A MAC layer proocol for wireless neworks wih asymmeric links Guoqiang Wang, Damla Turgu, Ladislau Bölöni, Yongchang Ji, Dan C. Marinescu School of Elecrical Engineering and Compuer Science, Universiy of Cenral Florida Orlando, FL 32816-2450 Absrac We inroduce AsyMAC, a MAC layer proocol for wireless neworks wih asymmeric links and sudy a proocol sack consising of AsyMAC and he A 4 LP rouing proocol. The wo proocols are able o mainain conneciviy where he sandard IEEE 802.11 MAC proocol coupled wih eiher AODV or OLSR rouing proocols may loose conneciviy. A comparaive sudy shows ha AsyMAC improves on wo previously proposed proocols accuracy in deermining he nodes o be silenced o preven collisions. Key words: Asymmeric link, AsyMAC, heerogeneous MANET 1 Inroducion Asymmeric links are presen in wireless neworks for a variey of physical, logical, operaional, and legal reasons: (a) The ransmission range is limied by he node hardware. The hardware properies of he node (for insance, he anenna or he RF circuis) deermine he maximum ransmission range. The differen ransmission ranges of he nodes lead o asymmeric links, which canno be avoided excep by physically changing he nodes hardware componens, for insance by insalling a differen anenna. (b) Power limiaion. Differen nodes may have differen power consrains. For insance, node A may have sufficien power reserves and a ransmission range enabling i o reach node B; however, node B has limied power, and eiher (i) canno reach node A, or (ii) may choose no o reach node A o save Preprin submied o Elsevier Science 13 April 2007
power. The wo scenarios influence he design of he proocols in differen ways. In he second scenario, node B is capable o reach node A and we could exploi his capabiliy for shor ransmissions when necessary, e.g., during a nework seup phase. (c) Inerference. Node A can reach node B and node B can reach node A, bu if node B would ransmi a a power level sufficien o reach node A, i would inerfere wih node C who migh be a licensed user of he specrum. This scenario is criical for ransmiers which aemp o opporunisically exploi unused pars of he licensed specrum (such as unused elevision channels). Even if operaing in he unlicensed bands, dynamic specrum managemen arrangemens migh have given he prioriy o node C, hus node B needs o refrain from sending a a power level above a given hreshold. (d) Sealh consideraions. Node A and node B aemp o communicae and hey wish o hide he exisence or he exac locaion of node B from node O. One way o achieve his is o resric he ransmission power of node B o he minimum and/or ransmi on frequencies which make locaion deecion more difficul. This is especially imporan in miliary/balefield applicaions where low probabiliy of deecion (LPD) is an imporan consideraion [20, 21]. (e) Dynamic specrum managemen. In he emerging field of sofware defined radios, he nodes can ransmi virually in any band across he specrum, bu hey need o share he specrum wih devices belonging o licensed operaors as well as devices wih limied flexibiliy. Once any of he reasons discussed previously force a link o be unidirecional addiional consrains, e.g., he need for a reverse pah beween some pairs of nodes may cause oher links o change heir saus and operae in a unidirecional mode, even when here is no oher reason for unidirecionaliy. Inabiliy of some MAC proocols o exploi he asymmery of some of he communicaion channels could lead o an inefficien bandwidh uilizaion, or, in he wors case, o inabiliy o connec some of he nodes. To exploi he asymmeric links, he proocols mus be able o deliver he acknowledgemens back o he sender in a direcion opposie o he direcion of he asymmeric link. Furhermore, he problem of hidden nodes appears more ofen and in more complex forms han in he case of symmeric links. Depending wheher he rouing proocol of he wireless ad hoc nework is able o handle asymmeric links, he MAC proocol migh need o hide he exisence of asymmeric links wih a symmeric overlay. The challenge for a MAC layer proocol able o exploi asymmeric links is o solve he hard problems menioned above, while keeping he cos incurred lower han he benefis obained from he uilizaion of he asymmeric links. MAC proocols for asymmeric links were previously proposed by Poojary e 2
al. [15], Fujii e al. [6] and ohers. In his paper, we inroduce a new proocol, AsyMAC (asymmeric MAC) ha uses a geomeric analysis of he hidden node problem in he presence of asymmeric links for a more precise deerminaion of he nodes which need o be silenced during a ransmission. Informally, a hidden node is one ha can inerfere wih he recepion of a daa packe wihou he knowledge of he sender. As a noe, here is a difference beween he concep of a proocol, as he collecion of feaures necessary o implemen neworking a a cerain layer, and algorihm, which refers o he implemenaion of a specific funcionaliy. In his paper, when we refer o a proocol, we concenrae on he subse of funcionaliy necessary o implemen he asymmeric links, hus he erms algorihm and proocol will be used inerchangeably. The paper is organized as follows. Relaed work is presened in Secion 2. Secion 3 presens AsyMAC proocol in every aspecs. Secion 4 describes he simulaion environmen and presens he resuls of he simulaion sudy of he effec of nework load, nework mobiliy and number of nodes. We conclude in Secion 5. 2 Relaed Work MAC layer proocols allow a group of users o share a communicaion medium in a fair, sable, and efficien way. A MAC layer proocol for wireless ad hoc neworks mus address several specific problems: (1) Mobiliy - he connecion beween nodes can become unsable because of he independen movemen of he nodes; (2) Higher error raes - a wireless channel has a higher Bi Error Rae (BER) han a wired nework; (3) Inabiliy o deec collisions during some periods of ime - wireless ransceivers work in a half-duplex mode; nodes do no lisen when alk and do no alk when lisen. The sender is unable o deec he collision and he receiver is unable o noify he sender of he collision during he ransmission of a packe. Collision avoidance is almos mandaory. Carrier Sensing Muliple Access (CSMA) [10], requires every node o sense he channel before ransmiing, and if he channel is busy, refrain from ransmiing a packe. CSMA reduces he possibiliy of collisions in he viciniy of he sender. Muliple Access Collision Avoidance (MACA) [9] and is varian MACAW [2] are alernaive medium access conrol schemes for wireless ad hoc neworks ha aim o solve he hidden node problem by reducing he possibiliy of collisions in he viciniy of he receiver. 3
