cung cp cc quyn u tin khc nhau:

SIFS - Short Inter Frame Space, c s dng phn chia cc truyn dn thuc mt hi thoi n (v.d. Ack - on), v l Khng gian khung Inter ti thiu v lun c nhiu nht mt trm n truyn ti thi gian cho trc, do n c quyn u tin i vi tt c cc trm khc. l mt gi tr c nh trn lp vt l v c tnh ton theo cch

m trm pht truyn ngc li nhn kiu v kh nng gii m gi vo, trong lp vt l chun IEEE 802.11 FH gi tr ny c thit lp 28 micr - giy.

PIFS - Point Cooordination IFS, c s dng bi im truy cp (hoc Point Coordinator, c gi trong trng hp ny), c truy cp ti mi trng trc mi trm khc. Gi tr ny l SIFS cng vi mt khe thi gian (s c nh ngha sau), v d 78 micr - giy. DIFS - Distributed IFS, l khng gian khung Inter c s dng bi mt trm sn sng bt u mt truyn dn mi, m l c tnh ton l PIFS cng thm mt khe thi gian, v d 128 micr - giy.

EIFS - Extended IFS, L mt IFS di hn c s dng bi mt trm nhn mt gi khng hiu, n cn ngn trm (trm m khng hiu thng tin khong thi gian Cm bin sng mang o) khi xung t vi mt gi tng lai thuc hi thoi hin thi.

3.2.7. Gii thut Exponential Backoff

Backoff l mt phng php ni ting gii quyt cc tranh ginh gia cc trm khc nhau mun truy cp mi trng, phng php yu cu mi trm chn mt s ngu nhin (n) gia 0 v mt s cho trc, v i s khe thi gian ny trc khi truy cp mi trng, n lun kim tra liu c mt trm khc truy cp mi trng trc khng.

Khe thi gian c nh ngha theo cch m mt trm s lun c kh nng xc nh liu trm khc truy cp mi trng ti thi gian bt u ca khe trc khng. iu ny lm gim bt xc sut xung t i mt na.

Exponential Backoff c ngha rng mi ln trm chn mt khe thi gian v xy ra xung t, n s tng gi tr theo ly tha mt cch ngu nhin.

Chun IEEE 802.11 chun nh ngha gii thut Exponential Backoff c thc hin trong cc trng hp sau y:

Nu khi trm cm bin mi trng trc truyn gi u tin, v mi trng ang bn Sau mi ln truyn li Sau mt ln truyn thnh cng

Trng hp duy nht khi c ch ny khng c s dng l khi trm quyt nh truyn mt gi mi v mi trng rnh cho nhiu hn DIFS.

Exponential backoff khin cc nt chu kh ch lu hn khi mc xung t cao.

bit time: thi gian truyn 1 bit. n l s ln xung t khi truyn mt frame no .

sau n ln xung t, nt s i 512 x K bit time ri truyn li; K c chn ngu

nhin trong tp {0,1,2,,2m 1} vi m:=min (n,10).

Hnh 3.5 biu din s c ch truy cp:

Hnh 3.5. S c ch truy cp 3.2.8. So snh kiu C s h tng v kiu Ad Hoc

C hai phng php lm vic khc nhau cho thit b chun IEEE 802.11: Ad Hoc (tp hp cc dch v c bn c lp, IBSS) v C s h tng (tp hp cc dch v c m rng, ESS). Mt mng Ad Hoc thng thng l mt mng tn ti trong mt thi gian hu hn gia hai hoc nhiu hn hai thit b v tuyn m khng c ni thng qua mt im truy cp (AP) ti mt mng ni dy. V d, hai ngi dng laptop mun chia s cc file s thit lp mt mng Ad Hoc s dng cc card NIC thch hp chun IEEE 802.11 v chia s cc file qua mi trng WM m khng cn phng tin truyn thng ngoi no (nh a mm, cc card flash).

Kiu C s h tng gi thit c mt mt hoc nhiu hn cc AP bc cu phng tin truyn thng khng dy vi phng tin ni dy truyn thng (hnh 3.6). AP iu khin vic chng thc v lin kt trm ti mng khng dy. Nhiu AP c ni bi mt h phn phi (DS) m rng phm vi ca mng khng dy ra nhiu vng ln hn. Trong cc ci t tiu biu, DS n gin l c s h tng mng IP hin hu. Vi mc ch bo mt, ngi ta thng s dng cc mng LAN o

(VLAN) tch ring lu thng mng khng dy vi lu thng mng khc trn DS. Mc d chun IEEE 802.11 cho php cc trm v tuyn lin kt chuyn mch ng t im truy cp ny n im truy cp khc, nhng n khng iu khin cch trm thc hin. Kt qu l, cc thi hnh ca nh cung cp khc nhau ni chung khng tng tc vi nhau trong ng cnh ny. Ti thi im hin nay, kh nng thc hin kiu hot ng ny yu cu mt gii php nh cung cp n.

