A Fair Access Mechanism Based on TXOP in IEEE e Wireless Networks

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1 11 Internatonal Journal of Communcaton Networks and Informaton Securty (IJCNIS) Vol. 8, No. 1, Aprl 16 A Far Access Mechansm Based on TXOP n IEEE 8.11e Wreless Networks Marjan Yazdan 1, Maryam Kamal, Neda Moghm * and Mahdeh Ghazvn 4 1,, Department of Informaton Technology Engneerng, Faculty of Computer Engneerng, Unversty of Isfahan, Isfahan, Iran 4 Computer Engneerng Department, Faculty of Engneerng, Shahd Bahonar Unversty of Kerman, Kerman, Iran, marjan.yazdan@lve.com, m.kamal6@yahoo.com, n.moghm@eng.u.ac.r, mghazvn@uk.ac.r Abstract: IEEE 8.11e s an extenson of IEEE 8.11 that provdes Qualty of Servce (QoS) for the applcatons wth dfferent servce requrements. Ths standard makes use of several parameters such as contenton wndow; nter frame space tme and transmsson opportunty to create servce dfferentaton n the network. Transmsson opportunty (TXOP), whch s the focus pont of ths paper, s the tme nterval durng whch a staton s allowed to transmt packets wthout any further contenton. As the fxed amounts of TXOPs are allocated to dfferent statons, unfarness appears n the network. And when users wth dfferent data rates exst, IEEE 8.11e WLANs face the lack of farness n the network. Because the hgher data rate statons transfer more data compared to the lower rate ones. Several mechansms have been proposed to solve ths problem by generatng new TXOPs adaptve to the network's traffc condton. In ths paper, some proposed mechansms are evaluated and accordng to ther evaluated strengths and weaknesses, a new mechansm s proposed for TXOP deteraton n IEEE 8.11e wreless networks. The new algorthm consders data rate, channel error rate and data packet lengths to calculate adaptve TXOPs for the statons. The smulaton results show that the proposed algorthm leads to better farness. It also acheves hgher throughput and lower delays n the network. Keywords: IEEE 8.11e, MAC, TXOP, Farness 1. Introducton IEEE 8.11 s a set of standards for the mplementaton and communcaton of computers n wreless local area network (WLAN) n the.4,.6 and 5 GHz frequency bands. 8.11e commttee s responsble to provde Qualty of Servce (QoS) n wreless networks and a mechansm called HCF 1 s proposed for ths purpose. HCF has two access methods[1]: - Enhanced Dstrbuted Channel Access (EDCA or EDCF ) - Hybrd Coordnaton Channel Access (HCCA) An mportant feature of HCF s the exstence of four access category (AC) queues and eght traffc stream (TS) queues n the MAC layer. When a frame arrves at ts MAC layer, t s tagged wth a traffc prorty dentfer (TID), consderng ts QoS requrement. TIDs can take the values of to 15 and the frames wth TID values of to are mapped n to four ACs. Frames placed n these queues use EDCF access rules. On the other hand, frames wth TID values of 8 to 15 are mapped nto eght TS queues and use HCF controlled channel access rules. The reason of separatng TS queues from AC queues s to support strct parameterzed * Correspondng author. Tel.: Hybrd Coordnaton Functon Enhanced Dstrbuted Channel Access Enhanced Dstrbuted Channel Functon QoS at TS queues whle prortzed QoS s supported at AC queues[1]. Another mportant feature of HCF s the transmsson opportunty (TXOP). TXOP s the tme nterval durng whch a partcular STA s permtted to transmt packets wthout contenton. Frames that are transmtted by a staton n each TXOP are separated by SIFS 4. The TXOP s called ether EDCF-TXOP, f t s obtaned by wnnng a successful EDCF contenton or polled-txop, f t s obtaned by recevng a QoS CF-poll frame from the QoS-enhanced AP (QAP). The maxmum value of TXOP s called TXOP Lmt, whch s detered by QAP [1].. EDCA EDCA s desgned to support the contenton-based prortzed QoS. Fgure 1 shows the structure of EDCF. AC 8.11e: up to 8 user prortes (UPs) per QSTA 8 Ups mappng to 4 Access categores (AC) (AIFSN) CW CWmax Fgure 1. EDCA proposed by 8.11e [1] Each QoS-enhanced STA (QSTA) has 4 access category queues (ACs) to support 8 user prortes (UPs). Therefore, one or more UPs are mapped to the same AC queue [1, ]. 4 Short Inter Frame Space AC1 AC AC (AIFSN1) CW1 CWmax1 (AIFSN) CW CWmax Scheduler (resolve nternal collson) Transmsson attempt (AIFSN) CW CWmax

