Lecture 8 IEEE DCF Performance

Transcription

1 Lecture 8 IEEE82. DCF Perforace IEEE82. DCF Basc Access Mechas A stato wth a ew packet to trast otors the chael actvty. If the chael s dle for a perod of te equal to a dstrbuted terfrae space (DIFS), the stato trasts. If the chael s sesed busy (ether edately or durg the DIFS), the stato perssts to otor the chael utl t s easured dle for a DIFS. The stato geerates a rado backoff terval before trasttg to ze the probablty of collso wth packets beg trastted by other statos. (CA = Collso Avodace) I addto, to avod chael capture, a stato ust wat a rado backoff te betwee two cosecutve ew packet trasssos, eve f the edu s sesed dle the DIFS te

2 IEEE82. DCF Basc Access Mechas For effcecy reasos, DCF eploys a dscrete-te backoff scale. The te edately followg a dle DIFS s slotted, ad a stato s allowed to trast oly at the begg of each slot te. The slot te sze, σ, s set equal to the te eeded at ay stato to detect the trassso of a packet fro ay other stato. DCF adopts a expoetal backoff schee. At each packet trassso, the backoff te CW s uforly chose the rage (CW,CW ax ). The value CW s called coteto wdow, ad depeds o the uber of trasssos faled for the packet. At the frst trassso attept, s set equal to a value CW called u coteto wdow. After each usuccessful trassso, s doubled, up to a axu value CWax IEEE82. DCF Basc Access Mechas The backoff te couter s decreeted as log as the chael s sesed dle, froze whe a trassso s detected o the chael, ad reactvated whe the chael s sesed dle aga for ore tha a DIFS. The stato trasts whe the backoff te reaches zero. Sce the CSMA/CA does ot rely o the capablty of the statos to detect a collso by hearg ther ow trassso, a ACK s trastted by the destato stato to sgal the successful packet recepto. The ACK s edately trastted at the ed of the packet, after a perod of te called short terfrae space (SIFS). The SIFS (plus the propagato delay) s shorter tha a DIFS, o other stato s able to detect the chael dle for a DIFS utl the ed of the ACK

3 IEEE82. DCF Basc Access Mechas If the trasttg stato does ot receve the ACK wth a specfed ACK Teout, or t detects the trassso of a dfferet packet o the chael, t reschedules the packet trassso accordg to the gve backoff rules. Stato A Stato B Packet B DIFS SIFS DIFS Packet A ACK w w- w-2 SIFS DIFS DIFS Medu busy ACK Packet B Slot te: σ Slot te: froze backoff te IEEE82. DCF RTS-CTS Access Mechas A stato that wats to trast a packet, wats utl the chael s sesed dle for a DIFS, follows the backoff rules explaed above, ad the, stead of the packet, prelarly trasts a specal short frae called request to sed (RTS). Whe the recevg stato detects a RTS frae, t respods, after a SIFS, wth a clear to sed (CTS) frae. The trasttg stato s allowed to trast ts packet oly f the CTS frae s correctly receved. The fraes RTS ad CTS carry the forato of the legth of the packet to be trastted. Ths forato ca be read by ay lsteg stato, whch s the able to update a etwork allocato vector (NAV) cotag the forato of the perod of te whch the chael wll rea busy

4 IEEE82. DCF RTS-CTS Access Mechas Whe a stato s hdde fro ether the trasttg or the recevg stato, by detectg just oe frae aog the RTS ad CTS fraes, t ca sutably delay further trassso, ad thus avod collso. The RTS/CTS echas s very effectve ters of syste perforace, especally whe large packets are cosdered, as t reduces the legth of the fraes volved the coteto process. If perfect chael sesg would be possble, the collso could oly happe aog the short RTS packets ad thus the te lost due to collsos would be uch shorter copared to the basc access echas IEEE82. DCF Perforace Cosder saturated traffc codtos, whch all the users always have packet ready for trassso. The chael s assued to be perfect (o packet drop, o capture) Let τ deote the probablty that a gve stato trasts a packet radoly chose slot te. A collso happes wth probablty p. I order to ake the aalyss tractable, we ake the approxato that the collso probablty s costat ad the sae for all statos regardless of the trasssos already suffered

5 IEEE82. DCF Perforace Let b(t) deote the backoff te couter for a gve stato at dscrete slot te t. The slot te correspods to the te staces whe the backoff couter value s chaged. Stato A Stato B Packet B DIFS SIFS DIFS Packet A ACK w w- w-2 SIFS DIFS DIFS Medu busy ACK Slot te: σ Slot te: froze backoff te Packet B t t+ t+2 t+3 t IEEE82. DCF Perforace Let us defe W=CW CW ax =2 CW W =2 W, =,,2, Let s(t) deote the backoff stage of the stato at slot te t. Sce the collso probablty s p s assued to be depedet of the uber of retrassso attept, we ca o odel the behavour of the MAC protocol as two desoal Markov cha

6 IEEE82. DCF Perforace, W -2, W - G. Bach, Perforace of the IEEE82. Dstrbuted Coordato Fucto, IEEE Joural o Seleceted Areas Coucatos, Vol. 8, No. 3, March 2 IEEE82. DCF Perforace Let P{,k j,l}=pr{s(t+)=,b(t+)=k s(t)=j,b(t)=l} The state trasto probabltes the Markov cha ca thus be wrtte as Pr { kk,, + } =, k (, W 2), (, ) Pr{, k,} = ( p), k (, W ), (, ) W Pr { k,, } = p, k (, W ), (, ) W Pr { k,,} = p, k (, W ) W

