How to outperform IEEE802.11: Interference Aware (IA) MAC

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1 How to outpefom IEEE802.11: Intefeence Awae (IA) MAC Daniela Maniezzo, Piepaolo egamo, Matteo Cesana, Maio Gela CS Dept. - Univesity of Califonia Los Angeles - UCLA, Califonia, USA Engineeing Dept., Feaa Univesity, Italy Dipatimento di Elettonica e Infomazione, Politecnico di Milano, Italy maniezzo@ucla.edu, pbegamo@ing.unife.it, cesana@elet.polimi.it, gela@cs.ucla.edu Abstact Wieless netwoks ae nowadays vey popula. They ae mostly based on IEEE potocol which is known to show low pefomance in ad hoc netwoks. Also when infastuctued netwoks (many access points ae placed) ae consideed, a single tansmission could block many adio cells. In this wok, we popose Intefeence Awae (IA) MAC, a modified vesion of standad IEEE The main idea is not to set the Netwok Allocation Vecto (NAV) on the eception of each RTS and CTS, but only when it is stictly equied, i.e., when a concuent tansmission could actually destoy anothe tansmission. To make it feasible, we popose to inset some infomation about intefeence and eceived powe levels into IEEE contol packets. We show the effectiveness of IA MAC by means of an analytical investigation. I. INTRODUCTION The ecent success of wieless technologies has boosted the development of wieless netwoking. oth industy and academia have tuned thei attention to this aea, attacted the fome by intiguing eseach issues, the latte by maket evenues. Among the numeous standads fo wieless communications, the IEEE802.11b [1] is the one with the highest utilization at the moment. IEEE802.11b defines a MAC (Medium Access Contol) laye, MAC management potocols and sevices, and thee physical (PHY) layes espectively based on IR, FHSS at 2.4 GHz and DSSS at 2.4 GHz. The goal of the standad is to delive sevices peviously found only in wied netwoks, within mobile uses with high thoughput, high eliability and continuous netwok connection. Fo this eason, IEEE based netwoks ae often efeed to as Wieless LANs. The wieless envionment poses some challenging poblems the netwok design has to cope with. Fistly, the adio channel is pone to eos and tempoay failues which ae not encounteed in the wied wold, secondly, the channel is shaed and esouces ae often scace. In this scenaio, the employed access contol schemes ae key points fo achieving effectiveness. As fa as the MAC laye is concened, two diffeent schemes ae standadized: the Point Coodination Function (PCF) and the Distibuted Coodination Function (DCF). While PCF consists of a centalized polling based access scheme, the DCF defines a distibuted access algoithm fo both infastuctue and ad hoc wieless LANs [1], [2]. As fa as unicast data packet tansfe is concened, DCF defines two access methods. The fist one is based on a two-way handshake pocedue (DATA/ACK), the second one adopts a fou-way handshake pocedue, whee the DATA/ACK phase is peceded by a channel pobing/acquiing phase called RTS/CTS (Request To Send/ Clea To Send). oth of the above methods implement a multiple access scheme based on Caie Sensing with Collision Avoidance (CSMA/CA). asically, each node senses the channel befoe tansmitting the fist fame of the handshake; if the channel is sensed idle fo a cetain peiod of time called DIFS (Distibuted InteFame Space), the node stats tansmitting the fist fame of the handshake, othewise the node waits fo the channel to be idle fo DIFS and daws a andom additional backoff time to avoid possible collisions when the channel becomes fee (Collision Avoidance). A futhe channel contol mechanism is applied in the fou-ways handshake pocedue. Accoding to this scheme, each contol fame (RTS/CTS) bings infomation about the duation of the stating communication. Evey othe node oveheaing that fame is pevented fom accessing the channel fo all the duation of the ongoing communications by setting a pope paamete called NAV (Netwok Allocation Vecto). This pocedue is often efeed to as Vitual Caie Sensing. In this pape we popose a novel MAC laye fo Wieless LANs, named Intefeence Awae MAC (IA-MAC) [3], which extends the capabilities of the basic IEEE in envionments with high intefeence, both in ad hoc and in infastuctued mode [4], [5]. In Section II we focus on the open poblems of wieless netwoks, in paticula the IEEE based ones, and we give an oveview of the poposed appoaches of solutions. Section III summaizes the basics of the poposed IA-MAC potocol, while Section IV gives a mathematical model to analyze the pefomance of the potocol itself and in Section V some esults ae shown. Finally, Section VI concludes the pape.

