An ACK-based polling strategy for supporting high performance and QoS in IEEE 802.11 wireless networks

Computer Communications 29 (2006) 358–371 www.elsevier.com/locate/comcom

An ACK-based polling strategy for supporting high performance and QoS in IEEE 802.11 wireless networks Shiann-Tsong Sheu*, Yun-Yen Shih, Yue-Ru Chuang Department of Electrical Engineering, Tamkang University, Tamsui, Taipei County, Taiwan 251, ROC Received 1 March 2004; revised 6 April 2005; accepted 19 April 2005 Available online 23 May 2005

Abstract The infrastructure architecture of wireless local area networks (WLANs) has been widely established in many environments to provide convenient multimedia services. However, in this standard operation, arbitrary channel contention and frequent handshaking significantly affect data transmission efficiency between AP and stations (STAs). This paper proposes an ACK-based polling strategy (APS) to reduce the overheads of channel contention and frequent handshaking via adaptively arbitrating and scheduling the transmission sequence of STAs. That is, the proposed APS makes AP be able to defer the ACK frames replied to the STAs, which still have more data in their buffers, in order to temporarily terminate their subsequent contention accesses. A terminated STA is permitted to transmit data frame again only when it receives the ACK frame replied from AP. Using the ACK-based polling mechanism, the overheads of channel contention and frequent handshaking are reduced and the network goodput is improved. Using the ACK frames, the APS can be further enhanced to support the quality of service (QoS) for various multimedia applications. Simulation results demonstrate that the APS with enhanced QoS function is able to efficiently cope with various transmission requirements in multimedia WLANs. q 2005 Elsevier B.V. All rights reserved. Keywords: Infrastructure; ACK-based polling strategy (APS); QoS; Multimedia WLANs

1. Introduction In recent years, IEEE 802.11 infrastructure wireless local area networks (WLANs) have been widely established in campuses, public places and indoor environments to provide convenient data transmissions between mobile devices and Internet. Nowadays, the growing requirements of mobile communication equipments and multimedia applications drive the WLANs to be capable of supporting quality of service (QoS) for various multimedia applications (such as IP telephones, video on demands, video conferences, and interactive Internet games, and so on). However, the actual WLAN performance always falls short of people’s expectation, and the low bandwidth utilization results in unsatisfactory quality of wireless transmission. In literatures, various enhanced MAC protocols have been proposed to improve the transmission efficiency in * Corresponding author. Tel.: C886 22 6261202; fax: C886 22 6209814. E-mail address: [email protected] (S.-T. Sheu).

0140-3664/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.comcom.2005.04.009

WLANs [1–5]. Paper [1] concluded that the PCF operation obtains better performance than the DCF operation when traffic load is heavy. However, the pure PCF operation provides less flexibility to cope with varying traffic load due to an amount of bandwidth may be wasted on transmitting polling command frames. There is a simple collision resolution mechanism proposed to improve the performance of IEEE 802.11 DCF called GDCF [2]. The GDCF defers the contention window (CW) back to the minimal value after successful transmission. STAs with GDCF are permitted to reduce CW by a specified value after they have successfully and consecutively transmitted a sufficient number of data frames. However, the scheme of suspending CWs cannot completely alleviate contention situations. Besides, overheads caused by GDCF (applying a relatively larger CW size) also degrade the transmission efficiency. These overheads could be the control frames (i.e. RTS and CTS frames) in RTS/CTS handshaking mode or be the collisions of long data frames. Although the throughput of GDCF is better than that of standard protocol, the PCF scheme still outperforms GDCF. Among these proposed strategies, the receiver-initiated strategies, which reverse the standard handshaking sequence

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between sender and receiver [6], are attractive because they use fewer control frames than the standard MAC protocol to accomplish one data transmission [3–5]. The protocol proposed in papers [4,5] is named as receiver-initiated multiple access (RIMA) protocol. In RIMA, if STA A attempts to transmit data frame to STA B, then STA A first issues the Ready-to-Receive (RTR) control frame to STA B to query whether STA B has a data frame destining to STA A. At the moment, if STA B just has such data frame, it sends the data frame immediately and then waits for the acknowledgment (ACK) frame replied from STA A. Thus, STA A can send the ACK frame and its data frame to STA B sequentially. After STA B replies the ACK frame to STA A, the data exchange is finished. In this case, two data frames are transmitted and three control frames are used (i.e. one RTR frame and two ACK frames). Contrarily, if STA B has no data frame destining to STA A when it receives the RTR frame, then STA B will treat the RTR frame as the ordinary request-to-send (RTS) frame and replies the clear-to-send (CTS) frame to STA A (just likes the standard protocol does). From the mentioned earlier, the RIMA can perform superior than the standard MAC protocol. However, since neither sender or receiver can announce the total period of data transmissions only using a control frame (i.e. the RTR), their neighbors may interfere the ongoing data transmissions due to the hidden node situation [7]. Therefore, the proposed channel access protocol in this paper will solve the hidden node problem meanwhile sustaining high channel utilization. As the growing of WLANs, the emerging QoS demands push the amendment of IEEE 802.11e QoS enhancement standard [8] to be defined for supporting multimedia streams over WLANs. The IEEE 802.11e specification introduces the hybrid coordination function (HCF), which is backward compatible with DCF and PCF. By using parameterized QoS information, the HCF makes STAs to access the wireless medium with different priority levels. In HCF, the contention-based channel access operation (similar to DCF) is named as the enhanced distributed channel access (EDCA), and the contention-free channel access operation (similar to PCF) is named as the HCF controlled channel access (HCCA). The EDCA controls the arbitration interframe space (AIFS) and the contention window size (CW) to provide higher channel access probability for high priority data frames than that for low priority ones. However, since the random process of selecting backoff window, the QoS of a high priority multimedia stream could be temporarily unsatisfactory, especially in heavy load conditions [9,10]. Based on the aspect, polling based schemes are more capable of providing QoS guarantee due to the property of centralized control. Additionally, in HCF, a hybrid coordinator (HC) is used to collect the QoS parameters of STAs and allocate transmission opportunities (TXOPs) to STAs via polling. Unfortunately, it is complicated for WLAN users to precisely specify the QoS parameters of their applications, and the scheduling algorithm needed in HC is

