Free communication anytime, anywhere can be a desire for wireless network communication in recent years has been rapid development. It can be equipped with wireless interfaces, portable computer or personal digital assistants to achieve movement in communication. Most current mobile communication needs of wired infrastructure (such as base stations) support can be achieved. To be able to place in the absence of a fixed base station to communicate, a new network technologies - Ad Hoc Network came into being. Ad Hoc networks do not need the support of cable infrastructure, through the mobile host free networking for communication. Ad Hoc networks appear to promote people to achieve in any environment, Xia's free communication of the process, It also brings military communications, disaster relief and temporary communications provide an effective solution.
1, AdHoc network concept
Ad Hoc network is a wired infrastructure does not support the mobile network, network nodes are mobile hosts pose. Ad Hoc Networks initially used in military field, its research originated in the battlefield environment, data communication packet radio network project funded by the DARPA, was followed in 1983 and 1994 the network survivability can be adapted to SURAN (Survivable Adaptive Network) and Global Mobile Information Systems GloMo (Global Information System) research project. As wireless communication and terminal technology continues to evolve, Ad Hoc Networks Zai civilian environment have also been developed, such as the need for basic ?? Meiyou regions where wired Lin Shi traffic, can be readily accessible in the build Ad Hoc network.
In the Ad Hoc network, when two mobile host (host A in Figure 1, and B) in communication with each other within the coverage area, they can communicate directly. However, due to the mobile host's communication range is limited, if two far distance between the host (Figure 1, the host A and C) to communicate, you need to move between them, the forwarding host B can be achieved. Therefore, in Ad Hoc network, the host or router at the same time, to carry out routing and forwarding packets to find work. In Ad Hoc networks, each host's communication range is limited, so by the multi-hop routing general composition of the data transmitted through multiple hosts to reach the destination. It is also known as Ad Hoc networks multi-hop wireless network. The structure shown in Figure 2.
Ad Hoc network can be viewed as mobile communications and computer network of the cross. In Ad Hoc networks, the use of computer network packet exchange mechanism, rather than circuit switching mechanism. Communication is usually the host portable computer, personal digital assistants (PDA) and other mobile devices. Ad Hoc network is different from the current Internet environment in the mobile IP network. In mobile IP networks, mobile host can be a fixed wired network, wireless link and dial-up lines, access to the network, and in the Ad Hoc network, there is only one kind of wireless link connection. In mobile IP networks, mobile hosts through base stations adjacent to the support of cable infrastructure to communicate, in the base stations and base station (agents and agents) are among the cable networks still use the traditional Internet routing protocols. The Ad Hoc networks do not support these facilities. In addition, mobile IP network, mobile hosts do not have the routing function, just an ordinary communication terminal. When the mobile host from one zone to another zone does not change the network topology, while the Ad Hoc network, mobile hosts move will lead to topology change.
2, Ad Hoc Network Features
Ad Hoc network as a new networking mode, with the following features.
2.1 Network Independence
Ad Hoc networks relatively conventional communications network, the biggest difference is that you can at any time, any place without the support of network infrastructure hardware, fast built up a mobile communications network. Its creation does not depend on the existing network communications facilities, has a certain independence. Ad Hoc Networks This feature is suitable for disaster relief, remote communications applications.
2.2 Dynamic network topology changes
In the Ad Hoc network, mobile hosts can move freely in the network. Mobile host will lead to the link between hosts to increase or disappear, the relationship between the host constantly changing. In ad hoc networks, the host or router may also, therefore, moving continuously network topology will change, but change the manner and speed are unpredictable. For conventional networks, the network topology is relatively stable.
2.3 The limited bandwidth of wireless communication
In the Ad Hoc network without wired infrastructure support, all communications between hosts via wireless transmission to complete. Since the physical characteristics of wireless channel itself, which provides the network bandwidth is relatively much lower cable channel. In addition, considering that the competition resulting from the collision shared wireless channel, signal attenuation, noise and other factors, the actual available bandwidth of mobile devices is far smaller than the theoretical value of the maximum bandwidth.
2.4 The limited host energy
In the Ad Hoc network, the host were a number of mobile devices, such as PDA, portable computer or Pocket PC. As the host state may be in constant movement, the host's energy mainly by the battery, the Ad Hoc networks have limited energy characteristics.
