Mobile Adhoc Network (MANET) is defined as a combination of mobile nodes that lack a fixed infrastructure and is quickly deployable under any circumstances. These nodes have self-aware architecture and are able to move in multiple directions, which renders it dynamic topology. Its dynamicity makes routing in MANET rather challenging compared to fixed wired networks. This paper aims to perform a comparative study on the three categories of MANET routing protocol by comparing their characteristics and operations, as well as their strength and weaknesses.
Mobile Ad-hoc Network (MANET) is defined as a combination of mobile nodes that can keep in touch with one another despite the lack of a core centralized administrator or fixed infrastructure [
Over the years, there are many different routing protocols that have been developed for MANET. In general, these protocols can be categorized into three types: proactive, reactive and geographical routing protocols. This paper presents a comparative study of these three categories of MANET routing protocols. The presentation of the paper is organized as follows. Section 2 classifies multiple MANET routing protocols and provides a brief overview of several protocols in each category. Section 3 presents a comparison between the MANET routing protocols. An analysis of MANET routing protocols in terms of their characteristics, operation, strengths and weaknesses is presented in this section. It also highlights the drawbacks of these routing protocols to identify the areas that can be improved. Section 4 concludes the present comparative study of routing protocols in mobile ad-hoc networks.
In MANET, routing protocols can be classified into Proactive Routing Protocols, Reactive Routing Protocols, and Geographical Routing Protocols [
Proactive or table-driven routing protocols aim to keep up-to-date routing information flowing throughout a network between all nodes. As a means of preserving a consistent network connection, proactive routing protocols require every node to support at least one table which contains routing information. These nodes then react to the variations in the topology of the network by distributing the most current information through the network. This type of protocols is unique compared to others in term of the manner which the alterations to the network’s structure are transmitted and also the amount of routing-related tables that are required. The benefit of proactive routing protocols is the median delay time per packet that can be decreased. In these protocols, route information is present and accessible in the table whenever it is required. Nevertheless, in maintaining up-to-date routing information, proactive protocols uninterruptedly employ a significant share of network capacity. This makes such routing protocols unsuitable for reconfigurable mobile ad-hoc networks [
The OLSR is an optimized pure link state algorithm with proactive nature which allows it to ensure the availability of the routes when required. Hop-by-hop mechanism is utilized to forward packets, which is one of the main characteristics of any MANET routing protocol [
amount of traffic to be generated, which may take up network resources and reduce the performance of the network. Multi Point Rely (MPR) is a unique feature of MPR that is able to minimize the number of rebroadcasting nodes and, in turn, reduce the number of control messages generated during an update. With MPR, nodes are able to exchange topological information in a periodical manner without having to generate a large amount of traffic [
1-and-1 hop symmetrical information is used by the MPR selection process to recalculate the MPR set. When a change in 1 or 2 hop neighborhood’s topology is detected, then the MPR recalculation will occur. The route to each known destination is recalculated and updated when the updated information is received [
Perkins and Bhagwat introduced Destination-Sequenced Distance-Vector (DSDV) [
any two routes share a similar sequence number [
WRP is regarded as a part of the general class of path-finding algorithms [
• Distance table
• Routing table
• Link-cost table
• Message retransmission list (MRL) table
WRP utilizes periodic update message transmissions adjacent to a node. The nodes in the response list of new messages (which is formed using MRL) should in turn acknowledge it. If there are no changes from the previous update, then the nodes in the response list will send an idle HELLO message to confirm connectivity. A node is empowered to decide whether or not to update its routing table post-receiving an updated message from nearby while looking for a superior path with the updated information it receives. In case the node obtains a superior path, this information will be relayed to the original node for table updates. After being acknowledged, the original node will proceed to update its MRL. Every time the consistency of the routing information is being examined by the nodes present in the protocol, it helps reduce routing loops and determine the best routing solution within the network [
