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Figure 1 shows a simple ad-hoc network with three nodes. The node 3 is not within transmitter range of node 1. However the middle node 2 or 4 can be used to forward packets between the nodes. These middle nodes are acting as a router and the four nodes have formed an ad-hoc network.
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This is to be sure that the network will not collapse just because one of the mobile nodes moves out of transmitter range of the others. Nodes are able to enter/leave the network as their movable characteristics. Because of the limited transmitter range of the nodes, multiple hops may be needed to reach other nodes. Every node willing to participate in an ad-hoc network must be willing to forward packets for other nodes. Thus every node acts both as a host and as a router. A node can be viewed as an abstract entity consisting of a router and a set of mobile hosts (Figure 1). A router is an entity, which, among other things runs a routing protocol.
We can formalize the above statement by defining an ad hoc network as an autonomous system of mobile nodes (serving as routers) connected by wireless links, which forms a communication network in the form of an arbitrary graph with mobile nodes. These nodes may be located in or on airplanes, ships, trucks, cars, perhaps even on people or very small devices. This is in contrast to the well-known single hop cellular network model that supports the needs of wireless communication by installing base stations as access points. In these cellular networks, communications between two mobile nodes completely rely on the fixed base stations and the wired backbone. In a MANET, no such infrastructure exists due to dynamic network topology formation in an unpredictable manner since nodes are free to move.
CHARACTERISTICS OF MANET
MANETs have several salient characteristics:
1) Dynamic topologies: Nodes are free to move arbitrarily; thus, the network topology--which is typically multi-hop--may change randomly and rapidly at unpredictable times, and may consist of both bidirectional and unidirectional links.
2) Bandwidth-constrained, variable capacity links: Wireless links will continue to have significantly lower capacity than their hardwired counterparts. In addition, the realized throughput of wireless communications--after accounting for the effects of multiple access, fading, noise, and interference conditions ,etc.--is often much less than a radio's maximum transmission rate.
One effect of the relatively low to moderate link capacities is that congestion is typically the norm rather than the exception, i.e. aggregate application demand will likely approach or exceed network capacity frequently. As the mobile network is often simply an extension of the fixed network infrastructure, mobile ad hoc users will demand similar services. These demands will continue to increase as multimedia computing and collaborative networking applications rise. Performance concerns for protocol design which extend beyond those guiding the design of routing within the higher-speed, semi-static topology of the fixed Internet.
3) Energy-constrained operation: Some or all of the nodes in a MANET may rely on batteries or other exhaustible means for their energy. For these nodes, the most important system design criteria for optimization may be energy conservation.
4) Limited physical security: Mobile wireless networks are generally more prone to physical security threats than are fixed- cable nets. The increased possibility of eavesdropping, spoofing,
and denial-of-service attacks should be carefully considered. Existing link security techniques are often applied within wireless networks to reduce security threats. As a benefit, the decentralized nature of network control in MANETs provides additional robustness against the single points of failure of more centralized approaches.
An ad hoc application is a self-organizing application consisting of mobile devices forming a peer-to-peer network where communications are possible because of proximity of the devices within a physical distance. MANET can be used to form the basic infrastructure for ad hoc applications. There are many applications to ad hoc networks. As a matter of fact, any day-to-day application such as electronic email and file transfer can be considered to be easily deployable within an ad hoc network environment. Web services are also possible in case any node in the network can serve as a gateway to the outside world. In this discussion, we need not emphasize the wide range of military applications possible with ad hoc networks. Not to mention, the technology was initially developed keeping in mind the military applications, such as battlefield in an unknown territory where an infrastructure network is almost impossible to have or maintain. In such situations, the ad hoc networks having self-organizing capability can be effectively used where other technologies either fail or cannot be deployed effectively. Advanced features of wireless mobile systems, including data rates compatible with multimedia applications, global roaming capability, and coordination with other network structures, are enabling new applications.
Some typical applications are shown in table below:
â€¢Supports tactical network for military
â€¢ Automated battle fields.