Distributed network characteristics of 2.5
In Ad Hoc networks have no central control node, host connectivity through a distributed protocol. Once one or some of the nodes in the network fails, the remaining nodes can still work properly.
2.6 The short life cycle
Ad Hoc network is mainly used for temporary communication needs, the relative with the wired network, it is generally short survival time.
Limited physical security 2.7
Mobile networks are generally more vulnerable than the physical fixed network security attacks, vulnerable to eavesdropping, deception and denial of service attacks. Some of the existing link security technology has been used to reduce the wireless network security attacks. However, the Distributed Ad Hoc network characteristics relative to the centralized network has some survivability.
3, Ad Hoc network applications
Ad Hoc network, a wide range of applications, in general, it can be used for the following occasions:
- where no wired communication infrastructure, communication facilities without the establishment of hardware or cable communications facilities were damaged.
- need characteristics of network traffic distributed environment.
- lack of existing wired communication infrastructure needs to quickly establish a temporary communications network environment.
- as the survival of a strong back-up network.
Ad Hoc Network Technology originally to meet the needs of military applications, military communications systems need to have survivability, and mobility from the group. During the war, communication systems are vulnerable to enemy attack, therefore, need communication system can withstand a certain degree of attack. The use of a centralized communication system, once the communication center is damaged, will lead to paralysis of the entire system. Distributed system can guarantee part of the communication node or link disconnected, and the rest can continue to work. During the war, the battlefield is difficult to ensure reliable cable communications facilities, therefore, by their combination of communication nodes to form a communication system is necessary. In addition, mobility is an important part of combat effectiveness, which requires communication systems based on rapid formation and removal of the war demand.
Ad Hoc network of military communications systems to meet those needs. Ad Hoc networks using distributed technology, no central control node management. When a network failure of some node or link, other nodes can continue through the relevant technical communication. Ad Hoc network consists of mobile nodes in their free combination, does not rely on wired devices, therefore, has a strong group of self, it is suitable for the harsh battlefield communications environment. Ad Hoc networks is simple and has high mobility. At present, some developed countries for the warfighter with a cutting-edge personal communication systems, in harsh war environment, it difficult for the wired communication mechanism or ?? IP mechanism Lai completed communication tasks, but can be achieved through the Ad Hoc Wangluo to. Therefore, the study of the Ad Hoc network development of military communications systems has important application value and long-term significance.
In recent years, Ad Hoc network research in civil and commercial sectors will also be taken seriously. In the civil area, Ad Hoc network can be used for disaster relief. In the event of floods, earthquakes, cable communications facilities are ?? was met with destruction and not the normal communication, Tong Guo Ad Hoc Wangluokeyi Kuaisu De establishment of emergency communications networks to ensure the smooth progress of the rescue, communications Xu Qiu Jin Ji task completion. Ad Hoc network can be used for communications in remote or underdeveloped areas. In these areas, due to cost, geography and other factors are often not wired communication infrastructure, Ad Hoc network can solve the communication problems in these environments. Ad Hoc network can also be used for temporary communication needs, such as business meetings that need to communicate with each other exchange between participants in the existing wired communication systems to meet the communication needs of the case, can be accomplished through the Ad Hoc network communication task.
Ad Hoc networks are also popular in the research field concerned, in recent years, the basic network of the International Conference on Ad Hoc networks are subject, as mobile technology continues to develop and people's increasing demand for freedom of communication, Ad Hoc network will be more concern, more rapid development and popularization.
4, with comparison of other mobile communication systems
4.1 Cellular Systems
Cellular system is the most extensive coverage of public land mobile communication systems. In cellular systems, the coverage area is generally divided into a number of similar cellular cell. Within each district to set a fixed base station, to provide users with access and information forwarding service. Between mobile users and mobile users and non-communication between mobile users need to be carried out through the base station. Base stations are generally connected to the main line through the cable from the switch form the backbone of the exchange network. Cellular system is a kind of connect to the network, once a channel is assigned to a user, usually the channel may have been the users. Cellular systems are generally used for voice communications.
Cluster system and cellular system similar to a kind of connection is Di Wang Luo, are generally private networks, size small Tigong main voice communications for mobile users.
