Information Dissemination of Location Dependent Data in Consideration ofConnectivity and Network partition in Mobile Ad Hoc Sensor Networks

Gen Tsuchida Susumu IshiharaGraduate School of Science and Technology

Shizuoka University3-5-1 Johoku, Naka-ku, Hamamatsu, 432-8561 Japan

{gen, ishihara}@ishilab.net

Abstract

In mobile ad hoc networks, due to the movement ofhosts,obstacles, etc, it is difficult for mobile hosts to access thedata on other hosts. To solve this problem, replica distri­bution methods which distribute the replicas of data itemsto other hosts and maintain them have been proposed. Wehave proposed LDR (Link-aware and Density-based rear­rangement) method for dynamic rearrangement of replicasof location dependent data. In the LDR method, replicasof the location dependent data items are placed on mobilehosts which are the neighbors ofbranch points of the ad hocnetworks and the density of replicas are controlled accord­ing to the degree of the network topology. We evaluated thepeiformance of the LDR method by simulation. The sim­ulation results show that the LDR method achieves higherpeiformance than other methods when the distance betweenthe requesting host and the location where the data item isassociated is long and the host moving velocity is high.

1. Introduction

Recently, services dealing location dependent informa­tion are widely deployed. For example there are pedestriannavigation systems using cell-phones, car navigation sys­tems with VICS (Vehicle Information and CommunicationSystem), etc. Therefore, the management of geographicalinformation has been attracting research interest.

Services sharing location dependent information onwireless ad hoc networks [1] composed only mobile hostsis highly expected, because they do not require large andcostly infrastructure. For example, services that exchangelocation-dependent information such as traffic information,shopping information or natural disaster information be­tween mobile hosts or vehicles can be achieved using thisadvantage. We call such system SOLA (system for Shar-

ing Objects with Location Information on Ad hoc net­works)(Fig.l). We assume such location dependent datahandled by SOLA are generated by mobile hosts and asso­ciated with the location where the data item was generated.

Considering mobile hosts share data items in wireless adhoc networks, it is difficult to retain accessibility to dataon other hosts due to link disconnections caused by move­ments of hosts. This fact hampers the practical use of adhoc networks. The accessibility to the data object ownedby a mobile host from other hosts must be maintained evenif link disconnections occur between these hosts. For thesake of this, replica distribution methods which distributereplicas of data items to other mobile hosts and maintainthem have been proposed. For example, Hara has proposeda method based on an assumption that all mobile hosts onthe network know the probabilities of data access to eachitem and the probabilities do not change [3]. AdditionallyHara extended this method to support update of data [4], toselect replica allocation host using link state information [5]and to save power consumption [10]. Yin proposed a cachealgorithm for data maintained by fixed servers on ad hocnetworks [14].

We have proposed Skip Copy (SC) method[12] andAdaptive Skip Copy (ASC) method[13] especially formaintaining and sharing location-dependent data collectedby sensors and accessed using geocasting[8] of requestingmessages on server-less ad hoc networks. Such kind of datawill be used in personal navigation systems, car navigationsystems and cooperative work on disaster relief, etc. SC andASC methods are based on an assumption that mobile hostsdo not know the IDs of other hosts which have the replica ofa requested location-dependent data item and mobile hostssend request messages to multiple hosts near the locationwhere the requested data item is associated in order to ac­cess the data item. However, replicas are placed on onlymobile hosts on a reply route when SC or ASC methodis used. If replica holding mobile hosts on a previous re-

166

r Fire disaster

Figure 1. Sharing Objects with Location infor­mation on Ad-hoc networks (SOLA)

ply route move, replicas on "old routes" may not be usedand the request traffic increase because the previous replyroutes cannot be used and the request routes tend to be longto reach replica holding mobile hosts.

