Enabling Cooperation in Mobile Ad Hoc NetworksSantosh Kulkarni*, Pratap S. Prasadt and Prathima Agrawalt

*Computer Science and Software Engineering, Auburn University, Auburn, AL 36849.tElectrical and Computer Engineering, Auburn University, Auburn, AL 36849.Email: santosh@auburn.edu, prasaps@auburn.edu, pagrawal@eng.auburn.edu

Abstract-Spatial diversity achieved via nodal cooperation isknown to alleviate the ill effects of channel fading in wirelessnetworks. But to exploit this phenomenon the idea of node coop­eration needs to be extended to other layers of the protocol stack,especially the MAC layer. This paper takes a close look at SynergyMAC, a cooperative MAC protocol, proposed for wireless mobilead hoc networks. The studied protocol also leverages on themulti-rate capability of 802.11b to allow nodes with low SNRto their destination make use of intermediate relays, to transmitdata at rates higher than otherwise possible. The performanceimprovement achieved by this protocol in comparison to 802.11bis then evaluated through extensive simulations.

I. INTRODUCTION

Advances in wireless technologies have liberated end-usersfrom the geographical constraints of wired networks. Sincethe advent of IEEE's 802.11 standard [1], Mobile Ad hocNetworks (MANET) have gained widespread acceptance inproviding self-configuring wireless networks for mobile de­vices. Performance of such networks is however severelyaffected when radio waves experience fading. Although spatialdiversity is known to minimize the ill effects of fading, realiz­ing it generally requires incorporation of newer technologiessuch as Multiple Input Multiple Output (MIMO) systems. But,it is impractical to equip every node in a network with multipleantennae, primarily due to size and energy constraints.

Recent research on cooperative communication [2] [3] [4][5] [6] demonstrates that spatial diversity can also be achievedby exploiting some key characteristics of the wireless medium.Because of the broadcast nature of the medium, any signaltransmitted on the channel is overheard by all nodes withinrange. If such nodes were to retransmit the overheard signalto destination rather than discarding it, the destination wouldeffectively receive extra observations of the source signal,resulting in diversity. In short, cooperative system can be seenas a virtual antenna array, where each antenna in the arraycorresponds to an assisting neighbor [7]. As shown in [8],such nodal cooperation is able to significantly enhance theperformance of MANETs. However, to exploit the diversityrealized at the physical layer, the idea of node cooperationneeds to be extended to other layers of the protocol stack, es­pecially the MAC sub-layer. Further, if the cooperation awareMAC sub-layer is 802.11 b compatible, then even devices withlegacy hardware could potentially stand to gain from it.

But with 802.11 b, nodes in MANETs can experience fair­ness problems resulting from the protocol's multi-rate mod­ulation scheme. As shown in [9], if all nodes have uniformtraffic to/from a given node, lower data rate nodes use far more

channel time than higher data rate nodes. The lower data ratenodes also get poor service which reduces the bandwidth of thehigher data rate nodes [10], ultimately reducing the effectivethroughput of the entire network. In [11] it is shown that amulti-hop extension to 802.11b can alleviate this problem.Hence in this paper, the focus is not only on the design ofa new 802.11b compatible MAC for MANETs, but also onincorporating into it the multi-hop extension proposed in [11].

II. SYSTEM OVERVIEW

IEEE 802.11b supports transmission rates of 1Mbps(BPSK),2Mbps(QPSK), 5.5Mbps(CCK5.5) and 11Mbps(CCK11). Theprotocol provides access to a shared wireless medium pri­marily through a contention-based access mechanism, calledthe Distributed Coordination Function (DCF). DCF is basedon Carrier Sense Multiple Access with Collision Avoidance(CSMAlCA) under which a node with data to transmit, has tofirst sense the wireless medium to determine if it is free. It alsoemploys virtual carrier sensing by using frames like Requestto Send (RTS) and Clear to Send (CTS). These control framesset the Network Allocation Vector (NAV) using which nodes inthe network are able to avoid collisions from hidden terminals.If the data frame following the control frames is receivederror free, the destination node sends an acknowledgement(ACK) frame back to source. 802.11b modulates all controlframes and the header part of data frames using BPSK at1Mbps. The modulation scheme used for the payload partof the data frame is indicated in the PHY header of thetransmitted frame. Figure 1, obtained from [12], shows theBit Error Rate (BER) vs. Signal to Noise Ratio (SNR) fordifferent modulation schemes of 802.11 b. These are empiricalcurves provided by Intersil for their HFA3861B chip. Fromthe figure we can derive that given a BER one can findthe most suitable modulation scheme to use based on thereceived SNR. Because path loss determines SNR, we can alsoderive that higher transmission rates are possible only whencommunicating nodes are sufficiently close. If the nodes arefar apart, then utilizing an intermediate relay could result inan average rate that is greater than otherwise possible.