4.3 Satellite Communication System
Communications satellite communication system the most extensive, every corner of the world can provide communication services. In this system, satellites play a similar function with the base station. Satellite communications systems by satellite location could be divided into geostationary orbit, in orbit and low orbit 3. The high cost of satellite communications systems, transmission delay large enough bandwidth limited.
The mobile communication system needs to wired network communications infrastructure support, such as base stations, switches, satellites. The establishment and operation of these facilities require substantial human and material resources, so costs are higher, while the construction cycle is long. Ad Hoc network without base stations support themselves by the host network, therefore, the network set up low cost, while time is short, usually only a few seconds or minutes. The communication system is not direct communication between mobile terminals and mobile terminals have only send and receive functions do not have forwarding. The Ad Hoc networks constituted by the mobile host, mobile hosts can communicate directly, while the mobile host not only send and receive data, but also forward data. In addition, the present mobile communication system is mainly to provide users with voice communications, usually in circuit switching, topology stable. The Ad Hoc networks using packet forwarding technology, mainly to provide data communications services for users, easy to change the topology.
5, mobile IP, and the combination of Ad Hoc Networks
For mobile and all IP network is the development of two major trends of today. With the increasingly widespread use of mobile phones and people can provide information on the movement increasingly high demand, people want more access to the Internet anytime, anywhere.
The Ad Hoc networks, network is generated at any time and are easy to structure, with no need to pre-existing network of support, is widely used is simple. But this network has a strong independent, it can stand alone, its characteristics and its use of active, on-demand driven routing protocols make it difficult to communicate with the Internet, to interact with information purposes. To achieve the Ad Hoc network for mobile hosts in different mobile Ad Hoc networks and ready access to the Internet, we use the Mobile IP protocol can be in different network roaming feature, combined with Mobile IP and Ad Hoc networks, which MIPMANET, provided a way to use demand-driven Ad Hoc network routing mechanisms, mobile IP to provide the reverse tunnel proxy address and access to the Internet Ad Hoc network solutions. Figure 3 for the Ad Hoc network access the Internet model.
Of the management problems facing the Ad Hoc network
Ad Hoc network is such that the management of many complex than wired networks because of the dynamic changes in network topology, network management is also required dynamic auto-configuration. But also taking into account the mobile node itself constraints, such as energy limited, link status change and the limited Cunchu ability, therefore, to management agreements Ji Zhenggewangluo bring the load to consider including. Finally one has to consider network management applicability to different environments.
Ad Hoc network management specific problem to be solved for the following:
- Network Management Protocol is an important task is to make network management to know the network topology. In the wired network, because the network did not change frequently, so this easy to do. However, in mobile networks, node mobility result in topology changes too frequently, network nodes need to periodically collect the connection information, which will undoubtedly increase the network load.
- most of the nodes use battery-powered, so make sure the load limit network management minimum to conserve energy. To minimize the number of nodes sending and receiving and handling, but it is in need of regular updating of the topology contradictory.
- the limited energy and mobility of nodes may lead to the network node at any time, and this required timely aware of the network management protocol nodes leave and join, and update the topology.
- changes in the wireless environment, great signal quality. Signals of recession and network congestion will cause the misconception that the node has left, therefore, must be able to distinguish between network due to node mobility or link quality of the reasons as disconnected. Physical layer network management must be asked, but this would violate the OSI hierarchical management structure.
- Ad Hoc Networks are usually used in military, therefore, to prevent eavesdropping, destruction and invasion. Therefore, network management requires a combination of encryption and authentication process.
Clear from the Ad Hoc network and network management is different from the traditional network, to be addressed include how to effectively gather network topology information, how to handle dynamic network configuration and security and confidentiality.
Ad Hoc network is a novel type of mobile computer network, It can run as an independent network, but also can serve as a network of facilities that currently has a fixed form of a supplement. Its own unique, given its huge prospects for development. Research in Ad Hoc networks there are still many problems to be solved: the design of energy-saving strategies, security, multicast and QoS support extended features of the routing agreements and the Ad Hoc network, network management. Will focus on the Ad Hoc network dedicated to network monitoring, node mobility management, survivability, and security management, management research.