The document discusses using a mathematical model to analyze how adding supplementary sensor nodes near base stations could increase network lifetime by reducing the energy burden on nodes closest to the base stations. The results show that for some networks, adding only a limited number of extra nodes could quadruple network lifetime. However, the potential gain depends heavily on the existing fraction of nodes near the base stations.

1. _
A Simple and Efficient Method to Mitigate the Hot Spot
Problem in Wireless Sensor Networks
Helena Rivas, Thiemo Voigt, Adam Dunkels
Swedish Institute ofComputer Science, Box 1263, SE-164 29 Kista, Sweden
{hefena, thiemo, adam}Gsics. se
Abstract. Much work on wireless sensor networks deals with or considers the
hot spot problem, i.e., the problem that the sensor nodes closest to the base station
are critical for the lifetime of the sensor network because these nodes need to
relay more packet than nodes further away from the base station. Since it is often
assumed that sensor nodes will become inexpensive, a simple solution to the hot
spot problem is to place additional sensor nodes around the base stations. Using a
simple mathematical model we discuss the possible performance gains of adding
these supplementary nodes. Our results show that for certain networks only a
limited number of additional nodes are required to fourfold network lifetime. We
also show that the possible gain depends heavily on the fraction of nodes already
present in the vicinity of the base station.
I Introduction
The main task of most wireless sensor networks is to collect data and send it in a multi-
hop fashion to a base station. While forwarding data in a multi-hop fashion to the base
station is often more energy-efficient than transmitting the data directly from the sens-
ing node to the base station, a potential disadvantage of the multi-hop strategy is that
the nodes close to the base station must forward much more packets than nodes further
away from the base station. Therefore, these nodes "typically die at an early stage" [7].
This is sometimes called the hot spot problem []. Without adding extra nodes or redis-
tributing the available energy, this problem is hard to solve. For example, Perillo et al.
have shown that varying the transmission power of nodes, even considering unlimited
transmission ranges, does not solve the hot spot problem [7].
At the same time, it is also envisioned that sensor nodes will become "extremely
inexpensive" [5]. While beyond a certain node density, adding additional nodes does not
provide any improvement regarding sensing, communication or coverage [3], adding
nodes might obviously help to increase the lifetime of a sensor network while providing
the same service to its users, i.e. leveraging sensor values from the same number of
nodes.
In this paper, we study the benefit of adding extra nodes to a sensor network using
a mathematical model we developed previously [2]. Our results show that for certain
networks only a limited number of additional nodes are required to fourfold network
lifetime. We also show that the possible gain depends heavily on the fraction of nodes
already present in the vicinity of the base station.

2. NE RK ARC HITECTURES
D PROTOCOL STACK
Jun Zheng
Southeast Univ ersity, China
2.1 INTROD
Network architectures d protocols are important aspects in the design of wire-
less sensor networks ( ) [1]. Due to the severe energy constraint of sensor
nodes, network archi design has a big impact on the energy consumption
and thus the operati lifetime of the whole network. On the other hand, a
of a large number of sensor nodes that are denselv
ion and collaborate to accomplish a sensing task. It
protocols to implement various network control and
example, synchronization, self-confi.guration, medium
r networks because they do not consider the energy,
constraints in sensor nodes. On the other hand, most
Perspective, Edited by Jun Zheng and Abbas Janralipour
and Electronics Engineers
sensor network consis
deployed in a sensing
requires a suite of
management luncti
access control, routing, ta aggregation, node localization, and network securitv.
However, existing net protqiols for traditional wireless networks. for
example, cellular s and mobile ad hoc networks (MANETs), cannot be
applied directly to sen
computation, and
sensor networks are ication specific and have different application require-
ments. For these new suite of network protocols is required, which take
into account not only resource constraints in sensor nodes. but also the
Wireless Sewor Networks: A
Copynght @ 2009 Institute
19

3. -
i:
NETWORK ARCHI
ll
1l
li
icrunes
AND PRorocol srAcK
li
li
J
this purpose, it is imPortant
flesign for WSNs.
fn network architectures and
br node structure and tYPical
ldiscuss the classification of
! a protocol stack for sensor
rthis chapter.
I
lessi
I
ll
;.j.irrtrfle rtS of different network applications.
ir:rne a protocol stack to facilitate the
I-lis chapter introduces fundamental
:, ",r'rol stack of WSNs. We wiII flrst introduce se., :rlLrr JL<rw^ vr rr ur tr
,,-r.:trr n€twork architectures in Section 2.2, thei
-i:.-or networks in Section2.3, and finally descrit
' :: A L-rrks in Section 2.4. Section 2.5 will summariz
2.2 NETWORK ARCHITECTURES FOR WIR
sENSOR NETWORKS
.: lhis section, first we introduce the structure of a
:ipical network architectures for WSNs.
2.2.1 Sensor Node Structure
nsor node and then describe
scnsor node typically consists of four basic
-c.sinq unit, a communication unit, and a power it, which is shown in Fig.2.1'
,lr' sensing unit usually consists of one or
!l phenomenon and generate
ifneaPCs convert the analog
processing unit. The Process-
:);rvcrters (ADCs). The sensors observe the phys
..:aioq signals based on the observed phenomenori
.'""r[ into digital signals, which are then fed to th
:.r unit usually consists of a microcontroller or
,..s... lntel's StrongARM microprocessor and I
,ilrich provides intelligent control to the sensor I
L-rnsists of a short-range radio for performing
'rcr a radio channel. The power unit consists of
Power Unit
Processing Unit
Sensing Unit
nicroprocessor with memoty
mel's AVR microProcessor),
0
Fig. 2.1 Sensor node
nents: a sensing unit, a Pro-
and analog-to-digital
The communication unit
. transmission and recePtion
battery for suPPlYing Power

4. i
NETWORK ARCHITECTURES LESS SENSOR NETWORKS
to drive all other in the system. In addition, a sensor node can also
be equipped with some units, depending on specific applications. For
example, a global positioni system (GPS) may be needed in some applications
ion for network operation. A motor may be neededthat require location i
21
2.2,2 Network Arch
A sensor network of a large number of sensor nodes denselv
deployed in a region of i , and one or more data sinks or base stations that
are located close to or i the sensing region, as shown in Fig.2.2.The sink(s)
sends queries or comma
to move sensor nodes rn
a small module with low
2.3. However, long-dista
tion. In sensor networks.
required transmission powi
sion distance.Therefore. it i
sion distance in order to
sensing tasks. All these units should be biiilt into
consumption and low production cost.
ransmission is costly in terms of energy consump-
nergy consumed for communication is much higher
exponentially with the increase of transmis-
to reduce the amount of traffic and transmis-
energy savings and prolong network lifetime.
Sensing region
node
sensor nodes collaborate to
to the sink(s). Meanwhile,:
the sensor nodes in the sensing region while the
rmplish the sensing task and send the sensed data
sink(s) also serves as a gateway to outside net-
works, for example, the I
simple processing on the
et. It collects data from the sensor nodeq performs
data, and then sends relevant information (or
the processed data) via th
information.
rnet to the users who requested it or use the
To send data to the sin each sensor node can use single-hop long-distance
transmission, which leads single-hop network architecture, as shown in Fig.
than that for sensing and For example, the energy sonsumed for
transferring one bit of dati a receiver at 100m away is equal to that needed
]. The ratio of energy consumption for communi-to execute 3.000 instruct
cating 1 bit over the wi medium to that for processing the same bit could
be in the range of 1,000- [3,4]. Furthermore, the energy consunied for
transmission dominates I energy consumed for communication and the
Sensor network architecture.

5. 22 NETWORK ARCHITEC
Fig. 2.3 Single-hoP neovork
are close to each other, which makes it feasible to use
tion. In multihop communication, a sensor node transmi
the sink via one or more intermediate nodes' which can
sumption for communication. The architectglg- of.1 1
organized into two types: flat and hierarchical [5l,which a
tnic sections (Sections 2.2.2.L and2.2.2.2).
For this purpose, multihop short-distance communication
rort tensoi networks, sensor nodes are densely deploye
PROTOCOL STACK
highly preferred. ln
and neighbor nodes
listance communica-
s sensed data toward
the energY con-
network can be
described in the next
2.2.2.1 Flat Architecture. In aflatnetwork,each plays the same role
rs Due to the large
ntifier to each node
in a sensor network. For this reason' data gathering is ally accomPlished bY
fiuery to all nodes in
that have the data
node communicates
as relays Frgute 2.4
2.2.2.2 Hierarchical Architedure, In a sensor nodes
nd their data to the
itting the data to
rhe sink. A node with lower energy can be used to Ithe sensing task and
send the sensed data to its cluster head at short distance, a node with higher
ta from its cluster
process can not onlY
rcduce the energy consumption for communication, but balance traffic load
ll sensor nodes have
in performing a sensing task and all sensor nodes are
number of sensor nodes, it is not feasible to assign a glob
using data-centric routing, where the data sink transmits
the iensing region via flooding and only the sensor no
matching the query will respond to the sink' Each
uith the sink via a multihop path and uses its peer
illustrates the typical architecture of a flat network'
are organized into clusters, where the cluster members
.-lusteiheads while the cluster heads serve as relays for I
energy can be selected as a cluster head to process
in.rnb"tt and transmit the processed data to the sink'
ancl improve scalability when the network size grows Si
rhe same transmission capability, clustering must be p y performed in

6. WIRELESS SENSOR NETWORKS
Sink
2.4 Flat network architecture.
@ Clusterhead
o cluster member
2.5 Single-hop clustering architecture'
:i
,
order to balance the
tion can be
to the sink and i
The major
how to organize t
strategies Accordi
cluster heads, a
architecture or a
respectively [8].
sensor network can
':
flc load aqlpng all sensor nodes. Moreon"r, s,u aggrega-
: cluster hiads to reduce the amount of datd'transmitted
the energy efficiency of the network [6]. f
with clustering is how to select the cluster heads and
rsters [7]. In this context, there are mariy clustering
the distance between the cluster members and their
organized into a single-tier clustering architecture or a
itecture.
Figure 2.7 illustrates an example of*he msltitier
** ,i;
multitier clustering

7. 24
cnteria, WSNs can be classified into different
ng to different
NETWORK ,ARCHITECT AND PROTOCOL STACK
@ Clusterhead
O Cluster member
Fig. 2.5 Multihop clustering
Sink
(D Tier2 cluserhead
O Tier 1 clutcrhead
O Tier 0 cluster member
Fig. 2.7 Multitier clustering
clustering architecture [9].To address the clustering m, a variety of cluster-
rng algorithms have been proposed in the literature [7- iThe reader is referred
ro Chapter 6 for a comprehensive introduction of these g algorithms.
2,3 CTASSIFICATIONS OF WIRELESS SENSOR KS
SNs are application specific.A sensor network is usua deployed for a specific
arlplication.and thus has some different characte

8. CLASSIFICATIONS OF WIRE SENSOR NETWORKS
Static and Mobile
sensor network ca
nodes are static wi
etvvork. According to the mobility of sensor nodes, a
static or mobile. In a static sensor network, all sensor
out movement, which is the case for many applications'
inistic sensor network, the positions of sensor nodes
25
However.some
sensing task. A wi
example of mobile sensor networks [14]. Compared
tworks, whic.h is simpJer to co::trol and easier to imple-
mobile sensor networks must consider the mobilityment, the design,
effect, which incri the cornplexity of implementation' ,
ardetsminktir Ne*vark According to the depioy-
a sensor network can be deterministic or nondeter-
ior applications requite mobile nodes to at*onrplish a
less bi,osensor network using autonomously controlled
animals is a typici
wifl staficsensorn
are preplanned a
be used in some
are flxed once deployed. This type of network can only
ited situations, where the preplanned deployment is
possible.In most uations, however, it is difficult to depioy sensor nodes
in a preplannedi because of the harsh or hostile environments.
Instead. sensor are randomly deployed without preplanning and
engineering. Obvi nondeterministic networks are more scalable and
flexible, but requi higher control complexity.
. Static-Sink and M ite-Sink Network. A data sink in a sensor network can
be static or a static-sink network, the sink(s) is static with a fixed
to or inside a sensing region. All sensor nodes sendposition located
their sensed data the sink(s). Obviously, a static sink makes the network
simpler to contro t it would cause the hotspot effect [5].The amount of
traffic that sensor are required to forward increases dramatically as
the distance to data sink becomes smaller. As a result, sensor nodes
nk tend to die early, thus resulting in network partition
normal network operation. In a mobile-sink network,
in the sensing region to collect data from sensor
the traffic load of sensor nodes and alleviate the
ment of sensor
ministic. In a d
closest to the datl
hotspot effect in
Single-Sink and
sink ormultiple s
and even disrupti
the sink(s) moves
nodes, which canl
network.
Network. A sensor network can haye a single
In a single-sink network,there is only one sink located
close to or inside I
to this sink. In a i
different positi
send their data closest sink. which can effectivelv balance the traffic
load of sensor n and alleviate the hotspot effect in the network.
. Single-Hop and
between asensor
Network According to the number of hops
sensing resion.All sensor nodes send their sensed data
ttisink-neffiork, there may be several sinks located in
lose to or inside the sensing region. Sensor nodes can
and the data sink, a sensor network can be classified
into single-hop multihop. In a single-hop network, all sensor nodes
transmit their sen data directly to the sink,which makes network control
simpler to imple nt. However, this requires long-range wireless commu-
nication. which is in terms of both etergy consumption and hardware

9. 26 NETWORK ARCHITEC
implementation. The furthest nodes from the
quickly than those close to the sink. Also. the
network may increase rapidly with the increase o
would cause more collisions, and thus inerease
delivery latency. In a multihop network, sensor
data to the sink using short-range wireless
intermediate nodes Each intermediate node
forward the data along a multihop pa!S. Morec
be performed at an intermediate node [6 efimina
can reduce the total amount of traffic in the ne
energy efficiency of the network. In general, a
simpler network architecture and thus ii easier to
applications in small sensing areas with sparsely
Multihop networks have a wider range of applicat
control complexity.
S elf- Reconfigurab le and Non- Self-Configurable N r
configurability of sensor nodes, a sensor network
or non-self-conligurable. In a non-self-configurab
have no ability to organize themselves into a net
to rely on a central controller to control each s
information from them. Therefore, this type of
for small-scale networks In most sensor nit*o,
are able to autonomous$ organize and mainta
themselves and collaboratively accomplish a sens
such self-configurability is suitable for large_sca
complicated sensing tasks.
Homogeneous and Heterogeneous Network.
nodes have the same capabilities, a sensor
or heterogeneous [15]. In a homogeneous networ
the same capabilities in terms of energy, computal
trast, a heterogeneous network has some sophisti
are equipped with more processing and commun
normal sensor nodes. In this case, the network can
and communication tasks to those sophisticated nc
its energy efficiency and thus prolong the lifetime.
2.4 PROTOCOL STACK FOR WIRELESS SENSOR
The protocol stack for WSNs consists of five protocol la
data link layer, network layer, transport layer, and applic
Fig. 2.8. The application layer contains a viriety of ai
generate various sensor network applications. The ti
for reliable data delivery required o-y the application network layer is
AND PROTOCOT STACK
will die much more
traffic Ioad in the
network size, which
consumption and
transmit their sensed
ion via one or more
perform routing and
data aggregation can
redundancy, which
and thus improve the
-hop network has
ntrol.It is suitable for
ployed sensor nodes.
at the cost of higher
According to the
be self-configurable
twork, sensor nodes
ik. Instead, they have
node and collect
ks is only suitable
', sensor nodes
;heir connectivity by
itask. A network with
betworks to perform
ng to whether sensor
be homogeneous
ll sensor nodes have
; and storage. In con-
sensor nodes that
ing capabilities than
more processing
in order to improve
: the physical layer,
Iayer, as shown in
Jayer protocols to
layer is responsible