Throughput and delay analysis in an ad hoc wireless LAN using a token passing access method

*Correspondence to: Antonio Freitas, Equipe Reseaux et Protocoles du Limos, IUT-Universite d’Auvergne, B.P. 86,63172 Aubiere cedex, France.

sE-mail: [email protected]

Copyright ( 2000 John Wiley & Sons, Ltd.

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMSInt. J. Commun. Syst. 2000; 13:639—653

Throughput and delay analysis in an ad hoc wireless LANusing a token passing access method

Antonio Freitas*,s and Michel Misson

Equipe Re& seaux et Protocoles du Limos, IUT-Universite& d+Auvergne, B.P. 86, 63172 Aubie% re cedex, France

SUMMARY

Research activities on wireless LANs, in particular the standardization work dealing with medium accessmethods (MAC sub-layer) give priority to CSMA principles. In this paper we are concerned with perfor-mance evaluation of a token passing access method applied to a wireless LAN without infrastructure (ad hocnetwork), composed of nomadic stations. Typically, in an ad hoc network, not all stations are within range ofeach other. Communications between hidden terminals make use of one or several relay nodes. The routingtechnique is based on using the local view which is specific to each station and updated thanks toobservations made of the traffic ‘visible’ to each station. The study of the performances of such a networkreveals a correlation between the throughput and end-to-end delay versus the mesh connectivity character-istics and the wealth of the knowledge of the local views. Copyright ( 2000 John Wiley & Sons, Ltd.

KEY WORDS: nomadic station; wireless LAN; ad hoc networking; token passing; mesh connectivity

1. INTRODUCTION

In the wide wireless communications domain (GSM, DECT, UMTS, etc.) wireless local areanetworks (WLAN) need specific research as has been shown by the work of the IEEE 802.11normalization committee [1] and the ETSI committee with the Hiperlan project [2].

This research work relates to the specifications of the physical layer (radio or infrared), themedium access method, routing and addressing mechanisms. The medium access control (MAC)layer protocol specified by the standardization committees is based on CSMA principles inconjunction with priority mechanisms. Many studies have been carried out into the performanceanalysis of this access method [3,4].

This paper deals with performance analysis for a wireless LAN without infrastructure usinga medium access method based on token passing principles. We will discuss the consequenceswhich result from the characteristics of the mesh connectivity on the throughtput and on thetransit delay of such a WLAN. This study is presented as follows: the context and our objectivesare given in Section 2, the principles of the token passing WMAC method we propose are

described in Section 3. The simulation conditions and the complete results obtained by thesimulation are presented in Section 4.

2. INVESTIGATION DOMAIN

2.1. Short-range wireless communications

To satisfy large numbers of applications in the wireless local communication domain, neighbour-ing but distinct local networks must be able to coexist. So, these networks have to share thewireless medium (Hertzian domain, infrared spectrum, etc.) in a clever way. The free and usablefrequency bandwidths of the electromagnetic spectrum are a limited resource. It is not possible toshare them by assigning each network a frequency bandwidth, a time slot, a spreading code, etc.,acording to the medium access method selected.

A frequency bandwidth, a time slot, a spreading code, etc., are resources which have to beallocated according to the geographical situation of the competing networks. Such an objectiveleads us to introduce clusters that overlap communication areas. The smaller the cluster size, themore efficient this method of allocating resources becomes [5]. The resulting principle consists insharing the same medium with short-range links in order to make it easy to reuse the frequencybandwidth. The range of a transmitter in a given communication area depends mostly on theemitting power, on the transmitter sensitivity, on the radiation directivity and on the localenvironment (surrounding noise, obstacles, etc.).

With portable equipment, the emitting energy is limited by the available power which dependson the battery size, capacity and the emitted power regulations. The diffused signal is subjected tothe multipath phenomenon due to radiation, reflection and/or diffraction on the obstacles of thecommunication area. So this limits the maximum throughput allowed on the medium. Moreover,the channel state is not stationary, the range varies with changes in the environment, surroundingnoise, background topography for mobile communication, etc. Already used in the radio domainbut beginning to be used for infrared [6], the spread spectrum modulation techniques allow us tolimit the interferences coming either from surrounding noise, or from a communication rival onthe same medium.

