A performance analysis of block ACK scheme for IEEE 802.11e networks

Computer Networks 54 (2010) 2468–2481

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Computer Networks

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A performance analysis of block ACK scheme for IEEE 802.11e networks

Hyewon Lee a, Ilenia Tinnirello b, Jeonggyun Yu c, Sunghyun Choi a,*

a School of Electrical Engineering and INMC, Seoul National University, Seoul 151-744, Republic of Koreab Universita‘ Palermo, Dipartimento di Ingegneria Elettrica, Vaile delle Scienze, 90128 Palermo, Italyc Samsung Electronics, Suwon 443-742, Republic of Korea

a r t i c l e i n f o

Article history:Received 21 October 2009Received in revised form 2 March 2010Accepted 1 April 2010Available online 14 April 2010Responsible Editor: I.F. Akyildiz

Keywords:WLANIEEE 802.11eBlock ACKPerformance analysis

1389-1286/$ - see front matter � 2010 Elsevier B.Vdoi:10.1016/j.comnet.2010.04.001

* Corresponding author.E-mail addresses: [email protected] (H. Le

tti.unipa.it (I. Tinnirello), [email protected] (S. Choi).

a b s t r a c t

The demand for the IEEE 802.11 wireless local-area networks (WLANs) has been drasticallyincreasing along with many emerging applications and services over WLAN. However, theIEEE 802.11 medium access control (MAC) is known to be limited in terms of its through-put performance due to the high MAC overhead, such as interframe spaces (IFS) orper-frame based acknowledgement (ACK) frame transmissions. The IEEE 802.11e MACintroduces the block ACK scheme for improving the system efficiency of the WLAN. Usingthe block ACK scheme can reduce the ACK transmission overhead by integrating multipleACKs for a number of data frames into a bitmap that is contained in a block ACK frame, thusincreasing the MAC efficiency.

In this paper, we mathematically analyze the throughput and delay performance of theIEEE 802.11e block ACK scheme in an erroneous channel environment. Our extensive ns-2simulation results validate the accuracy of our analytical model and they further demon-strate that the block ACK scheme enhances the MAC throughput performance at the costof the resequencing delay at the receiving buffer.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Numerous applications and services over the IEEE802.11 wireless LANs (WLAN)—voice and video over WLAN,and the WLAN-based mesh network—have emerged re-cently. Accordingly, the demand for high-speed WLANshas also been drastically increasing. However, the IEEE802.11 medium access control (MAC) is known to have ahigh overhead, e.g., MAC headers, per-frame basis acknowl-edgement (ACK) frame transmissions, and collisions due tothe channel access method which is based on the carriersense multiple access with collision avoidance (CSMA/CA).

A new mechanism is introduced as a part of the IEEE802.11e MAC standard [1] in order to enhance the systemefficiency. That is, a transmitter gains a granted time dura-

. All rights reserved.

e), ilenia.tinnirello@com (J. Yu), schoi@

tion for the medium access, i.e., transmission opportunity(TXOP), and transmits a data burst, which refers to theset of the data frames transmitted in a TXOP in this paper.Then a receiver aggregates the multiple ACK frames of thereceived data burst. This new mechanism is referred to asthe block ACK scheme. The data frames in a data burst areseparately transmitted by the short interframe spaces(SIFS) without any intermediate immediate ACKs, and asingle control frame, which is referred to as the blockACK (BA), selectively acknowledges or negatively acknowl-edges all the transmitted data frames in a data burst atonce by using a bitmap.

There have been recent studies aimed at mathemati-cally analyzing the IEEE 802.11e block ACK scheme andevaluating its performance [2–7]. The performance of theblock ACK scheme is evaluated in [2–4] via simulationsand testbed implementations, while detailed mathematicalanalysis for the throughput and delay performance ofthe block ACK scheme is not provided. Simulation andanalytical results in [5] present the throughput and delay

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H. Lee et al. / Computer Networks 54 (2010) 2468–2481 2469

performance, but the authors consider only frame lossesdue to buffer drops, not due to channel errors. This paperpresents a firm mathematical analysis that is accompaniedwith simulation results in realistic network environments,and the results provide an insight for the system perfor-mance of the block ACK scheme considering various sys-tem parameters.

In [7], the throughput performance of the block ACKscheme over an error-free channel is mathematicallyanalyzed, and the effect of the request-to-send (RTS)/clear-to-send (CTS) exchange on the performance is alsostudied. In this paper, we extend this work by consideringthe additive white Gaussian noise (AWGN) channel. More-over, we evaluate the delay performance of the block ACKscheme according to our mathematical model and ns-2simulations.

Tianji et al. analyze the throughput performance of theblock ACK scheme over a noisy channel in [6]. However,the authors do not consider the data burst protectionmechanism which is mandated in [1]. Note that whenthe data burst protection mechanism is employed, eithera successful exchange of a data frame and the correspond-ing immediate ACK, or an RTS/CTS handshake, should pre-cede data frame transmissions in a data burst. If the databurst protection mechanism is not used, the transmittermay continue to transmit the whole data burst in spite ofthe collision of the head-of-burst (HOB) frame, and hence,all the subsequent frame transmissions continue to collide.The absence of the data burst protection mechanismapparently leads to severe degradation of the throughputperformance [6,7]. To our best knowledge, this is the firststudy that mathematically analyzes both the throughputand delay performance of the IEEE 802.11e block ACKscheme over a noisy channel, considering the data burstprotection scheme.

The remainder of this paper is organized as follows.Section 2 introduces the IEEE 802.11e, TXOP and blockACK mechanisms. Our analytical models that describe thethroughput and delay performances of the block ACKscheme are presented in Sections 3 and 4, respectively. InSection 5, our analytical models are validated via ns-2simulations, and we then evaluate how the performanceof the block ACK scheme is affected by various factors suchas the frame error rate (FER), data burst size, and numberof contending stations. Finally, we conclude this paper inSection 6.

1 Note that an MSDU is a data unit that arrives at the MAC layer from thehigher layer, and that one (or more than one, if fragmentation is employed)MAC protocol data unit (s) (MPDUs) are generated from a single MSDU atthe MAC layer.

2. Channel access and transmission operations

In distributed WLAN environments that are based onthe IEEE 802.11 distributed coordination function (DCF),a common wireless medium (channel) is shared by a num-ber of associated stations without any centralized coordi-nation. The management of the common medium isspecified by two aspects: (1) multiple access resolution,i.e., the rules that govern how a given station acquiresthe right to use the channel; and (2) channel transmissionoperations, i.e., the rules that govern how a station thatwins a contention performs transmissions without losingcontrol over the channel. In this section, we briefly de-

scribe the substantial innovations that are introduced bythe IEEE 802.11e extension regarding the second aspect.

