Performance evaluation of TDMA and CSMA protocols

1 Performance evaluation of TDMA versus CSMA-based protocols in SINR modelsSkander Banaouas Paul M¨u hlethalerINRIA Rocquencourt INRIA RocquencourtLe Chesnay FRANCE Le Chesnay FRANCEIskander.Banaouas@inria.fr Paul.Muhlethaler@inria.frAbstract—In this paper,we compare Time Division and Car-rier Sense Multiple Access schemes(TDMA and CSMA)in a Signal to Noise and Interference Ratio(SINR)-based model. We assume that the network nodes are distributed a Poisson process evenly distributed and have pending packets.First we study the distribution of the SINR in TDMA-based protocols where the same time-slot is reserved in given radius of n hops (n=2in most of these protocols).The same study is carried for CSMA-based access protocols where the reuse is controlled with the carrier-sense threshold.We then compare the total throughput of TDMA and CSMA based access protocols in an outage model.In this model a packet is considered as correctly received if its SINR is above a given threshold.Finally,we conduct the same comparison between TDMA and CSMA-based access protocols in a model of adaptive coding.In this model, the actual SINR rules the transmission coding.We idealize the performance of this technique using the Shannon’s well know formula W=W0log(1+SINR).I.I NTRODUCTIONThe need for connectivity in multihop networks give rise to large amount of studies particularly regarding routing.A large number of proposals have been made e.g.[1],[2],[3],[4]and a few of them are now standards[5],[6].The access to the radio medium is also a very challenging problem in mobile ad-hoc networks.Access protocol have to offer as many access opportunities as possible while also minimizing the number of collisions as much as possible.Carrier sense techniques such as CSMA are widely used in multihop ad-hoc networks.They are really simple schemes to implement and are also widespread because they are the basis of the proeminent IEEE802.11standard.However, these techniques are known to suffer from serious drawbacks such as:unpredictability,unfairness,hidden-collisions,ex-posed nodes,etc.It has been shown in[7]that QoS constraints could be successfully handled with CSMA-based approaches if additional mechanisms such as admission control,reservation and prioritization scheme are added to the network.But most proposals offering Quality of Service(QoS)in multihop ad hoc networks e.g.[8],[9]use TDMA-based approaches.One or more defined time-slots are made available for each node to transmit data on the radio ually the gain of these time-slots is organized through a reservation procedure which may be difficult to use in practice.Most papers on QoS in multihop ad hoc network study whether the traffic requirements are actually satisfied by a given protocol.We believe that,unfortunately,the models used at the physical and MAC layers are not always completely accurate.The few contributions that in-vestigate CSMA-based approaches usually use simulations to investigate QoS requirements.But these simulations are most often carried out with tools which employ only approximate models1.The papers that study TDMA-based approaches usually consider that the reservation scheme which assigns the time-slots to the transmitting nodes is perfect and that there is no collision whatsoever.This assumption,as will be shown in this paper,may be wrong.Models using Signal over Interference plus Noise Ratio (SINR)as the key indicator are much more accurate in multihop adhoc networks.The aim of this paper is indeed to study TDMA and CSMA-based access protocols with SINR-based models.Since current technology usually requires a large SINR to correctly receive a packet,wefirst study the distribution of the SINR with TDMA and CSMA-based access protocols.To compare the two approaches,we then study the numbers of successful transmission in the outage model which implies a SINR above a given threshold.We also consider an adaptive transmission model whose performance is represented by Shannon’s well-known formula W=W0log(1+SINR)II.R ELATED W ORKSNumerous protocols have been proposed for TDMA based access schemes in Ad Hoc networks e.g.NAMA[10],USAP [11],[12]and ASAP[13].These protocols share the idea of reserving timeslots in the2-hop neighborhood.In other words, the same time-slot used by a transmitter can not be reused in a 2-hop radius of the transmitter.This reservation rule is used to avoid collision with neighbor nodes or with hidden nodes.This reservation rule can easily be extended to the reservation in the n-hop radius of the transmitter.However,it is generally the 2-hop reservation which is used in most published protocols. The aboved mentionned papers contains no study of concurrent transmission in the network.The assumption is that if the timeslot is not re-used in a2-hop neighborhood then the packets sent in this timeslot is correctly received.The model used in these articles ignores the well accepted model based of the SINR creterion to evaluate the reception of the packet. 1ns-2only considers the strongest interferer to qualify the actual reception of a packet and neglects the other interferersEuropean Wireless 2009There have many studies of Ad Hoc networks using the IEEE802.11CSMA technique to share the radio medium[?]