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History based adaptive backoff (hbab) ieee 802.11 mac protocol

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  • 1. Communication Networks and Services Research Conference History Based Adaptive Backoff (HBAB) IEEE 802.11 MAC Protocol Qassim Nasir (1) and Maali Albalt (2) Department of Electrical and Computer Engineering College of Engineering- University Of Sharjah (1) nasir@sharjah.ac.ae , (2) maali@sharjah.ac.ae Abstract transmission without taking into consideration the network conditions. A Mobile Ad Hoc Network (MANET) is a collection Many researchers were motivated to enhance the of mobile nodes interconnected by wireless media. performance of the IEEE 802.11 through modifying Several protocols have been proposed to manage the BEB algorithm [1]-[11]. Most of the prior work in multiple access to the shared wireless medium in this area have changed or modified the BEB algorithm MANETs, with the IEEE 802.11 being the most such that it provides relative priority among two or implemented protocol. In this work, a History-Based more traffic classes [1]-[4]. This solves the intra-class Adaptive Backoff (HBAB) MAC Protocol is proposed contention problem since the class with the least to provide a better QoS performance. HBAB modifies specified CW value would access the channel first. the original IEEE 802.11Binary Exponential Backoff However, it does not solve the inter-class contention (BEB) algorithm (which is used to control the problem since a number of stations wishing to send contention window in the case of collisions) to reflect packets of the same priority class may still contend and the network status and condition. Inspired by the collide (in case their backoff timers expire adaptive delta modulation scheme that is used in simultaneously). The latter problem is solved by communications, HBAB considers the history of past determining what to do until a successful transmission trials for transmission. Simulations show a significant takes place, or simply how to increment the value of improvement in performance measures (around 15% in the contention window in the case of a busy channel. packet delivery fraction and up to 50% in average The suggested backoff algorithms were mostly slight packet delay) with using HBAB compared to the variations or scales of the BEB algorithm for each original backoff algorithm of IEEE 802.11. traffic class. However, the way how to decrement the CW was unaddressed explicitly and hence assumed to 1. Introduction remain the same as the original BEB algorithm (the CW would reset to CWmin upon a successful Wireless network access of mobile devices such as transmission). smart phones and laptops has recently become more It can be said that the above works solved the feasible and popular due to the incredible contention problem from a priority point of view developments in wireless technologies, with IEEE through determining which class should access the 802.11 Wireless Local Area Networks (WLANs) being medium first. But that is not enough since the sudden one of the most implemented standards. Since the CW reset to CWmin may cause several collisions, wireless medium is shared by all transmitting stations which requires addressing the contention problem from in range, IEEE 802.11 has to control medium access a congestion point of view. More specifically, when a among contending stations so as to minimize the effect station succeeds in transmitting a packet at a given of collisions on the performance of the network. To CW, that doesn’t mean a decrease in congestion, but it achieve that, 802.11 adopts a binary exponential means arriving at a convenient CW value [5]. backoff (BEB) algorithm that exponentially increases a This finding inspired some researchers to adopt a station’s waiting time if the medium is found busy, and different approach of looking at the backoff algorithm, resets to a minimum value right after a successful which is what to do after a successful transmission transmission. The BEB algorithm is considered takes place, or simply how to decrement the CW. “memory-less” since it resets the Contention Window Several works on slowly decreasing the CW value (CW) value to the minimum right after a successful were proposed [5]-[11]. [6] and [7] suggested an exponential decrease in the CW value upon a978-0-7695-3135-9/08 $25.00 © 2008 IEEE 533 539 537DOI 10.1109/CNSR.2008.20
