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Wireless mobile ad hoc networks are considered as networks without any physical connection. They don't have any specific topology due to mobility of nodes. There is no stationary infrastructure like base stations therefore every mobile host work as router in order to maintain all the information about connecting networks. Hence we cant use static routing protocols. we need dynamic protocols which can be implemented in such networks.
Several routing protocols have been suggested for ad hoc networks. In this review paper we will study,analyze and will see the advantages as well as disadvantages of some of the protocols such as DSDV,WRP,AODV,ABR,SSA and DSR. Where DSDV and WRP are proactive(table driven) protocol while AODV,ABR,SSA and DSR are reactive(on demand) protocols.
Routing is the act of moving information from a source to a destination in a network. The routing concept basically involves, two activities: firstly, determining optimal routing paths and secondly, transferring the information groups (called packets) through a internetwork. The later concept is called as packet switching which is straight forward, and the path determination could be very complex.
Ad-hoc networks are wireless networks where nodes communicate with each other using multi-hop links. The nodes in an ad-hoc network can be a laptop, PDA, or any other device capable of transmitting and receiving information. Nodes act both as an end system (for transmitting and receiving the data) and also as a router (to allow traffic to pass through) which results in multi-hop routing.
Routing in ad hoc networks has been a challenging task ever since the wireless networks came into existence. The major reason for this is the constant change in network topology because of high degree of node mobility.
A number of protocols have been developed for accomplish this task. Some of them are Destination Sequenced Distance Vector (DSDV),Wireless Routing Protocol(WRP), Ad-hoc On-Demand Distance Vector (AODV), and Dynamic Source Routing (DSR).Signal Stability-Based Adoptive Routing Protocol(SSA), Associatively Based Routing Protocol(ABR).All of these protocols are studied and analyzed in further sections.
1. CLASSIFICATION OF ROUTING PROTOCOLS IN MANET
1.1 Table-Driven routing protocols(Proactive)
These protocols are also called as proactive protocols since they maintain the routing information even before it is needed  . Each and every node in the network maintains routing information to every other node in the network. Routes information is generally kept in the routing tables and is periodically updated as the network topology changes.
There exist some differences between the protocols that come under this category depending on the routing information being updated in each routing table. Furthermore, these routing protocols maintain different number of tables.The proactive protocols are not suitable for larger networks, as they need to maintain node entries for each and every node in the routing table of every node. This causes more overhead in the routing table leading to consumption of more bandwidth.
1.2 On Demand routing protocols(Reactive)
These protocols are also called reactive protocols since they don't maintain routing information or routing activity at the network nodes if there is no communication. If a node wants to send a packet to another node then this protocol searches for the route in an on-demand manner and establishes the connection in order to transmit and receive the packet . The route discovery usually occurs by flooding the route request packets throughout the network.
DSDV WRP AODV DSR ABR SSA
Ad-Hoc Routing Protocols
Figure 1: Categorization of Ad-hoc Routing Protocols
2. Table-Driven routing protocols(Proactive)
2.1 Destination Sequenced Distance Vector (DSDV) Protocol
The destination sequenced distance vector routing protocol is a proactive routing protocol. This protocol adds a new attribute, sequence number, to each route table entry at each node.
Routing table is maintained at each node and with the help of this table, node transmits the packets to other nodes in the network .
Each node in the network maintains routing table for the transmission of the packets and to connect different stations in the network. These stations have the list for all the available destinations, and the number of hops required to reach each destination in the routing table. The routing entry is tagged with a sequence number which come from the destination station. In order to maintain the consistency, each station transmits and updates its routing table periodically. The packets being broadcasted between stations indicate which stations are accessible and how many hops are required to reach that particular station.
Routing information is advertised by broadcasting or multicasting the packets which are transmitted periodically as when the nodes move within the network. The DSDV protocol requires that each mobile station in the network must constantly, advertise to each of its neighbors, its own routing table . Since, the entries in the table my change very quickly, the advertisement should be made frequently to ensure that every node can locate its neighbors in the network. This agreement is placed, to ensure the shortest number of hops for a route to a destination; in this way the node can exchange its data even if there is no direct communication link.
The data broadcast by each node will contain its new sequence number and the following information for each new route:
(a)The destination address
(b)The number of hops required to reach the destination and
(c)The new sequence number, originally stamped by the destination
The transmitted routing tables will also contain the hardware address, network address of the mobile host transmitting them. The routing tables will contain the sequence number created by the transmitter and hence the most new destination sequence number is preferred as the basis for making for transmitting the packet. This new sequence number is also updated to all the hosts in the network which may decide on how to maintain the routing entry for that originating mobile host.
ADVANTAGES AND DISADVANTAGES
DSDV protocol guarantees loop free paths . 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.
One of the disadvantage is the Wastage of bandwidth due to unnecessary advertising of routing information even if there is no change in the network topology .It doesn't support Multi path Routing.
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.
2.2 WIRELESS ROUTING PROTOCOL(WRP)
WRP is a table based protocol which maintain routing information among all nodes in the network. Each node in the network maintains four table : (a) distance table, (b) routing table, (c) link-cost table, and (d) message transmission list(MLR) table. Each entry of the MLR contains the sequence number of the update message. The MLR also records which updates is an update message need to be retransmitted and which neighbours should acknowledge the retransmission .
Nodes inform each other about link changes through an update message. This message is sent between the neighbours only and has the list of updates(the destination and the distance to that destination) and a list of responses telling which node should acknowledge the update. In case of link loss nodes send update messages to their neighbours to update their table entries and also check for new possible paths. New paths are also send back to the original nodes so that they can also update their table entries.