2 II. OPEN PROLEMS AND PROPOSED SOLUTIONS IEEE was oiginally devised explicitly fo a single Access Point scenaio whee all the mobile nodes ae within the tansmitting ange of one anothe. In this envionment, the IEEE medium access contol potocol, which tends to avoid the intefeence, is able to achieve high efficiency. Poblems aise when IEEE is used both in a pue ad hoc mode, whee the basic IEEE mode is not able to exploit any spatial euse, and in infastuctued cellula-like scenaio whee the connectivity is povided by diffeent APs with ovelapping tansmission anges, i.e. with non negligible intefeence [6]. The efficiency of IEEE based netwoks can be damatically impaied by the well known hidden and exposed teminal poblems [7]. As a matte of fact, the fou-way handshake with the Vitual Caie Sensing solves only patially the hidden teminal poblem, and, to the best of ou knowledge, the exposed teminal one is still a pitfall and can deeply affect the pefomance of multi-hop ad hoc netwoks based on IEEE [8]. Diffeent solutions have been poposed in the liteatue to counteact these shotcomings of the IEEE standad. In paticula, a majo effot has been done in the development of an efficient medium access contol potocol able to exploit spatial euse and allow paallel feasible communications [9]. The wok on this topic deals pimaily with the modification of the numeous times the IEEE MAC level [10], [11] with the pupose to achieve a bette shaing of the common esouce among all the uses. Velayutham and Wang [12] poposed a distibuted scheduling algoithm fo IEEE netwoks able to solve the exposed teminal poblem and consequently incease the spatial euse. Recently, some woks have appeaed aiming at impoving the IEEE MAC laye by exploiting the captue at the physical laye, that is to say the capacity of coectly eceiving a tansmission even if in pesence of intefeing communications [13], [14]. Like in the cellula systems, the captue effect is exploited in ode to incease the euse and consequently the bandwidth efficiency of the wieless system [15]. Ou wok follows this second appoach. In details, we popose to enhance the infomation caied by the CTS packets (and in a moe complex appoach, also by RTS packets) so that each tansmitting/eceiving node can have an estimate on the intefeence it geneates on all the othe ongoing tansmission, and decides to actually tansmit/eceive if such an intefeence is contolled. The basic pinciples of ou mechanism ae simila to the ones intoduced in the Powe Contolled Multiple Access (PCMA) poposed by Monks et al. [16]. oth potocols ty to exploit the captue effect to incease the spatial euse, and, like PCMA, IA-MAC is based on a coopeation pinciple, i.e., no station is allowed to tansmit/eceive if its tansmission/eception can destoy ongoing tansmissions. On the othe hand, ou scheme diffes fom the PCMA in at least two significant aspects: (a) IA-MAC does not implement any powe contol, ou attention is focused on the access pat athe than on the powe management, which can be ticky in a distibuted envionment like the ad hoc one, (b) PCMA uses an additional adio channel in ode to tansmit a busy tone with infomation on the intefeence levels, while in IA- MAC the estimation of the intefeence is done on the same channel used fo data taffic; because of the last eason, IA- MAC seems to be moe cost effective and bette suited fo wieless envionments in which the battey consumption is a constaint. III. IA-MAC ASICS In the following, IA-MAC is explained by consideing that IEEE DCF is known by the eade, even if a bief desciption will be given. In ode to educe the pobability of two stations colliding on a eceive because they cannot hea each othe (hidden teminal poblem), the IEEE standad use the Request to Send/Clea to Send (RTS/CTS) handshake; a station equiing to tansmit a packet fist tansmits a shot contol packet called RTS (Request To Send), which includes the souce, destination, and the duation of the following fame; the taget station eplies (if the medium is fee) with a contol packet called CTS (Clea to Send), which includes the same duation infomation. All stations eceiving eithe the RTS and/o the CTS, efain tansmission fo the given duation. This mechanism educes the pobability of a collision on the eceive aea by a station that is hidden fom the tansmitte (and it does not hea the RTS), since the station heas the CTS and eseves the medium as busy until the end of the tansmission. The duation infomation on the RTS also potects the tansmitte aea fom collisions duing the ACK (fom stations that ae out of ange of the acknowledging station). RTS and CTS ae used in the IEEE only if the width of the