â€¢ Search and rescue operations
â€¢ Disaster recovery
â€¢ Replacement of fixed infrastructure in case of environmental
â€¢ Policing and fire fighting
â€¢ Supporting doctors and nurses in hospitals
Commercial and civilian
â€¢ E-commerce: electronic payments anytime and anywhere
â€¢ Business: dynamic database access, mobile offices
â€¢ Vehicular services: road or accident guidance, transmission of
road and weather conditions, taxi cab network, inter-vehicle
â€¢ Sports stadiums, trade fairs, shopping malls
â€¢ Networks of visitors at airports
Home and enterprise
â€¢ Home/office wireless networking
â€¢ Conferences, meeting rooms
â€¢ Personal area networks (PAN), Personal networks (PN)
â€¢ Networks at construction sites
â€¢ Universities and campus settings
â€¢ Virtual classrooms
â€¢ Ad hoc communications during meetings or lectures
â€¢ Multi-user games
â€¢ Wireless P2P networking
â€¢ Outdoor Internet access
â€¢ Robotic pets
â€¢ Theme parks
â€¢ Home applications: smart sensors and actuators embedded in
â€¢ Body area networks (BAN)
â€¢ Data tracking of environmental conditions, animal
movements, chemical/biological detection
Context aware services
â€¢ Follow-on services: call-forwarding, mobile workspace
â€¢ Information services: location specific services, time
â€¢ Infotainment: touristic information
â€¢ Extending cellular network access
â€¢ Linking up with the Internet, intranets, etc.
As mobile ad hoc networks are characterized by a multi-hop network topology that can change frequently due to mobility, efficient routing protocols are needed to establish communication paths between nodes, without causing excessive control traffic overhead or computational burden on the power constrained devices . A large number of solutions have already been proposed, some of them being subject to standardization within the IETF. A number of proposed solutions attempt to have an up-to-date route to all other nodes at all times. To this end, these protocols exchange routing control information periodically and on topological changes. These protocols, which are called proactive routing protocols, are typically modified versions of traditional link state or distance vector routing protocols encountered in wired networks, adapted to the specific requirements of the dynamic mobile ad hoc network environment. Most of the time, it is not necessary to have an up-to-date route to all other nodes. Therefore, reactive routing protocols only set up routes to nodes they communicate with and these routes are kept alive as long as they are needed. Combinations of proactive and reactive protocols, where nearby routes (for example, maximum two hops) are kept up-to-date proactively, while far-away routes are set up reactively, are also possible and fall in the category of hybrid routing protocols. A completely different approach is taken by the location-based routing protocols, where packet forwarding is based on the location of a node's communication partner. Location information services provide nodes with the location of the others, so packets can be forwarded in the direction of the destination.
Desirable Properties of Routing Protocols
If the conventional routing protocols do not meet our demands, we need a new routing protocol. The question is what properties such protocols should have? These are some of the properties  that are desirable:
The protocol should of course be distributed. It should not be dependent on a centralized controlling node. This is the case even for stationary networks. The difference is that nodes in an ad-hoc network can enter/leave the network very easily and because of mobility the network can be partitioned.
To improve the overall performance, we want the routing protocol to guarantee that the routes supplied are loop-free. This avoids any waste of bandwidth or CPU consumption.
Demand based operation
To minimize the control overhead in the network and thus not wasting network resources more than necessary, the protocol should be reactive. This means that the protocol should only react when needed and that the protocol should not periodically broadcast control information.
Unidirectional link support
The radio environment can cause the formation of unidirectional links. Utilization of these links and not only the bi-directional links improves the routing protocol performance.
The radio environment is especially vulnerable to impersonation attacks, so to ensure the wanted behavior from the routing protocol, we need some sort of preventive security measures. Authentication and encryption is probably the way to go and the problem here lies within distributing keys among the nodes in the ad-hoc network. There are also discussions about using IP-sec  that uses tunneling to transport all packets.
The nodes in an ad-hoc network can be laptops and thin clients, such as PDAs that are very limited in battery power and therefore uses some sort of stand-by mode to save power. It is therefore important that the routing protocol has support for these sleep-modes.