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also another complicated task. Therefore, it is still an open issue of scheduling access sequence and access periods for wireless traffics with different QoS requirements [11]. This paper proposes a more simple ACK-based polling strategy (APS), which combines the advantages of the standard contention function of DCF and the standard polling function of PCF, to minimize the redundant control frames as well as the collision probability in infrastructure WLANs [12]. The APS basically follows the operations of IEEE 802.11 standard protocol but alters the transmission sequence defined in standard. The APS makes access point (AP) be able to defer the ACK frames replied to the STAs, which still have more data in their buffers, to temporarily terminate their subsequent contention accesses. In other words, an STA is forced to stop following channel access unless it receives the replied ACK frame from AP. By deferring these ACK frames, most of data transmissions or contentions from STAs are controlled by AP. The transmission behavior is similar to the polling operation in PCF or HCCA but is more flexible than PCF and is less complicated than HCCA. On the other hand, if the STA has no more frames in its buffer while it is transmitting a data frame to AP, then AP replies the ACK frame immediately as the standard protocol does. Thus, the proposed APS not only keeps the features of DCF and PCF, but also reduces the overheads of channel contention and frequent handshaking in WLANs. Furthermore, the AP with the APS can flexibly schedule the deferred ACK frames (i.e. polling sequence) according to the priorities or QoS parameters to provide differentiated services in WLANs. The rest of the paper is organized as follows. Section 2 introduces how the APS works in the infrastructure architecture. Section 3 analyzes the system goodput of the APS. Section 4 describes the enhanced APS using scheduling mechanism and the fake-ACK strategy to provide the QoS function. Section 5 presents the simulation model and results to evaluate the performance of the APS. Finally, some conclusion remarks are given in Section 6.

2. The ACK-based polling strategy (APS) The APS breaks the transmission continuity of DATA and ACK frames. The ACK frames replied from AP can be deferrable and are regarded as the ‘polling’ frames to invite the corresponding STAs to send their next data frames in a collision-free manner [12]. The consecutive transmission sequence of frames becomes ‘ACK-DATA’, and those STAs, which have successfully transmitted data frames to AP and still have more data in their buffers, are terminated to access the channel unless they get authorizations from AP (i.e. receiving the replied ACK frames from AP). Each time AP receives a data frame from an STA, it first checks whether the sender still has more data frames to send. The ‘more data frames’ information is presented in the ‘More Data’ field in MAC header of the transmitted data

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frame. If the buffer of this STA becomes empty (i.e. without setting the ‘More Data’ bit), AP immediately replies the ACK frame. Otherwise, AP defers the ACK frame and schedules its appropriate transmission time. However, using the deferring operation, an ambiguous situation may occur at STAs. That is, both frame error/loss and an excessively deferred period cause the same timeout event. An STA cannot distinguish these two timeout conditions. To avoid the ambiguity, the time interval of AP deferring the ACK frame should be no longer than the remaining lifetime (RLT) of the received data frame (i.e. the expected ACK timeout value of this data frame). Obviously, considering different RLTs of various traffic types, the APS can be further designed to provide QoS requirements (such as delay bound and delay jitter requirements), which will be discussed in Section 4. To distinguish the standard ACK frame (without polling signal) from the deferred ACK frame (with polling signal), when AP receives a data frame from an STA and detects the more data information from the MAC header, AP should defer the ACK frame for a time interval and then reply the deferred ACK frame with the set ‘More Data’ field. Thus, when the waiting STA receives the ACK frame, it can immediately send its next data frame to AP without contention. Otherwise, AP will reply the standard ACK frame (without setting this field), and the STA needs to contend with other STAs for its next data transmission. 2.1. The APS superframe An APS superframe structure consists of three time periods: the beacon transmission period, the registration period (RP, for contention access) and the polling period (PP, for contention-free access). The superframe always starts from the beacon transmission period and followed by RP. During RP, AP and STAs follow the CSMA/CA scheme to transmit frames. By collecting the deferred ACK frames, a scheduled list of STAs is established in AP. After RP is PP. During PP, the AP sends the deferred ACK frames back to those STAs according to the scheduled result and then receives the data frames from those STAs. If an STA still has more data in its buffer after it has been polled and has transmitted its data frame to AP, then AP will hold the last ACK frame again and schedule it in the PP of the next superframe. We note that if AP has data frames to transmit, then the transmitted data frames can be sent via either CSMA/CA method in RP or be scheduled together with the deferred ACK frames in PP. In the latter case, the piggyback method can be applied if AP just has data frame destining to the polled STA. As specified by IEEE 802.11 standard, notations {w0, w1, w2, w3, w4, w5} are defined to represent the six backoff windows (exponentially growing from 32b to 1024b, where b represents a standard backoff time slot). Let notation T_RP denote the time period of registration period (RP). T_RP is initially set to a minimal value, denoted as T_RPmin.

Similar to standard, T_RPmin equals to DIFSCw0. If there is any STA accesses channel (even though collision occurrence) during the current RP, the AP will extend the T_RP value of the RP in the next superframe (denoted as T_RPnew). That is, T_RPnew Z DIFS C minf2 !ðT_RPcurr K DIFSÞ; w5 g where T_RPcurr represents the time period of the current RP. Notably, the T_RP is bounded by (DIFSCw5) as mentioned above. Contrarily, if channel is idle (i.e. no one accesses channel) during the current RP, then the T_RPnew value is reset to T_RPmin in order to control the bandwidth utilization. Before AP ending the current PP and starting a new RP (with the time period of T_RPnew), it first checks whether the deferred ACK frames still queued in its scheduled list are going to occur timeout after the period of T_RPnew. That is, RLT! DIFS C T_RPnew C 2 !SIFS C TACK C TDATA ; where RLT represents the remaining lifetime of a deferred ACK frame, and TACK and TDATA are the required transmission and propagation delays of this deferred ACK frame and a polled data frame, respectively. If the timeout event of this deferred ACK frame is going to happen, the AP needs to extend the PP to first reply this ACK frame. Fig. 1 shows an example of applying APS in a WLAN with an AP and four STAs. In this example, we only focus on the APS operations in RP and PP, and simply omit the beacon transmission period at the beginning of each STA3

STA1

AP

STA2

RP #1 DIFS RTS SIFS CTS SIFS DATA (more) PP #1 SIFS ACK (more) DATA (more) SIFS

STA4

T_RP1 = DIFS+w0

RP #2 RTS CTS DATA (no more) ACK (no more) RTS CTS DATA (more)

T_RP2 = DIFS+w1

PP #2 ACK (more) DATA (more) ACK (more) DATA (no more) ACK (no more) RP #3 T_RP3 = DIFS+w2 PP #3 ACK (more) DATA (no more) ACK (no more) RP #4

RTS CTS DATA (no more) ACK (no more)

T_RP4 = DIFS+w0

Fig. 1. An example of the ACK-based polling strategy (APS).

