The whole life cycle of ad-hoc networks could be categorized into first, second, third and fourth generation ad-hoc networks systems. Present ad-hoc networks systems are considered the fourth generation. The first generation goes back to 1972. At that time, they are called PRNET (Packet Radio Network). The second generation of ad-hoc networks emerged in 1980's, when the ad-hoc networks systems were further enhanced and implemented as a part of the SURAN (Survivable Adaptive Radio Networks) program. Since mid 1990's, a lot of work has been done on the ad-hoc standards. Within the IETF, the MANET working group was born, and made effort to standardize routing protocols for ad-hoc networks. Meanwhile, the IEEE 802.11 subcommittee standardized a medium access protocol that was based on collision avoidance and tolerated hidden terminals, for building mobile ad-hoc network prototypes. The fourth generation of cellular communication systems, generally known as 4G, is the emerging technology of future wireless networks. Some researchers define 4G as a significant improvement of 3G where current cellular networks' issues will be solved and data transfer will play more significant role. For others, 4G unifies cellular and wireless local area networks and introduces new routing techniques, efficient solutions for sharing dedicated frequency band, and increases mobility and bandwidth capacity.

Like 4G project, the MANET (Mobile Ad-hoc Network) is also in a developing stage. While defining the MANET standard, the Internet Engineering Task Force is working on routing techniques, like Ad-hoc On- Demand Distance Vector (AODV) defined in RFC 3561 and Optimized Link State Routing Protocol (OLSR) defined in RFC 3626, allowing selfconfiguring network of mobile nodes with routing capabilities. MANET standardizes the static and mobile techniques of creating mesh networks using available wireless technology.

New wireless communication technologies are expected to significantly influence the design and implementation of MANETs in the military environment. Since the future technology combining wireless local networks and cellular networks is more and more being referred to, and defined as the fourth generation (4G) of communication systems, it is critical to understand the meaning of 4G and its potential in influencing wireless networks, particularly MANET. Since we should assume low infrastructure of the mobile ad-hoc networks in the hostile military environment, 4G could be an answer to offer significant solutions for mobile MANETs to achieve high quality transmissions and constant connectivity. However, the implementation of 4G may be significantly more complicated than in the civil environment due to the unique specification and requirement of the military environment. The authors introduce the term 4GM@4GW that describes possible issues of the fourth generation of cellular network implementation in the fourth generation of warfare.

Mobile networking is one of the most important technologies supporting pervasive computing. During the last decade, advances in both hardware and software techniques have resulted in mobile hosts and wireless networking common and miscellaneous. Generally there are two distinct approaches for enabling wireless mobile units to communicate with each other:

• Infrastructured: Wireless Mobile Networks have traditionally been based on the cellular concept and relied on good infrastructure support, in which mobile devices communicate with access points like base stations connected to the fixed network infrastructure. Typical examples of this kind of wireless networks are GSM, UMTS, WLL, WLAN, etc.
• Infrastructureless: As to infrastructureless approach, the mobile wireless network is commonly known as a mobile ad hoc network (MANET). A MANET is a collection of wireless nodes that can dynamically form a network to exchange information without using any pre-existing fixed network infrastructure. This is a ver y important part of communication technology that supports truly pervasive computing, because in many contexts information exchange between mobile units cannot rely on any fixed network infrastructure, but on rapid configuration of a wireless connections on-the-fly. Wireless ad hoc networks themselves are an independent, wide area of research and applications, instead of being only just a complement of the cellular system.

In this paper, the authors describe the fundamental problems of ad hoc networking by giving its related research background including the concept, features, and its applications in future wireless network especially in the hostile military environment. Some of the key research issues for ad hoc networking technology are discussed in detail that are expected to promote the development and accelerate the commercial applications of the MANET technology.

1.1 MANET Concept

A mobile ad hoc network is a collection of wireless nodes that can dynamically be set up anywhere and anytime without using any pre-existing network infrastructure. It is an autonomous system in which mobile hosts connected by wireless links are free to move randomly and often act as routers at the same time. The traffic types in ad hoc networks are quite different from those in an infrastructured wireless network , including:

• Peer-to-Peer: Communication between two nodes which are within one hop. Network traffic (Bps) is usually consistent.
• Remote-to-Remote: Communication between two nodes beyond a single hop but which maintain a stable route between them. This may be the result of several nodes staying within communication range of each other in a single area or possibly moving as a group. The traffic is similar to standard network traffic.
• Dynamic Traffic: This occurs when nodes are dynamic and moving around. Routes must be reconstructed. This results in a poor connectivity and network activity in short bursts.