. These studies generally use simulators to evaluate the perfor-mance of the network.In most case often these simulation studies encompass an effective SINR model even though this model may be simpler than exact model.For instance,in ns2 [14]the reception of a packet is evaluated with the SINR where only the strongest interferer is considered whereas the exact model takes into account the sum of all the interferers.[A fouiller qualnet et autres simulateurs]It is worth noticing that the protocols based on TDMA and usually foreseen to meet QoS requirement generally use a binary criterion to evaluate the actual reception of packet whereas the protocols with a CSMA or random access spirit usually use more accurate models following a SINR approach. Calculating the distribution of the SINR using mathematical models is difficult.For random techniques such as CSMA the distribution of the simulaneously transmitting nodes is very difficult to obtain.The constraints on the pattern of transmitting nodes is difficult to model.Thus deriving accurate formulas to evaluate the SINR in Ad Hoc networks using random access techniques does appear within reach.The same remark may be made for control based access techniques such as TDMA.The spatial distribution of concur-rent transmitters is also difficult to express in a mathematically tractable model[footnot avec Mattern].As a matter of fact,it is difficult to compute the spatial distribution of nodes which are at least n hops aways from each other in a multihop Ad Hoc network.Consequently,deriving the distribution of the SINR in such a network is unlikely.To the best of the authors’knowledge,the only situation in which an exact formula can be derived for the SINR distribu-tion is the case of spatial Aloha[15].If we assume that the network nodes are distributed according to a Poisson process of intensityλ,the transmitting nodes are also a Poisson process of intensityλp where p is the Aloha transmission probability. The Laplace transform of the shot noise(or equivalently total interference)can be obtained with a closed formula.Thefirst contribution of this paper is to compute the distribution of the SINR for TDMA-based access technique in an Ad Hoc network where the nodes locations follow a Poisson distribution.We perform this evaluation through extensive simulations.The second contribution of this paper is to calculate the same distribution of the SINR for CSMA-based access tech-niques using the same assumption for the nodes locations and then to compare this distribution with that obtained for the SINR in TDMA based access technique.The third contribution is to compare the efficiency of TDMA-based and CSMA-based access techniques using an outage model.In this model a packet is counted as received if its SINR is above a given threshold.This study evaluates the density of successful transmissions in the network.For both approaches the access schemes must be optimized.Finally we contribution we compare the efficiency of TDMA-based access techniques and CSMA based access techniques using a criterion derived from Shannon’s well-known formula W=W0log(1+SINR).III.S IMULATION MODELThe network nodes are distributed according to a spatial Poisson process.Unless otherwise specified we assume that the nodes are evenly distributed,the node density being denoted asλ.We use a propagation model which takes into account the decay of the power with the distance denoted by R between the source node and the destination node.This decay is equal to:1Rβwhereβis the path-loss exponent.We also consider a fading F i,in other words the power received at the recepient node is :P=P0F iRβ.In this study we study the case where the fading is negligible F i≡1.We also consider a fading following an exponential law.This represents the case where there is no light of sight path between the transmitter and the receiver,(see e.g.[16, p.50and501]).The actual propagation between these two nodes is built using many independent paths.We also consider a fading which follows a log-normal law i.e.e F i follows a Gaussian distribution.This situation models the case where various objects between the two communicating nodes create a shadowing effect.The last distribution for the fading that we consider is the Nagakami distribution.Thefirst model we study is based on a outage model. We assume that a transmission is a success if during the transmission the ratio of the power received by the recepient node divided by the interference I induced by the simultaneous transmission plus the noiseνis greater than T.In other words we must have:F iRβjjRβj+ν>Twhere F j denotes the fading of transmitter j and R j the distance between the transmitting node j and the recepient node.The second model we consider assumes that there is an adaptive coding of the transmissions.When the SINR is high the bit-rate can be high but when the SINR is low the bit-rate must also be low.This behaviour is usually modeled using Shannon’s law:W=W0logF iRβj=iF jRβj+ν+1.In our model,the network is highly loaded:all the nodes have pending packets.A.TDMA based access protocolsWhen a time-slot is used by a transmitter S a straighforward computation of the SINR shows that the same time-slot can not be used within a given neighborhood of S,this neighborhood being defined by the capturethreshold.Most TDMA-based pro-tocols for multihop ad hoc networks