5.江韶珊-a minimum hop routing protocol for wireless sensor networks

7/28/2019 5. -A Minimum Hop Routing Protocol for Wireless Sensor Networks

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A Minimum Hop Routing Protocol for Wireless Sensor Networks


Shao-Shan Chiang1 Chih-Hung Huang2 Kuang-Chiung Chang1

1Department of Electrical Engineering2Department of Information Management

Lunghwa University of Science and Technology

Abstract

A wireless sensor network is a data communication system that consists of from several

to thousands of tiny wireless sensor nodes. These battery-powered sensor nodes cooperate

with each other to accomplish data transmission. A variety of wireless sensor networks havebeen developed for different applications in the recent years. In this paper, a data routing

protocol designed for wireless sensor networks is proposed. Due to the lifetime of such a

network depends entirely on the lifetime of the battery, the proposed routing protocol

chooses hop counts and battery power levels as metrics in order to conserve as much energy

as possible, both in computations and data communications. In addition, when some of the

nodes run out of battery power, the routing protocol could effectively adapt the change and

find an alternative path. Simulation results demonstrate the robustness and the

energy-efficiency of the proposed routing protocol.

Keywords: wireless sensor network, routing protocol, multihop relay, energy-efficiency.

I. Introduction

Recent advances in micro fabrications

and wireless communications technology

have enabled the development of low-cost,

small-sized, low power-consuming,

multifunctional sensor nodes, which are

capable of communicating wirelessly in

short distances with each other. A wireless

sensor network could consist of from several

to thousands of these tiny wireless sensor

nodes, which can be added and removed

conveniently to the network whenever it is

necessary. It achieves the goals of low-cost

and flexibility of deployment. Wireless

sensor networks are being developed for a

wide range of civil and military applications,

such as health care [1], bridge monitoring

[2], and home automation [3].

In a wireless sensor network, a sensor

node cooperates with neighbor nodes to

accomplish data transmission. This is so

called the multihop relay. When such a

network is used in a specific application, it

usually scatters sensors in a specific area to

collect data. By means of a multihop relay

scheme, the collected data is transmitted to a

base station for further analysis or for

remote monitoring and controlling.

Therefore, a routing protocol is needed to

ensure the message delivery. Even if there


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are node failures, a robust routing protocol

could dynamically adjust the transmission

path accordingly. There has been much

research in the field of network layer

protocols for wireless sensor networks, suchas flooding [4,5], directed diffusion [6],

sequential assignment routing [7],

gradient-based routing [8] to mention a few.

An important issue on wireless sensor

networks is to find an optimal routing path

to transmit the collected data from a source

node to the sink. Because wireless sensor

networks are different from one to another,the optimal path could be defined in

different ways. For example, energy

dissipation, radio coverage range, and

number of nodes used are all possible

factors, or called metrics, to consider. When

deciding an optimal path, these metrics can

be taken into account separately or in any

combinations. As the number of the sensor

nodes increases, a management scheme to

choose from those metrics becomes more

important.

Figure 1. Routing metrics of the paths.

Assuming a data packet is to be

transmitted from source node A to

destination nodeF as shown in Fig. 1, where

Cij denotes the cost from node i to nodej,

andPi denotes the available power of nodei.

There are three possible paths {ABF},{ABDF}, and {ACEF}. By

summing the costs and counting the hops

along each route, the total cost of {ABF}

is 10 with two hops, the total cost of

{ABDF} is 14 with three hops, and the

total cost of {ACEF} is 8 with threehops. Consequently, the optimal path from

node A to node F will be different

depending on the metric chosen. By

choosing the minimum hop counts as the

only metric, the route {ABF} is the

optimal one with only two hops. On the

other hand, by choosing the total cost as the

metric, {ACEF} is optimal with a costof 8. Lastly, if we choose the maximum

available power as the metric, {ABDF}

would be the optimal path because the total

available power along this path is maximal.

