Performance Limits and Analysis of Contention-based IEEE MAC

Transcription

1 Performance Lmts and Analyss of Contenton-based IEEE 8211 MAC Shao-Cheng Wang and Ahmed Helmy Department of Electrcal Engneerng Unversty of Southern Calforna (shaochew Abstract Recent advance n IEEE 8211 based standard has pushed the wreless bandwdth up to 6Mbps whle keepng the same wreless medum access control (MAC) schemes for full backward compatblty However t has been shown that the neffcent protocol overhead casts a theoretcal throughput upper lmt and delay lower lmt for the IEEE 8211 based protocols even the wreless data rate goes to nfntely hgh Such lmts are mportant to understand the bottleneck of the current technology and develop nsght for protocol performance mprovements hs paper uses a queung system approach to extend the dscussons of IEEE 8211 protocol throughput and delay lmts to the stuaton that arbtrary non-saturated background traffc s present n the network We derve analytcal models to quantfy the lmts for Dstrbuted Coordnaton Functon (DCF) of legacy 8211a/b/g and Enhanced Dstrbuted Coordnaton Access (EDCA) of IEEE 8211e We fnd such lmts are functons of the underlyng MAC layer backoff parameters and algorthms and are hghly dependent on the load that background traffc nects nto the network Surprsngly dependng on the rate of arrval traffc the packet delay lmt may become unbounded such that no delay senstve servces can be operated under such condton Moreover we also dscuss the effects of dfferent frame aggregaton schemes on the performance lmts when data rate s nfnte he developed model and analyss provde a comprehensve understandng of the performance lmtatons for IEEE 8211 MAC and are useful n gaugng the expected QoS for the purposes such as admsson control Keywords-IEEE 8211 MAC DCF EDCA Performance Analyss Non-saturaton hroughput Delay I INRODUCION In recent years the IEEE 8211-based [1] wreless local area networks (WLANs) namely IEEE 8211b [2] 8211g [3] and 8211a [4] have been ncreasngly popular n provdng low-cost hgh-bandwdth (up to 54Mbps) wreless connectons Wth the growng demands of hgher bandwdth for applcatons such as hgh-defnton vdeo streamng network storage and onlne gamng the ndustry has been workng to seek hgher data rate (HDR) extensons [5]-[7] to the famly of IEEE 8211 specfcatons Earler ths year IEEE Workng Group meetng approved the frst draft of IEEE 8211n [8] n whch the data rate s expected to be as hgh as 6Mbps Moreover the 8211n specfcaton adopts the same medum access control (MAC) schemes to ensure backward compatblty wth exstng IEEE 8211 specfcatons he ndustry also seeks advancement n provdng better Qualty-of-Servce (QoS) at the MAC layer A QoS amendment of IEEE 8211 MAC IEEE 8211e [9] ams to provde servce dfferentatons to dfferent traffc types In partcular the Enhanced Dstrbuted Channel Access (EDCA) contenton-based medum access mproves the legacy IEEE 8211 Dstrbuted Coordnaton Functon (DCF) by provdng dfferentated medum contenton opportuntes to hgh prorty traffc Despte the efforts on advancng data rate and QoS of IEEE 8211 an analyss of theoretcal throughput and delay lmt was frst dscussed n [1] by Xao and Rosdahl he paper emphaszed on the 8211 MAC overhead effectveness and proved the exstence of theoretcal throughput and delay lmts for IEEE 8211 DCF protocol he authors concluded that gven that the PHY data rate has advanced to nfntely hgh and only one staton transmts n the deal channel condton the mnmum tme requred for completng one packet transmsson task s bounded by PHY and MAC headers as well as MAC layer backoff watng tme and consequently bounds the maxmum achevable throughput and mnmum achevable packet delay In [11] the authors extended the dervaton of packet transmsson tme to consder collsons and backoff freeze n the case that multple statons transmt n saturaton mode However the results n [1] and [11] only represent the throughput and delay lmt n aforementoned specal cases but are unsutable to real-world scenaros whch typcally consst of multple wreless statons operatng n non-saturaton mode Besdes the delay analyss presented n [11] only consders the medum access delay and fals to address the queung delay for the watng tme packets spent when backlogged On the other hand as the models used n [1] and [11] are only applcable to legacy IEEE 8211 DCF t s also mportant to expand the exploratons of theoretcal lmts to the QoS enhanced IEEE 8211e specfcaton In partcular t s essental to answer the followng questons: wll the smlar performance boundares exst n the EDCA MAC protocol? If so how to quantfy such boundares n dfferent prortzed traffc categores and what are the subsequent mpacts n fulfllng the QoS requrements promsed by IEEE 8211e EDCA? herefore ths paper ams to provde a comprehensve understandng of the performance lmtatons on throughput and total system delay of both IEEE 8211 DCF and EDCA MAC protocols wth arbtrary amount of non-saturated competng traffc Such analyss s crtcal n pnpontng the performance bottleneck of state-of-the-art IEEE 8211 technologes and n developng nsght for future protocol performance mprovements We propose a queung system pont of vew to drectly analyze the access dynamcs of 8211 contenton-based MAC he proposed model works wth any saturated or non-saturated underlyng competng traffc patterns Each wreless staton s modeled as a queung 1

