Mobility of Wsn

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A Data Delivery Mechanism to Support Mobile

Users in Wireless Sensor Networks

Euisin Lee, Younghwan Choi, Soochang Park, Donghun Lee, and Sang-Ha KimDepartment of Computer Engineering

Chungnam National University220 Gung-dong, Yuseong-gu, Daejeon, 305 764, Republic of Korea

Emails: {eslee, yhchoi, winter, dhlee}@cclab.cnu.ac.kr and [email protected]

Abstract — Wireless sensor networks traditionally consist of

sensors perceiving data and sinks gathering the data. In addition,

users receive required information from the sinks via

infrastructure networks. The users, however, should receive the

information from the sinks through multi hop communications

of disseminated sensor nodes if such users move into the sensor

networks without infrastructure networks. Unlikely, the

previous works only considered mobility of sinks, which function

users. Nevertheless, it is difficult for such approaches about

mobility to exploit the existing data-centric routing algorithms

and also for the mobile sinks to function as gateways to connect

with infrastructure. To improve the shortcomings, we suggest a

novel viewpoint of mobility for wireless sensor networks and

propose a novel architecture and mechanism to support the

mobility with multiple static sinks in this paper. The multiple

static sinks, which are connected with each other via

infrastructure, provide high throughput and low latency.

Furthermore, they improve hotspot problems and prolong

network lifetime. The proposed mechanism finally is evaluated

by simulation results about throughput, latency, and network

lifetime.

Index Terms — wireless sensor networks, mobility, users, and

multiple static sinks

1. Introduction

Wireless sensor networks traditionally consist of sensors

perceiving data and sinks gathering the data. In addition,users receive required information from the sinks viainfrastructure networks. A mobility model in wireless sensor

networks, mentioned above, can classify according to eachobjects. Namely, it is a mobility of sensor node, a mobility of a sink, and a mobility of a user. Hence, deciding a mobility of

what kind of object for sensor networks that suits sensor networks according to various applications is important.

Recently, applications transmitting data to moving users

inside sensor fields such as rescue in disaster area or maneuver in the war zone are on the rise in large-scale sensor

networks [5]. But, until now, such researches support amobility of user by only two forms for these applications.First form is same with figure 2. It supports a mobility of a

user on the assumption that the user communicates directlysinks through infrastructure networks, namely, internet likecommunication systems in traditional sensor networks [1].

But, in applications such as rescues in disaster area or maneuvers in the war zone, circumstances without

infrastructure networks except sensor networks are moreactually. Because, infrastructure networks in these

applications cannot be used because they are damaged as aresult of the war or the disaster. Hence an assumption that auser and a sink can directly communicate through internet has

a problem that is not actually. Therefore communication between the user and the sink inside sensor fields is supported

by only sensor nodes.Second form is same with figure 3. It identifies a user with a

sink. So it supports a mobility of the user by reflectingmovement of the user with the direct movement of the sink

[5~9]. But, researches for this form have also various problems. First of all, they cannot use existing effective datacollection algorithms [2][3][4] between a sink and sensor

nodes based on data in static sink sensor networks. Because,such algorithms can hardly be exploited due to location changeof sink which collects data if sinks in sensor networks have

Figure 1. Typical wireless sensor networks model

Figure 2. A model to support

mobility of user through infra

structure networks, namely,

internet & satellite.

Figure 3. Mobile sink model

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in the outskirts of the war zone deploy sensor nodes in the war zone and collect locations and movements of enemies fromsensor nodes. Through collected data, the headquarters

elaborate a plan of operations and delivery the operations tosoldiers in the war zone. Then, soldiers carry out the

operations and by extension, will collect directly data fromsensor nodes to obtain the latest information.

3. Description of Mechanism

A. Overview of Mechanism

In our mechanism, if a user intends to obtain information onmoving inside sensor networks, the user disseminates interestto the nearest sink via sensor nodes, and then the user receivesresults of interest from the sink. Also, if the nearest sink of theuser changes the user requests the results to new the nearest

sink and receives the results from the new sink.

This paper makes the following assumptions:• A user can communicate multiple static sinks through

only sensor nodes, because networks within sensor fieldsare without infrastructure networks.

• Multiple static sinks are deployed in an arbitrary

position in the outskirts of sensor fields connected with

infrastructure networks as internet.

• Multiple static sinks can directly communicate other

sinks via infrastructure networks.

• The data which one sink collects is aggregated by the

sink. All sinks share the aggregated data via infrastructurenetworks.

To implement the proposed mechanism, we need to addressthe following phases: dissemination of sink announcementmessage, interest dissemination of user, data collection of sink,

information sharing of multiple static sinks, mobility supportof user, and information propagation of sink. We detail each phase to next section.