Energy-efficiency is one of the most

important metrics in the routing design for

wireless sensor networks because the battery

power of each sensor node is limited. There

are two strategies to deduce the energy

dissipation for the networks. One is to

control the variable-range transmission

power of the sensor nodes [9]. The other is

to develop energy-efficient routing protocols

based on common-range transmission, i.e.,

nodes broadcast using the same transmission

power level without any power control [4-8,

10].

In this paper, we present an

energy-efficient, scalable and robust routing

algorithm in the data link layer for wireless

sensor networks. Our approach is to flood a

special setup packet to establish a local

routing table for every node in the network

before data transmission actually takes place.

The routing table consists of parent, sibling,and child nodes, together with their


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identification numbers and energy levels,

within one hop distance. Based on the

routing table, each sensor node can

determine the best next-hop node, which has

the highest energy level, to relay themessage.

The rest of the paper is organized as

follows. Section II is a brief literature review

on this topic. Section III describes the

system model of the proposed routing

scheme in details. The system

implementations and experimental results

are discussed in Section IV. Some

conclusions are given in Section V.

I I. Related Works

Classic flooding is an old technique that

can also be used for routing in sensor

networks [4,5]. When a sensor node detects

an event, it broadcasts the message to other

sensor nodes within its radio transmission

distance, which is called one hop. Again, all

the receiving nodes broadcast the message to

all of their neighbor nodes within one hop,

except the node from which they receive the

message. This process is recursively

performed until the message reaches the sink

node, which is a designated node directly

linked to the base station or until a

maximum number of hops for the message

is reached. This routing method has several

deficiencies such as implosion, overlap, and

resource blindness [5].

A directed data dissemination paradigm

is proposed in [6], where the sink node

sends out an interest, which is a task

description, to all sensors by a diffusion or a

flooding mechanism. Each receiving nodethen stores the interest in its cache if no

matching entry exists in the cache already.

The interest entry contains a timestamp and

several gradient fields. The timestamp marks

the time when the last matching interest was

received, and the gradient, up to one perneighbor, indicates the neighbor node from

which the interest was received. As the

interest diffuses throughout the entire sensor

network, the gradients from the source back

to the sink node are set up. Then, any data

matching the interest is sent back toward the

sink node through paths that the gradients

describe.

The sequential assignment routing

(SAR) algorithm creates multiple trees, or

multiple paths from each node to the sink

node, in a wireless sensor network, where

the root of each tree is a one-hop neighbor

from the sink node [7]. Each tree grows

from every root node by successively

branching to neighbors with higher hop

distances while avoiding nodes with very

low quality of service (QoS) and energy

resources. At the end of this tree-building

procedure, most nodes belong to multiple

trees. This allows a sensor node to choose a

tree to relay its data back to the sink node.

There are two parameters, energy resources

and additive QoS, associated with each path

back to the sink node. The SAR algorithm

selects the path based on the two parameters

of each path. As a result, each sensor node

would select the most energy-efficient path

to route the data back to the sink node.

The authors in [8] proposed a variant of

directed diffusion, called gradient-based

routing (GBR). The key idea in GBR is to

memorize the number of hops when aninterest is diffused through the whole


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network. As such, each node can calculate a

parameter called the height of the node,

which is the minimum number of hops to

reach the base station. The difference

between the heights of a node and itsneighbor is considered as the gradient on

that link. Then the data packet is forwarded

to a link with the largest gradient. To avoid

overusing a particular node, when the nodes

energy drops below a certain threshold, it

increases its height so that other sensors are

discouraged from sending data to that node.

Note that when broadcasts are transmitted atthe same power level, a minimum hop

routing is equivalent to a minimum energy

routing [5].