2 system wth the packet generaton process as the arrval process and the varable amount of tme a packet spends on MAC layer medum contenton as the packet servce process he packet throughput and delay bound s then derved wth nfntely hgh operatng data rate he results are valdated through extensve smulatons under varous network loadng and operaton condtons he challenges to such analyss are n modelng the dynamc nteractons between the arrval pattern of the consdered node and the sgnfcantly varable amount of network delay ncurred by the backoff collson and re-transmsson procedures under dfferent background traffc load level Our paper makes the followng contrbutons: We construct a lghtweght mathematcal model for characterzng the throughput and delay lmts and performance of contenton-based IEEE 8211 MAC he proposed model enables us to systematcally explore the effects of backoff settngs arrval processes competng traffc characterstcs and frame aggregaton schemes on theoretcal throughput and delay lmt of dfferent versons of IEEE 8211 MAC protocols ncludng legacy DCF and QoS enhanced EDCA We dscover a performance bottleneck of the 8211 DCF and EDCA under the presence of background traffc: there s a turnng pont when packets arrve faster than the packet servce rate packet delay becomes unboundedly hgh beyond such network condton he rest of the paper s outlned as follows Secton II provdes background nformaton for IEEE8211 MAC standard Secton III descrbes the queung system based mathematcal model for packet throughput and delay of IEEE 8211 MAC Evaluatons and smulaton comparsons of the proposed model s presented n Secton IV Secton V concludes and provdes future work drectons II BACKGROUND In ths secton we brefly revew the legacy IEEE 8211 DCF MAC protocol and the enhanced IEEE 8211e EDCA We also descrbe the dfferences among 8211b 8211g and 8211a We hghlght the dfferent backoff settngs ncludng the specal protecton feature for 8211g devces to nteroperate wth 8211b devces whch affect the dervaton of the proposed model A DCF and EDCA of 8211 Standard he DCF of IEEE 8211 s a lsten-before-talk medum access scheme based on the Carrer Sense Multple Access wth Collson Avodance (CSMA/CA) protocol Before ntatng any packet delvery the staton detects the wreless medum to be dle for a mnmum duraton called DCF Interframe Space (DIFS) he staton randomly select the backoff tmer nterval from [ CW mn ] number of slot_tme where slot_tme s a parameter depends on the underlyng physcal layer (PHY) and then enters the backoff process Durng the count-down of backoff tmer f the staton senses the medum busy t stops decrementng the tmer and does not reactvate the paused value untl the channel s sensed dle agan for more than a DIFS At the tmer expraton the staton s free to access the medum for packet transmsson Upon recevng an acknowledgement frame the transmsson AC AC_BK AC_BE AC_VI AC_VO CWmn CWmax AIFSn DIFS/AIFS Busy medum AIFS[] = AIFSn[] *Slot_tmeSIFS AIFS[] DIFS PIFS SIFS Defer Access Contenton wndow Backoff Slots Slot_tme Fgure 1 IEEE 8211e EDCA Next Frame Select slot and decrement backoff as long as medum s dle ABLE I IMING PARAMEERS OF 8211A 8211G AND 8211B SANDARD 8211a 8211g (pure/hybrd) 8211b Slotme 9µs 9µs/2µs 2µs SIFS 16µs 16µs/1us 1µs DIFS 34µs 34µs/5µs 5µs p 16µs 16µs/72µs* 72µs* PHY 4µs 4µs/24µs 24µs CW mn Supported Data Bt Rate (Mbps) p : transmsson tme of physcal preambles PHY : transmsson tme of PHY header *short preamble mechansm / s consdered essful; the CW s reset to mnmum CW mn and the staton stands-by for the next packet arrval he transmsson s consdered faled f no acknowledgement s receved wthn a specfed tmeout; the staton repeats the backoff process wth CW selecton range doubled up to maxmum contenton wndow CW max If the transmsson has been re-tred for up to RetryLmt tmes the packet wll be dscarded and the CW s reset to CW mn he EDCF s a varant of DCF and provdes prortzed Qualty-of-Servce (QoS) support among dfferent traffc types Each QoS-enhanced staton (QSA) maps the packets arrvng at MAC layer nto four dfferent access categores (ACs) and assgns a set of backoff parameters namely Arbtraton IFS (AIFS) CW mn and CW max to each AC As llustrated n Fgure 1 each AC uses ts own backoff parameters to contend for the wreless medum by the same backoff rules as legacy DCF statons n the prevous paragraph he AIFS[AC] determned by AIFS[AC] = AIFSn[AC]slot_tme asifsme replaces the fxed DIFS n DCF Shorter AIFS[AC] n hgher prorty AC provdes hgh prorty traffc earler tmng to unfreeze the paused tmer after each busy wat perod On the other hand smaller CW szes probablstcally provde shorter backoff stages to hgh prorty traffc More detaled descrpton of DCF and EDCF can be found n [2] and [9] respectvely B IEEE 8211b 8211g and 8211a he IEEE 8211b 8211a and 8211g are hgher-speed physcal layer (PHY) extensons of the IEEE 8211 standard hey all use the same DCF medum access mechansm descrbed n prevous secton Note that on the other hand 2