In some cases the kind of medium allows the limits of the communication area to becontrolled. For example, millimetric waves in the 60 GHz bandwidth have propagationproperties close to those of optical waves such as infrared waves. The opacity of manyobstacles (ground, ceiling, wall, etc.) to microwaves and infrared means that transmissioncan be naturally confined within a volume such as an office or a meeting room for example.The confidentiality of data is preserved and several neighbouring networks can then coexistwithout any damage.

Using such a wireless short-range medium leads naturally to consider three kinds of configura-tions for local communications as shown in Figure 1:

f Full connected WLAN (Figure 1(a)): All the stations are in range of each other. This rule musthold in spite of the possible variations in range of the wireless links during the functioning ofthe network. A solution consists in making the throughput vary to maintain a full connectivitydespite important variations in the channel state [7]. Gfeller proposes an infrared transmissionsystem whose throughput varies according to the surrounding noise (sun light, electric light,etc.).

640 A. FREITAS AND M. MISSON

Copyright ( 2000 John Wiley & Sons, Ltd. Int. J. Commun. Syst. 2000; 13:639—653

Figure 1. WLANs configurations.

f Infrastructured WLAN (Figure 1(b)): A wired or wireless backbone LAN increases the size ofthe communication area, thereby allowing the interconnection of out of range stations. This isthe typical infrastructure retained by 802.11 IEEE Standard defining the medium accesscontrol protocol and the physical specifications of radio and infrared medium. This kind ofconfiguration requires the communication area to be equipped beforehand by means ofa backbone infrastructure.

f Ad hoc WLAN (Figure 1(c)): For such a network, the interconnection of all the stations, evenhidden stations, has to be made without any infrastructure. In this case, a source station can beobliged to use one or several intermediary stations as a relay to join the target station. Thisapproach is that of the ETSI standard of the Hiperlan Network defining a radio LAN. Ad hocWLANs allow a quick setting up of a set of telecommunicating devices typically for temporaryuse, during a meeting or within a protected historic site for example.

It should be observed that a fully connected WLAN is a particular favourable case which isevoked by the two standards previously quoted: 802.11 IEEE standard and Hiperlan.

This article deals with the performance evaluation of a token medium access method for anad hoc WLAN. Before tackling the technical part of this study, the generic problematics of themedium access for an ad hoc WLAN have to be introduced.

2.2. Medium access in an ad hoc WLAN

In the wireless transmission domain, the characteristics of the dispersion of the power (energy) ofthe emitted signal (diffusion, obstacle) are so complex that it is often impossible for a station todetect a collision. The emitted energy of a station blinds it so that it cannot detect other emissionactivity, this is known as the near—far effect. So the access method carrier sense multipleaccess/collision detection (CSMA/CD) widely used for wired LANs is very difficult to transpose towireless LANs [8].

A lot of work has shown [4] that using CSMA/CA principles (collision avoidance) will reduceprobability of collision between stations competing for the access channel in order to transmit.The CSMA/CA method was selected by IEEE 802.11 committee, and a more sophisticatedderived version elimination yield non-preemptive multiple access (EY-NPMA) is specific forHiperlan.

This kind of method loses its efficiency in an ad hoc topology case. An example of thisphenomenon is shown in Figure 2.

TOKEN PASSING ACCESS METHOD 641


Figure 2. Hidden terminal contention illustration.

Let A and B be two stations equipped with transmitters respectively covering Zones A and Bcommunication areas with an overlapping zone. Let station X be in the overlapping zone, so inrange of A and B. Let Station Y and Station Z be in the range of station B.