data packet to be tansmitted is geate than a cetain theshold. In this pape, we conside to set this theshold to zeo, i.e., each tansmission equies the RTS/CTS handshake. IA-MAC behaves exactly as IEEE apat fom the changes in the following. The fist subsections explain the basic impovement of IA-MAC to IEEE In the last subsection, othe impovements unde investigation ae biefly discussed. A. RTS and CTS In ou poposal, each node uses always the fou way handshake, i.e., the RTS (Request To Send) and CTS (Clea To Send) ae always sent by the nodes involved in the communication. This is necessay in ode to estimate the level of noise and intefeence. Moeove, we equie a vey slight modification of the CTS packet. CTS packet includes the same infomation of the standad potocol. In addition, we include two new heade fields: SINR: the expeienced SINR (Signal to Intefeence and Noise Ratio) at the time of the eception of the RTS,

3 Fig. 1. to D. C D A A is sending a packet to when C should decide whethe to tansmit since I P RT S /SINR. If SINR ext is below a cetain theshold (γ), C efains fom tansmitting by setting its NAV, othewise it can send its own RTS. The value of γ should be designed in such a way to take into account that moe than one node could incease the expeienced intefeence on the ongoing communications. P R : the eception powe of the RTS packet. Since the quantities could be expessed in dmw (d milliwatt), the ange values can be expessed in 1 byte fo each field. These values ae used by the node which oveheas the CTS to decide whethe to set o not the NAV, accoding to the ules descibed in the following.. Physical and Vitual Channel Sensing With espect to IEEE802.11, an IA-MAC compliant node does not pefom the physical channel sensing phase but always esots to the Vitual Caie Sensing. Nevetheless, the potocol belongs to CSMA family, due to the vitual caie sensing, i.e., the value of the Netwok Allocation Vecto (NAV) detemines whethe the channel is busy fo a oncoming tansmission and fo the backoff counte. Thus, nodes decement thei backoff counte only when the channel is logically sensed busy and not when a tansmission is head by the node. The backoff counte follows the othe ules of the standad potocol (it is decemented afte a Distibuted InteFame Space time since the channel was vitually tested fee if the last eception has been coect, etc). C. Netwok Allocation Vecto (NAV) NAV is always set upon the eception of RTS, as in the standad potocol. Each node which eceives the RTS (but the ecipient node) sets up its NAV. NAV is not always set on the eception of CTS. If a node eceives a CTS fom anothe node, it can compute the impact of a tansmission on the othe node SINR. Fo bette undestanding the potocol, let us conside fou nodes (A,, C, D) in a ow (see Figue 1). A is in adio coveage only with, with A and C, C with and D and D only with C. A sent a RTS packet to and answeed with a CTS. Node C eceives a CTS packet fom (in etun to a RTS fom A), it can estimate how an its own tansmission could affect the eception of. Let P RT S be the powe eceived by on the RTS sent by A, and let I be the sum of noise and intefeence peceived by. insets in its CTS the infomation of SINR P RT S and P RT S /I. C oveheas the CTS sent by. Let PC CT S be the powe C eceives on the CTS sent by. In ode to estimate the SINR would get if C stats tansmitting, C consides the channel symmetic, i.e., would eceive a packet fom C with the same powe PC CT S peceive a value of SINR equal to: SINR ext P RT S I + P C CT S. Thus, C estimates that would P RT S SINR + PC CT S, SINR P RT S D. A Paticula Situation Let us conside the same situation of Figue 1: tansmitted its CTS to A. Suppose C has a packet to be deliveed to. In the standad IEEE802.11, C would efain the tansmission upon eception of the CTS fom. With IA-MAC, C could ty to send an RTS to, unless it has to set the NAV because of the ules descibed above. In ode to avoid such a situation, the nodes should ecod in thei memoy the ID of the sende of the CTS they ovehea and the tansmission duation (which is inseted in the standad CTS packet). In this case, the node could simply set the NAV and eset it when the duation time expies. E. Impovements IA-MAC has bette pefomance with espect to standad IEEE fo two easons, at least: it pemits paallel eliable tansmissions which ae not allowed by the basic standad, and the backoff times expie faste, since the physical caie sensing is disabled. Convesely, IA-MAC uses always RTS/CTS, which is a easonable assumption if we conside an ad hoc scenaio. In Section IV, an estimation of the pefomance gain is deived though mathematical analysis. Futue