Benefit of Selecting Number of Active Mesh Routers in Disaster Oriented Wireless Mesh Network

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1 Journal of Software Engineering and Appliations, 2012, 5, 3-41 doi:10.423/jsea b008 Published Online Deeber 2012 ( Benefit of Seleting Nuber of Ative Mesh Routers in Disaster Oriented Wireless Mesh Network Panu Avakul 1, Hiroki Nishiyaa 1, Nei Kato 1, Yoshitaka Shiizu 2, Tooaki Kuagai 2 1 Graduate Shool of Inforation Sienes, Tohoku University, Sendai, Japan; 2 NTT Network Innovation Laboratories, NTT Corporation, Yokosuka, Japan. Eail: panu0014@it.eei.tohoku.a.jp, bigtree@it.eei.tohoku.a.jp, kato@it.eei.tohoku.a.jp, shiizu.yoshitaka@lab.ntt.o.jp, kuagai.tooaki@lab.ntt.o.jp Reeived 2012 ABSTRACT Couniations is one of the ost ritial fators in disaster reovery proess. However, after a ajor disaster, existing ouniations infrastrutures ay be heavily daaged or even opletely unusable. It is neessary that ouniationsare to be proptly restored to the disaster area, whih is the goal of our national projet. The projet ais to build three tiers wireless esh network fro reaining wireless aess points in order to provide ouniations servies to the disaster area. This work introdues a unique ultiple tiers wireless esh network projet. In addition, this work also illustrates erits in optiizing the nuber of esh routersin order to ahieve the optiu perforane by presenting both theoretial and siulation results of a speifi senario of ultiple tiers wireless esh network. Keywords: Wireless Mesh Network; Throughput; Interferene; Collision; WiFi 1. Introdution Following the 2011 Tohoku earthquake and tsunai, the field of eergeny network deployent has gaineduh attention fro researh institutions all over Japan. This is beause drasti disaster suh as earthquake or tsunaiwill inapaitate the existing ouniations infrastruture. Therefore, it is iportant to be able to restore ouniations in the disaster area using whatever eans neessary. In addition, it isentioned by [1] that it is ore opliated to deploy a new ouniations syste where the previous syste still exists. An ongoing national projet ais to restore ouniations within the disaster area using the reaining resoures, naely the reaining wireless aess points, to onstruts a wireless esh bakbone and to provide ouniations to users in the affeted area who are referred to as Mesh Client (MC). Wireless Mesh Network (WMN) is a type of infrastruture where eah partiipating node or a Mesh Router (MR) both sends and relays inforation siilar to an ad-ho network with the exeption that MR does not usually have power onstraint or obility proble. In addition, any MRs are also likely to at as wireless Aess Point (AP) and provide ouniations servie to MCs within its area siilar to that of an ordinary wireless AP. WMN has been gaining attention fro researh ounities due to the low-ost and rapid deployent, whihare suitable properties for an eergeny network role. In addition, it also inherits any other valuable harateristis fro ad-ho network suh deentralized design and distributed ouniations[2]. In addition to being able to relay inforation between MRs, MR is also usually equipped with an addition radio interfae to ats as a wireless AP andto provide ouniations servies to users within its area. This iplies that the MCs will be able to onnet to a network like onneting to WiFi hot spot, but the ouniation is relayed through the wireless esh bakbone instead of the traditional wired onnetion.due to this unique harateristi of this ultiple tiers WMN, the nuber of ative MRs plays a signifiant role in deterining the perforane of the whole network. It was entioned that our projet relies extensively on the reaining wireless APs that are already deployed in an unontrolled anner,and nuber of ative MRs an affet the perforane in ultiple-tiers of the projet.therefore, in order to deploy the projet suessfully with the optiu perforane, we need to optiize the nuber of MRs.This work briefly introdues the national projet and investigates the effet of using the optiu nuber of MRs in a unique WMN projet. We introdue a theoretial perforane of a speifi senario of ultiple-tier WMN, whih is baked up by siulation results. The reaining setions of this paper are divided into following: Setion 2introdues related works and dis-

