Part B | 15.5
382 Part B Autonomous Ocean Vehicles, Subsystems and Control
the request-to-send/clear-to-send type (RTS/CTS) prior
to transmitting the longer data packets.
In the past years there have been major developments on the medium access control (MAC) layer, with
focus on protocol design that is not ignorant of the high
acoustic latency, but strives to overcome it in an efficient manner or even take advantage of it. Examples
of such protocols include the distance-aware collision
avoidance protocol (DACAP) [15.94], T-Lohi [15.95],
propagation-delay-tolerant Aloha [15.96], and many
others, e.g., [15.97, 98]. For example, DACAP is based
on a standard control packet exchange to secure the link
for data transmission, with the addition of a warning
signal that interrupts an on-going transmission that is
bound to fail. T-Lohi uses dedicated collision-avoidance
tones to announce data transmission, thus enabling all
those who overhear the tones to count the number of active nodes, which, in turn, is used to optimize channel
access.
15.5.8 Routing
and Cross-Layer Integration
Work on higher layer protocols has focused in particular on routing. Geographical routing, which exploits
the knowledge of the nodes’ locations to determine
the best next relay so as to minimize the total energy
consumption, was addressed in [15.99]. A distributed
geographical routing protocol with integrated power
control was proposed in [15.81]. Distributed routing for
delay-sensitive and delay-insensitive applications was
investigated in [15.100], while a distributed procedure
for initial neighbor discovery was proposed in [15.101].
Transport control protocols have been considered to
a much lesser extent, since end-to-end retransmission
suffers greatly from long delays and high packet error
rate.
Cross-layer design plays an important role in acoustic systems, both between adjacent network layers and
between adjacent system functions. Examples of crosslayer design include frequency allocation and power
control integration with MAC and routing [15.81];
topology/sleep control for energy-efficiency in fixed
networks [15.56, 98, 102]; packet size selection for optimizing the throughput/energy performance of MAC
protocols [15.103], and optimization of rateless packet
coding (network coding) for use with half-duplex
acoustic links [15.104]. An example of cross-function
design is integration of localization/navigation with
acoustic communications. The use of acoustic modems
as traveling beacons that help to localize AUVs by measuring the relative propagation delays in a network is
described in [15.92]. Such localization may, in turn,
support geographical routing protocols in which location information is used to find the best routes through
the network.
The issues of mobility and delay-tolerance have received particular attention. Channel access in a network
composed of both fixed and mobile nodes was studied
in [15.102], via a simulation analysis that compares the
Aloha and the DACAP protocols. Both were found to
provide satisfactory performance, which is further enhanced for mobile applications by simple modifications
such as back-to-back repetitions. [15.105] investigated
routing in a delay-tolerant network (DTN), comparing
two classes of protocols, one based on spray-and-wait
principle and the other based on resource allocation
for intentional DTN. In conditions of high load, the
latter was found to outperform the former as well as
basic flooding in terms of packet delivery ratio regardless of mobility conditions, while the former showed
better performance in terms of average delivery ratio.
[15.106] considered a swarm of free-floating mobile
sensor nodes that need to report their measurements
to a central station. [15.107] considered location-based
routing in a mobile network. To improve the performance, the nodes do not use the current knowledge
of the destination’s location, but instead form a prediction based on the history of motion. [15.108] also
investigated location-based routing, which it enhanced
by inclusion of a link quality metric into the decisionmaking process. Finally, [15.109] explored the idea of
waiting for a favorable link to present itself before forwarding a packet. In other words, an AUV may not wish
to waste its energy transmitting to a relay far away, but
may instead want to wait for a better opportunity. Once
such an opportunity presents itself, highly directional
transmission is exploited to focus the energy in a desired direction.
These references offer a valuable first step towards
understanding the network behavior in the presence
of long acoustic delay and frequency-dependent attenuation. They identify vulnerable points of existing
protocols and offer clever solutions to recover the performance. However, although there is a common understanding that mobility needs special care in an acoustic
setting, the analysis is often conducted via simulation,
using a time-invariant propagation model. This fact emphasizes the need for statistical channel models that
will reflect temporal as well as spatial variability of
acoustic channel and enable the development of more
accurate simulation tools.