2.1. IEEE 802.11e EDCA

The IEEE 802.11e defines the enhanced distributedchannel access (EDCA) [1] in order to provide differenti-ated services among contending stations. The IEEE802.11e EDCA includes two main features as follows: (1)differentiating the probability of channel accesses amongthe contending stations (or traffic types, exactly speaking);and (2) defining the transmission unit based on the chan-nel holding time. In this paper, we skip the details of thechannel access functions, because we focus on the innova-tive aspects of the transmission and acknowledgementoperations. The channel holding time of the second featureis based on two new channel utilization rules, the TXOPconcept and block ACK mechanism.

2.1.1. Transmission opportunity (TXOP)Under the IEEE 802.11e EDCA, not every channel access

necessarily results in a single MAC service data unit(MSDU)1 transmission. Instead, a station that wins a conten-tion attains a TXOP, during which one or more MPDUs (pos-sibly from different MSDUs) can be separately transmittedvia a SIFS interval, and the set of MPDUs that are transmittedin a TXOP is referred to as data bursts in this paper. A databurst appears to be a single instance of the medium accessto other stations, due to the separation of a SIFS interval,which is shorter than a distributed interframe space (DIFS)interval.

A TXOP is defined by its starting time and maximumduration. The starting time is represented by the timewhen the medium is determined to be available to theTXOP owner under the EDCA access rules. The TXOP limit,which is expressed in an 11 bit-field in a unit of 32 ls withthe maximum limit of 65,536 ls, defines the maximumduration of a TXOP that is determined and announced bythe access point (AP) via the beacon frames.

As a part of the TXOP operation, the IEEE 802.11e stan-dard mandates the data burst protection mechanism forthe channel access in order to quickly identify collisionsand avoid continual data frame losses of a data burst. Thatis, each TXOP of a transmitter–receiver pair is only grantedafter completing a successful exchange of either data/ACKor RTS/CTS frames, which compel the other stations to defertheir channel access to the end of the TXOP. This is enabledby the physical and virtual carrier sense mechanisms [8].We refer to the channel access modes that are initiated withthe data/ACK and RTS/CTS exchanges as the basic access andthe RTS/CTS access, respectively, depending on which frameexchange is used for the burst protection mechanism.

2.1.2. Block ACK mechanismIn the IEEE 802.11e, a new acknowledgement scheme is

defined in order to reduce bandwidth wastage due to ACK

2470 H. Lee et al. / Computer Networks 54 (2010) 2468–2481

transmissions. Basically, the block ACK mechanism allowsa number of MPDUs to be separately transmitted back-to-back via a SIFS interval, and the block ACK mechanismalso allows these MPDUs to be acknowledged by a singleaggregated ACK frame, viz., the block ACK frame (BA). Ablock ACK frame stores information about the receptionof corresponding MPDUs by using a bitmap, and a blockACK frame is transmitted as a response to an explicit trans-mitter request. This request is performed by a new controlframe that is called the block ACK request frame (BAR).2

Both the block ACK request and block ACK frame are trans-mitted at the same transmission rate that is used for the cor-responding data frame transmissions.

Before using the block ACK mechanism, the transmitter(originator) and receiver (recipient) have to set up this newacknowledgement policy by exchanging the add block ACKrequest (ADDBA request) and add block ACK response (ADD-BA response) messages. After such an initialization, theMPDUs that constitute a data block, which is defined tobe a group of MPDUs that are acknowledged by a blockACK frame, are transmitted and acknowledged. The maxi-mum number of data frames in a data block is specifiedin the initial setup phase, according to the receiving buffersize for re-ordering at the receiver, as well as the numberof MPDUs that are generated from 64 MSDUs, which isthe equivalent of 12� 64 ¼ 768 MPDUs, since the maxi-mum number of MPDUs (fragments) from a single MSDUis 12.

It should be noted that the data block size representsthe transmission window of a selective automatic repeatrequest (ARQ) scheme with aggregated ACKs, and it doesnot depend on the TXOP limit that determines the numberof MPDUs in a data burst. Therefore, a data block can becomposed of several data bursts, i.e., several TXOPs mightbe required in order to complete the transmission of a datablock. Then, the BAR of the data block is either transmittedin the last TXOP which concludes the block, or it is trans-mitted in the subsequent TXOP if the remaining TXOPduration of the last TXOP is not long enough to accommo-date the BAR.

As a response to a BAR, a BA should be received beforethe expiration of the block ACK timeout. If there is no re-sponse, then the BAR is retransmitted until all the framesin a data block are discarded due to the expiration of theirlifetime limits. Note that the data frames that are sent viathe block ACK mechanism are not subject to the retry limit,but dropped only when their lifetime expires.

2.2. Examples of channel utilization

In summary, Fig. 1a and b show examples of data blocktransmissions with a block size of 4 for the basic and RTS/CTS channel access modes, respectively, by plotting only

2 Actually, there are two types of block ACK mechanisms, namely, theimmediate and delayed versions. In the delayed block ACK, a block ACKrequest is answered with an immediate normal ACK frame by the receiver,and then a block ACK frame is sent by the receiver with the highest possiblepriority in a subsequent TXOP of the receiver. In this paper, we consider theimmediate block ACK, i.e., a block ACK request is replied to by animmediate block ACK frame, since it is superior to the delayed block ACK interms of the bandwidth utilization and frame delay performance.

consecutive TXOPs of a given station. In the figures, a datablock is assumed to correspond to a data burst, i.e., eachgranted TXOP duration is long enough to allow the trans-mission of a data block, and is terminated by exchangingthe BAR and BA frames. The time interval between twoconsecutive data bursts is represented by the area withthe slanted lines, and this includes the backoff expirationtime and time wasted in HOB frame collisions andcorruptions.

3. Throughput analysis

In the following, we consider a WLAN with N stationsthat employ the same physical layer (PHY) transmissionrates. Since we are not concerned about the differentiatedchannel access of the IEEE 802.11e in this work, we assumethat all stations generate traffic of the same priority withthe same payload size, and hence, they have the sameprobability of winning the channel contention. We con-sider that all the traffic is generated toward a fixed destina-tion, i.e., an AP, and stations work in saturated conditions,i.e., data frames are always available in their transmissionbuffers. We also assume that the lifetime of each dataframe is infinite, so that a data frame is repeatedly retrans-mitted until its delivery is successful. Finally, we assumethat stations experience identical channel conditions, andso they suffer from the same channel error probability ofdata frame transmissions.