III. SYNERGY MAC PROTOCOL

The cooperative MAC proposed in this section is based on802.l1b's DCF mechanism. The main goal of this protocolis to mitigate the ill effects of fading by achieving spatialdiversity. The following are its assumptions.

• Transmission power for all nodes in the network is fixed.

B. The RTS Frame

When a source node N s wants to send L octets of data todestination Nd' it consults its Synergy Table and calculatesthe time needed to transmit all those octets using directtransmission. Following this, node N s begins to sense theshared channel for wireless activity. If the channel is foundto be idle for distributed inter-frame space (DIFS) time andN s has completed the required backoff procedure, an RTSframe is sent to the destination N d , reserving the channel forthe time needed for direct transmission. Figure 2(a) shows theexchange of control frames in Synergy MAC protocol.

According to [13], the More Fragments bit field in 802.11bframe header is set to 0 on all frames other than thosedata or management frames that have .another fragment oftheir current MAC Service Data Unit (MSDU) or MACManagement Protocol Data Unit (MMPDU) to follow. Thismeans that control frames in 802.11b are never fragmentedand consequently always have their More Fragments bit set toO. It is therefore feasible for Synergy MAC to use this bit todistinguish its control frames from those of standard 802.11b's.Apart from setting its More Fragments bit to 1, Synergy MACrequires no other change to the legacy RTS frame format.

C. Relay Identification

When an intermediate node N r , overhears an RTS transmis­sion between source N s and destination N d, it estimates thelength of the subsequent data frame based on the transmissionduration obtained from the Duration field of the overheardframe and the rate of data transmission Rsd , between nodes N sand N d obtained from its Synergy Table. Next, N r computesthe time required to transmit the same data frame over twohops with itself acting as the relay. If the data frame is L octetslong, the transmission time via N r ignoring overhead and thecontention time is 8L/ R sr +8L/ R rd where, R sr is the rate ofdata transmission between N s and Nr and Rrd is the rate ofdata transmission between N r and Nd. N r obtains both R srand Rrd from its Synergy Table. Such two hop transmissionvia Nr is efficient only if 8L/R sr + 8L/R rd < 8L/R sd . Ifthis is indeed the case, node Nr will indicate its availabilityfor cooperation by transmitting a self addressed CTS framewith Duration set to 8L / R sr + 8L/ Rrd' after at least shortinter-frame space (SIFS) time. Like with the overheard RTS,node N r sets the More Fragments bit to 1 in its CTS-to-selfframe header (CTSr). To resolve potential collisions betweenmany candidate relays, the CTSr from all eligible nodes aregoverned by a contention window. The contention window sizeused by candidate relays for transmitting their CTSr is smallwhen compared to that used by source nodes for transmittingtheir data frames. Moreover, candidate relays shall alwayschoose their random slot time within [1, CWr ] for transmittingtheir CTSr' The candidate that picks the lowest slot in thewindow wins while the remaining candidate relays update theirNAV based on the Duration contained in the winning CTSframe. In IBSS, nodes can choose CWr =CWmin .

Though the candidate nodes could have used any new frameformat to announce their availability, using CTSr to accom-

10.1IEEE 80211 b BER vs SNR

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• Channel between any two nodes in network is symmetric.• Threshold SNR for each modulation scheme is predefined

and stored in a physical mode table on every node.• Transmitting nodes choose data modulation scheme based

on their received SNR.• Control frames like RTS, CTS and ACK are overheard

by other nodes besides the transmitter and the receiver.

The following subsections present the underlying details.