In [11], we proposed Link-aware and Density-basedrearrangement (LDR) method for dynamic relocation ofreplica to improve the issue of the SC and the ASC method.In this paper we rename it to Link-aware and Density-basedRearrangement method. In the LDR method, replicas areplaced on mobile hosts which are the neighbors of branchpoints of the network topology. The density of replicasare controlled according to the host density, the number ofneighbor hosts. In [11], we briefly evaluated the perfor­mance of the LDR method. However, we did not evaluatedthe effect of the moving speed of hosts, the distance be­tween requesting hosts and the destination of the requestmessage. In this paper, we evaluate the performance ofthe LDR method in detail and present that the LDR methodachieves higher access success rate than other methods es­pecially when the moving speed of hosts is fast and the dis­tance between requesting hosts and the destination of therequest messages is long.

The remainder of the paper is organized as follows. Sec­tion 2 gives a brief overview of the Skip Copy (SC) methodand the Adaptive Skip Copy (ASC) method. In section 3,we explain the LDR method. Section 4 presents the sim­ulation conditions, and in section 5 we show the results ofthe simulation and discuss the characteristics of the LDRmethod. Finally, we summarize this paper in section 6.

2. Replica Distribution Method of Location De­pendent Data

In this section, we describe our previously proposedreplica distribution method of location-dependent data. Wefocus on a system which mobile hosts share location­dependent data obtained by cameras, sensors, etc. on wire­less ad hoc networks. We call such a system SOLA (systemfor Sharing Objects with Location information on Ad-hocnetworks) (Fig. 1). We assume location dependent data (e.g.cars traffic information, information in disaster area, etc.)

handled by SOLA are generated by mobile hosts and as­sociated with the location where the data item was gener­ated. We previously proposed Skip Copy (SC) method andAdaptive Skip Copy (ASC) method for replica distributionmethod of location-dependent data under SOLA environ­ments.

2.1. Assumptions

In SOLA, we assume the following condition.

• Mobile hosts collect and share location-dependent in­formation on an ad hoc network.

• Each host tracks its location using GPS, etc.

• There is no specific data server in the ad hoc network.Each host does not know which host has a particulardata item.

• Each host uses data items on other mobile hosts whenit does not have suitable ones locally. When a hostwanting to know information associated to a locationdoes not have a data item about the location, it sends arequest messages to hosts around the location by geo­cast.

• Each host has limited amount of data storage.

• Each mobile host generates data items associated withits current position. Each data item has its generationtime and expiration time.

2.2. Skip Copy (SC) method

In the SC method, replicas of location dependent dataitems collected by mobile hosts are distributed as follows.When a data item is generated on a mobile host, the replicaof the data item is sent to mobile hosts near the generatorof the data item using flooding, and distributed sparsely ac­cording to the hop count c from the generator of the dataitem within the replica distribution range R from the birthplace of the data item. The density of the replicas is con­trolled by the skip parameter Sr. If the hop count c satisfiesa condition c mod Sr = 0, the receiver of the replica holdsit, otherwise disposes it after forwarding it to the neigh­boring hosts. Adding to this, the placement of replicasare dynamically rearranged when they are forwarded as re­ply messages to the requesting hosts. Hosts which forwardthe reply messages hold the replica if the hop count Cr be­tween the replying host and the forwarding host satisfies Cr

mod Sr = 0 and the forwarding host is within Rr from thebirthplace of the data item.

By using the SC method, the replicas of a data itemare distributed sparsely around the location where they are

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Requesting Host

..Requesting Host

s= 2 (a= 2{b(b)

.........• Request message --~ Reply message OReplica data on A

Figure 2. Replica rearrangement by SCmethod (sr=2)

generated (birthplace), and replicas stay near the locationeven hosts that have replicas move far away from the birth­place. These help location dependent data can be accessedusing geocasting even if there are no fixed servers. Figure2 shows an example of the rearrangement of replicas usingSC method when Sr = 2.

In the SC method, when a host sends a request to a lo­cation where a data item was generated from away, the re­quest message tends to be lost and the response time tendsto become long because replicas are still far away from therequesting hosts. To solve this problem, we proposed theAdaptive Skip Copy (ASC) method.