2 1 Internatonal Journal of Communcaton Networks and Informaton Securty (IJCNIS) Vol. 8, No. 1, Aprl 16 Table 1 shows the mappng between ACs and UPs. These eght knds of applcatons do not usually transmt frames smultaneously. Therefore, ths mappng s very useful and MAC layer overhead s reduced. Fewer queues are also necessary for the mplementaton of ACs compared to those that are used for the UPs mplementaton. Each AC queue works as an ndependent DCF 5 STA and uses ts own back off parameters [1, ]. Table 1.User Prorty (UP) and Access Category (AC) Mappng [1] U D Desgnaton Servce type P e AC Not defned Background (BK) Best Effort (BE) Excellent Effort (EE) Controlled Load (CL) VI (Vdeo <1ms latency and jtter) VO (Vdeo <1ms latency and jtter) Network Control (NC) 1 Best Effort Best Effort Best Effort Vdeo Probe Vdeo Vdeo Vdeo Vdeo Two man methods have been ntroduced n EDCF to provde dfferent levels of Qualty of Servce[1]: Usng dfferent Inter Frame Space (IFS) szes for dfferent ACs. Allocaton of dfferent CW 6 szes to dfferent ACs. Smulatons have shown that although nternal collsons are reduced n EDCF, external collsons between dfferent QSTAs wth the same prortes are stll hgh. CW can alter between a mum and maxmum value and t s doubled after each unsuccessful transmsson attempt untl a pre-defned maxmum value of CW max s reached. After each successful transmsson, CW s reset to a fxed mum value of CW []. CW AC, The default values of AIFSN [ AC ], [ ] CW max [ AC ] and [ ] TXOP Lmt AC are announced by the QAP n beacon frames, and IEEE 8.11e standard allows the QAP to adapt these parameters dynamcally accordng to the network condtons. To mprove throughput, EDCF packet burstng can be used. It means that once a QSTA gans an EDCF-TXOP, t s allowed to send more than one frame wthout contendng for the medum any more. After accessng the medum, QSTA can transmt several frames tll the channel access tme does not exceed the TXOP lmt bound. SIFS s used between packet bursts so that no other QSTA nterrupts the packet burstng and f collson occurs, the EDCF burstng s ended. Ths mechansm can ncrease throughput by multple transmssons, usng SIFS 8 and burst acknowledgements. It can also reduce the network overhead [1, ]. 5 Dstrbuted Coordnaton Functon 6 Contenton Wndow QoS Access Pont 8 Short Inter Frame Space.1 EDCA parameters There are some EDCA parameters that can be adjusted to create dfferent levels of servce n IEEE 8.11e wreless networks. These parameters are[4]: - Contenton Wndow - The arbtrary nter frame space(aifs) - TXOP Table shows the default EDCA parameters for dfferent ACs. To make more servce dfferentaton, dfferent CW szes are consdered for dfferent ACs. The dfference of CW max and CW should not be too hgh for the two hgher prortes ACs (voce and vdeo) because ths wll lead to ncreased delay n the network. Due to the delay senstvty, droppng packets s preferred to watng for a transmsson opportunty, when the network s congested [4]. Table. Default EDCA parameters for each AC [4] A I TXOP lmt TXOP lmt AC F CW CW max S a PHY b PHY N (ms) (ms) Prorty AC_V Prorty 1 AC_V1 Prorty AC_BE Prorty AC_BK max max When CW s small, delay wll become less and more transmsson opportuntes wll be provded. However, a small CW causes a hgher collson probablty. When the number of hgh prorty traffc streams ncreases, the effect of CW becomes smaller, because more collsons occur between the hgh prorty flows [4]. ACK [ ] [ ] [ ] [ 1] [ ]+ [ ]+ [ 1]+ [ ]+ Fgure. Channel access mechansm of the 8.11e EDCA scheme[5] AIFS s a new nter-frame space tme that s dfferent for each AC. It s the mum tme that the medum remans dle before startng a back off. The arbtrary nter-frame space number (AIFSN) s used to calculate AIFS. AIFSN shows the number of slots that a staton should wat after SIFS and before back off or before startng ts transmsson.