Hnh 3.6. So snh kiu Ad Hoc v kiu c s h tng. 3.3. L thuyt v mng khng dy

3.3.1. Cch mt trm ni vi mt cell hin hu (BSS)

Khi mt trm mun truy cp mt BSS hin hu (hoc sau ch bt ngun, ch ngh, hoc ch l i vo vng BSS), trm cn c thng tin ng b t im truy cp (hoc t cc trm khc khi trong kiu Ad - hoc).

Trm nhn thng tin ny theo mt trong s hai cch sau:

1. Qut b ng: Trong trng hp ny trm i nhn mt khung n hiu (Beacon) t AP, (khung n hiu l mt khung tun hon cha thng tin ng b c gi bi AP).

2. Qut tch cc: Trong trng hp ny trm c gng tm mt im truy cp bng cch truyn cc khung yu cu d (Probe) v ch p li thng tin d t AP.

Hai phng php u hp l, v mi mt phng php c chn phi hi ho gia kh nng tiu th in v kh nng thc hin.

3.3.1.1. Qu trnh chng thc Mi khi trm tm thy mt im truy cp, n s quyt nh ni cc BSS, n thc hin thng qua qu trnh chng thc, l s trao i thng tin ln nhau gia AP v trm, m mi bn chng minh s nhn bit mt khu cho.

3.3.1.2. Qu trnh lin kt Khi trm c xc nhn, sau n s khi ng qu trnh lin kt, y l s trao i thng tin v cc trm v cc BSS, v n cho php thc hin DSS (tp hp cc AP bit v tr hin thi ca trm). Ch sau khi qu trnh lin kt c hon thnh, th mt trm mi c kh nng pht v nhn cc khung d liu.

3.3.2. Roaming

Roaming l qu trnh chuyn ng t cell ny (hoc BSS) n cell khc vi mt kt ni cht.

Chc nng ny tng t nh cc in thoi t bo, nhng c hai khc bit chnh:

Trong mt h thng mng LAN da trn cc gi, s chuyn tip gia cc cell c thc hin gia cc truyn dn gi, ngc vi k thut in thoi trong s chuyn tip xut hin trong thi gian mt cuc ni chuyn in thoi, iu ny lm roaming mng LAN d hn mt t.

Trong mt h thng ting ni, mt gin on tm thi khng nh hng cuc ni chuyn, trong khi trong mt gi da vo mi trng, n s gim ng k kh nng thc hin v s chuyn tip c thc hin bi cc giao thc lp trn.

Chun IEEE 802.11 khng nh ngha cch roaming c thc hin, nhng nh ngha cc cng c c bn cho n, iu ny bao gm s qut tch cc/b ng, v mt qu trnh ti lin kt, trong mt trm roaming t im truy cp ny sang im truy cp khc s c lin kt vi mt im truy cp mi.

3.3.3. Gi ng b

Cc trm cn gi ng b, gi cho nhy tn c ng b v cc chc nng khc nh tit kim nng lng. Trong mt c s h tng BSS iu ny c thc hin bi tt c cc trm cp nht cc ng h ca chng theo ng h ca AP, s dng c ch sau:

AP truyn cc khung tun hon gi l cc khung bo hiu, cc khung ny cha gi tr ca ng h AP ti thi im truyn (Ch rng y l thi im khi truyn dn tht s xut hin v khng phi l thi im truyn khi n c t vo hng i truyn, v khung bo hiu c truyn s dng cc quy tc CSMA, nn truyn dn tr mt cch ng k).

Cc trm thu kim tra gi tr ng h ca chng thi im nhn, sa cha n gi ng b vi ng h ca AP, iu ny ngn nga s tri ng h gy ra do mt ng b sau vi gi hot ng.

3.3.4. Tit kim nng lng

Mng LAN khng dy tiu biu lin quan n cc ng dng di ng, v trong cc kiu ng dng ny ngun pin l mt ngun nhanh ht, l l do ti sao chun IEEE 802.11 trc tip hng vo vn tit kim nng lng v nh ngha c c ch cho php cc trm i vo trong ch ngh ngi cho cc thi hn di m khng mt thng tin.

tng chnh ng sau c ch tit kim nng lng l AP duy tr mt bn ghi c cp nht ti cc trm hin thi ang lm vic trong ch tit kim nng lng v nh m cc gi c gi ti cc trm ny cho n khi c trm yu cu nhn cc gi bng cch gi mt yu cu kim tra tun t, hoc cho n khi chng thay i thao tc ca n.

AP cng truyn nh k (mt phn ca cc khung bo hiu) thng tin v trm tit kim nng lng no c cc khung c nh m AP, nh vy cc trm ny cn phi c nh thc nhn mt trong s cc khung bo hiu v nu mt ch bo cho bit c mt khung c lu tr ti AP i phn pht, th trm cn phi trong trng thi hot ng v gi mt thng bo kim tra tun t cho AP c cc khung ny.

Qung b v pht thanh c lu tr bi AP, v c truyn mt thi im c bit trc (mi DTIM), ti tt c trm tit kim nng lng mun nhn kim khung ny cn phi hot ng.

