CHAPTER3 - C H APT ER 3: WI RELESS LOCAL AREA N ET WORKS I...

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Unformatted text preview: C H APT ER 3: WI RELESS LOCAL AREA N ET WORKS I . F. Akyi l di z Br oa dba nd & Wi r el ess N etwor ki ng La bor a tor y School of El ectr i ca l a nd Computer Engi neer i ng Geor gi a I nsti tute of T echnol ogy T el : 404-894-5141; Fa x: 404-894-7883 E ma i l : i a n@ ece.ga tech.edu Web: http://www.ece.ga tech.edu/r esea r ch/l a bs/bwn WLAN s a r e H OT !!! s WLAN s (Wi Fi s) WLAN a r e becomi ng T H E a ccess poi nts to connect to the I nter net p oi a ccess chea per , fa ster a nd s Wi r el ess I nter net Wi fa r ther r ea chi ng fa I FA’2008 ECE4607 2 WLAN s (Some Fa cts) T he number of publ i c Wi -Fi hot spots wi l l gr ow t o 167K by 2008, up fr om vi r tua l l y ni l i n 2001. ( Ga r tner D a ta quest pr ojecti ons) Wi r el ess publ i c a r ea a ccess ser vi ce r evenue i s Wi expected to gr ow fr om $3.9 mi l l i on i n 2001 to $2 bi l l i on i n 2008. bi I FA’2008 ECE4607 3 M a r kets of I nter est Ai r por t, Conventi on Center s, Ca fes, T r a i n Sta ti ons, Ai a nd Ai r pl a nes L uftha nsa /Br i ti sh Ai r wa ys offer ser vi ce ($15-30); Luftha N i ppon Ai r wa ys, Si nga por e Ai r l i nes, Chi na Ai r l i nes.. Wa shi ngton Sta te Fer r i es offer Wi Fi ser vi ce on Wa va r i ous commuter r outes va H a r bor Ba y Fer r y l i ne i n CA pr ovi des untether ed I nter net a ccess on i ts l one r oute. I FA’2008 ECE4607 4 M a r kets of I nter est O n the open sea s, Whea t Wi r el ess Ser vi ces i n Reston, On V a ., i s ta r geti ng owner s of r ecr ea ti ona l ya chts a nd commer ci a l vessel s (T el eSea Gol d ser vi ce pl a n for boa ts wi thi n 30 mi l es or so of shor e; $500 monthl y on t op of a n i nsta l l a ti on fee of $7.5K) L i mousi ne compa ni es to pr ovi de I nter net a ccess to Li customer s i n Boston, Chi ca go, LA, N Y, SF a nd Wa shi ngton. (I nM oti on T echnol ogy) . ECE4607 I FA’2008 5 WLAN s – Cur r ent U se WLAN C ur I FA’2008 H ome wi r el ess networ ks Enter pr i se wi r el ess networ ks P ubl i c a ccess Publ H ospi ta l s Wa r ehouses Wa Consul ti ng a nd a udi t tea ms D yna mi c envi r onments, a d a genci es, etc. U ni ver si ti es H i stor i c bui l di ngs, ol der bui l di ngs M eeti ng r ooms Reta i l stor es R esta ur a nts a nd ca r r enta l a genci es Resta ECE4607 6 Some Fa cts By 2006, mor e tha n 1/3r d of I nter net user s wi l l ha ve By I nter net connecti vi ty thr ough a wi r el ess ena bl ed devi ce ( 750 mi l l i on user s)!!! (Sour ce: I nter ma r ket gr oup) ( Sour B y the yea r 2004 r evenue fr om wi r el ess da ta r ea ched mor e By t ha n $34B, a nd by the yea r 2010 the number of wi r el ess da ta subscr i ber s wi l l hi t 3B!! da I FA’2008 ECE4607 7 D esi gn Goa l s for Wi r el ess LAN s Gl ( must – G l oba l , sea ml ess oper a ti on (must sel l i n a l l countr i es) Low – L ow power for ba tter y use ( power sa vi ng modes a nd power ma na gement functi ons) p ower – N o speci a l per mi ssi ons or l i censes needed to use them Robust (a – R obust tr a nsmi ssi on technol ogy (a voi d i nter fer ence) – Si mpl i fi ed sponta neous cooper a ti on a t meeti ngs I FA’2008 ECE4607 8 D esi gn Goa l s for Wi r el ess LAN s – Ea sy to use for ever yone, si mpl e ma na gement Pr – P r otecti on of i nvestment i n wi r ed networ ks ( i nter oper a bl e wi th wi r ed LAN s) wi – – – Secur i ty (no one shoul d be a bl e to r ea d my da ta ), Secur ( no P r i va cy (no one shoul d be a bl e to col l ect user pr ofi l es), Pr ( no Sa fety (l ow r a di a ti on) Sa (l I FA’2008 ECE4607 