shallow water acoustic networks full report
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21-01-2010, 07:41 PM

.doc   Shallow water Acoustic Networks full report.DOC (Size: 479 KB / Downloads: 132)
In the last two decades, underwater acoustic communications has
experienced significant progress. The traditional approach for ocean-
bottom or ocean-column monitoring is to deploy oceanographic sensors,
record the data, and recover the instruments. But this approach failed
in real-time monitoring. The ideal solution for real-time monitoring of
selected ocean areas for long periods of time is to connect various
instruments through wireless links within a network structure. Basic
underwater acoustic networks are formed by establishing bidirectional
acoustic communication between nodes such as autonomous underwater
vehicles (AUVs) and fixed sensors. The network is then connected to a
surface station, which can further be connected to terrestrial networks
such as the Internet.


In the last two decades, underwater acoustic communications has
experienced significant progress. Communication systems with increased
bit-rate and reliability now enable real-time point-to-point links
between underwater nodes such as ocean bottom sensors and autonomous
underwater vehicles (AUVs). Current researches focused on combining
various point-to-point links within a network structure to meet the
emerging demand for applications such as environmental data collection,
offshore exploration, pollution monitoring, and military surveillance.
The traditional approach for ocean-bottom or ocean-column monitoring is
to deploy oceanographic sensors, record the data, and recover the
instruments. This approach has several disadvantages:
¢ The recorded data cannot be recovered until the end of the
mission, which can be several months.
¢ There is no interactive communication between the underwater
instrument and the onshore user. Therefore, it is not possible to
reconfigure the system as interesting events occur.
¢ If a failure occurs before recovery, data acquisition may stop
or all the data may be lost.
The ideal solution for real-time monitoring of selected ocean
areas for long periods of time is to connect various instruments
through wireless links within a network structure. Basic underwater
acoustic networks are formed by establishing bidirectional acoustic
communication between nodes such as autonomous underwater vehicles
(AUVs) and fixed sensors. The network is then connected to a surface
station, which can further be connected to terrestrial networks such as
the Internet, through an Ri link. Onshore users can extract real-time
data from multiple underwater instruments. After evaluating the
obtained data, they can send control messages to individual
instruments. Since data is not stored in the underwater instruments,
data loss is prevented as long as isolated node failures can be
circumvented by reconfiguring the network. A major constraint of
underwater acoustic (UWA) networks is the limited energy supply.
Whereas the batteries of a wireless modem can easily be replaced on
land-based systems, the replacement of an underwater modem battery
involves ship time and the retrieval of the modem from the ocean bottom
which is costly and time consuming. Therefore, transmission energy is
precious in underwater applications. Network protocols should conserve
energy by reducing the number of retransmissions, powering down between
transactions, and minimizing the energy required for transmission.
Some underwater applications require the network to be deployed
quickly without substantial planning, such as in rescue and recovery
missions. Therefore the network should be able to determine the node
locations and configure itself automatically to provide an efficient
data communication environment. Also, if the channel conditions change
or some of the nodes fail during the mission, the network should be
capable of reconfiguring itself dynamically to continue its operation.

Unlike digital communications through radio channels where data
are transmitted by means of electromagnetic waves, acoustic waves are
primarily used in underwater channels. The propagation speed of
acoustic waves in UWA channels is five orders of magnitude less than
that of radio waves. This low propagation speed increases the latency
of a packet network. If high latency is overlooked in the design of
network protocols for UWA applications, it can reduce the throughput of
a network considerably. The available bandwidth of a UWA channel
depends critically on transmission loss which increases with both range
and frequency and severely limits the available bandwidth. Within this
limited bandwidth, the acoustic signals are subjected to time varying
multi path which may result in severe inter symbol interference (ISI)
and large Doppler shills and spreads, relative to radio channels
especially in shallow water channels. Multi path propagation and
Doppler effects degrade acoustic signals and limit data throughput.
Special processing techniques are needed to combat these channel
impairments. Until the beginning of the last decade, due to the channel
characteristics of UW channels, modem development was focused on
employing non-coherent frequency shift keying (FSK) signals for
achieving reliable communication. Although non- coherent FSK systems
are effective in UWA channels, their low bandwidth makes them
inappropriate for high-data-rate applications such as multi-user
networks. The need high-throughput long-range systems have resulted in
a focus towards coherent modulation techniques. Today, with the
availability of powerful digital signal processing devices, we are able
to employ fully coherent phase shift keying (PSK) modulation in
underwater communications. A summary of acoustic models is listed in
table below:

Type Non-coherent Non-coherent Non-coherent Coherent
Year 1984 1991 1997 1989 1993
Data rate (b/s) 1200 1250 2400-2600 500,000 600-300
Bandwidth (kHz) 5 10 5 125 0.3-1
Range (km) 3.0 S 2.0 D 10.0 D-5.0 S 0.06 D 89S-203D
Table 1: A summary of performance matrix for some UWA models presented
in the literature. S indicates a shallow water result, while D
indicates a deep-water result generally a vertical channel.
As the data rate and the range of the systems increase, the
complexity of the algorithms grows beyond the capacity of current DSP
hardware. Current research is focused on DSP algorithms with decreased
complexity and multi-user modems that can operate in a network
The new Acoustic Underwater Modems bring wireless underwater
communication to a new level of performance. The new modems offer
significant improvement in performance and reliability over the
conventional technology. The modems are based on the patented S2C-
Technology opening a new dimension of underwater communication. New and
enhanced DSP modulation/demodulation, in combination with new standards
in hard- and software design, are the basis for significant
improvements, including increased data reliability, extremely higher
data rates and reduced system size and weight.

Acoustic Modem

Two types of applications have guided the evolution of
underwater networks. One is gathering of environmental data, and the
other is surveillance of an underwater area. Typically, the network
consists of several types of sensors, some of which are mounted on
fixed moorings; the others are mounted on freely moving vehicles. This
type of network is called an Autonomous Ocean Sampling Network (AOSN)
where the word sampling implies collecting the samples of oceanographic
parameters such as temperature, salinity, and underwater currents. For
surveillance application, the network consists of a large number of
sensors typically bottom-mounted or on slowly crawling robots, that can
be quickly deployed, and whose task is to map a shallow water area. An
example of such a network is Seaweb.