3 1 Internatonal Journal of Communcaton Networks and Informaton Securty (IJCNIS) Vol. 8, No. 1, Aprl 16 Back off tme for each AC s the sum of AIFS and a random number between zero and CW. Frst of all, CW s set equal to CW for each AC and after each collson, CW s doubled untl t reaches CW max [6]. Increasng AIFS decreases the system throughput because statons must wat longer to access the medum. Ths effect becomes stronger when network load ncreases, because AIFS occurs after each transmsson. Thus, larger AIFS has negatve effects on the network under heavy load condton. So, t should be kept as small as possble. TXOP s a bounded tme nterval whch s gven to each staton. Durng ths nterval, each staton can send as many frames as possble wthout any competton wth other statons. There are two knds of TXOP: 1- The TXOP lmt used n HCCA, whch s called HCCA TXOP lmt. HCCA TXOP s unque for each QSTA and t s based on the requrements of QSTA. - The TXOP lmt used n EDCA, whch s called EDCA TXOP lmt. EDCA TXOP lmt s announced n the beacon frames that are sent perodcally by the access pont. It has a deterstc value for each access category and s dfferent from the TXOP of the other access categores. The focus of the present study s on EDCA TXOP lmt. When there are mult-rate transmssons n IEEE8.11e WLANs, fxed TXOP leads to unfarness. If a fxed TXOP s consdered for all statons of an AC, the statons wth hgher data rates can transfer more data compared to the ones wth lower data rates. The reason s that the number of transmtted packets n any gven perod of tme depends on the data rate. To solve ths problem, several mechansms have been proposed to generate TXOPs adaptve to network traffc condtons. In ths paper, the performance of some of these mechansms wll be evaluated and compared. Also, a new mechansm s proposed that helps to mprove the drawbacks of the evaluated approaches. In secton III, a number of adjustng TXOP algorthms are ntroduced and ther advantage and dsadvantages are dscussed. Wth the dsadvantages n d, a new method s proposed n secton IV to mprove the network performance. The smulaton of the proposed method and ts results are presented n Secton V and secton VI s devoted to the concluson of paper.. Related works In the adjustng TXOP algorthms, dfferent solutons are proposed to assgn larger TXOPs to the statons wth lower data rates compared to the hgh data rate ones and n ths way, they want to provde farness n the network. It s shown that equalzng the channel access tme wll lead to the throughput adaptaton wth the nodes' transmsson rates n a mult-rate WLANs []. The authors n [8] ntroduced a Dynamc TXOP (DTXOP) algorthm whch enhances farness between upstream and downstream resource allocatons n W-F networks. The authors n [9] proposed another dynamc algorthm wth the same name of DTXOP that TXOP s perodcally updated for each AC accordng to the traffc condtons. Ther smulatons showed that DTXOP mantaned farness between upstream and downstream flows and mproved throughput and delay as well. In [1], TXOP s perodcally adjusted accordng to the present traffc condton of each AC by calculatng the number of statons nvolved n each AC and packet loss rates for each connecton. Mn et al. [11] proposed a dynamc TXOP that s adjusted accordng to the condton of the statons' queues. In TBD-TXOP [11, 1] method, TXOP wll be equal to ts default value as long as the queue length s less than the threshold. But f the queue length exceeds the threshold, TXOP wll be ncreased. The value of new TXOP should not be too large because large TXOPs often cause large fluctuatons n the performance and unfarness wll occur. Feng et al.[1] set TXOPs by usng a RED lke mechansm. Queue length that s a reflecton of network load s used for TXOP adjustment n ths algorthm. RED s a method of buffer management, n whch packet loss probablty ncreases lnearly wth the average queue length. Traffc condtons are montored n QAP and statons. Smlar to RED mechansm, f queue length s less than ts low threshold, the lower value s assgned to TXOP and f the queue length s more than ths threshold, TXOP ncreases lnearly wth the average queue length. If the queue length s greater than the upper threshold, the maxmum TXOP wll be used. These algorthms have focused on mprovng the QoS of vdeo streams, smlar to [14]. Through smulaton, Suzuk et al.