It is obvious that the network performance is affectedby two different aspects: (1) the probability of having asuccessful channel access grant, i.e., TXOP; and (2) thechannel’s utilization efficiency. The first aspect, which rep-resents the probability that a single station wins a channelcontention, depends on the number of competing stationsand FER. The second aspect, which represents the over-heads that are required for data delivery, is a function ofthe access mode (i.e., basic or RTS/CTS), acknowledge pol-icy (i.e., immediate or block ACK), block size, and FER.

3.1. Slotted channel model

For our analysis, we use the same slotted time modelthat is introduced in [9]. In fact, we can continue to assumethat channel accesses are performed only at discrete times,i.e., after an integer multiple of backoff slots from the lastchannel activity, by embedding all the transmissions of agiven data burst (or block) into a single busy slot.

Different from [9] in which all the busy slots end withan idle DIFS time, we differentiate the time that is requiredfor resuming the backoff process, according to the errorevent of the last received frame. An erroneous framereception desynchronizes the end of each busy slot as fol-lows. In fact, as stated in the standard [8], an extended IFS(EIFS) interval3 must be used instead of a DIFS interval forthe following channel contention whenever a cyclic redun-dancy check (CRC) fails or a carrier synchronization is lost.

3 The EIFS interval is computed by adding the normal DIFS to the timerequired for transmitting an ACK frame at the lowest PHY transmissionrate.

Fig. 1. An illustration of a burst transmission for the basic access and the RTS/CTS access.


If each station experiences independent channel conditions,then the reception outcomes of a given frame may differacross stations. Thus, at the end of the channel activity, eachstation can employ the EIFS or DIFS depending on the (un)detection of a CRC error.

The correct modeling of the alternating interframespaces has complications in terms of channel slotting, be-cause using heterogeneous interframe spaces (EIFS or DIFS)across stations leads to loss of synchronization among thebackoff processes of the contending stations. That is,whenever the AP experiences a CRC error and the mediumis released in our network scenario, stations that success-fully received the same frame restart the backoff count-down earlier than others. The early backoff resumption ofstations that did not detect the CRC error gives priority tothem, and this reduces the contention level (i.e., the colli-sion probability) of channel accesses that follow failedtransmissions.

In our analysis model, we avoid modeling this phenom-enon by (1) avoiding channel captures; and (2) assumingthat all stations in a WLAN, including the destined AP aswell as the overhearing stations, experience the sameCRC event that is revealed at the AP. With this hypothesis,the channel slots are synchronized for all stations. How-ever, in our simulation results, we consider both cases inwhich CRC events at stations are dependent on and inde-pendent of the AP’s, and we qualitatively discuss theirperformances.

Fig. 2 shows an example of the slotted channel model-ing of the basic access mode in which the channel grants

are MPDU-based (DCF) and TXOP-based (EDCA). The framecorruptions are indicated by the cross-marked boxes,which could be due to either collisions or channel errors.Even though we only present the basic access mode, thefigure demonstrates that modeling of the virtual slot timesis independent of both channel access modes (i.e., RTS/CTSor basic access) and channel utilizations (i.e., single or mul-tiple MPDU transmissions, normal or block ACK), andhence, the analytical model in [9] can easily be extendedto the block ACK scheme.

3.2. Throughput derivation

Our derivation of the throughput model is based on theassumption that channel errors do not corrupt controlframes, i.e., the ACK, RTS/CTS, BA, and BAR frames. Sincethe ACK, RTS, and CTS frames are usually transmitted atlower transmission rates, and the BAR and BA frames areshorter than the data frames, this should be a reasonableassumption in many practical environments.

Given our slotted channel model, each station tries toobtain a TXOP at the beginning of an arbitrary virtualslot with probability s [9]. Then depending on the num-ber of stations that simultaneously access the channeland frame error, we can classify each slot as one of thefollowing:

� Idle slot, which occurs whenever all the stations are pre-vented from accessing the channel because of the ongo-ing backoff countdown;

Table 1PHY payload and header transmission times assuming IEEE 802.11b (in ls).

Parameter Transmission time (ls)

TPHY 192TDATA TPHY + (224 + L)/rTACK TPHY + 112/r*

TRTS TPHY + 160/r*

TCTS TPHY + 112/r*

TBAR TPHY + 192/rTBA TPHY + 1216/r

Fig. 2. Slotted channel model.


� Burst slot, which occurs whenever only one stationaccesses the channel and the HOB transmission of thatstation is successfully acknowledged without a channelerror; and� Corruption slot, which occurs whenever an HOB frame

experiences a collision due to simultaneous channelaccesses of more than one station or a channel error.

We denote the occurrence probabilities of idle, burst,and corruption slots as Pidle, Pb, and Pc, respectively. LetPe represent the FER, i.e., the probability that a data frameis corrupted due to channel errors. Let Pe,HOB be the proba-bility of an HOB transmission failure due to channel errors,and let Ps be the probability that only a single stationaccesses the channel in a given slot, thus resulting in Pb = Ps

(1 � Pe,HOB). Note that according to our previous assump-tion that control frames are error free, Pe,HOB is equal to 0in the RTS/CTS access mode, while Pe,HOB = Pe in the basicaccess mode. Given the average bits that are delivered ina data burst, E[PT], the system throughput, S, is determinedby

S ¼ PbE½PT�Pidlerþ PbTb þ PcTc

¼ PbE½PT�E½slot� ; ð1Þ

where r is the backoff slot duration, Tb is the data bursttransmission time, Tc is the average wasted time due toHOB transmission failures, which are caused by either col-lisions or channel errors, and E½slot� is the average virtualslot duration.

As discussed earlier, the slot occurrence probabilities Pi-

dle, Ps and Pc do not depend on whether the block ACKscheme is enabled, and hence, they can immediately be de-rived on the basis of the well-known results that are givenin [9]. As in [9], each MAC instance is summarized by achannel access probability s, which is a function of thetransmission failure probability p. In our case, p dependson the probability of simultaneous accesses as well asHOB errors that are due to channel errors, i.e.,

p ¼ 1� ð1� sÞN�1ð1� Pe;HOBÞ; ð2Þ

where the coupling of p and s directly follows the equa-tions in [9].

In the 802.11b WLANs, different modulation schemeswith correspondingly different transmission rates (i.e., 1,2, 5.5, and 11 Mbps) are available. Let r and r* be the em-ployed frame transmission rates of the data and controlframes, respectively. Let L be the length of an MSDU in bits.Moreover, let TDATA, TACK, TRTS, TCTS, TBAR, and TBA be thetransmission times (in ls) of an MPDU, ACK, RTS, CTS,BAR, and BA frames, respectively. Each frame includes acommon physical header, so that its duration TPHY has tobe added to the frame transmission time; all the time dura-tions are summarized in Table 1.

