Dynamic Topology Control to Reduce Interference in MANETs

Transcription

1 Dynamic Topology Contol to Reduce Intefeence in MANETs Hwee Xian TAN 1,2 and Winston K. G. SEAH 2,1 {stuhxt, 1 Depatment of Compute Science, School of Computing, National Univesity of Singapoe 3 Science Dive 2, Singapoe Institute fo Infocomm Reseach, Agency fo Science Technology and Reseach 21 Heng Mui Keng Teace, Singapoe ABSTRACT The pesence of tansient netwok links and mobility of nodes in MANETs (Mobile Ad Hoc Netwoks) poses a hefty challenge fo such netwoks to scale and pefom efficiently when subjected to vaying netwok conditions. As such, nodes in a MANET can make use of topology contol which is the delibeate adjustment of cetain system paametes such as antenna diection and tansmission powe to fom a paticula netwok topology as a means of impoving the outing potocol pefomance. In this pape, we study some of the methods of topology contol used in pevious wok and popose the Citical Neighbou (CN) scheme, which adaptively adjusts the tansmission powe of individual nodes accoding to oute and taffic demands, to educe the level of intefeence amongst nodes in the netwok. This helps to incease spatial euse of the channel bandwidth and educes the numbe of collisions, esulting in highe thoughput and lowe end-to-end delay. We implement ou CN scheme on, and simulation esults highlight that ou scheme is able to achieve bette pefomance than the unmodified vesion. Keywods: Topology contol, adaptive potocols, scalability, tansmission ange, intefeence. 1 INTRODUCTION Mobile Ad Hoc Netwoks (MANETs) ae wieless netwoks that offe multi-hop connectivity between selfconfiguing and self-oganizing mobile hosts. These netwoks ae chaacteized by [1] dynamic netwok topology, lack of cental administation and limited esouces such as powe and bandwidth. The netwok size of a MANET is given by the total numbe of nodes in the netwok, which is a fixed numbe (assuming that no nodes ente o leave the netwok). Lage netwoks ae depicted by a sizeable numbe of nodes of up to 1, while small netwoks coespond to netwoks with vey few nodes within the same teain size. While the numbe of nodes may be a good estimate of netwok density in homogenously distibuted netwoks, this value may not be indicative of any specific netwok chaacteistics in a heteogeneous netwok whee nodes ae andomly positioned in the teain. Fom the liteatue we can identify a few key poblems associated with netwok topology, that ae faced by outing potocols in ad hoc netwoks with vaying netwok sizes and densities. In spase netwoks with low node densities, netwok patitions [2] may be fomed when mobile hosts move with contasting pattens and cause the netwok to divide into two o moe disconnected potions. This can lead to low connectivity and lack of outes to tageted destinations, esulting in highe packet loss and highe contol ovehead. Although path lengths might be compaatively shote, this is because outes to futhe destinations cannot be discoveed. The thoughput is also lowe in such patitioned netwoks. As node densities incease, it may be possible to establish path outes to destinations that ae futhe away. Inevitably, the fequency of link beakages within active outes will also incease. Moe Route Eo (RERR) messages will have to be tansmitted to nodes which utilize these boken links, esulting in highe ate of occuence of oute epais. These factos will lead to highe contol ovehead, inceased congestion, geate contention fo bandwidth and lowe packet delivey atio. Dense netwoks ae also chaacteized by highe node degees, whee each node has moe neighbous within its tansmission ange. This implies highe connectivity between nodes in the netwok, which can lowe the mean numbe of hops needed between a souce and its destination and impove the data delivey atio. Howeve, a high node degee can also esult in moe collisions between neighbouing nodes, which means that moe enegy is wasted at the adio level. Nevetheless, the netwok size and density ae often invaiable paametes in an ad hoc envionment. The main focus of pevious wok has been to educe the outing oveheads caused by outing potocols in dense netwoks that have unifom node densities and hieachical outing methods which can incu additional ovehead. These methods do not take into account the dynamic netwok topologies due to node mobility in MANETs, and ae inefficient fo heteogeneous netwoks which may not have unifom node distibutions. In this pape, we develop a scheme to impove the scalability of a outing potocol using topology contol. The poposed Citical Neighbou (CN) scheme dynamically adjusts the tansmission powe of individual nodes accoding to oute and taffic demands. This educes the

