IEEE E: QOS PROVISIONING AT THE MAC LAYER YANG XIAO, THE UNIVERSITY OF MEMPHIS

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1 ACCEPTED FROM O PEN C ALL IEEE E: QOS PROVISIONING AT THE MAC LAYER YANG XIAO, THE UNIVERSITY OF MEMPHIS ess AIFS[j] AIFS[] PIFS SIFS AIFS[] Content 0 to CW Bac Slot Select s The emergng IEEE e standard provdes QoS features and multmeda support to the exstng b and a wreless standards, whle mantanng full bacward compatblty wth these standards. ABSTRACT Ths artcle ntroduces the emergng IEEE e standard to support qualty of servce at the medum access control layer. Both the contenton-based and contenton-free centrally controlled channel access mechansms are ntroduced by descrbng not only the MAC protocol operatons and parameters, but also the call admsson technques and schedulng algorthm that have been desgned for IEEE e. Fnally, we provde smulaton results amed to hghlght the capablty of the EDCF to dfferentate the traffc classes. INTRODUCTION Net access through hotspots at arports, hotels, and coffee shops, va the hgh-speed wreless Internet access servce nown as WF, s rapdly becomng common. Recently, AT&T, IBM, and Intel joned forces to form Cometa Networs, an ambtous effort to start deployng wreless access ponts n 2003, wth 20,000 WF hot spots set up throughout the country by ths year. It was predcted that the number of hotspots could jump to 80,000 wthn four years. Wreless local area networs (WLANs) are becomng ubqutous and ncreasngly reled on as IEEE products become successful n the maret. In wreless home and offce networs where voce, vdeo, and audo wll be delvered, qualty of servce (QoS) and multmeda support are crtcal, and are essental ngredents to offer customers vdeo on demand, audo on demand, voce over IP, and hgh-speed Internet access. Consumer electroncs companes, such as Panasonc and Sharp, are loong to offer home wreless networng devces n whch QoS s a crtcal element. In the near future, all consumer electronc devces at home, such as VCRs, TVs, and mcrowave ovens, can be equpped for and connected to QoS-enabled wreless networs. One of the most promsng home networng canddates s IEEE WLAN. IEEE medum access control (MAC) employs a mandatory contenton-based channel access functon called the dstrbuted coordnaton functon (DCF)and an optonal centrally controlled channel access functon called the pont coordnaton functon (PCF) [1]. The DCF adopts carrer sense multple access wth collson avodance (CSMA/CA) and bnary exponental bacoff. It s treated as the wreless verson of the most successful LAN, IEEE (Ethernet), whch adopts CSMA wth collson detecton (CSMA/CD) and bnary exponental bacoff. Both IEEE DCF and IEEE enable fast nstallaton wth mnmal management and mantenance costs, and are very robust protocols for best effort servce. However, the current DCF s unsutable for multmeda applcatons wth QoS requrements [2]. Even though the PCF can provde some lmted QoS support, t s barely mplemented n today s products due to ts complexty and neffcency for normal data transmsson. To support MAC-level QoS, the IEEE Worng Group s currently worng on IEEE e [3 7], whch s n the fnal stage. The emergng IEEE e standard provdes QoS features and multmeda support to the exstng b [8] and a [9] wreless standards, whle mantanng full bacward compatblty wth these standards. In order to support multmeda applcatons such as voce and vdeo over the wreless medum, the IEEE e MAC employs a channel access functon called the hybrd coordnaton functon (HCF), whch ncludes contenton-based channel access and a contenton-free centrally controlled channel access mechansm. The contenton-based channel access s also referred to as the enhanced dstrbuted coordnaton functon (EDCF). The EDCF provdes dfferentated and dstrbuted access to the wreless medum for multple user prortes. The centrally controlled channel access of HCF s based on a pollng mechansm wth some enhanced QoS-specfc mechansms and frame subtypes to allow QoS data transfers durng collson-free perods. In ths artcle we frst ntroduce the orgnal IEEE MAC protocol, and then IEEE e. Fnally, we evaluate the EDCF dfferentated mechansm wth smulatons /04/$ IEEE IEEE Wreless Communcatons June 2004