Fig. 1. BER vs. SNR for different 802.11b PHY modes

A. The Synergy Table

After associating themselves into a basic service set (BSS)nodes in the network start listening for control and dataframes sent out by other nodes on the shared channel. This isrequired by 802.11b, as all nodes in the network need to updatetheir NAV. Additionally, Synergy MAC requires each node tomaintain a Synergy Table which helps determine the node'sability to volunteer as relay during cooperation. Each row inthis table has five fields. The first field of this table stores theID (MAC address) of the source followed by the Time thatthe last frame from that node was heard. The third field is usedto record the data rate that can be used between source andcurrent node and is denoted by R sr . The fourth field storesthe ID of the destination followed by Rsd which representsthe data rate used between the source and the destination. Thetable gets updated in the following manner:

When any transmission between other nodes is overheardby a node (Nr ), it checks if the transmitting node (Ns ) isalready in its Synergy Table. If not, a new row is added forthe sender and is identified by its ID. Then N r computes therelative channel condition between the sender and itself bymeasuring the received power level (in dB). Path loss can becalculated by subtracting this from the transmission power (indB), which is fixed for all nodes. By checking its physicalmode table, N r can find the data rate between N s and itselfand use this value to update the rate R sr for N s . If any dataframe between source N s and destination N d is overheard byN r , it detects the transmission rate used, by looking into thePHY header of the data frame, which is always transmitted at1Mbps. This value along with the destination's ID is used toupdate the Rsd field for N s . The Time field is updated everytime a frame from N s is overheard by N r .

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plish this task has its benefits. Not only is CTSr compatiblewith 802.11b as mentioned in [13], it also serves the purposeof reserving the medium for the duration of cooperation. Inaddition to this, a CTSr lets the source and destination knowthe identity of the assisting relay Nr .

D. The CTS Frame

After receiving the initial RTS frame from source, thedestination waits to overhear the CTSr transmitted by thewinning relay. If the CTSr from the relay N r is overheardby the destination Nd, it sends out a CTS frame (CTSd)to source N s after SIFS time, reserving the channel for thetime needed to complete a two hop transmission via N r . If aCTSr is not overheard within a period of CTSRelayTimeout,Nd still sends out a CTSd frame to source, but this timereserving the channel for the time needed to complete adirect transmission. In case of the former, Synergy MAC setsthe More Fragments bit in the CTSd frame header to 1,requesting the source to use relay assisted transmission forits subsequent data frame. As the latter case is similar to thatof legacy 802.11b, the More Fragments bit in the responseheader remains set to O. In situations where the destination isa legacy 802.11 device, a CTS response to a Synergy MAC'sRTS, is sent immediately after SIFS time. Contending relayswould overhear this response and update their NAV, as if thedestination had picked the lowest slot in CWr .

E. Cooperative Communication

Once node N s receives a CTSd frame from destination N d,it starts transmitting its data frame after SIFS time with theDuration field set to CTSd'S estimate duration. If CTSd'SMore Fragments bit was set to 1, N s sends the data frame toN r using rate R sr . Node N r then checks the CRe field of thereceived data frame and if correct, forwards the frame to Nd,using rate Rrd after SIFS time. If CTSd'S More Fragmentsbit was set to 0, N s sends the data frame directly to N d usingrate R sd It is possible that node Ns does not overhear a CTSrfrom the winning relay before receiving a CTSd from N dwith its More Fragments bit set to 1. This might occur dueto drastic change in channel condition between N s and N r

during control frame exchange. But because Nd had overhearda CTSr from relay, its Duration estimate in CTSd would be

(b) NAV update mechanism in Synergy MAC

far less than the Duration contained in the initial RTS frame.If this is the case, source N s resorts to fragmenting its data,based on CTSd'S Duration and direct transmission rate Rsdin order to maintain consistency of the NAV. After receivingthe data frame, destination N d responds back to N s with anACK frame indicating a successful reception. Otherwise N d

stays idle in which case N s notices the failure of transmissionafter a timeout period and starts backing off exponentially.

F. NAV Mechanism

According to [13], all nodes receiving a valid frame exceptthe one whose MAC address is equal to the RA (ReceiverAddress) mentioned in the frame header, are required toupdate their NAV with the information received in the frame'sDuration field. When compared to [13], Synergy MAC differsslightly in the way its NAV is calculated. The Duration carriedin a Synergy MAC RTS header is the time in microsecondsrequired to transmit the pending data frame using directtransmission from source N s to destination N d, plus oneCTS frame, one ACK frame, a relay timeout and three SIFSintervals as shown below.