The authors in [10] proposed a

single-path with repair routing (SWR)

scheme. The SWR scheme uses a

flood-based path setup approach to establish

a routing table for each sensor node in the

network. The routing table contains the hop

count from the current sensor node to the

sink node, parent nodes, and hop counts of

the parent nodes. Rather than searching

multiple paths for delivering data packets to

the sink node, SWR searches a single

optimal path with repair, in which data is

forwarded along a pre-established path to

save energy, and a high delivery ratio is

achieved by path repair whenever a broken

link during delivery is detected. In the SWR

scheme, a sensor node can skip over path

break by only using the already existing

routing information in its neighborhood.

I I I. System Model

Our system model consists of a basestation, a sink node, and a number of

wireless sensor nodes, as shown in Fig. 2.

The base station can be located in any

convenient place within or far from the field.

The sink node is a specially designed sensor

node that has more memory than othersensor nodes and is connected to the base

station through a wired or wireless link. The

sensor nodes are densely distributed in the

field so that the radio coverage of any sensor

node covers at least one other node. While

in the network communications, the sink

node takes commands from the base station

and transmits them to other sensor nodeswirelessly. On the other hand, the sink node

also collects data from other nodes and

delivers them to the base station. All the

sensor nodes are battery-powered except the

sink node, because the sink node is the most

frequently used node in this sensor network.

When deployed, the transmission power of

each sensor node is regulated so that its

radio coverage is fixed, despite the decay of

battery power, until the battery is exhausted.

It is required that every sensor node, with a

unique identification number (ID), can

monitor its own battery power level.

base station

sink node

Figure 2. The system model.

There are two phases in the proposedrouting method: the routing table


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establishment phase and the data routing

phase. As soon as the sensor network is

activated, it gets into the routing table

establishment phase in which the flooding

[4,5] technique is applied to establish a localrouting table for each sensor node. The

routing tables will be used in the data

routing phase and will be updated only when

the network topology changes, i.e. when any

sensor fails or a new sensor is added. The

data routing phase consists of three

processes: data routing, table updating, and

command routing. In the data routing phase,

as an event is detected, the data packets can

be transmitted only to the designated nodes

and then to the sink node based on the

routing tables. On the other hand, the sink

node takes commands from the base station

and forwards to the specific nodes or floods

to all sensor nodes. In any case at the data

routing phase, the energy consumption is

very low.

In the following, the algorithms to

establish and to update the routing tables are

described, followed by the algorithms to

route data and command packets.

3.1. The Establishment of Routing Tables

In the sensor network initialization, the

base station requests the sink node to start a

routing table establishment process for the

entire network. Then, the sink node

broadcasts a setup packet to all sensor nodes

within its transmission range. The format of

the setup packet is defined as

Setup.ID.hop.energylevel, where the

Setup indicates that it is a setup packet

and should be flooded throughout the entirenetwork, ID is the ID number of the

sending node, hop represents the hop

count between the sending node and the sink

node, and energylevel is the battery

energy level of the sending node. For the

setup packet broadcasted by sink node, thehop count value is set to 0 and the

energylevel value is set to a very large

value.

(a)

(b)

Figure 3. (a) The setup packet sent from the

sink node. (b) The setup packet is flooded one

hop away.

After receiving the setup packet, all the

receiving nodes within one hop from the

sink node set their hop counts with

increment 1, as shown in Fig. 3(a), and mark

the sink node as their parent node by

recording its hop count, ID number and

energy level on their routing tables. Then,

each node with hop count l, called a relaying


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node, in turn broadcasts a newly constructed

setup packet, as also shown in Fig. 3(b). The

new setup packet has the same format as that

was sent by the sink node, but with the

relaying nodes ID number and energy levelinstead, and the hop count is now set to 1.

This broadcasting process repeats

hop-by-hop until all the nodes in the

network have been notified.

As also shown in Fig. 3(b), the setup

packets, either sent by the sink node or any

of the relaying nodes, may be received by

any other sensor nodes within one hop. Onthe other hand, every sensor node may

receive more than one setup packet from its

neighbor nodes.

Figure 4. Flowchart of the routing table

establishment.