There are three basic topologies that can be used to
interconnect network nodes which are centralized, distributed and
multihop topology. In a centralized network a communication between
nodes takes place through a central station, which is sometimes called
the hub of the network. The network is connected to a backbone at this
central station. This configuration is suitable for deep water networks
where surface buoy with both an acoustic and an RF modem acts as the
hub and controls the communication to and from ocean bottom
instruments. A major disadvantage of this configuration is the presence
of a single failure point. If the hub fails, the entire network shuts
down. Also due to the limited range of a single modem, the network
cannot cover large areas.
The next two topologies belong to peer-to-peer networks. A
fully connected peer-to-peer topology provides point-to-point links
between every node of the network. Such a topology eliminates the need
for routing. However, the output power needed for communicating with
widely separated nodes is excessive. Also, a node that is trying to
send packets to a far-end node can over power and interfere with the
communication between neighboring nodes which is called the near-far
Multihop peer-to-peer networks are formed by establishing
communication links between neighboring nodes. Messages are transferred
source to destination hopping packets from node to node. Routing of the
messages is handled by intelligent algorithms that can adapt to change
in conditions. Multihop networks can cover relatively larger areas
since the range of the network is determined by the number of nodes
rather than the modem range. One of the UWA network design goals is to
minimize the energy consumption while providing reliable connectivity
between the nodes in the network and the backbone. The network topology
is an important parameter that determines energy consumption. The
strategy that minimizes energy consumption is multihop peer-to-peer
topology. The price paid for the decrease in energy consumption is the
need for a sophisticated communication protocol and an increase in
packet delay. Therefore, special attention should be given to
applications that are sensitive to delays.
In many information networks, including UWA networks,
communication is busty and the amount of time that a user spends
transmitting over the channel is usually smaller than the amount of
time it stays idle. Thus, network users should share the available
frequency and time in an efficient manner by means of a multiple access
method. Frequency-division multiple access (FDMA), divides the
available frequency bands into sub-bands and assigns each sub-band to
an individual user. Due to the severe bandwidth limitations and
vulnerability of narrow band systems to fading FDMA systems do not
provide an efficient solution for UWA applications.
Instead of dividing a frequency band, time-division multiple access
(TDMA) divides a time interval, called a frame, into time-slots.
Collision of packets from adjacent time- slots is prevented by
including guard times that are proportional to the propagation-delay
present in a channel. TDMA systems require very precise synchronization
for proper utilization of the time-slots. High latency present in UWA
channels require long guard times that limits the efficiency of TDMA.
Also, establishing a common timing reference is a difficult task.
Code-division multiple access (CDMA) allows multiple users to
transmit simultaneously over the entire frequency band. Signals from
different users are distinguished by means of pseudo-noise (PN) codes
that are used for spreading the user messages. The large bandwidth of
CDMA channels not only provide resistance to frequency selective
fading, but may also take advantage of the time diversity present in
the UWA channel by employing rake filters at the receiver in the case
of direct sequence CDMA. Spread spectrum signals can he used for
resolving collision at the receiver by using multi-user detectors in
this way the number of retransmissions and energy requirements of the
system are reduced. This property both reduces battery consumption and
increases the throughput of the network. Hence CDMA appears to be the
most suitable multiple access technique for shallow water acoustic
There are two basic methods used for routing packets through an
information network. They are virtual circuit routing and datagram
switching. In virtual circuit routing all the packets of a transaction
follow the same path through the network and datagram routing where
packets are allowed to pass through different paths. Networks using
virtual circuits decide on the path of the communication at the
beginning of the transaction. In datagram switching each node that is
involved in a transaction makes a routing decision, which is to
determine the next hop of the packet.
Many of the routing methods are based on the shortest path
algorithm. In this method each link in the network is assigned a cost
which is a function of the physical distance and the level of
congestion. The routing algorithm tries to find the shortest path (i.e.
the path with the lowest cost) from a source node to a destination
node. In the distributed implementation, each node determines the cost
of sending a data packet to its neighbors and shares this information
with the other nodes of the network. In this way every node maintains a
database that reflects the cost of pos routes. For routing let us
consider the most general problem where network nodes are allowed to