Let us suppose station A has to transmit a frame to station Z at time t. We are here faced witha multi-hop communication case. This frame has to transit through station X then throughStation B. At the same time B has to send a frame to Y. If station A and station B follow theCSMA/CA listen before talk principle, they can transmit because the medium is detected free. Thesimultaneous activity of stations A and B prevents station X from receiving anything. Station X isunder a collision effect but this collision is not known either by station A or by Station B. Stationslocated in an overlapping area are so exposed to the effects of collisions, which is all the moreserious as these stations have an essential part to play in multi-hop topology. A frame sent bystation A to station Z must follow the path first through station X then through station B. Thisforwarding mechanism greatly increases the traffic managed by the stations within the overlap-ping areas.

To limit the impact of this phenomenon on the functioning and the throughput of the network,IEEE 802.11 standard introduces the short frame mechanism: request to send/clear to send(RTS/CTS). This gives a source station and its target station the possibility of negociating thechannel reservation during the time necessary for data frame exchange.

As far as Hiperlan is concerned, the signals emitted during the phase of access channelnegociation, flood all of the network stations. The Hiperlan frames are composed of two parts:a short header transmitted at weak throughput (low bit rate) and wide range (1 Mbps) and a bodytransmitted at high throughput (high bit rate) and short range (20 Mbps). During the headerlow-speed transmission phase, the network is considered as following a full connected configur-ation.

Moreover, the random aspect of these access methods does not really fit with some applicationsneeding a fair distribution of the access right and/or a bounded channel access time. This is thecase for some real-time industrial applications or for some multimedia applications usingbounded and uniform delays. IEEE 802.11 introduces the PCF function (point co-ordinationfunction) to allow time bounded services typically in an infrastructured network without overlap-ping areas. The EY-NPMA medium access method of Hiperlan manages several priority levels toprovide this kind of quality of service (QoS). Moreover, the access channel negotiation betweentwo stations having the same priority level remains unpredictable.



So, we have chosen to study an access method managing a token passing from one station toanother in a deterministic way. At time t the token holder is the only station which can transmit.

The two normalized access methods for wired networks using the token passing technique arethe IEEE 802.4 (token bus) protocol and IEEE 802.5 (token ring) protocol. For IEEE 802.5protocol stations are organised following a physical ring consisting of a point-to-point linksuccession. This standard induces physical positioning constraints which are incompatible withour hypothesis. The IEEE 802. 4 has to be implemented on a bus topology. A token giving theright to speak goes from one station to the following one according to a logical ring. We havechosen to transpose the mechanisms of this protocol for a multi-hop topology.

The MAC sublayer of the OSI model for local networks provides point-to-point data transferservices and point to multipoint (broadcast) data transfer services. To provide such services ona multihop topology it is necessary to carry out a routing technique at MAC level for commun-ications between hidden terminals [9].

The aim of this paper is to study the effects of the characteristics of meshing (mesh connectivityrate, network diameter, etc.) on the performances of a token-passing-based MAC method in an adhoc network. To do this, it is necessary to make some hypothesis about the functioning rules ofthe access method and the routing frame strategy.

3. MAC LAYER CHARACTERISTICS OF THE STUDIED NETWORK

3.1. Token passing access method principles

The chosen medium access method is a transposition of IEEE 802.4 normalized protocolmechanisms known as the ‘Token Bus’ specification [10]. The basic principle of IEEE 802.4protocol is to make a token circulate from station to station, on a logical ring in order to sharefairly an access right to the channel for a fixed length of time. This technique allows a fair anddeterministic sharing of channel access.

Originally developed for a bus topology (fully connected network), this normalized protocolhas to be greatly modified in order for it to be used for a partially meshed wireless networkconfiguration (the typical network topology studied here). The resulting mechanisms are de-scribed in References [11,12], here are the main principles:

The token follows a path built when the token is created, and this path has to be modifiedfor every new comer or when a station leaves the network. This trajectory of the token isthe default path corresponding to a spanning tree which allows all stations on the network tobe reached at least once during a logical ring round trip. This path is used by a station everytime it wants to reach a station that does not appear to be in range. The token is used by a stationto transmit a data frame only if this token has been passed from the previous station on thelogical ring.