  • 2. successful transmission instead of resetting to CWmin, defines two medium access functions: a mandatorybut they assumed a fixed scale of decrease without distributed coordination function (DCF) and antaking the network conditions into account. That might optional point coordination function (PCF) [2].result in underutilizing the channel if it was idle orreturning to a congestion state if the network had not 2.1. DCFyet relieved from a previous congestion. This function uses a carrier sense multiple access While [5] and [8] suggested slow decrease backoff with collision avoidance (CSMA/CA) mechanism toalgorithms to adapt to the network load, there was control access to the shared wireless medium. Beforeanother proposal to assume a p-persistent MAC initiating a transmission, each STA is required to senseprotocol in which the station would transmit with a the medium for a time interval called DCF interframeprobability p and refrain form transmitting with a space (DIFS) and perform a binary exponential backoffprobability 1-p [9][10]. That p-value was calculated in procedure using an initial random Contention Windowruntime and updated after each transmission to reflect (CW) value. Only if the medium remains idle after thethe current number of active stations in [9] or the backoff timer expires, the station is allowed toaverage time the channel is idle or busy in [10] among transmit. A positive acknowledgment (ACK) is used toother conditions that affect the network load. In both notify the sender that the frame has been successfullythe slow decrease and p-persistent cases, complex received. If an ACK is not received within a timecomputations were needed to update the p-value and to period of ACKTimeout, the sender assumes that thereestimate the network load, respectively. Complex is a collision and schedules a retransmission bycomputations also mean high power consumption, entering the backoff process again until the maximumwhich is in many cases considered unaffordable in the retransmission limit is reached [2].wireless ad hoc networks context. A maximum of 7 retransmissions (4 This paper proposes a novel approach to slowly retransmissions) for short frames (long frames) areincrease and decrease the CW value based on the allowed before the frame is dropped. The basic accessbusyness of the channel, i.e. MAC layer transmission procedure is depicted in Figure 1 (a). To furtherretrials. The suggested algorithm to predict the decrease the overhead caused by frame collision andcontention window size, HBAB, is similar to the hidden terminal effects, Request-To-Send (RTS) andadaptive delta modulation algorithm (which is used to Clear-To-Send (CTS) frames may also be exchangedpredict the step size in communications). before the data transmission. The paper is organized as follows. Section 2 It is worth noting that collision avoidance isreviews the IEEE 802.11 MAC protocol and backoff achieved by a virtual carrier sense mechanism.algorithm. Section 3 presents the HBAB IEEE 802.11 Whether the channel is idle or busy is not solelyMAC Protocol. Section 4 discusses performance determined by the physical carrier sense result, but alsoevaluation of the proposed HBAB against the standard by the value of the Network Allocation Vector (NAV)BEB IEEE 802.11. Section 5 concludes the paper and timer maintained by the MAC. A duration value isprovides directions for future work. included in each frame that is transmitted by a station and indicates how long the transmission will last,2. IEEE 802.11 and Backoff including any subsequent acknowledgments and fragments. Each station in the vicinity of the To start with, let’s briefly review the IEEE 802.11 transmitting station receives the frame and uses thestandard [12]. It defines two basic network duration value to update its NAV. Therefore the NAVconfiguration modes: the infrastructure mode, where value indicates how long another station has access toall stations (STAs) have to go through a central access the wireless medium no matter what is the real activitypoint (AP) device in order to talk to each other, and the of that station.ad hoc mode, where any STA can directly talk to theothers without the need for a centralized device. The 2.1.2 Backoff Algorithms. The BEB algorithm isIEEE 802.11 standard defines the specifications of used by the IEEE 802.11 to control access to the sharedboth the physical (PHY) and medium access control wireless medium among contending stations. This is(MAC) layers to construct a WLAN using either done through adjusting the contention window sizeconfiguration mode. While the 802.11 PHY layer based on the current medium status. Figures 1 and 2defines the signaling and modulation properties of the show how the DCF employs the BEB algorithm withprotocol, the 802.11 MAC layer controls access to the or without having CTS/RTS [2].wireless medium, this is shared by all stations in the When a station has some data to send, it senses thetransmission range. To achieve that, the 802.11 MAC channel to determine whether it is idle. If the medium 534 540 538