B. Dissemination of sink announcement messageTo the initial stage of sensor network, if a sink is located in

an arbitrary position in the outskirts of sensor fields which isconnected with infrastructure networks as internet it has

flooded a sink announcement message to announce itself inside the whole sensor fields like figure 5. As a result of

flooding a sink announcement message, every sensor nodeshave known hop counts and next hop neighbor sensor node to

each sink. And every sensor nodes have known the nearestsink from location of themselves through hop counts to eachsink.

C. Interest dissemination of the user While moving inside the sensor fields, if a user wants to

collect a data from sensor fields, the user selects the nearestsensor node from location of itself as first agent. And the user delivers an interest to the first agent. The first agent which is

delivered the interest from user forwards the interest to a nexthop neighbor node toward the nearest sink. The next hopsensor node which is delivered the interest also forwards a

next hop neighbor node toward the nearest sink. This processis continued until the sink. So, the sink receives the interest of the user. Also, a back route from the sink to the user for the

interest has established through this process.

D. Data collection of the sink Sensor networks with a static sink are a network that

sensing data from sensor nodes should be transmitted to the

static sink through multi-hop communication. Routingalgorithms to collect data in sensor networks with a static sink are used in scenario of various types, for example, a scenario

generating data by periods, a scenario generating a minorityevent, and a scenario detecting a moving object, etc.

Hence, a user will can select and use the most appropriaterouting algorithm with static sink according to a scenario.Such research was advanced already plentifully [2][3][4]. Sowe will not mention anymore in this paper. Therefore, we use

one in the existing routing algorithm as the routing algorithmto collect data in this paper.

E. Information sharing of multiple static sinksAs shown in Fig. 1, a sink in typical sensor networks takes

charge of the function as gateways for connection withinfrastructure networks [1]. And various papers in relation tomultiple static sinks also indicate the connection between a

sink and an infrastructure network and the connection betweenall sinks as an assumption [10 - 13]. Therefore, in this paper, itis a sufficient propriety that all sinks placed in the edge of a

sensor field can communicate with the other sinks via

Figure 5. Dissemination of sink announcement

message

Figure 6. Mobility support of the user Figure 7. Information propagation of the sink

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infrastructure networks. Hence, in proposed mechanism, asink which is delivered an interest from a user collects datafrom sensor fields and aggregates the collected data. Next, the

sink will share aggregated information with the other sinksthrough infrastructure networks.

F. Mobility support of the user

The user may move to other place after sending interest tosink by agent. In this case, the user selects another agent andcreates a new connection from the original agent to newselected agent. The user can receive the aggregatedinformation from sink through this connection. Thus, mobility

of the user is guaranteed.The user can obtain the information of neighbor nodes in

radio range of the first agent by reply message received from

the first agent. If the user moves out of RF range of the firstagent, then the user will retransmit the agent selection messagewith same RF range of the agent. All sensor nodes which have

received this message send a reply message which contains theinformation of neighbor node in their radio range to the user.

As shown in Figure 6, the user checks whether there is sensor nodes which are a neighbor node both in RF range of firstagent and in RF range of the sensor node sent the replymessages. Among the sensor nodes which have the connection

node in their radio range, the user selects the nearest sensor node from user as second agent. The user settles this selected

sensor node as second agent and informs the connection nodes between the first agent and the second agent to the secondagent. Second agent creates the connection to the first agent

through the connection node informed by user. By this way,the path from the first agent to the user can be createdaccording to movement of user.

G. Information propagation of the sink A sink delivers the aggregated information of the collected

data to first agent through the connected path. The first agentdelivers the aggregated information to last agent through theconnection of agents. Last agent deliveries the aggregated

information to the user.As shown in figure 7, if the nearest sink from the user

changes, the user requests the information to the new nearestsink and receives the information from the new nearest sink.

4. Performance Evaluation

In this section, we evaluate the performance of a proposedmechanism through simulations. We first describe our simulation model and simulation metrics. We next evaluate

how environment factors and control parameters affect the performance of a proposed mechanism.

A. Simulation Model and Metrics

We implement the proposed mechanism in the Qualnetver.3.8 [14]. A sensor node’s transmitting and receiving power consumption rate are 0.66W and 0.39W respectively. Thetransceiver in the simulation has a 200m radio range. Eachinterest packet is 36 bytes long and the data packet has 64

bytes. The sensor network consists of 50 sensor nodes, whichare uniformly deployed in a 1000m x 1000m field.