Nodes senses the existence of their neighbours by receiving acknowledgement and other messages. Lack of messages indicates the failure of link. When a node receives a hallow message from a new node it adds that node in its routing table  and also send a new copy of its routing table to that node.
3. On Demand routing protocols(Reactive)
3.1 Ad-hoc On-demand Distance Vector (AODV) and Dynamic Source
AODV is a combination of DSR and DSDV protocols. It borrows the basic on-demand mechanism of route discovery and route maintenance from DSR and the use of hop-to-hop routing and periodicity from DSDV .Being a reactive protocol it only needs to maintain the route information of active paths.Inthis protocol each node have the routing table of next hop which contains the destinations to which it currently has route. The entry in the routing table expires if it is not being used or reactivated after a specific time.
In AODV,when a source node wants to send a packet to a destination but does not find a route available it initiates a route discovery operation, in which the source broadcasts the route request(RREQ)packets .A RREQ contains the addresses of source and destination, the broadcast identity which is used as identifier,the last seen sequence number of the destination as well as the source node's sequence number. It nodes maintain a cache to keep record of RREQs it received and also the path back to the originator of that RREQ.When the node that has the route to destination receives the RREQ,it checks the destination sequence number it currently knows and the one specified in RREQ.To make sure that the routing information is new a route reply(RREP) packet is send back to source. Upon receiving this packet each intermediate node update its routing table .
Operation of DSR is almost same as AODV the difference is that DSR does not rely on routing state in network instead it uses source routing in which a data packet carries the whole path to be traversed while in AODV the source node and the intermediate nodes stores the information of next-hop corresponding to each flow for data packet transmission  .The RREQ collects the IP addresses of all the nodes that it has passed on the way to destination. The destination sends a route reply by source routing back to the source of the request, which provide the source route to the destination.
ADVANTAGES AND DISADVANTAGES
DSR is a reactive protocol in which there is no need to periodically flood the network with table update messages as required in table driven approach. This protocol performs well in low mobility environments but performance degrades as mobility increases.Connection setup delay is less in AODV. One of the disadvantages is that intermediate nodes can cause inconsistent routes if the source sequence number is very old and also multiple route reply packets in response to single route request packet can cause heavy control overload.
3.2 ASSOSIATIVITY BASED ROUTING PROTOCOL(ABR)
It is distributed routing protocol which selects route based on stability of the wireless link. It is a beacon based, on-demand protocol .Stability or instability is determined by counting the beacons that a node receives from its neighbour.
A source node floods route request packet throughout the network if a route is not available in its route cache. A route request packet contains the path it has traversed and number of beacons count for the corresponding nodes in the path. When the first route request reaches the destination, the destination waits for the time period Troute select time to receive multiple route requests through different paths and then decide the path which is most stable one.If two paths have stable the same proportion of stable links it will select the shorter one.
If a link break occurs at an intermediate node closer to the source ,which detects the break,initiates a local route repair process by broadcasting route repair packet termed as local query(LQ) broadcast with a limited time to live(TTL).In this way a broken link is bypassed locally without resending the RR packet.
ADVANTAGES AND DISADVANTAGES
In ABR,stable routes are preferred as compared to shorter one which results in few path breaks which intern reduces the extent of flooding. One of the disadvantage is that the chosen path may be the longer one.
3.3 SIGNAL STABILITY-BASED ADOPTIVE ROUTING PROTOCOL(SSA)
SSA is on demand routing protocol that uses signal stability for finding the stable routes. It is also a beacon based protocol. This protocol has two parts:dynamic routing protocol(DRP) that maintains the routing table and forwarding protocol(FP) that performs the actual routing to forward a packet to its destination.
Each node maintains a signal stability table (SST) which is based on signal strengths of its neighbour beacons.This table is used by the nodes to forward a RR packet to the stable links.If the request fails to reach the destination then it will flood route request without taking the stability of routes into account.
But unlike other routing protocols, the nodes will process only those requests which received over a strong link.Then the destination will initiates a route reply packet to tell the source about the detected route.
ADVANTAGES AND DISADVANTAGES
More stable route selection as compared to others.But if a request fails then a path finding process is initiated without taking into account the stability,which consumes a significant amount of band width and increases oath setup time. Also it increases the path length as shorter path may be ignored due to instability.
RESULTS: COMPARISON OF ON-DEMAND REACTIVE PROTOCOLS AND
PROACTIVE ROUTING PROTOCOLS
In general, on-demand reactive protocols are more efficient than proactive ones. On demand protocols minimize the control overhead as well as power consumption because routes are established only when required. While proactive protocols require constant route updates to keep up to date information; in addition, maintain multiple routes that might never be needed, adding unnecessary routing overheads. As routing information is constantly updated in the proactive protocols, routes to every destination are always available and up to date, in this way we can minimize end to end delay. For on-demand protocols, the source node has to wait for the route to be discovered before it can communicate with other nodes. This latency in route discovery might be intolerable for real-time communications.
In this paper we have provided descriptions of several routing schemes proposed for ad-hoc mobile networks. We have also provided a classification of these schemes according the routing strategy.i.e., table driven and on demand. Protocols which fall under these categories are being discussed. We have presented a comparison of these two categories of routing protocols, as well as advantages and disadvantages are discussed. Finally we have identified possible applications and challenges facing ad-hoc wireless
While it is not clear that any particular algorithm or class of algorithm is the best for all scenarios, each protocol has definite advantages and disadvantages and
has certain situations for which it is well suited. The field of ad-hoc mobile networks is rapidly growing and challenging, and while there are still many challenges that need to be met, it is likely that such networks will see wide-spread use within the next few years.