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superframe. RP starts after a channel idle period of DIFS, and the following PP starts after a channel idle period of SIFS. Namely, AP uses different inter-frame spaces (IFSs) to start and terminate RP and PP. In Fig. 1, four STAs have data frames destining to AP. Initially, the T_RP1 is set to T_RPmin (ZDIFSCw0). After STA1 successfully sending its data frame in the first RP (RP#1), AP holds the ACK frame of STA1 since the ‘More Data’ field of this data frame is set. In the first PP (PP#1), AP sends the deferred ACK frame with polling signal to STA1. When AP receives the returned data frame from STA1, it holds the ACK frame again since STA1 still has more data in its buffer, and this deferred ACK frame will be sent back in the second PP (PP#2). In order to make sure the consecutive transmissions of ACK and DATA frames in APS, the inter-frame space between them is SIFS. In the second and third RPs, the T_RP2 and T_RP3 are extended to (DIFSCw1) and (DIFSCw2) respectively because the channel accesses occur in the first and the second RPs. Contrarily, the T_RP4 is reset to the initial value (T_RPmin) because no channel access occurs in the third RP. In the second RP, STA2 first acquires channel and it has only a data frame. Following the standard protocol, the complete 4-way handshaking (RTS-CTS-DATA-ACK) is applied. Subsequently, STA3 sends a data frame, in which the ‘More Data’ field is set, to AP and the AP also puts the ACK frame of STA3 into the scheduled list. Then AP will poll STA1 and STA3 sequentially in the second PP. Here, since STA3 has only one more data frame, it will receive the replied ACK frame immediately after data transmission, and then AP removes STA3 from its scheduled list. Based on the dynamic adaptation, the DCF operation will be gradually switched to the PCF operation while the traffic loads of STAs increase. 2.2. The restriction of deferred ACK frame In standard MAC protocol, an STA can be aware that the previous data transmission is successful if it receives the immediately replied ACK frame. However, in APS, since time interval between the data frame and its ACK frame is extended, we need to design a new timeout value for STAs to detect the transmission result. An STA starts a timeout timer as soon as it transmits a data frame to AP. Similarly, AP also employs a corresponding timeout timer for this STA and starts it when AP receives this data frame. The initial timeout values in both of STA and AP are SIFS (neglects the transmission and propagation delays) if the STA has no more data in its buffer; otherwise, the expected timeout value equals the remaining lifetime (RLT) of this transmitted data frame. Data frames with different service types have their own specified lifetimes, as shown in Fig. 2. In the MAC frame format of the standard, the type value ‘10’ is designed for various data frames and the range of the subtype values from 1000 to 1111 is reserved for future use. Due to the small of

361

to defer the ACK

AP removes the deferred ACK

the deferred ACK (polling command) RP

PP

RP

PP

X

AP

RTS STA

t

ACK (more)

CTS

Data 1 (w/more)

Data 2 (w/more)

t STA drops the timeout Data 2

lifetime of Data 1 Data 1 arrival

Data 1 timeout lifetime of Data 2 Data 2 arrival

Data 2 timeout

Fig. 2. An example illustrates how AP/STA handles the ACK/Data frames with and without timeout.

available range, the RLTs of data frames need to be quantized into eight levels and are presented in the subtype filed. Hence, when an STA transmits a data frame with more data information to AP, the STA fills the proper RLT value (denoted as RLTSTA) into the subtype field. Considering the average transmission and propagation delay a, when AP receives a data frame carrying more data information, it calculates a newly remaining lifetime (denoted as RLTAP) according to the received RLTSTA and uses this value of RLTAP to schedule the deferred ACK frame. The relationship between RLTSTA and RLTAP can be presented as follows: RLTAP Z RLTSTA K 2a: Based on these obtained values of RLTAP, AP can schedule the polling sequence (i.e. the scheduled list) by exploiting some strategies, such as user priorities and QoS specifications, and so on. Obeying the scheduled result, AP will automatically allot a sufficient time interval for PP to service these deferred ACK frames. However, if the lifetime of any deferred ACK frame has expired due to some reasons (e.g. system traffic loads become heavy), then AP and the STA will concurrently remove the expired ACK frame and the data frame, as shown in Fig. 2. 2.3. The ACK identification problem (AIP) This subsection presents the ACK identification problem (AIP) in the APS. As shown in Fig. 3(a), after AP replies the deferred ACK frame (with polling signal) to STA in PP, the data frame (denoted as Data 1) with more data information is sent from STA to AP but is corrupted or lost due to some interference sources. Then AP will retransmit the deferred ACK frame to the STA again if the remaining lifetime (RLTAP) is not expired. However, since the STA cannot identify the retransmitted ACK frame, the STA may send the undesired data frame (say Data 2) to AP. This phenomenon is the AIP. A sequence number can be additionally added to the ACK frame to identify which data frame is acknowledged. Considering the transmission window size in MAC layer is often set to one, we use

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Fig. 3. ACK frames are numbered to solve the ACK identification problem (AIP). (a) Without a sequence bit. (b) With a sequence bit.

the single bit of ‘Order/Rsrv’ subfield in ACK frame as the sequence bit to clearly indicate which data frame is being acknowledged, as shown in Fig. 3(b).

3. System goodput analyses of the APS This section presents the analysis of the system goodput obtained from the APS. Notation N represents the number of nodes in a WLAN (including STAs and AP). The control frame lengths of RTS, CTS and ACK are denoted as R, C and A bytes, respectively. For simplicity, the data frame length is a fixed value of L (bytes) and the system frame arrival rate is L (1/ms). Thus, the total traffic load of the WLAN can be calculated as (8!L!L) Mbps. The transmission rates of the data frames and the control frames are denoted as Rd and Rc Mbps, respectively. Thus, the required transmission delays of Data, RTS, CTS and ACK are ttd (Z8L/Rd), ttr (Z8R/Rc), ttc (Z8C/Rc) and tta (Z8A/Rc), respectively. Hence, a successful data transmission period (DTP), as shown in Fig. 4, is calculated as follows: DTPZ(ttrCSIFSCttcCSIFSCttd) ms, where

the average propagation delay is neglected. Contrarily, if a collision occurs during data transmission, then the wasted period (WP) is ttr. As mentioned in Section 2, the set of notations {w0, w1, w2, w3, w4, w5} is defined to represent the six backoff windows. Following the CSMA/CA method, the collision probability (Pck(wi)) of k frames contending for data transmission in the i-th backoff window (wi) can be derived by recursion and be obtained as follows: 8
  kK2   kK1 1 1 wi K1 > 1 > > ! C ! > wi > 2 wi >  wi > > > 1 > > Cð1 KPcðkK1Þ ðwi ÞÞ! > > w > >  <
kK2i  Pck ðwi Þ Z C PcðkK1Þ ðwi Þ K w1i > > > > !Ck ; kR3 > > > > >1 > > > ; k Z2 > > > : wi 0; k Z 1;

Fig. 4. An analysis model of the APS MAC protocol.