2 amount of ovelapping intefeences between nodes and allows moe simultaneous tansfe of packets in the netwok. Hence, pefomance of the potocol can be geatly impoved, especially fo dense netwoks. The est of the pape is oganized as follows: The next section discusses elated wok and motivation. Section 3 descibes some of the pevious wok on topology contol. In Section 4, we povide a detailed outline of ou CN scheme. Simulation esults and analysis ae pesented in Section 5. We conclude with diections fo futue wok in Section 6. 2 RELATED WORK AND MOTIVATION Thee has been much published wok in the liteatue which studies the issue of scalability faced by MANETs. Lee, Roye and Pekins [3] examine the scalability of AODV using diffeent combinations of modifications which can be integated into geneic eactive outing potocols. These modifications include the expanding ing seach fo Route Requests (RREQs), quey localization and local epai of link beaks in active outes. Simulation esults show that local epai is moe effective in impoving the pefomance of lage scale netwoks of up to 1 nodes, but it may waste pocessing powe and incu additional ovehead because moe than one epais fo the same oute can occu concuently. [4] intoduces two new schemes Fisheye State Routing (FSR) and Hieachical State Routing (HSR) which aim to addess the shotcomings of existing poactive and eactive potocols via a hieachical appoach. As compaed to flat, table diven outing schemes, the poposed solutions ae able to scale bette at the cost of inceased outing inaccuacy and complexity. As compaed to eactive schemes, the solutions ae able to povide lowe contol ovehead and lowe latency but at the cost of outing table stoage ovehead. Passive Clusteing (PC) and Landmak Routing ae suggested in [5] as techniques to ovecome scalability poblems faced by conventional poactive link state outing potocols. Passive clusteing is a cluste fomation potocol that can dynamically econfigue clustes duing mobility and topological changes. Landmak outing uses tuncated local outing tables and summaized outing infomation fo emote goups of nodes. Both techniques help to educe the outing table size and the contol ovehead effectively fo dense and lage scale netwoks. The existing methods which attempt to impove the scalability of outing potocols use hieachical outing stategies o mechanisms which incu additional oveheads. In addition, they ae unable to adapt to apid changes in netwok envionments, which is typical of MANETs. As such, thee is a need fo an adaptive scheme that is sensitive to netwok dynamics and can impove the scalability of MANET outing potocols. 3 TOPOLOGY CONTROL IN MANETS The netwok topology is defined as the set of communication links between node pais used explicitly o implicitly by a outing mechanism. Due to the indeteministic natue of MANETs, the topology of the netwok is often dependent on a numbe of dynamic factos such as taffic pattens, node mobility, noise, intefeence, antenna sensitivity and tansmission powe of nodes. Methods fo netwok topology contol and thei effects on netwok pefomance have been extensively studied in ecent yeas. The topology of a netwok can have a temendous effect on the pefomance of the netwok because it influences the way which packets ae outed to thei destinations. Spase netwoks face the possibility of being patitioned while dense netwoks ae subject to high collision ates and bandwidth contention amongst nodes. Futhemoe, nodes in MANETs ae constantly in motion, making it hade to pedict distibution pattens and handle outing decisions. Ramanathan and Rosales-Hain [6] discuss two decentalized heuistics that adjust the tansmission powes of nodes accoding to topological changes, and which attempt to maintain a connected topology using minimum powe. Howeve, these heuistics do not pefom well in wost-case scenaios and povide poo appoximations in powe minimization. Jia, Li and Du [7] examine the enegy efficient QoS (Quality of Sevice) topology contol poblem and ties to find an appopiate netwok topology that can meet cetain QoS equiements such as bandwidth and delay constaints while minimizing the maximum tansmitting ange of nodes. Thee ae pos and cons elated to the extent of tansmission anges coveed by nodes in MANETs. A node with high tansmission ange usually has a highe value of node degee which is the total numbe of nodes that ae within the tansmission adius. This means that thee ae moe one-hop neighbous that can be eached by that paticula node, which helps to decease the aveage path length and popagation delay duing data tansfe. Howeve, a high node degee can also lead to geate contention fo bandwidth, which educes the spatial euse of the channel access by nodes in the netwok and hence lowes the thoughput. Convesely, nodes with low tansmission anges tend to have lowe node degees and lowe connectivity. Although thee is highe spatial euse of the channel access, netwok patitions may fom, which could lead to lack of outes and lowe thoughput. As such, it appeas that thee ought to be an optimal node degee which netwoks should adhee to fo maximum pefomance. Accoding to Kleinock and Silveste [8], the optimal numbe of neighbous fo each node in a stationay netwok is six, and the tansmission adius should be adjusted accodingly to allow each node to maintain this node degee. Howeve, this analysis does not take into