3.2. Routing principle

3.2.1. Local view knowledge. The route given by the spanning tree is a means to reach anydestination station on the network from any source station. Following this spanning tree is aneasy and effective routing technique, but it is not always optimal. It is the case for example whenthe destination of the frame is a station in the range of the station which has to transmit or relay



this frame but this destination station is not the following one on the spanning tree trajectory. Toimprove this routing technique, each station has to know information about its local environ-ment. The frame format must include intermediary address fields and link quality fields with theaim of achieving both

— multihop routing,— the learning of the local environment (knowledge of the existence of oriented links).

So from network activity and medium listening, all stations know the list of their neighbours,stations in range (direct neighbours), and of the neighbours of their neighbours (indirect neigh-bours) [13]. This information is a part of the contents of the local view.

3.2.2. Routing strategy. Whenever a station has to transmit a frame or to relay a frame, thisstation consults its local view to decide on the path to reach the final destination. The followingrule is applied:

If a reliable path allowing the final destination to be reached is known, then the station sendsthe frame to the first station of this trajectory, otherwise this frame is routed to the next station ofthe token path.

Step by step, a frame reaches its target. But the end-to-end transit delay presentslarge variations (depending on the number of hops in the frame trajectory), so it is importantto set a limit to the time during which a station is token holder. In this aim, the station desti-nation acknowledges the frame and this acknowledgement is routed according to the samestrategy. In the following, the time needed for an acknowledged exchange is the end to enddelay.

4. SIMULATION AND RESULTS

A specific and configurable tool has been developed in order to evaluate by simulation thethroughput and transit delay performances of the token passing MAC method applied in anad hoc WLAN of nomadic stations.

4.1. Simulation model

4.1.1. Transceiver model. The communication zone is situated within a square room. A model ofthe transceiver is built, using an isochronous source located at the center of a circle of radiusP giving the range and the surface covered by the transceiver. It broadcasts to all the receiverssituated within the cell corresponding to this covered area. This simple model has been chosenduring a study dealing with in-house infrared communications [14].

The measurements of the room are expressed according to the range P of the transceiversthereby making this study as independent as possible of the type of medium (infrared, radio,micro-waves, etc.).

4.1.2. Network set up. The arrival of a new station that wishes to join the network issimulated by the random choice of a position (x, y) in the room. If this station is able to havea link with the network version already in service, the number of stations of the network increasesby one.



Figure 3. Example of a random configuration of 6 stations in a 3P square room.

The resulting configuration ramdomly obtained for N stations is characterized by a meshconnectivity rate

Mx"1

N](N!1)

N+i/1

DNi

(1)

with DNinumber of direct neighbours heard by station i.

The mesh connectivity rate (Mx) is the ratio between the number of existing links and thenumber of potential links. The rate is equal to 1 in the case of a full mesh connectivity (all thestations are within range of all the others). In the example Figure 3 the mesh connectivity rate isequal to 0.4.

4.1.3. Frame time length. The network activity that can be observed is made up of data frametransmission and MAC control frame transmission (token, acknowledgement, etc.). In a partiallymeshed network, frame relaying induces an extra activity that must be added to the two previousones.

In this simultation, the length of data frames is constant and equal to ¹, and the length of MACcontrol frames is constant and equal to ¹/10. The data frames are generated according toa Poisson law for the whole network and each data frame is put in the queue of a station chosenrandomly according to a uniform law. When a station is token holder, the token is used to senda data frame if the queue of this station is not empty, the acknowledgement is expected, then thetoken is passed to the next station on the logical ring.

The usual parameters, G and S, have been retained to define the offered load and thethroughput of the network, G represents the average number of frames proposed to the wholenetwork during the time needed for a frame serialisation (Frame time ¹ ) and S stands for theaverage number of frames crossing the network during a frame time [15].

4.2. First result

The aim of this study is to define the throughput performance according to the characteristics ofthe network mesh connectivity. The randomly obtained configurations are mainly studied fora network of 10 stations, because this number is thought to be significant for the applications ofad hoc WLANs quoted in Section 2.



Figure 4. Typical shape of throughput curves of token passing method.