The multiple static sinks are located in the outskirts of

sensor fields. The number of user is one. And the default speedof user is set to 10 m/sec. the user disseminates an interest at aninterval of 10 second. Every sensor nodes receive the interest

and generate only one sensing data for the interest. This isdefined as one interest round. Namely, one interest round is 10

second. The simulation lasts for 500 seconds.We use for metrics to evaluate the performance of the

proposed mechanism. The network lifetime is defined as thenumber of the interest round first sensor node die. The

residual energy is defined as the residual energy of sensor nodes at time, namely, the interest round that first sensor node

die. The data delivery ratio is the ratio of the number of successfully received reports at a user to the total number of reports generated by every sensor node. The delay is defined

as the average time between the time a sensor node transmits areport and the time a user receives the report.

We compare a static sink model without a mobile user to a

multiple static sinks model with a mobile user in the

simulation. We express a direct communication model between the user and the sink as ‘sink 1 and not user’ and a

user movement model as ‘sink 1 and user 1’ according to thenumber of sink in figures of performance evaluation. Here ‘1’is case that sink is one. If the number of sink changes, this

numerical value reflects in figures of performance evaluation.

B. Impact of the number of multiple static sinksWe first study the impact of the number of sinks on the

proposed mechanism’s performance. The number of sinks

varies from 1, 2, 3, to 4. And the number of sensor nodesvaries from 50, 100, 150, to 200. Sinks have a maximum speed

Figure 8. Network lifetime for the number of sink and sensor node Figure 9. Residual energy for the number of sink

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of 10mm/s. In this part, we compare one static sink without the

user to multiple static sinks with the user.Figure 8 shows the number of interest round, namely,

network lifetime. One static sink model without a mobile user

is of small number due to hotspot problem of sensor nodes

near the sink. But, the number of interest round of multiplestatic sinks model with mobile user is higher than the number of interest round of one static sink sensor network even thoughthe sinks are more than 3. Network lifetime, namely, interest

round, prolonged because energy consumption of sensor nodes became evenly. Figure 9 shows the residual energy attime that first sensor node dies in simulation circumstances of

50 sensor nodes As shown in Figure 9, the energyconsumption of sensor nodes become more evenly because itsolves hotspot problem due to addition of sink. Figure 10

shows the data delivery ratio. A model with mobile user islower than a model without user because it must deliveryinformation from sink to user. But, the hop count between sink

and user decreases due to addition of sink because data failsreduce. Therefore the data delivery ratio of a model withmobile user approaches a model without user. Figure 11 shows

the delay. The delay of a model with mobile user is longer thanthe delay of a model without user because it must delivery

information from sink to user. Therefore the data delivery ratioof a model with mobile user also approaches a model withoutuser.

C. Impact of the number of sensor nodesWe next evaluate the impact of the number of sensor nodes

on the proposed mechanism. The number of sensor nodes

varies from 50, 100, 150, to 200. As shown in Figure 8, 10, and11, the proposed mechanism never falls a performancenevertheless the number of sensor node increase.

D. Impact of the user’ mobility

We last evaluate the impact of the user’s moving speed onthe proposed mechanism. In the default simulation setting, wevary maximum speed of a user from 6, 8, 10, 12, to 20m/s. In

this part, we compare a network model of one static sink without the user to a network model of four static sinks withthe user. Figure 12 shows data delivery ratio when the user’

moving speed changes. Because the static sink model withoutthe user is not user, it indicates the same result independent of the user’s moving speed. While the multiple static sinks modelwith the user decrease according to increment of the user’smoving speed. But the data delivery ratio remains around 0.9 – 1.0 nevertheless the user move faster. Figure 13 shows the

delay about data delivery, which increases only slightly as the

user moves faster, because it increases the number of agentfrom the sink to the user. Figure 13 shows that the network

lifetime decreases as the user’ moving speed increases. Thefaster a user moves, the more a user needs the number of agentfor connection between the user and the sink.

5. Conclusion

In this paper, we propose a novel sensor network model and

a novel mechanism to support mobility of users in wirelesssensor networks based on multiple static sinks In proposednetwork model, because multiple static sinks can

Figure 11. Delay for the number of sink and sensor node Figure 12. Data delivery ratio for user speed

Figure 12. Data delivery ratio for user speed Figure 13. Delay for user speed

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communicate with the other sinks as short hop via

infrastructure networks, the user receives the information withhigher data delivery ratio and faster time. And the lifetime of the sensor networks increase because the balance energy

consumption of sensor nodes is possible.We verified that the lifetime of sensor networks is

prolonged because a use of multiple static sinks decreases a

consumption of sensor nodes. Also, we verified that a

performance about the data delivery ratio and the delay never falls nevertheless a communication between the user and the

sink for guaranteeing movement of the user is supported byonly sensor nodes without infrastructure networks, namely,internet.

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Mobility of Wsn