2.3. Adaptive Skip Copy (ASC) method

The ASC method is designed to reduce the responsetime for requests and achieve high accessibility to location­dependent data in environments where hosts frequently re­quest data items generated at a point distant from them­selves. By using the ASC method, replicas of data itemsare distributed sparsely around the location where they weregenerated in a similar fashion to the SC method. Adding tothis, the placement of replicas are dynamically rearrangedduring a data access in the almost same manner of the SCmethod. The ASC method selects reaarangement accordingto the hop count between a requesting host and the replyinghost, while the SC method regards only hop count betweena forwarding host and the replying host and the distancebetween the forwarding host and the birthplace of the dataitem. In the ASC method, skip parameter Sr is calculatedusing the following equation, while the SC method uses aconstant value of Sr'

(1)

In the equation (1), a is the number of replica holding hostsincluding the requesting host. h(Pr , Pa ) is the hop countbetween a requesting host Pr and a replying host Pa. Thisequation means that smaller rearrangement skip parameter

Figure 3. Replica rearrangement by ASCmethod

L Destination of a request message

(Location associated withthe requested data item)

...Request forwarding routeCD New requesting hostG> New requesting host

iii} Moved host• Replica holding hosts

Figure 4. Replica rearrangement by LORmethod

is used when a replica is found at a nearer host from a re­questing host. The minimum skip parameter Smin. is usedto control the usage of storage on mobile hosts.

3. Link-aware and Density-based Rearrange­ment (LDR) method

In the LDR method, replicas are placed on mobile hostswhich are neighbors of branch points on the reply routeand the density of replicas is controlled according to thehost density, the number of neighbor hosts, and hop countfrom a replica holding hosts on the replying route. Figure4 shows the example of the arrangement of replicas by theLDRmethod.

3.1. Assumption of LDR method

In the LDR method, each host sends hello messages to itsneighboring hosts so that its neighbors can know its currentposition periodically. Series of broadcast is used for for­warding reply messages. The replying route is the reverseroute of a route which was used for forwarding the requestmessage. Each reply message includes a data item, the IDof hosts on the reply route, hop count h from last replicastoring host, a replica conservation flag R H and host ill ofthe next reply forwarding host The initial value of RH isoff. If a host receives a reply message and the ID of the hostis specified as the next reply forwarding host, it forwardsthe message to its neighboring hosts.

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3.2. Information Dissemination Algorithmof LDR method

The first replying host sends a reply message (replica ofa data item) with replica conservation flag R H = off andh= O.

When a host i recieves the reply message, it increments hof the message and forwards the message to its neighbors ifit is on the reply forwarding route. If the RH of the messageis off and h and IM i I satisfy the following conditions, i setsthe RH of the message is on.

'" ® Reply forwarding hostIRv\l.<. ,0

~ "r - i -. • Replica storing hostI " I,,' \ 0 (Reply route)~ ¥,-;.. ~ 0 Replica storing host~ (Number of neighbor

I : ."0 hosts is less than 3)©.. 0 Replica storing host at''() probability P(k)

• Not store the replica(Neighbor of replica storinghost F on reply route)

_ Reply route

Figure 5. Example of rearrangement reoplicaby LOR method (Hmin = 3, D N = 3)

H min is a positive constant. M i is the set of the neighborsof i. After then, the host i forwards the replica.

If h = C r , i stores the replica and sets h = 0 beforeforwarding the replica. Cr is determined according to thenumber of hop count between the requesting host and thereplying host, e.g. the half of the hop count between them.

If RH of the received message is on, i stores the replicain its local storage. After then, i sets h = 0 and RH = offbefore forwarding the replica.

In the LDR method, replicas of data items are placed noton only hosts on reply route but on neighbor hosts of thehosts on the reply route. A neighbor host k of a reply for­warding host i may receive the reply message because replymessages are forwarded using broadcast. When k receivesthe reply message, it checks the list of its neighbors M k todetermine whether it stores the replica. If a next reply for­warding host in of the received replica is included in the list(in E M k ), k does not store the replica. Otherwise, k storesthe replica at probability P(k).