2 37 usses the struture of the projet. Setion 3shows theoretial disussion of the senario. Setion 4 presents the siulation results and disussion. Finally, Setion 5 presents the onlusion. 2. Bakground 2.1. Related Works Referene [3]presents Extree Networking Syste (ENS) arhitehture, whih is a three tiers network that is very siilar to our projet s arhitehture. Aording to the artile, ENS was experientally deployed on Noveber 2005, at San Diego and any useful eergeny response statistis, suh as traffi statisti, andperforane of the arhitehture, was olleted. The authors also present any hallenges in deploying wireless esh network in the ase of eergeny response. One of the ajor differene of ENS and our projet is that our projet plans to rely on existing inflastruture suh as reaining wireless APs. A ore detail explaination regarding our projet will be given in Setion 2.2. In addition to ENS presented by [3], authors of [4] also present another siilar network arhitehture to our projet alled the hybrid WMN. Hybrid WMN ais to enhanes the perforane of etapolian WMN with the addition of wired APs. While hybrid WMN is not intended for usage in an eergeny response situation, it is a great idea whih ay also help inrease the overall perforane of eergeny response WMN. Sine in an ideal situation, soe reaining wireless APs ay still have wired onnetion available. Referene[5] investigates the ipat of inter-ell interferene on WLAN perforane. The authors testbed experient shows that inter-ell inteferene an greatly redue the WLAN perforane under the TCP doinated oon offie traffi pattern. Referene [] presents a novel gateway seletion ethod for ultiple tiers WMN that fouses on deployent in disaster area. The ethod presented ultilize the onept of Collision Doain (CD) to hoose the ost suitable MR to at as Mesh Gateway (MG)in order to have the best overall syste throughput. Referene [7] presents a deployent evaluation of the Roofnet wireless esh network, whih is an unplanned WMN. The author suggests that having a denser MR ay help iprove the average throughput of the network due to the posibility of hosing shorter high quality links with the ost of having a higher average hop-ount. However, the author also shows that the perforane suffers fro ulti-hops transission due to inter-hop interferene. In addtition, [8]shows that in a ulti-hop WMN there is fairness proble where MRs that has a higher hop-ount to the gateway ahieve uh lower throughput than those with lower hop-ount Projet Struture and Charateristis Three Tiers Network As shown in Figure 1, the projet is oposed of several oponents. The Moveable and Deployable Resoure Units (MDRU) ats as a gateway that provides onnetions fro the syste to outside network suh as internet. The MR is an ordinary wireless AP whih is onfigured to at as wireless esh bakbone in order to provide ouniations servie to the MCs. MG is a speifi MR that ouniates diretly with the MDRU. The overall projet an be partitions into three separated tiers: network faility tier, esh tier, and lients tier. The network faility tier overs the ouniation between the MDRU(s) and MGs, whih is done in a speial 25GHz band. The esh tier is the wireless esh bakbone level where the MGs and MRs for a wireless esh bakbone network using the 5GHz band. The MGs at as esh portal to bridge the ouniation between the network faility tier and the esh tier. Finally, the lients tier is the level, whih eah MG and MR provide ouniations servie to MCs within its viinity over the 2.4GHz band. Sine the links at the network faility tier are assued to be high perforane links, this work fous on the perforane in esh tier and lients tier Unique Deployent A senario of how the projet will be deployed is the MDRU will be deploy to the disaster area prior to disaster. The MDRUis apable of arrying a ertain nuber of MGs in order to have soe ontrol over the topology of the syste. After arrival, the MDRU will onfigure any reaining wireless APs in the area into MRs or MGs and restore ouniations servie to MCs in the area Unontrolled MRs Plaeent Another ajor harateristi of the projet is that sine MRs are just oonly deployed wireless APs, and it is pointed out by [9] that oon APs are usually deployed in an unplanned or rando fashion. Therefore, there is a good hane that the deployent will not be optiized, and thus ausing the drop in perforane. Figure 1.The struture of the projet.