9 WI RELESS LAN s Ar chi tectur e I nfr a str uctur e N etwor k AP AP Wi r ed N etwor k AP: Access Poi nt AP Ad H oc N etwor k I FA’2008 ECE4607 10 Wi r el ess LAN - I EEE 802.11 Refer ence Wi Ar chi tectur e Ar T he ter mi nol ogy a nd some of the speci fi c fea tur es a r e uni que to thi s sta nda r d a nd a r e not r efl ected i n a l l commer ci a l pr oducts. sta H owever , i t i s useful to be f a mi l i a r wi th the sta nda r d si nce i ts fea tur es a r e r epr esenta ti ve of the Wi r el ess LAN ca pa bi l i ti es r equi r ed. I FA’2008 ECE4607 11 D eta i l ed Refer ence Ar chi tectur e of Wi r el ess LAN s L AN Sta ti on (ST A) – T er mi na l wi th a ccess mecha ni sms to the w i r el ess medi um a nd r a di o conta ct to the a ccess poi nt Ba si c Ser vi ce Set (BSS) Gr – G r oup of sta ti ons usi ng the sa me r a di o fr equency fr Access Poi nt Sta – Sta ti on i ntegr a ted i nto the wi r el ess LAN a nd the di str i buti on system Por ta l – Br i dge to other (wi r ed) networ ks D i str i buti on System – I nter connecti on networ k to for m one l ogi ca l networ k (ESS: Extended Ser vi ce Set) ba sed on sever a l BSS on 12 ST A1 BSS1 Access P oi nt Por ta l D i str i buti on System Access Point BSS2 STA2 STA3 ESS I FA’2008 ECE4607 Refer ence Ar chi tectur e B a si c Ser vi ce Set (BSS) consi sts of some number of sta ti ons wi th Ba t he sa me M AC pr otocol a nd competi ng for a ccess to the sa me sha r ed medi um. sha A B SS ma y be i sol a ted or i t ma y connect to a ba ckbone BSS di str i buti on system thr ough a n a ccess poi nt. di AP functi ons a s a br i dge. AP T he M AC pr otocol ma y be ful l y di str i buted or contr ol l ed by a centr a l coor di na ti on functi on housed i n the AP. I FA’2008 ECE4607 13 Refer ence Ar chi tectur e Ba si c Ser vi ce Set (BSS) <-> CELL E xtended Ser vi ce Set (ESS) consi sts of two or Extended m or e BSSs i nter connected by a di str i buti on system. D i str i buti on System a w i r ed ba ckbone LAN . E SS a ppea r s a s a si ngl e l ogi ca l LAN to the ESS l ogi ca l l i nk contr ol (LLC) l evel . I FA’2008 ECE4607 14 Refer ence Ar chi tectur e T hr ee types of sta ti ons ba sed on mobi l i ty: N o T r a nsi ti on: A sta ti on i s ei ther sta ti ona r y or moves onl y wi thi n the d i r ect communi ca ti on r a nge of the communi ca ti ng sta ti ons of a si ngl e BSS. si BSS T r a nsi ti on: A sta ti on moves fr om one BSS to a nother BSS wi thi n t he sa me ESS. ( I n thi s ca se, del i ver y of da ta to the sta ti on r equi r es tha t the a ddr essi ng ca pa bi l i ty be a bl e to r ecogni ze the new l oca ti on of the sta ti on). sta ESS T r a nsi ti on : A sta ti on moves fr om a BSS i n one ESS to a BSS w i thi n a nother ESS. I FA’2008 ECE4607 15 Pr otocol Ar chi tectur e M obi l e T er mi na l I nfr a str uctur e N etwor k Appl i ca ti on Fi xed T er mi na l Access Poi nt Appl i ca ti on T CP IP LLC M AC PH Y T CP IP LLC M AC PH Y 802.3 MAC 802.3 PHY LLC 802.3 M AC 802.3 PH Y I FA’2008 ECE4607 16 F a mi l y of Wi r el ess LAN (WLAN ) Fa Sta nda r ds 802.11 802.11 802.11a – 54M bps; 5GH z ( Ra ti fi ed i n 1999) 802.11b - 11M bps; 2.4GH z ( r a ti fi ed i n 1999) 802.11d - Addi ti ona l Regul a tor y D oma i ns 802.11e - Qua l i ty of Ser vi ce 802.11f - I nter -Access Poi nt Pr otocol (I APP) 802.11g - H i gher D a ta r a te (>20M bps) 2.4GH z 802.11h - D yna mi c Fr equency Sel ecti on a nd T r a nsmi t Power C ontr ol M echa ni sms – 802.11i - Authenti ca ti on a nd Secur i ty – 802.11n – Ver y H i gh Ba ndwi dth (10-20 ti mes mor e) – – – – – – – I FA’2008 ECE4607 17 WLAN : I EEE 802.11b D a ta r a te 1, – 