3.3. Example of LDR method

D N is a positive constant such that D N ~ 2. kM is a posi­tive constant.

h ~ H min

(IMkl < D N )

(IMkl ~ D N )

(2)

(3)

(4)

their neighboring hosts is the reply forwarding host F andthey are not specified as a reply forwarding host. Host Gstores the replica because F is not its neighbor and the num­ber of its neighboring hosts IMe I is less than D N. Hosts Hand I store the replica at probability P(H) and P(I) re­spectively, because they do not connect to host F and IMH Iand IMil equal D N

4. Simulation models

We evaluated the performance of the LDR method bysimulation using JiST/SWANS network simulator[6]. Inthis section, we explain the models used in our simulation.

First, 100-600 mobile hosts are placed at random in2000 m by 1000 m rectangle field. The field is divided into200 square-areas with 100m on a side. Following the ran­dom way-point model, each mobile host moves within thesimulation field. The moving speed of each mobile host ischosen randomly from range I-v m/second (v = 2,20) andthe pause time defined in the random way point model is 3seconds.

4.1. Data generation model

Each host generates a new data item associated with thecenter of its current sub-areas according to a Poisson modelwith a mean interval of 200 seconds. The size of reply andrequest messages are 1500 bytes and 128 bytes respectivelyincluding UDP and IP headers. TTL of each data item is500 seconds.

Figure 5 shows the example of replica rearrangement inthe LDR method with D N = 3, H min = 3. A replying hostR sends a reply message (replica of data item) with replicaconservation flag R H = off. Host E which satisfies condi­tion (2) and (3) forwards the reply message with RH = on.Host F which receives the reply message stores the replicabecause RH = on and F is specified as a reply forward­ing host. Then F forwards the replica with RH = off andh = O. Though hosts J and K receive the same messagefrom host E, they do not store the replica because one of

4.2. Data request model

To generate request messages from mobile hosts, weused two models: a priority-an-neighborhood model and aflat-priority model [12] .

In these models, the data access pattern is based on aZipj-like distribution [15], which has been frequently usedto model non-uniform distributions [14]. In this model, mo­bile hosts choose the areas to which they send requests ac­cording to the following rule. We assume the simulation

169

4.3. Evaluation Metrics

We used the following performance metrics for the eval­uation of the ASC method.

field is divided into 200 square sub-areas. Each mobile hostchooses one sub-area as the area in which it is interested. Inthe Zipj-like distribution, the access probability for the dataitem di associated with the ith sub-area is represented as

where ri is the distance between the current position of therequesting mobile host and the center of the ith sub-area.When 8 = 1, equation (5) follows a strict Zipf distribution.When 8 = 0, it follows a uniform distribution. Larger 8results in a more "skewed" access distribution. We set 8 to 1in the priority-on-neighborhood model and 8 to 0 in the fiat­priority model. If200 data items have already been acquiredby some mobile hosts, ith data item di (i = 1,2, ... ,200)is chosen with a probability Pr (i)

The generation of request messages follows Poissonmodel with a mean interval of 100 second. Request mes­sages are forwarded through Location Based Multicast(LBM) [8] so that the request messages reaches the chosensub-area.

Number of sub square-areas 100Data size[KB] 1.5Number of hosts 600 100 - 600v [mls] 20 2,20Pause Time [sec] 3Bandwidth [Mbps] 11Communication range [m] 105Data generates interval [sec] 200Data requests interval [sec] 100Hello interval[sec] 30 29-31TTTL [sec] 500

• ASC: Replica rearrangement using the ASC method.The number of replaced replicas a was 2, 3 and Smin

was 2.

