An Analytical Study of Variable Preamble Length-Based Broadcasting Scheme for WSNs

doi:10.4236/cn.2013.53B2095

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Communications and Network, 2013, 5, 517-523

http://dx.doi.org/10.4236/cn.2013.53B2095 Published Online September 2013 (http://www.scirp.org/journal/cn)

An Analytical Study of Variable Pr eamble Length-Based

Broadcasting Scheme for WSNs

Arun Kumar 1, Kai-Juan Wong2

1School of Computer Engineering, Nanyang Technological University, Singapore

2Singapore Institute of Technology, Singapore

Email: a [email protected], [email protected]

Received May 2013

ABSTRACT

In this paper, we present a detailed analytical study of previously proposed variable preamble length-based broadcasting

scheme for wireless sensor networks (WSNs). Analytical results show that in dense wireless sensor networks, broad-

casting a packet with a small size preamble can significantly improve the energy conservation on already limited battery

powered sensor nodes. For fairer comparison with variable preamble length-based broadcasting scheme, a comprehen-

sive analytical study for an existing probability-based broadcasting scheme is also presented. Analytically calculated

results show that variable preamble length-based broadcasting scheme is more energy-efficient than the probability-

based br oa dc asti ng scheme .

Keywords: Broadcast; Flooding; Analytical Model; Wireless Sensor Network

1. Introduction

A large-scale wireless sensor network (WSN) can consist

of thousands of sensor nodes and are helpful in many

situations [1], such as disaster relief missions and battle-

field communication facilities, infrastructure protection

and scientific exploration [1-3]. Wireless data transmis-

sion consumes more energy than data processing in a

sensor node. The situation becomes worse in dense wire-

less networks where broadcasting of data is necessary. In

such cases, transmission of a single bit may consume the

same amount of energy as that needed for processing

thousand operations in a typical sensor node.

Wireless sensor networks have broadcast as one of its

most fundamental service. Broadcast provides maximum

message propagation across the whole network and serves

high-level operations, making it critical to the overall

network design. Although broadcasting in wireless sen-

sor networks have many advantages, it can cause serious

problems like “broadcast storm problem” [4] , whi c h could

cause a lot of contention, redundant retransmission, col-

lision and most importantly, waste immense amount of

energy. A possible solution is that the sensor nodes al-

ternate between active and dormant states, which help

them to conserve energy and extend the network lifetime.

Simple broadcasting in this environment, where a node

goes to active and dormant state periodically in asyn-

chronous manner can be very challenging if there are not

enough nodes in the active state when a source wants to

send data.

As wireless communication is the major cause of en er-

gy consumption on sensor nodes, efforts have focused on

how to conserve energy especially at the medium access

control (MAC) layer. While a long preamble is used be-

fore the data packet in BMAC [5], we believe that a

small preamble is enough to achieve sufficient reachabil-

ity and throughput in dense wireless area networks. On

the other hand, it can dramatically save the energy re-

quired to broadcast a packet.

In this paper, we present an analytical study for varia-

ble preamble length-based broadcasting scheme [6]. An

Analytical study for an existing probability-based broad-

casting scheme is also presented and the scheme is im-

plemented to compare with the variable preamble length-

based scheme.

The rest of the paper is organized as follows. Section 2

introduces the motivation behind variable preamble length-

based broadcasting scheme. In Section 3, an analytical

study for variable preamble length-based and probabili-

ty-based broadcasting schemes is given. The parameters

used during the simulations and results are given in Sec-

tion 4. Section 5 concludes the paper and gives some

suggestions for future work.

2. Motivation and Variable Preamble

Length-Based Broadcasting

Wireless sensor networks have an important property that

A. KUMAR, K.-J. WONG

518

many sensor nodes alternate between active and dormant

states, helping them to conserve energy and extend net-

work lifetime. In dense area networks, there is a greater

chance of a node being in an active state to listen to the

channel at a particular time, when compared to a sparse

area network. Simple flooding in dense area networks,

where more nodes are active at a time near the relay node

will only cause energy consumption rather than covering

additional areas in the network, resulting in unnecessary

retransmission of packets and collisions.