[15] showed that sutable TXOP deteraton has the ablty of mprovng audo and vdeo qualty n the presence of channel errors. Some smulaton results and numercal analyses have shown that TXOP value should be chosen accordng to the buffer sze [16-]. The authors of [] desgned a dstrbuted approach, n whch each node measures ts throughput n a tme wndow. Then, t compares ts throughput wth the desred one and accordngly deteres ts TXOP value. A dynamc mult-step TXOP allocaton s presented n [4],based on the estmaton of channel condtons. In ths method and n each step, traffc s re-prortzed based on the network condtons and requrements of the traffc delays. Then, the new TXOP value wll be adjusted wth the estmaton of channel error, collson and successful transmsson probabltes. The author of [5] proposed a dstrbuted TXOP allocaton scheme based on the delay bound of multmeda traffc. In the proposed scheme, a staton checks the delay bound of each data packet n the queue, and allocates ts TXOP value to guarantee ther delay bounds. In [6], accordng to the dynamsms of WLAN networks and the number of nodes n the network, a game theoretc approach called GTXOP s proposed to detere TXOP dynamcally. GTXOP s defned based on the analytcal models of EDCA. In GTXOP, nodes can choose ther TXOP autonomously and users' QoS and overall network performance are both mproved. In [], a method called (DTAF) s proposed for the dynamc allocaton of TXOP to obtan farness n mult-rate 8.11e networks. The proposed method estmates the network traffc condtons usng the frequency of collson occurrence and TXOP wll be adjusted regardng the amount of competton n the network. Smulaton results showed a better farness and also a less number of attempts to retransmt n heavy traffc loads. But ths paper consdered

4 14 Internatonal Journal of Communcaton Networks and Informaton Securty (IJCNIS) Vol. 8, No. 1, Aprl 16 the network n saturaton condton wth only three statons n the sngle-hop mode. In [8], a method called adaptve opportunty (ATXOP) s proposed to solve the unfar problem n mult-rate IEEE 8.11e networks. In ths algorthm, an average total data rate n the network s calculated and then the rato of current data rate of statons to ths average data rate s calculated. Accordng to ths rato that s lower or hgher than one; TXOP wll be ncreased or decreased. Although ths algorthm could solve the problem of unfarness, packet sze s not consdered for the TXOP deteraton. Smulaton results showed that ths algorthm does not provde farness for the nodes wth dfferent packet szes. In fact; ths algorthm leads to farness only for the small packet szes not for the large packets. Also, the network s consdered error-free and the number of nodes s not consdered n TXOP calculaton as well. Actually, collson probablty has been gnored n the network. Therefore, when the number of nodes ncreases, collson wll ncrease and data traffc sent wll decrease. In [9], some parameters are defned to assgn new TXOPs to the statons wth dfferent data rates. These parameters are defned consderng the successful transmssons durng a sngle TXOP n a mult-rate IEEE 8.11e WLAN. The length of the packets s consdered for TXOP calculaton n [9] and the smulaton showed that ths algorthm provdes almost better farness when dfferent lengths of data packets are used. Smlar to the [8], the network s consdered error-free n [9] and the number of nodes s not consdered for the TXOP calculaton. So, when the number of nodes ncreases, collsons wll more frequently occur and data traffc sent wll decrease. In another nvestgated algorthm [], channel collson and error probablty are predcted and consdered n TXOP calculaton. Thereby, the throughput enhances and the delay decreases n the network. But dfferent data rates are not nvolved n the calculaton of new TXOP. Therefore, farness s not provded n a mult-rate condton. Nodes TXOPs are ncreased for almost the same amount that can only mprove the network throughput. 