2 IEEE MAC IEEE MAC employs a mandatory DCF and an optonal PCF. These functons determne when a staton (STA), operatng wthn a basc servce set (BSS) or ndependent BSS (IBSS), s permtted to transmt. There are two types of networs: an nfrastructure networ (BSS) n whch an access pont (AP) s present, and an ad hoc networ (IBSS) n whch an AP s not present. In a long run, tme s always dvded nto repetton ntervals called superframes. Each superframe starts wth a beacon frame, and the remanng tme s further dvded nto a contenton-free perod (CFP) and a contenton perod (CP). The DCF wors durng the CP, and the PCF wors durng the CFP. If the PCF s not actve, a superframe wll not nclude the CFP. However, the beacon frame s always sent no matter whether the PCF s actve or not. The beacon frame s a management frame for synchronzatons, power management, and delverng parameters. In a BSS, an AP sends beacon frames. In an IBSS, any moble staton that s confgured to start an IBSS wll begn sendng beacon frames. As other moble statons jon that IBSS, each staton that s a member of the IBSS s randomly chosen for the tas of sendng a beacon frame. Beacon frames are generated at regular ntervals called target beacon transmsson tmes. THE DISTRIBUTED COORDINATION FUNCTION The DCF defnes a basc access mechansm and an optonal request-to-send/clear-to-send (RTS/ CTS) mechansm. In the DCF a staton wth a frame to transmt montors the channel actvtes untl an dle perod equal to a dstrbuted nterframe space (DIFS) s detected. After sensng an dle DIFS, the staton wats for a random bacoff nterval before transmttng. The bacoff tme counter s decremented n terms of slot tme as long as the channel s sensed dle. The counter s stopped when a transmsson s detected on the channel, and reactvated when the channel s sensed dle agan for more than a DIFS. In ths manner, statons deferred from channel access because ther bacoff tme was larger than the bacoff tme of other statons are gven hgher prorty when they resume ther transmsson attempt. The staton transmts ts frame when the bacoff tme reaches zero. At each transmsson, the bacoff tme s unformly chosen n the range (0, CW 1) n terms of tmeslots, where CW s the current bacoff wndow sze. At the very frst transmsson attempt, CW equals the ntal bacoff wndow sze, CW mn. After each unsuccessful transmsson, CW s doubled untl a maxmum bacoff wndow sze value, CW max, s reached. After the destnaton staton successfully receves the frame, t transmts an acnowledgment frame (ACK) followng a short nterframe space (SIFS) tme. If the transmttng staton does not receve the ACK wthn a specfed ACK tmeout, or detects the transmsson of a dfferent frame on the channel, t reschedules the frame transmsson accordng to the prevous bacoff rules. The DCF provdes a channel access mechansm wth equal probabltes to all statons contendng for the same wreless medum. If an AP s present, STAs are not allowed to transmt frames to other STAs that are not APs. The above mechansm s called the basc access mechansm. In such a mechansm, the hdden node problem may happen: transmssons of a staton cannot be detected usng carrer sense by a second staton, but nterfere wth transmsson from the second staton to a thrd staton. To reduce the hdden staton problem, an optonal four-way data transmsson mechansm, RTS/CTS, s also defned n DCF. In RTS/CTS, before transmttng a data frame, a short RTS frame s transmtted. The RTS frame also follows the bacoff rules ntroduced above. If the RTS frame succeeds, the recever staton responds wth a short CTS frame. Then a data frame and an ACK frame wll follow. All four frames (RTS, CTS, data, ACK) are separated by an SIFS tme. In other words, the short RTS and CTS frames reserve the channel for the data frame transmsson that follows. Snce rado transmsson has a range, the range (denoted RA) of the source s RTS transmsson and the range (denoted RB) of the destnaton s CTS transmsson are overlapped but not equal. Therefore, after transmsson of the source s RTS and the destnaton s CTS, the channel s reserved for the data transmsson that follows, and any staton n ether RA or RB wll not transmt. A hdden staton of the source, who s n RB but not n RA, wll not nterfere wth the source data transmsson snce t hears the destnaton s CTS transmsson. THE POINT COORDINATION FUNCTION The PCF s an optonal centrally controlled channel access functon, whch provdes contenton-free (CF) frame transfer. The PCF s desgned to support tme-bounded servces, whch can provde lmted QoS. It logcally sts on top of the DCF and performs pollng, enablng polled statons to transmt wthout contendng