1, 2, 5.5, 11 M bps, dependi ng on SN R – U ser da ta r a te ma x. a ppr ox. 6 M bps T r a nsmi ssi on r a nge – 300m outdoor , 30m i ndoor – M a x. da ta r a te ~10m i ndoor Fr equency – Fr ee 2.4 GH z I SM -ba nd I FA’2008 ECE4607 18 WLAN : I EEE 802.11b Li mi ted Secur i ty Ava i l a bi l i ty M a ny pr oducts, ma ny vendor s Qua l i ty of Ser vi ce – T yp. Best effor t, no gua r a ntees (unl ess pol l i ng i s used, l i mi ted suppor t i n pr oducts) suppor I FA’2008 ECE4607 19 WLAN : I EEE 802.11b Speci a l Adva nta ges/D i sa dva nta ges – Adva nta ge: m a ny i nsta l l ed systems, l ot of exper i ence, a va i l a bl e wor l dwi de, fr ee I SM exper ba nd, ma ny vendor s, i ntegr a ted i n l a ptops, ba si mpl e system si – D i sa dva nta ge: h ea vy i nter fer ence on I SM -ba nd, no ser vi ce gua r a ntees, sl ow r el a ti ve speed onl y no I FA’2008 ECE4607 20 WLAN : I EEE 802.11a D a ta r a te – 6, 9, 12, 18, 24, 36, 48, 54 M bps, dependi ng on SN R – U ser thr oughput (1500 byte pa ckets): 5.3 (6), 18 (24), 24 (36), 32 (54) – 6, 12, 24 M bps ma nda tor y – 100m outdoor , 10m i ndoor E .g., 54 M bps up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to E.g., 30m, 18 up to 40 m, 12 up to 60 m T r a nsmi ssi on r a nge I FA’2008 ECE4607 21 WLAN : I EEE 802.11a Fr equency Li mi ted Secur i ty – Fr ee 5.15-5.25, 5.25-5.35, 5.725-5.825 GH z I SM -ba nd Qua l i ty of Ser vi ce – T yp. best effor t, no gua r a ntees (sa me a s a l l 802.11 pr oducts) pr Ava i l a bi l i ty – Some pr oducts, some vendor s I FA’2008 ECE4607 22 WLAN : I EEE 802.11a Speci a l Adva nta ges/D i sa dva nta ges – Adva nta ge: f i ts i nto 802.x sta nda r ds, fr ee I SM -ba nd, a va i l a bl e, si mpl e system, uses l ess cr owded 5 GH z ba nd cr – D i sa dva nta ge: str onger sha di ng due to hi gher fr equency, no QoS hi I FA’2008 23 ECE4607 WLAN s – 802.11 Compa ti bi l i ty WLAN C ompa 802.11a a nd 802.11b sha r e the sa me M AC l a yer Si gni fi ca nt di ffer ences a t the physi ca l l a yer . 802.11b: – 802.11b: 2.4 GH z, I SM ba nd, – 802.11a : 5 GH z, U -N I I ba nd Possi – P ossi bl e to oper a te both on the sa me networ k concur r entl y (usi ng the sa me a ccess poi nts) concur I nter oper a bi l i ty WECA – WECA (Wi r el ess Ether net Compa ti bi l i ty Al l i a nce): or ga ni za ti on behi nd Wi -Fi tha t cer ti fi es pr oducts meeti ng the 802.11b speci fi ca ti on m eeti I FA’2008 ECE4607 24 I FA’2008 ECE4607 25 I FA’2008 ECE4607 26 WLAN : I EEE 802.11 – Futur e D evel opments 802.11d: Regul a tor y D oma i n U pda te – compl eted 802.11d: compl 802.11e: M AC Enha ncements – QoS – ongoi ng 802.11e: ongoi E nha nce the cur r ent 802.11 M AC to expa nd suppor t for a ppl i ca ti ons wi th Qua l i ty of Ser vi ce r equi r ements, a nd i n t he ca pa bi l i ti es a nd effi ci ency of the pr otocol . 802.11f: I nter -Access Poi nt Pr otocol – ongoi ng ongoi E sta bl i sh a n I nter -Access Poi nt Pr otocol for da ta excha nge vi a the di str i buti on system. t he I FA’2008 ECE4607 27 WLAN : I EEE 802.11 – Futur e D evel opments 802.11g: D a ta Ra tes > 20 M bps a t 2.4 GH z; 54 M bps, OFD M – ongoi ng 54 802.11h: Spectr um M a na ged 802.11a – ongoi ng ongoi 802.11i : Enha nced Secur i ty M echa ni sms – ongoi ng ongoi E nha nce the cur r ent 802.11 M AC to pr ovi de i mpr ovements i n secur i ty. I FA’2008 ECE4607 28 WLAN : I EEE 802.11 – Futur e D evel opments 802.11n: U nder devel opment 802.11n: * A hi gh-ba ndwi dth extensi on hi * I ncr ea se of the speed of WLAN s by 10 to 20 ti mes * U ncl ea r when thi s wi l l be fi na l i zed Study Gr oups 5 G H z (ha r moni za ti on ET SI /I