To reduce the number of reactions to topological changes and congestion multiple routes could be used. If one route has become invalid, it is possible that another stored route could still be valid and thus saving the routing protocol from initiating another route discovery procedure.
Quality of service support
Some sort of Quality of Service support is probably necessary to incorporate into the routing protocol. This has a lot to do with what these networks will be used for. It could for instance be real-time traffic support.
None of the proposed protocols from MANET have all these properties, but it is necessary to remember that the protocols are still under development and are probably extended with more functionality. The primary function is still to find a route to the destination, not to find the best/optimal/shortest-path route.
Classification of Ad hoc Routing Protocol
Routing protocol in MANET can be classified into several ways depending upon their network structure, communication model, routing strategy, and state information and so on but most of these are done depending on routing strategy and network structure [3,10]. Based on the routing strategy the routing protocols can be classified into two parts: 1.Table driven and 2. Source initiated (on demand) while depending on the network structure these are classified as flat routing, hierarchical routing and geographic position assisted routing . Flat routing covers both routing protocols based on routing strategy. The classification of routing protocols is shown in the Figure.
Flat Routing Protocols
Flat routing protocols distribute information as needed to any router that can be reached or receive information. No effort is made to organize the network or its traffic, only to discover the best route hop by hop to a destination by any path. Think of this as all routers sitting on a flat geometric plane. These protocols are mainly divided into two classes, First one is proactive routing that is also known as table driven and other is reactive that's called on-demand routing protocols. Common thing is general for both protocol classes is that every node participates in routing play an equal role. These are further been classified based on their design principles; Proactive routing is mostly based on LS (Link State) while on-demand routing based on DV (Distance Vector).
Pro-Active /Table Driven routing Protocols
Proactive protocols continuously learn the topology of the network by exchanging topological information among the network nodes. Thus, when there is a need for a route to a destination, such route information is available immediately. If the network topology changes too frequently, the cost of maintaining the network might be very high. If the network activity is low, the information about actual topology might even not be used. Proactive protocols continuously evaluates the routes within the network so that when we are required to forward the packet route is already known and immediately ready for use. So there is no time delay. So a shortest path can be find without any time delay however these protocols are not suitable for very dense ad-hoc networks because in that condition problem of high traffic may arise. Several modifications of proactive protocols have been proposed for removing its shortcomings and use in ad-hoc networks. It maintains the unicast routes between all pair of nodes without considering of whether all routes are actually used or not.
Examples of Proactive MANET Protocols include:
Optimized Link State Routing (OLSR)
Fish-eye State Routing (FSR)
Destination-Sequence Distance Vector (DSDV)
Cluster-head Gateway Switch Routing Protocol (CGSR)
Reactive Routing Protocols
The reactive routing protocols are based on some sort of query-reply dialog. It is also called on demand routing. It is more efficient than proactive routing and most of the current work and modifications have been done in this type of routing for making it more and more better. The main idea behind this type of routing is to find a route between a source and destination whenever that route is needed whereas in proactive protocols we were maintaining all routes without regarding its state of use. So in reactive protocols we don't need to bother about the routes which are not being used currently. This type of routing is on demand. Discovering the route on demand avoids the cost of maintaining routes that are not being used and also controls the traffic of the network because it doesn't send excessive control messages which significantly create a large difference between proactive and reactive protocols. Time delay in reactive protocols is greater comparative to proactive types since routes are calculated when it is required.
Examples of Proactive MANET Protocols include:
Ad hoc On Demand Distance Vector (AODV)
Dynamic Source Routing Protocol (DSR)
Temporally Ordered Routing Algorithm (TORA)
Dynamic MANET On-Demand (DYMO)
Associativity Based Routing (ABR)
Signal Stability-Based Adaptive Routing (SSA)
Location-Aided Routing Protocol (LAR)
Hybrid Routing Protocols
Both of the proactive and reactive routing methods have some pros and cons. In hybrid routing a well combination of proactive and reactive routing methods are used which are better than the both used in isolation. It includes the advantages of both protocols. As an example facilitate the reactive routing protocol such as AODV with some proactive features by refreshing routes of active destinations which would definitely reduce the delay and overhead so refresh interval can improve the performance of the network and node. So these types of protocols can incorporate the facility of other protocols without compromising with its own advantages. Examples of hybrid protocols are Zone Routing Protocol (ZRP).