  • 3. remains idle for a specified time interval, called thedistributed inter-frame space (DIFS), the station isallowed to transmit. If the medium is busy, thetransmission is postponed until the ongoingtransmission concludes. Meanwhile, a slotted binaryexponential backoff procedure takes place: each slot isequal to DIFS, and the number of such slots isdetermined by a random value uniformly chosen in [0,CW -1], where CW is the current contention windowsize. That random value is used to initialize the backoff Figure 2: IEEE 802.11 DCF without RTS-CTS.timer, which keeps running as long as the channel issensed idle, paused when data transmission (initiatedby other stations) is in progress, and resumed when thechannel is sensed idle again for more than DIFS. The 3. History Based Adaptive Backoff (HBAB)time immediately following an idle DIFS is slotted, IEEE 802.11 MAC Protocolwith each slot equal to the time needed for any stationto detect the transmission of a frame (in the IEEE This paper proposes a novel backoff algorithm, the802.11 term, MAC Service Data Unit (MSDU)) from History Based Adaptive Backoff (HBAB) algorithm, inany other station. When the backoff timer expires, the which the history of the past trials for transmission isstation attempts to transmit a data frame at the taken into account. This approach is similar to whatbeginning of next slot. adaptive delta modulators do when deciding the step Finally, if the data frame is successfully received, size [13-15]. The HBAB algorithm checks the last Nthe receiver transmits an acknowledgment frame after a states of the medium (N=2 in this implementation), andspecified interval, called the short inter-frame space decides whether to increment or decrement the CW(SIFS), that is less than DIFS. If an acknowledgment is value based on the channels tendency to being free ornot received, the data frame is presumed to be lost, and busy. In other words, if the channel tends to be freea retransmission is scheduled. The value of CW is set (the most recent state(s) indicate(s) a free channel),to CWmin in the first transmission attempt, and is then the CW value is decreased; if the channel tends todoubled at each retransmission up to a pre-determined be busy (the most recent state(s) indicate(s) a busyvalue CWmax. Retransmissions for the same data channel), then the CW value is increased. The HBABframe can be made up to a pre-determined retry limit, algorithm fixes two parameters, α and β, which areL, times. Beyond that, the pending frame will be used to increase or decrease the new CW based on thedropped. In the case that the floor acquisition RTS- old CW value (that will automatically increase orCTS mechanism is used, the same procedure is decrease the backoff time). The paper does not discussconducted except that an RTS-CTS handshake the optimization of those two parameters and could beoperation precedes the DATA-ACK exchange (Figure a good future work. Table 1 shows the suggested CW1). [2] values per state check (0 indicates a busy channel and 1 indicates a free channel): State CW value Ex: CW value (with α=1 β=2) 00 CW=CWold* (α β) 2 CWold 01 CW=CWold* (α / β) 1/2 CWold 10 CW=CWold* (β / α) 2 CWold 11 CW=CWold* (1/ α β) 1/2 CWold Table 1: CW estimation algorithm in HBAB.Figure 1: IEEE 802.11 DCF with RTS-CTS. Figure 3 shows the operation of HBAB. The node tries to transmit a packet and in case of failure then a Channel Status Bit (CSB) will be set to ‘0’, otherwise will be set to ‘1’ and send the packet. The last three CSBs are collected to form the Channel Status (CS) 535 541 539
  • 4. word, e.g. if CS=001 then it represents that current Simulation time 600 stransmission trial is successful while the previous trials Packet size 512 byteswas failed. According to the value of this word, the Packet rate 4 packets/scontention window (CW) is estimated according to Mobility pattern Random waypoint withTable 1. The estimated CW can not be set below a max speed of 10 m/s andminimum CW or beyond the maximum configured by pause time of 50 sthe interface card hardware (the values of 31and 1023 Traffic type Constant bit rate (CBR)respectively are used in this paper). CWmin, CWmax 31, 1023 α,β 1.1, 1.9 Table 2: Simulation Parameters. For the mobile scenarios, the random waypoint model is used to model node mobility. In this model, a node chooses a random point in the network, and moves towards that point at a constant speed. The speeds are uniformly chosen between the minimum and maximum speeds set to 0 m/s and 10 m/s, respectively. When the node reaches its destination, it stays there for a certain pause time (fixed to be 50 seconds in this paper), after which it chooses another random destination point and repeats the process. All simulations last for 600 seconds. The data traffic is generated by Constant Bit Rate (CBR) sessions initiated between random source and destination pairs. To prevent the network from