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where Ck is a part of the collision probability, and it can be calculated as follows: jZkK2 X k Kj Ck Z k Kðj K1Þ jZ2 3 Q ðk KjÞjK2 ! gZkKjK1 ðw KgÞ i gZ1 h i 5; kR4: !P QgZkKlK1 lZkK2 lZ2 ðk KlÞ ! gZ1 ðwi KgÞ lZ2

In the second transmission period (TP12), an STA may be controlled by AP due to its lsquo;more data’ information. Consequently, in the beginning of this period, the number of free STAs (FS12) is [(NK2)C (1KPm11)], the mean number of accumulated frames (AF12) is (AF11C[TP11!L]K1), and the mean number of contending frames (CF12) is ( AF12 ; AF12 % FS12 CF12 Z : FS12; AF12 O FS12

Fig. 4 illustrates the analysis procedures of system goodput from the initial condition. Based on the recursion method, a basic recursion period contains a registration period (RP) and a polling period (PP). A RP consists of several transmission periods (TPs) and is controlled by the T-RP value described in Section 2. The TP is the time interval for a successful data transmission and it includes the collided time intervals, the idle time intervals for backoff and the interval of successful data transmission. Let FSij denote the number of free STAs at the beginning of the j-th TP in the i-th RP. (Free STA means the STA is not being scheduled by AP yet.) Initially, the system has (NK1) free STAs (i.e. FS11Z(NK1)), which can arbitrarily contend channel to transmit their data frames, but only a data frame arrives at the system. Namely, at the moment, both of the mean numbers of accumulated frames (AF11) and contending frames (CF11) in the system are one (AF11ZCF11Z1). Hence, the average backoff interval (BI11) of this data transmission is ((w0/2)!b), where b represents a standard backoff time slot. When the data frame is successfully transmitted, other data frames, which are still buffered in the system, are called as ‘more frames’ and the number of more frames (MF11) is calculated as follows:

Thus, the average backoff interval (BI12) of this period can be obtained as follows:

MF11 Z AF11 C ½ðDIFS C BI11 Þ !L K 1: In APS, the probability (Pm11) of an STA transmitting a data frame and still having more data in its buffer can be derived as follows:

 N K 1 MF11 Pm11 Z 1 K : N Thus, the complete time interval (CTI11) of a data transmission is Pm11!(DIFSCBI11CDTP)C(1KPm11)! (DIFSCBI11CDTPCSIFSCtta). Depending on the total traffic load, the value of CTI11 may be equal to or smaller than the system frame arrival interval (1/L). If the traffic load is too light to buffer any data frame in the system, then the transmission period must be increased to (1/L). Hence, the practical transmission period (TP11) should be determined as follows: ( CTI11 ; ½CTI11 !LR 1 TP11 Z : 1=L; otherwise

" ! jZi iZ5 Y X w0
w0  i 2 ! PcðCF12Þ ðwjK1 Þ BI12 Z C ! 2 2 iZ1 jZ1 # iZ4 Y P ðw Þ cðCF12Þ 5 C25 ! ! P ðw Þ 1 K PcðCF12Þ ðw5 Þ iZ0 cðCF12Þ i " jZi iZ5 Y X C ðWP C SIFSÞ ! PcðCF12Þ ðwjK1 Þ iZ1 jZ1

# iZ4 Y PcðCF12Þ ðw5 Þ C ! P ðw Þ : 1 K PcðCF12Þ ðw5 Þ iZ0 cðCF12Þ i When the data frame is transmitted, the number of more frames (MF12) in the system can be calculated as follows: MF12 Z AF12 C ½ðDIFS C BI12 Þ !L K 1: And the probability (Pm12) of an STA transmitting a data frame and still having more data in its buffer can be derived as follows:   N K 1 MF12 Pm12 Z 1 K : N Thus, the complete time interval (CTI12) of a data transmission is Pm12!(DIFSCBI 12CDTP)C (1KPm12)!(DIFSCBI12CDTPCSIFSCtta), and the practical transmission period (TP12) can be determined as follows: ( CTI12 ; ½ðTP11 C CTI12 Þ !L K 1R 1 TP12 Z : 1=L; otherwise In the g-th transmission period (TP1g), the number of free STAs (FS1g) is " # iZgK1 X ð1 K Pm1i Þ ; FS1g Z ðN K gÞ C iZ1

the mean number of accumulated frames (AF1g) is " ! # iZgK1 X TP1i !L K ðg K 1Þ; AF1g Z AF11 C iZ1

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where gR2. And the mean number of contending frames (CF1g) is ( AF1g ; AF1g % FS1g CF1g Z : FS1g; AF1g % FS1g Thus, the average backoff interval (BI1g) of this period can be obtained as follows: " ! jZi iZ5 Y X w0
w0  BI1g Z C 2i ! PcðCF1gÞ ðwjZ1 Þ ! 2 2 iZ1 jZ1 # iY Z4 PcðCF1gÞ ðw5 Þ 5 C2 ! ! P ðw Þ 1 K PcðCF1gÞ ðw5 Þ iZ0 cðCF1gÞ i " jZi iZ5 Y X C ðWP C SIFSÞ ! PcðCF1gÞ ðwjK1 Þ iZ1 jZ1

#

iZ4 Y PcðCF1gÞ ðw5 Þ C ! PcðCF1gÞ ðwi Þ : 1 K PcðCF1gÞ ðw5 Þ iZ0

When the data frame is transmitted, the number of more frames (MF1g) in the system can be calculated as follows: MF1g Z AF1g C ½ðDIFS C BI1g Þ !L K 1: And the probability (Pm1g) of an STA transmitting a data frame and still having more data in its buffer can be derived as follows:   N K 1 MF1g : Pm1g Z 1 K N Thus, the complete time interval (CTI1g) of a data transmission is Pm1g!(DIFSCBI 1gCDTP)C (1KPm1g)!(DIFSCBI1gCDTPCSIFSCtta), and the practical transmission period (TP1g) can be determined as follows: 8 " ! # iZgK1 X > < CTI1g ; TP1i CCTI1g !L KðgK1ÞR1 TP1g Z : iZ1 > : 1=L; otherwise

From the derived results, the first RP is terminated after g continuous TPs. From the constraint on T-RP1, the practical value of the first registration period (RP1) can be calculated as follows: ( ) iZg X RP1 Z max T K RP1 ; TP1i ; iZ1

where iZgK1 X

TP1i C BI1g % T K RP1

iZ1

and T-PR1Z32b.

