Modelling of underwater acoustic communication network

1. INTRODUCTION
The research on underwater acoustic networks (UAN) is gaining attention due to their important applications for military and commercial purposes. Underwater communication applications mostly involve long term monitoring of selected ocean areas. The traditional approach for ocean bottom monitoring is to deploy underwater sensors, record the data and recover the instruments. However this approach creates long delays in receiving the recorded information and if a failure occurs before the recovery, all the data is lost. The ideal solution for these applications is to establish real time communication between underwater instruments and a communication center within a network configuration .
Basic underwater networks (UAN) are formed by establishing a two way acoustic link between various instruments such as autonomous underwater vehicles and sensors. the network is then connected to backbone such as internet, through the RF link. This configuration creates an interactive environment where scientists can extract real time data from multiple underwater instruments. The data is transferred to the control station when it is available hence data loss is prevented until a failure occurs[2]. Underwater networks can also be used to increase the operation range of underwater vehicles. The feasible range of underwater vehicles is limited by the acoustic range of a single modem which varies from 10 to 90 km [2].However due to high cost involved in underwater devices it is necessary that the deployed network be highly reliable so as to avoid failure of monitoring mission due to failure of single or multiple devices.

From the communication point of view, underwater environment is much different from its terrestrial counterpart. Consequently, the research of UAN’s becomes different and exhibits unique features. It is because:
The attenuation of acoustic signals increase with frequency and range resulting in extremely small feasible band.
The propagation speed of acoustic wave is 1500m/sec which is several orders of magnitude lower than radio waves [3], thus giving large propagation delays.
The channel characteristics vary with time and highly depend on transmitter and receiver. The fluctuating nature of the channel causes distortion in the signals.
Due to the variable acoustic environment UAN differ in many aspects such as ranging from network topologies to protocols of all layers compared with the ground one.
2. NETWORK TOPOLOGIES
The network topology directly influences network capacity of the underwater channel which is severely limited. It becomes important to organize network topology in such a way that no congestion occurs or in other words designing of network topology with single point of failure should be avoided. Underwater networks can be composed of entirely fixed nodes, entirely mobile nodes or a mixture of both. The network topology typically need to be ad hoc in nature either because communicating nodes are moving or basic acoustic conditions change with time. There are three basic network topologies that can be used to interconnect network nodes [3].
(1)Centralized topologyIn this topology, each network host is connected to central station known as hub of the network. The network is connected to a backbone at this central station. Deep under water acoustic networks (UAN) has been tested using this configuration where a surface buoy with both an acoustic and RF modem acts as the hub and controls the communication to and from ocean bottom instruments. This topology is considered the easiest topology to design and implement .The advantage of this topology is the simplicity of adding additional nodes. A major disadvantage of this topology is the presence of single failure point.If the hub fails, the entire network goes down. Further, the network cannot cover large areas due to limited range of single modem.

(2)Distributed of point to point topology This topology provides point to point links between every node of the network. There is just one hop from a node to any other node, hence routing is not necessary. The major disadvantage of this configuration is that excessive power is needed for communicating with widely spread nodes. Further, near far problem [4] is much prominent in which a node can block signals of the neighboring node.
(3)Multihop topology In this topology nodes are involved to send a message from source node to destination. Hence routing is needed which is handled by intelligent algorithms that can adapt to changing conditions.Multihop networks can cover large areas since the range of the network is now determined by number of nodes rather than the range of the modem. The only problem with this topology is that of packet delay as the number of hops increase
3. MEDIA ACCESS TECHNIQUES
Due to scarce bandwidth, long propagation delay and high error rate, underwater nodes in a UAN have to share the available resources. The three basic access techniques are
(1) Frequency division multiple access(FDMA)FDMA divides the bandwidth into several subbands and assigns one of them to a particular user. The band is used by this user only till it is released.FDMA may not be efficient in underwater environment. The available bandwidth is extremely limited .By dividing the band into smaller sub bands , the coherence bandwidth of the transmission channel can be larger than FDMA subchannel.This will result in severe fading .another issue is that mechanism could in inefficient in bursty[ 4] traffic because bandwidth is fixed for each subband and cannot be adjusted [5] .