3.3.5. Cc kiu khung

C ba kiu khung chnh:

Khung d liu: cc khung c s dng truyn d liu Khung iu khin: cc khung c s dng iu khin truy cp ti mi trng (v d RTS, CTS, v ACK). Khung qun l: cc khung c truyn ging nh cc khung d liu trao i

thng tin qun l, nhng khng hng ti cho cc lp trn.

Mi kiu c chia nh ra thnh cc kiu nh hn khc nhau, ty theo chc nng ca chng.

3.3.6. Khun dng khung

Tt c cc khung chun IEEE 802.11 u c cc thnh phn sau y:

Hnh 3.7. Khun dng khung chun IEEE 802.11

3.3.6.1. Tin t (Preamble)

N ph thuc lp vt l, bao gm: Synch: Mt chui 80 bit 0 v 1 xen k, c s dng bi bo mt lp vt l la chn anten thch hp (nu tnh s phn tp c s dng) v nh hng ti vic sa li dch tn s trng thi vng ng b vi vic nh thi gian gi nhn c. SFD: Mt b nh ranh gii khung bt u, n gm 16 bit nh phn 0000 1100 1011

1101, c dng nh ngha nh thi khung.

3.3.6.2. u mc (Header) PLCP

u mc PLCP lun lun c truyn tc 1 Mbit/s v n cha thng tin Logic m s c s dng bi lp vt l gii m khung, gm c:

Chiu di t PLCP_PDU: biu din s byte cha trong gi, n c ch cho lp vt l pht hin ra chnh xc kt thc gi. Tng bo hiu PLCP: hin thi, n ch cha ng thng tin tc , c m ha tc 0.5 MBps, tng dn t 1Mbit/s ti 4.5 Mbit/s.

Trng kim tra li u mc: l trng pht hin sai st CRC 16 bit.

3.3.6.3. D liu MAC

Hnh 3.8 cho thy khun dng khung MAC chung, cc phn ca trng trn cc phn ca cc khung nh m t sau .

Hnh 3.8. Khun dng khung MAC

Trng iu khin khung (Frame Control) Trng iu khin khung cha ng thng tin sau: Hnh 3.9. Trng iu khin khung (Frame Control)

- Protocol Verson Trng ny gm 2 bit c kch thc khng i v xp t theo cc phin bn sau ca chun IEEE 802.11, v s c s dng nhn bit cc phin bn tng lai c th. Trong phin bn hin thi ca chun gi tr c nh l 0.

- ToDS

Bit ny l tp hp cc bit 1 khi khung c nh a ch ti AP hng n ti h phn phi (gm trng hp m trm ch t li khung ging vi BSS, v AP). Bit l tp hp cc bit 0 trong tt c cc khung khc.

- FromDS Bit ny l tp hp cc bit 1 khi khung ang n t h phn phi.

- More Fragments

Bit ny l tp hp cc bit 1 khi c nhiu on hn thuc cng khung theo sau on hin thi ny.

- Retry

Bit ny cho bit on ny l mt chuyn tip mt on trc c truyn, n s c s dng bi trm my thu on nhn bn sao c truyn ca cc khung m xut hin khi mt gi Chng thc b mt.

- Power mangenment

Bit ny cho bit kiu qun l nng lng trong trm sau khi truyn khung ny. N c s dng bi cc trm ang thay i trng thi t ch tit kim nng lng n ch hot ng hoc ngc li.

- More Data

Bit ny cng c s dng qun l nng lng v n c s dng bi AP cho bit rng c nhiu khung c nh m hn ti trm ny. Tm quyt nh s dng thng tin ny tip tc kim tra tun t hoc kiu ang thay i thm ch thay i sang ch hot ng.

- WEP

Bit ny cho bit rng thn khung c m ha theo gii thut WEP

- Order

Bit ny cho bit rng khung ny ang c gi s dng lp dch v Strictly - Order.

Khong thi gian/ID Trng ny c hai ngha ph thuc vo kiu khung:

Trong cc bn tin Kim tra tun t tit kim nng lng, th n l ID trm.

Trong tt c cc khung khc, n l gi tr khong thi gian c dng cho Tnh ton NAV.

Cc trng a ch

Mt khung cha ln trn ti 4 a ch ph thuc vo cc bit ToDS v FromDS c nh

ngha trong trng iu khin, nh sau:

a ch - 1 : Lun l a ch nhn (v d, trm trn BSS m nhn gi tc thi), nu bit ToDS c lp th y l a ch AP, nu bit ToDS c xa th n l a ch trm kt thc.

a ch - 2 : Lun lun l a ch my pht (v d, trm ang truyn gi vt l), nu bit FromDS c lp th y l a ch AP, nu c xa th n l a ch trm.

a ch - 3 : Trong hu ht cc trng hp cn li, mt a ch, trn mt khung vi bit FromDS c lp, sau a ch - 3 l a ch ngun gc, nu khung c bit ToDS lp, sau a ch - 3 l a ch ch.

a ch - 4 :c s dng trong trng hp c bit trong mt h phn phi khng dy c s dng, khung ang c truyn t im truy cp ny sang im truy cp khc, trong trng hp ny c cc bit ToDS ln cc bit FromDS c lp, v vy c a ch ch gc v a ch ngun gc u b mt.