impovements to the potocol we ae woking on will futhe incease the spatial euse and consequently the oveall gain. The basic idea is to elax the ule which wants the NAV to be set always upon eception of the RTS. We have found that in some situations the NAV can be unset even if the node has eceived a RTS. This can be done inseting the infomation on the intefeence level in the RTS as well, and exploiting the time divesity among paallel communications, that is to say cases whee collision can be avoided because the nodes within the same coveage ange ae in the same tansmitting/eceiving state. IV. AN ANALYTICAL MODEL OF IA-MAC In ode to compae the pefomance of the standad IEEE DCF and the poposed IA-MAC, we conside that each tansmission of a data packet is peceded by the RTS/CTS exchanging phase. In this pape, we focus on static ad hoc netwoks. Note that in most mobility scenaios, nodes do not move fo significant distances duing a packet tansmission time, thus, fo capacity analysis, we can assume mobile netwoks as effectively static. The status of a wieless link depends on seveal system and envionmental factos that affect sende and eceive anges. In geneal, a node tansmission ange is neithe fixed, no symmetic but it shows time and spatial vaiability. In this

4 y A1 A2 R A d q A2 x L Fig. 3. Gid Netwok. Fig. 2. Coveage and intefeence aeas. pape, a widely applied optimistic model has been used [17], [18]. Fistly, we intoduce the concepts of coveage ange and blocking ange, defined espectively as the aea whee the tansmissions of a given node can be coectly detected and the aea which is blocked by the tansmission of a contol packet (RTS/CTS). Secondly, we assume that, given a sample node, both its coveage ange and blocking ange can be depicted as a cicle of adius R and espectively, centeed in the position of the node itself. The value of R depends on the tansmission powe level used and on the popagation model adopted, while the value of depends on the access potocol to be used. It is clea to undestand how in the basic IEEE DCF, the coveage ange and the blocking ange ae the same, i.e. R. Finally, we assume unifom tansmitted powe and unifom popagation law thoughout the netwok, i.e. evey node has a coveage ange with a adius R. If IA-MAC is used, the blocking ange of the node sending a CTS is smalle than its coveage ange ( < R) as shown in Figue 2, whee two nodes A and ae epesented. Given the nodes A and in Figue 2, the aea of the geometic shape which is the union of the two cicumfeences centeed on the nodes ae: A tot(, d, R, 8q) A 1(R) + δ A2 (, d, R, q) >< 0 d R, πr R d+ p 2 (x d) q 2 dx+ >: 2 R R R2 x q 2 dx R < d R; whee R is the adius of the confeence A 1, δ A2 (, d, R, q) is the aea included in the cicumfeence A 2 with adius not included in A 1, d is the distance between A and, and q is the abscissa of the intesection points between the two cicumfeences. Supposing node A is the RTS sende and node answes with a CTS, using the standad IEEE the blocking anges of both nodes ae equal, with adius R, and the two coesponding cicumfeences have thei intesection in the (1) abscissa with the same distance fom the position of the two nodes (q d 2 ). Thus, in the standad IEEE DCF, RTS and CTS block the tansmissions of all the nodes within the aea given by the union of the two blocking anges centeed on the nodes A and : A std (d, R) A tot (R, d, R, d 2 ) [ πr 2 + d 2 4R2 d 2 + 2R 2 actan ] d 4R2 d 2 On the othe hand, if IA-MAC is applied in the same case, the CTS sent by the eceive blocks a smalle numbe of neighbos than the IEEE standad one. This is because some nodes that eceive only CTS packet do not set NAV accoding to the ules explained in Section III. The nodes that have to set the NAV, ae in the aea A IA (, d, R) A tot (, d, R, q IA ). Inseting the coect value fo the intesection point abscissa, q IA d2 2 +R 2 2d, in Eq. 1 we obtain: A IA(, d, R) 8 A tot(r, d, R, q IA) 0 d R, 1 >< ˆπ( 2 R 2 ) πr 2 2 h + a(, d, R) i R 2 d actan 2 2 +R 2 + R < d R; h i >: + 2 d actan R 2 (3) whee a(, d, R) 4d 2 R 2 (d R 2 ) 2. If we conside a mesh (o gid) netwok with distance between two contiguous nodes (see Figue 3), in the IEEE standad case the numbe of simultaneous tansmissions in the netwok is given by: N std (d, R). (2) L 2 / 2 A std (d, R)/ 2 L 2 A std (d, R) ; (4) whee L is the side of the squaed netwok aea. Note that the esult is not elated to, which means it is not elated to the numbe of nodes in the netwok. This is because the blocked aea does not depend on the numbe of the nodes o on its density. Obviously, the esult depends on L because it is a function of the total available aea L 2.

