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Wireless field and mobile communication has gone through an unexpected growth during the past decades and has been so incredibly popular in the past few decades, specifically around the 1990's when wireless devices became popular and were adapted to make devices mobility a reality. As the wireless networks and its popularity of mobile devices increases, a wireless ad hoc network is now one of the very useful and very active fields of research and communication. Recently, wireless local area network (LAN) hot spots is emerging with an increasing number, making it easier for users with computers (laptop) to go on the internet from places like the airports, railway station, hotels and some public locations. Broadband internet access set up in most homes for sharing access between computers. Second generation networks are evolving to third generation, which offers higher data rates and personalise services.
However, all these networks are still the conventional type of wireless networks, conventional in the aspect that as prerequisites, a network infrastructure with centralised administration which is fixed is required for their operation. This consumes a lot of time, maintenance and cost a lot for the initial set-up. An increasing number of devices such as laptops, smart phones, personal digital assistants etc are built with wireless interfaces which are of short range. Every day, these devices are getting smaller, faster in performance, cheaper and more user friendly. Mobile communication is somewhat evolving in a new alternative way in which the mobile devices form a self-organising and self-administering wireless network which is called the mobile ad hoc network (MANET). This report will cover an overview of the history of MANET and their routing protocols. Then will present some of its challenging issues.
History of MANET
Ad-hoc networks lifecycle can be categorised into three generation ad-hoc networks systems i.e. first, second, and third. Presently, the ad-hoc systems in use are considered to be the third generation. The first generation MANET goes back to the year 1972. At that time, they were called PRNET (Packet Radio Networks). The history of ad-hoc networks can be dated back to the DoD1-sponsored Packet Radio Network research for military purposes in the 1970s, involving the program in the early 1980s known as the Survivable Adaptive Radio Networks (SURAN). Approaches for medium access control and some sort of distance-vector routing PRNET were used on a trial basis to provide different network capabilities in a combat environment.
The second generation of ad-hoc networks emerged in the 1980s, when ad-hoc network systems were enhanced further and implemented as a part of the SURAN program. This makes a packet-switched network available to the mobile battlefield in an environment which has no infrastructure. The second generation program proves to be very useful in improving radios' performance by making them cheaper, smaller and also resilient to electronic attacks. In the 1990's, the concept of commercial ad-hoc networks arrived with notebook computers and other type of communication devices. During this period, the collection idea of mobile nodes was proposed at many research conferences. Since the middles of 1990, ad-hoc standards have gone through a lot of work. Within the Internet Engineering Task Force (IETF), the MANET working group was set up and they made effort to make routing protocols which would be standardized for ad-hoc networks. At this same time, the IEEE 802.11 sub-committee standardized a medium access protocol that was on collision avoidance and tolerated hidden terminals for building mobile ad hoc network prototypes out of notebook and 802.11 PCMIA cards. Mobile wireless networks are of two kinds. The first is known as infrastructured networks with fixed and wired gateways. Applications of this type of wireless network include wireless local area networks (WLANs). This is a "one-hop" network. The second type of mobile wireless network is the infrastructureless mobile network, commonly known as the MANET. MANET is usually a self0organising, self-configuring "multi-hop" network which does not require any fixed infrastructure. In such network, all nodes are dynamically and arbitrarily located, and are required to forward packets for other nodes in order to deliver data across the network.
The most fundamental research in MANET is the routing and it must deal with limitation such as high power consumption, low bandwidth, high error rates and unpredictable movements of nodes. Generally, the current routing protocols for MANET can be categorized as:
Re-active (source-initiated on-demand driven) and
The most common proactive routing protocols are Destination-Sequenced Distance Vector (DSDV) and Wireless Routing Protocol (WRP). They are seen to attempt to maintain consistent, up-to-date routing information of the whole network. These keep track of routes for all destinations and enjoy having the advantages of experiencing minimal initial delay in communication with destination which are arbitrary. When application starts, a route can be immediately selected from the routing table. Such protocols are called proactive because they store route information even before it is needed. Re-active routing protocols try to eliminate the conventional routing tables and consequently reduce the need for updating these tables to track changes in the network topology. In contrast to proactive routing which maintain all up-to-date at every node, routes are created only when desired by the source node in re-active protocols. When a source requires to a destination, it has to establish a route by route discovery procedure, maintain it by some form of route maintenance procedure until either the route is no longer desired or it becomes inaccessible, and finally tear down it by route deletion procedure. Some reactive protocols are Cluster Based Routing Protocol (CBRP), Ad Hoc On-Demand Distance Vector (AODV)  and Dynamic Source Routing (DSR). Hybrid routing protocols aggregates a set of nodes into zones in the network topology. Then, the network is partitioned into zones and proactive approach is used within each zone to maintain routing information. To route packets between different zones, the reactive approach is used. Consequently, in hybrid schemes, a route to a destination that is in the same zone is established without delay, while a route discovery and a route maintenance procedure is required for destinations that are in other zones. The zone routing protocol (ZRP)  and zone based hierarchical link state (ZHLS) routing protocol provide a compromise on scalability issue in relation to the frequency of end-to-end connection, the total number of nodes, and the frequency of topology change. Furthermore, these protocols can provide a better trade-off between communication overhead and delay, but this trade-off is subjected to the size of a zone and the dynamics of a zone. Thus, the hybrid approach is an appropriate candidate for routing in a large network.
