A Framework for Analysis of Connectivity and Performance Bounds in Ad Hoc Networks and Its Application to a Slotted-ALOHA Scenario

Transcription

1 JOURNAL OF COMMUNICATIONS SOFTWARE AND SYSTEMS, VOL., NO., SEPTEMBER A Framework for Aalysis of Coectivity ad Performace Bouds i Ad Hoc Networks ad Its Applicatio to a Slotted-ALOHA Sceario Fabrizio Graelli, Member, IEEE Abstract: I this paper, a framework is proposed to aalyse the problems of coectivity ad performace i ad-hoc etworks through a aalytical approach. To this aim, available results regardig the applicatio of percolatio theory to the study of coectivity i ad-hoc etworks are exploited oitly with commuicatio theory models i order to derive the cofiguratio of etwork parameters that esures log rage coectivity amog odes ad the correspodig available capacity o the wireless medium. The framework is the applied to a slotted ALOHA ad-hoc etwork. Theoretical ad umerical results validate the approach ad allow the derivatio of iterestig desig priciples for ad-hoc etworks that cosider the impact of physical ad MAC-level parameters o etwork coectivity ad ed-to-ed performace. Idex terms: ad-hoc etworks, performace evaluatio, slottedaloha I. INTRODUCTION Mobile ad-hoc etworks (MANET cosist of wireless hosts that commuicate with each other i the absece of a fixed ifrastructure [8]. They ca be used i a wide plethora of applicatios, ragig from tactical operatios, to establish quickly military commuicatios durig the deploymet of forces i ukow ad hostile terrai; to sesor etworks, for commuicatio betwee itelliget sesors mouted o mobile platforms. I the last applicatio, mobile ad hoc etworks are likely to achieve wide deploymet i the ear future because they greatly exted the ability to moitor ad cotrol the physical eviromet from remote locatios. I ad-hoc wireless etworks, effective exploitatio of the shared trasmissio resources ad power cotrol are two key elemets represetig research challeges. Not all hosts are i fact withi the trasmissio rage of each other ad commuicatio is achieved by multi-hop routig, where itermediate odes cooperate by forwardig packets betwee two hosts. Available badwidth o the wireless medium is partitioed / shared by all the odes i a give rage ad access to the medium is usually performed i a distributed fashio. Moreover, employmet of batteries i ad-hoc etworks odes imposes strog costraits o eergy cosumptio ad Mauscript received May 3, 004; revised May 09, 005 ad July, 005. The paper was preseted i part at the Coferece o Software, Telecommuicatios ad Computer Networks (SoftCOM 004. F. Graelli is with DIT Uiversity of Treto, I Treto, Italy ( fabrizio.graelli@uit.it. Origial scietific paper trasmissio power. Key poit for the existece of the etwork is i fact the life of each ode, which should be maximized, ad thus eergy savig. At the preset momet, a relevat scietific effort is devoted to the desig ad implemetatio of ad-hoc etworks, ad i particular the developmet of useful models as tools for comparig differet desig priciples or solutios represets a ope issue. I such framework, the usage of percolatio theory usig a physical model for trasmissio based o the sigal to oise ratio was recetly proposed [] for studyig capacity ad coectivity issues i ad hoc etworks i presece of iterferece. A iterestig approach is preseted i [], which cosiders a extesio of the model to the case where iterferece amog cotemporary odes trasmissio is mitigated through the itroductio of a costat (γ to iclude the employmet of orthogoal CDMA codes, as a way to effectively reduce the impact of iterferig sigals. The preset work focuses o the issues related to resource ad power allocatio / maagemet i ad-hoc etworks, i order to provide guidelies for the desig ad cofiguratio of ad-hoc etworks uder costraits of etwork coectivity ad performace bouds of the resultig architecture. From a methodological poit-of-view, the paper represets a further geeralizatio of the work preseted i [] by itroducig a framework that cosiders i additio to physical parameters (trasmissio power, modulatio parameters, etc. the existece of the medium access cotrol (MAC protocol, that coordiates ad maages access to the wireless medium. Differet MAC strategies have a relevat impact o ad-hoc etwork coectivity ad performace. The framework is the applied i a sceario where the available badwidth is shared amog all odes ad the MAC protocol employs a slotted-aloha strategy. Results will aalyse the cofiguratios of parameters (ode spatial desity, trasmissio power, etc. of the etwork that grat coectivity betwee two distat odes. The structure of the paper is as follows: sectio II itroduces the framework ad the proposed model, while the relevat theoretical results are derived i sectio III. Fially, sectio IV presets some umerical results ad sectio V draws coclusios ad outlies future work o the topic. II. THE PROPOSED APPROACH A. The Proposed Framework The cosidered sceario is a multi-hop ad-hoc etwork of ifiite extesio, where odes are spatially distributed /05/ CCIS