move. This situation can be viewed as an underwater network with both
fixed ocean bottom sensors and AUVs. The instruments temporarily form a
network without the aid of any pre-existing infrastructure.
In ad hoc networks, the main problem is to obtain the most
recent state of each individual link in the network to decide on the
best route for a packet. However if the communication medium is highly
variable as in the shallow water acoustic channel the number of routing
updates can be very high. Current research on routing focuses on
reducing the overhead added by routing messages while at the same time
finding the best path which are two conflicting requirements. Also, the
effect of long propagation delays and channel asymmetries caused by
power control arc issues that need to be addressed when designing
network routing protocols to UWA channels.
There are various media access control (MAC) protocols that can
be employed to avoid information loss in UWA networks due to packet
collisions. We shall focus on the MACA protocol and a variation of this
The MACA protocol, proposed by Karn used two signaling packets
called Request- to-Send (RTS) and Clear-to-Send (CTS). When A wants to
send a message to B it first issues an RTS command. If B receives the
RTS it sends back a CTS command. As soon as A receives CTS it begins
transmission of the data packet. The nodes can probe the channel during
the RTS-CTS exchange. The channel state information can be used to set
the physical layer parameters such as output power and modulation type.
These properties of the MACA protocol are essential for efficient UWA
network design. It provides information for reliable communication with
minimum energy consumption and has the ability to avoid collisions
before they occur. RTS-CTS exchange adds overhead, but the reduction of
retransmissions can compensate for this, increase.
The MACA protocol ensures the reliability of the end-to-end
link with the network layer. If some packets of a message are lost due
to errors, the final destination node will ask the originating source
to retransmit the lost packets. On highly reliable links this approach
increases throughput, since it eliminates the need to send individual
acknowledgments for each hop. In case of poor-quality communication
channels, a message will most probably contain erroneous packets.
Recovering the errors in a data packet at the network layer will
require excessive delay. Generally, error correction is better
performed at the data link layer for channels of low reliability such
as radio or shallow water acoustic channels.
The performance and reliability of the MACA protocol may be
improved by creating error free, reliable, point-to-point links with
the data link control (DLC) layer. For this purpose, Bhargavan proposed
the MACAW protocol where an acknowledgment (ACK) packet is transmitted
after each successful transaction. Including an extra packet in the
transaction increases the overhead, which decreases the throughput.
However, it is shown in that, for radio channels, the guiding
throughput exceeds the increase in overhead. This result may also apply
to UWA channels. The MACAW protocol ignores power control and
asymmetries that occur. Its performance under power control needs to be
investigated. Also, the effect of adding more overhead to the protocol
in an environment where propagation delays are excessive needs to he
Automatic Repeat Request (ARQ) is used to detect errors in the
data link control layer and then to request the retransmission of
erroneous packets. The simplest ARQ scheme that can be directly
employed in a half-duplex UWA channel is a stop-and- wait ARQ where the
source of the packet waits for an ACK from the destination node for the
confirmation ofâ„¢ error-free packet transmission. Since the channel is
not utilized during the round trip propagation time, this ARQ scheme
has low throughput. In go-back-N and selective repeat ARQ schemes,
nodes transmit packets and receive ACKs at the same time and therefore
require full duplex links. Dividing the limited bandwidth of the UWA
channels into two channels for full duplex operation can significantly
reduce the data rate of the physical layer. However the effect on the
overall network throughput needs to be investigated. The selective
repeat ARQ scheme can be modified to work on half duplex UWA channels.
Instead of acknowledging each packet individually at reception time,
the receiver will wait for N packet durations and send an ACK packet
with the id numbers of packets received without errors. Accordingly the
source of the packets will send N packets and wait for the ACK. Then
the source will send another group of N packets that contains the
acknowledged packets and new packets.
Acknowledgements can be handled in two possible ways. In the
first approach which is called positive acknowledgement, upon reception
of an error-free, the destination node will send an ACK packet to
source node. If the source does not receive an ACK packet before preset
time out duration, it will retransmit the data packet. In the case of a
negative acknowledgement, the destination sends a packet if it receives
a corrupted packet or does not receive a scheduled data packet. A
negative acknowledgement may help to
conserve energy by eliminating the need to send explicit ACK packets
and retransmission of data packets in case of a lost ACK packet. When
combined with a MACA type MAC protocol, the negative acknowledgement
scheme may provide highly reliable point-to-point links due to the
information obtained during RTS-CTS exchange.

