Interference Aware Channel Assignment Scheme in Multichannel Wireless Mesh Networks

Transcription

1 Iterferece Aware Chael Assigmet Scheme i Multichael Wireless Mesh Networks Sumyeg Kim Departmet of Computer Software Egieerig Kumoh Natioal Istitute of Techology Gum South Korea Abstract Wireless mesh etworks (WMNs) are gaiig sigificat attetio to provide wireless broadbad service. Nodes i a wireless mesh etwork ca commuicate with each other directly or through oe or more itermediate odes. Because of multi-hop trasmissios with multiple cotedig ad competig chaels, performace of wireless mesh etworks decreases. Supportig high performace is a importat challege i multi-hop mesh etworks. Nassiri et al. proposed a Molecular MAC protocol for autoomic assigmet ad use of multiple chaels to improve etwork performace. I the Molecular MAC protocol, odes are either uclei or electros i a atom. Neighborig atoms use orthogoal chaels to operate i parallel data trasmissios. Each ucleus selects a idle chael that is ot curretly beig occupied by its eighborig atoms with the assistace from electros i the same atom. However, this protocol has the followig drawback; sice a ucleus allocates a chael with help from the electros i its ow trasmissio rage, it is ot able to recogize the existece of those atoms i the iterferece rage. Therefore, allocatig the same chael to eighborig atoms results i the deterioratio of etwork performace. I order to resolve this problem, we propose a chael allocatio scheme with the iterferece issue take ito accout. Based o various simulatio results, the proposed scheme was verified that differet chaels could be allocated to those eighborig atoms i the iterferece rage. Keywords iterferece; chael assigmet; multichael; mesh etwork I. INTRODUCTION A wireless mesh etwork (WMN) is a cost-effective access etwork architecture. It is a promisig wireless techology for umerous applicatios. It is gaiig sigificat attetio as a possible way for Iteret service providers (ISPs) ad carriers i order to provide wireless broadbad service. I WMNs, odes are cosisted of mesh routers ad mesh cliets [1]. Mesh routers have miimal mobility ad form the backboe of WMNs. Whe a ode is withi the trasmissio rage of aother ode, they are cosidered as eighbors, ad there is a wireless lik betwee them. Some odes, called gateways, are coected to the wired etwork, which coects the WMNs to rest of the Iteret. The packets set by ed users travel through wireless mesh etworks over multiple hops. Ad gateway odes relay traffic to ad from the wired Iteret. The performace of multi-hop commuicatio quickly reduces as the umber of hops becomes larger due to itra-flow ad iter-flow iterfereces [2-]. This is because a wireless lik is shared amog eighborig odes. The itra-flow iterferece occurs because odes are i each other's iterferece rage alog the path of the same flow. Nodes experiece differet amout of chael cotetio as show i Fig. 1. Iter-flow iterferece occurs sice multiple flows o the same chaels betwee differet routig paths operate ad compete for the medium as show i Fig Fig. 1. Itra-flow iterferece Fig. 2. Iter-flow iterferece 3 Iter-flow iterferece betwee 3 ad With a sigle chael, odes operate o the same chael. Therefore, etwork performace decreases. Nodes ca be equipped with multiple radios ad chaels. This meas there is a uique frequecy used for each wireless hop, ad thus eables separatio of wireless isio domai ad reduces the iterferece ad cotetio. This ca sigificatly improve etwork performace without badwidth degradatio. The desig of the MAC protocol is the most likely challege i WMNs. A iterestig problem i WMNs is how to efficietly utilize multiple chaels. Several MAC protocols for hadlig multiple chaels have bee proposed i the literature [5-17]. The algorithms proposed i [5] select chaels for the mesh radios to miimize iterferece withi the mesh etwork ad betwee the mesh etwork ad co-located wireless etworks. A ew method for the iterferece estimatio is suggested. All available chaels are periodically moitored by each mesh ode ad measured iformatio of iteral chael usage ad exteral iterferece is shared with mesh odes withi This research was supported by Basic Sciece Research Program through the Natioal Research Foudatio of Korea (NRF) fuded by the Miistry of Educatio, Sciece ad Techology ( ). 75 P a g e