The Floor Acquisiion Muliple Access (FAMA) [7] proocol consiss of boh carrier sensing and a collision avoidance handshake beween sender and receiver of a packe. Once he conrol of he channel is assigned o one node, all oher nodes in he nework should become silen. Carrier Sensing Muliple Access based on Collision Avoidance (CSMA/CA), he combinaion of CSMA and MACA, is considered a varian of FAMA proocols. The IEEE 802.11 sandard [8] is he bes-known insance of CSMA/CA. In a wireless nework wih symmeric links, a hidden node is one ou of range of he sender, bu in he range of he receiver. The soluion provided by he 802.11 MAC o he hidden node problem is he RTS/CTS handshake mechanism. [11] analyzes he effeciveness of RTS/CTS handshake mechanism, and indicaes ha some of he hidden nodes may no be covered by he receiver due o he fac ha i requires much lower power o inerrup a packe recepion han o successfully deliver a packe. RTS DATA CTS ACK CTS s r k RTS DATA CTS XCTS CTS s r k ACK j (a) (b) Fig. 1. (a) Hidden node problem in a classical wireless nework wih mobile nodes. All links are assumed o be bidirecional. A hidden node is a node ou of he range of he source and in he range of he receiver node. k is a hidden node for a ransmission from node s o node r. (b) Hidden node problem in a heerogeneous wireless nework wih mobile nodes. A hidden node is a node ou of he range of he sender and whose range covers he receiver. k is a hidden node for ransmission from node s o node r. We can define a hidden node in wireless ad hoc neworks wih asymmeric links as a node ou of he range of he sender and whose range covers he receiver (See Figure 1(b)). Thus, a hidden node is hidden from he sender and possibly from he receiver as well. The RTS/CTS handshake mechanism is no a soluion for such neworks since a CTS packe may no be able o reach hidden nodes. Several soluions o he hidden node problem in wireless ad hoc neworks wih asymmeric links are discussed in he lieraure. Poojary e al. [15] propose ha a node rebroadcass a CTS packe if i is received from a low-power node. To decrease he probabiliy of collisions, each node wais a random number (1... 6) of SIFS (Shor Iner-Frame Spacing) periods before ransmiing a CTS packe. Fujii e al. [6] made several improvemens relaive o [15]: (i) no only 4
CTS bu also RTS packes are rebroadcased; (ii) nodes wih a CTS packe o rebroadcas, firs sense he channel and ransmi only if he channel is no busy; and (iii) only high-power nodes rebroadcas RTS or CTS packes. The soluions proposed by [15] and [6] can lead o inefficien use of he channel if nodes are misclassified as hidden nodes. In such siuaions, nodes ha could have been acive are silenced due o misclassificaion, severely degrading he channel uilizaion. [15] and [6] rouinely assume rouing over symmeric links so ha he sender is able o receive boh CTS and ACK packes. In he presence of asymmeric links, however, he sender migh no receive he CTS or ACK packes, hus he sender canno rigger he ransmission of DATA packes, and does no know wheher a ransmission was successful or no. Bao e al. [1] propose a collision-free dynamic channel access scheduling algorihm PANAMA. Two scheduling algorihms are proposed for neworks wih unidirecional links, NAMA-UN ha is node acivaion oriened and suppors broadcas raffic efficienly, and PAMA-UN ha is link acivaion oriened and is more suiable for relaying unicas raffic. The channel access is allocaed for NAMA-UN and PAMA-UN alernaively, wih each scheduling algorihm lasing for a fixed amoun of ime. In PANAMA, he sender node is able o deec he hidden node ha also aemps o relay raffic o he receiver. The winner of a conenion is he node wih higher prioriy. When he link from he hidden node o he receiver is unidirecional, he hidden node may no be aware of he sender. In hese cases, he hidden node is auomaically considered as having a higher prioriy. The proocols considered previously are based on he modificaion of he MAC proocol. In conras, he Sub Rouing Layer (SRL) projec [17, 16] handles asymmeric neworks by adding an inermediary layer beween he MAC and nework layers. This layer parially isolaes he rouing proocol from he MAC layer, alhough i sill allows he rouing proocol o direcly conac he MAC layer. For unidirecional links, reverse pahs are compued using he Reverse Disribued Bellman-Ford algorihm. The SRL implemenaion also signals he deecion of new neighbors and he loss of (unidirecional) links. A MAC layer proocol able o uilize asymmeric links should be sacked ogeher wih rouing proocols ha can uilize asymmeric links as well. A 4 LP [19] is a locaion-aware and power-aware rouing proocol designed for ad hoc neworks wih asymmeric links. In A 4 LP, neighbors are re-classified as In-bound, Ou-bound, and In/Ou-bound neighbors due o he asymmery of links. A 4 LP is composed by a neighbor discovery proocol, a pah discovery proocol, and a pah mainenance mechanism. A 4 LP proposes an advanced flooding echnique - m-limied forwarding. Receivers can re-broadcas a packe only if is finess value exceeds a predefined hreshold, specified by he sender. The finess funcion used by m-limied forwarding can be uned o minimize he power consumpion, maximize he sabiliy of he roues, minimize he 5