4. Proposed algorthm In the proposed algorthm, a new TXOP deteraton formula s gven to mprove farness and throughput together n dfferent network condtons. Ths formula takes almost all the effectve factors n to consderaton for the TXOP calculaton. The followng parameters are defned n ths regard: To consder the mpact of dfferent transmsson rates on TXOP deteraton, RF and f [ ] are defned [8]. RF j ( _ rate ) j ATXOP = data _ rate j Avg data (1) Equaton (1) represents the rato of staton j s current data rate to the average possble data rates n the network. TXOP [ ] f ATXOP [ ] = () RF j Accordng to equaton (), a staton wth a lower data rate has a better chance of channel access compared to the statons wth hgher data rates. If data rate s the only assumed parameter for TXOP deteraton and packet sze s dsregarded, nodes wth dfferent data rates wll acheve unequal access to the medum.in fact; ths algorthm leads to farness only for the small packets not for the large ones. In addton, channel condton nformaton should also be consdered for better network performance. To apply the effect of channel condton on the TXOP deteraton P s s used. Ps can be calculated accordng to Equaton (), wth the use of P c and error probablty due to the collson, and probablty of channel error [5]. P e. Pc stands for the Pe shows the # SuccessfulTransmts Ps = = ( 1 Pc )( 1 Pe ) () # AttemptedTransmts To calculate the probablty of channel error, the network s run several tmes and the bt error rate s detered for each node. The average of these rates s consdered as the error probablty of each node. Collson probablty s estmated as the rato of the number of busy slots due to the others transmsson to the total number of slots. As t s obvous, collson probablty depends on the number of nodes and t s estmated as shown n paper [19]. The proposed dynamc TXOP allocaton s done n each staton accordng to the channel condton. When a new flow arrves at a staton, t starts wth the estmaton of successful transmsson probablty ( P s ). Then, the estmated probablty P s wll be compared to the threshold of δ. Ths threshold s assumed to consder the channel status and s detered expermentally. Usng a comprehensve smulaton, the optmum value of.54 s obtaned for δ. If P probablty s larger than the threshold, TXOP wll be s ncreased and f P s s smaller than δ, TXOP wll be decreased. The pseudo-code of dynamc TXOP allocaton procedure s gven below: f ( P > δ ) s = ( 1+ ) [ ] ( Ps δ ) = ( ) [ ] TX OP P f else f s ATXOP TX OP P f s ATXOP New TX OP = PacketSze ( 1 α ) Past TXOP + α TXOP 1 Each tme a new TXOP s calculated accordng to s P and the prevous amount of TXOP. The smoothng factor (α) s equal to.8 and a proporton of packet length s consdered n the equaton to consder the mpact of dfferent packet szes on the TXOP deteraton. Regardng Equaton (), dfferent data rates are consdered and accordng to the pseudo-code, channel error rate and data packet lengths are also nvolved n the new TXOP calculaton. Therefore, all of the prevously dscussed effectve factors are consdered n the proposed algorthm and t s expected to acheve better throughput and delay usng the proposed algorthm, compared to the other methods.

5 15 Internatonal Journal of Communcaton Networks and Informaton Securty (IJCNIS) Vol. 8, No. 1, Aprl Smulaton OPNET s used to smulate and evaluate the performance of the proposed algorthm. Its performance wll be compared wth that of the standard IEEE 8.11e EDCA. Three fxed nodes wth dfferent data rates of Mbps, 5.5Mbps and 11 Mbps are smulated that send data to the same fxed destnaton. TXOP of the statons wth AC1 and AC traffc s assumed equal to MSDU 9 and each tme they send a sngle data unt dsregardng ther data rate. Therefore, (AC) vdeo and (AC) voce Traffcs are consdered n ths smulaton. OPNET parameters are set accordng to Table. It s noteworthy that each smulaton was run for 1 dfferent Seeds. The average of the results s shown n the followng fgures. Table.Smulaton parameters Network Sze Start Tme (s) ON State Tme (s) OFF State Tme (s) Inter arrval Tme (s) Traffc Type of Servce Physcal Characterstc Short Retry Lmt Long retry Lmt Transmt Power Data Rate AC(CW,CW MAX) AC(CW,CW MAX) AC1(CW,CW MAX) AC(CW,CW MAX) m m Constant() Constant(6) Constant(1) Exponental(.41) Interactve Voce Or Interactve Multmeda Drect Sequence.5 1,,5.5,11 Mbps (,15) (15.1) (1,1) (1,1) data traffc. Therefore the proposed method acheves better farness. As mentoned before, packet length s consdered for the TXOP calculaton n the proposed algorthm. Therefore, as t was predctable, Fgures 1-1 confrmed that ths algorthm s relatvely far for dfferent packet lengths. Fgure. Data Traffc Sent at AC for the data packets of 14 bts long Fgure 4.Data Traffc Sent at AC for the data