Bng 3.2 tng kt cc cch dng a ch khc nhau theo cch thit lp bit ToDS v bit FromDS:

Bng 3.2: Tng kt cc cch dng a ch

3.3.6.4. iu khin ni tip Trng iu khin ni tip c dng biu din th t cc on khc nhau thuc khung, nhn bit cc gi sao, n gm c hai trng con: trng S on v trng S ni tip, m nh ngha khung v s on trong khung.

3.3.7. Cc khung nh dng ph bin nht

3.3.7.1. Khun dng khung RTS Khung RTS nh sau:

Hnh 3.10: Khun dng khung RTS

RA ca khung RTS l a ch STA, trong mi trng khng dy, n c dnh nhn d liu tip theo hoc khung qun l mt cch tc thi.

TA l a ch ca STA pht khung RTS.

Gi tr Khong thi gian l thi gian, tnh theo micr - giy, c yu cu truyn d liu lin tip hoc khung qun l, cng vi mt khung CTS, cng mt khung ACK, cng ba khong SIFS.

3.3.7.2. Khun dng khung CTS Khung CTS nh sau:

Hnh 3.11: nh dng khung CTS

a ch my thu (RA) ca khung CTS c copy t trng a ch my pht (TA) ca khung RTS ngay trc n mt p ng CTS no . Gi tr Khong thi gian l gi tr thu c t trng Khong thi gian ca khung RTS ngay trc , tr thi gian (tnh theo micr - giy) c yu cu pht khung CTS v khong SIFS.

3.3.7.3. Khun dng khung ACK Khung ACK nh sau:

Hnh 3.12: nh dng khung ACK

a ch My thu ca khung ACK c sao chp t trng a ch-2 ca khung ngay trc .

Nu nhiu bit on hn c xa (0) trong trng iu khin khung ca khung trc , th gi tr Khong thi gian l 0, nu khng th gi tr Khong thi gian thu c t trng Khong thi gian ca khung trc , tr i thi gian (tnh theo micr - giy) c pht khung ACK v khong SIFS ca n.

3.3.8. Hm Phi hp im (PCF)

Bn cnh Hm Phi hp Phn tn c bn, c mt Hm Phi hp im chn, m s dng thc hin cc dch v bin - thi gian, nh ting ni hoc truyn video. Hm Phi hp im lm cho im truy cp s dng quyn u tin cao hn bng cch s dng mt Khng gian khung Inter (PIFS) nh hn.

Bng cch s dng cao hn ny quyn u tin truy cp, cc vn im truy cp kim tra tun t yu cu ca cc trm truyn d liu, do iu khin vic truy cp mi trng. cho php cho cc trm bnh thng kh nng vn cn truy cp mi trng, c mt chun b m im truy cp phi li thi gian cho Truy cp Phn tn trong gia PCF.

3.3.9. Cc mng Ad-hoc

Trong mt s trng hp cc ngi dng mun lp mt mng LAN khng dy m khng c mt c s h tng (c bit hn khng c mt im truy cp), iu ny bao gm truyn file gia hai ngi dng my notebook, cuc hp gia cc cng tc vin bn ngoi vn phng, vn vn.

Chun IEEE 802.11 gii quyt cc nhu cu ny bng cch nh ngha mt m hnh hot ng

Ad-hoc, trong trng hp ny khng c im truy cp no hoc phn no tnh nng ca n c thc hin bi cc trm ngi dng cui (nh to bo hiu, ng b, ), v cc chc nng khc khng c h tr (nh t li gia hai trm khng nm trong phm vi, hoc tit kim nng lng).

3.4. Cc tiu chun trong b tiu chun IEEE 802.11

3.4.1. IEEE 802.11

Nm 1997, IEEE a ra chun WLAN u tin c gi l 802.11 theo tn ca nhm gim st s pht trin ca chun ny. Lc ny, 802.11 s dng tn s 2,4GHz v dng k thut tri ph trc tip (Direct-Sequence Spread Spectrum-DSSS) nhng ch h tr bng thng ti a l 2Mbps tc kh chm cho hu ht cc ng dng. V l do , cc sn phm chun khng dy ny khng cn c sn xut na.

3.4.2. IEEE 802.11b

T thng 6 nm 1999, IEEE bt u m rng chun 802.11 ban u v to ra cc c t k thut cho 802.11b. Chun 802.11b h tr bng thng ln n 11Mbps, ngang vi tc Ethernet thi by gi. y l chun WLAN u tin c chp nhn trn th trng, s dng tn s 2,4 GHz. Chun 802.11b s dng k thut iu ch kha m b (Complementary Code Keying - CCK) v dng k thut tri ph trc tip ging nh chun 802.11 nguyn bn. Vi li th v tn s (bng tn nghip d ISM 2,4GHz), cc hng sn xut s dng tn s ny gim chi ph sn xut. Nhng khi y, tnh trng "ln xn" li xy ra, 802.11b c th b nhiu do l vi sng, in thoi m bng con v cc dng c khc cng s dng tn s 2,4GHz. Tuy nhin, bng cch lp t 802.11b khong cch hp l s d dng trnh c nhiu.

u im ca 802.11b l gi thp, tm ph sng tt v khng d b che khut.

