1、2 附 录 一、 英文原文 A Brief Overview of ad hoc Networks: Challenges and Directions One 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 netwo
2、rk 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 co
3、mmunicate 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
4、 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
5、 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 reconn
6、ection 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
7、 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 headquart
8、ers. History The 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 batt
9、lefield elements in an infrastructureless, hostile environment (soldiers, tanks, aircraft, etc., forming the 3 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 c
10、ommunications 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
11、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 wa
12、s 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 s
13、elf-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,
14、 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 protocol
15、s 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 stan
16、dards that addressed and benefited ad hoc networking. Open Problems Despite 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 re
17、ly 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
18、 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
19、 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 4 ch
20、ange 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
21、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 acco
22、unt 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 th
23、an 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 k
24、inds 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 processe
25、s 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 applicat
26、ion 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 h
27、oc 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
28、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 il
29、l-defined. The Future Imagine 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
30、 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