(2) Time division multiple access(TDMA) In this multiple access scheme time frame is divided into slots and each slot is assigned one individual user. Each user transmits in the assigned slot. The advantage of TDMA is power saving which is extremely critical in underwater environment. Since each user transmit only in its assigned slot, transmitter could be turned off during the idle period to save energy.TDMA is also flexible in the way that data rate of users can be increased on demand. The same hardware can be used to transmit and no extra hardware is needed e.g. to add another time slot for a user.
The disadvantage of TDMA is that it has larger overload than FDMA which means guard times are included in order to avoid collisions from neighbors. Further, TDMA requires strict time synchronization. The significant difference in propagation delays cause large idle times resulting in decrease in throughput.
(3) Code division multiple access CDMA This multiple access method is the widely deployed scheme based on spread spectrum. It allows users to transmit signalsall the time with all available bandwidth. Signals from users are distinguished by means of spreading code. This code is orthogonal to the spreading codes used by other users. There are two spreading techniques namely direct sequence spread spectrum(DS) and frequency hopping spread spectrum(FH).In the former case the spread code is multiplied directly(linear modulation) in order to spread the original bits while in latter case, the carrier frequency of a user is changed according to the pattern of the spread code.
Following are the main advantages of CDMA
(a)It has higher efficiency and throughput than FDMA and CDMA [3].
(b)CDMA is very effective against jamming, multipath interference and any other interference that appears deterministic [6].
(c)Switching from signal to signal for a transmitter or receiver can be easily done by changing the spread codes. Thus CDMA is flexible.
(d)In DS system, fine time resolution of spreading codes provides the possibility of coherently combining multipath arrivals using rake receiver. The rake receiver identifies three strongest multipath signals and combines them to one powerful signal. If the resolvable multipath components fade independently, it is possible to extract a time diversity gain present in the channel [5].
(e)Increased communication security.
Due to above mentioned reasons, CDMA and spread spectrum signaling appear to be promising multiple access method for shallow water acoustic networks.
4. MAC PROTOCOLS FOR UNDERWATER NETWORKS
A lot of media access control (MAC) protocols for underwater networks have been explored such as ALOHA, slotted ALOHA and CSMA. The most significant protocols among underwater networks seem to be CSMA/CA .
Carrier sense media access with collision avoidance (CSMA/CA)
The scarce resources of channel can be utilized much better if users sense the channel before transmitting a packet. This protocol uses two signaling packets called request to send (RTS) and clear to send (CTS). When a device intends to send a packet, it first senses whether another station is already transmitting (carrier sense). If no transmission is sensed, the device will issue RTS signal which contains the length of the message to be sent. If the recipient station senses that the medium is clear, it sends a clear to signal (CTS) which also contains the length of the message to be transmitted. As soon as the station wishing to transmit receives the CTS signal, it sends the actual data packet to its intended recipient. If the transmitting station does not receive the CTS signal in reply, it begins the RTS procedure. The controlling signal CTS should be heard by all the nodes within the range of the receiver node which in turn means that this protocol relies on the symmetry of the channel. It becomes essential to send CTS from a higher level to ensure that all the nodes within the range can hear it. This protocol can be used as a basis of media access protocol for underwater networks. It provides information for power control algorithms as nodes learn the minimum power level needed for reliable communication by trial and error.
5. NETWORK LAYER
Single hop transmission becomes inefficient if the range of the network becomes large. In that case multihop transmission is needed to relay the information from source to destination. It has also been proved that in underwater networks multihop transmission is more efficient in terms of power consumption [7].
The network layer is responsible for routing packets from source to destination when multihop is needed. There are two methods of routing namely virtual circuit routing and packet switched routing.
In virtual circuit routing, a communication path is decided before the data transmission takes place. Based on resource optimizing algorithm, the system decides which route to follow. For the whole transmission time session between two communicating entities is dedicated and exclusive, and released only when the session terminates.
In packet switching, the packets are sent towards the destination irrespective of each other. There is no pre determined path and each packet has to find its own route. Each node is involved in routing the packets in order to determine the next hop of the packet.