3 IEEE 8211e s the MAC QoS enhancement amendment to the IEEE 8211 standard and can be ncorporated wth any of the three hgher-speed PHY extensons he detaled operatonal parameter settngs of the three versons of standard are summarzed n able I Compared to IEEE 8211b IEEE 8211a offers hgher data rate shorter PHY header MAC slot tme and lower mnmum contenton wndow On the other hand IEEE 8211g operates at the same band as 8211b and supports data rates and PHY/MAC parameters of both 8211b and 8211a When there are no 8211b statons present n the networ all 8211g statons utlze the same PHY/MAC settngs specfed n 8211a In the case when 8211g statons co-exst wth 8211b statons n the networ 8211g-capable statons shall swtch to longer 2ms slot tme n order to be synchronzed wth the tmng of 8211b statons In addton whenever 8211g-capable statons use OFDM modulated hgh rate to transmt DAA and ACK frames a specal protecton RS-CS or CS-toself exchange formed n 8211b decodable control frames must precede the data frame n order to ensure nteroperablty As we wll show later these parameters have substantal effects n theoretcal protocol performance lmts III ANALYSIS MODEL In ths secton we derve the theoretcal throughput and delay lmt wth non-saturaton background traffc for IEEE 8211 DCF and EDCA contenton-based wreless medum access methods We consder the nfrastructure Basc Servce Set (BSS) scenaro whch conssts of multple wreless nodes and a base staton connected wth wred networks Followng the best-case scenaro phlosophy n [1] and [11] we make the followng assumptons: 1) he wreless channel s deal wthout errors 2) All nodes are wthn carrer sensng range of each other 3) All nodes use the basc access operaton (no RS/CS) for shorter transmsson cycles he key dea to our analyss s to model the MAC layer tmng dynamcs from packet arrvng nto the sendng staton untl the packet receved by the ntended node as a G/G/1 queung system he theoretcal throughput and delay lmt are thus derved wth nfnte data rate We wll show that even wth nfntely hgh data rate the overhead of background packets causng non-neglgble amount of tme n the backoff stages s the domnant factor that bounds the MAC layer throughput and delay lmt In the extended verson [18] we show that our model can also be appled to quantfy the throughput and delay performance n practcal scenaros such as fnte data rate and non-deal wreless channel A Packet Arrvals Dependng on the applcaton layer the traffc arrvng at each wreless staton can be characterzed wth dfferent probablstc models In our proposed model we treat the packet arrvals as the general arrval process of G/G/1 queue For specal case arrval process such as Voce over IP (VoIP) wth determnstc arrval rate t can be treated as D/G/1 queue n our model B MAC Layer Servce me he packet servce tme of the proposed model s defned as MAC layer servce tme: the tme duraton from the nstant slot that a packet becomes the head of the transmsson queue and starts MAC layer contenton backoff process to the nstant that the packet s essfully receved or beng dropped because of maxmum retry lmt has reached As shown n Fgure 2 we model MAC layer servce tme by analyzng the duraton and occurrng probablty of dfferent events take place at backoff stages 1) When the backoff tmer decrements ether no transmsson s sensed n the tme slot and slot (the length of one tme slot) elapses or the slot s sensed busy wth busy taken Here busy s the average tme nterval the wreless medum beng occuped by background traffc transmssons We defne P busy be the probablty that at a gven tme slot the backoff tmer s frozen due to busy medum n carrer sensng he occurrng probablty of dle slot s smply 1- P busy 2) When the backoff tmer expres (e decrements to zero) the attempt of packet transmsson mght ether fal or eed In the falure case whch occurs wth probablty P fal fal s taken Note that n the case of deal wreless channel P fal equals P busy because transmsson attempt colldes only when the slot s supposed to be busy 3) In the ess case t takes for the packet transmsson process and then the packet s consdered served and therefore departs the queue he probablty of transmsson attempt eeds s smply 1- P fal Note that we assume that P busy (and P fal ) s constant n steady-state and ndependent of the backoff stages of the node under consderaton (e the tagged node) 1 As a result nodes can obtan P busy (and P fal ) by montorng the channel actvty and gatherng the long-term statstcs of the rato that medum s busy over all tme slots [12] Lkewse busy and fal can also be obtaned by channel actvty montorng On the other hand busy fal and can be formulated by consderng the frame exchanges and MAC layer tmng parameters nvolved n a essful or collded transmsson cycle For example can be expressed by the duraton of DAA and ACK frame for pure 8211a/b/g traffc or by the duraton of CS DAA and ACK frame when the tagged node operates at hybrd 8211g envronment and has CS-toself protecton turned on: pure 8 LDAA = p PHY SIFS R δ DAA (1a) 8 L ACK P PHY DIFS R δ ACK 1 Prevous work has shown that ths assumpton has very meager effects on model accuracy [12] Backoff stage Backoff stage 1 busy busy busy fal suc ransmsson by other nodes Collson wth other nodes Successful ransmsson Of the tagged nodes Fgure 2 Packet transmsson and collson events durng MAC backoff 3