A realization of underwater acoustic networking is the U.S.
Navyâ„¢s experimental Telesonar and Seaweb program. Telesonar links
interconnect distributed underwater nodes, potentially integrating them
as a unified resource and extending naval net centric operations into
the underwater battle space. Seaweb provides a command control,
communications, and navigation infrastructure for coordinating
autonomous nodes to accomplish given missions in arbitrary ocean
environments. More generally Seaweb networking is applicable for
oceanographic telemetry, underwater vehicle control, and other uses of
underwater wireless digital communications.
Telesonar and Seaweb experimentations address the many aspects
of this problem including propagation, signaling, transducers, modem
electronics, networking command-centre interfacing and transmission
security. The major sea tests have included Seawebs ˜98,™99 and 2000.


Figure 1: Seaweb underwater acoustic networking enables data telemetry
and remote control for deployable autonomous distributed systems (DADS)
and other autonomous peripherals. Gateways to manned control centers
include radio links to space onshore and telesonar links to ships.
Telesonar acoustic links from the digital network of fixed and
mobile nodes. Operational objectives mandate reliability, energy
efficiency, deploys ability interoperability flexibility, affordability
and security. Thus, telesonar links must be environmentally and
situationally adaptive, with provision for bidirectional asymmetry. The
Seaweb backbone is a set of autonomous stationary nodes (e.g. sensor
nodes, repeater nodes and master nodes) collect data from the sensor
nodes and forward to the gateways and vice-versa. Seaweb peripherals
include mobile nodes (e.g. AUVs).
Odyssey is a low-cost AUV specifically developed by the
Massachusetts Institute of Technology, SeaGrant Office for the AOSN
Program. Constructed to operate at full ocean depth, Odyssey was
designed from the beginning to be both highly capable and inexpensive
to mass-produce. At less than two meters in length and not requiring
any special handling equipment for launch and recovery, Odyssey can
transit at several knots for up to 20 hours due to its ultra-low
hydrodynamic drag profile and efficient propulsion system, yielding a
very respectable range and oceanographic mission profile. An integral
part of the Odyssey is its powerful onboard computer, which is based
upon a commercial 68030 processor board. This computer executes a
control program based upon a flexible high-level behavioral language
developed at MIT, and supports vehicle control in a wide range of
conditions and mission profiles. New mission profiles are quickly
configured, tested (via a simulator developed by the Charles Stark
Draper Laboratory) and entered into the computer's library. A
sophisticated acoustic modem (developed by the Woods Hole Oceanographic
Institute) is an integral part of the system and is used to support
reliable two-way digital communications. A large fraction of Odyssey's
internal volume is available for mission sensors. Odyssey is a mature
technology which has been successfully deployed and operated in many
types of ocean environments, including the arctic.

Seaweb gateways connect with command centers submerged afloat,
ashore and aloft including access to terrestrial, airborne, and space-
based networks. For example the telesonobuoy serves as a radio /
acoustic interface permitting satellites and maritime aircraft to
communicate submerged autonomous systems. Similarly submarines can
access off board systems with telesonar signaling. A Seaweb server
resides at the manned command centers and is an interfaced underwater
Seaweb development involves periodic concentration of resources
in prolonged ocean network. The annual Seaweb experiments are designed
to validate system analysis purposively evolve critical technology
areas such that the state of the art advances with greater reliability,
functionality and quality of service. The objective of the Seaweb
experiments is to implement and test telesonar modems in networked
configurations where various modulation and networking algorithms can
be exercised, compared and conclusions drawn. In the long term, the
goal is to provide for a self-configuring network of distributed nodes,
with the network links adapting to the prevailing environment through
automatic selection of the optimum transmit parameters.

Figure 2: The Seaweb 2000 incrementally exercise the telesonar
handshake protocol in a network context. The 17-node Seaweb network
delivered oceanographic data from sensor nodes to gateway nodes with
line-of-sight packet radio and via cellular telephone modem. The 10m
waters of Buzzards Bay produced forward- scattered propagation with
l0ms multi path signal dispersion.
The Seaweb ˜98,™99 and 2000 operating area is the readily
accessible waters of Buzzards Bay, Massachusetts, charted in Fig 2. An
expanse of 5-15 m shallow water is available for large-area network
coverage with convenient line-of-sight radio contact to laboratory
facilities in Western Cape Cod. The shipping channel extending from the
Bourne canal provides periodic episodes of high shipping noise useful
for stressing the link signal-to-noise ratio (SNR) margins. Seaweb
development demands attention to the underline critical issues of
adverse transmission channel, asynchronous networking, battery-energy
efficiency, transmission security, information throughput and cost.
Knowledge of the fundamental constraints on telesonar technology is
converted into increasingly sophisticated moderns. The MAC layer,
Seaweb employs an MACA handshake protocol uniquely suited to wireless
half-duplex networking. The handshaking process permits addressing,
ranging, channel estimation, adaptive modulation and power control.
Since the network in consideration is an ad hoc network, an
initialization algorithm is needed to establish preliminary connections
autonomously. This algorithm is based on polling and as such it
guarantees connectivity to all the nodes that are acoustically
reachable by at least one of their nearest neighbors. During
initialization the nodes create neighbor tables. These tables contain a
list of each nodes neighbors and a quality measure of their links,
which can be received SNR from the corresponding neighbor. The neighbor
tables are then collected by the master node and a routing tree is
formed. Optimum routes are determined with the help of a genetic
algorithm-based routing protocol. The routing protocol tries to
maximize the lifetime of battery-powered network by minimizing the
total energy consumption of the network. The minimum energy required to
establish reliable communication between two nodes is used as the link
distance metric. A master node collects the link cost information from
the network nodes, determines optimum roots, and sends the routing
information hack to the nodes. The optimization algorithm favors
multihop links to the expense of increased delay.
The performance of acoustic links between nodes can degrade and
even a link can he permanently lost due to a node failure. In such
cases the network should be able to adapt itself to the changing
conditions without interrupting the packet transfer ibis robustness can
be obtained by updating the routes periodically.
In the current design the master node creates a routine tree
depending on neighbor labels reported by its nodes. If a node reports
that a link performance has degraded or it is no longer available, the
master node selects new routes that take the place of the failed link.
The changes in the routing tree are reported to all related nodes. This
procedure ensures that nodes wonâ„¢t attempt to use a failure link. In
this way unnecessary transmissions that increase battery consumption
are avoided.
The media access protocol for Seaweb is based on MACA protocol
which uses R CTS-DATA exchange. The network employs the stop-and-wait
ARQ scheme. If the source cannot receive CTS from the destination after
a predetermined time interval it repeats RTS. If after K trials of RTS,
the source cannot receive CTS it decides that link is no longer
available and returns to low power state. If the source receives CTS,
it immediately transmits the data packet. The RTS / CTS exchange is
used to determine the channel conditions, and this information is used
to set the acoustic modem parameters such as output power level. An ACK
signal is send by the destination upon receipt of a correct data packet
to provide positive acknowledgment to the source in the data link
layer. The protocol can also handle negative acknowledgments depending
on the operation mode selected by the user. Figure 3 illustrates the
MAC protocol.

