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Mobile Ad Hoc is a self constructive network of mobile nodes connected by wireless link to form at arbitrary topology exclusive of existing infrastructure. MANET's network topology unpredictable and change quit frequently due to its dynamic nature. MANET's networks are similar to traditional networks however posses few challenges mainly due to mobility of communicating nodes. This essay report illustrates the investigation of different mobility models and challenges of mobile ad hoc networks faces during communication.
//The vital growth of online multimedia services in digital ecosystem increased the potential uses of Mobile Ad Hoc Networks. Mobile Ad Hoc Networks gained popularity over the last decade as of its highly dynamic nature and usefulness. The rapid deployment and self configuring attributes enhanced its acceptance in wide variety of use such as battle field, disaster relief, surveillances.
The Mobile Ad Hoc Network builds networks instinctively without the aid of infrastructure. The each node in the network acts as end node or intermediate node //due to decentralize nature of Mobile Ad Hoc Networks  . However, with advent of wireless technology the use of mobile ad hoc networks increased, due to its robustness but the various major constraints such as limited battery charge, frequently change of topology and security issues makes its use susceptible.
The mobility models uses in MANET's network to define the movement of wireless mobile node .
Types of Mobility Models
There are two types of mobility models
• Single entity mobility model
• Group nodes mobility model
In single entity mobility model each node moves independently in the network area whereas in group mobility model, nodes are believe to be in a group and the mobility of node is often reflective of the movement pattern of whole group. The distinctive feature of every mobility model is to guarantee that the mobile node will not travel outside the network boundary .
Random based Mobility Models
In this type of mobility models nodes move freely in the network area without any constraint. The speed and direction of nodes in random based mobility are chosen randomly.
Different type of Random based Mobility Model
• Random Waypoint Mobility Model
• Random Walk Mobility Model
• Random Point Group Mobility Model
• Freeway Mobility Model
• Manhattan Mobility Model
Random Waypoint Mobility Model
Random Way Mobility model is uncomplicated and mostly used to evaluate performance of mobile ad hoc networks. These types of mobility model include pause time between change direction and speed. Whenever node starts to travel, it stays in one location for a while until pause time expires. After the pause time expires its randomly selects the next destination in a simulation area and opt a speed uniformly distribution between maximum and minimum speed. The chosen speed by mobile node reflects the degree of mobility. Then the mobile node starts travelling to destination node. As soon as mobile node reaches destination node, it wait again for expire of pause time before initiating the same process .
Geographical Restriction: The movement of mobile node in this model is restricted along the street or freeway. Geographical map may define this kind of restrictions.
Spatial Dependency: Every node in Random Waypoint Mobility Model moves independently of others.
Temporal Dependency: The current velocity of mobile node is dependent on previous velocity in Random Waypoint Mobility Model. However, spontaneously the velocity at two different time slots is independent in this model .
Random Walk Mobility Model
Random Walk Mobility Model is same as Random Waypoint Mobility model but at a trip transition, instant it also chooses direction including of trip duration and speed. The mobile node travels in a predefined direction with the opted speed and trip time. Every movement of a node in a Random Walk Mobility Model occurs either for given constant interval or constant distance travelled. Whenever, a mobile node reaches at boundary, it then bounce off the boundary with an angle determined by incoming direction. This mobility model is memory less memory pattern because it does not retain information concerning its past values and speed values .
This model is memory less because it does not preserve past knowledge of node speed and direction.
This mobility model includes zero pause time and in every time interval node changes their speed and direction.
//The mobility of node is analyzed by fixing the reference frame of one with respect to another as the link or connectivity between the two mobile nodes is dependent on relative movement of node. Furthermore, for every movement of node, the reference frame of other node is translated an equal distance in the opposite direction .
Random Point Group Mobility Model
This type of mobility model represents multiple nodes whose actions are completely independent of each other. The group mobility model includes group leader who determine the groups motion behaviour. //Initially, every node of the group equivalently distributed in the locality of group leader. Subsequently, at each instant every node has direction and speed that is derived by randomly deviating from that of the group leader. Each node deviates its velocity both speed and direction randomly according to the group leader  .
Every node in this model deviate its direction and speed randomly according to group leader .
In Random Point Group Mobility Model, the groups are categorized by physical proximity either all the nodes are close to the reference point .
Freeway Mobility Model
Freeway mobility model imitate the motion behaviour of mobile node on freeway. It can be used in exchanging traffic status or tracking a vehicle on freeway . Each node is restricted on its own freeway however; the velocity of node is dependent on previous velocity.
Every node in this model restricted to its lane on the freeway.
The velocity of node is temporarily reliant on its subsequent velocity.
The following node cannot exceed the proceeding node if both nodes are in same freeway lane with safety distance .
Manhattan (Drunk) Mobility Model
This mobility model is similar to city section model however; it does not label the target point to reach. Based on, distribution probability every time new direction chooses from the available ones. The node speed can be changed as a different stochastic process or according to development constraints  . The Manhattan mobility model focuses on nodes moving on horizontal or vertical streets and doesn't allow the nodes to move non-horizontal or non-vertical streets. Furthermore, this model also imposes geographical restrictions though it offers flexibility for the node to change the direction .