3 38 3. Effet of Nuber of MRs 3.1. Effet of Nuber of MRs CD is an iportant onept that an be used to estiate the apaity of WMN as shown in [10]. Sine wireless links share the sae ediu, it is neessary that only one devie within the sae interfering range should be transitting at any given tie to have a suessful transission. In another word, if a ertain link n is ative, any other link within the sae interfering range of link n ust be inative in order for transission at link n to be suessful. A CD is defined by [10] to be a set of links that have to be inative for a transission at a ertain link to be suessful inluding the transitting link itself. Figure 2 illustrates a hain topology of a two tiers WMN where there are six MRs, whih an only transit to an adjaent MR. The rightost devie is the MG that is assued to be the sink of all traffi. Eah MR has to transit G aount of traffi fro its own lients tier and any other aount of traffi forwarded by its upstrea MR(s). For exaple, MR2 has to forward G aount of traffi fro its lients tier plus what MR1 forwarded whih is another G to the total of 2G aount of traffi. It is assued that the interfering range is two ties the transission range; therefore, the CD of link between MR4 and MR5 is a set of {(MR2, MR3),(MR3, MR4),(MR4, MR5),(MR5, MR),(MR, MG)}. The bottlenek ollision doain (BCD) is defined by [10] to be the CD that has to forward the ost traffi. For instane, the CD of link MR4 and MR5 has to forward all traffi forwarded by eah link within the set of its CD. Therefore, CD of link MR4 and MR5 has to forward total of 2G+3G+4G+5G+G = 20G. The sae alulation an also be done on CD of other links, but in the ase of senario in Figure 2,CD of link MR4 and MR5 has to forward the ost traffi. Hene, it is the BCD of this hain topology. Aording to the previous alulation, CD of link between MR4 and MR5 has to forward 20G aount of traffi. However, the apaity is bound by the MAC layer apaity C, thus an be represent by Figure 2. A hain topology where eah MR sends the sae aount of traffi to the MG. 20G C. (1) By solving inequality in Equation (1), we an see that the axiu throughput available to eah MR or Gax is C Gax (2) 20 Fro the analysis above, we an see that the ore general ase of Equation (2) is Gax C AMT (3) whereamt is the total aount of traffi forwarded by BCD divided by G, whih will be 20 in the ase of CD of link MR4 and MR Considered Senario In order to show the effet of nuber of MRs, we onsider following senario illustrates in Figure 3(a) where there are one MG, three MRs and six MCs. Soe properties of the senario are as following: All MCs an onnet to any available MRs.(All MCs are within the ouniation range of any MRs.) Eah MR operates in different hannel in a way that they do not have inter-ell interferene introdued in [5]. The esh tier links operate in one oon hannel. Eah MRs will always have the sae nuber of MCs within its ell. In addition, this senario an be narrow down to three sub-senarios as follow: Figure 3. (a) Topology for onsidered senario; (b) Subsenario 1, onsisting of one ative MR; () Sub-senario 2, onsisting of two ative MRs; (d) Sub-senario 3, onsisting of three ative MRs.

4 39 1) Only MR1 is ative and all MCs are assoiated with MR1 as illustrated in Figure 3(b). 2) Either MR1 and MR2, or MR1 and MR3 are ative. In this sub-senario, eah MR will have to serve three MCs as illustrated in Figure 3(). 3) All MRs are ative and eah has to serve two MCs as illustrated in Figure 3(d). It is possible to use the onept of BCD to estiate the axiu throughput in both the esh tier and the lients tier in eah of these sub-senarios so that we an see the effet of nuber of MRs in this senario Sub-senario 1 In the lients tier, MR1 serves all six MCs as wireless AP. Sine all devies used only one hannel within a given ell, we an easily onluded that the CD of eah link is the sae set that ontain all links; thus all links are BCD (all links need to transitted sae aount of traffi.)by using Equation (3) wherecis the MAC layer apaity of the lients tier or C and AMTequals to, beause there are six MC eah offering 1G traffi.we an onluded that G ax (throughput available to eah MC) of lients tier or C is C G (4) In the esh tier, there is only MR1 and the