Although the block ACK mechanism is very flexible aswe described in the previous section, in this paper we donot consider any data block that is split across multipleTXOPs. Accordingly, whenever the block ACK mechanismis employed, each TXOP always starts with a data/ACK orRTS/CTS exchange depending on the channel access mode,accommodates as many data frames as possible within aTXOP limit, and finally concludes with a BAR and BA trans-mission. This TXOP operation is justified by the followingreasons: (1) splitting a data block across multiple TXOPsintuitively deteriorates the delay performance since a dataframe takes long time to be acknowledged; and (2) giving

4 We assume that the successfully received frames are stacked in thereceiving buffer, and the receiver delivers the first contiguous sequence-runof the data frames to the upper layer on the reception of the BAR frame.


up a granted TXOP spoils the resource utilization efficiencysince a station looses a guaranteed chance to access thechannel. Consequently, a data block corresponds to a singledata burst that is embedded in a single burst slot of ourmodel. We denote the number of data frames that can beaccommodated in a single TXOP, i.e., the data block size,as B. The transmitter maintains the data block size B bysubstituting the previously acknowledged frames withnew ones that need to be transmitted, as shown in Fig. 1.Thus, a data burst duration Tb of a burst slot is given by

Tb ¼ OS þ ðTDATA þ SIFSÞ � Bþ OE; ð3Þ

where OS is the overhead that is required for starting a databurst, which depends on the channel access mode, and OE

represents the overhead that is required for ending theburst transmission by a BAR/BA exchange. Specifically,the starting overhead OS is given by

OS ¼TACK þ SIFS; Basic access;TRTS þ TCTS þ 2SIFS; RTS=CTS access;

�ð4Þ

and the ending overhead OE is given by

OE ¼ TBAR þ SIFSþ TBA þ DIFS: ð5ÞWhen a CRC error is revealed by the AP on the HOB

frame due to either channel errors or collisions, in ourmodel, all the other stations reveal the same CRC errorand they defer the resumption of their backoff by an EIFSinterval, that is,

Tc ¼ THOB þ EIFS; ð6Þ

where THOB represents TDATA or TRTS depending on the em-ployed channel access mode.

Finally, considering that each data frame experiences anindependent channel error rate Pe, and each data block hasa size B, we express the average payload bits that are deliv-ered in a data burst as

PT ¼ L � fðB� dÞð1� PeÞ þ dg; ð7Þ

where d is an indicator that is set to 1 in the basic accessmode, and 0 in the RTS/CTS access mode. Note that in thebasic access mode, each data burst in a burst slot alwaysincludes at least one uncorrupted HOB data frame, whichis acknowledged by a normal ACK, and d represents thisexceptional HOB data frame.

4. Delay analysis

In [10] and [11], the authors analyze the delay perfor-mance of the IEEE 802.11 DCF via Little’s law and Markovchain model, respectively. However, these results cannotbe directly applied to the block ACK scheme, because ofthe in-order delivery requirement in the standard [1].

For example, in Fig. 1a, let us assume that Frame 3 is re-ceived in error within the first data block transmission.Even though Frame 4 is correctly received at the receiver,this frame cannot be delivered to the upper layer of the re-cipient MAC, since Frame 3 must be delivered to the upperlayer first according to the in-order delivery constraint.That is, the delivery of Frame 4 is delayed until Frame 3is correctly received in subsequent data block transmis-sions. We refer to this delay as the resequencing delay,

which is defined by the elapsed time from the time epochof successful frame reception at the receiving buffer to thetime epoch of frame delivery to the upper layer.4

Regarding this resequencing delay, we define the delay ofa data frame as the elapsed time from the time epoch ofinitial frame transmission at the transmitter to the timeepoch of frame delivery to the upper layer of the recipientMAC. Note that the delay we defined does not include thequeuing delay at the transmission buffer, since it is mean-ingless in the scenario where the network is saturated.

In the following analysis, even though the standard lim-its the number of MSDUs that are acknowledged via a BAframe to 64, we assume that a BA frame can acknowledgeas many MSDUs as required. This reduces the complexityof our analytical model, since a transmitter needs onlyone BA frame in order to acknowledge reception of the cor-responding MSDUs. In Section 5, we validate this assump-tion via our simulation results by demonstrating that thenumber of MSDUs that are acknowledged by a BA framerarely exceeds the limit of 64.

In order to compute the delay of a frame, we focus on atagged frame in the transmission buffer. Let J be a discreterandom variable that has a positive integer value, whichrepresents the position of the tagged frame within the datablock of its initial transmission attempt. Obviously, J be-longs to the range [1,B]. When J = 1, the tagged frame isin the first position of the transmission buffer (i.e., head-of-line (HOL) frame) during the contention phase, whichprecedes the burst slot for the initial transmission attemptof the tagged frame. Therefore the tagged frame experi-ences the contention delay as an HOL frame. On the otherhand, when J > 1, the frame reaches the first position of thetransmission buffer (i.e., it becomes an HOL frame) in theperiod when a burst slot has already been granted, and thisimplies that the tagged frame does not experience the con-tention delay during the initial transmission attempt.However, the frame with J > 1 eventually experiences thecontention delay during retransmissions.

Consequently, we readily see that the delay of thetagged frame depends on J. Therefore, first, we derive theprobability distribution of J; second, we derive the numberof (re) transmission attempts of the tagged frame; and fi-nally, we formulate the frame delay of the frame J = j inthe following sections.

4.1. Probability mass function of J

We define a probability p(x,y) where x frames are cor-rupted due to channel errors with the probability Pe, froma data block that is composed of y frames. Considering anexceptional HOB data frame transmission that is denotedby d depending on the channel access mode, p(x,y) is givenby

pðx; yÞ ¼y� d

x

� �Px

eð1� PeÞy�d�x; ð8Þ

where x ¼ 0;1; . . . ; b; y.


Let M be a discrete random variable that has a non-neg-ative integer denoting the number of erroneous dataframes in a data block of size B with frame error probabilityPe, and considering an exceptional HOB data framedepending on the channel access mode. The probabilitymass function of M is expressed as

pMðmÞ ¼ pðm;BÞ ¼B� d

m

� �Pm

e ð1� PeÞB�d�m; ð9Þ

where m ¼ 0;1; . . . ;m;B. In Eq. (9), we assume thatB� 1

B

� �is 0 so that pM (B) = 0 in the basic access mode,

i.e., at least one data frame is successfully delivered viaan immediate ACK.

Given that m frames are corrupted from B frames thatare transmitted in the previous data block, only positionsfrom m + 1 to B are available for new frames in the nextdata block, due to retransmissions of m frames, i.e., J is uni-formly distributed within a range of [m + 1,B]. Therefore,given m, the conditional probability mass function of J is

pJðjjmÞ ¼1

B�m; ð10Þ

where m ¼ 0;1; . . . ;m;B� 1, and j ¼ 1;2; . . . ;m;B�m.Note that in order for J to be valid, i.e., J > 0, m in Eq. (10)should be less than B, since none of the new frames canbe processed when every frame of the previous data blockneeds to be retransmitted.