2 iterferece rage. Duarte et al. use game theory to desig a systematic approach to utilize partially overlapped chaels i WMNs while miimizig the adverse effect of adjacet chael iterferece [6]. I [7], both cetralized ad distributed algorithms are preseted, which aim to miimize the umber of pairs of liks that are iterferig. Molecular MAC protocol was proposed to orgaize the mesh etwork accordig to the molecular aalogy [-12]. It divides the etwork ito atoms with ucleus odes operatig o fixed chaels ad electros that dyamically switch chaels betwee eighbor uclei. Electros ca be shared by umerous atoms. Whe a ucleus selects a chael, it receives help from the electros i its ow trasmissio rage. The, electros deliver the iformatio about those chaels used by their paret atoms to the ucleus. Because a atom caot be aware of the existece of other eighborig atoms if there are o electros shared amog them, the ucleus may allocate the same chael to such atoms. Although the proposed scheme aturally adopts a fudametal operatio priciple of the Molecular MAC protocol, it modifies a chael allocatio method. The proposed scheme is somewhat similar to the Molecular MAC protocol method i that a chael is selected by the assistace of electros. The, a isio probability is applied to allocate a ew chael i order to recogize ay iterferece ad avoid ay overlapped chael allocatio. The paper is orgaized as follows. We give a brief itroductio of the Molecular MAC protocol ad its chael assigmet problem i Sectio II. I Sectio III, the proposed scheme is preseted i detail. I Sectio IV, performace studies are carried out through simulatio results. Fially, we draw a coclusio i Sectio V. II. RELATED WORK This Sectio is desiged to briefly specify the Molecular MAC protocol ad the touch o certai problem relatig to the chael allocatio. A. Molecular MAC Protocol The IEEE wireless etwork fuctios well i the ifrastructure mode. I additio, it ca provide a fair badwidth to all users by slightly modifyig a chael access method. Nevertheless, the IEEE etwork may icur umerous problems o a multi-hop etwork. The Molecular MAC protocol expads the IEEE etwork i order to trasmit data packets o the multi-hop etwork effectively. Sice the IEEE access method works well o a sigle-hop etwork, the Molecular MAC protocol divides a wireless mesh etwork ito differet spatially distributed atoms. Each atom uses a chael ot used by other eighborig atoms. A atom is composed of oe ucleus ad several electros, ad a ucleus selects a chael to be used by its ow atom. Ay ode withi a atom s boudary plays a role as a electro ad belogs to a eighborig atom. A electro directly commuicates with its uclei; however, sice there is o direct lik betwee electros, a direct commuicatio amog them caot be implemeted. I additio, due to o direct lik amog ucle direct commuicatios amog them are also ot possible. Therefore, the commuicatio amog eighborig electros ca be hadled by uclei while the commuicatio amog uclei ca be executed by eighborig electros. I the Molecular MAC protocol, each ode is assiged with the role as a ucleus or a electro while each ucleus selects a chael to be used by its ow atom. Accordigly, each ode forms a shortest path-spaig tree to a gateway ode liked to a wired Iteret. After a tree is formed, the odes with a eveumbered depth ad a odd-umbered depth are assiged with the roles of a ucleus ad a electro, respectively. After such roles are assiged, each ucleus requests its ow electros for chael iformatio. Chael iformatio icludes a list of active chaels ad their activities. A list of active chaels icludes the umbers of active chaels i the correspodig electros paret atoms. A chael activity is a parameter that is desiged to idicate how may data packets are trasmitted o each chael i the active chael list, which is expressed i the umber of packets trasmitted. Requested by a ucleus, each electro makes up a list of active chaels, measures each chael s activities, ad accordigly respods to the uclei. The, a ucleus receives certai resposes from all eighborig electros ad accordigly selects a chael accordig to a subsequet rule. 1) After a list of active chaels is received from all eighborig electros, a chael is radomly chose out of those o-active chaels. 