Energy efficient MAC protocols are used to conserve

energy of sensor nodes in wireless sensor networks . BMAC

[5] is an energy-saving, distributed, asynchronous and

random-access MAC protocol best suited for wireless

sensor networks. BMAC employs the idea of “preamble

listening” to conserve energy, whereby nodes would du-

ty-cycle the radio receivers by periodically sampling the

channel for activity. In order to transmit a packet, a node

would transmit a long preamble to inform its neighbors

of impending data. This allows B-MAC to support asyn-

chronous and random-access transmission of data pack-

ets. However, a long preamble contains no useful data

and in dense wireless networks it is the major cause of

energy wastage. Preamble length TPreamble in BMAC must

be such that

Preamble Interval

TT>

where TPreamble is the duration of preamble length and

TInterval is the wakeup interval of the nodes.

Variable preamble leng th-based broadcast uses a small

preamble before the data packet in dense wireless sensor

networks [6]. Figure 1 shows the difference between (a)

broadcast using the long preamble before the data pack et

and (b) broadcast using variable preamble length in asyn-

chronous duty cyc led wireless sen sor network. The small

preamble can clearly affect the overall performance of

the broadcasting scheme in dense networks. In case of

small preamble being sent before the data packet, this

preamble wil l only be list ene d by a subs et of nodes present

in the radio range of a relay node or sender node. The

subset of sensor nodes that receives the data packet then

rebroadcasts the packet using the same preamble length.

This practice of sending a small preamble before the data

packet reduces the number of rebroadcasts as well as the

overall energy consumption in the network, as only a

subset of nodes will listen to the preamble and receive

the data packet. Sensor nod es will rebroadcast the packet

only after a random delay.

(a)

(b)

Figure 1. Shows the difference between (a) Broadcast using the long preamble before the data packet and (b) Broadcast using

variable pr eamble length in as ynchronous duty cycled wireless s ensor networks.

A. KUMAR, K.-J. WONG

519

In dense area networks, even small preambles used in

variable preamble length-based broadcast [6] can give

sufficient data dissemination and dramatically save the

energy of the node. In sparse area network, a compara-

tively large preamble will b e required due to the fact tha t

there are lesser chances that a node is active in the neigh-

borhood of the relaying node or the sender node. These

small preambles will save most of the rebroadcasts and

conserve maximum energy in dense area networks. In

case of small preamble being used the preamble length

TPreamble must be such that

Preamble Interval

TT≤

where TPreamble is the duration of the preamble and TInterval

is the wakeup interval of the nodes.

3. Analytical Models

Most of the energy consumption on a node is because of

the node’s radio power consumption. Therefore, analyti-

cal models focus on the radio power consumption of

nodes. Some equations in this section are due to [7].

Table 1 lists the abbreviations and units used in the

equations for the calculation of the energy consumption.

Furthermore, to simplify the comparisons, the analyti-

cal study is based on the following conditions:

• All equations are normalized to one second.

• All the scenarios have same number of nodes and

transmission conditions.

• Both the broadcasting algorithms use preambles be-

fore the data packet but the size of the preamble dif-

fers substantially. The following equation s are used to

calculate the duration of TPreamble and TS_Preamble for the

probability-based and variable preamble length-based

broadcasting schemes respectively:

PreambleIntervalSample Guard

TT TT= ++

(1)

_S PreambleVariableSampleGuard

TTTT= ++

(2)

• To perform channel sampling from an idle state, a

node has to turn its radio on and then sample the

channel. Thus, the time taken to perform channel sam-

pling is calculated to be:

SampleIRx RSSI

T TT= +

(3)

• A node will turn on its receiver and wait for the Radio

Strength Signal Information (RSSI) value to stabilize

before transmitting a packet. Then the node will check

whether the channel is free for the entire duration of

Tcsma. For simplifying the comparison between the

different protocols, the channel is always assumed to

be clear, and therefore no back-offs were considered

during channel contention. At the MAC layer, for

both the broadcasting protocols, the total time, TCSMA,

in which the radio receiver is turned on to perform

CSMA is calculated as follows:

( )

CSMAIRx RSSIcsmaPkts

TTT TN=++ ∗

(4)

Table 1. Notations in the analytical model.