error raes or he number of reransmissions. By avoiding a full broadcas, m- limied forwarding reduces he cos of pah discovery. A 4 LP is a hybrid ad hoc rouing proocol, combining feaures of boh pro-acive and on-demand proocols. The roues o In-, Ou-, and In/Ou-bound neighbors are mainained by periodic neighbor updae and immediaely available upon reques, while he roues o oher nodes in he nework are obained by a pah discovery proocol. In he following secions, we inroduce a new MAC layer proocol for ad hoc neworks wih asymmeric links (AsyMAC). AsyMAC currenly works wih A 4 LP, since hey share he process of neighbor discovery and neighbor mainenance. 3 The asymmeric MAC (AsyMAC) proocol 3.1 Topological consideraions The handling of he hidden nodes is an essenial problem for wireless MAC proocols operaing in he presence of asymmeric links. We inroduce opological conceps necessary o define a hidden node of a nework wih asymmeric links. The connecion beween wo nodes is described by he Boolean reachabiliy funcion R(i, j, ) which can be inerpreed as follows: a node i can send a packe o node j a ime if and only if R(i, j, ) = rue. A link beween wo nodes is symmeric if R(i, j, ) = R(j, i, ) = rue. Noe, ha he reachabiliy is a ime varying funcion; he connecion can be affeced by various channel condiions, fading, he mobiliy of he node or he mobiliy of he obsacles in he field. We assume ha every node is aware of he curren values of he reachabiliy funcion beween iself and he neighboring nodes (in boh direcion). In he A 4 LP/AsyMAC proocol sack, i is he role of he neighbor discovery proocol of A 4 LP o find hese values and keep hem up-o-dae. Alhough neighbor discovery is a common feaure of ad hoc rouing proocols, mos proocols will no deec oubound neighbors, because he confirmaion message will no reach back o he originaing node. Asymmeric rouing proocols, such as A 4 LP have a provision o roue back he confirmaion messages even in he absence of a direc link, hus allowing he discovery of he full asymmeric reachabiliy marix. In he following definiions we omi he ime parameer even hough all he ses are variable in ime, a fac which needs o be considered by he proocols relying on hem. 6
We define a series of opological conceps relaed o communicaion in he presence of asymmeric links and illusrae for he simple scenario in Figure 2; a sender node s sends a packe o he receiver node r in he viciniy of nodes 1..9. The circles cenered a s and r show he ransmission ranges of he sender and he receiver, respecively. The reachabiliy informaion of oher nodes is shown by direced lines; o avoid cluering he figure we do no include he links no relevan o he scenario. In his simple scenario we assume ha he asymmeric links are caused by he nodes having differen ransmission ranges and he ransmission range is a disk; his is no necessarily rue in real life scenarios, and our definiions do no assume a uni disk model. 9 5 4 s 1 r 2 3 8 7 6 Fig. 2. An illusraion for opology conceps. The ransmission ranges of he sender s and he receiver r are refleced by he circles cenered a hem. The parial reachabiliy informaion of oher nodes is shown by direced lines. Definiion 1 A se of m nodes i 1, i 2,... i m N are in an m-pary proxy se if each node can reach he oher m 1 nodes eiher direcly or hrough a subse of he oher m 2 members. For insance, in he scenario in Figure 2 he hree pary proxy ses are {r, 1, 6}, {r, 2, 6}, {r, 2, 7}, {r, 3, 7}, {r, 3, 8}, {r, 4, 8}, and {r, 4, 9}. Definiion 2 Call he viciniy of node i, V i he se of all nodes ha could be reached from node i. V i = {j R(i, j)} (1) In our scenario, he viciniy of he receiver node r is V r = {1, 2, 3, 4, 5}. Definiion 3 Call H sr he se of hidden nodes of a ransmission T sr. H sr includes nodes ha are no reachable from he sender, bu from which he receiver is reachable: H sr = {k R(s, k) R(k, r)} (2) 7
Noe ha H sr are he hidden nodes for he ransmission of he DATA packes, while H rs are he hidden nodes for he ransmission of ACK packes. In our scenario, he hidden nodes of he ransmission from source node s o receiver node r are H sr = {2, 3, 4, 6, 7, 8, 9}. Definiion 4 Call P 3 i he hree-pary proxy se coverage of node i. P 3 i is he se of nodes which are eiher reachable by node i direcly or paricipae in a hree-pary proxy se wih node i and a hird node. P 3 i = {k R(i, k) j (R(i, j) R(j, k) R(k, i))} (3) In he scenario of Figure 2 he hree-pary proxy se coverage of node r is P 3 r = {1, 2, 3, 4, 5, 6, 7, 8, 9}. Definiion 5 Call H3 sr he hidden nodes of a ransmission T sr in he hreepary proxy se coverage of node r. The se H3 sr includes hidden nodes covered by P 3 r. H3 sr = H sr P 3 r (4) In our scenario, he hidden nodes in he hree-pary proxy se coverage of r are H3 sr = {2, 3, 4, 6, 7, 8, 9}. Definiion 6 Call XH3 sr he exended hidden nodes of a ransmission T sr in hree-pary proxy se coverage of node r. The se XH3 sr includes nodes in H3 sr no covered by V r. XH3 sr = H3 sr V r (5) In he scenario of Figure 2, he exended hidden nodes of he ransmission from source node s o receiver node r are XH3 sr = {6, 7, 8, 9}. Definiion 7 Call XHR3 sr he exended hidden nodes relay se of a ransmission T sr in hree-pary proxy se coverage of node r. XHR3 sr includes all nodes in P 3 r ha could relay raffic from node r o nodes belonging o XH3 sr. XHR3 sr = {j j V r k XH3sr (R(j, k))} (6) The exended hidden nodes relay se of he ransmission from s o r on he example scenario is XHR3 sr = {1, 2, 3, 4}. Definiion 8 Call mxhr3 sr a minimal exended hidden nodes relay se of a ransmission T sr in hree-pary proxy se coverage of node r. mxhr3 sr includes a subse of nodes from XHR3 r (mxhr3 r XHR3 r ) such ha (i) he node r can relay raffic o any node in XH3 sr hrough some nodes from 8
mxhr3 sr and (ii) he removal of any node from mxhr3 sr makes some nodes in XH3 sr unreacheable from r. k XH3sr j mxhr3sr (R(j, k)) (7) and j mxhr3sr k XH3sr j mxhr3sr {j }(R(j, k)) (8) Noe ha mxhr3 sr may no be unique, and differen minimal exended hidden nodes relay ses could conain a differen number of nodes. Call {mxhr3 sr } he se ha conains all possible ses of mxhr3 sr. For insance, in our scenario here are wo possible minimal exended hidden nodes relay ses: mxhr3 sr = {2, 4} and mxhr3 sr = {1, 3, 4}. Also noe ha he wo ses have a differen number of nodes. Definiion 9 Call MXHR3 sr he minimum exended hidden nodes relay se of a ransmission T sr in hree-pary proxy se coverage of node r. MXHR3 sr is he insance of mxhr3 sr wih he smalles number of nodes. Call {MXHR3 sr } he se ha conains all possible ses of MXHR3 sr. In our scenario, we need o simply pick he smalles of he possible mxhr3 sr ses, which in our case will be MXHR3 sr = {2, 4}. We noe ha all he definiions provided above are consrucive, providing heir own implemenaion mehodology. Every se is defined based on he cascade of definiions preceding i, and all of hem can be reduced o he reachabiliy marix R(i, j). 3.2 Deerminaion of he ses in AsyMAC The ses V r and P 3 r of node r are he direc resuls of he neighbor discovery proocol of A 4 LP. Based on which, we can deermine he ses in AsyMAC. (1) H sr includes all he hidden nodes of a ransmission T sr, which migh be ouside of he hree-pary proxy coverage of node r (P 3 r ), hus he complee se of nodes of H sr may no be found and is no mainained. (2) The members of H3 sr can be found by removing from he se P 3 r he nodes ha can be reached by he oher peer of he ransmission. Noe ha in A 4 LP/AsyMAC, he reachabiliy informaion of wo neighbors of a node can be calculaed based on heir locaions and ransmission ranges. (3) XH3 sr is obained by XH3 sr = H3 sr V r. 9
