外文翻译：ad-hoc网络的一个简要概述：挑战与研究方向.doc

附 录一、 英文原文A Brief Overview of ad hoc Networks:Challenges and DirectionsOne of the most vibrant and active “new” fields today is that of ad hoc networks. Significant research in this area has been ongoing for nearly 30 years, also under the names packet radio or multi-hop networks. ad hoc network is a (possibly mobile) collection of communications devices (nodes) that wish to communicate, but have no fixed infrastructure available, and have no pre-determined organization of available links. Individual nodes are responsible for dynamically discovering which other nodes they can directly communicate with. Ad hoc networking is a multi-layer problem. The physical layer must adapt to rapid changes in link characteristics. The multiple access control (MAC) layer needs to minimize collisions, allow fair access, and semi-reliably transport data over the shared wireless links in the presence of rapid changes and hidden or exposed terminals. The network layer needs to determine and distribute information used to calculate paths in a way that maintains efficiency when links change often and bandwidth is at a premium. Its also needs to integrate smoothly with traditional, non ad hoc-aware internetworks and perform functions such as auto-configuration in this changing environment. The transport layer must be able to handle delay and packet loss statistics that are very different than wired networks. Finally, applications need to be designed to handle frequent disconnection and reconnection with peer applications as well as widely varying delay and packet loss characteristics.Ad hoc networks are suited for use in situations where infrastructure is either not available, not trusted, or should not be relied on in times of emergency. A few examples include: military solders in the field; sensors scattered throughout a city for biological detection; an infrastructureless network of notebook computers in a conference or campus setting; the forestry or lumber industry; rare animal tracking; space exploration; undersea operations; and temporary offices such as campaign headquarters.HistoryThe history of ad hoc networks can be traced back to 1972 and the DoD-sponsored Packet Radio Network (PRNET), which evolved into the Survivable Adaptive Radio Networks(SURAN) program in the early 1980s l. The goal of these programs was to provide packetswitched networking to mobile battlefield elements in an infrastructureless, hostile environment (soldiers, tanks, aircraft, etc., forming the nodes in the network).In the early 1990s a spate of new developments signaled a new phase in ad hoc networking. Notebook computers became popular, as did open-source software, and viable communications equipment based on RF and infrared. The idea of an infrstructureless collection of mobile hosts was proposed in two conference papers 2,3, and the IEEE 802.11 subcommittee adopted the term “ad hoc networks.” The concept of commercial (non-military) ad hoc networking had arrived. Other novel non-military possibilities were suggested (as mentioned in the introduction), and interest grew.At around the same time, the DoD continued from where it left off, funding programs such as the Global Mobile Information Systems(GloMo), and the Near-term Digital Radio(NTDR). The goal of GloMo was to provide office-environment Ethernet-type multimedia connectivity anytime, anywhere, in handheld devices. Channel access approaches were now in the CSMA/CA and TDMA molds, and several novel routing and topology control schemes were developed. The NTDR used clustering and linkstate routing, and self-organized into a two-tier ad hoc network. Now used