packets of bts long Usng the proposed algorthm, the TXOPs calculated for AC and AC n dfferent data rates and packet szes are shown n Table 4. Packet sze (Bytes) Mbps 5.5 Mbps 11 Mbps Table 4: New TXOPs for AC and AC New TXOP for AC (voce) New TXOP for AC n ms (vdeo) n ms Fgure 5. Data Traffc Sent at AC for the data packets of 14 bts long The AC's data traffc sent, acheved by our proposed algorthm, s shown n Fgures and 4 n dfferent data rates. Data packets are consdered 14 and bts long n Fgure and 4 respectvely. The AC's data traffc sent, acheved by our proposed algorthm, s shown n Fgures 5 and 6 n dfferent data rates. Data packets are consdered 14 and bts long n Fgure 5 and 6 respectvely. These mprovements are because of consderng data rate, channel error rate and data packet lengths n the TXOP calculatons. Fgures -6 show that traffcs wth dfferent data rates n each access category have almost the same chance to send 9 MAC Protocol Data Unt Fgure 6. Data Traffc Sent at AC for the data packets of bts long The average throughput s equal to the total number of bts (n bts/sec) forwarded from wreless LAN layers to the hgher layers n all the WLAN nodes. Fgures and 8

6 16 Internatonal Journal of Communcaton Networks and Informaton Securty (IJCNIS) Vol. 8, No. 1, Aprl 16 suggest that the throughput of the network n AC and AC classes of traffc has been ncreased through usng the proposed algorthm n comparson wth fxed TXOP usage n EDCA standard. Ths means that the proposed algorthm has mproved both the throughput and farness n the network. Fgure 1. Delay comparson of the fxed TXOP and the proposed TXOP at AC 6. Conclusons Fgure. Throughput comparson of the fxed TXOP and the proposed TXOP at AC Fgure 8. Throughput comparson of the fxed TXOP and the proposed TXOP at AC The comparson of these methods n terms of the end-toend delay s shown n Fgures 9 and 1. The end-to-end delay refers to the tme taken for a packet to be transmtted from the source to the destnaton. It s noteworthy that the end-to-end delay does not nclude the delays of the lost packets, whch are dropped due to the successve collsons. However, end-to-end delay resulted n the proposed algorthm s smaller than the delays resulted from EDCA at both AC and AC traffc classes. Fgure 9. Delay comparson of the fxed TXOP and the proposed TXOP at AC When dfferent data rates are used n IEEE 8.11e WLANs, unfarness problem occurs. The reason s that equal TXOP or transmsson opportunty s allocated to the statons, wthout any data rate consderaton. The statons wth hgher data rates can send more traffc n the network compared to the lower rate ones. Several algorthms have been proposed to solve ths problem. They detere the new TXOP adaptve to the network's traffc condton. Some of these protocols were nvestgated n ths paper and ther advantages and dsadvantages were dscussed. Consderng the drawbacks, an algorthm was proposed that takes dfferent effectve factors n to account for the TXOP deteraton. The smulaton results showed that the proposed algorthm leads to better farness and network throughput. Furthermore, packets delay mproves. References [1] Q. N, L. Romdhan, and T. Turlett, "A Survey of QoS Enhancements for IEEE 8.11 Wreless LAN," Wreless Communcatons and Moble Computng, vol. 4, pp , 4. [] I. W. Group, "IEEE 8.11, IEEE Standard for Informaton Technology Telecommuncatons and Informaton Exchange Between Systems Local and Metropoltan area Networks Specfc Requrements. Part 11: Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons,". [] M. Ghazvn and N. Movahedna, "A game theory based contenton wndow adjustment for IEEE 8.11 under heavy load," Internatonal Journal of Communcaton Networks and Informaton Securty (IJCNIS), vol. 5, 1. [4] A. Matnlaur, "Farness and transmsson opportunty lmt n IEEE 8.11 e enhanced dstrbuted channel access," Msc., Faculty of Electroncs, Communcatons and Automaton Networkng Laboratory HELSINKI UNIVERSITY OF TECHNOLOGY 8. [5] K. Ju, D. Lee, and K. Chung, "Dynamc TXOP allocaton to support QoS based on channel condtons n wreless networks," n Computng Technology and Informaton Management (ICCM), 1 8th Internatonal Conference on, 1, pp [6] Y. Dehb, H. Benaboud, and N. Mkou, "A Geometrc Dstrbuton for Tme n IEEE 8.11 DCF: An Analytcal Study," Internatonal Journal of

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