A previous study [16] has shown that in a population of 104 configurations of 10 stationsrandomly spread in a 2P square room, the most frequent mesh connectivity rate of the obtainednetworks is equal to 0.58. Let us then take two typical configurations with 10 nodes (N"10), thefirst with Mx"1 and the second with Mx"0.58. Let us evaluate by simulation the throughputS versus the offered load G varying from 0 to 1, for each configuration. The measured pointsresulting from this simulation are given in Figure 4.

Two parts can be distinguished:

— a linear part when the throughput is equal to the offered load, S"G,— and a steady saturation part reached when the offered load is greater than the channel

capacity, corresponding to the useful maximun throughput S.!9

.

Here we find the characteristics of a wired ‘Token Bus’ network: a linear part, S"G, followedby a saturation threshold for G'S

.!9. On the classical wired ‘token bus’ throughput graph, this

threshold indicates that a part of the bandwidth is used up for the medium access rightmanagement. This maximum throughput of the corresponding theoretical model is deduced fromthe respective length of the data frame and the token frame (theoretical S

.!9"1!(10/11) for our

set of parameters).In the case of a fully connected WLAN, a configuration with Mx"1, the Token Passing

WMAC method we propose uses up a part of the channel bandwidth for the transmission of thetoken and acknowledgement frames, S

.!9"0.84. In the case of a multihop WLAN, for instance

a configuration with Mx"0.58, the forwarding function uses, moreover, a part of the channelbandwidth to route the frames during communications between hidden terminals, S

.!9"0.55.

Let us take the configuration with Mx"0.58 as our reference. The obtained result leadsnaturally to the following question: is this graph characteristic of a mesh connectivity rate equalto 0.58? If we consider another configuration of 10 stations in a 2P square room, with a meshconnectivity rate equal to 0.58, will we obtain the same graph?

The observation of the obtained graph shows a slight S.!9

divergence when G is greater thanthe channel capacity. It is therefore necessary to study the S

.!9divergence for a given G'S

.!9applied to the reference configuration.



Figure 5. S.!9

divergence for the reference configuration.

Figure 6. S.!9

divergence for any configuration with N"10 and Mx"0.58.

4.3. Result convergence

The S.!9

distribution, given in Figure 5, obtained from 1000 tests of traffic simulation withG"0.9, is a narrow peak whose shape can be classed as a normal distribution. The narrowness ofthis peak indicates that a good degree of accuracy can be obtained for the average of S

.!9for

a given configuration. The width half-way up depends on the characteristics of the random eventgenerator which has the job of producing an offered load G following a Poisson law andproducing station pairs given by a uniform distribution.

The narrow shape of the curve given in Figure 5 shows that the forwarding and the mediumaccess mechanisms allow a throughput to be achieved which converges for a given G.

4.4. Incidence of mesh connectivity characteristics on channel capacity

Now, let us study S.!9

convergence for any 10 node network characterized by a 0.58 meshconnectivity rate within a 2P square room.

The S.!9

distribution, represented by the curve (b) in Figure 6, indicates a wider dispersion andan asymmetry. S

.!9can vary in a range of [0.46—0.60], so the average value is not known with



Figure 7. S.!9

divergence for any configuration with N"10, Mx"0.58 and Rx"8.5.

a great precision. We can conclude that the mesh connectivity rate does not provide enoughprecision for the forwarding conditions.

Hence, it was decided to improve this study by considering a parameter expressing theconditions under which forwarding is carried out. We consider the elements used to optimizethe number of hops necessary for the transit of a frame, and specifically the wealth of theknowledge of the local view.

A station deduces:

— the existence of neighbours by listening to the network traffic,— the existence of indirect neighbours by analysing the received frames.

The list of the neighbours (direct and indirect neighbours) of a station is memorized for eachstation in a table and constitutes a part of the local view. The wealth of this local view depends onthe size of this list.

Let DINibe the number of direct and indirect neighbours of station i. Let Rx be the average

number of direct and indirect neighbours of all stations:

Rx"1

N

N+i/1

DINi

(2)

For instance Rx"N!1 if the network is a full connected WLAN or if for each station allothers are direct or indirect neighbours. This parameter characterizes the routing conditions onmultihop communications.