2 36 JOURNAL OF COMMUNICATIONS SOFTWARE AND SYSTEMS, VOL., NO., SEPTEMBER 005 accordig to a Poisso distributio of costat desity λ, like i []. λ represets the average umber of odes per square meter. Each ode trasmits usig a power level P i, selected i the rage [0, P t ] ad tued i such a way to reach its eighbors. P t is the maximum available trasmissio power of each ode. Fially, x i is a vector represetig the positio of the i-th ode (two-dimesioal i the case cosidered throughout the paper, while L(x is the atteuatio fuctio experieced by a sigal o a distace equal to x. L(x depeds from the specific propagatio model uder exam. I case the access to the medium is perfectly scheduled (oe trasmissio at a time, o iterferece like i Time Divisio or Frequecy Divisio Multiple Access, trasmissio betwee ode i ad ode is successful if the received sigalto-oise ratio is higher tha a threshold β: Pi ( T where T is the thermal oise power, give by FkT 0 B (F is the oise figure, k is the Boltzma s costat, B the trasmissio badwidth ad T 0 the temperature of the eviromet []. Eq.( models the well-kow priciples of commuicatio systems, that require a miimum ratio betwee received sigal power ad oise i order to allow effective decodig at the receiver side. The threshold β depeds o several desig aspects: the employed modulatio, the trasmissio / receptio schemes, the usage of error correctio schemes, etc. I a more realistic sceario, other odes will access the medium through the selected MAC strategy ad thus potetially geeratig iterferece to the trasmissio betwee odes i ad. Therefore, we eed to ehace Eq.( by itroducig the cotributio of the other odes ad obtaiig the followig relatioship: Pi ( T g P L x ( MAC k ( k odes where g MAC is the MAC-level factor of merit of the etwork ad represets a multiplicative term (betwee 0 ad that models the mitigatio effects o iterferece of the specific MAC strategy employed for accessig the medium. It ca be easily see that for g MAC = (perfect schedulig: TDMA, FDMA, Eq.( becomes Eq.(. It should be oticed that Eq.( cotais both the costraits o the physical iterferece level ad medium access cotrol efficiecy to achieve successful trasmissio o a give lik. B. The Cosidered Sceario We restrict our aalysis to the case of a perfect wireless chael (the effect of trasmissio o the chael is oly atteuatio, characterized by the followig propagatio model: L x = x 3 (3 ( 6 A relevat aspect i the framework of ad-hoc etworks is the Medium Access Cotrol (MAC protocol, sice it defies the procedures for sharig the wireless medium commo to all termials belogig to the etwork. More i detail, access to commuicatio resources i ad-hoc etworks is based o two priciples: resource sharig (the wireless medium is shared amog odes i a eighborhood defied o the basis of the trasmissio rage ad space-divisio (iterferece is limited to eighborig odes. Several approaches have bee proposed so far, startig from ALOHA ad CSMA (for geeric radio commuicatios up to ad-hoc etwork specific protocols such as MACA [0], FAMA [], MACAW [], MACA-BI [3], etc. For a good, comprehesive ad up-todate review of existig medium access cotrol schemes for ad hoc etworks ad ope challeges, the reader should refer to [9]. The remaider of the paper is focused o a Slotted-ALOHA medium access cotrol strategy, because it is the simplest ad least computatioal extesive, while maitaiig the features of radom access ad distributed implemetatio of the moder schemes. Furthermore, the cosidered sceario allows to provide closed-form ad easy-to-iterpret solutios to the problem of plaig ad-hoc etworks. However, the framework remais valid also for other MAC schemes, give the possibility of estimatig the term g MAC i Eq.