Nhc im ca 802.11b l tc thp; c th b nhiu bi cc thit b gia dng.

3.4.3. IEEE 802.11a

Song hnh vi 802.11b, IEEE tip tc a ra chun m rng th hai cng da vo 802.11 u tin - 802.11a. Chun 802.11a s dng tn s 5GHz, tc 54Mbps trnh c can nhiu t cc thit b dn dng. ng thi, chun 802.11a cng s dng k thut tri ph khc vi chun 802.11b - k thut tri ph theo phng php a phn chia tn s trc giao (Orthogonal Frequency Division Multiplexing-OFDM). y c coi l k thut tri hn so vi tri ph trc tip (DSSS). Do chi ph cao hn, 802.11a thng ch c s dng trong cc mng doanh nghip, ngc li, 802.11b thch hp hn cho nhu cu gia nh. Tuy nhin, do tn s cao hn tn s ca chun 802.11b nn tn hiu ca 802.11a gp nhiu kh khn hn khi xuyn tng v cc vt cn khc.

Do 802.11a v 802.11b s dng tn s khc nhau, hai cng ngh ny khng tng thch vi nhau. Mt vi hng sn xut bt u cho ra i sn phm "lai" 802.11a/b, nhng cc sn phm ny ch n thun l cung cp 2 chun sng Wi-Fi cng lc (my trm dng chun no th kt ni theo chunf).

u im ca 802.11a l tc nhanh; trnh xuyn nhiu bi cc thit b khc.

Nhc im ca 802.11a l gi thnh cao; tm ph sng ngn hn v d b che khut.

3.4.4. IEEE 802.11d

Chun IEEE 802.11d l mt chun IEEE b sung lp s iu khin truy cp (MAC) vo chun IEEE 802.11 y mnh kh nng s dng rng mng WLAN chun IEEE 802.11. N s cho php cc im truy cp truyn thng thng tin trn cc knh v tuyn dng c vi cc mc cng sut chp nhn c cho cc thit b khch hng. Cc thit b s t ng iu chnh da vo cc yu cu a l.

Mc ch 802.11d l s thm cc c tnh v cc hn ch cho php mng WLAN hot ng theo cc quy tc ca cc nc ny. Cc nh sn xut thit b khng mun to ra mt s a dng rng ln ca cc sn phm v cc ngi dng chuyn bit theo quc gia m ngi i du lch khng mun mt ti y cc card PC mng WLAN chuyn bit theo quc gia. Hu qu s l cc gii php b chuyn bit theo quc gia.

3.4.5. IEEE 802.1x (Tbd)

Chun IEEE 802.1x (Yu cu mt nh cung cp dch v RADIUS) cung cp cc doanh nghip & cc nh ring mt gii php chng thc bo mt, bin i c s dng k thut ti kha (re - keying) ng, s chng thc tn v mt khu ngi dng v chng thc ln nhau. K thut ti kha ng, m trong sut vi ngi dng, loi tr phn phi kha khng bo mt v s chi phI thi gian v ngn nga cc tn cng lin quan n cc kha WEP tnh. S chng thc trn nn ngi dng loi tr cc l bo mt xut hin t thit b b trm hoc mt khi s chng thc trn nn thit b c s dng, v s chng thc ln nhau gim nh tn cng da vo cc im truy cp lu c. ng thi, v s chng thc chun IEEE 802.1x thng qua mt c s d liu RADIUS, n cng chia thang d dng iu khin cc s lng ngi dng mng WLAN ang gia tng.

3.4.6. IEEE 802.11i

y l tn ca nhm lm vic IEEE dnh cho chun ha bo mt mng WLAN. Bo mt chun IEEE 802.11i c mt khung lm vic c da vo RSN (C ch Bo mt tng cng).

RSN gm c hai phn:

1. C ch ring ca d liu

2. Qun l lin kt bo mt.

C ch ring ca d liu h tr hai s c xng: TKIP v AES. TKIP (S ton vn kha thi gian) l mt gii php ngn hn m nh ngha phn mm v cho WEP cung cp mt mc ring t d liu thch hp ti thiu. AES hoc AES - OCB (Advanced Encryption Standard and Offset Codebook) l mt s ring t d liu mnh m v l mt gii php thi hn lu hn.

Qun l lin kt bo mt c nh a ch bi:

a) Cc th tc m phn RSN

b) S Chng thc chun IEEE 802.1x

c) Qun l kha chun IEEE 802.1x.

Cc chun ang c nh ngha cng tn ti mt cch t nhin cc mng pre - RSN m hin thi c trin khai. Chun ny khng k vng s c thng qua cho n khi kt thc nm 2003.