The study which has therefore been redone for configurations characterized by a given meshconnectivity rate (Mx"0.58) and an average given number of neighbours (Rx"8.5) givesa distribution whose shape is relatively sharp and more symmetric as shown Figure 7. This kindof configuration shows a throughput that is relatively homogeneous.

A classification of configurations carried out by recurrence (as described in Reference [16]),taking into account an average mesh connectivity rate and an average number of neighbours is



Figure 8. Throughput versus offered load according to different Mx and Rx parameters.

going to be used as a basis for evaluating the performances in throughput and end to end delay onthis kind of wireless network without infrastructure. The studied configurations (Mx, Rx) are thefollowing:

— (Mx"1, Rx"9), full connected configuration,— (Mx"0.78, Rx"9), typical configuration in a 1.5P square room,— (Mx"0.58, Rx"8.5), typical configuration in a 2P square room,— (Mx"0.38, Rx"6.5), typical configuration in a 3P square room.

4.5. Throughput according to the mesh connectivity rate and the wealth of the local view

The results have been obtained from simulations carried out for an offered load of 3500 dataframes for each point.

The (a) curve of Figure 8 obtained for completely meshed configurations has the highestsaturation threshold because it is simply the result of the consumption of the bandwidthnecessary for the passing of the token and the acknowlegdment.

The other curves of Figure 8 have thresholds which show the existence of the traffic necessaryfor the passing of the token but also of a traffic corresponding to the forwarding of somedata frames. The less dense the mesh connectivity, the greater this latter type of traffic, sincethe number of frames that must be forwarded increases when the mesh connectivity ratedecreases.

This is illustrated by the fact that the value of the saturation threshold of the curves of Figure 8increases at the same time as the mesh connectivity rate of the network increases. For the samemesh connectivity rate the classification obtained by taking the average number of neighboursallows the dispersion of the results obtained for S to be reduced.

4.6. End to end delay according to the mesh connectivity rate and the wealth of the local view

The study of the influence of the network characteristics on the end-to-end delays was carried outon the same basis. Figure 9 shows the evolution of the end to end transit time versus thethroughput for configurations with different mesh connectivity rates presented above. This delay



Figure 9. End-to-end delay versus throughput according to different Mx and Rx parameters.

starts when a frame is put in the queue of the MAC layer of the emitting station, and ends whenthis frame has been received by the target station. From this set of curves there emerges a typicalshape where G is low, the transit delay D increases exponentially (linear curve for a log. scale) thenfrom a threshold value which depends on the characteristics of the network mesh connectivity,the increase in end to end delay showing an avalanche effect.

When the offered load G reaches and becomes greater than the channel capacity, this delay ismeanly depending on the time spent in the queue of the emitting station. Let the configurationwith a mesh connectivity rate be equal to 0.58 giving the curve drawn with a continuous line; letA(x, y) be a point of this curve near the S

.!9asymptote (about 0.55), when G'x the extra delay is

resulting from the overload of the queue of the transmitting stations. The time needed by thedeterministic mechanism for channel access and routing does not directly appear in this figure.

4.7. Comparison of the results obtained for the proposed token passing WMAC method with publishedresults of comparable methods

4.7.1. Token passing variants. Different studies deal with the use of a token for wireless commun-ications between stations within a small location area [17], even sometimes as part of Europeanproject [18]. These studies generally concern configurations which are not always fully connectedbut hidden terminals are then indirect neighbours needing 2 hops to be reachable. In our study,the number of hops needed to join a neighbour is only bounded by the number of stations minusone (N!1). The configurations we are dealing with are then more general.

In spite of these slight differences if we just look at the hypothesis, the throughput curves versusthe offered load for these token passing access methods present first a linear part followed bya plateau. For the case of a 0.78 meshed configuration within a 1.5P square room, if we considertwo randomly chosen stations they can be:

— in range direct neighbours (one hop)— indirect neighbours needing two hops— neighbours needing one extra hop, this case having a very low probability of existing for

reasons of elementary geometry.