(. I Slotted-ALOHA [4], the available resources are partitioed ito equal-size time slots, ad odes are allowed to trasmit i ay slot without cotrol but oly o the begiig of each time slot. If ust oe ode accesses a give slot ad o errors are itroduced, trasmissio is successful ad the receiver seds a ackowledgemet back to the seder i order to cofirm the data. ACKs are ot cosidered i the paper, supposig they are very small ad have a very low probability of iterferig with other trasmissios. The advatage of the approach is that uder specific circumstaces which will be defied i the followig sectios it provides a simple yet effective exploitatio of the available resources, sice i priciple resources are assiged to the odes that eed them. I fact, i the case oly oe ode access the medium i a give time slot, it gais full access to the available chael. I order to evaluate g MAC, the probability that a give statio/ode accesses the shared chael i a give slot is labeled p (both slot time ad p are equal for ay ode. The probability that more tha ode accesses the medium durig a give slot (i.e. the probability of collisios is the give by N Pr{ collisios } = ( p (4 where N is the umber of odes i the local eighborhood (icludig ode i. As a cosequece, the odes that ca potetially iterfere with the trasmissio of ode i is N-. Startig from Eq.(4, it is possible to write N g = p (5 MAC [ ( ] Ad fially, by substitutig (5 i (, we fid a updated model for successful trasmissio, which cotais a set of physical ad MAC-level desig parameters: T P L ( xi Pk i N [ ( p ] L( x odes k (6

3 GRANELLI: A FRAMEWORK FOR ANALYSIS OF CONNECTIVITY AND PERFORMANCE BOUNDS IN AD HOC NETWORKS 37 Eq.(6 represets the costrait for successful trasmissio betwee ode i ad ode i the case of employig the slotted-aloha access disciplie. Sice odes are Poisso-distributed i the space with average λ, it is possible to estimate the maximum umber of potetially iterferig odes by cosiderig the oes withi the maximum trasmissio rage r max. From [] it is kow that, if L ( x = x, the eglectig mutual itereferece it is possible to evaluate r max as: P max β t r = I (7 m T where I m is the uitary distace costat (equal to meter. As a cosequece: P λ π max λ π β t N = r = I (8 m T 3. THEORETICAL RESULTS Eq.(6 is the same as the first coditio preseted i [, N γ = p, we obtai: Eq.(]. I fact, if we substitute ( Pk L( xk P i (9 T γ odes As a cosequece, it is possible to exploit the results preseted i [] as a startig poit ad geeralize for further aalysis of coectivity problems i ad-hoc etworks. A. Percolatio The mai result is that we ca apply the percolatio theory [3] to characterize coectivity of the ad-hoc etwork, i.e. to discrimiate uder which coditios the etwork is made of discoected clusters of odes or distat odes are eabled to trasfer data (full coectivity. More i details, we are iterested i studyig for which cofiguratios of the etwork parameters it is theoretically possible to set up log rage coectios. From results i [] we kow that: A percolatio threshold exists for γ = 0 ( p = 0 A percolatio threshold also exists for γ < γ, with γ < β. I this case, the value of γ ca oly be approximately determied through umerical simulatios. I our case, sice the case p = 0 is straightforward ad could be cosidered as a referece case, we focus o the secod case: N γ = p < γ (0 p > exp p < ( l( γ ( N ( l( γ l γ exp = exp ( N λπrmax Eq.( cofirms the empirical cosideratio that if p is too large, the o log rage commuicatio is possible due to the high umber of collisios o the wireless medium. By kowig it, it is possible to evaluate the critical probability of medium access by a sigle statio, give by: ( l γ p = exp (3 λπrmax Aother result is the critical ode desity above which coectivity is grated. I this case, startig from ( it is possible to fid that: l( γ λπ r > (4 max l p Pt λ > π β T Pt λ = π β T ( l l l l ( γ ( p ( γ ( p Figure depicts the behaviour of parameters of the etwork. (a (5 (6 p ad λ agaist the (b Fig.. Behaviour of p (a ad λ (b i the (λ,β ad (p,β spaces, respectively (P t =00mW, N 0 =0-6, =3, γ =0.0. B. Coectivity The maximum umber of odes N c that a ode ca be locally coected is give by []:

4 38 JOURNAL OF COMMUNICATIONS SOFTWARE AND SYSTEMS, VOL., NO., SEPTEMBER 005 Nc = β γ β Nc β [ ( ] λ πrmax p N [ ( p ] (7 (8 As a cosequece, we ca defie the coectio desity δ as the product betwee the ode desity λ ad the maximum umber of coectios N c of a sigle ode: coectios δ = λ N (9 c square meter The coectio desity represets the umber of coectios per uit area that ca be supported by the cosidered ad-hoc etwork. C. Perfomace Bouds Recallig Hartley-Shao s law for the capacity of a commuicatio chael [5], the capacity C of a chael ca be calculated i o the basis of the available badwidth B ad the sigal-to-oise ratio at the receiver S/N with the followig formula: C = B log ( S N (0 I the cosidered sceario, it is the possible to calculate the theoretical maximum capacity related to ay of the liks withi a trasmissio path (ed-to-ed EE as: = B log T P L Ci, ( xi Pk i [ ( ] ( N p L x odes k ( The total capacity over the etire trasmissio path is give by the miimum capacity o all the liks. Cosiderig the available ed-to-ed paths where Eq.(6 is verified o every poit-to-poit lik, it is possible to evaluate the ed-to-ed capacity of ay existig multi-hop path through the etwork: CE E = mi { Ci, } > B log( β ( i, Eq.( defies the miimum chael capacity associated to ay lik amog two odes i the ad-hoc etwork that ca be achieved i our sceario (i the case percolatio is verified ad thus it represets the miimum capacity over a established ed-to-ed path through the ad-hoc etwork. This result is geeral, sice it is valid as log as Eq.(6 holds. Aother iterestig result is the possibility of computig a estimate of the ed-to-ed bit error rate (BER by usig a method similar to the oe preseted i [6]. I our sceario, cosiderig a biary phase shift keyig (BPSK modulatio: BER E E Q T Pi [ g MAC ] Pk L( xk odes N h (3 where N h is the umber of hops over the ed-to-ed trasmissio path. Agai, it is possible to fid a lower boud for BER EE by usig the costrait i Eq.(6: BER E E,mi N [ Q( β ] h (4 Eq.( ad (4 allows oit estimatio of the chael bitrate ad the BER performace o the ed-to-ed data coectio i the cosidered wireless ad-hoc etwork. Kowig such iformatio ad the format of the MAC-level frame it is possible to derive the ed-to-ed frame error rate (FER EE. Furthermore, the trasmissio rate over the ed-toed data path will be derived from ( as = γ C (5 RE E MAC PHY E E where γ PHY itroduces sub-optimal performace (with respect to Hartley-Shao law due to physical level modulatio / codig schemes, while MAC takes ito accout reductio i data rate due to the employed MAC strategy. It must be uderlied that the proposed framework provides useful results also i the case of differet MAC strategies, as log as it remais possible to characterize the etwork behaviour cosistet with formula (. IV. NUMERICAL RESULTS AND DISCUSSION This sectio cotais umerical results used for uderliig some desig priciples for ad-hoc etworks. First, umerical results are related to the problem of achievig full etwork coectivity. The employed algorithm (implemeted i the Matlab eviromet [7] is the followig: the ode topology is radomly geerated usig a Poissoia distributio with parameter λ; for each ode, the potetial iterferece derivig by the odes i that time slot is computed ad the correspodig sigal-to-oise ratio for ay receivig ode is computed; if the sigal-to-oise is higher tha β, the coectio is available betwee the cosidered ode couple. (a (b

5 GRANELLI: A FRAMEWORK FOR ANALYSIS OF CONNECTIVITY AND PERFORMANCE BOUNDS IN AD HOC NETWORKS 39 Fig.. No. of coectios per ode agaist p : P t =0mW (a, P t =0µW (b. (a (b (c Fig. 3. Topology ad available coectios for differet cofiguratios of the etwork parameters: N=0, p=0., P t =0mW (a-b, N=00, p=0.05, P t =0µW (c-d. The other parameters are N 0 =0-6, =3, β=. (d Figure depicts the average umber of coectios per ode usig fixed trasmissio power ad variable probability of accessig the medium p. The differet power levels employed i the experimets are used to simulate two situatios: (a high trasmissio power, with maily siglehop coectios; (b low trasmissio power, with multi-hop paths. Coectivity is achieved i both cases for low values of p, with a sharper behaviour i case (a derivig from the impact of collisios eve by far odes. Furthermore, from the aalysis of Fig., it is possible to fid a approximate value of the percolatio threshold o the basis of the umber of odes i the etwork. Table I summarizes the empirical evaluatio of the threshold value p that eables etwork coectivity. It ca be oticed that i the case preseted i fig.-a, the critical value of p that eables coectivity decreases as the umber of odes (ad cosequetly collisios icreases, while i case (b the behaviour is differet, due to the fact that the rage of iterferece is much more reduced. For more clarity, figure 3 presets a sapshot of the ode topology ad the correspodig available liks. The proposed framework allows evaluatio of the impact of several trasmissio ad medium access cotrol parameters o the performace of the ad-hoc etwork. I the followig paragraphs, performace of the etwork i terms of ormalized ed-to-ed capacity (capacity divided by occupied badwidth ad bit error rate are preseted. For sake of geerality, results are preseted as fuctio of the MAC-level factor of merit g MAC (simply refereced as g for clarity. TABLE I. THRESHOLD VALUE P FOR ACHIEVING CONNECTIVITY. No. of odes Critical value of p 0, P t =0mW (fig.a >0.3 30, P t =0mW (fig.a 0.<p<0.5 50, P t =0mW (fig.a 0. 00, P t =0mW (fig.a 0.05<p<0. 0, P t =0µW (fig.b 0.05<p<0. 30, P t =0µW (fig.b 0.5<p<0. 50, P t =0µW (fig.b 0.05<p<0. 00, P t =0µW (fig.b 0.05<p<0. Figure 4 depicts the ormalized ed-to-ed capacity or spectral efficiecy [bits/sec Hz] (i.e. the ed-to-ed capacity