4 11g _ hybrd CCK CCK 8 L = p PHY R CS CS OFDM OFDM 8 L p PHY R OFDM OFDM 8 L p PHY R δ SIFS DAA DAA ACK ACK δ SIFS δ DIFS (1b) where L CS L DAA and L ACK s the sze (n bytes) of CS DAA and ACK frame respectvely R CS R DAA and R ACK s the data rate (n bps) of CS DAA and ACK respectvely SIFS s the mandatory Short IFS nserted between frames σ s the propagaton delay In the case when the attempt of packet transmsson fals consderng the ACK tmeout effect fal s expressed wth the longest data frame nvolved n the collson In other words the L DAA n Equaton 1a and 1b s the sze (n bytes) of the longest data frame nvolved n the collson he duraton of busy slot busy can be expressed ether by when the busy slot s occuped by essful transmsson of the background traffc or by fal when the busy slot s occuped by packet collsons In the case when the wreless nodes operate at fnte data rates busy can be collected by the long-term statstcs of channel actvty montorng Fnally note that wth nfntely hgh data rate the tme duraton to carry the payload of CS DAA and ACK frames become nfntesmal As a result dependng on the network operates n pure 8211a/b/g or n hybrd 8211b/g envronment fal and busy can be expressed by pure = = = 2 2 2δ SIFS DIFS = p pure fal PHY pure busy 11g _ hybrd 11g _ hybrd 11g _ hybrd = fal = busy 11b 11b 11g _ pure 11g _ pure p PHY 2 p 2PHY (2a) (2b) 3δ 2SIFS DIFS where p and PHY s the preamble and PHY layer overhead SIFS s the mandatory Short IFS nserted between frames δ s the propagaton delay C hroughput and Delay Model of Legacy DCF o derve the throughput and delay of IEEE 8211 MAC we need to construct the detaled servce tme dstrbuton whch wll then be appled to standard queung theory model In ths subsecton we derve the detaled servce tme dstrbuton by carefully examnng the varable amount of tme spent on busy and slent slots and the correspondng occurrng probabltes throughout the backoff stages We frst defne the occurrng probablty Fk n k that n any sngle backoff stage wth backoff tmer selected from to W there are exactly k busy tme slots and (n-k) dle slots s 1 k k Fk n k = Ck Pbusy (1 Pbusy ) k n W (3) W Moreover we know that such combnaton of number of busy and dle slots can be a cumulatve effect from essve backoff stages herefore we then defne an ntermedate term k S for probablty of backoff counter beng frozen k tmes and dle (n-k) tmes that up to back off stage S = F k n W for = S = P k 1 fal m= for S 1 m F 1 m k m k n W 1 For stage ths ntermedate term equals Equaton 3 For stage greater than zero (e =12 m) ths ntermedate term ncludes all possble cases from combnaton of prevous stage(s) and the current stage whch result n k busy slots and (n-k) dle slots Fnally for all m backoff stages the overall probablty of backoff counter beng frozen exactly k tmes and dle (n-k) tmes s S k n k = (1 P fal ) m 2 = S k k n k S n m 1 k n k m = W 1 Snce Equaton 5 covers all possble combnatons of busy and dle slot tme we have the probablty dstrbuton functon (pdf) of MAC layer servce tme B t n the sequences of tme ponts at the multples of the busy medum tme ( busy ) and slot tme ( slot ) plus the essful transmsson tme ( ) [ ] ProbB t = S for t = k* busy ( )* k n m slot ( W 1) 1 Furthermore the probablty generatng functon (pgf) of B t can be expressed wth a reasonable system clock unt eg n µs or n system slot tme [ ] B( z) = ProbBt z (7) = S z k= S 1 1 z busy t S 1 11 z busy 1 slot S 2 2z We know that the average MAC layer servce tme can be obtaned by B'(1) the frst dervatve of B(z) at z=1 Hence wth the LDAA bytes long payload n IEEE 8211 DAA frame maxmum achevable throughput can be expressed by 8 LDAA hroughput = (8) B'(1) On the other hand to derve the total packet delay we can apply standard dscrete tme queung theory [13]-[15] wth the statstcal characterstcs of the arrval and servce process Here we assume the frst and second moment of the arrval dstrbuton are known and can be expressed n closed form A'(1) and A"(1) represent the frst and second dervatve of the pgf of arrval dstrbuton A(z) at z=1 respectvely Accordng to [13] f the arrval process s a general ndependent (GI) arrval process e the numbers of packets enterng the system durng the consecutve tme unts are busy (4) (5) (6) 4

5 assumed to be ndependent and dentcally dstrbuted (d) the mean system tme e packet delay n our case of GI/G/1 queue system can be expressed as Delay GI / G /1 2 [ A' (1)] B"(1) A"(1) B' (1) = 1 X B'(1) (9) 2[1 A'(1) B'(1)] where B'(1)and B"(1) are the frst and second dervatve of the pgf of MAC layer servce tme e P serv (z) at z=1 X s the mean dstance of the arrval pont from the start of the unt tme slot When unt tme s small X s neglgble However Equaton 9 s not applcable for applcatons wth determnstc arrval process eg VoIP herefore we refer to [14] for models of dscrete-tme D/G/1 queues he average delay of such system s A'(1)( A'(1) 1) B"(1) 1 Delay D G /1 = B'(1) (1) 2( A'(1) B'(1)) 1 z N 1 / r= 1 where N s the nter-arrval tme n system tme unt of the determnstc arrval process Z r are the roots of solvng zn- B(z)= on or nsde the unt crcle Fnally for packet arrvals that are nether General Independent process nor Determnstc process [15] provdes an upper bound of the system watng tme A"(1) B"(1) W 2[ A' (1) B'(1) ] (11) And hence the upper bound of total system delay s Delay B'(1) (12) G / G /1 W D hroughput and Delay Model of EDCA he QoS-enabled IEEE 8211e EDCA mechansm provdes prortzed medum access by assgnng dfferent AIFS and backoff wndow parameters (CW mn and CW max ) to dfferent traffc categores In partcular AIFS provdes advanced opportunty to hgh prorty traffc to access