express this neighborhoodcondition in terms of number of hops from the source nodeS.These protocols usually assume that a time-slot can not be shared within m hops from the transmitter,generally m=2. In other words,two transmitters using the same time-slot must be at least n hops away from each other.Thus n=m+1and the usual value for n is3.We call this rule for the time-slots the reservation rule at distance n.In our simulation we do not consider a particular TDMA-based access protocol.We only assume the simulated TDMA-based access protocol follows the reservation rule at distance n.We build the set of simultaneous transmitting nodes by random draws.The set of nodes S tdma which are transmitting in the same time-slot is built as follows.First we select a node s1at random among the nodes in the Poisson point process and we initialize S tdma with this node S tdma={s1}.We then randomly pick another node s2and verify that this node follows the reservation rule i.e.is at least n hops away from every node in S tdma.If this holds,this node is added to S tdma. This procedure terminates when all the nodes in the point process have been tested.To measure the performance of TDMA based access proto-cols,we assume that a node in S tdma transmits to a random neighbor and we compute the SINR for this transmission.By considering numerous network configurations,we measure the reception probability using a capture threshold T.B.CSMA-based access protocolsIn CSMA-based access protocols,access is governed by a threshold,often called carrier-sense threshold,denoted here C cs.Before transmitting a packet,a node senses the channel and measures its level of power.If this level is below C cs then the packet is transmitted;if not the node goes in back-off mode and will attempt again later.As in TDMA-based access protocols,we will not go into the details of the access protocol.We will only assume that all the simultaneous transmitting nodes follow the rule based on the carrier sensing.The set of nodes S csma that are transmitting at the same time is computed as S tdma.We select a node in the network and add this node to S csma if it satisfies the carrier sense rule.To measure the performance of CSMA-based access proto-cols,we assume that a node in S csma transmits to a random neighbor.Thus it is easy to compute the reception probability for a given capture threshold T if we use a carrier sense threshold C cs.IV.S IMULATION R ESULTSIn this section,unless otherwise specified,we use the following values.We assume that the network density is λ=0.01and that the nodes are evenly distributed over a 100m×100m area.We set the transmission radius to15m. The default path-loss exponent is assumed to beβ=4.A.SINR in TDMA-based multihop access protocolsWe study the SINR with respect to the minimum reuse distance n(in number of hops)of the TDMA-based multihopCaptureprobabilityCapture thresholdNo fadingFig.1.Probability of capture with respect to the capture threshold T for TDMA-based protocol for various value of n.No fading.23456789103 3.54 4.5 5miminumreusedistancenpath loss exponent betaNo fadingT = 2T = 5T = 10Fig.2.Probability of capture with respect to the capture threshold T for TDMA-based protocol for various value of n.Exponential fading of mean1access protocols.We assume here that there is no fading i.e F i=1In Figure1we show the probability of capture when T is from1to10.We see that for n=3the capture probability is small fo a capture threshold T=10.Thus for n=3we can not employ a transmission technique that requires a capture threshold T=10.A suitable capture threshold T would be less than or equal to2.For n=4the capture probability is also too small to transmitt with T=10,and we must use a capture threshold T≤5.For n=5,the capture probability can be acceptable for T≤10.We can see that the improvement between n=5and n=6is very small.In Figure2we study the minimum distance for reuse to obtain a probability of capture greater than0.9for various value of the path-loss exponent.We use three values of the capture threshold T=2,T=5and T=10.We observe that T=10leads to large values of n which are difficult to obtain in practice.We also note that reaching a probability of capture close to1is not possible with TDMA-based multihop access protocols.B.SINR in CSMA-based multihop access protocolsIn Figure3we show the probability of capture with CSMA-based multihop access protocols.We see that the tuning is veryC a p t u r e p r o b a b i l i t yCapture thresholdNo fadingFig.3.Probability of capture with respect to the capture threshold T for CSMA-based protocol and various values of the carrier sense C cs easy to obtain large SINR contrary to the case of TDMA-based access protocols.This can be explain because a larger dynamic is easily available with CSMA-based protocol whereas the dynamic achievable in TDMA-based proposed is much more constrained.We observe that for a defer threshold less than 10−6the capture probability is always greater than parison of CSMA and TDMA-based access protocols in an outage SINR modelIn this subsection we evaluate the number of successful transmissions in the network when we use CSMA-based or TDMA-based access protocols.We suppose that a successful transmission is granted when its SINR is above a capture threshold T .We vary this capture threshold between 