6 40 JOURNAL OF COMMUNICATIONS SOFTWARE AND SYSTEMS, VOL., NO., SEPTEMBER 005 per uit of frequecy for differet levels of the sigal-tooise ratio at the receiver. I the figure, the x-axis represets the ratio betwee the received power ad termal oise, which is cosidered fixed, while the impact of iterferece by other odes ad merit of MAC strategy are cosidered i differet curves. The ideal case cosiders perfect schedulig i access to the shared resource, which eglects mutual iterferece amog eiborig odes ad brigs to higher capacity; completely radom access is the worst case, where o coordiatio amog odes occurs. The figure uderlies that capacity variatio is at maximum 0%. Furthermore, as a compariso, capacity is reported i the case of fixed TDMA (Time Divisio Multiple Access, where resource are equally divided amog ad 4 users i the same eighborhood. Reduced performace i spectral efficecy is the price for complete ad rigid schedulig ad resource sharig leadig to perfect coordiatio i medium access ad o iterferece. cotrol, which suggest the usage either of a reliable MAC strategy ad wider rage, or a less complex MAC but also shorter rage. BER Ideal (g= g=0.5 g=0. Radom (g= Sigal-to-Noise ratio [db] Fig. 5. BER performace agaist sigal-to-oise ratio for differet levels of MAC performace. Aother relevat outcome of the proposed umerical aalysis is that MAC performace has limited impact i the ed-to-ed capacity of the etwork, while it dramatically affects trasmissio performace i terms of the bit error ratio. Fig. 4. Normalized capacity of the etwork for differet sigal-tooise ratios ad differet MAC performace. Fig. 4 allows to coclude that i the cosidered sceario the factor of merit of the selected MAC strategy does ot impact much o the ed-to-ed capacity of the ad-hoc etwork (aroud 0% at most, while the philosophy employed by the MAC protocol is more relevat: resource reservatio / preassigmet (like i TDMA brig less advatage tha cotetio-based solutios (upper curves. Figure 5 provides a iterestig isight o the impact of the MAC factor of merit o the BER performace of the multihop etwork. By usig Eq. (3 it is possible to uderlie that the employmet of a reliable medium access cotrol strategy ca dramatically improve the performace of the ad-hoc etwork, eve of several orders of magitude. Summarizig, the MAC factor of merit (or probability of accessig the medium, i the cosidered slotted-aloha case oitly with physical trasmissio parameters has a strog impact o the coectivity properties of the resultig ad-hoc etwork (see fig. ad table I. This is due to the critical trade-off betwee trasmissio rage ad iterferece V. CONCLUSIONS AND FUTURE WORK The paper itroduced a framework for the theoretical aalysis of coectivity ad performace issues i a wireless ad-hoc etwork. The applicatio of the percolatio theory oitly with commuicatio theory priciples allowed to derive some relevat ad geeral aalytical relatioships amog the etwork parameters, ad the correspodig performace bouds i several scearios. The paper presets the applicatio of such framework i the case of a slotted- ALOHA medium access cotrol strategy. Numerical aalysis of the derived relatioships ad simulatios are used i order to derive desig priciples for ad-hoc etworks that cosider the impact of physical ad MAC-level parameters o etwork coectivity ad ed-to-ed performace. Mai outcome, show through graphical comparisos, is that the MAC factor of merit (i.e. the capability to coordiate medium access i a give eighborhood has a dramatic impact o the bit error ratio, while i terms of spectral efficiecy the higher gai depeds o usig cotetio rather tha schedulig i medium access. Future work will be devoted i studyig ad itroducig i the model additioal MAC- ad physical level characteristics, such as sub-optimal trasmissio performace, protocol overhead, ad lik-level retrasmissios. REFERENCES [] P. Gupta, P.R. Kumar, The capacity of Wireless Networks, IEEE Tras. o Iformatio Theory, Vol. 46, No., pp , March 000.