5 Using IA-MAC in the same netwok configuation of Figue 3, the numbe of possible simultaneous tansmissions ae: /R0.5 /R0.6 /R0.7 /R0.8 /R0.9 N IA (, d, R) L 2 / 2 A IA (, d, R)/ 2 L 2 A IA (d, R). (5) We define the capacity gain G of the IA-MAC fo the netwok topology consideed as the atio between N IA (, d, R) and N std (d, R): Capacity Gain G(, d, R) N IA(, d, R) N std (d, R) { πr 2 +b(d,r,r) πr d R, 2 πr 2 +b(d,r,r) πr 2 +b(d,r,) R < d R; (6) d/r Fig. 4. Capacity Gain vs. sende-eceive distance. [ 1 whee: b(, d, R) 2 π( 2 R 2 ) + a(, d, R) ] [ ] [ ] + R 2 actan + 2 actan (basically: d 2 2 +R 2 d R 2 b(, d, R) A IA (, d, R) πr 2 when R < d R). Note that G is neithe function of no of L. The deived esult is geneic fo a mesh netwok. V. PERFORMANCE RESULTS In this Section some analytical esults ae shown in ode to veify the impovement of the poposed scheme with espect to the basic access scheme. The pefomances have been evaluated by consideing diffeent value of (adius fom the eceive in which nodes set thei NAV upon the eception of a CTS packet). asically, in the eal envionment, is a function of the theshold γ discussed in Section III. Fo a single fixed value, we evaluate the capacity gain G(, d, R) of the netwok with espect to the distance d between the sende and the eceive nodes. Figue 4 shows the capacity gain G as a function of d, fo diffeent values of. Since d R (othewise the eceive does not hea the RTS) and R (the maximum tansmit adius is equal to R), the vaiables of the function G ae nomalized to R. Thus, the plots show G( R, d R, 1). When d R 0, the two cicles ae ovelapping and the gain is vey low. When d R 1, IEEE blocks the lagest aea, while IA-MAC can show its effectiveness. The smalle, the geate G, because the blocked aea is educed vey much. Fo a given value of R the gain G changes its inceasing tend when d R 1 R, because of the conditions in Eq. 6. In the best situation, the netwok capacity obtained with IA- MAC is about 40% geate than IEEE Geate gains could be achieved if smalle values of R ae consideed. Actually, if values of R < 1 2 ae consideed, it means that being some nodes close to the eceive than to the tansmitte, they ae not blocked and this does not make sense. Assuming a unifom distibution of the distances between souce and destination d in the inteval [0, R], the stochastic aveage of the capacity gain function G(, d, R) is given by: Capacity Gain Fig /R Stochastic Aveage Capacity Gain vs. Receive Tansmission Radius. Γ(, R) 1 R + 1 R R 0 R R πr 2 + b(d, R, R) πr 2 dd + πr 2 + b(d, R, R) πr 2 dd. (7) + b(d, R, ) Figue 5 shows the esults of the numeical computation of the integal Γ( R, 1). The vaiables ae nomalized as above. The pefomance inceases with the nomalized blocking adius R 1 2 (the captue effect is consideed pefect) is about 26%. With a moe ealistic value of R ( R 7 10 ), the incease is about 20%, which definitely shows the effectiveness of the poposal. VI. CONCLUSION AND RESEARCH IN PROGRESS In this pape we have pesented an effective impovement to IEEE MAC potocol. The poposed scheme, Intefeence Awae IA-MAC, achieves a highe spatial euse in the netwok by allowing feasible paallel tansmissions. To