EEE) – cl osed GH R a di o Resour ce M ea sur ements – sta r ted sta H i gh T hr oughput – sta r ted sta I FA’2008 ECE4607 29 WLAN s – 802.11 Pr otocol Ar chi tectur e Rea l T i me T r a ffi c N or ma l D a ta T r a ffi c (Asynchr onous) M AC Poi nt Coor di na ti on F uncti on (PCF) D i str i buted Coor di na ti on Functi on (D CF) Physi ca l La yer (PH Y) I FA’2008 ECE4607 30 I EEE 802.11: PH YSI CAL LAYERPhysi ca l M edi um Speci fi ca ti on P hysi T hr ee Physi ca l M edi a : I N FRARED N a r r owba nd M i cr owa ve Spr ea d Spectr um I FA’2008 ECE4607 31 I nfr a r ed I nfr a r ed si gna l s used to tr a nsmi t da ta (si mi l a r to T V r emotes!) H i gher da ta r a tes possi bl e (tha n spr ea d spectr um) L i ne of si ght poi nt-to-poi nt confi gur a ti on r equi r ed (or r efl ecti on sur fa ce tha t r efl ects si gna l s) ( or T oo sensi ti ve to obsta cl es, l i ne-of-si ght r equi r ement, etc. I FA’2008 ECE4607 32 I nfr a r ed 850-950 nm, di ffuse l i ght (to a l l ow poi nt-to850-950 mul ti poi nt communi ca ti on) m ul 10 m ma xi mum r a nge wi th no sunl i ght or hea t i nter fer ence I FA’2008 ECE4607 33 N a r r owba nd M i cr owa ve T ypi ca l l y used to l i nk two WLAN s together (e.g., to l i nk WLAN s i n two bui l di ngs) t wo M i cr owa ve di shes r equi r ed a t both ends of l i nk U nl i ke spr ea d spectr um whi ch oper a tes i n the unl i censed I SM ba nd, na r r owba nd mi cr owa ve r equi r es FCC l i censi ng na Excl usi ve l i cense typi ca l l y effecti ve wi thi n a 17.5 mi l e r a di us E xcl I FA’2008 ECE4607 34 Spr ea d Spectr um D i str i buted si gna l s over mul ti pl e fr equenci es (to a voi d ea vesdr oppi ng or ja mmi ng) Fr equency H oppi ng Spr ea d Spectr um (FH SS) Fr – Sender tr a nsmi ts over a seemi ngl y r a ndom ser i es of fr equenci es – I ntended r ecei ver a wa r e of sequence of fr equenci es a nd hops a ccor di ngl y Al – Al l ows the coexi stence of mul ti pl e networ ks i n the sa me a r ea by usi ng di ffer ent hoppi ng sequences u si I FA’2008 ECE4607 35 Spr ea d Spectr um D i r ect Sequence Spr ea d Spectr um (D SSS) Sender – Sender tr a nsmi ts r edunda nt i nfor ma ti on ca l l ed “chi ps” between a ctua l da ta bi ts “chi – I ntended r ecei ver a wa r e of spr ea d r emoves r edunda nt i nfor ma ti on a ccor di ngl y I FA’2008 ECE4607 36 C a n we use CSM A a s M AC Pr otocol Ca for Wi r el ess LAN s? f or H i dden a nd Exposed T er mi na l P r obl ems ma ke the use of CSM A a n i neffi ci ent techni que!! ma I FA’2008 ECE4607 37 H i dden T er mi na l Pr obl em Col l i si on A B C A ta l ks to B ta C senses the cha nnel senses C d oes not hea r A’s tr a nsmi ssi on (out of r a nge) ( out C ta l ks to B ta Si gna l s fr om A a nd B col l i de ECE4607 38 I FA’2008 Exposed T er mi na l Pr obl em N ot possi bl e A B C D B ta l ks to A ta C wa nts to ta l k to D wa C senses cha nnel a nd fi nds i t to be busy senses C sta ys qui et (when i t coul d ha ve i dea l l y sta tr a nsmi tted) tr ECE4607 39 I FA’2008 H i dden a nd Exposed T er mi na l Pr obl em I mpl i ca ti ons Pr H i dden T er mi na l – M or e col l i si ons – Wa sta ge of r esour ces Exposed T er mi na l E xposed – U nder uti l i za ti on of cha nnel – Lower effecti ve thr oughput I FA’2008 ECE4607 40 WLAN s – I EEE 802.11 Pr otocol Ar chi tectur e Rea l T i me T r a ffi c C ontenti on Fr ee Ser vi ce N or ma l D a ta T r a ffi c (Asynchr onous) Contenti on Ser vi ce M AC Poi nt Coor di na ti on F uncti on (PCF) D i str i buted Coor di na ti on Functi on (D CF) Physi ca l La yer (PH Y) I FA’2008 ECE4607 41 D i str i buted M ode: D i str i buted Coor di na ti on Functi