for the channel. It has hgher prorty than the DCF by adoptng a shorter nterframe space (IFS) called the pont nterframe space (PIFS): SIFS < PIFS < DIFS. If the PCF s mplemented, a CFP under the PCF and a CP under the DCF alternate over tme. A CFP and a CP form a superframe. The AP where the pont coordnator (PC) s normally located senses the medum dle for a PIFS nterval, and then transmts a beacon frame to ntate a CFP (.e., to ntate a superframe). After a SIFS tme, the PC sends a poll frame to a staton to as to transmt a frame. The poll frame may or may not nclude data to that staton. After recevng the poll frame from the PC, the staton wth a frame to transmt may choose to transmt a frame after a SIFS tme. When the destnaton staton receves the frame, an ACK s returned to the source staton after a SIFS tme. The PC wats a PIFS nterval followng the ACK frame before pollng another staton or termnatng the CFP by transmttng a CF-End frame. If the PC receves no response from the polled staton for a PIFS nterval, the PC can poll the next staton or termnate the CFP by transmttng a CF-End frame. In a long run, tme s always dvded nto repetton ntervals called superframes. Each superframe starts wth a beacon frame, and the remanng tme s further dvded nto a contenton-free perod and a contenton perod. IEEE Wreless Communcatons June

3 The PCF cannot provde good QoS support snce t lacs an admsson functon to control channel access from statons. Therefore, when the traffc load s too hgh, all exstng traffcs may be degraded. 1 In an earler verson of the IEEE e draft, up to eght queues were mplemented nstead of four before the concept of ACs was ntroduced. 0 AIFS[0] BO[0] Access categores 1 AIFS[1] BO[1] Vrtual collson handler Fgure 1. Vrtual transmsson queues, where BO[] stands for the bacoff counter for AC. The PCF cannot provde good QoS support snce t lacs an admsson functon to control channel access from statons. Therefore, when the traffc load s too hgh, all exstng traffc may be degraded. IEEE E IEEE e provdes a channel access functon, the HCF, to support applcatons wth QoS requrements. The HCF ncludes both contenton-based and centrally controlled channel access. Contenton-based access of the HCF s also referred to as the EDCF. In ths artcle we refer to the centrally controlled channel access of the HCF as the controlled HCF. QoS enhancements are avalable to QoS enhanced statons (QSTAs) that can be assocated wth a QoS enhanced access pont (QAP) n a QoS BSS (QBSS) or n a QoS IBSS (QIBSS) wthout a QAP. Snce a QSTA mplements a superset of STA functonalty, the QSTA may assocate wth a non-qos AP n a non-qos BSS to be bacward compatble by provdng non- QoS MAC data servce. Note that the term non- QoS stands for the orgnal MAC ntroduced n the prevous secton. Smlar to the orgnal MAC, a CFP under the controlled-hcf and a CP under the EDCF alternate over tme, and tme s always dvded nto superframes. Each superframe starts wth a beacon frame, and the remanng tme s further dvded nto a CFP and a CP. The EDCF wors durng the CP, and the controlled HCF wors durng CFP. A new concept, transmsson opportunty (), s ntroduced n IEEE e for both the EDCF and controlled HCF. A s a tme perod when a STA has the rght to ntate transmssons onto the wreless medum. It s defned by a startng tme and a maxmum duraton. A STA cannot transmt a frame 2 AIFS[2] BO[2] 3 AIFS[3] BO[3] Prorty AC Desgnaton 1 0 Best effort 2 0 Best effort 0 0 Best effort 3 1 Vdeo probe 4 2 Vdeo 5 2 Vdeo 6 3 Voce 7 3 Voce Table 1. Prorty to access category mappng. that extends beyond a. If a frame s too large to be transmtted n a, t should be fragmented nto smaller frames. A s used by both the EDCF and the controlled HCF, and can be obtaned both when the medum s avalable under the EDCF rules and when the staton receves a poll. Non-AP QSTAs may send requests durng polled s or EDCF s usng the QoS control feld n the frame drected to the QAP, wth the request duraton or queue sze ndcated to the QAP. THE ENHANCED DCF The EDCF s based on dfferentatng prortes at whch traffc s to be delvered and wors wth four access categores (ACs), 1 whch are vrtual DCFs, as shown n Fg. 1, where each AC acheves a dfferentated channel access. Ths dfferentaton s acheved through varyng the amount of tme a STA wll sense the channel to be dle and the length of the contenton wndow durng bacoff. The EDCF supports eght dfferent prortes, whch