Examples of Hybrid Protocols are:
Zone Routing Protocol (ZRP)
Wireless Ad hoc Routing Protocol (WARP)
Hierarchical Routing Protocols
A hierarchical control structure is employed by the protocols which fall in this class . The nodes located in a common scope (may be defined by their distances to each other) in the network are grouped together into a cluster, so that the network is defined as clusters. The nodes of a specific cluster elect a cluster head who coordinates the work between the different nodes in the cluster. This clustering can be extended to a multi level hierarchy. A very important example of this class is Cluster-Head Gateway Switch Routing Protocol (CGSR). A hierarchical addressing scheme is required .
Hierarchical State Routing (HSR)
Zone Routing Protocol (ZRP)
Cluster-head Gateway Switch Routing Protocol (CGSR)
Landmark Ad hoc Routing Protocol (LANMAR)
Geographical Routing Protocols
Geographic routing protocols scale better for ad hoc networks mainly for two reasons: 1) there is no necessity to keep routing tables up-to-date and 2) no need to have a global view of the network topology and its changes. Therefore, geographic routing protocols have attracted a lot of attention in the field of routing protocols for MANETs. These geographic approaches allow routers to be nearly stateless because forwarding decisions are based on location information of the destination and the location information of all one-hop neighbours. Most of these protocols keep state only about the local topology (i.e., neighbor's location information). No routing table
is constructed. As a result, establishment and maintenance of routes are not required, reducing the overhead considerably.
Some Geographical Routing Protocols are:
Geographic Addressing and Routing (GeoCast)
Distance Routing Effect Algorithm for Mobility (DREAM)
Greedy Perimeter Stateless Routing (GPSR)
Description of Some Popular Proactive Routing Protocols:
Destination-sequenced distance vector (DSDV):
DSDV  is the most obvious proactive protocol. It is based on Bellman ford algorithm. It removed the shortcomings (loops, count to infinity problem) of contemporary distance vector protocol which was not suited for ad-hoc networks. It is a destination based distance vector routing protocol in which every node maintains a routing table. This routing table contains all available destinations, the next node to reach to destination, and the no of hops between it. Whenever any node changes its position it broadcast the routing updates to the other nodes. Sequence number is used to avoid loop problems.
Keeping the simplicity of distance vector protocol it guarantees loop freeness it reacts immediately on topology changes. Since the route for destination is always available at the routing table of each node so there is no latency caused by route discovery. But broadcasting of routing updates may cause high traffic load between the nodes if the density of the nodes are high. So this protocol is best suited if the density of the ad-hoc network is low. However if the mobility of the node is too high broadcasting updates may cause time delay.
Advantages of DSDV:
DSDV protocol guarantees loop free paths.
Count to infinity problem is reduced in DSDV.
· We can avoid extra trace with incremental updates instead of full dump updates. The path selection in DSDV maintains only the best path instead of maintaining multiple paths to every destination; with this the amount of space in routing table is reduced.
Limitations of DSDV:
Wastage of bandwidth due to unnecessary advertising of routing information even if there is no change in the network topology.
DSDV doesn't support Multi path Routing.
In DSDV it is difficult to determine a time delay for the advertisement of routes and also it is difficult to maintain the routing table's advertisement for larger network.
In DSDV each and every host in the network should maintain a routing table for advertising. But for larger network this would lead to overhead, which consumes more bandwidth.
