History Based Probabilistic Backoff Algorithm

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1 America Joural of Egieerig ad Applied Scieces, 2012, 5 (3), ISSN: Rajagopala ad Mala, This ope access article is distributed uder a Creative Commos Attributio (CC-BY) 3.0 licese doi: /ajeassp Published Olie 5 (3) 2012 ( History Based Probabilistic Backoff Naredra Rajagopala ad C. Mala Departmet of Computer Sciece ad Egieerig, Natioal Istitute of Techology, Trichy, Idia Received , Revised ; Accepted ABSTRACT Performace of Wireless LAN ca be improved at each layer of the protocol stack with respect to eergy efficiecy. The Media Access Cotrol layer is resposible for the key fuctios like access cotrol ad flow cotrol. Durig cotetio, Backoff algorithm is used to gai access to the medium with miimum probability of collisio. After studyig differet variatios of back off algorithms that have bee proposed, a ew variat called History based Probabilistic Backoff is proposed. Through mathematical aalysis ad simulatio results usig NS-2, it is see that proposed History based Probabilistic Backoff algorithm performs better tha Biary Expoetial Backoff algorithm. Keywords: Wide Area Networks (WANs), Local Area Networks (LANs), Wireless Fidelity (WiFi), Istitutio of Electrical ad Electroics Egieers (IEEE), Carrier Sese Multiple Access with Collisio Detectio (CSMA/CD) 1. INTRODUCTION Withi a short spa of time, commuicatio etworks have goe through rapid evolutio sice its iceptio. Network systems are classified ito Local Area Networks (LANs), Wide Area Networks (WANs) ad Metropolita Area Networks (MANs) based upo its area of coverage. LAN is a importat classificatio which is widely used to coect stadaloe systems i offices or i istitutios. The Istitutio of Electrical ad Electroics Egieers (IEEE) is a techical professioal society which stadardizes specificatios. IEEE came up with differet stadards for differet LAN techologies such as IEEE for wired etheret LANs, IEEE for wired toke rig LANs, IEEE (Schiller, 2008; Wu et al., 2001; 2003) for wireless LANs. Mobility has become oe of the ecessary requiremets for users. IEEE b also kow as Wireless Fidelity (WiFi) is oe of the widely used stadards today. Media access cotrol is a importat fuctio i ay Local Area Network. Access Cotrol is the mai focus i this work. Cocurret access of the shared medium by may statios would lead to collisio ad retrasmissio of data frames, resultig i degradatio of the etwork efficiecy. Access cotrol is a protocol by which decisio is made as to who will access the medium at a give poit of time. Aother importat fuctio hadled by the data lik layer is flow cotrol. The seder ad the receiver come to a uderstadig o how much of data ca be set, so that it is coveiet for both the seder ad the receiver to optimize the badwidth utilizatio. This fuctio is called flow cotrol. The ature of the wired ad wireless media differs cosiderably. Wired medium is scalable with the usage of higher badwidth cables ad ot easily subjected to iterferece ad disturbace. The sigal stregth is uiform throughout the medium makig it possible to sese for collisio wheever two or more statios access it simultaeously. The protocol used for media access cotrol i IEEE is Carrier Sese Multiple Access with Collisio Detectio (CSMA/CD) (Schiller, 2008). But i the case of a wireless medium the frequecy rage is fixed ad also highly vulerable to iterferece ad disturbace, makig the same access cotrol specificatios usuitable for both media. I wireless LANs the sigal stregth decreases proportioal to the Correspodig Author: Naredra Rajagopala, Departmet of Computer Sciece ad Egieerig, Natioal Istitute of Techology, Trichy, Idia Sciece Publicatios 230