are further mapped nto four ACs, shown n Table 1. ACs are acheved by dfferentatng the arbtraton nterframe space (AIFS), ntal wndow sze, and maxmum wndow sze. For AC ( = 0, 3), the ntal bacoff wndow sze s CW mn [], the maxmum bacoff wndow sze s CW max [], and the AIFS s AIFS[]. For AC ( = 0, 3), the bacoff mechansm s smlar to the orgnal MAC: after each unsuccessful transmsson, CW[] s doubled untl a maxmum bacoff wndow sze value CW max [] s reached. For 0 < j 3, we have CW mn [] CW mn [j], CW max [] CW max [j], and AIFS[] AIFS[j], and at least one of the above nequaltes must be not equal to. In other words, the EDCF employs AIFS[], CW mn [], and CW max [] (all for = 0,, 3) nstead of DIFS, CW mn, and CW max, respectvely. If one AC has a smaller AIFS or CW mn or CW max, the AC s traffc has a better chance of accessng the wreless medum earler. Fgure 2 shows the EDCF tmng dagram, where three ACs are shown:, j, and. Fgure 1 shows four transmsson queues mplemented n a staton, and each queue supports one AC, behavng roughly as a sngle DCF entty n the orgnal IEEE MAC. It s assumed that a payload from a hgher layer s labeled wth a prorty value and enqueued nto the correspondng 74 IEEE Wreless Communcatons June 2004

4 Immedate access when Medum s free AIFS[]/ AIFS[j]/ AIFS[] AIFS[]/ AIFS[j]/ AIFS[] Busy medum Defer access Fgure 2. The EDCF tmng dagram. AIFS[j] AIFS[] PIFS SIFS AIFS[] queue accordng to the mappng n Table 1. Each queue acts as an ndependent MAC entty and performs the same DCF functon, wth a dfferent nterframe space (AIFS[]), a dfferent ntal wndow sze (CW mn []), and a dfferent maxmum wndow sze (CW max []). Each queue has ts own bacoff counter (BO[]), whch acts ndependently n the same way as the orgnal DCF bacoff counter. If there s more than one queue fnshng the bacoff at the same tme, t s called an nternal collson, whch s resolved as follows. The hghest AC frame s chosen to transmt by the vrtual collson handler. Other lower AC frames whose bacoff counters also reach zero wll ncrease ther bacoff counters wth CW mn [] ( = 0,, 3) accordngly. In other words, other lower AC frames whose bacoff counters also reach zero do not treat ths nternal collson as a real collson after adjustng the bacoff counters. Furthermore, we have AIFS[] PIFS. The values of AIFS[] ( = 0,, 3), CW mn [] ( = 0,, 3), and CW max [] ( = 0,, 3) are referred to as the EDCF parameters, announced by the QAP va perodcally transmtted beacon frames. The QAP can also adaptvely adjust these EDCF parameters based on the networ traffc condtons. DISTRIBUTED ADMISSION CONTROL FOR EDCF The QoS parameter set element (QPSE) provdes nformaton needed by QSTAs for proper operaton of the QoS faclty durng the CP. The QPSE ncludes CW mn [], CW max [], AIFS[], Lmt[], Budget[], Load[], and SurplusFactor[] for ( = 0, 3). For the AC, Lmt[] specfes the tme lmt on s; Budget[] specfes the addtonal amount of tme avalable durng the next beacon nterval; Load[] specfes the amount of tme used durng the prevous beacon nterval; and SurplusFactor[] represents the rato of over-thear bandwdth reserved to the bandwdth of the transported frames requred for successful transmsson. The QPSE s calculated by the QAP for each beacon nterval and embedded nto the next beacon frame transmtted to each QSTA. When the transmsson budget for an AC s depleted, new QSTAs cannot gan transmsson tme, whle exstng QSTAs cannot ncrease the Contenton wndow from 0 to CW mn Bacoff wndow Slot tme Next frame Select slot and decrement bacoff as long as medum s dle transmsson tme they are already usng. Ths mechansm protects exstng flows. The QAP shall measure the amount of tme occuped by transmssons for each AC durng the beacon perod, ncludng assocated SIFS and ACK tmes f applcable. The QAP shall mantan a set of counters TxTme[], whch shall be set to zero mmedately followng transmsson of a beacon. For each data frame transmsson (ether upln or downln), the QAP shall add to the TxTme counter correspondng to the AC of that frame a tme equal to the frame transmsson tme and all overhead nvolved (SIFS tme, MAC and PHY overhead, ACK, etc.). The QAP shall transmt n each beacon the Budget and SurplusFactor for each AC, contaned n QPSE. The QAP wll set - Budget[] to be Budget[] (1) =Max(ATL[] TxTme[]*SurplusFactor[],0). The varable ATL[] s the maxmum amount tme that may be spent on transmssons of AC per beacon nterval. Each QSTA has to