The flowchart in Fig. 4 demonstrates

the workflow of a sensor node receiving a

setup packet. The following algorithm

describes the establishment of the routing

table for sensor nodej when receiving asetup packet from sensor nodei, where hop(i)

denotes the hop count of sensor node i.

Routing table establishment algorithm

IF hop(j) =empty

Set hop(j) =hop(i) +1.

Record nodei into the parent list.

Transmit a new setup packet.

ELSE

IF hop(j) >hop(i)

Record nodei into the parent list.

ELSEIF hop(j) =hop(i)

Record nodei into the sibling list.

ELSE

Record nodei into the child list.END

END

Referring to Fig. 3(b), nodes F, G and H

record node A as their parent node, and node

E records both nodes A and B as its parent

nodes. Also, the sink node records nodes A,

B, C, and D as its child nodes, and bothnodes B and D record node A as a sibling

node while nodes A, C and D record node B

as a sibling node.

Flooding the setup packet in this way

may cause several problems such as

collision and incorrect setting of the hop

count. If several nodes broadcast the packet

at the same time, they will collide to eachother. To prevent collisions, all nodes

receiving the setup packet for the first time


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have to wait for a fixed time plus a random

time, like the back-off algorithm used in

Ethernet, before relaying the setup packet.

The duration of the fixed time is

proportional to the hop count value in thefirst received packet. The design of the fixed

time may lengthen the time needed to

establish the routing tables. However, it can

prevent the receiving sensor node from

setting its hop count to a higher value than it

should be. In other words, once a receiving

node has set its hop count, it is impossible to

receive any setup packet that has smaller

hop count value. To further reduce the

amount of setup packets flooded in the

entire network, every sensor node only

relays the setup packet once.

A routing table consists of three rows. All

the parent, sibling and child nodes, together

with their ID numbers and energy levels, are

put at the bottom, middle and upper row,

respectively. As a result, the memory

capacity needed for the sensor node is very

low because only a local routing table needs

to be stored. Besides, searching for an

optimal path for data routing is merely by

looking up the local routing table stored,

therefore the demand of calculation ability is

relatively low, compared to other classical

methods. The correctness of routing table

can be easily verified when the routing table

establishment phase is completed. The hop

count values of all the parent, sibling and

child nodes should be three consecutive

numbers. If it is not true, this sensor node

resets the current routing table and acts as a

newly added node to establish its routing

table (see the next subsection).

3.2. Deletion and Addition of Sensor

Nodes

When the battery power of a sensor

node falls below a predetermined level, it

sends a delete packet to all the sensor nodeslisted in its routing table. In this situation,

each receiving node simply deletes the

dying-out node from its routing table. The

deletion of a dead node is part of the node

updating process illustrated in Fig. 5.

Figure 5. The node updating sub-process.

As time goes by, sensor nodes will

gradually be energy-exhausted and be

deleted from the routing tables one after

another. Once the number of deleted nodes

increases so that the sensor network

becomes disconnected, new sensor nodes

must be added on in order to maintain the

connectivity of the network. The proceduresof the new sensor node and its neighbor


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nodes in the node addition process are

shown respectively in Fig. 5 and Fig. 6.

Figure 6. Workflow of the new sensor node in

the node addition process.

When a new sensor node appears in the

network, it has to establish its own routing

table and updates routing tables of the

sensor nodes within its one hop distance.

The new sensor node broadcasts an add

packet in the format Add.ID.energylevel,

where the Add indicates it is an add

packet and should be answered, to notify the

sensor nodes within one hop, as node N

shown in Fig. 7. Every receiving node

replies an ACK packet with its hop count,

energy level and ID number destined to the

new sensor node after a random time delay.

Waiting for a predefined time interval to

ensure that all receiving sensor nodes have

responded, the new sensor nodeN generates

its routing table according to the followingcriteria.