3.4.7. IEEE 802.11g

Nm 2002 v 2003, cc sn phm WLAN h tr chun mi hn c gi l 802.11g ni ln trn th trng; chun ny c gng kt hp tt nht 802.11a v 802.11b. 802.11g h tr bng thng

54Mbps v s dng tn s 2,4GHz cho phm vi ph sng ln hn. 802.11g tng thch ngc vi 802.11b, ngha l cc AP 802.11g s lm vic vi card mng Wi-Fi chunp802.11b...

Thng 7/2003, IEEE ph chun 802.11g. Chun ny cng s dng phng thc iu ch OFDM tng t 802.11a nhng li dng tn s 2,4GHz ging vi chun 802.11b. iu th v l chun ny vn t tc 54Mbps v c kh nng tng thch ngc vi chun 802.11b ang ph bin.

u im ca 802.11g l tc nhanh, tm ph sng tt v khng d b che khut.

Nhc im ca 802.11g l gi cao hn 802.11b; c th b nhiu bi cc thit b gia dng.

3.4.8. IEEE 802.11h

Chun ny c dng chu u, di tn 5 Ghz. N cung cp tnh nng s la chn knh ng v iu khin cng sut truyn dn TPC, nhm trnh can nhiu. chu u ngi ta ch yu s dng thng tin v tinh, nn phn ln cc quc gia y s dng ch s dng Wireless LAN

trong nh (Indoor). Chun ny ang giai on chun ha.

3.4.9. IEEE 802.11n

Do tnh tin dng v d trin khai, mng WLAN ngy cng thm nhp khp ni phc v nhu cu trao i thng tin v gii tr. Vi nhu cu ngy cng cao cp, tc 11 Mbps ca chun 802.11b, 54Mbps ca chun 802.11a/g d rt hp dn nhng cng cha tha yu cu ca ngi dng. p ng nhu cu ngy cng cao hn , IEEE hnh thnh nhm lm vic pht trin chun 802.11n. Phm vi lm vic ca nhm ny l nh ngha iu chnh lp vt l v lp MAC chuyn giao mt gi tr nh nht 100Mbps thng qua mt im truy nhp dch v MAC (SAP).

802.11n s dng mt quan im pht trin ang s dng ti cc cng ngh hin hu trong khi gii thiu cng ngh mi chng cung cp hiu qu lm vic s pht trin thy c yu cu ca vic pht trin nhng ng dng. S dng li nhng cng ngh qu bu nh OFDM, m ha sa li t pha trc, an xen v nh x iu bin bin cu phng c sa v gia li, gi c gim xung v hon ton tng thch vi cc cng ngh trc .

So vi cc chun trc, c t k thut ca 802.11n "thong" hn nhiu: c nhiu ch ty chn, nhiu cu hnh c th cho ra sn phm c cc mc tc ti a khc nhau. (Trc y, tt c cc sn phm 802.11b phi c tc 11Mbps; 802.11a v 802.11g phi c tc 54Mbps). iu ny vch ra ranh gii v hiu nng trn mi thit b 802.11n: cc nh sn xut c th tng hoc iu chnh kh nng h tr ng dng, mc gi... ng vi mi ty chn, 802.11n c th h tr tc ln n 600Mbps, nhng phn cng WLAN khng nht thit phi p dng tt c cc ty chn. V d, nm 2006, hu ht thit b phn cng WLAN 802.11n 1.0 h tr tc 300Mbps.

Mt trong nhng iu mong i nht ca ngi dng thit b u cui Wi-Fi khng g khc ngoi tc v tm ph sng. Theo c t k thut, chun 802.11n c tc l thuyt ln n 600Mbps (cao hn 10 ln chun 802.11g) v vng ph sng rng khong 250m (cao hn chun 802.11g gn 2 ln, 140m). Hai c im then cht ny gip vic s dng cc ng dng trong mi trng mng Wi-Fi c ci tin ng k, phc v tt cho nhu cu gii tr a phng tin, nhiu ngi dng c th xem phim cht lng cao (HD, Full HD, Full HD 3D...), gi in thoi qua mng Internet (VoIP), ti tp tin dung lng ln ng thi... m cht lng dch v v tin cy vn lun t mc cao.

Hnh 3.13:Logo chng nhn sn phm t chun 802.11n

Bn cnh , chun 802.11n vn m bo kh nng tng thch ngc vi cc sn phm trc , chng hn, nu sn phm Wi-Fi chun n s dng ng thi hai tn s 2,4GHz v 5GHz th s tng thch ngc vi cc sn phm chun 802.11a/b/g.

Chun 802.11n c IEEE ph duyt a vo s dng chnh thc v cng c Hip hi Wi-Fi (Wi-Fi Alliance) kim nh v cp chng nhn cho cc sn phm t chun. Chng nhn chun Wi-Fi 802.11n l bc cp nht thm mt s tnh nng ty chn cho 802.11n d tho 2.0 (draft 2.0, xem thm bi vit ID: A0905_100) c Wi-Fi Alliance bt u t hi thng 6/2007; cc yu cu c bn (bng tng, tc , MIMO, cc nh dng khung, kh nng tng thch ngc) khng thay i. y l tin vui cho nhng ai ang s hu thit b t chng nhn 802.11n draft 2.0. Chng nhn Wi-Fi n vn m bo cho hn 700 sn phm c cp chng nhn draft 2.0 trc y (gm my tnh, thit b in t tiu dng nh tivi, my ch a phng tin (media server) v cc thit b mng). Tt c thit b c cp chng nhn dng draft n c iu kin s dng logo "Wi-Fi CERTIFIED n" m khng cn phi kim tra li (xem hnh 3.13).