Parameter I Default value I Range

Table 1. Simulation parameters

• LDR: The LDR method. Replica rearrangement pa­rameters were set as follows: D N = 3; H min = 2,3;kM = 1; Hello interval =30 seconds; Hello TTL =30seconds; Cr was set to the half of the number of thehop count between a requesting host and the replyinghost.

storage size of each mobile host is 10 data items. If a hostis specified as a replica holder, it disposes a replica of theoldest data item in its storage. We also evaluated the caseswhen the storage size is larger than 10, the results showedthe same tendencies.

We compared the following replica rearrangement meth­ods.

(5)P (i) - l/rl (0 ::; 8 ::; 1),r - ",200 1/ .e

L..-j=l rJ

• Access Success rate As

Here R c is the number of requests and A c is the num­ber of successful responses during the whole simula­tion.

• Request traffic Treq and reply traffic Treply

Treq and Trep are the number of packets sent for therequest and reply messages, respectively.

• Responce Time TD

This is the average time from when a host sends re­quest message to the time when the requesting host re­ceives a response.

As = A cRc

(6) • SC: Replica rearrangement using the SC method.Replica placement range R r was 300m and skip pa­rameter Sr was 2.

• Path: The Path replication method which is used inunstructured pure Peer-to-peer (P2P) networks[2, 9].Requested data items are replicated on all hosts alongthe data transmission path between a requesting hostand a replying host.

• Owner: The Owner replication method which is usedin unstructured pure P2P networks [2, 9]. Only thesender of a request stores the replica of a requesteddata item.

5.1. Effect of velocity of mobile hostsTable 1 shows the simulation parameters.

5.1.1. Access Success Rate

5. Results and Discussion

We ran the simulations for 10000 seconds in the simu­lation time. Data collected at the first 1000 seconds of thesimulation were neglected in order to avoid the effect ofthe initial state. Hereafter, we show the results when the

Figure 6 shows As of various number of hosts when fiat­priority model (8 = 0) is used. When the number of mo­bile hosts is less than 200, the difference of As among allmodels is small. If the density of mobile host is low, re­quest messages would be lost because of unstable connec­tivity between mobile hosts. Thus, As is small in all cases.

170

"LDR Hmin=2" --+-­"LDR-Hmin=3" ---)0(--­

"ASC a=2""ASC-a=3" -------8

- "SC" ---.--"PATH" - -0---

"Owner" ---_e---

15

10

20 ,--c::-:----,-----,,--,---------,---,-------,"LDR Hmin=2" ---+--"LDR-Hmin=3" ---x---

"ASC a=2" ----'Jk--­

"ASC-a=3" -------8

- "SC" ---.-­"PATH" - -0---

"Owner" ----e---

15

Number of hosts

Figure 7. Request traffic vs. Number of hosts(Flat-priority model)

10

Number of hosts

20 ,------,---,--------,---,--------,

o"==_----'--__-'----_-----'-__--'----_----.J100 200 300 400 500 600

(b) v=20m/s

0"==-----'-----'----------'-----'------100 200 300 400 500 600

(a) v=2m/s

"LDR Hmin=2" --+­"LDR-Hmin=3" --->(--­

'-ASC a=2" ---*--­"ASC-a=3" -------8

- "SCM ---.-­"PATH" - -0--­

"Owner" ----e---

0.2

0.4

0.8

0.6

0100 200 300 400 500 600

Number of hosts

(a) v=2m/s

"LDR Hmin=2" --+-"LDR- Hmin=3" ---x---

'-ASC a=2" ---elf---

0.8 "ASC=a=3" -------8

"SCM ---.--

W"PATH" - -0---

"Owner" ---_e---

~

0.6

]~

0.4

<:

0.2

0100 200 300 400 500 600

Number of hosts

(b) v=20m/s

Figure 6. Access Success Rate vs. Numberof hosts (Flat-priority model)