Underwater networks may have entirely fixed nodes (ocean bottom sensors) or completely mobile nodes (autonomous underwater vehicles).These instrumentstemporarily form a network without the aid on any pre existing infrastructure.These are called ad hoc networks [3].The main problem in ad hoc networks is obtaining most recent individual link state in the network, so as to decide best route for the packets. However, in case communication medium is highly variable such as shallow water acoustic channel, the number of routing updates can be very high. Some of the routing protocols that can be used in underwater acoustic networks are as follows [3]:
(1)DSDV (Destination sequenced distance vector) In this routing algorithm every node maintains a routing table of all available destinations, number of hops to reach the destination and the sequence number assigned by the destination node. The sequence number is used to distinguish stale routes from new routes and thus avoids the formation of loops. If a node receives new information, it uses the latest sequence number .If the sequence number is same as the one already in the table, the route with better metric will be used. The nodes periodically transmit their routing tables to their neighbors. If a node detects any route to the destination broken, then its hop number is set to infinity and its sequence number is increased. The disadvantage of DSDV is that its routing tables need to be updated regularly which wastes batter and small bandwidth even when the network is idle. further, if topology of network changes, a new sequence number needs to be added hence DSDV is not suitable for highly dynamic networks.
(2)DSR (Dynamic source routing) Instead on relying on the routing table at intermediate node, DSR makes use of source routing. The sender knows the complete hop by hop route to destination with these routes stored in a route cache. The route for each packet is included in its header. The node which receives the packet checks the header for the next hop and forwards packet. Route discovery works by flooding the network with route requests (RREQ) packets. On receiving the RREQ each node rebroadcasts it, unless it is the destination or it has route to the destination in its route cache. This protocol works well in static and low mobility environments.
(3)AODV (Ad hoc on demand distance vector This protocol establishes route to the destination only on demand and does not require nodes to maintain routing tables of destinations that are not actively used. Routes are discovered and maintained by route requests (RREQ), route replies (RREP) and route errors (RERR).
AODV uses destination sequence numbers on route updates which guarantees loop free path and gives the view of several fresh routes. The advantage of AODV is that it creates no extra traffic for communication along existing links by lowering the number of messages, thus conserving capacity of the network. Also , distance vector routing is simple and does not require much calculation. However time to establish connection and initial establishment of a route is much longer than the other approaches.
6. RELATED WORK
The interest in underwater networks and the consequent research has exponentially grown in recent years. Network Simulation and testing of underwater acoustic networks is relatively a new area, however there already exists some effort in this area. The authors of [2] compare the performances of DSDV, DSR and AODV with regards to following parameters
(1)Total throughputIt is the average rate of successful message delivery over a communication channel and is expressed as bits per second. Throughput is the very important metric in underwater acoustics because of very limited bandwidth.
Fig 1: Total throughput for DSDV, DSR and AODV routing protocols [2]
The above figure shows the total throughput plotted against the offered load. It can easily be concluded that AODV has the best performance and maximum throughput, whereas DSR routing protocol is the worst.
(1) Total packet delivery ratio It is the ratio between the number of packets sent out by the source and the number of packets correctly received by the corresponding destination. It is calculated by averaging time passed from the time a data packet is generated and when the packet is received by the destination.
Fig 2: Total delivered packets for DSDV, DSR and AODV routing protocols [2]
The above figure total delivered packets versus the offered load. The plot indicates that the DSR and DSDV have best performance when the offered load is below 0.1 pkt/sec, and AODV protocol is worse when the offered load is 0.1 pkt/sec. however when the offered load increases AODV protocol gives the best performance compared to DSR and DSDV.
(1)Average end to end delay It is the delay in the arrival of packet calculated by averaging the time that passes the time a data packet is generated to when it arrives at its final destination. Figure (3) shows the plot of average end to end delay versus the load offered. The minimum end to end delay is achieved by AODV protocol .DSR is the worst routing protocol having an average delay of 115 sec.In general minimum delay is achieved by all routing protocols when the offered load is small.
Fig 3: Total average end to end delay for DSR, DSDV and AODV protocols. [2]
In the above mentioned system it can be concluded that AODV routing protocolachieves maximum throughput and has best performance compared with DSDV and DSR routing protocols. It also gives minimum end to end delay when compared with other protocols. The best performance was achieved when offered load was decreased resulting in increase in packet delivery rate and decrease in average end to end delay.
7. CONCLUSION & FUTURE PROSPECTS

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