Cc ci tin cng ngh ca chun 802.11n: a) OFDM tt hn

Yu cu u tin l phi s dng phng php ghp knh phn chia tng s trc giao OFDM c pht trin trn da trn 802.11a/g, s dng tc m ln nht cao hn v nhp di thng rng hn. iu c ngha l 802.11n h tr mt OFDM tt hn. Nhng thay i gip tng tc ln 65 Mbps so vi 52Mbps ca chun 802.11a v 802.11g.

Ci thin hiu nng ca MIMO: Mt trong nhng thnh phn c bit rng ri nht trong c t k thut ca bn d tho l MIMO. MIMO tn dng hin tng t nhin ca sng trung tn c gi l a ng: thng tin c pht xuyn qua tng, ca s v cc vt chn khc, anten thu tn hiu nhiu ln qua cc b nh tuyn khc nhau cc thi im khc nhau. Do , tn hiu a ng nguyn gc c th b "bp mo" dn n kh gii m v ko theo hiu nng Wi-Fi km. MIMO khai thc hin tng a ng vi k thut a phn chia theo khng gian (space-division multiplexing). Thit b pht WLAN chia gi d liu ra thnh nhiu phn, mi phn c gi l chui d liu (Spatial Stream) v pht tng chui d liu qua cc anten ring r n cc anten thu.

Hnh 3.14: Cng ngh MIMO

Hin ti, 802.11n cung cp n 4 chui d liu, cho d phn cng khng yu cu h tr nhiu nh th. (xem hnh 3.15)

Mi mu tng ng vi mt chui d liu

Hnh 3.15 :Cc chui d liu ca 802.11n Gp i s lng chui d liu ng ngha vi vic tng gp i tc , tuy nhin s ko theo cng sut tiu th tng, kh nng m rng km hn v gi thnh sn phm cao hn. Trong khi c t k thut 802.11n yu cu phi c ch tit kim nng lng (MIMO power-save). iu ny c ngha l ch nn s dng k thut a ng khi t c li ch v hiu nng.

Cc c im ni bt ca MIMO

C 2 tnh nng trong c t k thut 802.11n nhm tp trung ci thin hiu nng MIMO: cc to bp sng (Beam-forming) v s phn tp (Diversity). Beam-forming l k thut iu chnh tn hiu trc tip trn anten, gip tng vng ph sng v hiu sut bng cch hn ch nhiu. Diversity khai thc trn nhiu anten bng cch tng hp cc tn hiu u ra hoc chn tn hiu tt nht trong s cc anten. y l c t k thut quan trng do 802.11n c 4 anten, v th s gp phi trng hp thit b c s lng anten khc vi n. V v, my tnh xch tay dng 2 anten c th kt ni n access point (AP) c 3 anten. Trng hp ny, ch 2 chui d liu c dng d AP h tr n 3 chui d liu. Vi Diversity, thm cng nhiu anten cng tt. Thit b nhiu anten s c phm vi ph sng xa hn. V d, tn hiu pht ra ca 2 anten c th kt hp li thu mt chui d liu khong cch xa. tng ny c th c m rng kt hp cc tn hiu u ra ca 3 anten thu v 2 chui d liu c tc cao, vng ph sng rng... Diversity khng b gii hn trong

802.11n v c WLAN. Thc t, n c ci t cho cc sn phm chun 802.11a/b/g c 2 anten.

Cc c t k thut ch yu ca 802.11n (xem bng 3.3).

Bng 3.3: Cc c t k thut ch yu ca 802.11n

Tnh nng ngha Trng thi

OFDM tt hn H tr bng thng rng hn v tc m ha cao hn

tng tc t ti a 65Mbps Bt buc

a phn chia theo khng gian Ci tin hiu sut bng cch phn chia d liu thnh nhiu chui pht n nhiu anten Ty chn n ti a 4 chui d liu

Diversity Khai thc s c mt ca nhiu anten ci tin tm ph sng v tin cy. Hnh thc ny c thc thi khi s Ty chn n ti a 4 anten

lng anten u thu cao hn s lng anten u pht.