MG. However, the perforane should be alulated based on throughput available to eah MC rather than throughput available to eah MR. Therefore, the G ax of esh tiershould be divided by nuber of MCs served by eah MR or in this ase to forulate G of C G (5) where G the MAC layer apaity of esh tier Sub-senario 2 In this sub-senario, eah MR has toserve three MC. Therefore, using Equation (3) and AMT equal to 3 results in C of G C 3 However, in the esh tier there are two operating MR. We an learly see fro Figure 3() that CD of both links in the sub-senario are BCN and eah CD has to arry 1G + 2G = 3G aount of traffi. By using Equation (3) and AMT equals to 3 and the fat that eah MR serves three MCs, the resulting G is C G (7) 9 () 3.5. Sub-senario 3 The lients tier of this sub-senario has three total ative APs and eah has to serve two MCs. Using siilar analysis to sub-senario 1 and sub-senario 2 in previous setions, we an see that AMT is 2 thus resulting with C G (8) 2 In the esh tier, siilar to sub-senario 2, we an see fro Figure 3(d) that CD of any link inludes all links, and thus are BCNs. In this sub-senario any BCNs has to arry 1G+1G+3G=5G (AMT=5) aount of traffi and that eah MR serves 2 MCs, hene, resulting with C G (9) Senario Disussion Sine our projet is a ultiple tiers WMN, the perforane of the projet will be restrited by the tier with lower perforane. We an learly see fro Table 1 or Figure 4 that the perforane in the esh tier dereases with inreasing nuber of MR. This is beause of the interferene odeled by BCD onepts. On the other hand, the perforane in the lients tier benefits fro the extra apaity of additional MR operating in nonoverlapping hannel. Therefore, it is iportant to optiize this trade off, whih results fro the nuber of ativating MRsin the area, in order to provide the best axiu throughput to eah MC. All etris are suarized in Table 1 whilefigure 4 shows plots of G and C with different value of G for different a data rates (G-54 for 54Mbps, G-24 for 24Mbp, and G-12 for 12Mbps). Eah value of Cfor both C and G are experientally deterined by siulations for eah data rate. Aording to Figure 4, the axiu throughput available to eah MC is bounded by either G or C. Sine G and C rely on eah other, the lower value of the two will be the upper bound of the axiu throughput. Therefore, by using this onept, it is possible to estiate the optiu nuber of MRs that will give the axiu average throughput to eah MC. We an easily see fro Figure 4 Table 1. Suary of all values of this setion. Sub-senario G C C 9 C 10 Metris G C C 3 C 2

5 40 Figure 4. Theoretial value of G and G for different data rate. that the optiu value of MR used for a with 54Mbps data rate is three MRs.This is beause the apaity in the esh tier is large enough to aoodate the extra apaity in the lients tier, whih results fro having three APs operate in nonoverlapping hannel. 4. Siulation Results and Disussion Siulations are onduted to onfir this effet disussed in setion 3. The siulation senarios are set up in Qualnet 5.1 with topologies siilar to that of Figure 3(b), () and (d). Eah MR is set up to operate as AP in different nonoverlappinghannel in the lients tier;thus, interferene between eah MR will not exist within the lients tier. In additional to lients tier interfae, eah MR also has another interfae for ouniating in the esh tier. In esh tier, all MRs operate in one oon hannel, whih eans the ollision doains ontain all links. Eah MC sends CBR traffi to MG and thus all traffi only originate fro MCs and flow to MG. Figure 5 shows results of the siulation senarios where the line graphs are the theoretial results fro Figure 4,and the bar graphs show the siulation results of eah esh tier s data rate (54 Mbps in blue, 24 Mbps in redand 12 Mbps in green.) In the single MR senario, the siulated results represent the fat that the average throughputs are bounded by C, whih has lower value than all G of all data rate. The results of two operating MRs senario differ fro that of the single operating MR senario in that the average throughput per MC of 12Mbps ase should now be bounded by G with data rate of 12Mbps (in green).this is beause the result of G with data rate of 12Mbps is now lower than that of C as shown in Figure 