2 Naredra Rajagopala ad C. Mala / America Joural of Egieerig ad Applied Scieces 5 (3) (2012) square of the distace it travels, makig it practically ifeasible to sese a collisio as the retured sigal would be very mild which may go udetected. Carrier Sese Multiple Access with Collisio Avoidace (CSMA/CA) (Ziouva ad Atoakopoulos, 2003) is the protocol for access cotrol i IEEE The overview of IEEE MAC is explaied ext followed by discussio o the differet variatios of Backoff s. A overview of the proposed History Based Probabilistic Backoff is discussed ext followed by Mathematical aalysis of the proposed, Simulatio Results followed by coclusio Overview of IEEE Media Access Cotrol IEEE Media Access Cotrol supports asychroous ad time boud delivery of data frames. The stadard supports madatory Distributed Coordiatio Fuctio (DCF) (Wu et al., 2003; Ziouva ad Atoakopoulos, 2003) ad optioal Poit Coordiatio Fuctio (PCF) (Wu et al., 2003; Ziouva ad Atoakopoulos, 2003). I the case of PCF, a access poit elected as the moitor keeps pollig the other odes o whether they are iterested to access the medium. It allocates the medium to oe of the odes based upo priority criteria. PCF is ecessary for real time services. But ad-hoc etworks caot guaratee real time services as it is formed dyamically o the move by some statios ad do ot cotai a access poit. I DCF two techiques are used i data trasmissio. Oe techique is the basic two way hadshakig mechaism i which the seder seds the data frame ad the receiver seds a Positive Ackowledgemet (PACK) after receivig the data frame. If the seder does ot receive the PACK util timeout occurs, the frame is retrasmitted. This hadshakig mechaism is simple ad easy, but results i wastage of badwidth ad very low throughput due to hidde termial ad exposed termial problems (Schiller, 2008). If A, B ad C are three statios, statio A is trasmittig packet to statio B which is ot visible to statio C. Now if statio C also tries to sed a packet to statio B, it would lead to a collisio or iterferece at B. This problem is called hidde termial problem. Statio C uderstads that statios A ad B are i coversatio by sesig the medium ad refrais from sedig ay data to statio D, eve though statio D is ot i the iterferece rage of both statios A ad B. This is called as exposed termial problem. The other techique is a four way hadshakig i which Request To Sed (RTS) ad Clear To Sed (CTS) mechaisms are used to reserve the medium. This mechaism tries to overcome Sciece Publicatios 231 the hidde termial ad exposed termial problems. If statios A ad B are i coversatio, statio A seds RTS to statio B. Every statio receivig the RTS irrespective of whether it is the iteded recipiet or ot, must set their Network Allocatio Vector (NAV) (Biachi, 2000), which idicates the earliest time duratio to be elapsed to access the medium. Statio B respods with CTS, statio C uderstads that statios A ad B are i coversatio ad refrais from sedig data to statios A or B. If statio C eeds to commuicate with statio D which is out of iterferece rage of both the statios A ad B, the statio C seds RTS to statio D which respods with the CTS, eablig the commuicatio betwee them without disturbig statios A ad B. The access cotrol protocol CSMA/CA uses four way hadshakig with RTS/CTS. Eve though the probability of collisio of data frames is elimiated drastically due to the usage of CSMA/CA, there is a probability of collisio of cotrol frames, because two statios ca try to access the medium by sedig the RTS simultaeously, resultig i a collisio. To overcome this issue the Backoff algorithm is used. It specifies the time spa after which the system ca attempt to access the medium agai thus reducig the probability of collisio. Differet variatios of backoff algorithms are proposed i the literature Overview of Existig Back Off s Three importat variatios i the literature amely Biary Expoetial Backoff (Nasir ad Albalt, 2009; Shi et al., 2004; Chatzimisios et al., 2005; Moura ad Mariheiro, 2005), Modified Biary Expoetial Backoff (Shi et al., 2004) ad Probability Based Backoff (Rajagopala ad Mala, 2010), are discussed here Biary Expoetial Backoff Accordig to the Biary Expoetial Back off (BEB) algorithm: Step 1: If a seder statio tries to trasmit data to a receiver statio ad there is a collisio, the seder statio would radomly choose a value with uiform probability from its cotetio widow [0-]. Step 2: The Cotetio Widow () is set to mi iitially ad the doubled every successive collisio util it reaches max. Step 3: It remais at max util the trasmissio is successful or the umber of retries exceed the specified limit of 7 for RTS, wherei it is assumed that the statio is ureachable.