mantan the followng varables for each AC: TxCounter[], TxUsed[], TxLmt[], TxRemander[], and TxMemory[]. The varable TxUsed[] counts the amount of tme occuped on ar by transmssons, successful or otherwse, from ths staton for ths AC, ncludng assocated SIFS and ACK tmes f applcable. TxCounter[] counts successful transmssons. The STA shall not transmt a data frame f dong so would result n the value n Txused[] exceedng the value of TxLmt[]. If the QSTA s prevented from sendng a frame for ths reason, t may carry over the partal frame tme remander to the next beacon perod, by storng the remander n TxRemander[]: TxRemander[] = TxLmt[] TxUsed[]; otherwse, TxRemander[] = 0. At each target beacon transmsson tme, the TxMemory, TxLmt, and TxCounter state varables are updated accordng to the followng procedure: If Budget[] = 0 TxMemory[] shall be set to zero for new QSTAs that started transmsson wth ths AC n the last beacon nterval; all other QSTAs TxMemory[] remans unchanged If the Budget[] >0 When the transmsson budget for an AC s depleted, new QSTAs cannot gan transmsson tme, whle exstng QSTAs cannot ncrease the transmsson tme that they are already usng. Ths mechansm protects exstng flows. IEEE Wreless Communcatons June

5 DLP allows QSTAs to transmt frames drectly to another QSTA by settng up such data transfer when a QAP s present. The need for ths protocol s motvated by the fact that the ntended recpent may be n Power Save Mode, n whch case t can only be woen up by the QAP. TxMemory[] = f*txmemory[] + (1 f)* (TxCounter[]*SurplusFactor[] + Budget[]) TxCounter[] = 0 TxLmt[] = TxMemory[] + TxRemander[] where f s the dampng factor, whch does not affect the entrance of a new flow nto the system when enough budget s avalable, because the decreased Budget s offset by an ncreased TxCounter nstantaneously, so TxMemory does not change a lot. The dampng does affect TxMemory when a new flow starts up n another QSTA. In that case, the decreased Budget s not offset by an ncreased TxCounter; consequently, the TxMemory converges to the lower target value. The Budget used n ths calculaton shall be the budget most recently obtaned from the QAP. The TxCounter value shall be the value of the beacon perod before the perod that just ended. Tang the earler value accounts for the delay that occurs between the moment at whch the QAP determnes the Budget and the pont at whch ths budget s used n the above calculatons. The value TxCounter + Budget s the target to whch TxMemory converges. TxLmt s equal to TxMemory plus a possble capped remander. TxMemory memorzes the amount of resources the QSTA has been able to spend n a specfc AC. Once the budget s depleted (.e., Budget hovers around 0), TxMemory converges to TxCounter, whch s the lower lmt. Ths ensures that the QSTA can contnue consumng the same amount of resources n followng beacon perods. Dampng allows for some amount of fluctuaton to occur, but TxMemory cannot grow any further n the saturated state. Ths prevents new flows from enterng the specfc AC when t s saturated. A sutable ntal value for ths varable could be between 0 and Budget[]/SurplusFactor[]. QSTAs shall not ncrease ther TxLmt[] f they dd not transmt traffc wth the AC n the prevous beacon nterval. DIRECT LINK PROTOCOL Drect Ln Protocol (DLP) allows QSTAs to transmt frames drectly to other QSTAs by settng up such data transfer when a QAP s present. The need for ths protocol s motvated by the fact that the ntended recpent may be n power save mode, n whch case t can only be woen up by the QAP. DLP allows the sender and recever to exchange rate set and other nformaton. Furthermore, DLP messages can be used to attach securty nformaton elements. Ths protocol prohbts STAs gong nto power save for the actve duraton of the drect stream. DLP does not apply n an ad hoc networ, where frames are always sent drectly from one QSTA to another. A drect ln can be bult by the followng sequences: QSTA-1 that has data to send nvoes DLP and sends a DLP-request frame to the QAP. Ths request contans the rate set, and (extended) capabltes of QSTA-1, as well as the MAC addresses of QSTA-1 and QSTA-2. If QSTA-2 s assocated n the BSS, the QAP shall forward the DLP-request to the recpent, QSTA-2. If QSTA-2 accepts the drect stream, t shall send a DLP-response frame to the QAP. The QAP shall forward the DLP-response to QSTA-1, after whch the drect ln becomes actve and frames can be