being flooded with RREQs when the simulation starts, the start of each of the CBR sessions is time-staggered with a 5 second delay between sessions. Each session lasts until the end of the simulation. The metrics used in comparing protocol performance are 1) the Packet Delivery Fraction (PDF), which represents the ratio of the number of the successfully delivered data packets to their destinations versus the number of all data packets being sent; and 2) the average end to end delay, which measures the average required time in seconds to receive a packet . The following graphs (Figures 4 to 7) show theFigure 3: Operation of HBAB PDF and average delay in seconds of both the original and the modified IEEE 802.11 protocol (HBAB) with varying the number of connection (connections load),4. Performance Analysis and number of nodes (network density) respectively. The results clearly show the improvement of the delay The simulation is carried out in GloMoSim (Global with the implementation of the HBAB algorithmMobile system Simulator) 2.03 [16]. The GloMoSim (around 15% increase in PDF and up to 50% decreaselibrary is a scalable simulation environment for in average packet delay). In the light of these results, itwireless network systems using the parallel discrete- can be strongly suggested to consider modifying theevent simulation capability provided by PARSEC. Our backoff algorithm in order to provide QoS in the MACsimulation modelled a network of 50 mobile hosts layer.placed randomly within a 1000 X 1000 meter area. Figure 4 shows the PDF against the number ofRadio propagation range for each node was 250 meters connected nodes. This test studies the behavior of theand channel capacity was 2 Mbits/sec. two algorithms under different network load condition. It is clear that HBAB has better PDF (around 15%Parameter Value better) compared with the original 802.11 which usesArea 1000x1000 m an ordinary exponential Backoff.Number of nodes 50 536 542 540
  • 5. delay and shows almost the same improvement as the 90.00% 85.00% previous test case. 80.00% 75.00% 6 Average Delay (Sec) 70.00% PDF 802.11 65.00% 5 60.00% HBAB 55.00% 4 50.00% 802.11 3 45.00% HBAB 40.00% 2 0 10 20 30 40 50 60 1 # of Connections 0Figure 4: Packet Delivery Fraction (PDF) when 0 50 100 150 200 250varying the number of connections from 10 to 60. # of Nodes The average end to end delay has been also Figure 7: Average delay (in seconds) with varyingmeasured for both algorithms under different network the number of nodes from 10 to 250.load condition as it can be seen from figure (5). AgainHBAB has less average delay (up to 50%) compared to 5. Conclusionstandard BEB 802.11 MAC layer. This paper proposes the History-Based Adaptive 5 Backoff algorithm (HBAB), a simple-to-implement Average Delay (Sec) 4.5 backoff scheme that improves the performance of the 4 3.5 IEEE 802.11 DCF. The HBAB algorithm dynamically 3 802.11 adjusts the contention window based on the history of 2.5 2 HBAB past trials for transmission, thus adapting to the current 1.5 congestion level of the network.. It can be used in 1 MANETs to help achieve lower end-to-end delay and 0.5 0 better packet delivery fraction. The most important 0 10 20 30 40 50 60 characteristic of the HBAB scheme is its simplicity of # of Connections implementation in the widely deployed IEEE 802.11 WLANs. Simulation results show that HBAB achievesFigure 5: Average delay (in seconds) when varying better performance (around 15% increase in packetthe number of connections from 10 to 60. delivery fraction and up to 50% decrease in average packet delay) compared to the standard BEB, 90.00% especially when basic access scheme is employed for 80.00% high congested environment. The expense of deploying 70.00% the HBAB algorithm depends on the desired depth in PDF 60.00% 802.11 channel status history (1 bit for each saved channel HBAB state). HBAB algorithm is currently being tested using 50.00% a Linux-based testbed. 40.00% 30.00% 0 50 100 150 200 250 # of Nodes 10. References [1] D. Deng and R. Chang, "A priority scheme for IEEE 802.11 DCF access method ", IEICE Transaction onFigure 6: Packet Delivery Fraction (PDF) with Communications, vol.E82-B, No.1 Jan 99, pp. 96-102.varying the number of connections from 10 to 60. [2] M. Barry, A. Campbell, A. Veres, "Distributed Control Figures 6 and 7 show the performance of both Algorithms for Service Differentiation in Wireless Packetalgorithms under different network densities (the Networks", Proc. INFOCOM 2001, Anchorage, AK, USA,number of active nodes in the working area). HBAB Apr 2001.algorithm behave better in terms of PDF and average [3] V. Kanodia, C. Li, A. Sabharwal, B. Sadeghi, and E. Knightly, “Distributed Multi-Hop Scheduling and Medium 537 543 541
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