on (D CF) C oor * B a sed on the CSM A/CA pr otocol * U ses a contenti on a l gor i thm to pr ovi de a ccess to a l l tr a ffi c. * Or di na r y a synchr onous tr a ffi c uses D CF di r ectl y. Or I FA’2008 ECE4607 42 Coor di na ted M ode: Poi nt Coor di na ti o F uncti on (PCF) Functi * Suppor ts R ea l T i me T r a ffi c * B a sed on pol l i ng whi ch i s contr ol l ed by a C entr a l i zed Poi nt Coor di na tor . * U ses a centr a l i zed M AC a l gor i thm a nd pr ovi des contenti on-fr ee ser vi ce. * P CF i s bui l t on top of D CF a nd expl oi ts fea tur es of D CF to a ssur e a ccess for i ts user s. I FA’2008 ECE4607 43 D CF a nd PCF REM ARK: R EM B oth the D CF a nd PCF ca n oper a te concur r entl y wi thi n the sa me BSS to pr ovi de a l ter na ti ve contenti on a nd contenti on-fr ee per i ods p er I FA’2008 ECE4607 44 802.11 - M AC La yer Over vi ew (M – D FWM AC-D CF CSM A/CA (M a nda tor y) ( D i str i buted Founda ti on Wi r el ess M edi um Access (D Contr ol - D i str i buted Coor di na ted Functi on CSM A/CA) C ontr – D FWM AC-D CF w/RT S/CT S ( Opti ona l ) D i str i buted Founda ti on Wi r el ess M AC Avoi ds H i dden T er mi na l pr obl em ( Opti – D FWM AC- PCF (Opti ona l ) I FA’2008 ECE4607 Access poi nt pol l s ter mi na l s a ccor di ng to a l i st 45 D FWM AC-D CF C SM A/CA (FLOWCH ART ) ( FLOWCH ( D i str i buted Founda ti on Wi r el ess M edi um Access Contr ol (D D i str i buted Coor di na ted Functi on CSM A/CA (D FWM AC-D CF) for I EEE 802.11) I FA’2008 ECE4607 46 D FWM AC CSM A/CA C SM I FS I FS Contenti on Wi ndow (Ra ndomi zed Ba ck-off M echa ni sm) M edi um Busy ta ti on enses he cha nnel n d i t i s i dl e N ext Fr a me t D i r ect a ccess i f m edi um i s fr ee ≥ I FS Sl ot T i me I FA’2008 ECE4607 47 D FWM AC CSM A/CA C SM sta – A sta ti on wi th a fr a me to tr a nsmi t senses the medi um (cha nnel ). – I F I D LE w a i ts to see i f the cha nnel r ema i ns i dl e for a ti me equa l to I FS. I f so, the sta ti on ma y tr a nsmi t i mmedi a tel y. equa – I F BU SY ( ei ther beca use the sta ti on i ni ti a l l y fi nds the cha nnel busy or beca use the cha nnel becomes busy dur i ng the I FS i dl e t i me), the sta ti on defer s tr a nsmi ssi on a nd conti nues to moni tor t he I FA’2008 ECE4607 48 D FWM AC CSM A/CA C SM Once – O nce the cur r ent tr a nsmi ssi on i s over , the sta ti on del a ys a nother I FS. – I f the medi um r ema i ns i dl e for thi s per i od, the sta ti on ba cks off usi ng a bi na r y exponenti a l ba ckoff scheme a nd a ga i n keeps bi sensi ng the medi um. – T he sta ti on pi cks up a r a ndom number of sl ots (the i ni ti a l va l ue of ba ckoff counter ) wi thi n a contenti on wi ndow t o wa i t befor e tr a nsmi tti ng i ts fr a me. I FA’2008 ECE4607 49 D FWM AC CSM A/CA C SM • M AC r uns a r a ndom number gener a tor to set a BACKOFF CLOCK for ever y contendi ng sta ti on. contendi When • When the CON T EN T I ON WI N D OW sta r ts i n whi ch a l l sta ti ons ha vi ng p a ckets for tr a nsmi ssi on r un down thei r BACKOFF cl ocks. f or * T he fi r st sta ti on tha t expi r es i ts cl ock sta r ts tr a nsmi ssi on. Other • O ther ter mi na l s sense the new tr a nsmi ssi on a nd fr eeze thei r cl ocks to be r esta r ted a fter the compl eti on of the cur r ent tr a nsmi ssi on i n the next contenti on per i od. contenti I FA’2008 ECE4607 50 D FWM AC CSM A/CA Al gor i thm C SM – I f Col l i si ons (Contr ol or D a ta ) B i na r y exponenti a l i ncr ea se (doubl i ng) of CW; C W; L ength of ba ckoff ti me i s exponenti a l l y i ncr ea sed a s the sta ti on goes thr ough successi ve r etr a nsmi ssi ons. I FA’2008 