The major challenges faced by the internet architecture can be broadly classified as follow:
To provide end-to-end service abstractions that facilitates application development.
In incorporating emerging wireless network elements such as MDs, ad-hoc routers and embedded sensors in the existing protocol framework
These challenges occurs by a broad range of environments such as cellular data services, WiFi hot-spots, Info stations, mobile peer-to-peer, ad-hoc mesh networks for broadband access, sensor networks, pervasive systems and vehicular networks. These wireless application scenarios lead to the future internet as listed below:
1) Naming and addressing flexibility.
2) Mobility support for dynamic migration of end-users and network devices.
3) Location services that provide information on geographic position.
4) Self-organization and discovery for distributed control of network topology.
5) Security and privacy considerations for mobile nodes and open wireless channels.
6) Decentralized management for remote monitoring and control.
7) Cross-layer support for optimization of protocol performance.
8) Sensor network features such as aggregation, content routing and in-network Processing.
9) Cognitive radio support for networks with physical layer adaptation.
10) Economic incentives to encourage efficient sharing of resources.
The above listed MANET requirement represents a spectrum of network challenges. In the last few years, almost every aspects of MANET have been explored to some level of detail. Still, more questions have been come to exist which have not yet been answered. The major open problems are listed below:
Autonomous- No centralized administration entity is available to manage the operation of the different mobile nodes. Because MANET is a type of network that change location and can also configure itself, it can only do this because of the algorithm it uses in the mode of operation and what has already been installed in its operating system software. Thus, if a breakdown occurs, it would affect the network causing probably a devastating effect.
Bandwidth optimization- Wireless links have significantly lower capacity than the wire links.
Dynamic topology- Nodes are mobile and can be connected dynamically in an arbitrary manner. Links of the network vary timely and are based on proximity of one node to another.
Device discovery- Identifying relevant newly moved in nodes and informing about their existence need dynamic update to facilitate automatic optimal route selection.
Scalability- Scalability can be broadly defined as whether the network is able to provide an acceptable level of service even in the presence of a large number of nodes. When we talk about scalability in wired networks, we discuss the closely related capability as to how quickly network protocol control overhead increases as a function of an increase in the number of nodes and link changes. In proactive networks, scalability is mostly achieved by introducing routing and/or location hierarchy in the network or by limiting the scope of the control updates to locations close to the changed. In reactive ad hoc networks, techniques such as dynamically limiting the scope of the route requests and attempting local repairs to broken routes are rarely put into use.
Limited resources- Since ad hoc networks do not assume the availability of a fixed infrastructure, individual nodes may have to rely on portable, limited power sources. Surprisingly, there has been few published work in the area of energy-efficiency of ad hoc networks until very recently. Most exciting solution for saving energy in ad hoc network revolves around the reduction of power used by the radio transceiver. Mobile nodes rely on battery power, which is a scarce resource. Also storage capacity and power are severely limited.
Infrastructure-less and self operated- Self healing feature demands MANET should realign itself to blanket any node moving out of its range.
Limited physical security- Mobility implies higher security risks such as peer-to-peer network architecture or a shared wireless medium accessible to both legitimate network users and malicious attackers. Eavesdropping, spoofing and denial-of-service attacks should be considered.
Poor Transmission Quality- This is an inherent problem of wireless communication caused by several error sources that result in degradation of the received signal.
Network configuration- The whole MANET infrastructure is dynamic and is the reason for dynamic connection and disconnection of the variable links.
Ad hoc addressing- Challenges in standard addressing scheme to be implemented.
Topology maintenance- Updating information of dynamic links among nodes in MANETs is a major challenge
For mobile ad-hoc network to overcome most of its challenges it faces now, it can look at the call for both bandwidth and capacity, which implies calls for the need of higher frequency and spectral put to better use. Propagation, spectral reuse, and energy issues support a shift away from a single long wireless link to a mesh of short links. Wireless mesh networks evolved from the drawing board into reality. A lot of companies are now looking to get the technology and use it to propagate the need for many applications, providing broadband internet access, WLAN coverage and connectivity. The main set-back MANET have encounter so far is it complexity which is the combination between wireless technologies (with its flexibility and drawback) and the unusual role of each wireless node (as host and simultaneous router). The challenges are in largely unique and there have to be a considerable amount of research still yet to be undertaken for MANET to achieve its full potential.