the medum by shorten the amount of tme a staton defers access to the channel followng a busy tme slot However AIFS changes the way we construct the occurrng probablty of busy and dle slot combnatons herefore we need to redefne Equaton 3 In order to perform theoretcal throughput and delay analyss we agan assume the best-case scenaro hat s we assume that only the tagged node utlze the short AIFS traffc category e AC_VO or AC_VI wth AIFSn=2 whle all other competng traffc utlze the AC_BE traffc category wth AIFSn=3 In ths way only the tagged node has hgher advanced prorty to access the medum and thus s consdered as best-case scenaro Under ths settng what happens n the last backoff tme slot of the tagged node decdes two dfferent scenaros 1) When last backoff slot (cw=1) was an dle slot the transmsson s subect to collson 2) When last backoff slot was a busy slot the tagged node un-freezes the backoff tmer one tme slot before all other traffc As a result the backoff tmer of tagged node expres before all other traffc un-freeze the tmer and thus the transmsson s guaranteed to be essful wthout collson r Here we frst consder the case last backoff slot was an dle slot We defne the occurrng probablty FCCk n k that n any sngle backoff stage there are exactly k busy tme slots and (n-k) dle slots s FCC k k 1 n k 1 k n 2 k Pbusy (1 Pbusy ) 1 n k = C k W ( W 1) / n W (13) Note that ths formulaton dffers from Equaton 3 n the occurrng probablty of dle slots he very frst tme slot and the tme slots after busy slot always happen before all other traffc wth probablty 1 he slots other than these specal slots and busy slots n Equaton 3 are all classfed as dle slot wth occurrng probablty (1-P busy ) We then consder the case last backoff slot was a busy slot We defne the occurrng probablty FNCk n k that n any sngle backoff stage there are exactly k busy tme slots and (n-k) dle slots s FNC k n k = C W 1 k ( W 1 n k 1 k n 1 (1 ) 2 k k Pbusy Pbusy 1) / 2 1 n W (14) Smlarly n ths formulaton only (n-) dle slots happen wth probablty (1-P busy ) All other dle slots happen wth probablty 1 Subsequently the ntermedate terms defnng all possble cases from combnaton of prevous stage(s) and the current stage whch result n k busy slots and (n-k) dle slots follow smlar dervatons as Equatons 4 to 7 SCC SCC SNC SNC k = FCC = P for = FNC = P for k k ( W k 1 fal m= 1 m k 1 1 m SCC 1 k ( W fal m= 1) / m k FCC 1 n SCC n 1 n W [ ( W 1) / 2 1] k m 1) / m k FNC W 1 n W [ ( W 1) / 2 1] k m W (15) (16) Fnally for all m backoff stages the overall probablty of backoff counter beng frozen exactly k tmes and dle (n-k) tmes s 5

6 S = m 1 SNC (1 p) m 2 k SCC [ ( W 1) / 2 1] 1 n k SCC m 1 k (19) As a result we can plug n the obtaned probablty dstrbuton functon (pdf) of MAC layer servce tme B t and correspondng probablty generatng functon to Equatons 8-12 to get the theoretcal throughput and delay lmt for IEEE 8211e EDCA IV RESUL In ths secton we use the queung system based packet throughput and delay model to quantfy and explore the theoretcal throughput and delay lmts of dfferent IEEE 8211 MAC specfcatons A P busy and busy From the analyss model n Secton III we can see that MAC layer packet servce tme and subsequently throughput and packet delay s a drect functon of two parameters: P busy slot busy probablty and busy average slot busy nterval herefore t s mportant before we proceed to present the numercal evaluatons of the proposed model we frst quantfy and understand the mplcatons of these two parameters P busy s an ndcator for how busy the network s and t s usually a functon of number of nodes n the networ ther traffc patterns and correspondng traffc load It s obvous from Secton III that the buser the network s the more often a packet wll wat on busy slots and consequently the MAC layer servce tme s longer Unfortunately no exstng model can be used to quantfy P busy wth arbtrary number of nodes and traffc loads such that we mght have dffcultes n relatng the amount of P busy wth real-world scenaros and quantfable metrcs such as number of nodes or traffc loads 2 Nevertheless n real-world traffc scenaros P busy can always be obtaned by gatherng the long-term statstcs from channel actvty montorng We thus adapt the usage model scenaros suggested by IEEE 8211 ask Group N (Gn) [17] to further llustrate the relatonshp between P busy and realworld non-saturated traffc As summarzed n able II we use dfferent combnatons of hgh-bandwdth multmeda (vdeo and audo) and data networkng applcatons to emulate futurstc hgh-performance wreless network scenaros such as dgtal home dgtal offce and publc hotspots We then obtan P busy of each scenaro through smulatons Later we use these scenaros to evaluate the accuracy of the proposed packet delay and throughput model under partcular P busy On the other hand busy s an ndcator for how long a busy slot takes he longer busy s the longer t takes to wat on busy slots and consequently the MAC layer servce tme s longer Recall from Equaton 1 n Secton III t s obvous 2 If we assume all nodes transmt n saturaton mode then the models n [16] can accurately quantfy and related P busy wth number of nodes n the network Average packet servce tme (ms) Busyness rato 8211b 8211a/g short 8211e AC_VO AC_VO busy=252us 8211g hybrd slot busy=9us 8211g pure long busy=9us 8211e AC_VI long slot