(4) XHR3 sr includes all nodes in V r ha can reach a node in se XH3 sr. (5) The calculaion of {mxhr3 sr } is described in Algorihm 1. Algorihm 1 Calculaion of {mxhr3 sr } 1: {mxhr3 sr } = Φ; 2: Lis he complee permuaion of XHR3 sr, call i P. 3: 4: /* Find mxhr3 sr for each permuaion P i P. */ 5: for all permuaions P i P do 6: mxhr3 sr = Φ; 7: T = XH3 sr ; 8: found = false; 9: while P i Φ found = false do 10: remove he nex node p from P i, P i = P i {p}; 11: mxhr3 sr = mxhr3 sr {p}; 12: for all nodes T do 13: if R(p, ) hen 14: T = T {}; 15: end if 16: if T = Φ hen 17: found = rue; 18: break; 19: end if 20: end for 21: end while 22: add mxhr3 sr o {mxhr3 sr }; 23: end for 24: 25: /* Remove all ses M from {mxhr3 sr } if here exiss M {mxhr3 sr } such ha M M. */ 26: for all M {mxhr3 sr } do 27: for all M {mxhr3 sr } do 28: if M M hen 29: remove M from {mxhr3 sr }; 30: break; 31: end if 32: end for 33: end for 34: 35: reurn {mxhr3 sr }; (6) {MXHR3 sr } includes he se(s) in {mxhr3 sr } wih he smalles cardinaliy. During he process of consrucing {MXHR3 sr }, we can ignore he minimal exended hidden nodes se whose cardinaliy already exceeds he 10
achieved minimum value, which becomes our incenive o improve he algorihm. The calculaion of {MXHR3 sr } is described in Algorihm 2. Algorihm 2 Calculaion of {MXHR3 sr } 1: {MXHR3 sr } = Φ; 2: Lis he complee permuaion of XHR3 sr, call i P. 3: min cardinaliy = MAX; 4: 5: /* Find mxhr3 sr for each permuaion P i P. */ 6: for all permuaions P i P do 7: mxhr3 sr = Φ; 8: T = XH3 sr ; 9: found = false; 10: while P i Φ found = false mxhr3 sr < min cardinaliy do 11: 12: remove he nex node p from P i, P i = P i {p}; mxhr3 sr = mxhr3 sr {p}; 13: for all nodes T do 14: if R(p, ) hen 15: T = T {}; 16: end if 17: if T = Φ hen 18: found = rue; 19: break; 20: end if 21: end for 22: end while 23: 24: /* if mxhr3 sr is less han he curren achieved minimum cardinaliy, updae min cardinaliy and remove all elemens from {MXHR3 sr }. 25: if found = rue hen 26: if mxhr3 sr < min cardinaliy hen 27: min cardinaliy = mxhr3 sr ; 28: {MXHR3 sr } = Φ; 29: end if 30: add mxhr3 sr o {MXHR3 sr }; 31: end if 32: end for 33: 34: reurn {MXHR3 sr }; 11
3.3 Accuracy merics for node classificaion We inroduce a se of merics characerizing he abiliy of a MAC proocol o silence nodes which could cause collisions. Ideally, an algorihm should silence all nodes ha have he poenial o be hidden nodes, as well as nodes ha could poenially be affeced by he ransmission T sr. Assume here exiss an algorihm I which consrucs he se of all he nodes ha should be silenced during a ransmission T sr : S sr (I) = H sr H rs V s V r (9) In pracice, he se of nodes silenced by an algorihm F, S sr (F), migh conain nodes ha are silenced unnecessarily (misclassified) and migh lack nodes which should have been silenced (missed nodes). Call Misc sr (F) he misclassificaion raio of an algorihm F for a ransmission T sr. Misc sr (F) measures he raio of nodes ha are incorrecly silenced by F. Misc sr (F) = S sr(f) S sr (I) S sr (I) (10) Call Miss sr (F) he miss raio of an algorihm F for a ransmission T sr. Miss sr (F) measures he raio of nodes which are no silenced by he algorihm F, alhough hey should have been. Miss sr (F) = S sr(i) S sr (F) S sr (I) (11) Le M isc(f) and M iss(f) be he average misclassificaion raio and average miss raio of an algorihm F, respecively. The averages are compued over a nework N. Misc(F) = s,r N R(s,r) S sr (F) S sr (I), (12) s,r N R(s,r) S sr (I) and Miss(F) = s,r N R(s,r) S sr (I) S sr (F) s,r N R(s,r) S sr (I) (13) 3.4 A soluion o he hidden node problem In a wireless ad hoc nework wih asymmeric links, he sender may no be able o receive he CTS or ACK packes from he receiver. In such a case a 12
DATA packe, or he nex frame canno be sen. The IEEE 802.11 proocol assumes ha all he connecions are symmeric. Our proocol relaxes his assumpion, asymmeric links can be used provided ha hey are par of a hree-pary proxy se [19]. Our proocol reains he use of RTS, CTS, DATA and ACK frames defined in IEEE 802.11 sandard. In addiion, we inroduce four new frames: XRTS (Exended RTS), XCTS (Exended CTS), TCTS (Tunneled CTS), and TACK (Tunneled ACK). An ideal MAC layer proocol should be based upon a scheme ha delivers he RTS and CTS packes o all hidden nodes in H rs and H sr, respecively. However, such a scheme can be impracical because (i) a node may no have knowledge of all is In-bound neighbors; (ii) he number of hops needed o reach an In-bound neighbor migh be large, hus he ime penaly and he power consumpion required for he RTS/CTS diffusion migh ouweigh he benefis of a reduced probabiliy of collision. Our soluion is o send RTS and CTS packes o he nodes in H3 rs and H3 sr respecively. In his way, a considerable number of nodes ha are misclassified as hidden nodes by [15], referred o as proocol A, and [6], referred o as proocol B, are allowed o ransmi. Noe ha our approach does no idenify all hidden nodes, bu neiher mehods A or B are able o idenify all hidden nodes. 3.5 Node Saus In IEEE 802.11, when a node overhears a RTS or a CTS packe, i becomes silen and canno send any packe unil is NAV expires. This way, nodes in he relay se canno send XRTS/XCTS as hey should be in a silen sae afer overhearing he RTS/CTS packe. To resolve his dilemma, we replace he silen sae wih a quasi silen sae, in which a node is allowed o send conrol packes, excep RTS and CTS. The medium access conrol model proposed in his paper classifies a node as eiher idle, acive, quasi silen, or silen. When a node is idle, i is able o send or receive any ype of packes. When a node is acive, i is eiher sending or receiving a packe. When a node is in he quasi silen sae, i can eiher receive packes or send any packe ype excep RTS, CTS, or DATA. When a node is in he silen sae, he node can receive packes bu canno send any packe. 13