DurationRTS == 3Ts1FS + CTSRelayTimeout + TCTS +8L/ Rsd + TACK

This ensures that even if there is no intermediate nodeto volunteer, the data frame can be sent to the destinationby direct transmission using rate R sd . The Duration field insubsequent CTSr will be set as follows,

DurationCTsr == 4Ts1FS +TCTS +8L/R sr +8L/R rd +TACK

The Duration in the CTS d frame header is calculated basedon whether or not the destination overheard a CTSr. If it didoverhear, the value of the Duration via N r is set as,

DurationCTsd == 3TsIFS + 8L/R sr + 8L/Rrd + TACKElse, the Duration in the CTS d frame header is set as,DurationCTSd == 2Ts1FS +8L/Rsd +TACKFigure 2(b) illustrates the NAV update mechanism in Syn­

ergy MAC. Nodes that can overhear both RTS and CTSdframes (e.g. N l ) need to set their NAV duration accordingto the RTS frame first. Once the CTSr or CTSd frameis overheard, they need to reset the NAV according to theDuration contained in the new frame. Hidden terminals thatcan only overhear Nd'S transmissions (e.g. N 2 ) need to update

(d) Gain vs Pkt Size

(b) Gain vs No. of Nodes

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Fig. 4. Simulation results: Comparison of IEEE 802.11b with Synergy MAC

is between 48m and 67m, 2Mbps if distance is between 67mand 74m and 1Mbps for all other distances within 100m.Nodes which are farther than 100m are considered to be outof communication range.

The first experimental scenario consists of nodes randomlyuniformly distributed in a 400m X 400m rectangular area. Allnodes are static, operate in ad hoc mode and update theirSynergy Table with information overheard from neighboringnodes. Each node transmits data to a randomly selected desti­nation located within 100m of itself. At each node, data framesof length 1000 octets arrive at a rate of 200 frames per secondto keep the network heavily loaded. The experiment sets theminimum contention window size (CWmin ) to 31 slots anduses a maximum of 6 back off stages during retransmission.

Figure 4(a) shows the average saturated throughput achievedby both 802.11b and Synergy MAC for the above scenario.The graph shows that Synergy MAC is able to achieve muchhigher throughput than 802.11b under similar conditions. Thisis because Synergy MAC not only combats fading throughspatial diversity but allows nodes with low SNR to destinationmake use of intermediate nodes to achieve higher rates ofdata transfer than otherwise possible. The graph also revealsthat the throughput for 802.11b decreases with increase inthe number of nodes on the network. This is mainly due toexcessive collisions occurring on the shared channel. In case

Coop Coop UID SynergyMAC I MAC II MAC MAC

Characteristic

IEEE 802. II b based V- v' v' v'

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with legacy 802.1 .1

Employs three-way v' X X v'h.mdshake

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COMPARISON OF DIFH:'RENT COOPERATIVE MAC PROTOCOLS

their NAV on overhearing a CTSd. Terminals that can onlyoverhear N s ' s transmissions set their NAV according to theinitial RTS frame and update it on the subsequent Data frame.

In summary, Synergy MAC implements multi-hop extensionproposed in [11] by allowing nodes with low SNR to theirdestination make use of intermediate relays, to transmit dataat rates higher than otherwise possible. The protocol alsoachieves spatial diversity by ensuring that the destinationoverhears multiple copies of the original frame (source-relayand relay-destination transmissions).

Fig. 3. Comparison of different Cooperative MAC protocols

IV. RELATED WORK

Cooperation in wireless networks is a relatively new areaof research. To the best of our knowledge, there has beenno extensive study on cooperative MACs for MANETs. How­ever, we are aware of other 802.11 b based cooperative MACprotocols that have been proposed for infrastructure LANs.This section contrasts Synergy MAC against such protocols.In UTD MAC [14], the data frame transmitted by source issimultaneously made available at both relay and destination. Itis only when the destination fails in its reception attempt thatthe relay intervenes to re-send the data frame after RIFS dura­tion. Because the Duration in RTS and CTS remains unaltered,the protocol can lead to inconsistency in NAV propagation,resulting in collisions. Though Coop MAC I [10] employssimilar techniques as Synergy MAC for infrastructure LANs, itrequires considerable changes in the frame formats of 802.11brendering it incompatible with legacy implementations. CoopMAC II [10] on the other hand does not require any changesto 802.11b frame format but because it employs only a 2-wayhandshake, it can lead to collisions at the relay node. Alsoboth Coop MAC I and II identify their relay nodes a priori atsource and are vulnerable to change in its availability causeddue to mobility. A list of differences between these cooperativeMAC schemes is listed in Figure 3.