busy=26us slot busy=9us busy=9us Fgure 3 Average MAC layer servce tme of dfferent 8211 specfcatons heoretcalhroughput (Mbps) MU=2346 bytes 8211e AC_VO 8211e AC_VI 8211a/g short slot 8211g long slot 8211b Busyness rato Fgure 4 heoretcal throughput lmt of dfferent 8211 specfcatons that when we consder nfnte data rate busy s determned by the PHY and MAC overhead specfed n dfferent versons of 8211 standard Later we wll see how does dfferent length of busy potentally vares about an order of magntude for dfferent versons of 8211 standard mpact on throughput and delay lmts of IEEE 8211 MAC protocols B MAC Layer Packet Servce me o study the theoretcal packet throughput and delay lmt of IEEE 8211 MAC we frst examne the MAC layer packet servce tme Note that even wth nfntely hgh data rate ths s the mnmum requred tme that the packets need to wat durng MAC backoff due to fnte protocol overhead of background traffc n busy slots Fgure 3 plots the average MAC layer servce tme of dfferent versons of 8211 standard n the presence of nonsaturated background traffc We can see that packet servce tme of all 8211 specfcatons ncreases wth P busy In a network wth busness rato P busy as low as 5-6 the MAC layer packet servce tme can be n the order of tens of mllseconds As we wll see n later sectons ths sgnfcant amount of medum access tme lmts the achevable packet throughput and delay of IEEE 8211-based protocols Besdes we can refer the MAC parameters and overhead of dfferent IEEE 8211 specfcatons n able I and see such parameters do affect the packet servce tme sgnfcantly For example because the mnmum contenton wndow (CW mn ) of 8211g s only half of that of 8211b t takes roughly half of 6

7 the tme for a packet from 8211g staton to be served compared to a packet from 8211b staton Due to shorter busy packet servce tme of pure 8211a/g network s further decreased compared to 8211g staton n hybrd 8211 b/g networks Moreover through the medum access advantage of 8211e AIFS dfferentaton the packet servce tme n 8211e networks reduces by an order of magntude compare to legacy 8211 protocols On the other hand by comparng pure 8211g n long slot and short slot settng we fnd t nterestng that shorter slot tme does not help n reducng the packet servce tme too much It s because shorter slot tme only saves the tme spent n dle slots by couple of mcroseconds whch are relatvely nsgnfcant compared to the duraton of busy slots n hundreds of mcroseconds From the observatons we made above we fnd that through the progresson of recent PHY and MAC amendments on 8211 standard the mnmum medum access tme has been reduced sgnfcantly In the followng sectons we wll examne such effects on packet throughput and delay C heoretcal hroughput Lmt In ths secton we examne the theoretcal maxmum throughput of dfferent 8211 specfcatons wth nfnte data rate From prevous secton we know that even wth nfntely hgh data rate there s a mnmum medum access tme that the packets need to wat durng MAC backoff As a result the maxmum amount of data delvered n a gven perod of tme s bounded by ths mnmum medum access tme and thus a theoretcal throughput lmt of 8211 MAC exts Fgure 4 plots the theoretcal throughput lmts wth arbtrary background traffc at dfferent level of busyness rato When there s no background traffc e P busy = our result s consstent wth the theoretcal throughput lmt presented n [1] As P busy ncreases we can see the maxmum throughput decreases exponentally In a network wth P busy about 5 the throughput lmt has decreased for more than an order of magntude Moreover as the latest IEEE 8211n proposal ams to provde 1Mbps effectve throughput at the MAC layer our result ndcates that theoretcally even wth nfnte data rate such goal can only be acheved at the condton that the busyness rato of the network s less than 3 (or 2) wth 8211e AC_VO (or AC_VI) beng employed Such QoS performance boundary s not dentfed before D heoretcal Delay Lmt From the results we present above the exstence of fnte MAC servce tme n the case of nfntely hgh data rate bounds the mnmum packet delay that can be acheved by IEEE 8211-based MAC Moreover recall from Secton III dependng on the type of packet arrval process addtonal backlog watng tme wll be added to the total packet delay In ths secton by usng the queung system based MAC layer packet delay model we present the results of theoretcal delay lmt of dfferent 8211 MAC specfcatons n the presence of arbtrary background traffc Fgure 5 shows the theoretcal packet delay lmt of dfferent 8211 specfcatons wth nfnte operaton data rate Here we use the determnstc arrval process of a G 711 VoIP applcaton wth 1ms nter-arrval tme as a case study We can see the delay lmts ncrease exponentally as P busy ncreases It s because the MAC layer servce tme ncreases ABLE II GN USAGE MODELS IN HIGH PERFORMANCE NEWORKS Applcaton Offered Number of applcatons n raffc Load Gn-1: Gn-2: Gn-3: ype DIGIAL DIGIAL PUBLIC (Mbps) HOME OFFICE HOSPO VoIP 96 UDP Vdeo Conferencng 5 CP 1 1 /Vdeo Phone A/V Streamng 2-4 UDP 1 1 SDV 4 UDP HDV UDP 2 Internet Fle ransfer N/A CP 1 1 Local Fle ransfer N/A CP 1 2 P busy (wth unlmted data rate) heoretcal packet delay (ms) b D/G/1 nter-arrval=1ms 8211g (n hybrd 8211b/g networ long slot) 8211a short slot 8211e AC_VO Busyness rato Fgure 5 heoretcal delay lmt of dfferent 8211 specfcatons Average packet delay (ms) e AC_VI D/G/1 arrval nter-arrval=15ms nter-arrval=25ms nter-arrval=5ms nter-arrval=1ms Busyness rato Fgure 6 heoretcal delay lmt of 8211e AC_VI wth dfferent arrval process as the network get buser and thus ncreases the backlog watng tme sgnfcantly An observaton that worth specal attenton n ths fgure s that there s a pont where the packet delay becomes unbounded Recall from the queung model n Secton III ths s because the fact that whenever the queue servce tme approaches or even exceeds the packet arrval tme (e 1ms n ths case) the queue becomes unable to handle packets n a tmely manner and eventually packets become ndefntely backlogged he exstence of such turnng pont demonstrates an mportant performance lmtaton of 7