Figure 1 shows the examples of both infrastructured and infrastructureless ad hoc wireless networks.

Figure 1. Infrastructured and infrastructureless wireless networks

1.2 MANET Features

MANET has the following features:

• Autonomous terminal: In MANET, each mobile terminal is an autonomous node, which may function as both a host and a router. In other words, besides the basic processing ability as a host, the mobile nodes can also perform switching functions as a router. So usually endpoints and switches are indistinguishable in MANET.
• Distributed operation: Since there is no background network for the central control of the network operations, the control and management of the network is distributed among the terminals. The nodes involved in a MANET should collaborate among themselves and each node acts as a relay as needed, to implement functions e.g. security and routing.
• Multihop routing: Basic types of ad hoc routing algorithms can be single-hop and multihop, based on different link layer attributes and routing protocols. Single-hop MANET is simpler than multihop in terms of structure and implementation, with the cost of lesser functionality and applicability. When delivering data packets from a source to its destination out of the direct wireless transmission range, the packets should be forwarded via one or more intermediate nodes.
• Dynamic network topology: Since the nodes are mobile, the network topology may change rapidly and unpredictably and the connectivity among the terminals may vary with time. MANET should adapt to the traffic and propagation conditions as well as the mobility patterns of the mobile network nodes. The mobile nodes in the network dynamically establish routing among themselves as they move about, forming their own network on the fly. Moreover, a user in the MANET may not only operate within the ad hoc network, but may require access to a public fixed network (e.g. Internet).
• Fluctuating link capacity: The nature of high bit-error rates of wireless connection might be more profound in a MANET. One end-to-end path can be shared by several sessions. The channel over which the terminals communicate is subject to noise, fading, and interference, and has less bandwidth than a wired network. In some scenarios, the path between any pair of users can traverse multiple wireless links and the link themselves can be heterogeneous.
• Light-weight terminals: In most cases, the MANET nodes are mobile devices with less CPU processing capability, small memory size, and low power storage. Such devices need optimized algorithms and mechanisms that implement the computing and communicating functions.

1.3 MANET Application in Military

With the increase of portable devices as well as progress in wireless communication, ad hoc networking is gaining importance with the increasing number of widespread applications. Ad hoc networking can be applied anywhere where there is little or no communication infrastructure or the existing infrastructure is expensive or inconvenient to use. Ad hoc networking allows the devices to maintain connections to the network as well as easily adding and removing devices to and from the network. The set of applications for MANETs is diverse, ranging from largescale, mobile, highly dynamic networks, to small, static networks that are constrained by power sources. Besides the legacy applications that move from traditional infrastructured environment into the ad hoc context, a great deal of new services can and will be generated for the new environment. Typical applications include: Commercial sector, Local level, Personal Area Network (PAN) etc, but here the authors discussed its applications in future wireless network especially in the hostile military environment. By combining two hottest wireless network topics, 4G (the fourth generation of cellular communication systems) and MANET.

Military battlefield. Military equipment now routinely contains some sort of computer equipment. Ad hoc networking would allow the military to take advantage of commonplace network technology to maintain an information network between the soldiers, vehicles, and military information head quarters. The basic techniques of ad hoc network came from this field.

They present a single military scenario to show the importance of considering the power constraints of 4G MANET. The important aspect of 4G MANET in military environment is the fact that soldiers will become independent units where everybody will be able to communicate between themselves as well as with the headquarter using all possible means of wireless low infrastructure communication. Increased soldiers' mobility as well as quality of communication increase the total weight of equipments that soldiers have to carry with them on battlefield. The increased amount of technology increases the minimum amount of electrical power that has to be provided during military action for every soldier. Whereas 4G mobile devices implemented for military environment should not significantly increase the soldier's weight, the batteries weight might do so.