2to 10.In this approach we have a tradeoff.If the protocol is controlled to have a high density of transmission the interference level will be high and thus the capture probability will be low.On the other hand if the protocol is controlled to have a low density of transmission the interference level will be low and thus the capture probability will be high.Therefore the CSMA-based and TDMA-based access protocol require to be optimized to offer the larger number of successful transmissions.First we study TDMA-based access protocols.The number of successful transmissions in the network for 1000slots is shown in Figure 4.We observe that except for a capture threshold of T =10the maximum number of successful transmissions is obtained for n =3,in other words for T =2or T =5the gain in transmission density exceeds the loss caused by a lower transmission probability.For T =10the maximum number of successful transmissions is obtained for n =4.For large n the number of successful transmissions is approximately the same for T =2,T =5or T =10.The gain in the number of successful transmissions when we pass from T =10to T =5is of 23%.This gain is of 82%when we pass from T =10to T =2.In Figure 5we also study the number of successful transmis-sions in the network for 1000slots but for CSMA-based access protocols.For T =2the number of successful transmissions is reached for a carrier sense threshold of C cs =1.510−5,4000600080001000012000345 6 78N u m b e r o f s u c c e s s f u l l y t r a n s m i t t e d p a c k e t sMinimum distance reuseNo fadingTDMA T=2 TDMA T=5 TDMA T=10Fig.4.Number of successfully transmitted packets with respect to the minimum reuse distance n for TDMA and various values of capture thresholdsfor T =5the optimum is reached for C cs =510−6and for T =10the maximum is for C cs =10−6.In Figure 5,we have also shown the maximum of successful transmissions for TDMA-based access protocols for T =2,T =5and T =10.We can see in this Figure that TDMA-based access protocols slightly outperforms CSMA-based access protocols in the outage SINR model.The gain is of 4%for T =2,it is of 8%for T =5and 4%for T =10thus the gain remains moderate.parison of CSMA and TDMA-based access protocols in an adaptive transmission modelIn this subsection we evaluate the number of transmitted bits in the network with CSMA-based or TDMA-based access protocols.We use an adaptive transmission model in which the transmission automtically adapts to the value of the SINR.We represent the performance of this dynamic scheme by Shannon’s formula W =W 0log (1+SINR ).In this paper we assume that W 0≡1.The result of this comparison is provided in Figure 6.For TDMA-based access protocols the maximum of throughput is still obtained for a minimum reuse distance n =3which is also the optimum for the outage model except for T =10where the maximum is reached for n =4.For CSMA-based protocols the maximum of throughput is reached for C cs =10−5.Here again TDMA is sligthly better that CSMA,the gain is of 7%.E.DiscussionsIn this paper we have not presented simulation results with fading for space reason although we have investigate a few cases of slowly changing Rayleigh fading.With this fading the conclusions of the simulation remain the same as when there is no fading.Other types of fading should be also investigated.Moreover the overhead induced by the access protocols should also be incorporated in the models to build a fair comparison.The effect of the mobility on the performance should also be considered.1e-061e-05N u m b e r o f s u c c e s s f u l l y t r a n s m i t t e d p a c k e t sCarrier sense threshold C_csNo fadingFig.5.Number of successfully transmitted packets with respect to the carrier sense C cs for CSMA and various values of capture thresholds10001500 2000 2500 3000 3500 4000 3 45 6 7 82e-064e-066e-06 8e-06 1e-051.2e-05 1.4e-05T h r o u g h p u t t h r o u g h S h a n n o n ’s l a wMinimum distance reuseNo fadingCarrier sense threshold TDMACSMAFig.6.Throughput obtained with a dynamic coding and Shannon’s law with respect to the minimum reuse distance n for TDMA and with respect to the to the carrier sense C cs for CSMAV.C ONCLUSIONIn this study we have investigated TDMA-based and CSMA based access techniques in an Ad Hoc network where the nodes locations follow a Poisson distribution.We have used simulations to carry out a qualitive evaluation of the perfor-mance.First we have studied the distribution of the SINR with respect to the minimum reuse distance n for TDMA-based protocols.For CSMA-based protocols the distribution of the SINR is given with respect to the carrier sense C cs .The simulations show that reaching a high probability of capture when the capture threshold is high (≥10)is more difficult for TDMA-based protocols.For TDMA-based protocols where the the minimum reuse distance n is often 3,the transmission technique should be adapted to operate with a small capture threshold i.e between 2to 5.Then we have studied the performance of TDMA-based and CSMA-based protocols with an outage model where a transmission is successful if its SINR is above a given threshold.We have optimized the performance of the protocols with respect to the minimum reuse distance n and with repsect to the carrier sense threshod C cs .The 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