This model works on map with composition of horizontal and vertical streets. The mobile node legitimated to travel along the grid horizontal and vertical street grid on map. However, it also imposes restriction on node mobility .
The velocity of node can change as a separate stochastic process or according to scenario constraints  .
Performance influencing factors
//Mobile Ad Hoc Networks is challenging issue due to dynamic and distributed nature of mobile nodes required frequent observed updating of node information to route the packets appropriately however, there are proactive routing, reactive routing and geographical routing techniques to route the packet from source to destination efficiently .
Various factors affect the performance and effectiveness of Mobile Ad Hoc Networks
Exposed and hidden terminals
Error prone shared broadcast radio channel
The nodes movement in the Mobile Ad Hoc network changes frequently and makes the network topology highly dynamic. Hence, constantly change of network topology breaks the ongoing session and cause to disrupt the communication, the communication could be either from intermediate node in the path or from the end node .
The bandwidth rate in Mobile Ad Hoc Network is limited in contrary to wired networks and causes to reduce the overhead as much as possible. The limited bandwidth imposes challenge in routing protocol of Ad Hoc network to maintain information of highly dynamic network topology change. Furthermore, more bandwidth wastage results due to overhead of maintaining control information of frequent move of communication nodes .
Exposed and Hidden Terminal
When the receiving node bottlenecked by simultaneous receiving of packet from sender and other nodes those are not in direct communication but in its range cause to hidden terminal problem. Congestion occurs when both nodes start transmitting of packet at the same time without any information regarding transmission of each other. The exposed terminal problems refer to incapability of a node which is blocked due to transmission by nearby transmitting node to transmit to //one more nodes .
The two major challenges in Mobile Ad Hoc networks are limited processing power and restriction in battery source . Furthermore, essence of portability of mobile ad hoc networks also restricts the size and weight of communication nodes. Thus increasing of size and weight will make node bulky and less portable. Last but not least, malicious nodes in the topology can drain the battery and processing power of node by constantly flooding of junk routing messages, this kind of attack known as sleep deprivation .
Error Prone Shared Broadcast Radio Channel
The Mobile Ad Hoc Networks poses unique challenge due to broadcast nature of its radio signal. The wireless links have time varying characteristics in terms of link capacity and link error probability . The routing protocols in mobile ad hoc networks consult MAC (Media Access Control) layer for alternate route for better quality link. In addition to this loss of data and control packets also results as discussed above in hidden terminal. Furthermore, the bit error rate is more in wireless network in comparison to wired network due to unstable nature of radio channels .
Mobile ad hoc networks facilitates network connectivity between multi hop wireless channels and widely separated moving nodes through link layer protocols to ensure point to point connectivity and network layer protocol to extend the connectivity to multi hops. All the distributed protocols believe that every node is cooperative in coordination process. However, this assumption is unfortunately not true because cooperation assumed not enforced in MANET's, vulnerable nodes can easily disrupts network operation by violating protocol specifications. A malicious node in the network can impose denial service of attacks by injecting junk of packet in the network. Similarly, intruders along //established route may alter the packet data or control information .
MANET's Performance effects with Mobility of nodes
The mobility of nodes in MANET's is the key attribute of network. The real life mobility patterns in an autonomous system could be more complex depending upon mission of objective of mobile node. The more difficult is mobility pattern the more difficult it is to mobility model because more control information need to be included in the mobility model . The frequent movement of mobile nodes force network topology to change dynamically. Similarly, the performances of mobile ad hoc network vary significantly with different mobility models. Furthermore, varying of parameters of different mobility models also affects MANET's performance at a great extent . The mobility model selection may need communication traffic pattern over the network which significantly affects the network and protocol performance dramatically.
The investigation carried out by Bhavyesh et al. on impact of node mobility on MANET's routing protocol, concluded the performance of routing protocol typically depend upon mobility model used and study results of one mobility model cannot be applied to another model. The author concluded that for highly mobile network Dynamic Source Routing gives better result in contrary to DSDV. Moreover, Dynamic Source Routing(DSR) is faster to discover new route when the old route is disrupted as it call up route repair mechanism locally whereas in DSDV if no route found it drops packets .
In addition to this, experiment conducted by Suganya et al. on impact of mobility for QOS based secure MANET's, observed that Random Waypoint produces better results than the other mobility model of mobile ad hoc networks . Furthermore, the study carried out by Shams-Ul -Arfeen et al. Observed AODV reduces protocol overhead in HRM and HWM. Similarly, DSDV can be useful in HRM and HWM models because in these models nodes move slowly and may have higher delay in choosing next hop .
Mobile ad hoc networks gaining popularity in a great momentum recently due to its key attribute of mobility however, impotential performance of MANET's mobility models has been attracted various researchers for research. The high degree of node mobility and self configuration nature burdens the network with unpredictable topology change. In MANET's each model has its own importance and effectiveness with little performance imperfection. The performance imperfection could be at routing level either disruption in discovering of new route when old is disrupted or drop of packets when mobile node feel helpless to find new route. In addition to this, the potential effectiveness of MANET's also affected due to challenges it faces such as frequent mobility, security loopholes, bandwidth constraints, or resource constraints. //The MANET's followers should clutch on investigation to resolve the various flaws which affects its performance.