8 ABLE III 8211b 8211g 8211e AC_VI 8211e AC_VI COMPARISON OF PACKE DELAY FROM MODEL AND SIMULAIONS P Busy Delay Delay Error (Model) (Smulaton) (%) Gn ms 1945ms 275% Gn ms 312ms 367% Gn ms N/A Gn ms 957ms 392% Gn ms 145ms 143% Gn ms 13926ms 728% Gn ms 52ms 582% Gn ms 664ms 93% Gn ms 912ms 39% Gn ms 81ms 99% Gn ms 175ms 142% Gn ms 1773ms 139% 8211-based MAC that s not dscovered n prevous lterature: wth the presence of background traffc the packet delay of 8211-based MAC can be boundlessly hgh even wth nfnte operatng data rate Our model dentfes a network condton boundary such as P busy > 45 for 1msframe G 711 VoIP n 8211b networks that any delaysenstve applcaton may never be able to meet the delay requrement beyond such boundary Furthermore from prevous secton we know the MAC layer servce tme reduces as the MAC parameters such as CW mn protocol overhead and AIFS mprove from 8211b to 8211g to 8211a and then to 8211e herefore we see the turnng pont of boundless delay moves toward hgher busyness rato n the same order On the other hand we also explore the effects of arrval process on packet delay lmts partcularly for the turnng pont of boundless packet delay As the packet delay lmt of 8211e appears to be fnte n all level of busyness rato n Fgure 5 we plot Fgure 6 for the packet delay lmt of prortzed 8211e MAC wth decreasng arrval tme We can see a turnng pont of unbounded delay eventually emerges as the arrval tme decreases less than 25ms It s because when arrval tme decreases t poses strcter delay constrants to the queue system consequently results n packet delay lmt curve movng toward less busy envronments Out result ndcates the performance bottleneck also exsts for the latest QoS-enabled IEEE 8211e EDCA E Model Valdaton wth Gn Usage Scenaros We further valdate the accuracy of our model wth ns-2 smulatons of non-saturated Gn scenaros lsted n able II Wth the traffc characterstcs specfed n able II each Gn scenaro corresponds wth a partcular P busy value We collect smulaton results of packet delay for G711 1ms nter-arrval VoIP traffc and compare to the delay derved from the analytcal model under the same P busy value able III shows a close match between our model and smulatons (error < 1%) V DISCUSSION A Effects of Competng raffc Packet Data Rates and Payload Szes In real-world IEEE 8211-based wreless network deployments the operatng data rate s not only fnte t s changng dynamcally A wreless node usually degrade the Average packet delay (ms) e AC_VI D/G/1 nter-arrval=1ms Busyness Rato busy=26ms (8 Mbps) busy=351ms (54Mbps 1bytes) busy=939s (11Mbps 1bytes) busy=234ms (2Mbps 5bytes) busy=634ms (2Mbps 15bytes) busy=227ms (6Mbps 1bytes) busy=544ms (24Mbps 1bytes) busy=1738ms (55Mbps 1bytes) busy=434ms (2Mbps 1bytes) Fgure 7 Average packet delay lmt wth competng traffc operates at dfferent data bt rates and dfferent payload szes data bt rate (to ncorporate a more reslent modulaton scheme) due to ncreased dstance or obstructons such as walls between the access pont and the wreless node or due to repeated unessful frame transmssons Besdes the payload szes of the competng traffc also vary from tme to tme It s apparent from the equatons n secton III that t takes more tme for a wreless staton to wat on busy slots when other wreless nodes operate at slower data rate or longer payload sze As a result ncreased busy slot tme ncreases the servce tme and delay and results n sgnfcant performance degradaton hs problem becomes more mportant n hybrd IEEE 8211 networks As specfed n the latest IEEE 8211n proposal[8] the fastest data rate s expected as hgh as 6Mbps Meanwhle the network s backward compatble to legacy IEEE 8211a/b/g devces whch support data rate as low as 1Mbps In a network that the operatng data rates vary n such a wde range the resultng packet servce tme and consequently network throughput and delay may also fluctuate greatly herefore t s essental to nvestgate the performance under the scenaros that nodes operate at dfferent data bt rates and dfferent payload szes Fgure 7 plots the average packet delay of prortzed 8211e AC_VI access category wth competng traffc operates at dfferent data bt rates and dfferent payload szes We vary the data rate from nfnte to 2Mbps n whch cases the average busy slot length ncreases an order of magntude As a result we can see delay performance vares sgnfcantly For the cases the operatng data rate s hgher than 11Mbps (up to nfnte data rate) the average packet delay mantans below 1ms for all medum busyness rato When the operatng data rate drops to 55Mbps the turnng pont for boundless delay emerges In the case when all other nodes operate at 2Mbps the average packet delay mght have reached to an unacceptable level (> 1ms) wth busyness rato as lttle as 4 On the other hand when we fx the data rate at 2Mbps and ncrease the payload szes of background traffc we can see the delay turns to be nfntely hgh at even less busyness rato (P busy ~3) c 8