6 each this goal we popose to include some infomation on the Signal To Intefeence and Noise Ratio (SINR) and eceived powe levels into CTS heade and to slightly modify the actual IEEE DCF. Futhemoe, we tested the effectiveness of the the poposed potocol by developing an analytical model which is able to give some pefomance indices in tems of genealized netwok capacity. Unde the discussed assumptions, the incease of pefomance with espect to the basic IEEE eaches a maximum gain of 30%. Some peliminay ideas ae given on how to futhe impove IA-MAC by including infomation even in RTS packets and by taking into account the tansmission duations of the packets. To test the effectiveness of the potocol and the consistency of ou analytical wok, IA-MAC will be implemented in QualNet Simulato [19]. VII. ACKNOWLEDGEMENTS This wok has been patially funded by the UC Coe pogam Coe unde the sponsoship of ST Micoelectonics. Coesponding Autho: D. Maniezzo UCLA Depatment of Compute Science, H 3731, 420 Westwood Plaza, Los Angeles, CA 90024, USA, dmaniezzo@ieee.og. REFERENCES [1] [2] IEEE Standad , Wieless LAN media access contol (MAC) and physical laye (PHY) specifications, [3] D. Maniezzo, M. Cesana, M. Gela, IA-MAC: Intefeence Awae MAC fo WLANs, UCLA Compute Science Depatment Technical Repot [4] K.K. Leung,. McNai, L.J. Cimini, J.H. Wintes, Outdoo IEEE cellula netwoks: MAC potocol design and pefomance, ICC 2002, New Yok, NY, USA, Apil [5] P. Gupta, P.R. Kuma, The capacity of wieless netwoks, IEEE Tansactions on Infomation Theoy, Volume 46 Issue 2, Mach [6] Haitao Wu, Shiduan Cheng, Yong Peng, Keping Long, Jian Ma, Does the IEEE MAC potocol wok well in multihop wieless ad hoc netwoks?, IEEE Communication Magazine, Volume 39, Issue 6, June [7] Haitao Wu, Shiduan Cheng, Yong Peng, Keping Long, Jian Ma, IEEE distibuted coodination function (DCF): analysis and enhancement, ICC 2002, New Yok, USA, Apil [8] C.C. Chow, V.C.M. Leung, Pefomance of IEEE medium access contol potocol ove a wieless local aea netwok with distibuted adio bidges, WCNC 1999, New Oleans, LA, USA, Septembe [9] J. Deng and Z. Haas, Dual busy tone multiple access (DTMA)- a multiple access contol scheme fo ad hoc netwoks, IEEE Tansactions on Communications, Volume 50, Issue 6, June [10] V. haghavan, MACAW: a Media Access Potocol fo Wieless LAN s, SIGCOMM 1994, London, UK, Septembe, [11] F. Talucci, M. Gela, L. Fatta, MACA-I (MACA y Invitation)-a eceive oiented access potocol fo wieless multihop netwoks, PIMRC 1997, Oulu, Finland, Septembe [12] A. Velayutham, H. Wang, Solution to the Exposed Node Poblem of in Wieless Ad-Hoc Netwoks, vel/eseach/e-mac.pdf. [13] S.-L. Wu, Y.-C. Tseng, C.-Y. Lin and J.-P. Sheu, A Multi-Channel MAC Potocol with Powe Contol fo Multi-Hop Mobile Ad Hoc Netwoks, The Compute Jounal, vol. 45, [14] J.So, N. Vaidya, A Multi-channel MAC Potocol fo Ad Hoc Wieless Netwoks, Dept. of Electical and Compute Engeneeing, Univesity of Illinois, USA, Technical Repot, Januay [15] T. Rappapot, Wieless Communications: Pinciples and Pactice, Pentice Hall, New Jesey, [16] J.P. Monks, V. haghavan, W.-M.W. Hwu, A powe contolled multiple access potocol fo wieless packet netwoks, INFOCOM 2001, Anchoage, Alaska, Apil [17] D. Hong and S. Rappapot, Taffic models and pefomance analysis fo cellula mobile adio telephone systems with pioitized and nonpioitized handoff pocedues, IEEE Tansactions on Vehicula Technology, Vol. 35, no. 3, August [18] A.. McDonald and T. F. Znati, A Path Availability Model fo Wieless Ad-Hoc Netwoks, WCNC 1999, New Oleans, LA, USA, Septembe [19]