3 account the mobility effects that mobile hosts in MANETs expeience. The effects of tansmission powe on the pefomance of MANETs ae studied in [9], which also attempts to detemine the optimum node density fo maximizing thoughput. Simulations pefomed on AODV show that while thee does not exist a global optimum density, the node density should be popotional to the ate of mobility to achieve bette netwok pefomance. Futhemoe, inceasing the powe tansmission of nodes at high mobilities can esult in a highe pecentage of data packets eaching thei destinations, which consequently leads to highe thoughput. Nilson [1] descibes the pefomance analysis of taffic load and node density in ad hoc netwoks. Though simulations pefomed using the Modified Random Diection model, it was shown that the optimum node density depends on both the taffic load and the mobility of the nodes. In spase netwoks, high delivey ates can be achieved up to a cetain theshold (i.e. taffic satuation point), afte which it stats to decline. In dense netwoks, thee is usually highe packet delivey atio at the cost of moe powe consumption and channel bandwidth. Most of the pevious wok done on topology contol have been used to optimize powe consumption in netwoks, which is undeniably an impotant constaint faced by ad hoc netwoks. Howeve, this pape focuses on how topology contol of the netwok can be used to impove the scalability of the outing potocol by adjusting the tansmission powe of nodes accoding to netwok dynamics. 4 CRITICAL NEIGHBOUR SCHEME 4.1 Peliminaies We classify the neighbouing nodes of the node of inteest into two categoies: Citical nodes, C[x] = {set of nodes that ae equied to tansmit data} Non-citical nodes, N[x] = {set of nodes that ae not equied to tansmit data} Theefoe, the set of all neighbouing nodes of an abitay node x can be given as the union of the set of citical and non-citical nodes: A[x] = C[x] U N[x] Futhemoe, C[x] N[x] = Ø, which means that the set of citical and non-citical nodes ae two mutually exclusive sets. We use the Gound Reflection (o Two-Ray) model in ou simulations, which consides both the diect path and the gound eflected popagation path between the tansmitte and the eceive: P = P t h t 2 h 2 d 4 G t G whee P = eceived powe; P t = tansmitted powe; G t = antenna gain at the tansmitte; G = antenna gain at the eceive; h t = height of the tansmitte antenna; and h = height of the eceive antenna. We hence deive the estimated distance between node x and its i th citical neighbou as: E dist [x i ] = 4 ( P t h 2 t h 2 G t G P ) Taking G t = G = dbm = 1, E dist [x i ] = 4 ( P t h 2 2 t h P ) While evaluating the estimated distance as above, we keep in mind that this value may not be a tue eflection of the actual distance between any two nodes. This is because two nodes in close poximity may also be subject to intefeences fom the noise in the envionment, as well as fom suounding nodes that ae tansmitting data o contol packets. We also define the citical tansmission ange of an abitay node as the minimum distance equied to keep the set of citical nodes C[x] within connectivity: C txn [x] = Max(E dist [x i ], E dist [x i+1 ],, E dist [x n ]), 1 i n whee n is the total numbe of nodes belonging to the set C[x]; and E dist [x i ] is the estimated citical distance between node x and its i th citical neighbou. 4.2 Ou Adaptive Algoithm Nodes in wieless netwoks ae subject to intefeences fom neighbouing nodes within the tansmission and intefeence anges. These neighbouing nodes may cause highe delay due to contention of bandwidth, as well as highe packet loss due to collisions. The CN scheme (see Figue 2) attempts to educe the intefeence caused by adjacent nodes that ae not pat of the fowading outes of a paticula node. This is commonly known as the hidden/exposed teminal poblem as shown in Figue 1, whee a eceiving node may expeience intefeences fom othe adjacent nodes, esulting in packet loss. Thee have been attempts to solve this poblem in the liteatue, one of which includes the RTS/CTS dialogue which necessitates handshakes between the tansmitting and eceiving nodes that pecedes the actual tansmission. In the RTS/CTS scheme, a node that wants to tansmit data has to send a Request To Send (RTS) contol packet, which defes all nodes that hea the RTS fom accessing the channel fo a specified time peiod. The destination node esponds with a Clea To Send (CTS) contol packet upon eception of the RTS. Howeve, the use of the RTS/CTS dialogue will only eliminate collusions caused by nodes within the tansmission ange and not the intefeence ange.

