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Top1. Introduction
A Mobile Ad-hoc Network (MANET) is an autonomous collection of mobile users where the network topology may change rapidly and unpredictably over time. Mobile ad-hoc networks may well complement infrastructure-based wireless networks and allow mobile users to obtain access to interact directly with one another even when they are outside the coverage area of cellular networks or wireless LAN. Similarly, MANETs may enable communication between vehicles, sensors, laptops and other mobile equipment without the need to deploy a fixed infrastructure network. Numerous dedicated routing protocols have been proposed to establish and maintain reachability between communicating nodes in such dynamic environments.
Challenged networks or Intermittently Connected Networks arise from MANETs primarily as a result of node mobility but may also come into being as a result of disconnection due to power management or interference. As a result the network becomes partitioned. Unfortunately, with current ad hoc routing protocols, packets are not delivered if a network partition exists between the source and the destination when a message is originated. Certain applications, such as real-time, constant bit rate communication may require a connected path for meaningful communication. However, a number of other application classes benefit from the eventual and timely delivery of messages, especially in the case where frequent and numerous network partitions would prevent messages from ever being delivered end to end (Vahdat &
Becker, 2000). Examples of such networks include terrestrial mobile networks, exotic media networks, military ad-hoc networks and sensor networks. These challenged networks are characterized by high latency, bandwidth limitations, high error probability, node longevity, or path stability that are substantially worse than is typical of today’s TCP/IP based networks. (Burleigh, Hooke, Torgerson, Fall, Cerf, Durst et al., 2003; Cerf, Burleigh, Hooke, Torgerson, Durst, Scott et al., 2007; Fall, 2003).
Existing TCP/IP based network protocols could not be utilized for these challenged networks since they operate on a principle of providing end-to-end inter-process communication through a concatenation of dissimilar link-layer technologies. A number of key assumptions are made regarding the overall performance of the underlying links in order to achieve smooth operation: an end-to-end path exists between a data source and its peer, the maximum round-trip time between any node pairs in the network is not excessive, and the end-to-end path loss probability is small. Unfortunately, challenged networks which may violate one or more of these assumptions, are becoming important and may not be well served by the current end-to-end TCP/IP model.
When a direct contact routing approach is applied to this type of network, the message delivery delay will be very high (Wang & Wu, 2006) and then they will get dropped by the network if delay increases further. Since many nodes are mobile nodes they will increase the delivery ratio in wireless environment (Grossglauser & Tse, 2002). Hence by using the mobility property of the nodes, there were numbers of routing protocols proposed for these partially connected networks and they are divided into replication based and knowledge based (Jain, Fall, & Patra, 2004). Replication based protocols create multiple copies of a message. A common technique used to maximize the probability of a message successfully transferred is to replicate many copies of the message in hopes that one will succeed in reaching its destination as said in Vahdat and Becker (2000). In this kind of flooding protocol, each node tries to forward every message to every one of its neighbors except the source node. This results in every message eventually being delivered to all reachable parts of the network.