Reactive routing is also known as on-demand routing protocols. These protocols lack routing information or routing activity on the nodes in the network when communication is lacking or dismal. Unused routes are maintained with less overhead. Unfortunately, more time delays may be experienced initially. A route is searched for by the reactive protocol in an on-demand manner if a node intends to pass on a packet to another node. The packet is received and transmitted after forming a connection. Then, the request packets are dispersed within the networks, leading to a discovery of routes. There are two categories of reactive protocol; source routing and hop-by-hop routing. A complete source to the designated address is carried by the source routed on-demand protocols. The information contained in the header of each packet will be evaluated by every intermediate node when forwarding these packets. The intermediate nodes are not required to maintain updated routing data for each active route. In addition, neighbor connectivity through periodic beaconing messages is also not required in the database of the nodes. As each node has the potential to update its routing table in the presence of fresher topology information, the routes are therefore adaptable to the changing environment, which takes place dynamically in the MANETs. The data packets are forwarded over better and fresher routes this way [
The ABR [
1) Route discovery phase: The route discovery phase is a broadcast query and await-reply (BQ-REPLY) cycle. The source node broadcasts a BQ message seeking nodes possessing a route to the destination. A node will only pass a BQ request once. Upon receiving the BQ message, an intermediate node alters both the address and associativity ticks of the query packet. The upcoming node will delete the upstream nodes of its neighbors’ associativity tick entries while keeping the entry that is associated with itself and its corresponding upstream node. Every packet that reaches the destination will possess the associativity ticks of the nodes along the route, all the way from the source to the destination. Now, the destination can freely choose the best route via analysing the associativity ticks associated with each path. In the case of multiple paths possessing similar degree of association stability, the route having the least amount of hops will be chosen. Once a path has been determined, the destination passes forth a REPLY packet back to the source on this path. The nodes that the REPLY packet adhere to will serve to validate their respective routes, while other routes remain inactive, eschewing any chances of duplicated packets reaching the destination as well [
2) Route reconstruction (RRC) phase: RRC phase consists of partial route discovery, invalid route erasure, valid route updates, and new route discovery, depending on which node(s) along the route move. The movement of source nodes will precipitate a unique BQ-REPLY process due to the fact that the routing protocol is source- initiated. The route notification (RN) message deletes entries associated with routes and downstream nodes. When the destination moves, its immediate upstream node deletes its corresponding routes. A localized query (LQ [H]) process, where H refers to the hop count from the upstream node to the destination, will start for the purpose of confirming whether or not the node can be reached. If the destination gets the LQ packet, it will be prompted to choose the best partial route and REPLYs; otherwise, the initiating node times out and backtrack to the next upstream node. An RN message is dispatched to the adjacent upstream node to delete invalid routes and also inform it that the node must initiate the LQ [H] process. However, if the backtracking exceeds halfway to the source, the LQ process is terminated, and the source will restart the BQ process all over again [
3) Route deletion phase: When a route is no longer required, the source node will start a route delete (RD) broadcast. Each node present on the route will remove the route’s entry from their respective routing tables. The RD message is broadcasted indirectly, as the source node might be unaware of any alteration to its route during RRCs [
TORA is an adaptive routing protocol for highly dynamic mobile multi hop networks that are source initiated and based on link reversal algorithms [
Ad-hoc On-Demand Distance Vector Routing Protocol (AODV) [