9 We should keep n mnd that data bt rates and payload szes of competng traffc may change from tme to tme and are not controlled by any other node n the network In other words even the consdered node operates at the hghest possble data rate and uses the hghest prorty access category the performance of the consdered node can be constrant by the operatng characterstcs of other nodes n the network Such performance lmtaton s not properly dentfed and quantfed n prevous lterature B Performance Improvements on Frame Burstng and Block Acknowledgement From the observatons we made n prevous subsectons we know that the maor contrbutor to the delay n 8211 based networks s the delay ntroduced durng backoff stages It s especally neffcent n terms of channel utlzaton that such backoff medum contenton has to be repeated for every arrvng packet On the other hand the amendments of IEEE 8211 standard specfy two frame aggregaton schemes to mprove channel utlzatons by aggregatng multple transmssons n one medum contenton namely Frame- Burstng (FB) and Block Acknowledgment (BA) Frame-Burstng nserts a burst (say m number) of DAA frames and correspondng ACK frames back-to-back wthout ntatng another round of random backoff In addton Frame-Burstng does not requre any explct sgnalng between the source and recever nodes and hence can be mplemented n any IEEE 8211 based networks On the other hand Block ACK mtgates the neffcency of protocol overhead by placng a burst of DAA frames separated by a SIFS perod wthout beng acknowledged At the end of the burst the sender ntates an explct Block ACK Request (BAR) to enqure the number of frames essfully receved by the recever he recever then responds wth a Block ACK (BA) frame he number of frames n a BA burst (say n) s broadcasted by the access pont or pre-negotated between the sender and the recever It s obvous that wth the same number of data frames n a burst (m=n) BA transmts fewer frames and thus saves more overhead compared to FB Assumng the consdered node always has data packets to send we can express the theoretcal throughput of FB and BA aggregaton schemes by slght modfcatons n Equaton 8 put FB 8 LDAA m = (14) B'(1) ( SIFS) 2m p PHY 8 LDAA n putba = (15) B' (1) ( SIFS) ( n 2) p PHY Fgure 8 shows the theoretcal throughput of Frame-burstng and Block ACK schemes wth dfferent burst szes n the presences of non-saturated competng traffc In partcular we can see that wth the same burst sze m=n=16 the performance mprovements from BA s greater than that from FB n low busyness envronments he dfference n throughput mprovements between BA and FB however becomes nsgnfcant as the wreless medum becomes buser It s because the watng tme n backoff contenton becomes the domnant factor that lmts the theoretcal throughput under such network condtons hroughput (Mbps) VI MU=2346 bytes CONCLUSION In ths paper we nvestgate the theoretcal lmts of IEEE 8211 MAC throughput and delay performance n the presence of non-saturated background traffc A queung system based analytcal model s proposed to evaluate the throughput and delay bounded by PHY and MAC overhead and backoff watng tme even the operatng data rate s nfntely hgh We present a detaled analyss for theoretcal throughput and delay lmts n dfferent IEEE 8211 specfcatons We dentfy a performance bottleneck beyond whch the packet delay becomes nfntely hgh Such bottleneck exsts for all IEEE 8211 contenton-based DCF and EDCA MAC protocol although the exact turnng pont depends on the packet arrval pattern n consderaton We also show that such theoretcal lmts are functons of the MAC layer parameters the nodes operate on and the busyness of the wreless medum caused by competng traffc We study the effects of factors lke backoff contenton wndows protocol overhead system slot tme and nterframe space tme on throughput and delay lmts One of the key observatons s that the advanced medum access opportunty enabled by AIFSn n hgh prorty EDCA voce and vdeo access categores s the prmary contrbutor whch sgnfcantly mproves the QoS n terms of maxmum achevable throughput and mnmum achevable delay he effects of Frame-burstng and Block ACK frame aggregaton schemes on packet performance are also dscussed We plan to extend the analytc model to dscuss the packet delay performance of Frame-burstng and Block ACK schemes wth arbtrary arrval processes REFERENCES BA n=64 BA n=16 FB m=16 FB m=2 legacy 8211g Busyness rato Fgure 8 heoretcal throughput of Frame-burstng and Block ACK schemes wth dfferent burst szes [1] IEEE Standard for Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons Sep 1999 [2] IEEE Standard for Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons: Hgher Data Rate Extenson n the 24 GHz Band (8211b) Sep 1999 [3] IEEE Standard for Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons Amendment 4: Further Hgher Data Rate Extenson n the 24 GHz Band (8211g) June 23 [4] IEEE Standard for Wreless LAN Medum 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