1.3.1 4G MANET Implementation

The next generation of wireless technology requires understanding the future of warfare: the 4GW. 4GM@4GW is the idea of implementing the fourth generation of wireless technology into mobile ad-hoc network in the next generation military environment. To understand this concept better, the authors explain the 4GW term defined by William Lind, an American expert on military affairs, as the theory of the Forth Generation War. Lind addressed the history of warfare can be divided into four generations. The first generation of warfare is characterized by significance of man power; the winner had simply more soldiers. The second generation focused on firepower. During World War I, armies were staying on the line, shooting toward each other, and the winner was the one who could survive the hell by having access to more artillery. Focusing on speed and coordination, the idea of highly mobile military introduced by the Nazi Germany during World War II as flash war (ger. Blitzkrieg), illustrate the third generation where “attack relied on infiltration to bypass and collapse the enemy's combat forces rather than seeking to close with and destroy them”. The news coming from Iraq and Afghanistan show examples of the fourth generation of warfare known as 4GW, which is characterized by vanishing line between peace and war, and no existence of battle front. According to, the 4GW “includes all forms of conflict where the other side refuses to stand up and fight fair.” Additional technological key components of 4GW question the success of 4G MANET in military environment. 4GW is not only the new way of moving army forces using new artillery, but also the worldwide real-time information revolution, supported by all kinds of new technology, with cyber and net wars. In the world of priceless information, the transmission medium of data becomes a target of attack during any kind of conflict. It is a misconception that addition of efficient and unbreakable cryptography to 4G supported by MANET's routing protocols will be enough to create professional wireless communication for army. The first significant point of the 4GM@4GW is to aware about the wide spectrum of terrorist technological activity by an enemy and to indicate the possible actions done by the enemy that can prevent successful implementation of future wireless networks in military environment. 4GM@4GW will be supported by 4G using increased mobility, range and bandwidth, single terminal connectivity with all already existing military equipment, possibility of using any present technology maintaining communications on the territory of action, and supported by MANET's mobile mesh routing in case of lack infrastructure. At the same time, 4G(M@W) will be twisted by technical limitations, natural-geographical constraints, possible breakings of encryption code, and DoS (Denial of Service) attacks.

Technological limitations defined as physical constraints in terms of bandwidth, memory, and power, are the first significant point on the list of issues in 4GM@4GW. This paper defines the possibilities of increasing bandwidth using the forth generation of wireless communication. Currently accessible on market different kinds of memory allow us to assume that sufficient and low cost memories such as flash drives, should fit into military environment. However, the important issue arises from supplying military mobile electronic devices by sufficient amount of power that is dependent on: how 4G is going to be implemented, what routing protocol will be applied, in what urban geographical environment missions will take a place, and finally the duration of missions. Natural geographical constrains are another issue that can influence 4G(M@W). By increasing range and bandwidth, 4G lowers the possibility of existence of routing problems between nodes in MANET network, such as hidden terminal interference and signal's fading. But at the same, the shape of the land may limit the benefits of using 4G. For example, 4G in military environment will achieve different results when used on the deserts of Iraq, in Afghanistan Mountains, or in some very urban city with metal-concrete and electric barriers. This brings to the question if a single implementation of 4G(M@W) can success in all geographical environments? In 4GW where army forces have to be ready to combat in any time and in any place all over the world it would be critical to use one single model of a device that will be able to provide sufficient communication during military action without major dependencies on the shape of land.

Comparing to the implementation of 4G in the civil environment, 4GM@4GW is also constrained by enemy's action preventing succesfull communities of army forces and technological devices left on the battle field by enemies for the purpose of disrupting the wireless communication. Such an activity does not have to focus on decrypting information that is sent through air, but simply on preventing information to reach the soldiers by using DoS attacks. In many cases, successful DoS can be more important that decrypting information because of its immediate influence on the army's condition.

This section analyses key Research issues concerning MANET network layer routing strategies, including four selected key problems in MANET: X-cast routing, security & reliability, QoS, and interworking with outside IP networks. These issues are currently main challenges of ad hoc wireless networks. The lack of robust solutions to these problems prevents MANET from wide commercial deployment.

2.1 X-cast Routing Algorithms

As in the infrastructured wireless networks, all kinds of X-cast communication schemes should be supported in an ad hoc mobile environment. These include unicast, anycast, multicast, and broadcast. MANET also brings new X-cast modes into communications, e.g. geocast and contentbased. In particular, multicast is desirable to support multiparty wireless communications. Since the multicast tree is no longer static (i.e. its topology is subject to change over time), the multicast routing protocol must be able to cope with mobility, including multicast membership dynamics (e.g., leave and join). In a multihop ad hoc context, the routing problem becomes more complex because of the mobility of both hosts and routers. The random movement of the nodes and the uncertainty of path quality render the traditional routing protocols impractical. Trade-off between reactive and proactive schemes in terms of latency and overhead of route discovery and maintenance are to be considered depending on different traffic and mobility patterns. Issues to be taken into account include routing discovery and flooding, caching, data delivery, location-aided and power-aware, broadcast storm issue, route request and reverse path.