sent from QSTA-1 to QSTA-2 and from QSTA-2 to QSTA-1. When the drect ln s actve, QSTA-1 may use DLP probes to gauge the qualty of the ln between QSTA-1 and QSTA-2.The drect ln becomes nactve when no frames have been exchanged as part of the drect ln for the duraton of a DLPIdleTmeout. After the tmeout, frames wth destnaton QSTA-2 shall be sent va the QAP. THE CONTROLLED HCF The controlled HCF, the centrally controlled channel access functon, allows reservaton of transmsson opportuntes (s) wth a hybrd coordnator (HC), a type of PC handlng rules defned by the HCF. The HC, collocated wth a QAP, performs bandwdth management ncludng allocaton of s to QSTAs. Smlar to a PC, an HC can transmt the beacon frame to ntate CFP when t senses the medum dle for a PIFS nterval, and termnate the CFP by transmttng a CF-End frame when t senses the medum dle for a PIFS nterval. A non-ap QSTA, based on ts requrements, requests the HC for s for both ts own transmssons and transmssons from the HC to tself. Non-AP QSTAs may send requests usng the QoS control feld n the frame drected to the QAP, wth the request duraton or queue sze ndcated to the QAP. The HC, based on an admsson control polcy ntroduced n the next subsecton, ether accepts or rejects the request. If the request s accepted, t schedules s for the non-ap QSTA. For transmssons for the STA, HC polls a non- AP QSTA based on the parameters suppled by the non-ap QSTA at the tme of ts request. For transmssons to the non-ap QSTA, the HC queues the frames and delvers them perodcally, agan based on the parameters suppled by the non-ap QSTA. Ths mechansm s expected to be used for applcatons such as voce and vdeo, whch may need perodc servce from the HC. Non-QoS STAs may also assocate n a QBSS. All frames that are sent to non-qos STAs by an AP conform to the orgnal MAC ntroduced earler. ADMISSION CONTROL AND SCHEDULING FOR THE CONTROLLED HCF When the HC provdes controlled channel access to non-ap QSTAs, t s responsble for grantng or denyng pollng servce based on the admtted traffc streams. The behavor of the scheduler s as follows: The scheduler shall be mplemented such that, under the controlled HCF, all statons wth admtted traffc streams are offered s that satsfy the servce schedule. Specfcally, f a traffc stream s admtted by the HC, the scheduler shall send polls any- 76 IEEE Wreless Communcatons June 2004

6 where between the mnmum servce nterval and the maxmum servce nterval wthn the specfcaton nterval. The HC aggregates admtted traffc streams for a sngle QSTA and establshes a servce schedule for the QSTA. Durng any tme nterval [t1, t2], the cumulatve duraton shall be greater than the total tme requred to transmt all frames belongng to all the admtted streams, each arrvng at the mean data rate for the stream, over the perod [t1, t2 D]. The parameter D s set to the specfed maxmum servce nterval. If the maxmum servce nterval s not specfed, D s set to the delay bound. A mnmum set of traffc stream parameters shall be specfed durng the negotaton. The specfcaton of a mnmum set of parameters s requred so that the scheduler can determne a schedule for an admtted stream. These parameters are mean data rate, nomnal MSDU sze, and at least one of maxmum servce nterval and delay bound. Note that IEEE e specfes some mnmum mandatory requrements for admsson control and schedulng for the controlled HCF as dscussed above, and allows vendors to desgn ther own admsson control and scheduler based on some gudelnes. An nformatve example s provded for vendors consderaton n IEEE e as follows. An example of a traffc stream s defned by a set of parameters such as mean data rate, nomnal frame sze, and maxmum servce nterval or delay bound. The maxmum servce nterval s the maxmum nterval between the start of two successve QoS CF polls. All of these crtera affect the admssblty of a gven traffc stream, and any new traffc stream may be rejected. If both maxmum servce nterval and delay bound are specfed, uses the maxmum servce nterval for calculaton of the schedule. The schedule for an admtted stream s calculated n two steps: Calculaton of the scheduled servce nterval (SI) Calculaton of duraton for a gven SI for the stream Frst, calculate the mnmum of all maxmum SIs for all admtted streams. Let ths mnmum be m. Second, the scheduler chooses a number lower than m that s a submultple of the beacon nterval. Ths value s the scheduled SI for