Routing table establishment algorithm fora newly added sensor node

Receive ACK packets from all its neighbornode i, for i =1, 2, , m.

Set hop(N) =min{hop(i), i =1, 2, , m}+1.

Sort the responding nodes.

FOR i =1 to m

IF hop(i)


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count, energy level and ID number are sent

to all sensor nodes listed in the routing table.

The receiving sensor nodes append the new

sensor node into their routing tables

according to its hop count.

3.3. The Data Routing Scheme

The data routing scheme is developed to

ensure the data delivery based on the routing

tables. When a sensor node detects an event,

the collected data must be sent back to the

sink node and then to the base station by the

relay scheme. Since processing and

transmitting data packets both consume

energy, the purpose of the proposed data

routing scheme is not only to find a path to

the sink node, but also to conserve as much

battery energy as possible. Besides, to

extend the lifetime of the wireless sensor

network, it also needs to consider how to

avoid overuse of certain nodes in data

packet delivery process.

The proposed routing algorithm

determines an optimal path to the sink node

by choosing the hop count and the energy

level as the metrics. When several paths

exist, the sensor node with the most energy

should be chosen as the next recipient to

relay data packets. Every data packet has a

field specifying the designated recipients ID.

All nodes other than the designated recipient

would ignore the packet when receiving it.

Beside the designated recipient field, the

data packet also contains fields of the energy

level, ID number, and event description of

the sending node. Not only process and relay

the data packet, the designated recipient but

also must acknowledge the reception byreplying an ACK packet with its energy

level to ensure the delivery of the data

packet and to update the energy information

in the routing table. The algorithm to select

the next relay node in an energy-based

manner is described as follows, and theworkflow is shown in Fig. 8.

Data routing algorithm

IF a data packet is received

Reply an ACK packet back.

ELSEIF an NAK packet is received

Delete the replying node from the

routing table.

END

IF parent nodes are available

Forward the data packet to theparent node with the highest energy.

ELSEIF sibling nodes are available

Forward the data packet to thesibling node with the highestenergy.

ELSE

Reply an NAK packet back.

Broadcast a delete packet.

END

The node or path failure detection

mechanism is different from the mechanism

in [10] where the sensor node broadcasts apacket to all the neighbor nodes for help.

The sensor node in the proposed method

makes its own decision to opt a neighbor

node for an alternative path. Once a node or

path failure occurs, the failed node will be

removed from the routing tables of all its

neighbor nodes. It means the neighbor nodes

will not try to use the node or path to deliver

packets in order to conserve energy. The

method provided here not only guarantees


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data packet delivery but also saves energy.

Receive packet

Is a datapacket?

Listening

Choose the parentnode with the

highest energy

Is there anyparent node?

Yes

Yes

Is there anysibling node?

No

Choose the siblingnode with the

highest energy

Yes

Forward packet tothe sibling node

Reply NAK to the

sending node

No

Broadcast a deletepacket

Forward packet tothe parent node

Reply ACK to thesending node

Is an NAKpacket?

No

Yes

Delete thesending node fromthe routing table

Is an ACK

packet?

YesUpdate the routing

table

Other process

Is an ENQpacket?

No

YesReply the routing

table

No

No

Figure 8. The data routing process.

3.4. The Command Routing Scheme

There are two types of commands from

the sink node in the proposed scheme: one is

to all sensor nodes, namely, ComALL, and

the other is to a designated node. The

algorithm to deliver ComALL from the sink

node is similar to that of the routing table

establishment expect the command packet is

forwarded only to the child nodes in the

routing table. The delivery of ComALL

stops when the relaying node has no child

nodes. To reduce the amount of informationflooded in the network, when a relaying

node receives the same ComALL from

different parent nodes, it only relays the

command once.

Sending a command to a designated

node may use ComALL with the ID of thesensor node in it to tell who should respond.