2) If every chael is curretly occupied, a chael with the least activity is selected. Oce a ucleus allocates a chael, its eighborig electros use the chael. The correspodig electros use all chaels allocated by their atoms uclei. E1 E2 Atom 1 Atom 2 N1 E3 E N2 Chael 1 Chael 2 N Nucleus E Electro Fig. 3. Basic architecture of Molecular MAC protocol Fig. 3 illustrates a fudametal structure of the Molecular MAC protocol. As show i the figure, there are 2 atoms, 2 uclei ad 6 electros. The atom 1 icludes the ucleus N1 ad the electros (E1, E2, E3 ad E), ad uses the chael 1. The atom 2 icludes the ucleus N2 ad the electros (E3, E, E5 ad E6), ad uses the chael 2. The electros E3 ad E, shared by the two atoms, use both chael 1 ad chael 2. If a eighborig atom s ucleus requests iformatio for the allocatio of a chael, the correspodig electros trasmit their chael activity iformatio. Furthermore, the electros E1/E2, E3/E ad E5/E6 also trasmit the active chael lists icludig the chael 1, the chaels 1 & 2 ad the chael 2, respectively, to the ucleus. Oce all chaels are allocated, commuicatios are processed as follows: As show i the figure, whe certai data are to be trasmitted from E1 to E6, E5 E6 76 P a g e

3 E1 trasmits the data to N1 usig the chael 1 while N1 trasmits the date back to E3 or E. The, E3 or E trasmits the data to N2 usig the chael 2 while N2 trasmits the data back to E6. The eighborig electros, E1 ad E2, do ot directly commuicate with each other but commuicate via the ucleus N1. B. Chael Assigmet Problem i the Molecular MAC Protocol There is a problem associated with a chael allocatio scheme of the Molecular MAC protocol. The active chael list ad chael activity iformatio used by a ucleus for allocatig a chael are ot sufficiet. If there are o electros shared by eighborig atoms ad if such atoms exist i the same iterferece rage, the other atom s chael iformatio caot be idetified. Therefore, a idetical chael is allocated so that etwork performace ca be deteriorated due to the iterferece. I additio, sice each electro is located out of the other electro s trasmissio rage, the umber of packets trasmitted caot be properly measured, hece makig the chael activity iformatio iaccurate. As show i the Fig. 3, because the atoms 1 ad 2 share the electros E3 ad E, oe atom ca idetify the other atom s active chael whe the uclei N1 ad N2 allocate a chael. I this way, a overlapped chael allocatio ca be avoided. That is, the ucleus N1 selects the chael 1 sice it is aware that the chael 2 is curretly occupied by the atom 2. Similarly, the ucleus N2 is aware of the curret occupatio of the chael 1 by the atom 1 ad thereby selects the chael 2. However, as show i the Fig., because there is o electro directly liked betwee the atom 1 ad the atom 2, actives chaels occupied by the atoms caot be idetified. Therefore, the uclei N1 ad N2 may allocate the same chael. I the figure, the two atoms use the chael 1. E1 Atom 1 Atom 2 E3 E5 o this iformatio, a ucleus allocates a chael. I the Molecular MAC protocol, the activity iformatio, expressed i the umber of packets trasmitted, is applied. However, the proposed scheme utilizes the isio probability iformatio. Although the utilizatio of a chael is clearly a possible alterative, it is ot used sice it differs as per a access method (basic or RTS/CTS) or size of data packets. Because a isio probability is idepedet of a access methods or size of data packets, it is more efficiet to apply the probability values. Collisio probability is used as a simple ad effective estimate of the chael cotetio level. Whe a chael is to be allocated i the Molecular MAC protocol, a ucleus receives a list of active chaels from all eighborig electros ad the radomly selects a chael out of those o-active chaels. If every chael is curretly occupied, a chael with the least activity is selected. However, the proposed scheme uses the active chael list & isio probability iformatio, idetifies all o-active chaels ad the radomly selects a chael from those o-active chaels. I additio, if every chael is curretly occupied, a chael is chose based o the isio probability iformatio. Similar to the Molecular MAC protocol, the proposed scheme first selects a o-active chael usig a list of active chaels. The, it