Following the first RP is the first PP. In the beginning of the period, the mean number of accumulated frames (AFPP1b) is " ! # iZg X AFPP1e Z AF11 C TP1i C PP1 !L K g K SS1 ; iZ1

the number of STAs stayed in the scheduled list (SS1) is and  PiZg P . Among these scheduled STAs, the probability iZ1 m1i (PmPP1) of the STA, which is polled by AP and still has more data in its buffer, can be derived as follows:   SS1 K 1 uðAFPP1bKSS1 Þ PmPP1 Z 1 K ; SS1 where ( uðxÞ Z

x;

xR 0

0; x! 0

:

Thus, a single polling period (SPP), can be calculated as (2!SIFSCttaC ttd)C(1(PmPP1)!(SIFSCtta), and the practical value of the first polling period (PP1) equals (SPP!SS1). At the end of the first PP, the mean number of the accumulated frames (AFPP1e) can be obtained as follows: " ! # iZg X AFPP1e Z AF11 C TP1i C PP1 !L K g K SS1 ; iZ1

and the number of free STAs (FSPP1) is (NK1K[SS1! PmPP1]). Following the same analysis procedure, the number of free STAs (FSk1) in the beginning of the first transmission period (TPk1) in the k-th RP equals FSPP(k(1), the mean number of accumulated frames (AFk1) equals AFPP(k(1)e, and the mean number of contending frames (CFk1) is ( AFk1 ; AFk1 % FSk1 CFk1 Z : FSk1 ; AFk1 O FSk1 Thus, the average backoff interval (BIk1) of this period can be obtained as follows: " ! jZi iZ5 Y X w0
w0  i BIk1 Z C 2 ! PcðCFk1Þ ðwjK1 Þ ! 2 2 iZ1 jZ1 # iY Z4 PcðCFk1Þ ðw5 Þ 5 C2 ! ! P ðw Þ 1 K PcðCFk1Þ ðw5 Þ iZ0 cðCFk1Þ i " jZi iZ5 Y X C ðWP C SIFSÞ ! PcðCFk1Þ ðwjK1 Þ iZ1 jZ1

# iZ4 Y PcðCFk1Þ ðw5 Þ C ! P ðw Þ : 1 K PcðCFk1Þ ðw5 Þ iZ0 cðCFk1Þ i When the data frame is transmitted, the number of more frames (MFk1) in the system can be calculated as

S.-T. Sheu et al. / Computer Communications 29 (2006) 358–371

follows:

365

derived as follows: 

MFk1 Z AFk1 C ½ðDIFS C BIk1 Þ !L K 1: And the probability (Pmk1) of an STA transmitting a data frame and still having more data in its buffer can be derived as follows:   N K 1 MFk1 Pmk1 Z 1 K : N Thus, the complete time interval (CTIk1) of a data transmission is Pmk1!(DIFSCBIk1CDTP)C(1KPmk1)! (DIFSCBIk1CDTPCSIFSCtta), and the practical transmission period (TPk1) can be determined as follows: ( CTIk1 ; ½CTIk1 !LR 1 TPk1 Z : 1=L; otherwise In the g-th transmission period (TPkg), the number of free STAs (FSkg) is " # iZgK1 X FSkg Z ðN K gÞ C ð1 K Pmki Þ ; iZ1

Pmkg Z 1 K

N K1 N

MFkg

:

Thus, the complete time interval (CTIkg) of a data transmission is Pmkg!(DIFSCBIkgCDTP)C(1KPmkg)! (DIFSCBIkgCDTPCSIFSCtta), and the practical transmission period (TPkg) can be determined as follows: " ! # 8 iZgK1 X > > > TPki C CTIkg !L < CTIkg ; iZ1 TPkg Z : > Kðg K 1ÞR 1 > > : 1=L; otherwise From the derived results, the k-th RP is terminated after g continuous TPs. Based on the constraint of T-RPk, the practical value of the k-th registration period (RPk) can be calculated as follows: ( ) iZg X RPk Z max T K RPk ; TPki ; iZ1

the mean number of accumulated frames (AFkg) is " ! # iZgK1 X AFkg Z AFk1 C TPki !L K ðg K 1Þ; iZ1

where gR2. And the mean number of contending frames (CFkg) is ( AFkg ; AFkg % FSkg CFkg Z : FSkg; AFkg O FSkg Thus, the average backoff interval (BIkg) of this period can be obtained as follows: " ! jZi iZ5 Y X w0
w0  i BIkg Z C 2 ! PcðCFkgÞ ðwjK1 Þ ! 2 2 iZ1 jZ1 # iY Z4 PcðCFkgÞ ðw5 Þ 5 C2 ! ! P ðw Þ 1 K PcðCFkgÞ ðw5 Þ iZ0 cðCFkgÞ i " jZi iZ5 Y X C ðWP C SIFSÞ ! PcðCFkgÞ ðwjK1 Þ

where iZgK1 X

TPki C BIkg % T K RPk :

iZ1

Following the k-th RP is the k-th PP. In the beginning of the period, the mean number of accumulated frames (AFPPkb) is " ! # iZg X TPki !L K g; AFPPkb Z AFk1 C iZ1

and the number of STAs stayed in the scheduled list (SSk) PiZg is iZ1 Pmki . Among these scheduled STAs, the probability (PmPPk) of the STA, which is polled by AP and still has more data in its buffer, can be derived as follows:  PmPPk Z 1 K

iZ1 jZ1

# iZ4 Y PcðCFkgÞ ðw5 Þ C ! P ðw Þ : 1 K PcðCFkgÞ ðw5 Þ iZ0 cðCFkgÞ i When the data frame is transmitted, the number of more frames (MFkg) in the system can be calculated as follows: MFkg Z AFkg C ½ðDIFS C BIkg Þ !L K 1: And the probability (Pmkg) of an STA transmitting a data frame and still having more data in its buffer can be

SSk K 1 SSk

uðAFPPkbKSSk Þ

;

where ( uðxÞ Z

x;

xR 0

0;

x! 0

:

Thus, a single polling period (SPP) can be calculated as (2!SIFSCttaC ttd)C(1(PmPPk)!(SIFSCtta), and the practical value of the k-th polling period (PPk) equals (SPP!SSk). At the end of the k-th PP, the mean number of the accumulated frames (AFPPke) can be obtained as

366

S.-T. Sheu et al. / Computer Communications 29 (2006) 358–371

follows: AFPP1e Z AF11 C

"

iZg X

!