k 1 XRTS j 1 RTS s TCTS TACK RTS DATA CTS ACK j r CTS j 2 XCTS k 2 Fig. 3. Rouing over asymmeric links in a heerogeneous wireless ad hoc nework. Node s is he sender, r is he receiver, he link from node s o r is asymmeric, and node j is he proxy node ha can relay raffic o s for r. Nodes k 1 and k 2 are hidden nodes for ransmissions T rs and T sr, respecively. Nodes j 1 and j 2 are he proxy nodes ha can relay raffic from s o k 1 and from r o k 2, respecively. 3.6 Medium Access Conrol Model The medium access conrol (MAC) model of our proocol is based upon an exended four-way handshake (Figure 3). For shor daa frames, here is no need o iniiae a RTS/CTS handshake (see Figures 4 (a) and (b)). For long daa frames, we recognize several phases (see Figures 4 (c) and (d)): (1) Sensing. The sender s senses he medium. If i does no deec any raffic for a DIFS period, he sender sars he conenion phase; oherwise, i backs off for a random ime before i senses again. (2) Conenion. The sender s generaes a random γ [0, conenion window] slo ime. The sender s sars a ransmission if i does no deec any raffic for γ ime. (3) RTS ransmission. The sender s sends a RTS packe o he receiver r. The RTS packe specifies he NAV(RTS), link ype of L sr and MXHR3 rs. The link ype field is used o deermine wheher symmeric or asymmeric medium access conrol model is used. (4) CTS ransmission. The receiver r checks wheher he link is symmeric or no. If link L sr is symmeric, node r sends a CTS packe back o node s; oherwise, node r sends a TCTS packe o node s. A TCTS packe specifies boh he proxy node and he receiver r. The proxy node forwards he TCTS packe o he original sender s afer receiving i. A CTS/TCTS packe can be sen only afer sensing a free SIFS period. Insead of MXHR3 sr, MXHR3 rs MXHR3 sr is specified in he CTS/TCTS packe so ha every exended hidden node relay is included 14
only once. Thus, he duraion of XCTS/XRTS diffusion phase can be reduced. (5) XRTS/XCTS diffusion. All nodes ha overhear a RTS/CTS/TCTS packe eners a quasi silen sae. Afer he CTS ransmission phase, all exended hidden node relays ha are eiher specified in RTS or CTS/TCTS sars conenion for broadcasing XRTS/XCTS o is neighbors. When a node capures he medium, all oher nodes back-off for a random number of (1...4) SIFS periods, and coninue he conenion unil he XRTS/XCTS diffusion phase finishes. An XRTS/XCTS diffusion phase lass for 6 SIFS periods, afer which all nodes excep he proxy node become silen. (6) Daa ransmission. When he XRTS/XCTS diffusion phase finishes, he sender s sars sending DATA packes o he receiver r afer sensing a free SIFS period. (7) Acknowledgemen. Once he receiver r successfully received he DATA packe from he sender s, i replies wih an ACK if link L sr is symmeric, or a TACK packe if link L sr is asymmeric. An ACK/TACK packe can be sen only afer sensing a free SIFS period. When he sender s receives an ACK/TACK packe, i sars conending he medium for he nex frame. Meanwhile, he NAVs ha are reserved for his ransmission should expire. A any momen, if a node overhears a packe conaining new NAV informaion, i compares i wih he currenly sored NAV, and reains he NAV which expires laer. 4 Simulaion and case sudy We have implemened he AsyMAC proocol in NS-2 [3, 18], an objec-oriened even-driven simulaor developed a he Lawrence Berkeley Naional Laboraory, wih he CMU wireless exensions [13]. As AsyMAC requires a rouing proocol able o handle asymmeric links, we paired i wih A 4 LP rouing proocol o form a complee ad hoc neworking sack. In our experimens, we compare he A 4 LP/AsyMAC pair agains he sandard IEEE 802.11 proocol coupled wih AODV [14], a widely used on-demand ad hoc rouing proocol and he more recen OLSR [5] proocol. The simulaion resuls reflec he performance of he pair of he corresponding MAC and rouing proocols raher han he performance of he MAC or rouing proocols alone. We had chosen his experimenal seup because i provides he mos informaive comparison of real scenarios. We canno run a rouing proocol which does no suppor asymmeric links on op of Asy- MAC. On he oher hand, A 4 LP can be run on op of MAC proocols which do no suppor asymmeric links. However, A 4 LP has a higher overhead han 15
Node s DIFS CW DATA SIFS Node s Node r DIFS CW DATA SIFS ACK SIFS Node r ACK Node j TACK (a) (b) Node s DIFS CW RTS SIFS SIFS DATA SIFS Node r CTS random SIFSs 6SIFS ACK Node j random SIFSs XCTS Node j1 XRTS random SIFSs Node j2 XCTS (c) Node s DIFS CW RTS SIFS SIFS DATA SIFS Node r CTS SIFS 6SIFS random SIFSs ACK SIFS Node j TCTS random SIFSs XCTS TACK Node j1 XRTS random SIFSs Node j2 XCTS (d) Fig. 4. The medium access conrol model for he MAC layer proocol inroduced in his paper for he scenario in Figure 3. (a) The medium access conrol model of proposed MAC proocol for shor daa frames over a symmeric link. (b) The medium access conrol model of proposed MAC proocol for shor daa frames over an asymmeric link. (c) The medium access conrol model of proposed MAC proocol for long daa frames over a symmeric link. (d) The medium access conrol model of proposed MAC proocol for long daa frames over an asymmeric link. rouing proocols which assume symmeric connecions, hus an A 4 LP/802.11 combinaion would always perform somewha worse han combinaion such as OLSR/802.11, because we can ake advanage of he exisence of asymmeric links only if hey are suppored hroughou he sack. Thus, he only reasonable choices are o use eiher all symmeric proocols or all asymmeric-link 16