3 Naredra Rajagopala ad C. Mala / America Joural of Egieerig ad Applied Scieces 5 (3) (2012) Step 4: Wheever there is a successful trasmissio or the umber of retries exceed the limit, is reset to mi. This basic BEB algorithm assumes that the probability fuctio of successful trasmissio of a data frame is idepedet of whether the previous frame was successfully trasmitted or ot. But the probability fuctio of successful or usuccessful trasmissio of a data frame is depedet upo how may retries were attempted before the previous data frame was successfully trasmitted Modified Biary Expoetial Backoff Modified Biary Expoetial Backoff (MBEB) algorithm was proposed i (Shi et al., 2004) to the BEB algorithm i which the cotetio widow of previous trasmitted data frame is retaied for the curret frame: Step 1: If the trasmissio is successful, the the cotetio widow is reduced by half i.e., = /2, which would be used as value for ext frame trasmissio. Step 2: If the trasmissio is usuccessful, the the cotetio widow is doubled = 2 util it reaches max which is retaied as it is, for further retries util successful. Step 3: Whe the umber of retries exceed the limit, the is ot reset to mi, but retaied as it is, for the ext data frame trasmissio. Step 4: Every time whe the cotetio widow is updated, it is verified whether the value falls below mi, if it falls below mi, the it is seto mi +1. Step 5: The ew value is verified whether it exceeds max ad if it does, the the calculated ew value is igored ad is set to max Probability Based Back Off The Probability Based Backoff (PBB) is as follows: Step 1: The iitial value of Cotetio Widow is set to mi. The variables S ad C which represet umber of successful trasmissios ad umber of collisios respectively are iitialized to 0. Step 2: After each attempt to trasmit a data frame, S or C is icremeted accordigly ad the value of probability P is calculated usig P = C/(C +S). Sciece Publicatios 232 Step 3: The value of is calculated usig the formula = 1 + (2 P). Step 4: The value of i the case of successful trasmissio is calculated by the formula a ew = MAX[ mi + 1, Ad the value of i case of usuccessful trasmissio, by the formula: a ew = MIN[ max - 1, Step 5: Wheever the umber of retrasmissios exceed the umber of retries, Cotetio widow value is ot reset but carried forward to the ext frame trasmissio. The BEB algorithm assumes that the probability of a successful trasmissio of a data frame is idepedet of the previous trasmissios. Every ew trasmissio is iitialized to mi. This results i decreased throughput. The MBEB algorithm retais the value from the previous trasmissio, but agai it decreases or icrease the value expoetially, which ca deteriorate the performace. The PBB algorithm takes ito accout the total umber of successful trasmissios ad collisios to determie the value, but without cosideratio of the order of trasmissio History Based Probabilistic Backoff Accordig to Biary Expoetial Backoff, wheever a collisio occurs durig a data trasmissio, the cotetio widow size is doubled, or i other words, multiplied by 2 util successfully trasmitted or give up tryig for a maximum umber of attempts. I case of Modified Biary Expoetial Backoff, the cotetio widow size is doubled for every collisio util the maximum value is reached ad retaied the same for the allowed maximum umber of tries. I case of a successful trasmissio, the cotetio widow value is divided by two for every successful trasmissio util a miimum value is reached ad is retaied at the miimum value for subsequet attempts. The cotetio widow size is ot reset for every ew trasmissio, but is carried ahead from the previous trasmissio. It ca be mathematically represeted as: ext = prev 2 i case of successful trasmissio. ext = + prev /2 i case of collisio.