2.2 QoS Supporting Model

Just like in wired networks, QoS protocols can be used to prioritize data within ad hoc networks in order to reserve better connections for high data rate applications while still maintaining enough bandwidth for lower bit rate communication. The support of multimedia services will most likely be required within and throughout the MANET, for which different QoS classes (e.g. voice, video, audio, web, and data stream) are needed to facilitate the use of multimedia applications. In such a stochastic changing environment involving dynamic nodes, hidden terminals, and fluctuating link characteristics, supporting end-to-end QoS at different levels will be a great challenge that requires in-depth investigation. An adaptive QoS must be implemented over the traditional plain resource reservation to support the multimedia services. Special emphasis should be put on achieving a new QoS model for MANETs by taking into account the ad hoc features of the target networks: dynamic node roles, data flow granularity, traffic profile, etc.

2.3 Security, Reliability, and Availability Schemes

Security, reliability, and availability are three crucial aspect of MANET, especially in security-sensitive applications. Since ad hoc relies on wireless communication medium, it is important to deploy a security protocol to protect the privacy of transmissions. The requirements regarding confidentiality, integrity, and availability are the same as for any other public communication networks. However, the implementation schemes of key management, authentication, and authorization are quite different because there is no aid of a trusted third-party certification authority to create trusted relationships by exchanging private/public keys. Different types of threats and attacks against routing in MANET should be analysed leading to the requirement of ad hoc routing security, and advanced solutions are needed for the secure routing of MANET. Wireless communication is subject to many types of problems due to interference and poor signals. As for reliability and availability issues, besides low level error masking and recovery mechanisms (i.e. link layer error detection and correction coding), special attention should be paid to studying fault-tolerant routing algorithm. In multihop ad hoc wireless networks, there exists an inherent attribute of redundant routing paths between nodes. Exploiting this property, it's possible to provide a faulttolerant routing scheme, for increasing the reliability and security of the target routing algorithm. Since overhead occurs in this reliable-increasing algorithm, research should also study the tradeoff between performance and reliability in order to calculate the most efficient solution.

2.4 Internetworking Mechanisms

To integrate the two mobility management schemes in the domains of both traditional infrastructured wireless networks and the new mobile ad hoc networks is an important issue. The mobility mode of an ad hoc network is quite different from that of infrastructured networks. In infrastructured networks only the nodes (terminals) at the very edges (the last hop) of fixed networks are moving, whereas an ad hoc network can be completely mobile, since a device can serve both as router and host at the same time. Consequently, in an ad hoc network mobility is handled directly by the routing algorithm. In many cases, device accesses both within the ad hoc network and to public networks (e.g. the Internet) can be expected to form a universal communication scenario. In other words, a terminal in an ad hoc wireless network is able to connect to nodes outside the MANET while being itself also accessible by external nodes. The interworking between ad hoc and fixed networks is necessary. In particular, the coexistence and cooperation with the public IP based wireless networks is necessary to many contexts. The Mobile IP protocol for MANET should be deeply studied in order to give nodes in ad hoc networks the ability of accessing the Internet and other IP based networks to take advantage of the services of Mobile IP.

This article describes the fundamental issues and analyses key research problems of MANET. Firstly, the background information of MANET are introduced, including the MANET concept, features and application areas using 4G. The successful and safe implementation of the forth generation of wireless technology into the mobile ad-hoc network for the next generation military environment might face tough challenges. Finally, four key network layer research issues of MANET routing strategies are described in detail. The novel and advanced solutions to these issues are necessary to fulfill the requirements of wide commercial deployment of MANET. Mobile ad hoc networking is one of the most important and essential technologies that support future pervasive computing scenario. Currently MANET is becoming more and more interesting research topic and there are many research projects employed by academic and companies all over the world.

The future work includes: define physical constraints for military mobile devices, simulate the effectiveness of MANET's routing protocols in 4G environment to find the most optimal protocol for the 4GM@4GW.