aware ones in he full sack. A possible sudy would involve he comparison of beween asymmeric sacks, by subsiuing for AsyMAC he proocols described by [6] and [15]. In Subsecion 4.3, we have implemened he core decision algorihms of hese proocols for a comparison of he classificaion accuracy. However, here is no publicly available NS-2 implemenaion of hese proocols, and a fully funcional implemenaion of hese proocols is beyond he scope of his paper. Firs, we analyze he benefis of algorihms able o ake advanage of asymmeric links in he mainaining he conneciviy of a nework. Through he sudy of a specific scenario, we show ha a proocol sack composed by Asy- MAC and he A 4 LP rouing proocol is able o mainain conneciviy where he sandard IEEE 802.11 MAC proocol coupled wih AODV or OLSR loose conneciviy. Second, we perform a simulaion sudy in which we measure he performance of he A 4 LP/AsyMAC sack agains he AODV/802.11 and OLSR/802.11 sacks in a series of randomized mobile ad hoc nework scenarios wih realisic raffic source paerns. Finally, we compare AsyMAC agains wo previously proposed asymmeric MAC proocols in erms of he accuracy of he hidden node classificaion. 4.1 A conneciviy scenario In his secion, we briefly discuss an example illusraes he case when A 4 LP/AsyMAC uses asymmeric links o roue packes from each pair of nodes while boh AODV/802.11 and OLSR/802.11 fail o roue packes. The conneciviy scenario is given in Figure 5. The iniial posiion of nodes is depiced in he graph (a), which shows also he ransmission range and he disance beween he nodes. The graph (b) is a logical view of he above scenario. The nodes do no move during he simulaion. The forward and reverse roues are found and esablished by A 4 LP, and MAC layer acknowledgemens are assured by AsyMAC. For insance, node 5 is a proxy node ha forwards CTS and ACK packes for a unidirecional ransmission from node 1 o node 4 a MAC layer. In his scenario, he wo far-mos nodes 0 and 4 are exchanging packes. The packes are successfully delivered and acknowledged by A 4 LP/AsyMAC, while all packes are los by AODV/802.11 or OLSR/802.11 during he ransmission. 17
80 3 (40,120) 3 150 0 (0,60) (0,0) 72 120 110 72 2 (40,0) 80 95 100 80 1 90 4 50 40 (110,60) 72 95 5 (140,20) 55 (200,60) 0 2 1 5 4 (a) Physical nework (b) Logical nework Fig. 5. (a) The physical opology of he nework, where node 0 and 4 are exchanging packes. The numbers nex o he nodes indicae he posiion in he (x,y) forma and he ransmission range (underlined). The numbers on he links represen he disance beween he nodes. (b) The logical opology of a nework. 4.2 A sudy of alernaive proocol sacks in a mobile ad hoc nework The previous scenario provided an example when he A 4 LP/AsyMAC proocol mainained conneciviy, while he AODV/802.11 and OLSR/802.11 sacks did no. However, hese exreme cases migh be relaively rare. In he following, we compare hese proocol sacks in a series of simulaions involving an ad hoc nework wih mobile nodes in a more realisic seup. To describe he movemen of nodes in he sysem, we use he random waypoin model [4]. Each node randomly picks a desinaion on he map, moves o he desinaion a a consan speed, and hen pauses for cerain ime, he pause ime. Afer he pause ime, i coninues he movemen following he same paern. The nodes are classified ino four classes C1, C2, C3 and C4 wih differen ransmission ranges. The raffic paerns are generaed by consan bi rae (CBR) sources sending UDP packes. Each CBR source resides a one node and generaes packes for anoher node. Each CBR source is acive for a ime inerval called CBR duraion. Our simulaion allows a seup ime o allow nodes gaher cerain rouing informaion before generaing any raffic. Afer he seup ime, he simulaion ime is divided ino equal ime slices, called swiching inervals. During each swiching inerval, we generae CBR sources for differen pairs of senders and receivers. Table 1 illusraes he defaul seings and he range of he parameers for our simulaion experimens. To consruc 95% confidence inervals, each experimen was repeaed 20 imes for a pair of scenario and raffic paern, he wo elemens affecing he resuls of a performance sudy. This involves 200 individual runs for he each of he 3 sudies. The average simulaion ime for a single experimen was abou 3 hours, for a oal of 1800 hours of compuer ime. By observing he evoluion 18
Table 1 The defaul values and he range of he parameers for our simulaion sudies. Field Value Range simulaion area 500 500 (m 2 ) number of nodes 8(C1), 16(C2), 24(C3), 32(C4) 30-110 raio of nodes C1:C2:C3:C4 = 1:2:3:4 ransmission ranges 200(C1),150(C2), 100(C3),50(C4)(m) speed 1 (m/s) 1-10 (m/s) pause ime simulaion ime seup ime swiching inerval 15 (s) 300 (s) 20 (s) 10 (s) number of CBR sources 10 4-40 CBR packe size CBR sending rae CBR duraion 64 (byes) 512 (bps) 5 (s) of he average values and he calculaed confidence inervals afer 5, 10 and 20 repeiions, we noice ha a 20 repeiions he values reach quiescence, and fuure repeiions would provide only insignifican changes on he overall shape of he graphs. We are concerned wih he impac of node mobiliy, nework load, and nework densiy upon power consumpion, packe loss raio, and laency. For each randomly generaed scenario and raffic paerns, we run simulaion experimens covering AODV wih IEEE 802.11, OLSR wih IEEE 802.11, A 4 LP using 3-limied forwarding wih disance meric (A 4 LP-M3-F1/AsyMAC) wih AsyMAC, and A 4 LP using 3-limied forwarding wih he meric proposed in [19] (A 4 LP-M3-F2) wih AsyMAC. The influence of nework load The effec of he nework load upon he packe loss raio for wo sandard proocol sacks AODV/IEEE 802.11, OLSR/IEEE 802.11 and for A 4 LP-M3- F1/AsyMAC and A 4 LP-M3-F2/AsyMAC is summarized by he graphs in Figure 6. The raio of he packes los by AODV/802.11 is roughly wice he rae of he packes los by he oher proocols. The major reason is ha flooding, an inefficien broadcas soluion, is used in AODV/802.11 for finding a roue. Among he oher proocols, A 4 LP-M3-F2/AsyMAC performs he 19