4 Figue 1: Hidden/Exposed Teminal Poblem Tansmission ange Intefeence ange Figue 2: Citical Neighbou (CN) scheme The Citical Neighbou scheme complements the RTS/CTS dialogue to educe the collisions within the intefeence ange. By educing the tansmission ange of nodes such that they each the minimum distance equied to maintain connectivity with the neighbouing nodes that ae pat of the active outes, unnecessay intefeences expeienced by othe neighbouing nodes can be minimized. Ou adaptive Citical Neighbou (CN) scheme compises of thee main components: Measuement of estimated citical ange; Estimation of the ideal powe; and Adjustment of the ideal powe based on constaints. When peiodic beacons such as Hello packets in AODV ae eceived, the node will calculate the citical tansmission ange, C txn. This is then used to estimate and adjust the tansmission powe so that the pefomance of the outing potocol can be impoved. As such, we estimate the coesponding ideal tansmission powe as follows: C txn 4 P ideal = P min_ h 2 t h 2 G t G toleance_facto Since G t = G = dbm = 1, P ideal = P min_ C txn 4 h 2 2 t h toleance_facto whee P min_ is the minimum signal stength fo a packet to be eceived coectly; and toleance_facto is a pecentage which allows fo node mobility, as well as some noise and intefeences in the envionment. Afte obtaining the ideal powe, it is checked to ensue that it emains within a low powe theshold and a high powe theshold. This is to avoid nodes that ae cuently not involved in the sending of any data packets fom adjusting thei tansmission powes to a value that is too low to be of use late on, as well as to eliminate the possibility that nodes will adjust thei tansmission powes to a value that is too high, which could waste enegy esouces. Ou CN scheme is summaized in Figue 3. /* Handling of Hello packets */ FOR each Hello packet eceived { IF it is eceived fom a citical neighbou { Detemine the citical tansmission ange C txn ; Detemine the ideal tansmission powe P ideal ; IF P ideal is within constaints Adjust the tansmission powe accodingly; } Update outing and neighbou tables; } Figue 3: Pseudocode fo CN scheme 5 SIMULATION RESULTS & ANALYSIS We implemented ou CN scheme on, which is an enhanced vesion of AODV with Local Repai and un simulations on GloMoSim [11], which povides a scalable simulation platfom fo wieless netwoks. The Random Waypoint mobility model is used with minimum and maximum speeds of 1 ms -1 and 2 ms -1 espectively and the pause time is set to 3s. Nodes ae unifomly distibuted and CBR (Constant Bit Rate) taffic with data packets of size 512 bytes ae tansmitted at an aival ate of 1 packets pe second. The teain size is set to 2 2 metes. Each scenaio is also un with at least 5 diffeent seed numbes to minimize any abitay andomness. Ou simulations ae evaluated accoding to the following pefomance measues: Thoughput total numbe of successfully deliveed data (in kilobytes); Packet delivey atio total numbe of data packets eceived as a faction of the total numbe of data packets oiginated fom all the nodes in the netwok; End to end delay aveage time taken to tansmit a packet fom souce to destination; and Nomalized outing ovehead total numbe of contol packets as a faction of the total thoughput. Figue 4 to Figue 7 show the compaative esults between the pefomances of and the adaptive enhanced with the CN scheme unde vaying netwok sizes. In Figue 4, we obseve highe thoughput fo lage netwok sizes of moe than 5 nodes. Lage netwoks with many nodes tend to have highe node densities. This esults in moe ovelapping tansmission anges amongst the nodes, inceased numbe of collisions and highe packet loss. In ou CN scheme, we adaptively educe the tansmission ange of nodes such that thee is less ovelapping intefeences. This helps to incease the spatial euse of the channel bandwidth. Consequently, thee is highe thoughput in the netwok, which also esults in highe packet delivey atio as shown in Figue 5. Howeve, fo small netwok sizes of 5 nodes, the aveage netwok density is typically small and educing the tansmission adii of the nodes may esult in netwok patitions.