ECE4607 51 I nter -fr a me Spa ces (I FS) D I FS D I FS PI FS M edi um Busy SI FS Contenti on Wi ndow N ext Fr a me t D i r ect a ccess i f m edi um i s fr ee ≥ D I FS I FA’2008 ECE4607 52 D FWM AC-D CF CSM A/CA I FA’2008 ECE4607 53 I nter -fr a me Spa ces (I FS) P r i or i ti es a r e defi ned thr ough di ffer ent i nter fr a me spa ces – SI FS (Shor t I nter Fr a me Spa ci ng) H i ghest pr i or i ty pa ckets such a s ACK, CT S, p ol l i ng r esponse U sed for i mmedi a te r esponse a cti ons I FA’2008 ECE4607 54 I nter -fr a me Spa ces (I FS) – PI FS (PCF I FS) - Poi nt Coor di na ti on Functi on I nter F r a me spa ci ng M edi um pr i or i ty, for r ea l ti me ser vi ce usi ng P CF SI FS + One sl ot ti me U sed by centr a l i zed contr ol l er i n PCF scheme when I FA’2008 ECE4607 55 I nter -fr a me Spa ces (I FS) – D I FS (D CF, D i str i buted Coor di na ti on Functi on I FS) L owest pr i or i ty, for a synchr onous da ta ser vi ce SFI S + T wo sl ot ti mes U sed a s mi ni mum del a y of a synchr onous fr a mes contendi ng for a ccess contendi I FA’2008 ECE4607 56 D FWM AC-D CF CSM A/CA wi th ACK C SM – Sta ti on ha s to wa i t for D I FS befor e sendi ng da ta – Recei ver ACKs i mmedi a tel y (a fter wa i ti ng for SI FS < D I FS) i f the pa cket wa s r ecei ved cor r ectl y (CRC)) SI – Recei ver tr a nsmi ts ACK wi thout sensi ng the medi um. – I f ACK i s l ost, r etr a nsmi ssi on done. Automa – Automa ti c r etr a nsmi ssi on of da ta pa ckets i n ca se of tr a nsmi ssi on er r or s tr I FA’2008 ECE4607 57 D FWM AC-D CF CSM A/CA wi th ACK D I FS Sender D a ta SI FS Recei ver Other Sta ti ons ACK D I FS Contenti on W t Wa i ti ng T i me ECE4607 I FA’2008 Contenti on Wi ndow 58 D FWM AC-D CF C SM A/CA wi th RT S/CT S T r a nsmi tter sends a n RT S (Request T o Send) a fter medi um ha s RT been i dl e for ti me i nter va l mor e tha n D I FS. been R ecei ver r esponds wi th CT S (Cl ea r T o Send) a fter medi um ha s Recei ( Cl been i dl e for SI FS. been T hen da ta i s tr a nsmi tted. R T S/CT S i s used for r eser vi ng cha nnel for da ta tr a nsmi ssi on so RT tha t the col l i si on ca n onl y occur i n contr ol messa ge. t ha I FA’2008 ECE4607 59 D FWM AC-D CF C SM A/CA wi th RT S/CT S RT U se shor t si gna l i ng pa ckets f or Col l i si on Avoi da nce shor C ol – RT S (Request T o Send) Pa cket (20 Bytes): A sender r equests the r i ght to send fr om a r ecei ver wi th a shor t RT S pa cket befor e i t sends a da ta pa cket befor – CT S (Cl ea r T o Send) Pa cket (16 Bytes): T he r ecei ver gr a nts the r i ght to send a s soon a s i t i s r ea dy to r ecei ve T hey conta i n: (Sender Addr ess; Recei ver Addr ess; Pa cket Si ze) I FA’2008 ECE4607 60 D FWM AC-D CF D I FS C SM A wi th RT S/CT S Time SIFS SIFS RT S SIFS CT S Source D esti na ti on D a ta ACK DIFS Contenti on Wi ndow Other Defer Access Next Frame Ba ckoff After D efer I FA’2008 ECE4607 61 H i dden T er mi na l Pr obl em A RT S CT S D AT A B CT S C I FA’2008 A sends RT S sends B sends CT S sends C over hea r s CT S over C i nhi bi ts i ts own tr a nsmi tter A successful l y sends D AT A to B successful ECE4607 62 H i dden T er mi na l Pr obl em H ow does C know how l ong to wa i t befor e i t ca n a ttempt a tr a nsmi ssi on? tr A i ncl udes l ength of D AT A tha t i t wa nts to send i n the RT S pa cket pa B i ncl udes thi s i nfor ma ti on i n the CT S pa cket C , when i t over hea r s the CT S pa cket, r etr i eves the l ength i nfor ma ti on a nd uses i t to set the i nhi bi ti on ti me I FA’2008 ECE4607 63 Exposed T er mi na l Pr obl em A RT S CT S B RT S C Ca nnot hea r CT S T x not i nhi bi ted D B