60 55 50 AODV / IEEE 802.11 OLSR / IEEE 802.11 A4LP M3 F1 / AsyMAC A4LP M3 F2 / AsyMAC Packe Loss Raio (%) 45 40 35 30 25 20 15 10 1 2 3 4 5 6 7 8 9 10 Nework Load (Kbps) Fig. 6. Packe loss raio versus nework load. The raio of packes los by AODV/802.11 is roughly wice he raio of packes los by he oher proocols. Among he oher proocols, A 4 LP-M3-F2/AsyMAC performs he bes, followed by OLSR/802.11, which delivers more packes han A 4 LP-M3-F1/AsyMAC for similar scenarios and raffic paerns. bes, followed by OLSR/802.11, which delivers more packes han A 4 LP-M3- F1/AsyMAC for similar scenarios and raffic paerns. OLSR/802.11 is able o deliver packes only via symmeric links, hus packes are dropped if a leas one asymmeric link is on he criical pah; however, A 4 LP/AsyMAC is able o deliver hose packes. Our experimen also shows he meric we proposed in [19] (A 4 LP-M3-F2), a combined meric wih disance, power level and class informaion, provides beer performance han he disance only meric (A 4 LP-M3-F1) in heerogeneous mobile ad hoc neworks. In our sudy, he measured values have relaively large confidence inervals, and mos of hese confidence inervals overlap. This means ha we do no have 95% confidence ha for any paricular experimenal insance he given proocol will perform beer han he oher proocol. Indeed, if here are no (or very few) asymmeric links, he symmeric proocols will likely ouperform he asymmeric ones, due o he higher overhead of he asymmeric proocol. Unforunaely, he range of he measurable values for merics such as packe loss is very wide in some scenarios here migh be no packe loss, in oher ones, many of packes are los. This variabiliy is refleced in relaively large confidence inervals. We believe ha ofen when he average value of packe loss is lower for one of he proocols, he proocol will perform in average beer han he oher ones. The effec of he nework load upon he average laency for wo sandard proocol sacks AODV/IEEE 802.11, OLSR/IEEE 802.11 and for A 4 LP-M3- F1/AsyMAC and A 4 LP-M3-F2/AsyMAC is summarized by he graphs in Figure 7. The average laency of AODV/802.11 is much higher han ha of he 20
2000 AODV / IEEE 802.11 OLSR / IEEE 802.11 A4LP M3 F1 / AsyMAC A4LP M3 F2 / AsyMAC Average Laency (ms) 1500 1000 500 0 1 2 3 4 5 6 7 8 9 10 Nework Load (Kbps) Fig. 7. Average laency versus nework load. The average laency of AODV/802.11 is much higher han he oher proocols. Among he oher proocols, OLSR/802.11 has he shores laency. oher proocols. AODV is a reacive proocol which finds roues only when needed. A 4 LP is a hybrid proocol, roues o non-neighbors are sill discovered when needed, however, roues o cerain In-, Ou-, and In/Ou-bound neighbors are mainained proacively in a rouing able; his fac conribues o he reducion of he average packe delivery laency. OLSR/802.11 has he lowes average packe delivery laency, followed by A 4 LP-M3-F2/AsyMAC, and A 4 LP-M3-F1/AsyMAC. Noe, however, ha he average packe delivery laency is based only on he delivered packes. OLSR/802.11 drops more packes han A 4 LP-M3-F2/AsyMAC; hese are he packes which require a proocol able o deal wih asymmeric links. The packes ha could be delivered by A 4 LP-M3-F2/AsyMAC bu no by OLSR/802.11 generally have higher laency, and his could explain why he average packe delivery laency of A 4 LP-M3-F2/AsyMAC is higher han ha of OLSR/802.11. The influence of nework mobiliy The average packe loss raio versus node mobiliy is summarized in Figure 8. Wih he nework mobiliy increasing, he performances of A 4 LP- M3-F2/AsyMAC and OLSR/802.11 are degraded, while he performance of AODV/802.11 flucuaes beween 35% o 45%. AODV/802.11 performs he wors in case of ad hoc neworks wih low mobiliy, bu i ouperforms he oher proocols for highly mobile ad hoc neworks. The reason for his is ha for ad hoc neworks wih relaively high mobiliy, cached roues and neighbor informaion becomes sale rapidly, which degrades he performance of proacive (OLSR) or hybrid (A 4 LP) proocols bu no reacive (AODV) proo- 21
70 60 AODV / IEEE 802.11 OLSR / IEEE 802.11 A4LP M3 F1 / AsyMAC A4LP M3 F2 / AsyMAC Packe Loss Raio (%) 50 40 30 20 10 1 2 3 4 5 6 7 8 9 10 Node Mobiliy (m/s) Fig. 8. Packe loss raio versus node mobiliy. Wih he nework mobiliy increasing, he performances of A 4 LP-M3-F2 and OLSR/802.11 are degraded while he performance of AODV/802.11 flucuaes beween 35% and 45%. AODV/802.11 performs he wors in case of ad hoc neworks wih low mobiliy, bu i ouperforms he oher proocols for highly mobile ad hoc neworks. cols. However, A 4 LP-M3-F2/AsyMAC always ouperforms OLSR/802.11 and A 4 LP-M3-F1/AsyMAC a any nework mobiliy in erms of packe loss raio. 2500 2000 AODV / IEEE 802.11 OLSR / IEEE 802.11 A4LP M3 F1 / AsyMAC A4LP M3 F2 / AsyMAC Average Laency (ms) 1500 1000 500 0 1 2 3 4 5 6 7 8 9 10 Node Mobiliy (m/s) Fig. 9. Average laency versus node mobiliy. The average laency of AODV/802.11 is much higher han he oher proocols ha perform similarly. Figure 9 presens average packe delivery laency versus nework mobiliy. AODV, which is an on-demand proocol, shows abou he same, relaively long, laency irrespecive of he mobiliy of he nodes. For A 4 LP/AsyMAC and OLSR/802.11 he laency is increasing wih he mobiliy, as he proocols need addiional overhead o keep heir opology informaion up-o-dae. A he mobiliy of abou 10 m/s, AODV/802.11, A 4 LP-M3-F2/AsyMAC and A 4 LP- M3-F1/AsyMAC show abou he same laency. In hese ess, OLSR/802.11 22
ouperforms A 4 LP/AsyMAC because he amoun of opology daa i needs o mainain is lower, being resriced o he symmeric links only. This laency advanage comes a he cos of ignoring asymmeric links and herefore, poenially disconnecing nodes which would mainain conneciviy wih he A 4 LP/AsyMAC soluion. The influence of he number of nodes 90 80 70 AODV / IEEE 802.11 OLSR / IEEE 802.11 A4LP M3 F1 / AsyMAC A4LP M3 F2 / AsyMAC Packe Loss Raio (%) 60 50 40 30 20 10 30 40 50 60 70 80 90 100 110 Number of Nodes Fig. 10. Packe loss raio versus number of nodes. A 4 LP-M3-F2/AsyMAC delivers mos packes, followed by OLSR/802.11, A 4 LP-M3-F1/AsyMAC and AODV/802.11 for similar scenarios and raffic paerns. The packe loss raio decreases when he number of nodes increases. In he following se of experimens, we vary he number of nodes moving in he measuremen area. As he nodes have a limied range, when he number of nodes is oo low, some nodes migh loose conneciviy. Figure 10 illusraes he packe loss raio versus he number of nodes. For similar scenarios and raffic paerns, A 4 LP-M3-F2/AsyMAC delivers mos packes, followed by OLSR/802.11, A 4 LP-M3-F1/AsyMAC, and AODV/802.11. As he number of nodes in he nework increases, he nework conneciviy increases as well, hus he packe loss raio decreases. Figure 10 shows ha he packe loss raio decreases from roughly 40% o abou 10% as he number of nodes increases from 30 o 110. Figure 11 shows he average packe delivery laency versus he number of nodes. The average laency of AODV/802.11 is much higher han he oher proocols. For A 4 LP/AsyMAC and OLSR/802.11 he packe laency ends o decrease as he number of nodes increases. As he number of nodes in he nework increases, more neighbors and roues are found during he neighbor informaion exchange process, hus he packe delivery laency decreases. 23