V. SIMULATION RESULTS

Synergy MAC was implemented and tested using ns-2 simu­lator. All nodes in the experiments described below select theirmodulation scheme with BER ~ 10-5 . Based on path lossand Figure 1, this translates to transmission rates of 11 Mbps ifdistance between two nodes is within 48m, 5.5Mbps if distance

of Synergy MAC however, with more nodes in the network,there is a higher possibility for a node with low data rateto find an intermediate relay. This increased availability ofrelays not only offsets the degradation in performance causedby packet collisions but also leads to an increase in the overallthroughput achieved by Synergy MAC. The relative gain inthroughput of Synergy MAC expressed as percentage versusnumber of nodes in the network is shown in Figure 4(b).

The next experimental scenario investigates the effects ofMPDU length on Synergy MAC's performance. The scenarioretains most of the earlier experiment's setup but fixes thenumber of nodes in the rectangle to 40. Figure 4(c) givesthe throughput comparison for protocols 802.11b and SynergyMAC as the data frame length increases. Figure 4(d) showsthe corresponding relative gain in Synergy MAC's throughput.It is evident from these graphs that Synergy MAC performsbetter for larger data frames. In fact 802.11 b performs betterthan Synergy MAC for data frames that are less than 150 octetslong. This is because Synergy MAC has larger protocol over­head (mostly in form of CTSRelayTimeout) when comparedto 802.11b. Gains due to cooperation offsets these overheadsonly when the relayed data frame is sufficiently large.

The last scenario is designed to inspect the adaptability ofSynergy MAC in face of node mobility. The simulation setupconsists of the same rectangular surface of dimension 400m X400m but fixes the number of nodes to 20. As before, the nodesare all randomly uniformly distributed and have incomingtraffic rate of 200 frames per second. Also, each incomingframe is 1000 octets in length. Node mobility is simulatedusing Random Way-Point mobility model [15] with pausetime between successive movements fixed to 5 seconds. At thebeginning of the simulation each node randomly selects oneof its neighbors as the destination for its cbr traffic. As nodesstart to move, the modulation scheme used and consequentlythe transmission rate between each source-destination pairchanges due to varying SNR.

Figure 4(e) depicts the network throughput achieved byboth 802.11 b and Synergy MAC protocols when constituentnodes are all mobile. From the graph it is clear that SynergyMAC performs consistently better than 802.11b under mobilescenarios. It is interesting to note that Synergy MAC deliverswell even when nodes in the experiment are moving at speedsexceeding 20 meters per second. This is because not only isSynergy MAC quick in identifying a relay for subsequent datatransmission, but it also is able to do so dynamically, resultingin robust relay selection. Figure 4(f) shows the relative gainin throughput for Synergy MAC for the mobile scenariodescribed above.

From all the above results, it is clear that Synergy MACoffers vastly improved performance gains when compared toIEEE 802.11 b under similar conditions.

VI. CONCLUSIONS

In this paper, we studied Synergy MAC, a 802.11 b basedcooperative MAC protocol for mobile ad hoc networks. Sim­ulation studies have shown promising results that validate

the proposed benefits of enabling cooperation at the MACsub-layer. The proposed protocol uses cooperation betweenwireless nodes to realize spatial diversity in order to combatthe ill effects of signal fading. Also, the protocol helps alleviatesome of the fairness problems caused by multi-rate modulationschemes by allowing nodes with low SNR to destination utilizeintermediate relays in order to transmit data at rates higherthan otherwise possible. Simulation results show that SynergyMAC outperforms standard IEEE 802.11 b despite using thesame PHY under various scenarios.

Synergy MAC is also completely compatible with 802.11band can be easily extended to suit other versions of the 802.11standard. In future, we propose to extend this work to studythe energy overhead in relaying, cooperative relay selectionmodels and also address some of the security issues involved.

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