MIMO tit kim nng lng Gii hn cng sut tiu th bt li ca MIMO bng cch ch s dng nhiu anten khi cn thit Quy nh

Cc knh 40MHz Tng tc gp i bng cch tng rng bng thng t

20MHz ln 40MHz Ty chn

S kt hp Ci tin hiu sut bng cch cho php nhiu gi d liu pht tng tc gia s truyn thng trn cao Quy nh

Gim Inter-frame

Spacing (RIFS) Mt trong nhng c im chung trong thit k 802.11n-n l ci tin hiu sut. Thi gian tr trong vic pht OFDM ngn hn so vi 802.11a, 802.11g Quy nh

Ch

Greenfield Ci tin hiu sut bng cch loi cc thit b 802.11a/b/g ra khi mng 802.11n-n Ty chn hin thi

b) Ci tin lu lng v tc truyn d liu

Mt ty chn khc trong 802.11n d tho l nhn i tc bng cch tng bng thng knh truyn WLAN t 20MHz ln 40MHz. iu ny lm gim s lng knh gy bt li cho cc thit b khc. Tn s 2,4GHz c khng gian cho 3 knh 20MHz khng chng ln nhau (nonoverlapping), cn knh 40MHz khng c nhiu khng gian cho cc thit b khc tham gia vo mng hay truyn d liu trn cng khu vc vi chng. Do , vic chn knh 40MHz s ci thin hiu nng cho ton WLAN.

Cc c im k thut ca IEEE 802.11 (xem bng 3.4).

Bng 3.4: Cc c im k thut ca IEEE 802.11n

Cc chun 802.11a 802.11b 802.11g 802.11n

Nm ph chun Thng 7/1999 Thng 7/1999 Thng 6/2003 Thng 6/2007

Tc ti a 54Mbps 11Mbps 54Mbps 300Mbps

Khong

cch ti a 100m 100m 100m 150m

K thut iu ch OFDM DSSS hay CCK DSSS hay

CCK hay

OFDM DSSS hay CCK hay

OFDM

Di tn s trung tn

(RF) 5GHz 2,4GHZ 2,4GHZ 2,4GHz hay 5GHz

Chui d liu 1 1 1 1, 2, 3 hay 4

rng bng thng 20MHz 20MHz 20MHz 20MHz hay 40MHz

S knh khng

chng ln nhau 3 3 23 3 (2,4GHz)

23 (5GHz)

Ngun can nhiu Bluetooth, l vi sng, thit b quan st b t xa... Bluetooth, l vi sng, thit b quan st b t xa... in thoi m bng con Tng t 802.11b/g

(2,4GHz)Tng t

802.11a (5GHz)

c) Hot ng hiu qu cng WLAN hin hnh

c tnh k thut ca 802.11n d tho c kh nng tng thch vi chun trc . Access

Point 802.11n s tng thch ngc vi cc thit b chun 802.11a tn s 5GHz cng nh chun 802.11b v 802.11g tn s 2,4GHz. 802.11n s h tr tt hn vi ch "mixed" ca 802.11g. Thc t, hiu sut mng ph thuc vo t l bng thng c s dng pht d liu ch khng phi gi tin u tin (Overhead) hay cc giao thc c s dng qun l vic truyn thng mng. Mt trong cc tnh nng quan trng nht trong c t k thut 802.11n ci tin hiu nng ch hp nht (mixed mode) l s tp hp. Thay v gi tng frame d liu, my trm pht mt gi gm nhiu frame chung vi nhau. Do , vic tp hp d liu s hiu qu hn (xem hnh 3.16).

Hnh 3.16: S tp hp ci thin hiu qu trong ch Mixed

iu ny d dng cho cc thit b 802.11n cng tn ti vi 802.11g v 802.11a v tt c u dng phng thc iu ch OFDM. V th, c nhiu tnh nng trong c t k thut ca thit b nh tng hiu nng cho cc mng ch s dng OFDM. Chng hn nh tnh nng gim khong khng gia frame (Reduced Inter-Frame Spacing, hay RIFS), chi tit ny c thi gian tr hon gia 2 ln pht ngn.

t hiu nng tt, c t k thut ca 802.11n cung cp mt ch gi l "cnh ng xanh" (Greenfield) - ch cc thit b 802.11n hot ng trong mng.

d) Hin thc phn cng ca chun IEEE 802.11n Cc hng sn xut chip Wi-Fi ln nh Atheros, Broadcom xut xng cc chip h tr chun 802.11n v t c chng nhn ca Wi-Fi Alliance, chng hn chip BCM943224HMS, BCM94313HMGB ca Broadcom, chip AR5B195 ca Atheros. Cc chip ny h tr y 4

chun 802.11a/b/g/n vi rt nhiu tnh nng ty chn (Short Guard Interval, Greenfield Preamble, A-MPDU, STBC, 40MHz trn tn s 2,4GHz hay 5GHz...), ch bo mt WPA2 cao cp, tnh nng WMM (Wi-Fi Multimedia) h tr gii tr a phng tin v cc tnh nng tin ch khc nh ci t m ha Wi-Fi nhanh theo dng PIN (Personal Identification Number) hay PBC (Push button configuration)...

u, nhc im ca 802.11n:

u im ca 802.11n l tc nhanh nht, vng ph sng tt nht; tr khng ln hn chng nhiu t cc tc ng ca mi trng.

Nhc im ca 802.11n gi cao hn 802.11g; s dng nhiu lung tn hiu c th gy nhiu vi cc thit b 802.11b/g k cn.

Tuy nhin, vi u im nhiu hn khuyt im nh vy, tng lai chun IEEE 802.11n s ph bin trn ton cu v thay th cc chun IEEE 802.11 khc.