sends RT S to A (over hea r d by C) sends A sends CT S to B sends C ca nnot hea r A’s CT S ca C a ssumes A i s ei ther down or out of r a nge C does not i nhi bi t i ts tr a nsmi ssi ons to D d oes ECE4607 64 I FA’2008 Col l i si ons Sti l l possi bl e – RT S pa ckets ca n col l i de! Sti B i na r y exponenti a l ba ckoff per for med by sta ti ons tha t exper i ence RT S col l i si ons exper R T S col l i si ons not a s ba d a s da ta col l i si ons i n CSM A ( si nce RT S pa ckets a r e typi ca l l y much sma l l er tha n D AT A pa ckets) I FA’2008 ECE4607 65 D FWM AC-D CF C SM A/CA wi th RT S/CT S (N etwor k Al l oca ti on Vector (N AV)) (N B oth Physi ca l Ca r r i er Sensi ng a nd Vi r tua l C a r r i er P hysi Vi Sensi ng Sensi used i n 802.11 u sed I f ei ther functi on i ndi ca tes tha t the medi um i s busy, 802.11 tr ea ts the cha nnel to be busy 802.11 Vi r tua l Ca r r i er Sensi ng i s pr ovi ded by the Vi N AV (N etwor k Al l oca ti on Vector ) I FA’2008 ECE4607 66 D FWM AC-D CF C SM A/CA wi th RT S/CT S (N etwor k Al l oca ti on Vector (N AV)) (N M ost 802.11 fr a mes ca r r y a dur a ti on fi el d w hi ch i s used d ur to r eser ve the medi um for a fi xed ti me per i od T x sets the N AV to the ti me for whi ch i t expects to use the medi um t he Other sta ti ons sta r t counti ng down fr om N AV to 0 O ther As l ong a s N AV > 0, the medi um i s busy As I FA’2008 ECE4607 67 D FWM AC-D CF C SM A/CA wi th RT S/CT S (N etwor k Al l oca ti on Vector (N AV)) (N C H AN N EL VI RT U ALLY BU SY a N AV SI GN AL i s tur ned on! T he tr a nsmi ssi on wi l l be del a yed unti l the N AV si gna l ha s di sa ppea r ed. di When the cha nnel i s vi r tua l l y a va i l a bl e, then M AC checks for PH Y condi ti on of the cha nnel . condi I FA’2008 ECE4607 68 I l l ustr a ti on SI FS Sender RT S SI FS D AT A SI FS Recei ver CT S ACK N AV RT S CT S I FA’2008 ECE4607 69 CSM A/CA wi th RT S/CT S (N AV) RT CT to – I f r ecei ver r ecei ves RT S, i t sends CT S (Cl ea r to Send) a fter SI FS. CT d ur – C T S a ga i n conta i ns dur a ti on fi el d a nd a l l sta ti ons r ecei vi ng thi s p a cket need to a djust thei r N AV n eed Sender – Sender ca n now send da ta a fter SI FS, a cknowl edgement vi a ACK by r ecei ver a fter SI FS I FA’2008 ECE4607 70 CSM A/CA wi th RT S/CT S (N AV) Ever – E ver y sta ti on r ecei vi ng the R T S t ha t i s not a ddr essed to i t, w i l l go to the Vi r tua l Ca r r i er Sensi ng M ode f or the enti r e Vi p er i od i denti fi ed i n the RT C/CT S communi ca ti on, by setti ng thei r N AV si gna l on. setti – N et Al l oca ti on Vector (N AV) i s set i n a ccor da nce wi th the dur a ti on of the fi el d d ur (N – ( N AV speci fi es the ea r l i est poi nt a t whi ch the sta ti on ca n tr y to a ccess the medi um) tr I FA’2008 ECE4607 71 CSM A/CA wi th RT S/CT S (N AV) – T hus, the sour ce sta ti on sends i ts pa cket wi thout contenti on. contenti After – After compl eti on of the tr a nsmi ssi on, the d esti na ti on ter mi na l sends a n ACK a nd N AV si gna l i s ter mi na ted, openi ng the contenti on for other user s. u ser I FA’2008 ECE4607 72 CSM A/CA wi th RT S/CT S (N AV) DI FS Sender RT S da ta SI FS SI FS CT S SI FS Recei ver ACK N AV (RT S) Other Sta ti ons D I FS N AV (CT S) defer a ccess RT S t I FA’2008 ECE4607 Contenti on Wi ndow 73 D FWM AC-PCF (OPT I ON AL) ( OPT T he a ccess mecha ni sms pr esented so fa r ca nnot gua r a ntee a ma xi mum a ccess del a y or m i ni mum tr a nsmi ssi on ma ba ndwi dth. ba T o pr ovi de a ti me bounded ser vi ce, the sta nda r ds speci fy a Poi nt Coor di na ti on Functi on (PCF) on top of the D CF P oi