Finally, we can find the probability mass function of J byaveraging the conditional probability mass function of Jover m, i.e.,

pJðjÞ ¼Xj�1

m¼0

1B�m

� pMðmÞ1� pðB;BÞ ; ð11Þ

where j ¼ 1;2; . . . ;m;B. The last term of the right-hand-side of Eq. (11) represents the normalized probability massfunction of M to satisfy m < B.

4.2. Number of transmissions of tagged frame

Now we calculate the required number of (re) transmis-sions of the tagged frame, which consequently determinesthe actual delay. Let Kj be the number of required TXOPs todeliver the tagged frame with Jj, including retransmissionsthat follow the initial transmission. Kj is a positive integersuch that Kj P 1, and the tagged frame requires Kj succes-sive burst slots before being delivered to the upper layer of

Fig. 3. An illustrative example f

the recipient MAC. Note that the delivery of the taggedframe at the jth position implies that all the frames inthe preceding (j � 1) positions are successfully delivered.

Let us consider a case when a block of j frames are suc-cessfully transmitted in k TXOPs, and the number of trans-mission failed frames in the ith TXOP is ni. We also definen0 = j. Note that ni P nl for i < l 6 k, and nk = 0. Then, theprobability of successfully delivering a block of j framesin k TXOPs, given ni, is represented by

Qki¼1pðni;ni�1Þ, where

p(x,y) is defined in Eq. (8). The probability mass function ofKj is then the probability of successfully delivering a blockof j frames in Kj TXOPs for an arbitrary Kj, i.e.,

pKjðkjÞ ¼

Xn0�d

n1¼0

Xn1�d

n2¼0

� � �Xnkj�2�d

nkj�1¼0

Ykj

i¼1

pðni;ni�1Þ" #

; ð12Þ

where n0 = j and kj ¼ 1;2; . . . ;m. Now, we can numericallyobtain the average values, E½Kj�, which are used for the de-lay derivation.

4.3. Delay of tagged frame

In our slotted channel model, a successful burst slot ap-pears on average every 1

Pbvirtual slots, and these successful

virtual slots are shared by N stations. Therefore, a stationobtains a successful burst slot on average every N

Pbslots.

Let D(j) be the delay of the tagged frame with J = j. Thenthe average delay for the tagged frame where position Jis equal to 1, D(1), is given by

Dð1Þ ¼ E½K1�E½slot� NPb: ð13Þ

For the tagged frame in the other positions where J > 1, therequired contention time for acquiring the first burst slotdoes not need to be considered, since this frame becomesan HOL frame when the first burst slot, i.e., the first TXOP,has already been granted. Therefore D(j) for J > 1 is givenby

DðjÞ ¼ ðE½Kj� � 1ÞE½slot� NPbþ ðTDATA þ SIFSÞðB� jþ 1Þ þ OE;

ð14Þ

where j ¼ 2;3; . . . ;m;B. The last two additive terms of theright-hand-side of Eq. (14) represent the residual part ofthe first burst slot, which follows the beginning of thetagged frame transmission.

Fig. 3 provides a graphical representation of the delaymodel, by presenting contiguous TXOPs that are granted

or the delay computation.

5 Specifically, 7.725 ms and 7.684 ms in the basic and RTS/CTS accessmode, respectively, when the TXOP limit is 20.556 ms and the channel iserror-free.


to a given station, when the basic access mode is employedand J = 4. The corrupted frames are marked by the crossinglines and the tagged frame is illustrated by the grey box. Agiven station is allowed to transmit a data block after achannel contention that is denoted by the area with theslanted lines, and this contention includes the channelusages by other stations and collisions. The average timebetween the starts of two successive bursts is E½slot� N

Pb.

For a frame in position J > 1, the delay computation startsfrom the time in which the frame reaches the HOL positionin the transmission buffer, and ends at the reception of theBAR frame on the Kjth attempt, as indicated in Fig. 3. Thedelay accordingly includes only Kj � 1 contention phases,since the frame does not have to contend for the initialtransmission attempt when J > 1.

Finally, by averaging all J positions, the average delay iscomputed by

E½D� ¼XB

j¼1

DðjÞpJðjÞ � ðSIFSþ TBA þ DIFSÞ; ð15Þ

where we also subtract the residual part of the last burstslot, after the successful reception of the BAR frame, asshown in Fig. 3.

5. Performance evaluation

We now validate our analytical model via ns-2 simula-tions [12] and we study how the performance of the blockACK scheme is affected by various factors such as the FER,burst size, and channel access modes. We consider an IEEE802.11b WLAN that is composed of a single AP and a num-ber of associated stations that send their data frames to theAP. We assume that these stations use transmission ratesof 11 Mbps for data/BAR/BA transmissions and 1 Mbpsfor RTS/CTS/ACK transmissions. We also consider a fixedMSDU size of 1500 bytes. Each simulation result is ob-tained by averaging over 600 s simulation run. Dependingon whether or not all the stations experience the sameframe error events as the AP, in the following simulationresults, we present two different channel error events,i.e., synchronous and asynchronous frame errors.

5.1. Throughput performance

First, we verify the enhancement of the aggregatethroughput performance according to the block ACKscheme. Fig. 4 shows the aggregate throughput perfor-mance versus the TXOP limit for different channel accessmodes and frame error rates. It is well known that themaximum achievable aggregate throughput of the IEEE802.11 DCF is limited due to the fixed amount of overheadper frame, e.g., the PHY preamble length, SIFS, ACK trans-mission time, and backoff duration [13]. However, theblock ACK scheme with the TXOP operation permits multi-ple transmissions in the TXOP, and alleviates the per-frameACK overhead. Hence, the aggregate throughput is drasti-cally improved as the TXOP limit is extended. Specifically,the aggregate throughput increases in a step-wise mannerwhenever the extended TXOP limit allows one more dataframe to be transmitted, which is 1.315 ms including a SIFS

interval, in a data burst. However, the throughput gain ofthe block ACK scheme becomes marginal after the numberof data frames in a TXOP exceeds a certain threshold,around 10 data frames in the TXOP in our network sce-nario. This is because the MAC overhead, which is causedby the other factors, i.e., the SIFS timings and backoff dura-tion, disturbs the relaxation of the ACK overhead.