4 Naredra Rajagopala ad C. Mala / America Joural of Egieerig ad Applied Scieces 5 (3) (2012) I geeral, ext = prev 2 a, where is 1 i case of successful trasmissio ad-1 whe it is a collisio. The value of is static. This work proposes aalyses ad reasos out theoretically ad through simulatio the eed ad effect of expoetial icrease or decrease i the size of the cotetio widow by a factor of 2. Mathematically the value of is static ad either 1-1. If the cotetio widow is icreased by a factor more tha 2, the cotetio widow would be loger, leadig to higher latecy. If the cotetio widow is icreased by a factor less tha 1, the due to very short cotetio widow, it would result i may collisios ad decreased throughput. I the proposed algorithm, the value of is varied dyamically betwee-1 ad 1 depedig o the traffic load or collisio rate. The traffic load or the collisio rate is represeted by: Sciece Publicatios P = C/(C + S) + β where, variable C represets umber of collisios ad variable S represets successful trasmissio. Variable β is added to give weight o the order of collisio or successful trasmissio. The value of β is calculated by heuristic method by the equatio below. If T 1,T 2,T 3,... T( -1 ) is the trasmissios, the: For I = 1,2,3,4,,(-1): β ± (i ) ( i+ 1) mod ulo2 [( i 1)]2 = [ ] ± is used to idicate whether it is a successful trasmissio or a collisio. As P is a ratio, it varies betwee 0 ad 1. But the iterval of α is betwee -1 ad 1, twice of P. Hece the value of α is give by equatio: α = -1 + (p 2) After calculatig the value of α, the cotetio widow is calculated by equatio: = 2 α ew 1.7. The History based Probabilistic Backoff Step 1: The iitial value of Cotetio Widow is set to mi. The variables S ad C which represet umber of successful trasmissios ad umber of collisios respectively are iitialized to Step 2: After each attempt to trasmit a data frame, S or C is icremeted accordigly ad the value of probability P is calculated If P is the probability estimatio of T th. Trasmissio, the for i = 1, 2, 3, 4,..., ( 1): β ± (i ) ( i+ 1) mod ulo2 [ i 1]/2 = [ ] P = C/(C + S) P = P If P >0.80 OR P <0.20 Eq. 1a: Else Eq. 1b: P = C/(C + S) + β (1a) (1b) Step 3: The value of a is calculated usig the formula Eq. 2: α = -1 + (P 2) (2) Step 4: The value of is calculated by the formula Eq. 3: ew = MAX[ mi + 1, α (3) I the case of successful trasmissio ad Eq. 4: ew = MIN[ max - 1, α (4) For usuccessful trasmissio. Step 5: Wheever the umber of retrasmissios exceed the umber of retries, Cotetio widow value is ot reset but carried forward to the ext frame trasmissio: = ew 1.8. Mathematical Aalysis of BEB, MBEB, PBB ad HBPB Cosider a sceario i which three data frame trasmissio attempts have bee made, each of which may result i success or failure; success is represeted by S ad failure or collisio is represeted by C. Now whe there is a attempt to trasmit the fourth frame, the value of the cotetio widow is calculated usig the BEB algorithm, MBEB algorithm ad PBB algorithm. A comparative study is discussed i (Nasir ad Albalt, 2009; Shi et al., 2004; Chatzimisios et al., 2005) o

5 Naredra Rajagopala ad C. Mala / America Joural of Egieerig ad Applied Scieces 5 (3) (2012) BEB algorithm ad MBEB algorithm. (Biachi, 1998; 2000) discuss its effect o throughput. From the basic BEB algorithm, it is kow that the calculatio of the cotetio widow of a data frame does ot deped o previous trasmissios but oly o the successive usuccessful attempts BEB Cosider the scearios i Table 1, calculatig the values of the after 3 trasmissios usig basic BEB algorithm. Iitially let = mi. I Table 1, first row, trasmissio 1 resulted i collisio, = mi 2. Trasmissio 2 also resulted i a collisio, = mi 2 2. Similarly after third trasmissio, = mi 2 3, represeted as 2. I the secod row of Table 1. Agai, iitially, by the BEB algorithm, = mi. Further, it is observed that the first trasmissio ad secod trasmissio resulted i collisio deoted by C, but the third trasmissio was successful i trasmittig the frame: At trasmissio 2, = mi 2 At trasmissio 3, = mi 2 2 At trasmissio 4, = mi as reflected i Table 1 Similarly examiig the third row ad fourth row, it ca be observed that irrespective of the history of the trasmissio, if the previous trasmissio is