Route discovery mechanism begins when no valid route is found within the routing table of the source node. Route requests (RREQs) are sent to the network to search for the route to the destination. Receiving nodes create reverse routing entries towards the source for the purpose of sending possible reply packets later. A route reply (RREP) is dispatched by either the destination or intermediate node that is a validated route towards the destination. Nodes that received RREPs also create reverse routing entries towards the nodes that sent the RREPs. Often, each of the nodes along an active route will transmit HELLO messages to the neighboring nodes. If no HELLO message or data is received from a neighboring node after a period of time, the link is regarded as broken. If the destination of the route using this link is nearby the next hop from the neighbor, then a local repair process may be used to repair the route. If not, then a route error (RERR) message is sent to neighboring nodes, which then broadcasts the RERR message towards other nodes that may have routes affected by the broken link. If the route is needed by the affected source, the route discovery process will then be repeated [
Geographical routing [
Greedy Perimeter Stateless Routing (GPSR) is a novel routing protocol for wireless datagram networks that utilizes the location of the routers and its destination to decide on forwarding. GPSR decides on greedy forwarding decisions by utilizing the information regarding a router’s adjacent neighbors within the network’s topology. When a packet reaches a region within which greedy forwarding becomes impossible, the algorithm recovers itself via routing adjacent to the perimeter of the region. By remaining close to the local topology, the GPSR scales better in per-router state than shortest-path and ad-hoc routing protocols as the number of network destinations increases. Under the mobility’s frequent topological changes,
The location aided routing protocol fails to confirm a location-based routing protocol and proposed the usage of position information to improve the route discovery phase of reactive ad-hoc routing approaches. The location information is obtained by GPS by utilizing two flooding regions; forwarded and expected. The decrease in the search space will inevitably result in lesser route messages. When a source node intends to dispatch a data packet, it will first request for the location of the destination from the location service which causes contacting and tracking problems [
In GSR, source node computes the shortest path to the destination using dijkstra’s algorithm based on distance metrics. It computes the distance from the source to intermediate nodes through which data is to be forwarded [
In this section, an analysis of the reviewed MANET routing protocols in terms of their characteristics, operation, strengths and weaknesses is presented. This section also highlights the drawbacks of these routing protocols to identify the areas that can be improved
This subsection presents the comparison between the routing protocols reviewed in Section 2 above.
Routing class | Reactive [ | Geographical | Proactive [ |
---|---|---|---|
Routing structure | Mostly flat, except cluster-based routing | Greedy forwarding routing | Both Flat and hierarchical structures |
Availability of route | Determined when needed | Always available | Always available |
Control Traffic volume | Lower than proactive routing protocols | Generate less control traffic | Usually high |
Periodic updates | Not required. Some nodes may require periodic beacons. | Periodic beacons | Yes, some may use conditional. |
Route acquisition delay | High | Low | Low |
Storage Requirements | Depends on the number of routes kept or required. Usually lower than proactive protocols | The storage will be high since each node stores the locations | High |
Bandwidth requirement | Low | High | High |
Power requirement | Low | Low | High |
Scalability | Source routing protocols up to few hundred nodes. Point-to-point may scale higher. | Limited Scalability problem | Usually up to 100 nodes. |
Handling effects of mobility | Usually updates Associativity-Based Routing introduced localised broadcast query. AODV uses local route discovery | Constantly changing | Occur at fixed intervals and alters periodic updates based on mobility |
Quality of service support | Few can support QoS , Although most support shortest path | Provide a node location service | Mainly shortest path as the QoS metric |
Weaknesses | Have high latency, Flooding can lead to network clogging. | Short life of nodes in the networks due to the frequency of communication in each node. | Unsuitable for reconfigurable wireless ad-hoc network environment and not suitable for large networks. |
Strengths | Reduce the overheads because it does not need to maintain up-to-date information about the network. | Suitable for sensor networks. The mobility support can be facilitated. | Control traffic are constant, and routes are always available. |
Routing class | OLSR | DSDV | WRP |
---|---|---|---|
Multicast | No | Yes | No |
Number of tables | five | Two | Four |
Frequency of updates | Periodic | Periodic and as required | Periodic |
Weaknesses | For the control message increases when the numbers of mobile nodes are increased, need higher processing power, 2-hop neighbor knowledge required | Unsuitable for highly dynamic networks, requires battery power, High overhead | Consumes a lot of bandwidth, as well as the power of each node is required to stay life all times, High Memory Overhead |