Next we compare the basic and RTS/CTS access modesin order to assess the influence of the RTS/CTS exchangeon the aggregate throughput. Note that the RTS/CTS ex-change is also the overhead for data burst transmissionswhen the contention level is low. Therefore, when thereare few contending stations, as shown in Fig. 4a, the basicaccess mode outperforms the RTS/CTS access mode. How-ever, as the number of contending stations increases, theRTS/CTS exchange mitigates the collision overhead forthe channel access. As a result, the performance gap be-tween the basic and RTS/CTS access modes is inverted inFig. 4b. It should be also noted that the throughput gainof the RTS/CTS exchange is not significant even thoughthere are 50 stations in Fig. 4b. The RTS/CTS exchange issupposed to mitigate the collision overhead by reducingthe time that is wasted due to collisions via short RTSframe transmissions. In [14], the condition that the RTS/CTS exchange is beneficial is well described; the RTS/CTSexchange is advantageous only if the average virtual slottime of the RTS/CTS access mode is shorter than that ofthe basic access mode. In our network scenario, the differ-ence of the average virtual slot time is very small5 due tohigh data transmission rates (11 Mbps), low control frametransmission rates (1 Mbps), and TXOP operations. There-fore, the RTS/CTS access mode in Fig. 4b is little better thanthe basic access mode.

The long TXOP limit, i.e., the large data burst size, im-proves the throughput performance by prolonging thetime portion that is used for data transmissions. This longdata transmission time makes the contention overheadless affect the throughput performance, since the time por-tion that is wasted by MAC contentions becomes relativelysmall. Hence, the number of stations matters little to thethroughput performance in the long TXOP limit region, sothe throughput performances of 5 station and 50 stationcases are almost the same when the TXOP limit is 20 ms.Conversely, the contention overhead, which grows as thenumber of stations increases or the basic access mode isused, severely degrades the throughput performance ifthe TXOP limit is short. One can verify this phenomenonin Fig. 4 that the throughput loss of the basic access modein the 50 station scenario is worse than that of the RTS/CTSaccess mode.

The following provides the validation of our analyticalmodel and discussion on the asynchronous frame errorsthat are mentioned in Section 3. In Fig. 4, the analytical re-sults are well matched with the simulation results whenthe channel error events are synchronous, and this vali-dates our analytical model given the assumption of syn-chronous frame errors. However, if the channel error

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RTS/CTS access, simulation (sync), FER = 0.1RTS/CTS access, simulation (async), FER = 0.1

(b) 50 stations.

Fig. 4. Aggregate throughput performance versus TXOP limit.


events are asynchronous in the basic access mode so thatonly some of the stations find a CRC failure of an HOB dataframe, the analytical results of the basic access modeunderestimate the aggregate throughput compared withthe simulation results.

This mismatch is due to the varying contention levelthat is caused by asynchronous CRC failures of HOB frames.In reality, a station that receives an erroneous frame due toCRC failures or carrier synchronization losses must deferfor an EIFS interval, instead of a DIFS interval. Therefore,after an arbitrary frame transmission is completed, sta-tions that successfully received the frame can access thechannel after a DIFS interval, while the others access thechannel after an EIFS interval, which is longer than a DIFS

interval. The postponed channel access of these EIFS-defer-ring stations eventually reduces the contention level of thestations that contend after a DIFS interval, in contrast tothe case when all stations in a WLAN successfully/errone-ously receive a frame and contend after a(n) DIFS/EIFSinterval. The reduced contention level correspondingly en-hances the aggregate throughput of the basic access modewith asynchronous frame errors in Fig. 4.

On the other hand, there is no such disordered channelaccess in the RTS/CTS access mode, since we assume thatthe control frames, i.e., RTS, CTS, BAR, or BA frames, arenot corrupted due to channel errors. In fact, the EIFSdeferring occurs only for the first data frame that is usedfor the channel contention in the basic access mode, since


receiving BAR and BA frames, which are error-free, at theend of each data burst renews EIFS settings from erroneousdata frame receptions within a data burst transmission.Therefore the simulation and analytical results of theRTS/CTS access mode show the same throughputperformance.

As we discussed in Section 3, the correct modeling ofthis varying contention level of asynchronous channelerror events requires a complex mathematical modelthat is beyond the scope of this paper. Hence, we willaddress this problem in our future work. In the followingsimulation results, we only consider synchronous frameerrors of both the simulation and analytical results inorder to fully envisage the implications of our analyticalmodel.

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Fig. 5. Delay performance

5.2. Delay performance

Although the block ACK scheme enhances the through-put performance, it increases the average delay due to thebuffering operation of the recipient in an error-prone envi-ronment. Moreover, having a large number of frames in adata block causes a far worse delay performance, sincethe frame acknowledgement time is longer. Fig. 5 demon-strates the average delay for a frame delivery in a functionof the TXOP limit. Comparing the orders of Fig. 5a and byields that the frame delay is approximately proportionalto the number of contending nodes, and this observationcorresponds to our delay model in Eqs. (13) and (14). Thewell-matching simulation and analytical results also con-firm the accuracy of our analytical model in all cases.

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is, FER = 0n, FER = 0is, FER = 0n, FER = 0, FER = 0.1, FER = 0.1, FER = 0.1, FER = 0.1

stations.

12 14 16 18 20 22 (msec)

is, FER = 0n, FER = 0is, FER = 0n, FER = 0, FER = 0.1, FER = 0.1, FER = 0.1, FER = 0.1

stations.

versus TXOP limit.


The lower lines in Fig. 5a and b present the average de-lay as the TXOP is extended when the frame error rate iszero. The discrepancy between the trends of the twographs arises since dominant factors that determine theaverage delay depend on the number of contending sta-tions. Our average delay model is mainly composed oftwo delay factors, namely, (1) the channel access delayincluding channel contentions, and (2) the buffering delayat the receiving buffer caused by the block ACK operation.Note that E[Kj], i.e., the number of required burst transmis-sions for the delivery of the jth frame, is 1 in an error-freeenvironment, which implies that a data frame needs only asingle TXOP to be successfully delivered. Then according to

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Fig. 6. Delay performance and required TXOP

Eqs. (13) and (14), the first data frame suffers from the longchannel access delay due to contentions as well as the buf-fering delay, while the other data frames merely experi-ence the buffering delay within a TXOP as the channelaccess delay is negligible (SIFS timing). Consequently, ifthe contention overhead dominates the delay performance,then averaging the delay of data frames makes the longchannel access delay of the first data frame in a TXOP bedistributed across the other frames. This averaging affectsfurther as the data burst size grows, i.e., the large databurst size alleviates the average frame delay, as depictedin Fig. 5b and short TXOP limit region in Fig. 5a. On theother hand, when the buffering delay influences the frame

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s of individual frames, FER = 0.1, B = 15.


delay more than the contention delay, then the decrease ofthe average frame delay is compromised, and hence, theaverage frame delay slightly ascends as the TXOP limitincreases in Fig. 5a. Due to the same reason, the averagedelay in Fig. 5b eventually rises as the TXOP limit is furtherincreased, even though this is not demonstrated in Fig. 5bdue to the limited space.