uses the isio probability iformatio i order to idetify ay overlapped chael used by aother atom ad accordigly allocates a ew chael. I order to measure the isio probability, both a ucleus ad a electro maages the umber of packets ( ) set set ad the umber of packets ( ) ided. The ided ucleus maages these values for each electro. set( ad imply the umber of packets set ad ided( ided, respectively, for the ode i ad the ode j. I the variable (, oe represets a ucleus ad the other deotes a electro. Based o these values, a isio probability ( P ) ( j ) ca be computed as follows. E2 N1 E N2 E6 ided( P ( set( Chael 1 Chael 1 Fig.. Chael assigmet problem III. INTERFERENCE AWARE CHANNEL ASSIGNMENT (IACA) SCHEME I this paper, a ew method to resolve such issues ivolved with the allocatio of a chael i the Molecular MAC protocol is proposed. Although the proposed scheme aturally adopts a fudametal operatio priciple of the Molecular MAC protocol, it modifies a chael allocatio method. Based o the scheme proposed i this paper ad with the correspodig iterferece take ito accout, a list of active chaels is formed while their activities are measured. Based Note that P is always i the rage of [0, 1] ad ( estimatig the value is simple without ay additioal overheads or costs. If a statio does ot receive a ACK for the basic method or a CTS for the RTS/CTS method, the statio assumes the packet is ided. The statio does ot cout at all ay packets from other statios. Collisio probability has to be maitaied ad updated by each ode (ucleus ad electros i a atom). We assume isio probability is updated per each update period (UpdateTime) expressed i time. Each ode couts the umber of packets set ( set ) ad ided ( ) durig the th update period ad the calculates istataeous isio probability ( P curret ) usig (1). From this value, average 77 P a g e

4 a) Atom 1 is ot withi the iterferece rage of Atom 2 b) Some electros i Atom 1 is withi the iterferece rage of Atom 2 c) Nucleus ad some electros i Atom 1 is withi the iterferece rage d) Nucleus ad all electros i Atom 1 is withi the iterferece rage of Atom 2 Fig. 5. Four cases accordig to the iterferece rage of atom isio probability ( P ) is approximated by exploitig a movig averagig widow: P P (1 ) P 1 curret 1 where P is the average isio probability at the ed of the (-1)th period ad is a smoothig factor. A electro calculates a isio probability ad periodically trasmits it to a ucleus. As illustrated i the four cases of Fig. 5, the ucleus uses the isio probability ad allocates a ew chael i order to avoid a overlapped chael allocatio. Firstly, if the atom 1 is ot covered by the iterferece rage of the atom 2, the atom 1 is ot affected by the atom 2; therefore, the performace of the atom 1 is ot deteriorated, ad it is okay to cotiue to utilize the curret active chael. That is, there is o eed to cosider a isio probability. Secodly, if part of the electros icluded i the atom 1 is covered by the iterferece rage of the atom 2, a isio probability measured by the electros is greater tha that measured by the ucleus. Hece, if the two isio probabilities differ by more tha a threshold value, it implies that the curret active chael is also occupied by other eighborig atoms; therefore, a ew chael is allocated. However, if the differece is egligible, the effect of the iterferece is miimal, therefore makig the utilizatio of the curret chael feasible. Thirdly, if a ucleus ad some electros of the atom 1 are covered by the iterferece rage of the atom 2, a isio probability measured by the ucleus is similar to that measured by the electros. Therefore, similar to the secod case, a ew chael caot be allocated. However, this issue ca be cleared by usig a electro of the atom 1 ot covered by the iterferece rage of the atom 2 ad factorig i a isio probability measured by the electro. A isio probability measured by the oiterfered electro is less tha that measured by the ucleus. That is, if there is ay differece betwee the two isio probabilities, it implies that oe atom is affected by aother atom s iterferece; therefore, a ew chael is allocated. For the secod ad the third cases, if the isio probability measured by the ucleus differs from that measured by the electro by more tha a threshold value, a ew chael is allocated. That is, a ew chael ca be allocated if the followig formula is satisfied. I the variable (, oe represets a ucleus ad the other deotes a electro. P P ( ( j, i) where, is a threshold value for chagig chael. 7 P a g e