by the US Army,NTDR is the only “real” (non-prototypical) ad hoc network in use today.Spurred by the growing interest in ad hoc networking, a number of standards activities and commercial standards evolved in the mid to late90s.Within the IETF, the Mobile Ad hoc Networking(MANET) working group was horn, and sought to standardize routing protocols for ad hoc networks. The development of routing within the MANET working group and the larger community forked into reactive (routes ondemand) and proactive (routes ready-to-use) routing protocols 4. The 802.11 subcommittee standardized a medium access protocol that was based on collision avoidance and tolerated hidden terminals, making it usable, if not optimal,for building mobile ad hoc network prototypes out of notebooks and 802.11 PCMCIA cards.HIPERLAN and Bluetooth were some other standards that addressed and benefited ad hoc networking.Open ProblemsDespite the long history of ad hoc networking, there are still quite a number of problems that are open. Since ad hoc networks do not assume the availability of a fixed infrastructure, it follows that individual nodes may have to rely on portable, limited power sources. The idea of energy-efficiency therefore becomes an important problem in ad hoc networks. Surprisingly,there has been little published work in the area of energy-efficiency of ad hoc networks until fairly recently. Most existing solutions for saving energy in ad hoc networks revolve around the reduction of power used by the radio transceiver. At the MAC level and above, this is often done by selectively sending the receiver into a sleep mode, or by using a transmitter with variable output power (and proportionate input power draw) and selecting routes that require many short hops, instead of a few longer hops 8.The ability of fixed, wireless networks to satisfy quality of service (QoS) requirements is another open problem. Ad hoc networks further complicate the known QoS challenges in wireline networks with RF channel characteristics that often change unpredictably, along with the difficulty of sharing the channel medium with many neighbors, each with its own set of potentially changing QoS requirements. Reflecting the multi-layer nature of ad hoc networks, there are numerous attempts to improve the QoS problems from the service contracts 9 to the MAC layer. A promising method for satisfying QoS requirements is a more unified approach of cross-layer or vertical-layer integration. The idea is to violate many of the traditional layering styles to allow different parts of the stack to adapt to the environment in a way that takes into account the adaptation and available information at other layers. A similar multi-layer issue is that of security in ad hoc networks 10. Since nodes use the shared radio medium in a potentially insecure environment, they are susceptible to denial of service (DoS) attacks that are harder to track down than in wired networks. Also, since a large portion of the network nodes will be dynamically reorganizing and forwarding packets on behalf of others, ad hoc networks are particularly susceptible to the injection of bogus network control traffic. Finally, ad hoc networks can he victims of specialized kinds of security attacks such as DoS attacks that cause a node to use its transceiver so much that it depletes its battery.Robots and sensors also provide new hardware capabilities ripe for new methods of enhancing ad hoc efficiency. Robots, for example,have a tight integration between the processes of movement, decision-making, and networking that allow them to modify their actions while taking into account the effects on many different