7 GRANELLI: A FRAMEWORK FOR ANALYSIS OF CONNECTIVITY AND PERFORMANCE BOUNDS IN AD HOC NETWORKS [] O. Dousse, F. Baccelli, P. Thira, Impact of Iterfereces o Coectivity i Ad Hoc Networks, INFOCOM 003, pp [3] G. Grimmett, Percolatio, Spriger, 999. [4] W. Stalligs, Data ad Computer Commuicatios, MacMilla, VI ed., 003. [5] A.B. Carlso, Commuicatio Systems, McGraw-Hill, Chapter 5, pp [6] G. Ferrari, O.K. Toguz, Performace of Ad Hoc Wireless Networks with Aloha ad PR-CSMA MAC Protocols, GLOBECOM 003, Sa Fracisco (U.S.A., pp [7] Mathworks, [8] IETF Maet charter, [9] Hogqiag Zhai, Jiafeg Wag, Xiag Che, ad Yuguag Fag, Medium Access Cotrol i Mobile Ad Hoc Networks: Challeges ad Solutios, WCMC 005, 005. [0] P. Kar, MACA: a ew chael access method for packet radio, 9th Computer Networkig Coferece, pp , ARRL/CRRL Amateur Radio, 990. [] C. L. Fullmer ad J. Garcia-Lua-Aceves, Floor acquisitio multiple access (FAMA for packet-radio etworks, SIGCOMM 95, pp ACM, August 995. Cambridge, MA (USA. [] V. Bharghava, A. Demers, S. Sheker, ad L. Zhag, MACAW: A media access protocol for wireless LAN s, SIGCOMM 94, pp. 5. ACM, 994. [3] F. Talucci ad M. Gerla, MACA-BI (MACA by ivitatio: A receiver orieted access protocol for wireless multihop etworks, PIMRC 97, Helsiki (Filad. 4 Fabrizio Graelli was bor i Geoa i 97. He received the «Laurea» (M.Sc. degree i Electroic Egieerig from the Uiversity of Geoa, Italy, i 997, with a thesis o video codig, awarded with the TELECOM Italy prize, ad the Ph.D. i Telecommuicatios from the same uiversity, i 00. Sice 000 he is carryig o his teachig activity as Assistat Professor i Telecommuicatios at the Dept. of Iformatio ad Commuicatio Techology Uiversity of Treto (Italy. I August 004, he was visitig professor at the State Uiversity of Campias (Brasil. He is author or co-author of more tha 45 papers published i iteratioal ourals, books ad cofereces. Dr. Graelli is guest-editor of ACM Joural o Mobile Networks ad Applicatios, special issue o WLAN Optimizatio at the MAC ad Network Levels ad CoChair of 0th IEEE Workshop o Computer-Aided Modelig, Aalysis, ad Desig of Commuicatio Liks ad Networks (CAMAD 04. Dr. Graelli is Geeral Vice-Chair of the First Iteratioal Coferece o Wireless Iteret (WICON 05 ad Geeral Chair of th IEEE Workshop o Computer-Aided Modelig, Aalysis, ad Desig of Commuicatio Liks ad Networks (CAMAD 06.