5 Collisio Probability (IJACSA) Iteratioal Joural of Advaced Computer Sciece ad Applicatios, Fially, if a ucleus ad all electros of the atom 1 are covered by the iterferece rage of the atom 2, those isio probabilities measured by the ucleus ad the electros is similar; therefore, a ew chael caot be allocated because the (3) caot be satisfied. Still, the etwork performace is deteriorated due to the impact by the atom 2. I this case, the ucleus factors i a total isio probability ( P _ ). P total j total _ j ided( set( where, i represets a ucleus while j deotes a electro. I order to show the effect of the umber of odes o isio probability, we simulated a IEEE 02.11a etwork with trasmissio rates of 5 Mbps for data packets ad of 6 Mbps for cotrol packets such as RTS, CTS ad ACK, respectively. We assume that each ode always has packets to trasmit, ad all packets are of the same size. This eviromet is extreme case. I Fig., isio probability for the IEEE DCF gets higher as the umber of statios becomes larger. This is from the fact that all odes always try to access the chael ad make isios with oe aother Number of Statios Fig. 6. Collisio probability accordig to the umber of odes Fig. 6 shows such isio probabilities resulted from the worst-case sceario. That is, ay ormal eviromet other tha that of Fig. 6 geerates better results. Therefore, a specific value based o such results provides a upper boud for a isio probability. A ucleus determies whether to allocate a ew chael usig this value. For such determiatio, the ucleus maages the correspodig isio probabilities o a table-form basis. The ucleus is well aware of the umber of electros icluded i its ow atom. The, the ucleus compares the total isio probability computed i () with the value resulted from Fig. 6 ad the determies whether to allocate a ew chael. If the total isio probability is greater tha that i the isio probability table by more tha a threshold value, a ew chael is allocated. P total_ P, where, P, is the isio probability for electros i the table ad is a threshold value for chagig chael. If there is ay o-active chael, a ucleus selects a chael based o the procedures metioed above. However, if every chael is curretly occupied, the ucleus selects a chael with the lowest isio probability. IV. SIMULATION RESULTS I this sectio, we discuss the simulatio results of the proposed IACA scheme. To study the performace of the IACA scheme, we have implemeted it. We compare them to the results of the Molecular MAC protocol. We simulated a IEEE 02.11a etwork with trasmissio rates of 5 Mbps for data packets ad of 6 Mbps for cotrol packets such as RTS, CTS ad ACK, respectively. Nodes use a etwork iterface with the trasmissio rage of 10 uits ad the iterferece rage of 22 uits. The spaig tree costructio proceeds as follows. First, the etwork elects a gateway ode, which is coected to the wired Iteret. The, the other odes costruct a spaig tree rooted at the gateway ode. We place the gateway ode o the top-left corer ad radomly the other odes i a simulatio topology. Mai performace metrics of iterest are the umber of atoms, the umber of assiged chaels ad the umber of eighborig atoms. The umber of atoms idicates the umber of total atoms produced i a spaig tree of the simulatio topology. I additio, the umber of assiged chaels is the umber of such chaels allocated to uclei i the simulatio topology. That is, it is the miimum umber of chaels required to serve every ode i the simulatio topology. The umber of eighborig atoms represets the umber of those eighborig atoms whe a atom s ucleus selects a chael. Because each atom occupies oe chael, the umber of eighborig atoms meas the umber of active chaels i other eighborig atoms. All simulatio results are averaged over te simulatios. Table I shows the simulatio results (umber of eighborig atoms, umber of assiged chaels) whe there are 50 odes. For the Molecular MAC protocol, sice each ucleus factors i 3 eighborig atoms o average ad accordigly allocates a chael, approximately.1 chaels are required to serve every ode i the simulatio