Error-prone channel environments deteriorate the de-lay performance of the block ACK scheme due to the rese-quencing delay that occurs at the receiving buffer. Notethat in spite of the delay averaging effect that is explainedabove, a large data burst size over a noisy channel can stillincur buffering and resequencing operations in the recipi-ent, which dominate the delay performance of the blockACK scheme. The frame delay accordingly swells as thedata burst size increases, regardless of the contention level.The results in Fig. 5 also demonstrate that the RTS/CTS ac-cess mode incurs a worse frame delay then the basic accessmode in error-prone channel environments, because of de-layed acknowledgements of data frames when the firstdata frame is corrupted. For instance, if the first data frameis received in error in the basic access mode, then the fol-lowing 2 6 j 6 B data frames are not transmitted, since thetransmitter cannot attain a TXOP. However, if the RTS/CTSaccess mode is employed, then these frames are transmit-ted regardless of the successful reception of the first dataframe because the transmitter already holds the channelby exchanging RTS/CTS frames. The subsequent bufferingoperation of the transmitted frames increases the rese-quencing delay of these frames until the first data frameis successfully retransmitted in the following TXOPs, sothat the data block is reconstructed in an orderly manner.

In order to further study the delay performance of theindividual frames, we examine the frame delay as well asthe number of required TXOPs for delivery of each data

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Fig. 7. Evolution of the sequence gap of the receiv

frame in Fig. 6. The frame error rate is 0.1 and the databurst size is 15. Recall from Eq. (12) that the number of re-quired TXOPs for a successful frame delivery only dependson the frame error rate and channel access mode, while thenumber of required TXOPs is irrelevant to the contentionlevel. Fig. 6 shows that the first data frame of the basic ac-cess mode requires a single TXOP as it is acknowledged viaan immediate ACK, while this is not the case in the RTS/CTSaccess mode. Then, the number of required TXOPs mono-tonically increases as J increases, since the latter-locatedframes necessitate successful deliveries of the precedingframes. Moreover, even though the number of requiredTXOPs of J = 1 is close to 1, the actual frame delay is signif-icant, whereas the frame delay and number of requiredTXOPs of the other data frames show similar incrementaltrends. This complies with our delay model, where the ma-jor delay factor of the first data frame and the other dataframes is the channel contention and resequencing delayin an erroneous channel, respectively.

5.3. Bitmap size of a BA frame

In our analytical model, it is assumed that a BA framecan acknowledge an unlimited number of MSDUs. Wenow provide the simulation results to investigate the ac-tual number of MSDUs that are acknowledged by a BAframe, and show that the previous assumption on the bit-map size does not affect the numerical performance resultsin general network environments.

According to the standard [1], a BA frame conveys a bit-map that acknowledges a sequence run of at most 64MSDUs, which all have contiguous sequence numbers thatstart from the sequence number that is denoted in theBlock Ack Starting Sequence Control field of the BA frame.Therefore, a recipient can acknowledge MSDUs in its

160 180 200 time (sec)

Basic accessRTS/CTS access

ing buffer for 100 s, FER = 0.1, B = 15, N = 5.


receiving buffer at once by using a single BA frame only ifthe sequence gap between the first and the last MSDUsin the receiving buffer is less than 64; otherwise, the reci-pient continues to acknowledge by using the subsequentBA frame, and hence, the frame delay grows worseaccordingly.

The simulation results in Fig. 7 illustrate the evolutionof the sequence gap between the first and last MSDUs inthe AP’s receiving buffer that is dedicated to a specificstation. The frame error rate is 0.1 and the data block sizeis 15. We only demonstrate the WLAN with 5 stations dur-ing a 100-s simulation interval, because the results aresimilar for the WLAN with 50 stations and the other partsof the simulation period as well. The sequence gap of thebasic access mode is generally less than that of the RTS/CTS access mode, because the immediate ACK of the basicaccess mode mitigates the acknowledgement overhead forthe BA frame at the beginning of each TXOP. Furthermore,the sequence gap rarely exceeds 64, so we conclude thatthe current bitmap size of the standard is acceptable forgeneral network environments.

6. Conclusion

In this paper, we mathematically analyze the through-put and delay performances of the 802.11e block ACKscheme according to various channel access modes, con-sidering channel errors and resequencing delay at thereceiving buffer. We also verify the accuracy of the pro-posed analytical model by comparing the numerical resultswith the ns-2 simulation results.

Increasing the size of a data block improves the aggre-gate throughput performance with better efficiency forthe MAC level acknowledgement at the cost of a longerframe delay, due to postponed acknowledgements via theblock ACK scheme. The channel errors deteriorate thethroughput and delay performances because of retrans-missions of corrupted frames. Especially, exchanging RTS/CTS frames over an error-prone wireless channel furtherdegrades the delay performance considering the rese-quencing delay, and that is caused by the in-order deliveryconstraint for the standard compliant operation. We alsoshow that the average delay is approximately proportionalto the number of stations in a WLAN, since the chance towin the channel contention, which dominates the averagedelay, is also roughly proportional to the number of con-tending stations. Finally, we examine the sequence gap ofthe receiving buffer for the block ACK scheme, and confirmthat the bitmap size that is defined in the standard isreasonable.

Acknowledgements

This research was supported by MKE (The Ministry ofKnowledge Economy), Korea, under ITRC (InformationTechnology Research Center) support program supervisedby NIPA (National IT Industry Promotion Agency) (NIPA-2010-(C1090-1011-0004)).

References

[1] IEEE Std., IEEE 802.11e, Part 11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) Specifications: MediumAccess Control Quality of Service Enhancements, September 2005.

[2] O. Cabral, A. Segarra, F.J. Velez, Implementation of IEEE 802.11e blockacknowledgement policies based on the buffer size, in: Proceedingsof European Wireless Conference, Prague, Czech Republic, 2008, pp.1–7.

[3] R. Faruqui, S. Chani, A simulation study of block acknowledgementand TXOPs under varying channel conditions, in: Proceedings of IEEEInternational Multitopic Conference, Karachi, Pakistan, 2008, pp.286–289.

[4] Y. Jeong, S. Kakumanu, C.-L. Tsao, R. Sivakumar, VoIP over Wi-Finetworks: performance analysis and acceleration algorithms, MobileNetwork Appl. 14 (4) (2009) 365–551.

[5] G. Min, J. Hu, M.E. Woodward, A dynamic IEEE 802.11e TXOPSchemes in WLANs under self-similar traffic: performanceenhancement and analysis, in: Proceedings of IEEE ICC, Beijing,China, 2008, pp. 2632–2636.