all QSTAs wth admtted streams. When a new stream requests admsson, the admsson control process s done n three steps. Frst, calculate the number of frames that arrve at the mean data rate durng the scheduled SI. Second, calculate the duraton that needs to be allocated for the stream. Fnally, the admsson control unt (ACU) determnes that the stream can be admtted when the followng nequalty s satsfed, where s the number of exstng streams and + 1 s used as the ndex for the newly arrvng stream. T ndcates the beacon nterval and T CP s the tme used for EDCF traffc. + 1 j T TCP +. SI j = 1 SI T (2) All admtted streams have guaranteed access to the channel. For the calculaton of the duraton for an admtted stream, the smple scheduler uses the followng parameters: mean data rate (r) and nomnal MSDU sze (L) from the negotated traffc stream, the scheduled SI (SI) calculated above, physcal transmsson rate (R), sze of maxmum frame (2304 bytes, M), and overheads n tme unts (O). The physcal transmsson rate s the mnmum PHY rate negotated n the traffc stream. The overheads n tme ncludes nterframe spaces, ACKs, and CF polls. For the calculaton of the duraton for an admtted stream, the smple scheduler uses the followng parameters. Frst, the scheduler calculates the number of frames that arrved at the mean data rate durng the SI: N ; then the scheduler calculates the duraton as the maxmum of: Tme to transmt N frames at R Tme to transmt one maxmum sze MSDU at R (plus overheads): SI N = *ρ (3) L L * L O M = max( +, + (4) R R O ) An example of the schedulng s shown n Fg. 3. Stream from QSTA s admtted n Fg. 3a. The beacon nterval s 100 ms, and the maxmum SI for the stream s 60 ms. The scheduler calculates a scheduled SI equal to 50 ms usng the steps explaned above. The same process s repeated contnuously whle the maxmum SI for the admtted stream s smaller than current SI. If a new stream s admtted wth a maxmum SI smaller than the current SI, the scheduler needs to change the current SI to a smaller number than the maxmum SI of the newly admtted stream. Therefore, the duraton for the current admtted streams also needs to be recalculated wth the new SI. If a stream s dropped (Fg. 3c), the scheduler mght use the tme avalable to resume contenton. The scheduler mght also choose to move the s for the QSTAs followng the QSTA dropped to use the unused tme. However, ths last opton mght requre the announcement of a new schedule to all QSTAs. Fg. 3c shows the stuaton when a stream for QSTA j s removed. THE GROUP ACKNOWLEDGMENT MECHANISM Snce the wreless medum s error-prone, transmtted frames can easly be corrupted even wthout collsons. In the IEEE MAC protocol, each frame s acnowledged. Ths approach s very natural and robust, but ntroduces a great deal of overhead. In order to reduce the acnowledgment overhead, a new mechansm, called group acnowledgment (GA), s ntroduced n IEEE e. The GA mechansm allows a group of frames to be transmtted before any acnowledgment. After sendng a burst of frames, the sender sends a group acnowledgment request (GroupAcReq) frame, and the recever must respond by sendng the group acnowledgment The specfcaton of a mnmum set of parameters s requred so that the scheduler can determne a schedule for an admtted stream. These parameters are Mean Data Rate, Nomnal MSDU Sze and at least one of Maxmum Servce Interval and Delay Bound. IEEE Wreless Communcatons June

7 The EDCF provdes a prorty scheme by dfferentatng the nter-frame space, the ntal wndow sze, and the maxmum wndow sze. Such a prortzed QoS scheme s smple to mplement and s smlar to the DffServ model. j SI = 50 ms SI = 50 ms SI (a) Schedule for stream from QSTA "I" SI (b) Schedule for streams from QSTAs "I" to "" j SI = 50 ms SI (c) Reallocaton of s when a stream s dropped j Fgure 3. An example of the scheduler. (GroupAc) frame, n whch the correctly receved frames nformaton s ncluded. All the frames, ncludng the GroupAcReq and GroupAc frames, are separated by an SIFS nterval. A typcal GA sequence durng a CF poll) s shown n Fg. 4. The sender should frst wn a usng a channel access mechansm before startng a burst. The burst length s lmted, and the amount of state that must be ept by the recever of the recevng frames s bounded. The GroupAc frame has an Ac btmap feld that can acnowledge the burst of data frames, where each bt acnowledges one potental frame. If the GroupAc ndcates that a frame