5.1.2. Request and reply traffic

When the number of mobile hosts is more than 300, As ofthe LDR method is larger than that of other methods. Es­pecially, As of the LDR method is 10% higher than theother methods when v = 20m/ s. When mobile hosts movefast, the network topology of the ad hoc networks changesfrequently. If replica holding mobile hosts on a previousreply route move, replicas held by hosts on the old replyroute cannot be used. In other methods, replicas are placedon only hosts on the old reply routes. If hosts moves fast,the connectivity between the requesting host and the replicaholders may be lost at high probability. As a result, As be­comes small. However, in the LDR method, replicas of adata item are placed not only on hosts on the reply route butalso on the neighbor hosts of them. Thus, the connectivityof the replica holders and the requesting host may be keptat high probability even if the mobility of hosts is high. Asa result, As of the LDR method is larger than that of othermethods. On the other hand, when the velocity is small, net­work topology changes infrequently. Thus replicas on replyroute can be reached from a requesting host at high proba­bility. This results in the small difference of As among allreplica rearrangement methods.

Figure 7 shows Trequest of various number of hosts whenflat-priority model (8 = 0) is used. Other parameters are thesame as the previous section. We can see that the total re­quest traffic of all methods are almost same. We used LBMas a geocast scheme for sending request messages. Becausethis scheme is based on flooding, even if a host replies to arequest message and stops forwarding the request message,other hosts continue to forward the request message. Thusthe difference of request traffic among rearrangement meth­ods is small. If we use unicast-base geocasting scheme e.g.GPSR [7], the amount of request traffic will decrease.

Figure 8 shows Treply of various number of hosts. Com­paring the LDR method and other methods, the total replytraffic of the LDR method is larger than that of other meth­ods when the density of hosts is more than 400. This isbecause that the number of successful requests of the LDRmethod is larger than that of other methods. That is, moresuccessful accesses cause larger reply traffic.

5.1.3. Responce Time

Figure 9 shows TD of various number of hosts when flat­priority model (8 = 0) is used. When the density of hosts issmall, the response time of the LDR method is the shortest

171

600500400300200

"LOR Hmin=2" --+-­"LOR- Hmin=3" ---)(--­

"ASe a=2" ---*---"ASe-a=3" G

-"5(" :~-.c~-:~;_~;~~.:_:j"PATH" - ---0---

"Owner"

"LOR Hmin=2" --+-­"LOR-Hmin=3" ---)(---

"ASe a=2" ----;.tE---

"ASe-a=3" G-"Se" --.--

"PATH" - -:&j-i::.;;;:~!~d~ .:.:.:. l"Owner" _<

o~-~--~--~-~--~100

0.03

0.02

0.05

0.01

0.04

0.04

0.01

E 0.03i=oco0.~ 0.02

o~-----'---~--~-~-~100 200 300 400 500 600

(a) v=2m/s

0.06 ~-~--~--~-~--~

0.05 ~----,--~--~-~------,

Number of hosts

800000"LOR Hmin=2" --+--

700000"LOR- Hmin=3" ---)(---

"ASe a=2" ----;.tE---

"ASe=a=3" B

600000 "Se" --.--"PATH" - -&--

"()wner" -- -.--500000

400000

300000

200000

100000

0100 200 300 400 500 600

Number of hosts

(a) v=2m/s800000

"LOR Hmin=2" --+--"LOR- Hmin=3" ---)(---

700000 "ASe a=2" ---*---"Ase=a=3" D

600000 "Se" --.--"PATH" - -&--

"()wner" -- -.--

t500000

400000

i 300000

200000

100000

0100 200 300 400 500 600

Number of hosts

(b) v=20m/s

Number of hosts

(b) v=20m/s

Figure 8. Reply traffic vs. Number of hosts(Flat-priority model)

Figure 9. Responce time vs. Number of hosts(Flat-priority model)

of all methods. On the other hand, When the density ofhosts is high, the response time of the LDR method is longerthan that of the ASC method and the Path replication. Themain reason of this is that the higher access success rate Asof the LDR method. Because larger number of hosts replyto a request in the LDR method, contentions between replymessages from different hosts result in the long responsetime. To obtain high access success rate without increasingthe response time in high density network, we should tunethe parameters of the LDR method e.g. D N which controlsthe probability of holding replicas at the neighbor of branchpoints. Adding to this, using the information of two-hopneighbors to determine the probability would be useful.