5 Figue 6 shows the aveage delay fo the tansmission of data in the netwok. Unde the CN scheme, the educed tansmission anges allow fo moe spatial euse of the channel and data packets can each thei destinations faste. The nomalized outing ovehead, which is indicative of the efficiency of a outing potocol, is illustated in Figue 7. Ou adaptive CN scheme esults in lesse nomalized outing ovehead because each node has a smalle node degee. As such, duing popagation of contol packets, thee ae lesse nodes within the tansmission ange of the boadcasting node, esulting in less contol ovehead. thoughput (KB) pkt delivey atio delay (s) CBR connections, 3s pause time with CN numbe of nodes Figue 4: Thoughput vs netwok size 3 CBR connections, 3s pause time with CN numbe of nodes Figue 5: Packet delivey atio vs netwok size nomalized outing ovehead 3 CBR connections, 3s pause time with CN numbe of nodes Figue 6: End to end delay vs netwok size 3 CBR connections, 3s pause time 25 2 with CN numbe of nodes Figue 7: Nomalized outing ovehead vs netwok size We also compaed the pefomance of and ou adaptive schemes unde vaying taffic conditions. 2 nodes wee unifomly distibuted and simulated unde the Random Waypoint mobility model with the same mobility paametes as befoe. CBR taffic is also used with data packet sizes of 512 bytes tansmitted at intevals of 1 milliseconds. In Figue 8, we obseve that the thoughput of with the CN scheme is much highe than that of fo vaying. Unde the CN scheme, the tansmission ange of nodes is typically smalle. As such, moe nodes ae able to tansmit data at the same time and thus incease the thoughput of the netwok. The smalle tansmission ange also esults in a smalle intefeence ange; theefoe less packets ae lost o coupted, esulting in highe packet delivey atios as seen in Figue 9. Figue 1 and Figue 11 show the aveage end to end delay and the nomalized outing ovehead espectively. Ou CN scheme is able to pefom bette because of the inceased spatial euse of the channel esulting fom the educed tansmission and intefeence anges of individual nodes. In addition, this also esults in less nomalized outing ovehead because less nodes eceive boadcasted contol packets duing oute fomation and maintenance. We have used P min_ = -81 dbm and toleance_facto = 1.1 in all ou simulations. The value of P min_ is the minimum powe of the eceived packet in ode fo it to be eceived coectly by the destination (based on static paametes used fo the antenna gains and diection in GloMoSim). The toleance facto of 1.1 allows fo a 1% inaccuacy in detemining the citical tansmission ange, which could be influenced by both intefeences in the envionment (due to fading and noise) and node mobility. Having an allowance of 1% diffeence in the tansmission powe allows the nodes to maintain the same link even if they move about 1 metes (the minimum and maximum speeds of the cuent mobility model used in ou simulations ae 1 ms -1 and 2 ms -1 espectively) futhe away fom the node of inteest afte the citical tansmission ange was calculated in the pevious second. This value should be adjusted adaptively accoding to the speeds which nodes ae expected to move unde diffeent scenaios. thoughput (KB) nodes, 3s pause time with CN Figue 8: Thoughput vs data load