#

TP1i C PP1 !L K g K SS1 ;

iZ1

and the free STAs (FSPPk) is (NK1K[SSk!PmPPk]). Using these analytical results of the k-th recursion period, the system goodput (GP) of APS MAC protocol can be obtained as follows: GP Z

ðttd !gÞ C ðttd !SSk Þ !Rd : RPk C PPk

buffer, the RLTSTA will be carried in the currently transmitted data frame to notify AP. In APS, the currently transmitted data frame carries itself RLTSTA to notify AP. However, in QAPS, for supporting QoS requirements (such as delay bound, etc.), the currently transmitted data frame carries the RLTSTA of the next transmitted data frame, which is queued in the head of the STA’s buffer. Using the calculated RLTAP, the QoS scheduler will follow a scheduling algorithm to select a frame from the heads of these priority queues and send it to an STA, as shown in Fig. 5. 4.1. Scheduling algorithm

4. The extended APS for supporting QoS The flexibility of scheduling mechanism in the APS makes the potential of supporting QoS requirements for real-time traffics, and the enhanced version of the APS is called as the QAPS. Based on general differentiated services [8,13,14], the QAPS classifies various traffic types into several QoS classes (priorities). As mentioned in Section 2, these QoS requirements can be indicated in the subtype field of the data frame header. In paper [15], a simple QoS implementation, the earliest deadline first (EDF), permits system to first transmit the data frame with the shortest lifetime. The QAPS exploits the fundamental concept of the EDF to achieve the QoS scheduling. Fig. 5 presents an example of the scheduling mechanism with four priority (class) queues, where the highest/lowest priorities are priority 0 (class 0)/priority 3 (class 3) respectively. According to different traffic types, AP classifies and puts the deferred ACK frames into these four priority queues. Additionally, if AP also has data frames to transmit, it can either send its data frames in RP (contention) or put them into the corresponding priority queues together with those deferred ACK frames in PP (contention free). In each priority queue, all stored frames (data frames and deferred ACK frames) are sorted by the sequence sorter (SS) according to their RLTAP values. Similar to the operation of the APS, when an STA transmits a data frame to AP and it still has more data in its

Based on the structure of scheduling mechanism in Fig. 5, let HZ{h0, h1, h2, h3} be the set of the frames queued in the heads of these four priority queues. In the set H, a frame hp (0%p%3) with the smallest RLTAP value is first selected to be a candidate for transmission. Here, we set RLTAP(hi)ZN if the queue with priority i is empty. For the selected candidate, the QoS scheduler requires to check whether transmitting the candidate first will cause other frames in H with high priorities to be timeout. To do this, the candidate (hp) and the frames in H with higher priorities than the candidate are collected to form a subset H 0 Z{h0,.,hp} (p!3). After performing the check process, if none in H 0 will be timeout, the candidate can be transmitted. Otherwise, the candidate is dropped and a new candidate will be selected from H to do the same check process. The check process follows the rule of timeout urgency. Therefore, the frames in H 0 are first sorted according to their RLTAP values in increasing order and remove the frames, whose RLTAP values are N. Thus, a sorted subset RZ {r0,.,rq}, where q%p, is formed and the frame r0 is hp. Here, we define a complete transaction period a(x), which contains a pair of propagation delays of a frame x sent from AP and the other frame y replied from an STA. That is, a(x)Z2!SIFSCTframe(x)CTframe(y), and then a recursive function f(ri, t) is used to execute the check process: 8 0; if ðRLTAP ðri Þ! t C aðri ÞÞ > > > < 1; if ðði Z qÞ & ðRLTAP ðri ÞR t f ðri ; tÞ Z Caðr ÞÞÞ > i > > : f ðriC1 ; t C aðri ÞÞ; otherwise where ri is a frame in the subset R, and t represents a currently calculated system time. Based on the above procedures, the scheduling algorithm executed in the QoS scheduler can be described as follows. 4.1.1. Scheduling algorithm

Fig. 5. The structure of scheduling mechanism supporting QoS services in AP.

Step 1: Generate a set HZ{h0, h1, h2, h3}; Step 2: Select a candidate hp;

S.-T. Sheu et al. / Computer Communications 29 (2006) 358–371

(a) α (h0)=2 α (h1)=5 α (h2)=2 α (h3)=2 p=0

h0

p=1

h1

p=2

h2

p=3

h3

the frame h3 to guarantee the QoS requirements of frames h0, h1 and h2. Thus, the frame h2 (RLTAP(h2)Z6 time slots) is selected as the new candidate and the new transmission sequence is {h2, h1, h0}.

t=0 RLTAP(h0)=12 RLTAP(h1)=8 RLTAP(h2)=10 RLTAP(h3)=5

h3

h1 2

QoS scheduler

(b)

h2 h0 7

system time (t)

9 11 a proper schedule

u3 is first transmitted

t=0

α (h0)=2 α (h1)=5 α (h2)=2 α (h3)=2 p=0

h0

p=1

h1

p=2

h2

p=3

h3

367

RLTAP(h0)=12 RLTAP(h1)=8 RLTAP(h2)=6

x

RLTAP(h3)=5 h3 h2 h1 h0 system time (t) 9 11 2 4 an improper schedule

h2 h1 h0 (t) QoS scheduler 2 7 9 a proper schedule u3 is dropped

Fig. 6. Two scheduling examples of the QAPS.