Abbreviation Description Unit

TTx Time taken to transmit a data frame Sec

TRx Tim e taken to receive a data frame Sec

TS_Preamble T he duration for which the small preamble is sent Sec

TVariable The variable minimum time duration for which the small preamble should be sent Sec

TInterval Time between each sampling of the channel during channel listening Sec

TPreamble The minimum duration for which the preamble should be sent Sec

TGuard A guard time to allow for clock skews between nodes Sec

Tcsma Duration for which the channel has to be clear before the node can assume that the channel is free Sec

NNeigh Number of neighbors per node

NPkts Number of packets transmitted per second

TTxb Time taken to transmit a byte Sec

TRxTx, TITx Time taken for the radio to switch from Receive (Rx)/Idle to Transmit (Tx) mode Sec

TIRx Time taken for the radio to switch from Idle to Receive mode Sec

TRSSI Time for RSSI to provide a reading Sec

TSample Time taken from the idle state to sample the channel for activity Sec

PRx Power consumption of the radio in Receive mode mW

PTx Powe r consumption of the radio in Transmit mode mW

PIdle Power consum ption of the radio in Idle Mode mW

P Predefined probability with which a data packet is broadcasted [0.2, 1]

A. KUMAR, K.-J. WONG

520

• As in probab ility-based schemes nodes do not require

global topological information of the network to make

rebroadcast decisions. Every node is allowed to re-

broadcast a packet based on a predetermine d forward-

ing probability P. A random number between 0 and 1

is generated and if this random number is below or

equal to a predefined probability P then the packet is

rebroadcasted.

Random NumberP≤

(5)

• It is assumed that the transmitted signals will always

be received correctly. So the c hannel no ise i s not mod-

elled in the equations.

• In this analysis, for a fairer comparison of both the

broadcasting schemes, the equations and results are

based on “average case” scenarios for probability based

broadcasting scheme and on the “average or worst

case” scenarios for variable preamble length-based

broadcasting scheme.

3.1. Probability-Based Broadcasting Scheme

The total power consumption of the radio in the case of

probability-based broadcasting scheme, EProbBroadcast, is cal-

culated from the durations for which the radio receiver or

transmitter is turned on to perform the following opera-

tions: CSMA (4), transmission of the data packet (7),

reception of a data packet (8), channel listening (9) and

idling ( 10 ) .

ProbBroadcastCSMAProbBroadcast TxProbBroadcastRx

ProbBroadcast ListenProbBroadcast Idle

E EEE

=++

(6)

where, (ECSMA = TCSMA*PRx), (EProbBroadcast_Tx = TProbBroad-

cast_Tx*PTx), (EProbBroadcast_Rx = TProbBroadcast_Rx*PRx), (EProb-

Broadcast_Listen = TProbBroadcast_Listen*PRx) and (EProbBroadcast_Idle

= TProbBroadcast_Idle*PIdle).

Before transmitting a data packet, a node always sends

a long preamble followed by the data packet. Therefore,

the time, TProbBroadcast_Tx, for which the transmitter will be

turned on can be represented by following equation;

( )

_ProbBroadcast TxRxTxPreambleTxPkts

TTTTNP=++∗ ∗

(7)

To receive a data packet, nodes turn their receiver on if

any channel activity is detected during the periodic lis-

tening. These nodes will keep their receiver turned on

until a packet is received and the total time, TProbBroad-

cast_Rx, for which the node receiver is turned on for the

reception of a data packet is represented by following

equation:

( )

0.5 * **

ProbBroadcast Rx

PreambleRxPkts Neigh

TTNN P= +∗

(8)

For the remainder of the time, when a node is neither

receiving nor transmitting data, the node performs peri-

odic channel listening. The duration, TProbBroadcast_Listen, for

which the receiver is turned on for channel listening is

calculated as follows:

( )

()

ProbBroadcast Listen

ProbBroadcast TxProbBroadcastRxCSMAInterval

Sample

T TTT



=−−−





∗

(9)

Finally, the total time TProbBroadcast_Idle, for which a

node’s radio remains in an idle state, is calculated to be:

( )

ProbBroadcast Idle

ProbBroadcast TxProbBroadcastRxCSMA

ProbBroadcast Listen

TTT

=−−−

−

(10)

The above equations will be true only if the following

condition is satisfied:

TProbBroadcast_Listen ≥ 0

This is explained by the fact that a negative value for

TProbBroadcast_Listen implies that the bandwidth of the chan-

nel has been exceeded.

3.2. Variable Preamble Length-Based

Broadcasting Scheme

The total power consumption of a node in the case of

variable preamble length-based broadcasting scheme can

be calculated from the durations for which the node’s

radio receiver or transmitter is turned on to perform the

following functions: CSMA (4), transmission of the data

packet (12), reception of a data packet (13) or (14), chan-

nel listening (15) and idling (16).

PreaBroadcastCSMAPreaBroadcast TxPreaBroadcastRx

PreaBroadcast ListenPreaBroadcast Idle

E EEE

=++

(11)

where, (ECSMA = TCSMA*PRx), (EPreaBroadcast_Tx = TPreaBroad-

cast_Tx*PTx), (EPreaBroadcast_Rx = TPreaBroadcast_Rx*PRx), (EPrea-

Broadcast_Listen = TPreaBroadcast_Listen*PRx) and (EPreaBroadcast_Idle

= TPreaBroadcast_Idle*PIdle).

Before transmitting a data packet, a node will transmit

a small preamble of fixed size followed by a data packet.

This small preamble will always be less than the regular

preamble sent in probability based broadcasting scheme

as shown in Figure 1. The time which a node takes to

transmit a packet can be represented by following equa-

tion;

( )

PreaBroadcast TxRxTxSPreambleTxPkts

TT TTN=++

(12)

To receive a data packet, nodes will turn their receiver

on if any channel activity is detected during periodic lis-

tening. These nodes will keep their receiver turned on

until a packet is received and the total time, TPreaBroad-

A. KUMAR, K.-J. WONG

521

cast_Rx, for which the node receiver is turned on for the

reception of a data packet is represented by following

equation:

( )

0.5** *

PreaBroadcast RxS PreambleRxPktsNeigh

TT TNN= +

(13)

In the worst case (when the receiver checks the chan-

nel just when the transmitting node starts sending the

preamble) the total time, TPreaBroadcast_Rx, for which the

node receiver is turned on for the reception of a data

packet is represented by following equation:

( )

PreaBroadcast RxS PreambleRxPktsNeigh

TT TNN= +

(14)

For the remainder of the time, when a node is neither

receiving nor transmitting data, it performs periodic chan-

nel listening. The duration, TPreaBroadcast_Listen, for which

the receiver is turned on for channel listening is calcu-

lated as follows:

( )

()

PreaBroadcast Listen

PreaBroadcast TxPreaBroadcastRxCSMAInterval

Sample

T TTT



=−−−





(15)

Finally, the total time, TPreaBroadcast_Idle, for which a

node’s ra dio remains in the idle state, is calculated to be:

( )

PreaBroadcast Idle

PreaBroadcast TxPreaBroadcastRxCSMA

PreaBroadcast Listen

TTT

=−−−

−

(16)

As worst case assumptions are being used, the above

equations will be true only if the follow ing cond itions are

satisfied:

TInterval ≥ TS_Preamble + TIRx

TPreaBroadcast_Listen ≥ 0

The above mentioned conditions can be explained by

the fact that a negative value for TPreaBroadcast_Listen implies

that the bandwidth of the channel has been exceeded.

4. Simulation Parameters Used and Results

As the previously proposed variable preamble length-

based broadcasting scheme [6] has been experimentally

evaluated, we now present the preliminary results of the

broadcasting schemes based on analytical study.

Table 2 shows the parameters used for the calculation

of the energy consumption for the broadcasting schemes.

These parameters are determined based on the CC2420

transceiver [8].

Packet broadcasting rate is 1 packet/sec. A d ata packet

is of 32 bytes, time to transmit a byte i s 0 .000 032 sec and

the data rate is 250 kbps. For the evaluation of probabili-

ty-based broadcast the values of TPreamble are recalculated

according to the TInterval (0.01 - 0.05 sec) based on for-

mula (1).

Based on the parameters, the equations are solved to

determine the energy consumption of the broadcasting

nodes un de r different broadc asting c onditions.

4.1. Scenario 1

Figures 2-4 show the energy consumption by the node’s

radios to communicate a packet between the nodes, while

Table 2. Parameters Used To Emulate CC2420.

Abbreviation Unit

TRxTx 0.000192 Sec

TIRx 0.000192 Sec

TRSSI 0.000128 Sec

TSample 0.00032 Sec

TGuard 0.00068 Sec

Tcsma 0.001 Sec

PRx 62.1 mW

PTx 57.4 mW

PIdle 1.41 mW

Figure 2. Energy Consumption (mW) Vs TInterval (sec).

Figure 3. Energy Consumption (mW) Vs TInterval (sec).

A. KUMAR, K.-J. WONG

522

Figure 4. Energy Consumption (mW) Vs TInterval (sec).

the broadcasting node considers 10 nodes in the neigh-

bourhood. The power consumptions by the radios is cal-

culated for different sets of TInterval (0.01 - 0.05 sec).

Figure 2 shows the energy consumption (mW) versus

TInterval (sec). It is observed that as the value of TInterval

increases, the en ergy consumption increases linear ly. This

increase in the energy consumption is due to the long

preamble used f or increased TInterval. Broadcasting a packet

with a low probability shows less energy consumption as

compared to broadcasting a packet with high probability.

The difference in energy consumption of different prob-

abilities of broadcasting is clearly visib le at higher values

of TInterval.

Figures 3 and 4 show the energy consumption (mW)

versus TInterval (sec) for average and worst cases respec-

tively. The value of TInterval varies and the packets are

broadcasted with different preamble length (4 ms to 21

ms). It is observed that as the value of TInterval increases

the energy consumption decreases. Broadcasting a packet

with a small preamble shows less power consumption

compared to broadcasting a packet with a long preamble.

A long preamble always results in high transmission and

reception power consumption. The energy consumption

of the radios in both cases decreases linearly as the TInter-

val increases. This energy saving in case of high TInterval is

because of the small preambles used for broadcasting

packets.

4.2. Scenario 2

Figures 5-7 show the energy consumption by the node’s

radios to communicate a packet between the nod es, wh ile

TInterval is considered as 0.02 sec. The power consump-

tions by the radios is calculated for a range of neighbor-

ing nodes (1 - 10 nodes).

Figure 5 shows the energy consumption (mW) versus

number of neighbors. It is observed that as the number of

neighbours of a broadcasting node increases, the energy

consumption increases linearly. Broadcasting a packet

with a low probability shows less energy consumption as

Figure 5. Energy Consumption (mW) Vs Number of Neigh-

bors.

Figure 6. Energy Consumption (mW) Vs Number of Neigh-

bors.

Figure 7. Energy Consumption (mW) Vs Number of Neigh-

bors.

compared to broadcasting a packet with hig h pr obabili t y.

Figures 6 and 7 show the energy consumption (mW)

versus number of neighbors for average and worst cases

respectively. The number of neighbors of a broadcasting

node varies and the packets are broadcasted with differ-

ent preamble length (4 ms to 21 ms). It is observed that

as the number of neighbors of a broadcasting node in-

creases, the energy consumption increases linearly. Broad-

A. KUMAR, K.-J. WONG

523

casting a packet with a small preamble shows less power

consumption compared to broadcasting a packet with a

long preamble. A long preamble always results in high

transmission and reception power consumption. The energy

consumption of the radios in both cases increases as the

neighboring nodes increases. But a higher energy con-

sumption in worst case is due to the long preamble lis-

tening.

5. Conclusion and Future Work

In this paper, we have presented a comprehensive analyt-

ical model of the variable preamble length-based broad-

casting scheme for wireless sensor networks. An analyt-

ical model for an ex isting probability-based broadcasting

scheme is also presented. Analytical models do not con-

sider the circuit and CPU power consumption in the pre-

sented equations. Our analytical study emphasizes the

fact that a small preamble during broadcast can mitigate

the need for higher energy consumption. Analytical re-

sults show that variable preamble length-based broad-

casting scheme provides higher energy conservation in

dense area networks, when compared to the probability-

based scheme.

In future, we plan to collect these power consumption

results by implementing these broadcasting algorithms

on a physical test bed. Results obtained from analytical

studies, simulations and physical test bed will be used to

compare the broadcasting algorithms.

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