OLSR (Optimized Link State Routing Protocol):
Optimized Link State Routing OLSR  incorporates two optimizations over the conventional link state routing in ad hoc networks. Each node selects a set of neighbor nodes called multi-point relays (MPRs). Furthermore, when exchanging link-state routing information, a node lists only the connections to those neighbors that have selected it as MPR, i.e., its Multipoint Relay Selector set .Further, the link state updates are diffused throughout the network only using these MPRs thus significantly reducing the number of retransmissions. The MPRs of a node are basically the smallest set of neighbors who can effectively reach all the two hop neighbors of that node. The MPRs of a node changes with node mobility and are updated using periodic HELLO messaging. A source-destination route is basically a sequence of hops through the multipoint relay nodes. Routes selected are shortest hop as in the conventional link state algorithm. The protocol selects bi-directional links for routing.
Advantages of OLSR:
OLSR has less average end to end delay.
OLSR is a flat routing protocol, which does not need central administrative system to handle its routing process.
OLSR is well suited for an application which does not allow long delays in the transmission of data packets.
Limitations of OLSR:
OLSR needs more time re-discovering the broken link.
Wider delay distribution.
OLSR requires more power when discovering alternative route.
Description of Some Popular Reactive Routing Protocols:
DSR (Dynamic Source Routing Protocol):
DSR  is a loop-free, source based, on demand routing protocol. This protocol is source-initiated rather than hop-by-hop. This is particularly designed for use in multi hop wireless ad hoc networks of mobile nodes. Basically, DSR protocol does not need any existing network infrastructure or administration and this allows the network to be completely self-organizing and self-configuring. This protocol is composed of two essential parts of route discovery and route maintenance. Every node maintains a cache to store recently discovered paths. When a node desires to send a packet to some node, it first checks its entry in the cache. If it is there, then it uses that path to transmit the packet and also attach its source address on the packet. If it is not there in the cache or the entry in cache is expired, the sender broadcasts a route request packet to all of its neighbors asking for a path to the destination. The sender will be waiting till the route is discovered. During waiting time, the sender can perform other tasks such as sending/forwarding other packets. As the route request packet arrives to any of the nodes, they check from their neighbor or from their caches whether the destination asked is known or unknown. If route information is known, they send back a route reply packet to the destination otherwise they broadcast the same route request packet. When the route is discovered, the required packets will be transmitted by the sender on the discovered route. Also an entry in the cache will be inserted for the future use. The node will also maintain the age information of the entry so as to know whether the cache is fresh or not. When a data packet is received by any intermediate node, it first checks whether the packet is meant for itself or not. If it is meant for itself, the packet is received otherwise the same will be forwarded using the path attached on the data packet. Since in Ad hoc network, any link might fail anytime. Therefore, route maintenance process will constantly monitors and will also notify the nodes if there is any failure in the path. Consequently, the nodes will change the entries of their route cache.
Advantages of DSR:
One of the main benefits of DSR protocol is that there is no need to keep routing table so as to route a given data packet as the entire route is contained in the packet header.
Limitations of DSR:
The limitations of DSR protocol is that, it is not scalable to large networks and even requires significantly more processing resources than most other protocols. Basically, in order to obtain the routing information, each node must spend lot of time to process any control data it receives, even if it is not the intended recipient.
AODV (Ad hoc On Demand Distance Vector Protocol):
Ad hoc On Demand Distance Vector AODV  is a variation of Destination-Sequenced Distance-Vector (DSDV) routing protocol which is collectively based on DSDV and DSR. It aims to minimize the requirement of system-wide broadcasts to the greater extent. It does not maintain routes from every node to every other node in the network rather they are discovered as and when needed and are maintained only as long as they are required. The key steps used by AODV for establishment of unicast routes are Route discovery and Route maintenance.
i) Route Discovery
When a node wants to send a data packet to a destination node, the entries in route table are checked to ensure whether there is a current route to that destination node or not. If it is there, the data packet is forwarded to the appropriate next hop toward the destination. If it is not there, the route discovery process is initiated. AODV initiates a route discovery process using Route Request (RREQ) and Route Reply (RREP). The source node will create a RREQ packet containing its IP address, its current sequence number, the destination's IP address, the destination's last sequence number and broadcast ID. The broadcast ID is incremented each time the source node initiates RREQ. Basically, the sequence numbers are used to determine the timeliness of each data packet and the broadcast ID & the IP address together form a unique identifier for RREQ so as to uniquely identify each request. The requests are sent using RREQ message and the information in connection with creation of a route is sent back in RREP message. The source node broadcasts the RREQ packet to its neighbors and then sets a timer to wait for a reply. To process the RREQ, the node sets up a reverse route entry for the source node in its route table. This helps to know how to forward a RREP to the source. Basically a lifetime is associated with the reverse route entry and if this entry is not used within this lifetime, the route information is deleted. If the RREQ is lost during transmission, the source node is allowed to broadcast again using route discovery mechanism.