Step 3: while (RLTAP(hp)!N) Form a subset H 0 Z{h0,.,hp}; Generate a sorted subset RZ{r0,., rq}; if (f(r0, t)Z1) Transmit hp; else Drop hp; Generate a new set HZ{h0, h1, h2, h3}; Select a new candidate hp; Repeat Step 3; Fig. 6 illustrates two scheduling examples of the QAPS where the beginning system time (t) is zero. In Fig. 6(a), the values of RLTAP(hi) and a(hi) of frames h0, h1, h2, h3 are 12, 8, 10, 5 and 2, 5, 2, 2 time slots, respectively. Based on the scheduling algorithm, the first selected candidate is h3 because of the minimal RLTAP (RLTAP(h3)Z5 time slots). After executing consecutive check processes, the results show that f(h3,0)Zf(h1,2)Zf(h2,7)Zf(h0,9)Z1. Hence, these four head frames can be sequentially transmitted without timeout occurrence and the transmission sequence is {h3, h1, h2, h0}. Contrarily, in Fig. 6(b), the value of RLTAP(h2) is changed to 6 time slots, and a new transmission sequence becomes {h3, h2, h1, h0}. However, since the value of f(h1,4) is zero after executing check process, this means that the frame h1 with higher priority than h3 will be timeout. Therefore, the QoS scheduler drops

4.2. The fake-ACK mechanism The QAPS with polling schedule is able to support QoS requirements of different service types. However, if an STA has no more data in its buffer while it is transmitting a data frame to AP, the STA cannot be put in the scheduled list. Subsequently, the STA still needs to contend channel again for following data transmission, and then the required QoS is not easy to be guaranteed. To overcome the drawback, a fake-ACK mechanism is used to increase the possibility of an STA stayed in the scheduled list, as shown in Fig. 7. For real-time traffic types, such as voice, video and multimedia applications, when AP receives a data frame carrying the information of ‘no more data’ from an STA, AP will trigger a countdown timer for this STA. The initial value of the timer is a half of the default lifetime (LTdef) of this traffic type. When the timer counts down to zero and the system still operates in the PP. Then AP will automatically generate a fake-ACK frame for the STA and the value of its RLTAP is a half of the LTdef. AP puts this fake-ACK frame into a priority queue according to its traffic type, as shown in Fig. 5. Additionally, AP uses the maximal frame length to calculate the required transmission and propagation delays, which are carried in the ‘Duration/AID’ field of the fake-ACK frame to inform other STAs to update their NAV values. Thus, if the STA receives the fake-ACK frame (with polling signal) and it just has data stored in its buffer, then the STA can transmit its data frame to AP immediately. Otherwise, it transmits a data frame without payload to AP.

(RP)

(PP)

AP

t contention free (polling operation)

STA

Data 1 (no more data)

start a countdown timer AP

(RP)

ACK

lifetime (LT) of Data 2

Data 2 arrival ACK

generate a fakeACK after (LT/2)

contend channel

Data 2 timeout t

fake-ACK ACK polling operation

STA

Data 1 (no more data)

Data 2 (no more data)

t

drop Data 2

t successful transmission

Fig. 7. A comparison of the QAPS with and without the fake-ACK mechanism.

S.-T. Sheu et al. / Computer Communications 29 (2006) 358–371

5. Simultaion model and results 5.1. Simulation model and QoS definitions Referring to the paper [16] and the features of many famous application services in Internet, four kinds of QoS differentiated services, including voice, video, multimedia and background traffic, are investigated and simulated by the proposed QAPS, as shown in Table 1. The considered priorities of services from high to low are voice, video, multimedia, and background traffic. The first three traffic types are classified as real-time services, which require to be guaranteed the minimal data rates and maximal access delays. The background service is the best effort service. In simulations, the background traffic type is given a default lifetime (LTdef) of 1000 ms to indicate whether the data transmission is successful. In real-time services, senders will drop the overdue data frames, which do not receive their ACK frames before expiration. Contrarily, senders will retransmit the overdue data frames of the background traffic type. The simulation model follows the IEEE 802.11 specification to implement the proposed APS/QAPS and the standard DCF/PCF MAC protocols, as shown in Table 2. Other system assumptions are described as follows: † 10 fixed STAs and one AP are randomly distributed in the infrastructure environment. † The channel is noiseless and error-free. † The transmission and propagation delays, between two STAs or between STA and AP, are ignored. Table 1 Four traffic types considered in the simulation model Traffic types

Priority

Voice Video

0 1

Multimedia Background

Default lifetime (ms)

Max frame length

Application example

150 30

250 bytes 2 Kbytes

2

250

2 Kbytes

3

1,000

VoIP Video on demand Video conference FTP

2,312 bytes

100 90 80 70 60

Goodput (%)

368

Analysis (1500bytes) Simulation (1500bytes) Analysis (500bytes) Simulation (500bytes) Analysis (64bytes) Simulation (64bytes)

50 40 30 20 10 0 0.1

0.2

Normal value

Data frame rate Control frame rate A time slot (b) ASIFS ADIFS RTS frame length CTS frame length ACK frame length aCWmin aCWmax

2 Mbps 2 Mbps 20 ms 10 ms 50 ms 13.6 time slots (68 bytes) 12.4 time slots (62 bytes) 12.4 time slots (62 bytes) 32 time slots (32b) 1024 time slots (1024b)

0.8

0.9

1

† The encapsulation overheads of the PLCP header and the MAC header are considered. † The hidden terminal problem in a WLAN is considered. † The frame arrival rate of each STA or AP follows the Poisson distribution with a mean of L. † The frame length is an exponential distribution with a mean of L bytes. 5.2. Analyses verification Following the derived result of system goodput in Section 3, Fig. 8 shows the comparisons of system goodputs (GP) obtained by analysis and simulation, where the data frame lengths (L) are 64, 500 and 1500 bytes, respectively. Here, the performance of system goodput is defined as the ratio of an expected delivery period of data payload to an expected transmission period [17]. In each numerical result, the number of recursion times is 100 and the value of system goodput converges to a steady state. In simulation, the system arrival rate of data frames follows the Poisson distribution with a mean of L (1/ms), and the frame length is an exponential distribution with a mean of L (bytes). However, for simplicity, the system arrival rate and the 100

Goodput (%)

Parameter

0.4 0.5 0.6 0.7 Total traffic load

Fig. 8. The comparisons of system goodputs obtained by analysis and simulation under different data frame lengths (L).