system aspects 12. Similarly,sensors are often deployed in a way that makes their roles and capabilities redundant, suggesting new ways of combining application knowledge of delivered information with the routing layer 13.Finally, a problem that overarches all these others is the lack of well defined and widely accepted models for RF path attenuation, mobility, and traffic. These tightly interrelated models are needed for quantifying and comparing ad hoc system performance to a common baseline.The physical processes of refraction, reflection, and scattering of RF radiation is moderately well understood but difficult to quantify in detail when including a large number of complex objects such as foliage, cars, or buildings. In contrast,the pattern of movement of the nodes and the flow of traffic can certainly be easily described in detail, but the dependency on the target application, the lack of existing systems available for study, and the likely interactions between connectivity, movement, and user applications,causes these models to be ill-defined.The FutureImagine the following scenarios: a wireless mesh of rooftop-mounted ad hoc routers; an ad hoc network of cars for instant traffic and other information; sensors and robots forming a multimedia network that allows remote visualization and control; multiple airborne routers (from tiny robots to blimps) automatically providing connectivity and capacity where needed (e.g., at a football game); an ad hoc network of spacecraft around and in transit between the Earth and Mars. These may seem like science fiction, but are in fact ideas pursued seriously by the ad hoc research community. While only time can tell which of these imagined scenarios will become real, the above offers a glimpse into both the technological potential and the evolving state of the art. We discuss in this section the forces at play that are likely to shape the future of ad hoc networking, and discuss the directions in which it may evolve.To appreciate the role ad hoc networks are likely to play in the future, consider this: bandwidth-hungry applications and the laws of physics drive wireless architectures away from cellular toward ad hoc. This is because more capacity implies the need for a higher communications bandwidth and better spatial spectral reuse.Higher bandwidth is found at higher frequencies,where the propagation is dismal. Further, mobile devices have to be power-thrifty. Propagation,spectral reuse, and energy issues support a shift away from a single long wireless link (as in cellular) to a mesh of short links (as in ad hoc networks).That this might be the wave of the future is attested to by burgeoning startups, e.g.,Rooftop Communications (now part of Nokia),Mesh Networks, and Radiant Networks, that use a multihop mesh-based architecture in place of conventional 3G architectures.The other main impetus to ad hoc networks comes from the rapidly improving communications technologies.Wireless communication devices are getting smaller, cheaper, more sophisticated, and hence more ubiquitous.Exploitation of these technologies for better ad hoc networking gives rise to new problems that point to new research. For instance, the use of smart antennas in ad hoc networking requires new medium access and neighbor discovery protocols.The ability to dynamically alter spread spectrum codes, modulation schemes, and waveforms require corresponding innovations at the higher layers. Software radios, which represent an important change in radio architecture, offer more flexibility that is suitable for ad hoc networks.How are ad hoc networks likely to evolve? It is likely that the nodes themselves