ii) Route maintenance
As long as the route remains active, it will continue to be maintained. A route is considered active as long as there are data packets periodically travelling from the source to the destination along that path. Once the source stops sending data packets, the links will time out and eventually be deleted from the intermediate node routing tables. If a link break occurs while the route is active, the node upstream of the break propagates a route error (RERR) message to the source node to inform it of the now unreachable destination(s). After receiving the RERR, if the source node still desires the route, it can reinitiate route discovery.
Advantages of AODV
The benefits of AODV protocol are that it favors the least congested route instead of the shortest route and it also supports both unicast and multicast packet transmissions even for nodes in constant movement.
It also responds very quickly to the topological changes that affects the active routes.
AODV does not put any additional overheads on data packets as it does not make use of source routing.
Limitations of AODV
The limitation of AODV protocol is that it expects/requires that the nodes in the broadcast medium can detect each others' broadcasts. It is also possible that a valid route is expired and the determination of a reasonable expiry time is difficult. The reason behind this is that the nodes are mobile and their sending rates may differ widely and can change dynamically from node to node. In addition, as the size of network grows, various performance metrics begin decreasing.
AODV is vulnerable to various kinds of attacks as it based on the assumption that all nodes must cooperate and without their cooperation no route can be established.
The Dynamic MANET On-demand (DYMO):
The Dynamic MANET On-demand (DYMO)   routing protocol is a simple and fast routing protocol for multi hop networks. It discovers unicast routes among DYMO routers within the network in an on-demand fashion, offering improved convergence in dynamic topologies. To ensure the correctness of this protocol, digital signatures and hash chains are used . The basic operations of the DYMO protocol are route discovery and route management. When a source needs to send a data packet, it sends an RREQ to discover a route to that particular destination. After issuing an RREQ, the origin DYMO router waits for a route to be discovered. If a route is not obtained within RREQ waiting time, it may again try to discover a route by issuing another RREQ. To reduce congestion in a network, repeated attempts at route discovery for a particular target node should utilize an exponential back off. Data packets awaiting a route should be buffered by the source's DYMO router. This buffer should have a fixed limited size and older data packets should be discarded first. Buffering of data packets can have both positive and negative effects, and therefore buffer settings should be administratively configurable or intelligently controlled. If a route discovery has been attempted maximum times without receiving a route to the target node, all data packets intended for the corresponding target node are dropped from the buffer and a Destination Unreachable ICMP message is delivered to the source. When a data packet is to be forwarded and it cannot be delivered to the next-hop because no forwarding route for the IP Destination Address exists; an RERR is issued. Based on this condition, an ICMP Destination Unreachable message must not be generated unless this router is responsible for the IP Destination Address and that IP Destination Address is known to be unreachable. Moreover, an RERR should be issued after detecting a broken link of a forwarding route and quickly notify DYMO routers that a link break occurred and that certain routes are no longer available. If the route with the broken link has not been used recently, the RERR should not be generated.
Energy aware routing:
The aim of energy-aware routing protocols is to reduce energy consumption in transmission of packets between source and a destination, to avoid routing of packets through nodes with low residual energy, to optimize flooding of routing information over the network and to avoid interference and medium collisions. A single node failure in sensor networks is usually unimportant because it does not lead to a loss of sensing and communication coverage whereas ad-hoc networks are oriented towards personal communication and the loss of connectivity to any node is significant. there have been an increased interest in routing algorithms that take the energy levels of the nodes into account. Nodes in wireless ad hoc networks typically use batteries as their power source. These batteries often have a limited capacity, which is depleted with every transmission.