successful value is reset to mi, as show i rows 3 ad MBEB MBEB algorithm is applied to the same sceario ad the value after each trasmissio is determied ad tabulated i Table 2. Iitially at trasmissio 1, = mi, As trasmissio 1 resulted i collisio, = mi 2, As trasmissio 2 also a collisio, = mi 2 2, = = mi 2 3 represeted as 2 i Table 2. The sceario i the secod row differetiates MBEB algorithm from the BEB algorithm: As trasmissio 1 results i collisio, = mi 2 Trasmissio 2 also a collisio, = mi 2 2 Trasmissio 3 is successful, = /2 = mi 2 Sciece Publicatios 234 The iitial value of = mi at row 3. As the first trasmissio is a failure, = mi 2. As the secod trasmissio is a success, = /2 = mi Third trasmissio is a success, = MAX [ /2, mi +1] = mi +1. Similarly applyig the MBEB algorithm, the value at trasmissio 3 i rows 3 ad 4 would be = mi + 1 represeted as /2 i the Table PBB The PBB algorithm is applied to the give sceario ad the correspodig values are tabulated i Table 3. The iitial value of = mi. Cosiderig the first row, after the first usuccessful trasmissio: = mi 2 because P = 1. Hece a = 1 + (P 2) = 1. Thus = 2 a = mi 2 As trasmissio 2 has also eded i collisio, P = 1 = 2 3 = mi 2 2. Similarly for the fourth trasmissio: = MIN [ mi 2 3, max -1] represeted as 2 i Table 3 Table 1. BEB algorithm results Tras1 Tras2 Tras3 for Tras4 C C C W 2 C C S mi C S S mi S S S mi Table 2. MBEB algorithm results Tras1 Tras2 Tras3 for Tras4 C C C 2 C C S /2 C S S /2 S S S /2 Table 3. PBB algorithm results Tras1 Tras2 Tras3 for Tras4 C C C 2 C C S 3 C S S 3 2 S S S /2 2

6 Naredra Rajagopala ad C. Mala / America Joural of Egieerig ad Applied Scieces 5 (3) (2012) I the secod row after the first ad secod trasmissio which is similar to the first row: = mi 2 2 Trasmissio 3 is successful, P = 2/3. = a-1 + (2 (1-P) ) = -1/3. Therefore = 2 = mi /3 = mi /3, represeted as prev 2 i Table 3. I the third row, after the first usuccessful trasmissio, = mi 2, After secod trasmissio, P = 1/2, a = 1 + (2(1-P)) = 0. = 2 = = mi 2. Trasmissio 3 is successful, P = 1/3, = a-1 + (2 (1- P)) = 1/3. Therefore = 2 = mi 2 21/3 = 3 mi 24/3, represeted as prev 2 i Table 3. As all the trasmissios are successful trasmissios i row 4, P = 0, hece = 1+ (2 P) = -1, the value of = 2 = 2 1 = mi /2. But value is ot allowed to go below mi. Hece: = MAX[ mi + 1, /2] = mi + 1. = mi + 1, represeted by /2 i Table HBPB Cosiderig the same sceario as for the above algorithms, the iitial value of = mi Cosiderig the first row, after the first usuccessful trasmissio, = mi 2 because P = 1. Hece a = 1 + (P 2) = 1. Thus = 2 = mi 2 As trasmissio 2 has also eded i collisio, P = 1 = 2 = mi 2 2. Similarly for the fourth trasmissio, = MIN[ mi 2 3, max -1] represeted as 2 i Table 4. I the secod row of Table 4, The value of P is 2/3 ad β= = 0.04,= P = / = , ew = I the Third row of Table 4, the value of P is 1/3 ad β = = 0.14, = P = / = , ew = As all the trasmissios are successful trasmissios i row 4, P = 0, hece = 1+(2 P) = -1, The value of = 2 a = 2 1 = mi /2. But value is ot allowed to go below mi. Hece, = MAX[ mi + 1, /2] = mi + 1. = mi + 1, represeted by /2 i Table 4. The History Based Probabilistic Backoff (HBPB) is evaluated with Network Simulator NS-33 (Network Simulator. to compare with the basic Biary Expoetial Backoff (BEB) algorithm. Sciece Publicatios 235 Table 4. HBPB algorithm results Tras1 Tras2 Tras3 for Tras4 C C C 2 C C S C S S S S S 2 Table 5. Simulatio parameters Parameter Value Simulatio area m Simulatio Duratio 1000sec MAC Protocol Routig Protocol DSDV Traffic Type CBR RTS retry limit 7 Cotetio Widow ( max ) 1023 Cotetio Widow ( mi ) 31 Table 6. Retrasmissio failure cout BEB vs HBPB algorithm Number of Retrasmissio Retrasmissio Sl. No Nodes cout for BEB cout for HBPB Fig. 1. Retrasmissio Failures usig BEB Vs HBPB The Simulatio Parameters are give i Table 5. Compared to the basic biary expoetial backoff algorithm implemeted i NS-2.33, the two modules ic_cw () ad rst_cw () are modified to implemet the History Based Probabilistic Backoff. Global variables called Collisio Cout ad Trasmissio Cout are declared ad used to keep cout of successful trasmissios ad collisios. The umber of odes are varied from , for the same etwork cofiguratio.