Strengths | minimizes the size of protocol message and the number of rebroadcasting nodes during each route, Reduced control overhead and connection | Small amounts of bandwidth, loop free | provides the faster route convergence, Loop free |
Routing class | AODV | TORA | ABR |
---|---|---|---|
Multiple routes | No | Yes | No |
Route metric method | Freshest and Shortest path | Shortest path or next available | Strongest Associatively and Shortest path |
Route reconfiguration strategy | Erase route then source notification or local route repair | Link reversal and Route Repair | Localized Broadcast Query |
Weaknesses | Periodic beaconing result in excessive bandwidth consumption, the intermediate nodes might result in inconsistent routes, Scalability problems, Large delays and Hello messages | In large networks the overhead, consume a large bandwidth, Temporary routing loops and Overall complexity | Lack loops, deadlock, and packet duplicates, Scalability problems, High Overhead and Overall complexity |
Strengths | AODV has potentially less routing overheads, Adaptive to highly, Dynamic topologies and Low overhead | Able to rapidly build routes and decrease the communication’s overhead, Multiple routes | Longer-lived routes, Route stability |
Routing class | GSR | GPSR | LAR |
---|---|---|---|
Routing structure | Flat | Periodic beaconing | Location-based |
Routing metric | Shortest Path | Closest distance | Shortest Path |
Communication overhead | High | High | lower |
Weaknesses | High delay at high mobility and High Memory Overhead | Delay increases at high mobility, generates a large number of control packets for high speeds, don’t have better performance as the inter-beacon interval and group Leader is Single Point of Failure and low packet delivery ratio | Request the destination location, which might create contacting or tracking problem and Control complexity is higher than GPSR. |
Strengths | Shortest path to the destination and Localized updates | Guarantees a good Packet Delivery Ratio especially in the high density of nodes, Keeps a good rate of delivery in networks with high mobility, generates routing protocol traffic in quantity independent of the length of the routes through the network and low data forwarding overhead and local maxima can be found easily | Decrease the search space results in little route discovery messages and it has minimized the size of the route discovery process by defining the range of the destination node. |
Many researches have compared and analysed the characteristics and functionality of MANET routing protocols within these three categories. They observe that proactive routing protocols are unsuitable for reconfigurable wireless ad-hoc network environment due to the excessive use of the network capacity to maintain an up-to-data topological map on the entire network during the movement of nodes and network topology changes. In OLSR, the overhead for the control message increases when the numbers of mobile nodes increase. It will also need higher processing power compared to other protocols when trying to look for other routes [
WRP updates the message transmission to its adjacent neighbors, and the nodes within the response list of the updated message will acknowledge its receipt to the class of path algorithm. WRP provides the faster route convergence [
Reactive routing protocols have high latency due to the need to look for a route to the destination before data can be sent. Flooding can lead to network clogging, while RREP, RREQ & RERR messages lead to control overheads [
Geographical routing is suitable for sensor networks, where data aggregation is utilized to minimize transmissions to base station via the elimination of redundancy between packets from multiple sources. GPSR has low overhead for data forwarding and local maximums are easily found. The GSR and the source node compute the shortest path to the destination using Dijkstra’s algorithm, based on distance metrics [
This paper presents a comparative study of routing protocols in mobile ad-hoc networks. These protocols are divided into three: proactive or table-driven, reactive or on-demand, and geographical routing protocols. For each of these classes, we have reviewed several representative protocols. Each routing protocol has unique features. The main factor that distinguishes the protocols is the method of determining routes within source destination pairs. The drawbacks, strengths and weaknesses of each protocol have also been examined in this paper. Reactive routing protocols suffer from longer delays and proactive routing protocols have higher overhead. The geographical routing protocol is very suitable for sensor networks whereby data aggregation is effective in minimizing transmission towards the base station via the elimination of redundancy among packets of multiple sources.