This 0.78 meshed configuration can be compared to a configuration with 20 percent of brokenlinks in Reference [17]. The slight difference between the corresponding S

.!9can be explained by

the fact that the traffic induced by the token passing and the acknowledgements are both takeninto account in our study.

4.7.2. For the normalized medium access control CSMA/CA. In Reference [19] the author presentsthe study of performances (maximum throughput, etc.) of the IEEE 802.11 distributed co-ordination function (DCF) based on the CSMA/CA protocol according to different sets ofparameters (basic access, RTS/CTS mechanism, contention window, etc.) for a WLAN withouthidden terminals. For example for a basic service set of 10 stations (IEEE 802.11 simplestarchitecture) the obtained maximum throughput with a smart choice of the parameter set is equalto 0.84. This basic set service configuration can be compared with a full connected configuration(Mx"1) presented in Section 4.5 for 10 stations. The maximum throughput obtained for thesetwo configurations are very close, but for the CSMA MAC method the throughput decreases asthe offered load increases over the channel capacity because of the increasing probabilityof collision. Furthermore, the more the offered load increases, the more the CSMAmechanism defers transmitting frames. This induces an increasing end to end delay. Withthe token passing access method we defined above, the throughput is constant when the offeredload is greater than the channel capacity. The deterministic aspect of the mechanisms of ourmethod (MAC and routing) leads us to say that when the offered load is greater than the channelcapacity and even for a heavy load, the average time between two token holdings is the same foreach station. On the other hand, implementing CSMA seems to be easier than implementing ourmethod.

Starting from the basic CSMA scheme, Huang and Chen [3] have proved the interest of theCTS/RTS mechanism by means of simulation. In their paper the authors introduce an analysis toevaluate CSMA throughput in multicell WLAN by taking hidden terminals, radio interferenceand cell-overlapping into account. In this approach, a probability of being hidden is attached toeach terminal, which serves as a substitute for the meshing notion. The influence of the hiddenterminal activity on the observable throughput within a cell is shown. This approach is quitedifferent from what we have done in trying to make a serious comparison of the results.

4.7.3. For multihop configurations. We have been able to compare performances of the 802.11MAC protocol with those we obtained for a fully connected configuration. Concerning a multi-hop configuration, we are obliged to invoke Hiperlan as a relevant example. We have not beenable to compare our results with EY-MPMA MAC performances but this point is part of thescope of our future work.

5. CONCLUSION

The characteristics of throughput and of end to end delay for a wireless network withoutinfrastructure mainly depend on the characteristics of its mesh connectivity. Here we havequantified these effects for a network of 10 stations using a token access method and a specificforwarding strategy using the fact that a station can deduce the existence of its neighbours. Thediversity of performances obtained by simulation justifies the classification of the configurationsaccording to two criteria: the mesh connectivity rate and the average number of neighbours.



The knowledge of these criteria is mainly important when a newcomer puts his station intooperation so that given throughput and delay performances can be achieved. These parameterscould be given by a monitoring distributed function that is not yet available.

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AUTHORS’ BIOGRAPHIES

Antonio Freitas obtained his PhD in Computer Science at Blaise Pascal University,Clermont-Ferrand, France, in 1995. Since then he has been teaching networks in theInstitute of Technology of Auvergne University in Clermont-Ferrand. He is a memberof the ‘Reseaux Et Protocoles’ research team of the Computer Science Laboratory ofBlaise Pascal University. His current research interests are Wireless Local AreaNetworks and in particular WLAN without infrastructure.

Michel Misson obtained his PhD in Nuclear Physics at Blaise Pascal University,Clermont-Ferrand, France, in 1979. In 1983, he became a lecturer, teaching networksand system architecture in the Computer Sciences Department of the Institute ofTechnology in Clermont-Ferrand. He manages the team ‘Reseaux Et Protocoles’ ofthe Computer Science Laboratory of Blaise Pascal University. His current researchinterests are Wireless Local Area Network using short range links.



Throughput and delay analysis in an ad hoc wireless LAN using a token passing access method

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