382 Part B Autonomous Ocean Vehicles, Subsystems and Control
the request-to-send/clear-to-send type (RTS/CTS) prior
to transmitting the longer data packets.
In the past years there have been major developments on the medium access control (MAC) layer, with
focus on protocol design that is not ignorant of the high
acoustic latency, but strives to overcome it in an efficient manner or even take advantage of it. Examples
of such protocols include the distance-aware collision
avoidance protocol (DACAP) [15.94], T-Lohi [15.95],
propagation-delay-tolerant Aloha [15.96], and many
others, e.g., [15.97, 98]. For example, DACAP is based
on a standard control packet exchange to secure the link
for data transmission, with the addition of a warning
signal that interrupts an on-going transmission that is
bound to fail. T-Lohi uses dedicated collision-avoidance
tones to announce data transmission, thus enabling all
those who overhear the tones to count the number of active nodes, which, in turn, is used to optimize channel
access.
15.5.8 Routing
and Cross-Layer Integration
Work on higher layer protocols has focused in particular on routing. Geographical routing, which exploits
the knowledge of the nodes’ locations to determine
the best next relay so as to minimize the total energy
consumption, was addressed in [15.99]. A distributed
geographical routing protocol with integrated power
control was proposed in [15.81]. Distributed routing for
delay-sensitive and delay-insensitive applications was
investigated in [15.100], while a distributed procedure
for initial neighbor discovery was proposed in [15.101].
Transport control protocols have been considered to
a much lesser extent, since end-to-end retransmission
suffers greatly from long delays and high packet error
rate.
Cross-layer design plays an important role in acoustic systems, both between adjacent network layers and
between adjacent system functions. Examples of crosslayer design include frequency allocation and power
control integration with MAC and routing [15.81];
topology/sleep control for energy-efficiency in fixed
networks [15.56, 98, 102]; packet size selection for optimizing the throughput/energy performance of MAC
protocols [15.103], and optimization of rateless packet
coding (network coding) for use with half-duplex
acoustic links [15.104]. An example of cross-function
design is integration of localization/navigation with
acoustic communications. The use of acoustic modems
as traveling beacons that help to localize AUVs by measuring the relative propagation delays in a network is
described in [15.92]. Such localization may, in turn,
support geographical routing protocols in which location information is used to find the best routes through
the network.
The issues of mobility and delay-tolerance have received particular attention. Channel access in a network
composed of both fixed and mobile nodes was studied
in [15.102], via a simulation analysis that compares the
Aloha and the DACAP protocols. Both were found to
provide satisfactory performance, which is further enhanced for mobile applications by simple modifications
such as back-to-back repetitions. [15.105] investigated
routing in a delay-tolerant network (DTN), comparing
two classes of protocols, one based on spray-and-wait
principle and the other based on resource allocation
for intentional DTN. In conditions of high load, the
latter was found to outperform the former as well as
basic flooding in terms of packet delivery ratio regardless of mobility conditions, while the former showed
better performance in terms of average delivery ratio.
[15.106] considered a swarm of free-floating mobile
sensor nodes that need to report their measurements
to a central station. [15.107] considered location-based
routing in a mobile network. To improve the performance, the nodes do not use the current knowledge
of the destination’s location, but instead form a prediction based on the history of motion. [15.108] also
investigated location-based routing, which it enhanced
by inclusion of a link quality metric into the decisionmaking process. Finally, [15.109] explored the idea of
waiting for a favorable link to present itself before forwarding a packet. In other words, an AUV may not wish
to waste its energy transmitting to a relay far away, but
may instead want to wait for a better opportunity. Once
such an opportunity presents itself, highly directional
transmission is exploited to focus the energy in a desired direction.
These references offer a valuable first step towards
understanding the network behavior in the presence
of long acoustic delay and frequency-dependent attenuation. They identify vulnerable points of existing
protocols and offer clever solutions to recover the performance. However, although there is a common understanding that mobility needs special care in an acoustic
setting, the analysis is often conducted via simulation,
using a time-invariant propagation model. This fact emphasizes the need for statistical channel models that
will reflect temporal as well as spatial variability of
acoustic channel and enable the development of more
accurate simulation tools.