6 pkt delivey atio delay (s) nomalized outing ovehead 2 nodes, 3s pause time with CN Figue 9: Packet delivey atio vs data load 2 nodes, 3s pause time with CN Figue 1: End to end delay vs data load nodes, 3s pause time with CN Figue 11: Nomalized outing ovehead vs data load 6 CONCLUSION AND FUTURE WORK In this pape, we use dynamic topology contol to impove the scalability of outing potocols by educing intefeence via the Citical Neighbou (CN) scheme. The tansmission powe (and hence the tansmission ange) of nodes ae adjusted so that thee is educed ovelapping intefeence amongst neighbouing nodes. This not only helps to incease the spatial euse of the channel bandwidth, but also educes collisions among tansmitted packets, esulting in highe thoughput and bette pefomance. We have implemented ou schemes on top of, an enhanced vesion of AODV with local epai, and simulation esults have highlighted that ou adaptive schemes can impove seveal pefomance metics such as thoughput, aveage delay and nomalized outing ovehead. As pat of futue wok, we will study the effects of the CN scheme in lage netwok sizes. Ou continued eseach effots include the investigation of othe adaptive mechanisms to impove the scalability and pefomance of typical eactive outing potocols. REFERENCES [1] S. Coson and J. Macke, MANET: Routing Potocol Pefomance Issues and Evaluation Consideations, RFC251, [2] K. Wang and B. Li, Goup Mobility and Patition Pediction in Wieless Ad Hoc Netwoks, Poceedings of the IEEE Intenational Confeence on Communications (ICC22), New Yok City, New Yok, Ap 28 May 2, 22. [3] S. Lee, E. M. Roye and C. E. Pekins, Scalability Study of the Ad Hoc On Demand Distance Vecto Routing Potocol, Intenational Jounal of Netwok Management, Vol. 13, No. 2, Ma 23. [4] A. Iwata, C. Chiang, G. Pei, M. Gela and T. Chen, Scalable Routing Stategies fo Ad Hoc Wieless Netwoks, IEEE Jounal on Selected Aeas in Communications, Vol. 7, No. 8, Aug [5] X. Hong, M. Gela, Y. Xi, K. Xu and T. Kwon, Scalable Ad Hoc Routing in Lage, Dense Wieless Netwoks Using Clusteing and Landmaks, Poceedings of the IEEE Intenational Confeence on Communications, (ICC22), New Yok City, New Yok, Ap 22. [6] R. Ramanathan and R. Rosales-Hain, Topology Contol of Multihop Wieless Netwoks using Tansmit Powe Adjustment, Poceedings of the IEEE Confeence on Compute Communications (INFOCOM2), Ma 2. [7] X. Jia, D. Li and D. Du, QoS Topology Contol in Ad Hoc Wieless Netwoks, Poceedings of the IEEE Confeence on Compute Communications (INFOCOM24), Ma 24. [8] L. Kleinock and J. Silveste, Optimum Tansmission Radii fo Packet Radio Netwoks o Why Six is a Magic Numbe, Poceedings of the IEEE National Telecommunications Confeence, Bimingham, Alabama, Dec [9] E. M. Roye, P. M. Smith and L. E. Mose, An Analysis of the Optimum Node Density fo Ad Hoc Mobile Netwoks, Poceedings of the IEEE Intenational Confeence on Communications (INFOCOM21), Helsinki, Finland, 21. [1] A. Nilsson, Pefomance Analysis of Taffic Load and Density in Ad Hoc Netwoks, Poceedings of the 5 th Euopean Wieless 24: Mobile and Wieless Systems beyond 3G, Bacelona, Spain, Feb 24-27, 24. [11] L. Bajaj, M. Takai, R. Ahuja, K. Tang, R. Bagodia and M. Gela, GloMoSim: A Scalable Netwok Simulation Envionment, UCLA Compute Science Depatment Technical Repot 9927, 1999.