topology. However, because the proposed IACA scheme takes.1 eighborig atoms ito accout, roughly.7 chaels are eeded. The Molecular MAC protocol idetifies the umber of eighborig atoms with help from those electros i the trasmissio rage of a ucleus ad accordigly allocates a chael. However, the proposed scheme expads the scope up to the iterferece rage; therefore, it ca idetify more eighborig atoms ad accordigly allocate a chael. Because the proposed scheme cosiders 2.7-times more eighborig atoms tha the Molecular MAC protocol ad accordigly allocate a chael, about 2.1-times more chaels ca be allocated. Furthermore, i the Molecular MAC protocol, some 79 P a g e

6 Number of Atoms Number of Assiged Chaels Number of Neighborig Atoms (IJACSA) Iteratioal Joural of Advaced Computer Sciece ad Applicatios, eighborig atoms still allocates the same chael, hece deterioratig the performace of a etwork. O the other had, the proposed scheme prevets the allocatio of the same chael by those eighborig atoms i the iterferece rage ad thereby ehaces etwork performace. TABLE I. SIMULATION RESULTS FOR 50 NODES Number of eighborig atoms Number of assiged chaels Molecular MAC protocol IACA scheme.1.7 Figs. 7, ad 9 show how etwork performace is chaged as the umber of odes icreases. I particular, Fig. 7 idicates how the umber of those atoms produced i the simulatio topology liearly icreases i proportio to the umber of odes. If the umber of odes equals to 30, about 27% of total odes become uclei. As the umber of odes icrease, such percetage declies. For example, if the umber of odes is 70, approximate 1% of total odes become uclei. Fig. depicts the umber of those eighborig atoms to be cosidered whe each ucleus allocates a chael. I the Molecular MAC protocol, because the iformatio about eighborig atoms is ected from those electros i the trasmissio rage of a ucleus, the correspodig result value is quite low. O the other had, the proposed scheme further cosiders the iterferece rage ad accordigly ects the iformatio about eighborig atoms; therefore, the correspodig result value is relatively higher. As the umber of odes icreases, the umber of eighborig atoms for each ucleus accordigly icreases. Fig. 9 shows the umber of those chaels allocated to serve every ode i the simulatio topology. This result is similar to that of Fig.. As the umber of eighborig atoms icreases, the umber of chaels allocated accordigly icreases sice other chaels are allocated to avoid ay overlappig. Compared to the Molecular MAC protocol, the IACA scheme requires 1.9-times more ad 2.1-times more chaels if the umber of odes is 30 ad 70, respectively. Based o such results, a idetical chael is ot allocated to eighborig atoms, therefore ehacig the performace of a etwork Number of Nodes Fig. 7. Number of atoms accordig to the umber of odes Molecular MAC IACA Number of Nodes Fig.. Number of eighborig atoms accordig to the umber of odes Molecular MAC IACA Number of Nodes Fig. 9. Number of assiged chaels accordig to the umber of odes V. CONCLUSION A wireless mesh etwork is a promisig wireless techology for umerous applicatios. I mesh etworks with a sigle chael, the fuctioalities of odes are performed o the same chael. Therefore, etwork performace decreases. I order to icrease the performace, several MAC protocols have bee proposed based o multiple chaels. Molecular MAC protocol was proposed by adoptig a molecular aalogy. I this protocol, a ucleus assigs a chael for its atom with the assistace from electros i the same atom. It is ot able to recogize the existece of those atoms i the iterferece rage. Therefore, allocatig the same chael to eighborig atoms results i the deterioratio of etwork performace. I order to resolve this problem, we propose a ovel distributed iterferece aware chael assigmet scheme. The proposed scheme is somewhat similar to the Molecular MAC protocol method i that a chael is selected by the assistace of electros. The, a isio probability is applied to allocate a ew chael i order to recogize ay iterferece ad avoid ay overlapped chael allocatio. Simulatio results show that the proposed scheme was verified that differet chaels could be allocated to those eighborig atoms i the iterferece rage. 0 P a g e

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