will be smaller,cheaper, more capable and probably conformal,and come in all forms. Indoor ad hoc networks (perhaps based on Bluetooth, Wireless LAN, or similar technologies) will probably be used to connect smart appliances to the Internet. Mesh-based last-mile solutions will increase in popularity and may even be the dominant solution.Military ad hoc networks will have higher capacities and support multimedia applications,be more adaptive, stealthy, and evolve toward a system where all battlefield elements, mobile or stationary, are multimedia-networked. Finally, there is the utopian idea of a global infosphere where all network elements form a gigantic ad hoc wireless network using unlicensed spectrum, bypassing the existing infrastructure.While fascinating from a research viewpoint, the realization of this vision will depend not only on overcoming the capacity and other hurdles, but also the pragmatics of a cooperative network. Notwithstanding our predictions,however, like the Internet, which existed for more than 20 years before the World Wide Web came along, it may be a surprise killer app that shapes the future of ad hoc networking.二、英文翻译ad hoc网络的一个简要概述：挑战与研究方向 ad hoc网络已成为当今最为鲜明活跃的一个新的领域，在这个领域已经将近有30年的重点研究下命名为“分组无线网”或“多跳网络”。ad hoc网络是一个（可能是移动的）希望相互通信的没有固定基础设施的没有预先组织有效连接的通信设备（点）的集合。各个节点负责动态地发现其他的节点中哪些是可以与自己直接进行通信。ad hoc网络是一个多层问题。物理层必须与快速变化的链路特性相适应，媒体介质接入控制（MAC）层尽量减少碰撞，允许公平接入，在共享无线链路快速变化和存在隐藏或暴露终端条件下进行较可靠的传输数据，网络层需要确定和传输用于计算在链路质量时变和带宽有限条件下保持高效路由的信息，它也需要与传统的无线ad hoc互联网和执行功能（比如在不断变化的环境中自动配置）顺利的融合在一起，传输层需要处理与有线网络差别很大的延迟和分组数据丢失统计分析；最后，应用层需要处理与对等应用之间频繁的断连和重连以及变化范围较大的延迟和分组数据丢失特性。ad hoc网络适合在以下情况下使用，要么是没有固定的基础设施可用的场合，要么在紧急情况下没有依赖的场合，一些应用的例子包括：在野战军用通信应用方面；散布在一个城市的生物的传感器检测；在会议或校园内的一个无笔记本计算机的通信设施的网络；林业和木材工业；稀有动物跟踪；太空探索；海底作业；临时办公，比如总统的竞选。历史ad hoc网络技术的起源可以追溯到1972年的美国国防部高级研究计划局资助研究的战场环境中的无线分组数据网（PRNET）1，在20世纪80年代初发展成为具有抗毁性和自适应能力的网络（SURNAN）项目。该项目的目标是在没有现成基础设施可以利用、敌意环境中为移动战场节点（战士、坦克、飞机等）提供基于分组交换的网络。在20世纪90年代初一连串的新发展标志着ad hoc网络化进入一个新的阶段。笔记本计算机的流行和基于无线和红外技术通信设备的广泛出现，产生了计算机互联的要求，为无线ad hoc网络的应用提供了广阔的空间。在文献23中提到，IEEE 802.11委员会将这种无基础设施的移动主机网络采用术语“ad hoc 网络”一词，已经形成了商业 (非军事）ad hoc网络的概念，此外许多新型的非军事发展可能（简介中已经提到）也有了增长利益。大约在同一时间，美国国防部继续资助曾停止过的项目，诸如全球移动信息系统GloMo和近期数字化无线电(NTDR)。GloMo的目标是用手持设备为办公环境提供任何时间、任何地点的以太网类型的多媒体链接，采用CSMA/CA和TDMA作为信道接入方式，并开发了几种新的路由协议和拓扑控制方案。NTDR采用分群和链路状态路由，并自组为两层结构的ad hoc网络。NTDR是美国军方正在使用的实用网络，也是当今惟一的实用（非原型）网络。到了20世纪90年代中期，随着ad hoc组网热潮的到来，涌现了一系列标准活动和商业标准。在IETF成立了一个专门的移动ad hoc网络MANET工作组，专门负责研究和开发具有数百个节点的移动ad hoc网络的路由算法，并制定相应的标准，其开发的路由协议派生为先应式和反应式路由协议4。IEEE 802.11委员会对基于CA和容许隐藏终端的MAC协议进行了标准化，虽然不是最优，但可用于构建ad hoc网络原型。HIPERLAN和蓝牙技术也为ad hoc组网提供了一些技术设备。面临的挑战尽管ad hoc网络的历史悠久，仍然存在很多问题。由于ad hoc网络无法利用固定基础设施，因此，单个节点必须依靠可携带的有限电源。这种节能的想法在ad hoc网络已成为一个重要的问题。令人惊讶的是直到最近才有一些有关ad hoc网络的能源效率方面的发表著作。现有ad hoc 网络大多数节能解决方案围绕减少电台收发功率进行。在MAC及以上层可以选择将收信机设置为睡眠状态，或通过可变的发射机输出功率和选择需要更多短距离跳数取代少的长距离跳数的路由8实现。固定无线网络满足服务质量(QoS)能力要求是另一个要面对的问题。有线网络的已知服务质量QoS的挑战和RF信道特性的变化的不可预见性使得ad hoc网络进一步复杂化，同时与许多邻居共享信道也存在着困难，它们都有自己的随时设置和变化所需求的服务质量。考虑到ad hoc网络的多层特性，对MAC层的服务协议9作了很多改善服务质量问题的尝试。为了满足 QoS 要求的有效方式是一个更统一的跨层设计和分层设计结合，这个想法违反了各种允许不同地区的堆栈来适应环境的传统分层法，在某种程度上，考虑在其他层上的适应性和可利用的信息。在ad hoc网络中类似多层问题的论点是安全问题10。共享无线环境的潜在的不安全因素使得节点比有线网的更容易受到拒绝服务 (DoS) 攻击，更难追踪。此外，由于很大一部分网络节点的动态重配置和邻居分组转发容易被注入伪网络控制业务中。最后，ad hoc网络可能受到各种特定的安全攻击的受害者，如不停发射直至消耗完节点的电源。机器人与传感器网络已具有采用新方法提高网络效率的新硬件能力，这为提高ad hoc性能做好准备。机器人，举例来说，有一间紧密结合的过程中，决策、网络化等动作，让他们改变他们的行为而考虑到许多不同的系统方面12的影响。同样，传感器常常在某种程度上针对他们的角色与功能提出冗余信息的应用知识与交付路由层13相结合的新方法。最后，跨层切换设计是所有这些其他的问题中的一个，缺乏明确和通用模型的灵活性、射频路径衰减和通信。为量化需要特别的系统性能，我们对这些紧密相关的模型进行了分析比较，一个常见的诸如反射、折射、射频辐射和散射的基本的物理过程的理解是相当容易的,但却很难量化,包括大量的复杂的对象，如树叶、汽车、建筑。相比之下，与移动节点的通信流模式可以肯定很容易进行详细描述，但依赖于目标程序，缺乏现有系统的有效研究，连接性、 移动性和应用程序之间的可能相互作用，将导致现有模型的含糊不清。展望想象一下以下情况：一个无线网格的天台安装ad hoc路由器；ad hoc网络的汽车提供