2500 2000 AODV / IEEE 802.11 OLSR / IEEE 802.11 A4LP M3 F1 / AsyMAC A4LP M3 F2 / AsyMAC Average Laency (ms) 1500 1000 500 0 30 40 50 60 70 80 90 100 110 Number of Nodes Fig. 11. Average laency versus number of nodes. The average laency of AODV/802.11 is much higher han he oher proocols. The packe laency ends o decrease as he number of nodes increases for A 4 LP/AsyMAC and OLSR/802.11. 4.3 The accuracy of hidden node classificaion A node is misclassified as hidden if i is silenced by he algorihm while i should no be silenced. Misclassificaion reducing bandwidh uilizaion because i leads o unnecessary silencing of nodes which could have been ransmiing. A node is missed by he algorihm if i was no silenced alhough i should have been. Missed nodes lead o collisions. The more accurae is a proocol in classifying he nodes, he beer he bandwidh uilizaion. A useful measure of he global performance of an algorihm is he number of incorrec silencing decisions per ransmission - defined as he sum of misclassified and missed nodes. We compare he accuracy of he classificaion of our proposed AsyMAC proocol wih he accuracy of wo well known proocols which are performing he same classificaion [6, 15]. As a noe, he basic IEEE 802.11 proocol does no perform any classificaion of nodes. The simulaion environmen is an area of 500 500 meers. We populae our environmen wih a heerogeneous collecion of nodes belonging o he four main classes of wireless nodes C1, C2, C3, and C4 (see [12, 19]). The ransmission ranges are normally disribued random variables wih he mean 100, 75, 50, and 25 meers, respecively and he sandard deviaions for each class is 5 meers. The simulaion scenarios are creaed using a se of 40 o 120 nodes including an equal number of nodes for each class, uniformly disribued in he area. For each generaed scenario, we repea he experimen 1000 imes. The displacemen of nodes are disribued around an iniial posiion and he sandard deviaion is 20% of is ransmission range. 24
16 2.5 Misclassified nodes / ransmission 14 12 10 8 6 4 Proocol A Proocol B AsyMAC Missed nodes / ransmission 2 1.5 1 0.5 Proocol A Proocol B AsyMAC 2 0 40 50 60 70 80 90 100 110 120 Number of nodes 0 40 50 60 70 80 90 100 110 120 Number of nodes (a) Misclassified nodes (b) Missed nodes Incorrec silencing choices / ransmission 9 8 7 6 5 4 3 2 1 Proocol A Proocol B AsyMAC 0 40 50 60 70 80 90 100 110 120 Number of nodes (c) Incorrecly classified nodes Fig. 12. (a) The average misclassified nodes / ransmission as a funcion of he number of nodes. The AsyMAC proocol does no misclassify nodes in a saic nework. (b) The average missed nodes / ransmission for proocols A, B, and our approach, as a funcion of he number of nodes. (c) The average number of incorrec silencing decisions per ransmission for proocols A, B, and for our approach. The resuls of he simulaion are shown in Figure 12. The graph (a) shows he number of misclassified nodes per ransmission. The AsyMAC algorihm does no misclassify nodes in a saic nework, because in he process of hree-pary proxy se formaion, he nodes whose ransmission range does no reach he curren node are filered ou. However, misclassified nodes can appear wih he AsyMAC proocol if he nodes are highly mobile and he curren configuraion does no reflec he one deeced when he hree-pary proxy se was esablished. The graph (b) shows he missed nodes per ransmission. Here he AsyMAC proocol performs worse han he oher wo proocols considered, as i is considering only he hree-pary proxy ses, and ignores possible higher order proxy ses. However, he number of missed nodes is very small for all he hree proocols. Graph (c) shows he number of incorrec silencing decisions per ransmission. Here, he AsyMAC proocol emerges wih he lowes number of incorrec decisions, as is beer performance a misclassificaion 25
compensaes for he lower performance in regards o missed nodes. 5 Conclusions In his paper, we argue ha asymmery of he ransmission ranges in wireless neworks is a realiy and should be reaed as such. This asymmery makes reliable communicaion more difficul and complicaes medium access conrol, as well as nework layer proocols. The models of radiional muliple access neworks assumes ha all nodes share a single communicaion channel and have access o he feedback (success, idle slo, collision) from any ransmission. In his case, spliing algorihms allow sharing of he communicaion channel in a cooperaive environmen wih reasonable efficiency and fairness. This is no longer he case for wireless neworks wih asymmeric or unidirecional links, where he sender and he receiver do no share he feedback channel and hidden nodes may inerfere wih a ransmission. In case of neworks wih asymmeric links, hidden nodes may be ou of he reach of boh he sender and he receiver, bu heir ransmissions may inerfere wih he recepion of a packe by he inended desinaion. The problem of hidden nodes is furher complicaed because he feedback from he receiver in an RTS/CTS exchange may have o pass hrough several relay saions before reaching all he nodes expeced o be silen. Some of he soluions proposed in he lieraure reduce he probabiliy of a collision by requiring a larger han necessary se of nodes o be silen. In urn, his has negaive effecs upon he communicaion laency and he overall nework hroughpu. We propose a MAC layer proocol, AsyMAC, which reduces he number of nodes ha have o be silen bu, as all he oher schemes proposed, may miss some of he nodes which should have been classified as hidden. IEEE 802.11 assumes symmeric links beween each pair of nodes while Asy- MAC does no. For raffic over asymmeric links, AsyMAC relies on a proxy node in he hree-pary proxy se o relay acknowledgemens back o he sender so ha he reliabiliy is assured. Our MAC proocol reduces average packe loss raio and average packe delivery laency as asymmeric links are comprehensively uilized which dominae rouing in heerogeneous ad hoc neworks. We conduced a simulaion experimen using he NS-2 simulaor and compared he performance of AODV/IEEE 802.11, OLSR/IEEE 802.11, A 4 LP using 3- limied forwarding wih disance meric (A 4 LP-M3-F1) wih AsyMAC, and A 4 LP using 3-limied forwarding wih he meric proposed in [19] (A 4 LP-M3-26