APS (1500bytes) RIMA-DP (1500bytes) Standard (1500bytes) APS (64bytes) RIMA-DP (64bytes) Standard (64bytes)

80 Table 2 System parameters used in the simulation model

0.3

60 40 20 0 0.1

0.2

0.3

0.4 0.5 0.6 0.7 Total traffic load

0.8

0.9

1

Fig. 9. The comparisons of system goodputs obtained by the APS, the RIMA-DP and the standard MAC protocol under different mean frame lengths (LZ64 bytes and LZ500 bytes).

S.-T. Sheu et al. / Computer Communications 29 (2006) 358–371

(b) 100

Goodput improvment ratio (%)

(a)

Goodput (%)

80 60 40 20

APS Standard

0 50

250

450 650 850 1050 1250 1450 Mean Length (bytes)

369

80 70 60

1500bytes 500bytes 64bytes

50 40 30 20 10 0

0.1

0.2

0.3

0.4 0.5 0.6 0.7 Total traffic load

0.8

0.9

1

Fig. 10. The system goodput and goodput improvement ratio of APS and standard MAC protocol under different mean frame lengths and total traffic loads.

frame length of analysis are both fixed as L and L, respectively. From Fig. 8, we can see that the system goodputs of numerical results from analysis are close to that from simulations. 5.3. Simulation results Fig. 9 displays the comparison of system goodputs obtained by the APS, the RIMA-DP [5] and the standard MAC protocol under different frame lengths and traffic loads. Simulation result shows that the system goodput obtained from APS is higher than those of the RIMA-DP and the standard MAC protocol. The reason is that the APS can efficiently reduce the overheads of channel contention and frequent handshaking. We note that, as the frame length becomes shorter or traffic load becomes heavier, the transmission frequency will become higher and the overhead will also becomes larger. Hence, the improvement on goodput obtained by the APS will become more obvious. Fig. 10(a) presents the system goodput obtained by the APS and the standard MAC protocol, where the traffic load is 1.0 (i.e. 2 Mbps). The simulated mean frame length varies from 50 to 1450 bytes in a step of 50 bytes. In Fig. 10(b),

Standard deviation

4.0E-03

3.0E-03

APS (64bytes) Standard (64bytes) APS (1500bytes) Standard (1500bytes)

the system goodput is showed by ratio in order to display the improvement degree. The improvement ratio is calculated as (GPAPSKGPStd)!100/GPStd, where the GPAPS and GPStd are the system goodputs of the APS and the standard MAC protocol respectively. In the environment with LZ64 bytes, the goodput improvement ratio can reach 71.5% when the total traffic load is larger than or equal to 0.3. Such obvious improvement is caused by that, under a fixed traffic load, frames with short frame length will result in more data frames appearing in each STA. And, a large overhead of channel contention and frequent handshaking is generated in the operation of the standard MAC protocol. Contrarily, in APS, each STA with more data frames has a higher possibility to stay in the scheduled list of AP, and the improvement is obvious consequently. To evaluate the fairness of the proposed APS, we measure the standard deviations of goodputs of STAs obtained by IEEE 802.11 MAC protocol and APS. From Fig. 11, we can find that the curves of the APS are as smooth as those of the standard protocol under different mean frame lengths. Although APS combines the contention method and polling method, the channel access probabilities of STAs are still well controlled by APS. From Fig. 11, we conclude that the APS is a fairness protocol as the standard MAC protocol is. Finally, this study investigates the performances of the proposed QAPS with the fake-ACK mechanism.

Table 3 Four traffic types and related traffic loads in simulation model.

2.0E-03

Traffic types (QoS classes)

Number of STAs

1.0E-03

0.0E+00 0.1

0.2

0.3

0.4

0.5 0.6 0.7 Total traffic load

0.8

0.9

1

Fig. 11. The fairness comparison between APS and standard MAC protocol is displayed, under different mean frame lengths (LZ64 bytes and LZ 500 bytes).

Voice Video Multimedia Background

2 1 5 12

Downlink (from AP to STAs)

Uplink (from STAs to AP)

Each traffic load (Kbps)

Total traffic load (Kbps)

Each traffic load (Kbps)

Total traffic load (Kbps)

8 1500 32 800

16 1500 160 9600

8 1 32 100

16 1 160 1200

370

S.-T. Sheu et al. / Computer Communications 29 (2006) 358–371

simulation period. When the total traffic load is heavy, the data frames of video have a high probability of suffering from excessive access delay under the operation in standard MAC protocol.

6. Conclusions

Fig. 12. Comparisons of the average access delay obtained by the QAPS and the standard MAC protocol, where the simulated traffic types are voice, video, multimedia and background.

In simulations, the maximal frame length of each traffic type follows the value defined in Table 1. The access delay of a data frame is measured from the data arriving at a priority queue to it is being transmitted successfully. In this simulation model, as shown in Table 3, twenty STAs are designed to start performing different applications at different simulation time (in second). Initially, two STAs are first active with voice service. Subsequently, one STA begins to transmit video traffic at the 10th second, and five STAs begin to execute multimedia applications at the 20th second. Finally, the background traffic loads are generated from twelve STAs at the 30th second. Fig. 12 shows the results in 60 s of simulation period. To observe the maintenance of different QoS requirements, three QoS boundaries of voice, video and multimedia applications defined in Table 1, are plotted in Fig. 12. In Fig. 12(a), the QAPS can clearly separate four traffic services and maintain voice, video and multimedia traffic in their QoS requirements even though the total traffic loads become heavy. Contrarily, in Fig. 12(b), although the standard protocol can also provide the QoS requirements for voice, video and multimedia in light traffic loads (in the first 30 s). However, when the total traffic loads become heavy (after the 30th s), the QoS boundaries are not maintained any more. Notably, the video application may generate burst traffic during

This study proposed an ACK-based polling strategy (APS) in infrastructure architecture to provide the adaptively and dynamically polling function for AP. The strategy permits AP being able to defer the replied ACK frames of some STAs, which still have data frames in their buffers, and then uses them to be the control frames to poll these STAs. Thus, the STAs can continuously transmit their data frames to AP and avoid frequent contention and handshaking. Based on the adaptively polling function, the APS can be further extended to support QoS requirements of different traffic types. Different remaining lifetimes (RLT) and priorities of data frames are considered to schedule the polling sequence. Moreover, the fake-ACK mechanism is proposed to increase the possibilities of STAs stayed in the scheduled list. Simulation results showed that the proposed APS and its enhanced function (QAPS) could easily improve the transmission efficiency and support QoS requirements in the infrastructure WLANs.

Acknowledgements The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 92-2622-E-032-002CC3.

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