Figure 3: The source node starts the MC layer handshake protocol by
sending a RTS packet to the destination node. If the RTS packet is lost
in the channel, the source node retransmits the RTS packer after time
out duration equal to the round trip duration of a header-only packet,
and calculated using the range information in the neighbor tables. When
the destination node receives the RTS, it replies with a CTS packet.
Upon reception of the CTS packet by the source, the DATA packet, which
contains a header and the information, is sent to the destination. The
handshake is completed with the ACK packet sent by the destination to
denote error-free reception of the data packet.
If two nodes send an RTS to each other, unnecessary retries may
occur because both nodes will ignore the received RTS command. Each
node will then wait for another node to send CTS for time-out duration
and retransmit their RTS packet. This problem is solved by assigning
priority to the packets that are directed towards the master nodes.

An overview of basic principles and constraints in the design
of reliable shallow water acoustic networks that may be used for
transmitting data from a variety of undersea sensors to onshore
facilities. Major impediments in the design of such networks are
considered including severe power limitations imposed by battery power,
severe bandwidth limitation, channel characteristics including long
propagation times, multi path, and signal fading.
Multiple access methods, network protocols and routing
algorithms are also considered. Of the multiple access methods
considered it appears that CDMA achieved by either frequency hopping or
direct sequence provides the most robust method for the underwater
network environment. Currently under development are modems that
utilizes these types of spread-spectrum signals to provide multiple
access capability to the various nodes in the network. Simultaneous
with current modem development there are several investigations on the
design of routing algorithm and network protocols.
The design example of the shallow water network employed in
Seaweb embodies the power and the bandwidth constraints that are so
important in digital communication through underwater acoustic
channels. As an information system compatible with low bandwidth, high
latency and variable quality of service, Seaweb offers a blueprint for
the development of future shallow water acoustic networks. Over the
next decade, significant improvements are anticipated in the design and
implementation of shallow water acoustic networks as more experience is
gained through at-sea experiments and network simulations.
1. Computer Networks by Andrew S Tanenbaum
2. Acoustic Ocean Sampling Network, IEEE Network Magazine
3. Acoustic Communication, IEEE Communication Magazine


I express my sincere thanks to Prof. M.N Agnisarman Namboothiri (Head
of the Department, Computer Science and Engineering, MESCE), Mr.
Sminesh (Staff incharge) for their kind co-operation for presenting the
seminar and presentation.
I also extend my sincere thanks to all other members of the faculty of
Computer Science and Engineering Department and my friends for their
co-operation and encouragement.

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