7 IEEE82. DCF Perforace Let bk, = lt Pr { s( t) =, b( t) = k}, k (, W ), (, ) be the statoary state dstrbuto of the Markov cha. Cosder stage << Flow out = Flow -, bw, = p b, W p/w 2 -p bw = p b + bw = p b,,,w - W W p W k bk, = p b, () W, 2,,, It follows fro () that b, = pb,,2,...,, b, = pb, = (2) IEEE82. DCF Perforace For the last stage we have Flow out = Flow b = p b + p b = p b + b ( ) ( ) W,,,,, W W W 2 b = p b + p b + b = p b + b ( ) W, 2,, W,,, W W W W k b = p b + b k,,, W (3) -p -, It follows fro (2) ad (3) that p p p b = p( b + b ) b = b = p b = b p p p p/w,,,w - p/w,,,,,,, (4)

8 IEEE82. DCF Perforace For the frst stage =, we have b = p b ( ), W, W = 2 b = b + p b = p b ( ) ( ), W 2, W,, W = W = W k b b p b p b, k =, k+ + ( ), = ( ), W = W = -p (5),,,W - p, It thus follows fro (5) that ( ) b = p b b =,,, = = b, ( p) (6) IEEE82. DCF Perforace The state probabltes ust su to W k b = p b W k W k = k,, W bk, = p b, = b, W W b, pb, The probabltes ust su to W k W k W + 2 W W W bk, = b, = b, = b, = k= = k= W = k= W = ( 2 p) 2 b W +, = b, = ( 2p) W + + = = 2 2 = p p 2( 2 p)( p) b, = ( 2 p)( W + ) + pw 2 p ( ( ) )

9 IEEE82. DCF Perforace We ca ow express the probablty τ that a stato trasts durg a radoly chose slot te. The trassso occurs whe the backoff te couter s equal to zero, regardless of the backoff state, hece τ = b, =, = = b 2( 2 p) p ( 2 p)( W + ) + pw 2 p ( ( ) ) The probablty that there s collso s that ore tha oe user tres trassso at the sae slot te: p = ( τ ) IEEE82. DCF Perforace Probablty that at least oe user trasts Ptr ( τ ) = Probablty of succesful trassso codtoed that at least oe user trasts P = Pr exactly oe user trast at least oe user trast s { } { } τ ( τ) { } ( τ ) Pr exactly oe user trast = = Pr at least oe user trast

10 IEEE82. DCF Perforace The trhoughput s PP tr se{ P} S = P σ + PPT + P P T ( ) ( ) tr tr s s tr s c where δ s the slot legth (propagato delay) P s s the probablty of successful trassso a busy perod P tr s the probablty that there s trassso at gve slot E{P} s the expected legth of the packet T s s the average te the chael s busy case of succesful trassso T c s the s the te the chael s busy case of collso IEEE82. DCF Perforace Basc access ethod: Ts = H + E{ P} + SIFS+ σ + ACK + DIFS + σ T = H + E{ P} + DIFS+ σ c where H deotes the total legth of PHY ad MAC headers RTS-CTS hadshake T = RTS+ SIFS+ σ + CTS+ SIFS+ s c H + E{ P} + SIFS+ σ + ACK + DIFS + σ T = RTS+ DIFS+ σ G. Bach, Perforace of the IEEE82. Dstrbuted Coordato Fucto, IEEE Joural o Seleceted Areas Coucatos, Vol. 8, No. 3, March

11 IEEE82. DCF Perforace The throughput ca be wrtte as E{ P} S = σ ( Ptr ) Ptr + ( Ps ) Tc Ts + P s Hece, the throughput s axzed f P τ ( τ s ) = Tc Tc Tc ( Ptr ) Ptr + ( P s ) ( τ ) δ δ δ s axzed. Maxu occurs whe ( ( )) T = ( τ c ) τ ( τ ) δ IEEE82. DCF Perforace For low load τ << ( ) 2 2 ( τ ) τ + τ The trast probablty that axzes throughput s gve by Tc + 2( ) δ * τ = Tc ( ) Tc δ 2δ Hece, the throughput depeds oly o the etwork sze ad o the syste paraeters ad W

12 IEEE82. DCF Perforace The trassso probablty depeds o the paraeters ad W whch should be tued such that 2( 2 p) τ = ( 2 p)( W + ) + pw ( ( 2 p) ) Tc 2δ For =, we have 2 τ = ( W + ) Hece, the wdow sze should be selected as T W 2 c δ That s, the optal perforace s obtaed whe the wdow sze depeds o the uber of users ad the average trassso te of the packets IEEE82. DCF Perforace I practce, t s very dffcult to estate. BEB ca be vewed as a attept to estate the optal wdow sze, but t does ot do a very good job for large. A coparso betwee 82. BEB (oral) 82. CW opt, uses the optal CW BEB used 82. s far fro optal, especally for large uber of users for few statos the result s satsfactory uber of users chael utlzato IEEE82.a 82. BEB 82. CW opt cotrbuted by Thoas Nlsso, Ueå Uversty 2