7 Naredra Rajagopala ad C. Mala / America Joural of Egieerig ad Applied Scieces 5 (3) (2012) Fig. 2. Throughput usig BEB Vs HBPB The umber of RTS retrasmissio failures is logged for varyig umber of odes. The calculatio of β i the implemetatio of HBPB algorithm is limited to up to five trasmissios sice the sum of the rest is egligible. The retrasmissio failure cout is recorded i Table 6 ad also plotted i Graph 1 (Fig. 1). Also the throughput of both the algorithms are compared ad plotted i Graph 2 (Fig. 2). Sciece Publicatios 2. CONCLUSION The performace of BEB algorithm, MBEB algorithm ad PBB algorithm is aalyzed ad compared with HBPB algorithm mathematically. It ca be observed from the results tabulated that the value of Cotetio Widow grows expoetially i both the cases of BEB ad MBEB algorithms without takig the etwork collisio rate ito cosideratio. The PBB algorithm dyamically varies the value of takig the collisio rate ad the success rate of trasmissios ito cosideratio, makig it more adaptable to the etwork. A improvemet is proposed by itroducig β as a parameter i HBPB algorithm to cosider the order of collisios or successful trasmissios. It ca be iferred from the simulatio results that the proposed HBPB algorithm performs better tha the Biary Expoetial Backoff algorithm i terms of better throughput. It performs sigificatly better i the case of a cogested etwork. 3. REFERENCES Biachi, G., IEEE saturatio throughput aalysis. IEEE Commu. Lett., 2: DOI: / Biachi, G., Performace aalysis of the IEEE distributed coordiatio fuctio. IEEE J. Selected Area Commu., 18: DOI: / Chatzimisios, P., A.C. Boucouvalas, V. Vitsas, A. Vafiadis ad A. Ecoomidis et al., A simple ad effective backoff scheme for the IEEE MAC protocol. Bouremouth Uiversity, UK. Moura, J.A. ad R.N. Mariheiro, MAC approaches for QoS ehacemet i wireless LANs. Proc. JETC Netw. Simulator. Nasir, Q. ad M. Albalt, Improved backoff algorithm for IEEE etworks. Proceedigs of the Iteratioal Coferece o Networkig, Sesig ad Cotrol, Mar., IEEE Xplore Press, Okayama, pp: 1-6. DOI: /ICNSC Rajagopala, N. ad C. Mala, aalysis ad comparative study of differet backoff algorithms with probability based backoff algorithm. Recet Treds Netw. Security Appli., 89: DOI: / _34 Schiller, J.M., Mobile Commuicatios. 1st Ed., Pearso Educatio, Idia, ISBN-10: , pp: 512. Shi, H.J., D.R. Shi ad H.Y. You, A Efficiet Backoff Scheme for IEEE DCF. Persoal Wireless Commu., 3260: DOI: / _15 Wu, H., Y. Li, S. Cheg, Y. Peg ad K. Log, IEEE Distributed coordiatio fuctio: ehacemet ad aalysis. J. Comput. Sci. Techol., 18: DOI: /BF Wu, H., Y. Log ad S. Cheg, A simple model of IEEE wireless LAN. Proceedig of the Iteratioal Cofereces o Ifo-Tech ad Ifo- Net, Oct. 29-Nov. 01, IEEE Xplore Press, Beijig, pp: DOI: /ICII Ziouva, E. ad T. Atoakopoulos, The IEEE distributed coordiatio fuctio i smallscale ad-hoc wireless LANs. It. J. Wireless Iform. Netw., 10: DOI: /A: