Void-handling techniques for routing protocols in underwater sensor networks: survey
and challenges
Ghoreyshi, Seyed Mohammad; Shahrabi, Alireza; Boutaleb, Tuleen
Published in:
IEEE Communications Surveys and Tutorials
DOI:
10.1109/COMST.2017.2657881
Publication date:
2017
Document Version
Author accepted manuscript
Link to publication in ResearchOnline
Citation for published version (Harvard):
Ghoreyshi, SM, Shahrabi, A & Boutaleb, T 2017, 'Void-handling techniques for routing protocols in underwater
sensor networks: survey and challenges', IEEE Communications Surveys and Tutorials, vol. 19, no. 2, pp. 800-
827. https://doi.org/10.1109/COMST.2017.2657881
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1
Void Handling Techniques for Routing Protocols in
Underwater Sensor Networks: Survey and
Challenges
Seyed Mohammad Ghoreyshi, Alireza Shahrabi, Tuleen Boutaleb
School of Engineering and Built Environment
Glasgow Caledonian University
Glasgow, UK
{Seyed.MohammadGhoreyshi, A.Shahrabi, T.Boutaleb}@gcu.ac.uk
Abstract—From the view of routing protocols in Underwater
Sensor Networks (UWSNs), the presence of communication void,
where the packet cannot be forwarded further using the greedy
mode is perhaps the most challenging issue. In this paper, we
review the state of the art of void-handling techniques proposed
by underwater routing protocols. To this, we first review the void
problem and its negative impact on the category of the routing
protocols, which does not entail any void recovery technique. Af-
terwards, currently available void-handling techniques in UWSNs
are classified and investigated. They can be classified into two
main categories: location-based and depth-based techniques. The
advantages and disadvantages of each technique along with
the recent advances are then presented. Finally, we present a
qualitative comparison of these techniques and also propose some
possible future directions.
I. INTRODUCTION
Underwater acoustic sensor networks have obtained a con-
siderable attention to support aquatic applications such as
exploration of ocean resource, disaster prevention, intrusion
detection, military applications, and pollution monitoring [1]–
[4]. The sensors are distributed in different depths to collect
information and forward them to a destination, which may be
a sink, a group of sinks or an Autonomous Underwater Vehicle
(AUV) [5]–[8].
Different routing protocols are proposed to improve the
packet delivery with minimum energy and delay cost in
UWSNs, in which greedy routing protocols are the most
prominent approaches [9]–[11]. With the aid of localiza-
tion mechanisms, geographical greedy routing has become a
promising scheme in the sensor and ad hoc networks. Geo-
graphic greedy routing (also called position-based, or location-
based) is a routing principle, which relies on the geographic
position information to forward the data packets closer to the
destination in each hop [12]–[14].
In contrast to a table-driven (proactive) routing, which
requires large communication overhead to establish end-to-
end routes [15]–[17], geographical routing does not need to
discover and maintain the full path from the source to the
destination. In most cases, the geographic information of one-
hop or two hops has been held to route the packets. Thus,
there is no need for the routing tables and routing messages
to hold and update the route path. This unique feature makes
them scalable to be used in the large networks with many
nodes [9], [18]. Geocasting is another service which can be
supported by the geographic routing to deliver a packet to an
intended geographic region [19].
Geographic routing follows a greedy-forwarding strategy in
which every node looks for the closest neighbouring node to
the destination. However, greedy forwarding may fail because
of the communication voids or local maximum phenomenon
[20]–[22]. In this case, the forwarding node cannot find any
qualified node with a positive progress towards the destination,
so the packet may be dropped even though there is a valid path
from the sender to the destination.
As depicted in Fig. 1, nodes e is a void node since it has no
neighbouring node closer to the sinks S
1
and S
2
than itself.
Thus, in a greedy-forwarding strategy, if node e is selected
as one of the candidate nodes, it may obtain higher priority
to forward the packet, resulting in the packet suppression in
node d, which has a valid path to a sink. Without resolving
this issue, data packets may drop in the network, wasting the
network resources such as energy and bandwidth. Moreover,
the void problem is more challenging as it is unpredictable
when and where a void may occur.
A number of factors individually or a combination of them,
cause the void phenomenon, such as the sparse topology,
temporary obstacles, and unreliable nodes or links [20], [23].
Increasing the density of the network is a simple solution;
however, it is not possible all the time and, even so, it cannot
entirely eliminate the void problem. Therefore, in order to
improve the routing efficiency, many different techniques and
recovery methods are proposed to handle void problem in the
wireless and ad hoc networks [21], [24]–[27].
However, due to the different characteristics of UWSNs,
the terrestrial network techniques are completely useless and
can not be used directly in the underwater environment. This
is attributed to the fact that, first, all communications voids
in UWSNs are three-dimensional, which requires different
treatments than two-dimensional holes in the terrestrial net-
works [28]. Second, the mobility of most underwater nodes
makes the void mobile. A mobile void can also result from
the surrounding environment [29]. This can be the case of
a ship that navigates over an underwater network, it blocks
communications in the nearby area and thus generates a void
2
Void area
e
d
S1 S2
B
A
c
Fig. 1: A void area with respect to destinations S
1
and S
2
that moves along with the ship.
On the whole, the characteristics of underwater sensor
networks make them more difficult to cope with three-
dimensional and mobile voids in such an environment. There-
fore, specifically designing some efficient void-handling tech-
niques for the routing protocols in UWSNs is necessary. The
performance of these void-handling techniques depends on
many factors, such as the number of void nodes, network
dynamics, and number of destinations.
Generally, the routing strategies and void-handling tech-
niques can be categorised into two main groups: location-
based and depth-based. In the location-based category, the void
node is determined based on the geographical advancement of
the neighbouring nodes. A node is called a void node, if it
cannot find any other node with shorter euclidean distance
toward the destination. In the depth-based category, the void
node is determined based on the depth advancement of the
neighbouring nodes. Depth information indicates the vertical
distance from each node to the water surface. A node is a
void node if it cannot find any neighbouring node with the
lower depth than itself. Because of different features in these
categories, different void-handling techniques are required.
In this paper, our goal is to investigate the void-handling
techniques reported in the literature. To achieve this, we first
mention the characteristics of UWSNs in Section II. Then, in
Section III, we investigate the void problem in 3D UWSNs and
its ignorance impact on the protocols, which do not support
any void-handling technique. Then, we indicate the challenge
and required features to design and evaluate a void-handling
technique. In Section IV, we propose a classification for all
void-handling techniques in UWSNs. In Section V, almost all
currently reported void-handling techniques in the literature
are discussed in detail. The advantages and disadvantages of
each void-handling technique is then shown using different
examples and analysis. These void-handling techniques quali-
tatively are compared in terms of efficiency and cost in Section
VI. In Section VII, we identify some directions and guidelines
for the future research on the void-handling techniques in
UWSNs. Finally, in Section VIII, we conclude the paper.
II. CHARACTERISTICS OF UNDERWATER SENSOR
NETWORKS
The underwater sensor networks pose more severe situation
to cope with void regions.
A. Three-dimensionality and Node Movement
In contrast to the terrestrial networks, UWSNs are three-
dimensional, and sensors move with the water current. The
three-dimensional holes in the routing path can lead to more
packet failures. The hidden terminal problem in 3D UWSNs
is more intense due to the existence of more neighbouring
nodes in different directions [30]. Moreover, the topology
continuously changes with the nodes movement. The speed of
node depends on the water velocity, which varies at different
times [9], [10].
B. High and Variable Propagation Delay
In UWSNs, sensors use the acoustic waves for the under-
water communications. The speed of sound in underwater is
about 1500 m/s [31]. Thus, it causes a large propagation
delay is about to five orders of magnitude higher than that of
radio frequency (RF). The sound velocity varies based on the
different parameters such as temperature, salinity, and depth of
water [32]. It is critical to taken into account the propagation
delay in designing the void-handling techniques in UWSNs.
C. Limited Bandwidth
Due to features of acoustic waves and environmental noise,
the acoustic bandwidth is severely limited in UWSNs. The
available acoustic bandwidth depends on the communication
range and acoustic frequency. As a result of the limited
bandwidth, the data rate for underwater sensors can rarely
exceed 100 kbps [33]. Therefore, the limited bandwidth of
acoustic channels should be considered in designing of void-
handling techniques.
D. Path Loss
The underwater environment has higher path loss in compar-
ison to the terrestrial physical layer. The path loss results from
the attenuation and geometric spreading [34]. The attenuation
also results from the absorption of acoustic waves in water
[35]. Decreasing the traversed distance and increasing the
transmission power can reduce the path loss impact. Thus,
packet forwarding is more likely to be successful if packets
are relayed over multiple short distances instead of traversing
over long distances [30], [34].
3
Convex Void
b
c
d
e
f
g
Sink
Anchored Node
Relay Node
Radio Link
Acoustic Link
a
h
Fig. 2: A convex void in a single sink architecture
E. Noises
There are two kinds of noise which affect the acoustic com-
munications, including man-made noise and ambient noise.
Man-made noise is mainly generated by human activities like
using pumps and shipping. Ambient noise refers to natural
events such as seismic and tides [3], [35]. The main sources
of the noise include turbulence, shipping, waves and thermal
noise [34]. These noises lead to a lossy and noisy underwater
environment, which should be considered carefully in the void-
handling techniques.
F. Energy Consumption
Energy consumption is another primary concern in UWSNs
since it is hard to replace or recharge the sensor batteries.
In UWSNs, the energy consumed by the sensors is much
more than what is consumed by the terrestrial sensors [36],
[37]. Therefore, energy efficiency is an essential requirement
of void-handling techniques in UWSNs.
Due to these characteristics, the terrestrial void-handling
techniques are quite useless and can not be employed directly
in the underwater environment. Thus, it is required to develop
the void-handling techniques suitable for underwater acoustic
communications taking all the characteristics into account.
III. VOID PROBLEM AND CHALLENGES
In this section, first, we introduce the void problem and the
used terminology.
Sink
Anchored Node
Relay Node
Radio Link
Acoustic Link
Concave
Void
a
b c d
e
f
g
h i
j
Fig. 3: A concave void in a single sink architecture
A. Definitions and characteristics
An underwater sensor network has a 3D network topology
in which one or more sinks are located on the water surface
equipped with an acoustic modem for underwater communica-
tion and with a radio modem for out of water communication
[5]. Anchored nodes are located at the bottom of the ocean
in the predetermined locations to collect the information and
deliver them to a sink by using the relay nodes which are
located at different levels in between.
In a greedy-forwarding strategy, each forwarding node trans-
fers packets to a node closer than itself to the destination [9].
Given a sender node m
i
and a destination S, the advance of
a neighbouring node m
j
is defined as
ADV (m
j
) = D(m
j
, S) − D(m
i
, S) (1)
where D(m, S) denotes the Euclidean distance from node m
to destination S. In the location based routing, destination S is
considered as the closest sink to the sender node m
i
. [8], [38].
In the pressure-based routing, destination S is considered as
the water surface, and distance calculation is reduced to the
depth differences. Only the candidate nodes can participate in
the packet forwarding, which are within the following set
C
m
i
= {m
j
∈ N(m
i
) : ADV (m
j
) > 0} (2)
where N(m
i
) includes all the neighbouring nodes within the
transmission range of m
i
. If the candidate set, C
m
i
, is empty,
the node m
i
which cannot locate a qualified next-hop node
in greedy mode is called a void node, local maxima node, or
4
stuck node (These terms are interchangeably used throughout
this paper).
During the greedy mode of packet forwarding in geographic
routing, if a relay node cannot find any neighbouring node
with positive advancement toward the sink(s), it should switch
to the recovery mode to bypass the void area; otherwise, the
packet will be dropped [11], [25], [39], [40]. The void nodes
are generally located on the boundary of a void communication
area. In UWSNs, a void communication area is a three-
dimensional region between underwater nodes which is empty
of any nodes inside. A void area prevents communication
between some of the nodes in the network. The path between
the local maxima node and a non-local maxima node, where
greedy routing can be resumed, is called the recovery path.
The forwarding direction specifies whether a hole is a
communication void or not. In UWSNs, void areas are usually
considered as the holes between the relay nodes and water
surface where the sink(s) is located. For further clarification,
Fig. 2 shows a case in which there is a void area between node
e and sink a on the surface. If a greedy routing protocol does
not include any void-handling technique, the packet is dropped
by node e, while there are two valid paths from this node e to
the sink (e-d-c-b-a and e-f -g-h-a). Thus, node e is considered
as a void node with respect to the destination a while the
empty area between them is called a 3D void communication
area.
In a pressure-based model, a void node is defined in another
way. In this category, a node is called a void node if it is
located in a shallower depth than all of its neighbouring nodes
and it is not connected to any sink on the surface [41]. In
this case, a packet cannot make any upward progress toward
the surface. In Fig. 3, node e is a void node, since all of its
neighbouring nodes have higher depth. The trapped nodes are
those that are located down below the void node and involving
them in packet forwarding leads to getting stuck the packet
(e.g. b, c, d). The area in which the trapped nodes are located
called the trap area [42].
Voids emerge in the underwater environment in different
shapes and sizes. For instance, void areas are emerged in
convex and concave shapes in Figs. 2 and 3, respectively.
There might be cases where nodes seem to be connected to
each other in terms of transmission distance, but they cannot
communicate. This is due to the fact that some other factors
such as obstructions and underwater noises can disparage this
assumption [43]. Thus, nodes are connected to each other if the
transferred signal between them can be decoded without any
error. To this, Signal-to-Noise Ratio (SNR) over the traversed
distance should be higher than detection threshold at the
receiver side [34]. Furthermore, holes in water are not neces-
sarily distributed evenly. It can be formed by many factors such
as deployment model, energy depletion and movement pattern
of underwater nodes, etc [44]. Knowing such characteristics
is very useful when designing a void-handling technique.
Void areas in the terrestrial sensor network are usually fixed
because they consist of a set of static sensor nodes [20].
However, in UWSNs, by the movement of floating nodes
with the water current, void areas can gradually move to
other regions. Hence, mobility of the void communication area
hole
AB
v
w
x
y
s
Fig. 4: Impact of void problem on HH-VBF protocol
is another feature which should be taken into consideration
in this environment. Displacement speed of void area is
dependent on the velocity of the underwater nodes which is
not always so high. Nonetheless, high dynamics is not always
a negative factor for the routing protocols, because sometimes
the temporary voids can be vanished with the aid of the newly
arrived nodes [29].
B. Void-ignorance routing protocols
In this section, we briefly discuss the negative impact of void
on some well-known routing protocols which intentionally (for
the sake of simplicity) or ignorantly do not consider it. Some
routing protocols such as Vector-Based Forwarding (VBF),
and Hop-by-Hop Vector-Based Forwarding (HH-VBF) [45],
[46] are location-based greedy routing in which forwarding
nodes are selected within a virtual pipeline faced toward the
destination. However, no solution when facing with a void in
the pipeline is provided.
In VBF, packets are forwarded within a fixed virtual pipeline
between every pair of source (e.g. an anchored node at
the bottom of ocean) and destination. The performance of
VBF is dropped in the sparse networks, where candidate
nodes inside the pipeline can barely be found. In order to
increase the chance of finding packets in the pipeline, HH-VBF
requires a different pipeline at each hop originated from every
intermediate (relay) node. However, the void occurrence in the
pipeline toward the destination still remained as a problematic
issue. This problem is shown in Fig. 4. As can be seen, the
packets generated in source A can successfully be delivered
to the sink; however, packets generated in source B is stuck
in node V and dropped, though there exists a valid path like
(v-w-x-y-s) to the sink.
As another protocol, we can consider RDBF [47], which
similarly relies on the use of location-based coordinates but
with no void-handling technique. In this protocol, packets
are relayed through the nodes with the nearest geographical
distance to the sink node. RDBF does not limit the forwarding
nodes in a pipe, or other geometric shape; however, in facing
![Fig. 12: Route maintenance in RMTG [98]](/figures/fig-12-route-maintenance-in-rmtg-98-21eay9hd.webp)
![Fig. 13: Boundary Routing in RMTG [98]](/figures/fig-13-boundary-routing-in-rmtg-98-3hd3hb14.webp)