[6] T. Li, Q. Ni, T. Turletti, Y. Xiao, Performance analysis of the IEEE802.11e block ACK scheme in a noisy channel, in: Proceedings of IEEEBROADNET, Boston, Messachusetts, USA, 2005, pp. 511–517.

[7] I. Tinnirello, S. Choi, Efficiency analysis of burst transmissions withblock ACK in contention-based 802.11e WLANs, in: Proceedings ofIEEE ICC, Seoul, Korea, 2005, pp. 3455–3460.

[8] IEEE Std., IEEE 802.11-2007, Part 11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) Specifications (Revision ofIEEE Std. 802.11-1999), June 2007.

[9] G. Bianchi, Performance analysis of the IEEE 802.11 distributedcoordination function, IEEE J. Select. Area Commun. 18 (3) (2000)535–547.

[10] G. Bianchi, I. Tinnirello, Remarks on IEEE 802.11 DCF performanceanalysis, IEEE Commun. Lett. 9 (8) (2005) 765–767.

[11] P. Chatzimisios, A.C. Boucouvalas, V. Vitsas, IEEE 802.11 packet delay– a finite retry limit analysis, in: Proceedings of IEEE GLOBECOM, SanFrancisco, California, USA, 2003, pp. 950–954.

[12] The Network Simulator – ns-2. <http://www.isi.edu/nsnam/ns/>.[13] Y. Xiao, J. Rosdahl, Throughput and delay limits of IEEE 802.11, IEEE

Commun. Lett. 6 (8) (2002) 355–357.[14] I. Tinnirello, S. Choi, Y. Kim, Revisit of RTS/CTS exchange in high-

speed IEEE 802.11 networks, in: Proceedings of IEEE WoWMoM,Taormina, Italy, 2005.

Hyewon Lee received his B.E. degree fromSchool of Electrical Engineering from SeoulNational University (SNU), Seoul, Korea in2006. He is currently working toward hisPh.D. in the School of Electrical Engineering atSeoul National University, Seoul, Korea. Hisresearch interests include cognitive radios,3rd generation partnership project long termevolution (3GPP-LTE), performance evalua-tion for wireless networks, in particular, IEEE802.11 wireless local-area networks (WLANs).He is a student member of IEEE.

Ilenia Tinnirello has been Assistant Professorat the University of Palermo since January2005. She received the Laurea degree inElectronic Engineering and the Ph.D. in Com-munications, in April 2000 and February 2004,respectively. In 2004 she was visitingresearcher at the Seoul National University,Korea. In 2006 she was visiting researcher atthe Nanyang Technological University of Sin-gapore. Her research activity has mainlyfocused on wireless networks and in particu-lar on: multiple access algorithms with qual-

ity of service provisioning; cross-layer interactions between accesssolutions and physical layer; mobility management and load balancing inwireless packet networks.

http://www.isi.edu/nsnam/ns/

etworks 54 (2010) 2468–2481 2481

Jeonggyun Yu received his B.E. degree in theSchool of Electronic Engineering from Korea
University in August 2002 and Ph.D. at theSchool of Electrical Engineering, SeoulNational University (SNU) in February 2009,respectively. He is currently a Senior Engineerwith Samsung Electronics Co., Ltd., developingmobile/wireless access system. His researchinterests include QoS support, algorithmdevelopment, performance evaluation forwireless networks.
H. Lee et al. / Computer N

Sunghyun Choi is currently an AssociateProfessor at the School of Electrical Engi-neering, Seoul National University (SNU),Seoul, Korea. Before joining SNU in September2002, he was with Philips Research USA,Briarcliff Manor, New York, USA as a SeniorMember Research Staff and a project leaderfor three years. He received his B.S. (summacum laude) and M.S. degrees in electricalengineering from Korea Advanced Institute ofScience and Technology (KAIST) in 1992 and1994, respectively, and received Ph.D. at the

Department of Electrical Engineering and Computer Science, The Uni-versity of Michigan, Ann Arbor in September 1999.

His current research interests are in the area of wireless/mobile
networks with emphasis on wireless LAN/MAN/PAN, next-generationmobile networks, mesh networks, cognitive radios, resource manage-ment, data link layer protocols, and cross-layer approaches. He authored/
co-authored over 120 technical papers and book chapters in the areas ofwireless/mobile networks and communications. He has co-authored(with B.G. Lee) a book ‘‘Broadband Wireless Access and Local Networks:Mobile WiMAX and WiFi,” Artech House, 2008. He holds 15 US patents,nine European patents, and seven Korea patents, and has tens of patentspending. He has served as a General Co-Chair of COMSWARE 2008, and aTechnical Program Committee Co-Chair of ACM Multimedia 2007, IEEEWoWMoM 2007 and IEEE/Create-Net COMSWARE 2007. He was a Co-Chair of Cross-Layer Designs and Protocols Symposium in IWCMC 2006,2007, and 2008, the workshop co-chair of WILLOPAN 2006, the GeneralChair of ACMWMASH 2005, and a Technical Program Co-Chair for ACMWMASH 2004. He has also served on program and organization com-mittees of numerous leading wireless and networking conferencesincluding IEEE INFOCOM, IEEE SECON, IEEE MASS, and IEEE WoWMoM.He is also serving on the editorial boards of IEEE Transactions on MobileComputing, ACM SIGMOBILE Mobile Computing and CommunicationsReview (MC2R), and Journal of Communications and Networks (JCN). Heis serving and has served as a guest editor for IEEE Journal on SelectedAreas in Communications (JSAC), IEEE Wireless Communications, Perva-sive and Mobile Computing (PMC), ACM Wireless Networks (WINET),Wireless Personal Communications (WPC), and Wireless Communica-tions and Mobile Computing (WCMC). He gave a tutorial on IEEE 802.11in ACM MobiCom 2004 and IEEE ICC 2005. Since year 2000, he has been avoting member of IEEE 802.11 WLAN Working Group.

He has received a number of awards including the Young ScientistAward (awarded by the President of Korea) in 2008; IEEK/IEEE JointAward for Young IT Engineer of the Year 2007 in 2007; the OutstandingResearch Award in 2008 and the Best Teaching Award in 2006 both fromthe College of Engineering, Seoul National University; the Best PaperAward from IEEE WoWMoM 2008; and Recognition of Service Award in2005 and 2007 from ACM. He was a recipient of the Korea Foundation forAdvanced Studies (KFAS) Scholarship and the Korean Government Over-seas Scholarship during 1997–1999 and 1994–1997, respectively. He is asenior member of IEEE, and a member of ACM, KICS, IEEK, KIISE.

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A performance analysis of block ACK scheme for IEEE 802.11e networks