However, this mechanism requires the

participation of all the nodes in the network,

which may unnecessarily consume battery

power. Alternatively, the sink node could

build a global routing table showing the

entire tree structure of the network. The

global routing table can be established bysending an ENQ (enquiry) packet which is a

ComALL command to ask every sensor

node to reply its routing table to the sink

node. The procedure of generating the

global routing table is similar to the data

routing process, as shown in Fig. 8.

According to the global routing table, the

sink node could find a path to the destination

and fill the path into the command packet.

Due to the global routing table building

process is energy and time consuming, it is

not necessarily updated every time the sink

node sends a command. The node failure

detection mechanism in the data routing

algorithm could detect dead routes and

return the message to the sink node for

updating the global routing table.

IV. Simulation Results

We compare the energy-efficient

performance of the proposed method with

the classic flooding scheme and the SWR

scheme in this section. In this comparative

experiment, we used a simple abstract

energy model other than a particular one tocompute energy consumption in one data


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packet delivery. It is well known that in

wireless sensor network communications,

idle listening or signal transmitting/receiving

consumes much more energy of a sensor

node than calculation of an optimal path ormemory accessing does. Therefore, in this

experiment, the energy consumption of

calculation and memory access is not

considered. This model assumed that, from a

pure energy/bit standpoint, the sensor nodes

spent 1.0 and 2.0 energy units respectively

for transmitting and receiving one data bit.

Thus, for a data packet of 1 Kbit long, the

energy for relaying (transmitting and

receiving) one data packet was 3000 energy

units.

Experiments were carried out using

random network topologies. In each network

topology, sensor nodes were randomly

scattered in a fixed 5050 m2 deployed

area, and the sink node was located at the

lower left corner. The number of sensor

nodes changed from 50 to 100 with

increments of 10. Each sensor node had a

maximum transmission range of 10mand a

detection range of 5 m. The maximum hop

count was set 8 for all the routing schemes.

Before data delivery, the routing table had

been set up at each sensor node.The energy

dissipated in the routing table establishment

process was not included in the total energy

consumption, because both of the proposed

method and SWR use a technique similar to

the classic flooding method to establish the

routing table. At the time between the end of

the routing table establishment and

beginning of the data delivery, for each test,

a 10% of the sensor nodes were randomlyselected as failure nodes, in order to test the

robustness of the proposed algorithm. To

make a fair comparison, in each test, each

scheme used exactly the same network

topologies where 20 nodes were randomly

selected as source nodes. Each source nodesent one data packet to the sink node, using

the three different schemes. The size of the

data packet was 1 Kbit, including header.

With 20 successful deliveries of the data

packet, the average energy consumption per

data packet delivery for the proposed

algorithm is shown in Fig. 9. In SWR, a

sending node will broadcast a call-for-help

packet when its next relay node fails. Every

sensor node receiving the packet must

respond in order that the sending node finds

an alternative path. In contract, in the

proposed method, the sending node can

decide an alternative path directly from its

own routing table. Therefore, the proposed

method is more energy-efficient than SWR.Besides the energy saving, we see that, from

this figure, the proposed method is more

scalable.

Figure 9. Energy consumption of the proposed

method (real line), SWR (broken line) and the

classic flooding method (dashed line).


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V. Conclusions

We have presented an energy-efficient

routing scheme in the data link layer for

wireless sensor networks. The time

complicity of the proposed protocol is notdiscussed because it is beyond the scope of

this paper. In the proposed data routing

scheme, sensor nodes with more energy are

selected in data packet transmission relays.

Since the battery power of the network is

utilized efficiently and evenly among all the

sensor nodes, the lifetime of the wireless

sensor networks could be extended to itsoptimum. When any data packet

transmission fails due to sensor nodes fault

or battery exhaustion, the proposed scheme

would quickly adapt the change by updating

the routing tables and resending the packet

via a new optimal path. Therefore, the

proposed scheme is robust, energy-efficient

and scalable.

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5.江韶珊-a minimum hop routing protocol for wireless sensor networks

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