mecha ni sms. m echa U si ng PCF r equi r es a n Access Poi nt (AP) t ha t ca n contr ol Access m edi um a ccess a nd pol l s the si ngl e nodes. I FA’2008 ECE4607 74 D FWM AC- PCF (Opti ona l ) ( Opti – T he networ k i s confi gur ed such tha t a number of sta ti ons wi th ti me-sensi ti ve tr a ffi c a r e contr ol l ed by the P CF whi l e r ema i ni ng tr a ffi c contends for a ccess usi ng C SM A/CA. – T he PCF i ssues pol l s i n a r ound-r obi n f a shi on to a l l sta ti ons confi gur ed for pol l i ng. I FA’2008 ECE4607 75 D FWM AC- PCF (Opti ona l ) ( Opti – T he PCF ma kes use of PI FS when i ssuei ng pol l s. Beca – B eca use PI FS i s sma l l er tha n D I FS, the PCF ca n sei ze t he medi um a nd l ock out a l l a synchr onous tr a ffi c whi l e i t i ssues pol l s a nd r ecei ves r esponses. w hi I FA’2008 ECE4607 76 D FWM AC- PCF (Opti ona l ) ( Opti When – When a pol l i ssued, the pol l ed sta ti on ma y r espond usi ng SI FS. – I f the PCF r ecei ves a r esponse, i t i ssues a nother pol l usi ng PI FS. – I f no r esponse i s r ecei ved dur i ng the expected tur na r ound ti me, the PCF i ssues a pol l . t he I FA’2008 ECE4607 77 D FWM AC- PCF (Opti ona l ) T he use of the SU PERFRAM E (CON T EN T I ON FREE PERI OD (CFP)). ( CFP)). NAV I FA’2008 ECE4607 NAV 78 D FWM AC- PCF (Opti ona l ) ( Opti T he AP or ga ni zes a per i odi ca l contenti on-fr ee per i od (CFP) (SU PERFRAM E) for the r ea l -ti me i nfor ma ti on. ( SU T he AP coor di na tes r ea l -ti me da ta to be tr a nsmi tted a t the begi nni ng of ea ch CFP (SU PERFRAM E) a nd dur i ng those per i ods i t a r r a nges a n N AV si gna l for a l l other sta ti ons. T he l ength of the PCF per i od i s va r i a bl e, a nd i t onl y occupi es a p or ti on of the CFP (SU PERFRAM E). of I FA’2008 ECE4607 79 D FWM AC- PCF (Opti ona l ) ( Opti T he r est of the CFP (SU PERFRAM E) i s r el ea sed for contenti on a nd D CF pa ckets. contenti I f a D CF pa cket occupi es the cha nnel a nd does not compl ete befor e the sta r t of the next CFP (SU PERFRAM E) the sta r ti ng ti me of the CFP (super fr a me) wi l l defer . H owever , the N AV si gna l for a l l other ter mi na l s goes to I FA’2008 ECE4607 80 D FWM AC-PCF (I ) t0 t1 M edi um Busy Poi nt C oor di na tor Wi r el ess Sta ti ons Sta ti ons‘ N AV Super Fr a me SI FS SI FS PI FS D1 SI FS D2 SI FS U1 U2 N AV I FA’2008 ECE4607 81 D FWM AC-PCF I I t2 t3 t4 Poi nt C oor di na tor Wi r el ess Sta ti ons D3 PI FS D4 SI FS SI FS CF end U4 Sta ti ons‘ N AV N AV Contenti on Fr ee Per i od Contenti on Per i od 82 t I FA’2008 ECE4607 D FWM AC-PCF At ti me t 0 t he contenti on-fr ee per i od shoul d sta r t, but a nother sta ti on i s tr a nsmi tti ng da ta sta After the medi um ha s been i dl e, the PCF ha s to wa i t for PI FS befor e After a ccessi ng the medi um. T he PC now sends da ta D 1 t o the fi r st sta ti on. T he sta ti on ca n a nswer a fter SI FS. After wa i ti ng for SI FS, the PC ca n pol l the second sta ti on by After sendi ng D 2. sendi T he I FA’2008 second sta ti on r epl i es wi th U 2 ECE4607 83 D FWM AC-PCF Pol l i ng conti nues wi th the thi r d stati on whi ch has n othi ng to answer . After wai ti ng for PI FS, the PC i ssues an end m ar ker (CF end ), i ndi cati ng that the contenti on per i od m ay star t agai n. The cycl e star ts agai n wi th the next super fr ame I FA’2008 ECE4607 84 ...
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This note was uploaded on 02/16/2009 for the course ECE 4607 taught by Professor Akyldiz during the Spring '08 term at Georgia Institute of Technology.

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