was not receved correctly, the sender shall retry that frame subject to ts approprate retry lmt. Retransmtted burst data frames shall preserve ther orgnal relatve order. The recever shall mantan a burst acnowledgment record consstng of a transmtter address and a 32-octet btmap of receved frame sequence numbers. These hold the acnowledgment state of the burst data receved from that sender. PRELIMINARY RESULTS We evaluate the EDCF prorty scheme n ths secton va smulatons. The smulaton models had been developed based on the IEEE e draft [3], IEEE a standard [9], and OPNET WLAN smulaton model verson 8.0A. In our smulatons, dstrbuted admsson control s not consdered. A more complete study of prorty schemes can be found n [7]. We smulate three prorty classes n Fg. 5. Both the data rate and control rate are 6 Mb/s. The frame sze s fxed at 1024 bytes. Each staton/queue always has frames ready to send to acheve saturaton performance. The EDCF parameters are: AIFS[0] = PIFS + 1 µs; AIFS[1] = PIFS + 5 µs; AIFS[2] = DIFS; therefore, AIFS[0] < AIFS[1] < AIFS[2]; where PIFS = SIFS + SLOT = 25 µs and DIFS =SIFS + 2*SLOT = 34 µs. CW mn [0] = 16; CW mn [1] = 24; CW mn [2] = 32; therefore, CW mn [0]<CW mn [1]<CW mn [2]. Fgure 5 shows normalzed saturaton throughputs for three prorty classes over the number of actve queues. The number of actve queues n the fgure stands for the number of actve queues for each class. As the number of actve queues ncreases, throughputs for all classes decrease. As llustrated n the fgure, class 0 has a much better throughput than class 1, and class 1 has a much better throughput than class 2. Therefore, the EDCF prorty scheme s qute effectve. CPpoll Burst data Burst data Burst data Burst data Group ACK request Group ACK Fgure 4. A GA sequence. 78 IEEE Wreless Communcatons June 2004

8 CONCLUSIONS AND FUTURE WORK Ths artcle ntroduces the emergng IEEE e MAC, whch employs a contentonbased channel access EDCF and a centrally controlled channel access HCF. The controlled HCF s based on a pollng mechansm wth some enhanced QoS-specfc mechansms and frame subtypes to allow QoS data transfers durng CFP. The EDCF provdes a prorty scheme by dfferentatng the nterframe space, ntal wndow sze, and maxmum wndow sze. Such a prortzed QoS scheme s smple to mplement and s smlar to the dfferentated servces model. Dstrbuted admsson control under the EDCF, centrally controlled admsson control and a smple scheduler under the controlled HCF, the drect ln protocol, and the group acnowledgment mechansm are also ntroduced. Our prelmnary results show that IEEE e provdes very good servce dfferentaton. REFERENCES [1] IEEE WG, Part 11: Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcaton, Aug [2] Y. Xao, A Smple and Effectve Prorty Scheme for IEEE , IEEE Commun. Lett., vol. 7, no. 2, Feb. 2003, pp [3] IEEE WG, Draft Supplement to Part 11: Wreless Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons: Medum Access Control (MAC) Enhancements for Qualty of Servce (QoS), IEEE Std e/D3.3.2, Nov. 2002; draft supp. to IEEE Std , 1999 edton. [4] S. Mangold et al., IEEE e Wreless LAN for Qualty of Servce, Proc. Euro. Wreless 02, Florence, Italy, Feb [5] Y. Xao, Enhanced DCF of IEEE e to Support QoS, Proc. IEEE WCNC 03, New Orleans, LA, Mar [6] S. Cho et al., IEEE e Contenton-Based Channel Access (EDCF) Performance Evaluaton, Proc. IEEE ICC 03, Anchorage, AK, May [7] Y. Xao, Performance Analyss of Prorty Schemes for IEEE Wreless LANs, submtted to IEEE Trans. Wreless Commun., Number of actve queues Fgure 5. Normalzed saturaton throughputs of three classes. [8] IEEE b WG, Part 11: Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcaton: Hgh-Speed Physcal Layer Extenson n the 2.4 GHz Band, Sept [9] IEEE a WG, Part 11: Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcaton: Hgh-speed Physcal Layer n the 5 GHz Band, Sept BIOGRAPHIES YANG XIAO [M] (yangxao@eee.org) s an assstant professor n the Computer Scence Dvson at the Unversty of Memphs, Tennessee. Before jonng ths school n August 2002, he wored at Mcro Lnear as a MAC archtect nvolvng IEEE standard enhancement wor. He s a votng member of the IEEE Worng Group, and holds a Ph.D. degree n computer scence and engneerng from Wrght State Unversty, Dayton, Oho. Hs current research nterests nclude WLANs, WPANs, and moble cellular networs. Prorty 0 class Prorty 1 class Prorty 2 class IEEE Wreless Communcatons June