5.2. Effect of data request pattern

Figure 10 shows As of various number of hostswhen priority-on-neighborhood model (8 = 1) is used.When hosts gererate requests according to the priority­on-neighborhood model, the average distance between re­questing hosts and the destination of requests is shorterthan when they use the fiat-priority model. Thus the con­nectivity between replica holders and requesting hosts ishigher than the fiat priority model. The LDR method isdesigned to avoid the loss of the connectivity between thereplica holders and requesting hosts. Thus we have sup-

posed that the superiority of the LDR method is small withthe priority-on-neighborhood model. However, even inpriority-on-neighborhood-model, the LDR method outper­forms other methods, while the LDR method in fiat-prioritymodel achieves much more effect when the density of hostsis high. When the number of mobile hosts is more than 300,As of the LDR method is larger than that of other methods.

5.3. Effect of the Velocity

Comparing the case of v = 2m/sand v = 20m/ s in thePriority-on-neighborhood model(fig.lO), As of v = 20m/ sis smaller than v = 2m/ s when the number of mobilehosts is more than 400. In the Priority-on-neighborhoodmodel, replicas are placed at near the requesting host. How­ever, when the velocity of hosts is fast in the Priority-on­neighborhood model, replica holding hosts would moveaway from a requesting host. As a result, As of v = 20m/ sis smaller than v = 2m/sin the Priority-on-neighborhoodmodel.

On the other hand, in the fiat-priority model(fig.6), repli­cas are placed at far from the requesting host. When the ve­locity of hosts is fast, replica holding hosts and a requestinghost would get up close each other. However, connectivitybetween hosts is not stable when the velocity of host is fast.

172

Number of hosts

Number of hosts

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References

data item is generated. We will evaluate the influence ofboth replica distribution when the data item is generated andrearrangement of replicas using the LDR method.

600

600

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400

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300

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200

"LDR Hmin=2" --+­"LDR-Hmin=3" ---x--­

,~SC a=2" ---*--­"ASC-a=3" -------8

"5(" ---.--"PATH" - -&-­

"Owner" -- -.--

"LDR Hmin=2" --+­"LDR-Hmin=3" ---x--­

"ASC a=2" ---;.tE---

"ASCa=3" -0

-"SC" --.-­"PATH" - -&-­

"Owner" - -.--

(a) v=2m/s

oL-_----L__----'--__--'--__-'---_-----"

100

0"------'-----"------'------'---------"100

0.4

0.6

0.2

0.8

0.2

0.8

o

a:: 0.6

j

In this paper, we evaluated the Link-aware and Density­based Rearrangement (LDR) method, a replica rearrange­ment method for location-dependent data to maintain highavailability in mobile ad hoc networks. In the LDR method,replicas are placed at mobile hosts which are neighbors ofthe branch points on the reply route and the density of repli­cas are controlled according to the degree of the networktopology. Simulation results showed that the LDR methodachieves higher access success rate than conventional meth­ods when mobile hosts move fast and hosts request informa­tion related to locations far from themselves.

In this paper, we evaluated only the effect of rearrange­ment of replicas on the process of data requests. However,our method can be used for replica distribution when the

As a result, As of v = 20m/s equals that of v = 2m/ sexcluding the LDR method. In the LDR method, when thevelocity of hosts is fast, many replica holding mobile hostsspread far and wide because not only hosts on reply routesbut neighbor hosts of reply route hosts hold the replica.Adding to this, because request message is forwarded byLBM, if any on host receives the request message, forward­ing requests will success. As a result, when the velocity ofhosts is fast, the As of the LDR method is high.

Figure 10. Access Success Rate vs. Numberof hosts (Priority-on-neighborhood model)

(b) v=20m/s

6. Conclusion

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