It is important to consider this when designing routing protocols for ad hoc networks. Only finding the paths that require the least amount of power is not enough; some nodes might still be used much more than others, and will therefore be prematurely depleted.
A wireless sensor network is densely deployed with a large number of sensor nodes, each of which operates with limited battery power, while working with the self-organizing capability in the multi-hop environment. Since each node in the network plays both terminal node and routing node roles, a node cannot participate in the network if its battery power runs out. The increase of such dead nodes generates many network partitions and consequently, normal communication will be impossible as a sensor network. Thus, an important research issue is the development of an efficient batter-power management to increase the life cycle of the wireless sensor network .
The aim of energy-aware routing protocols is to reduce energy consumption in transmission of packets between a source and a destination, to avoid routing of packets through nodes with low residual energy, to optimize flooding of routing information over the network and to avoid interference and medium collisions. Many energy efficient routing protocol proposals were originally studied for sensor networks, where the limited energy of nodes is a strong constraint; in MANET, however, the requirements are different: a node has generally more hardware resources (capable of better performance, but consuming more energy) and the protocol must preserve the resources of every node in the network (not only a subset of them, because each node can be, at any time, source or destination of data). A single node failure in sensor networks is usually unimportant if it does not lead to a loss of sensing and communication coverage; ad-hoc networks, instead, are oriented towards personal communication and the loss of connectivity to any node is significant. In the routing protocol design of mobile nodes, many issues need to be considered in order to offer many important properties such as scalability, QoS support, security, low power consumption and so on. In this chapter we focus on the energy issues facing some important aspects going from the energy model definition for the computation of the energy consumption to energy-aware metrics definition and routing protocol design. If a network composed of mobile nodes communicating using a wireless radio and where each node can communicate with each other using the other mobile nodes as relay nodes is applied in a communication system, many challenging design issues need to be addressed. MANET technology became, in the last years, more commercial in comparison with the past where it was used for military purpose and this implies more additional features to offer to the end user with particular reference to quality of service, security and to node lifetime duration. In this chapter energy saving techniques at network layer and the routing strategies that allow a better energy expenditure and load distribution in order to prolong the network lifetime are considered.
A wireless network interface can be in one of the following four states: Transmit Receive, Idle or Sleep. Each state represents a different level of energy consumption.
Transmit: A node is transmitting a frame with some transmission power.
Receive: A node is receiving a frame with some reception power. That energy is consumed even if the frame is discarded by the node because it was intended for another destination, or it was not correctly decoded.
Idle (listening): Even when no messages are being transmitted over the medium, the nodes stay idle and keep listening the medium.
Sleep: when the radio is turned off and the node is not capable of detecting signals, no communication is possible. The node uses the power that is largely smaller than any other power
Energy aware metrics
The majority of energy efficient routing protocols for MANET try to reduce energy consumption by means of an energy efficient routing metric, used in routing table computation instead of the minimum-hop metric There are four possibilities to save power from the devices:
Minimal Energy Consumption per Packet
The energy consumption is the sum of power consumed on every hop in the path from a packet. The power consumption on a hop is a function of the distance between the neighbor and the load of this hop. So it is interesting to choose a route where the distance between the nodes isn't too long and also it is interesting to take a shorter route so there aren't too many hopes on the route where the power level gets down.
Maximize Network Connectivity
This metric tries to balance the load on all the nodes in the network. This assumes significance in environment where the network connectivity is to be ensured.
Minimum Variance in Node Power Levels
This metric proposes to distribute the load among all nodes so that the power consumption remains uniform to all nodes. This problem is very complex when the rate and size of data packets vary. When every node has the same level in power, you can be sure that the network functions longer. Because when there is a node which has to switch off because of the power level the whole network is in danger and it can break down the connectivity between the nodes.
4) Minimize Maximum Node Cost
This metric minimizes the maximum cost per nodes for a packet after routing a number of packets or after a specific period. So a node can be blocked for routing to save battery power. This metrics saves the connectivity from every node. When a node has been used several times for route, it blocks itself to save the power.
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