5. However, sine the average throughput per MC of C is still lower than those of G of 24Mbps and 54Mbps, the throughput per MC of those two ases are still bounded by C. Finally, in the Figure 5. Siulation results and theoretial results for different data rate. ase of 3 ative MRs, the theoretial results show that the siulation results should be bounded by G of eah ase. As shown in Figure 5, the siulation results learly follow those of the theoretial results. This shows that the theoretial results are aurate, and we an onluded that we an deterine the nuber of ative MR(s), whih would yield optiu nuber of MRs for eah G (of different data rate). Fro Figure 5, we an dedut that the ost optiized nuber of MRs in G equals to 54Mbps, 24Mbps, and 12Mbps are 3 MRs, 2 or 3 MRs, and 2 MR respetively. This is beause these obinations give the best per MC perforane. 5. Conlusion Previous setions show that in order for the projet to ahieves the optiu perforane, the set of ative MRs will need to be deterined to give the best possible perforane. In the future, we ai to apply this idea into a ore general ase of ultiple tiers WMN. In addition, any other iportant fators suh as hannel assignent in the esh tier, and inter-ell interferene present in [5] should also be onsidered when deiding optiu set of ative MRs. In this paper, we briefly explain the national projet The R&D on the reonfigurable ouniation resoure unit for disaster reovery, whih is a unique ultiple tiers WMN. We point out that by optiizing the nuber of ative MRs, we an ahieve the optiu perforaneby using the onept of BCD and the unique harateristi of ultiple tiers WMN. In addition, we also show both theoretial and siulation perforane based on a speifi senario of ultiple tiers WMN. Our results showed that by hoosing the right nuber or obination of ative MRs, we an optiize the perforane of the projet.

6 41. Aknowledgeents This work was onduted under the national projet, The R&D on the reonfigurable ouniation resoure unit for disaster reovery, supported by the Ministry of Internal Affairs and Couniations (MIC), Japan. REFERENCES [1] B.S. Manoj and Alexandra Hubenko Baker, Couniation hallenges in eergeny response, Couniations of the ACM Eergeny response inforation systes: eerging trends and tehnologies, Vol. 50, No. 3, 2007, pp doi: / [2] P. H. Pathak and R. Dutta, A Survey of Network Design Probles and Joint Design Approahes in Wireless Mesh Networks, IEEE Couniations Surveys & Tutorials, Vol. 13, No. 3, 2010, pp doi: /surv [3] B. Braunstein, T. Trible, R. Mishra, B. S. Manoj, L. Lenert and R. R. Rao, Challenges in Using of Distributed Wireless Mesh Networks, Proeedings of the 3 rd International ISCRAM Conferene, Newark, NJ, May 200 [4] W. Fu and D. P. Agrawal, Capaity of Hybrid Wireless Mesh Network with Rando APs, IEEE Transations on Mobile Coputing, Vol. 10, No. 12, doi: /tmc [5] M. A.Ergin, K. Raahandran and M.Gruteser, Understanding the effet of aess point density on wireless LAN perforane, Proeedings of the 13 th annual ACM international onferene on Mobile oputing and networking MobiCo, Montreal, 9-14 Septeber 2007, pp doi: / [] W. Liu, H. Nishiyaa, N. Kato, Y. Shiizu and T. Kuagai, A Novel Gateway Seletion Method to Maxiize the Syste Throughput of Wireless Mesh Network Deployed in Disaster Areas, 23 rd IEEE International Syposiu on Personal, Indoor and Mobile Radio Couniation PIMRC, Sydney, 9-12 Septeber [7] J. Biket, D. Aguayo, S. Biswas and R. Morris, Arhiteture and evaluation of an unplanned b esh network, MobiCo Proeeding of the 11 th annual international onferene on Mobile oputing and networking, Cologne, August 28- Septeber pp doi: / [8] J. Cap, J. Robinson, C. Steger and E. Knightly, Measureent Driven Deployent of a Two-Tier Urban Mesh Aess Network, MobiSys Proeeding of the 4 th international onferene on Mobile systes, appliations and servies, Uppsala, June 200. pp doi: / [9] A. Akella, G. Judd, S. Seshan and P. Steenkiste, Self-anageent in haoti wireless deployents, MobiCo Proeedings of the 11 th annual international onferene on Mobile oputing and networking, Cologne, August 28 - Septeber [10] J. Jun and M. L. Sihitiu, The Noinal Capaity of Wireless Mesh Networks, IEEE Wireless Couniations, Vol. 10, No. 5, 2003, pp doi: /mwc