Wireless multihop mesh networks无线多跳mesh网络Word文件下载.docx

1、Along with the expanding of the wireless nets scale, the requiring of wired connection for each access-point in WLAN makes it meet a lot of challenges and in convenience in the environment of lack of cable infrastructure. The traditional WLAN shows its insufficient gradually. The disadvantages of tr

2、aditional WLAN:a. The poor reliability: In the traditional WLAN, several users get access to the wireless net through one access-point directlyit is call “single-hop” networks. So long as one stoppage can breakdown the whole networks.Figure1. Traditional WLANb. The small coverage:The normal technolo

3、gy of “point-to-point” or “point-to-multipoint” uses the short networks, with the limitations of coverage, just like networks of 802.11 and Bluetooth.c. The poor scalability:As the joining and sharing data flow of access-points of extra nodes, the bandwidth of whole networks will come down gradually

4、.d. The bad quality of communication:Because of noise in link and mistakes in communication, the bandwidth will decrease as the distances increasing. Most kind of wireless networks have “blind points” in its effective distance, in which it shows the poor signal quality. The concept of wireless mesh

5、network （WMN）With the evolution of communication technique and catering to the need of the market, a new wireless network emerges, which is called “wireless mesh network”, short for “WMN”. Wireless mesh network is a flexible communication system to improve the efficiency and quality in single transm

6、ission. WMNs have emerged as a cost-effective alternative for large-area communications. WMNs have also become an important technology for constructing wireless networks beyond the 3G high-speed cell phone networks. A mesh network is an ad hoc network that employs one of two connection arrangements:

7、 full mesh topology or partial mesh topology. WMNs do not require wired infrastructure for long-haul communications, thus offering the major benefit of low-cost and flexible deployment and maintenance. WMNs, however, suffer from the same security vulnerabilities as conventional wireless networks. Be

8、cause data are transmitted in the form of radio waves in the air without physical boundaries, packet eavesdropping and packet intercepting can be carried out easily by attackers with the help of relatively cheap equipment. Moreover, wireless channels may be disturbed by noise and interference, which

9、 affect the quality of communications. WMNs are multi-hop networks, which are especially vulnerable to interference problems: Adjacent hops on the same path and neighboring paths could interference with each other. WMNs are distributed networks, which have additional issues on routing, authenticatio

10、n, and key management. Therefore, security is a major concern in WMNs. Ad hoc networks and Wireless mesh networksFigure2. Ad hoc networka. Ad hoc networksAd hoc networks are wireless networks that can be deployed quickly. A mobile ad hoc network is a potential solution for deploying a data and voice

11、 network for FRs in scenarios with limited or no infrastructure. All nodes in the network act as equal peers as Figure 2. An ad hoc network typically uses a suitable routing protocol such as AODV or DSR to allow multi-hop communication between the nodes. One or more nodes may act as gateways and con

12、nect the ad hoc network with the Internet or some other network. The gateways may use a wired or even a wireless back-haul to exchange data with an external network. Self-contained ad hoc networks may not even have a back-haul. Ad hoc networks generally have only single-radio nodes and can be consid

13、ered to be single-tier mesh networks.b. Wireless mesh networksIn the full mesh topology, each node is connected directly to each other node. In the partial mesh topology, nodes are connected to only some, but not all the other nodes. Wireless mesh networks thus combine a mesh topology with ad hoc wi

14、reless network characteristics. Mesh networks can be built on a mix of xed and mobile nodes interconnected via wireless links to form a multi-hop ad hoc network. Figure3. Wireless multi-hop mesh network of two tiersIn a sufficiently dense wireless mesh network, there is rarely a single point of fail

15、ure and the network is able to recover from the failure of an individual node by routing around it to other nodes in a direct or multi-hop manner. A mesh network allows nodes to communicate with other nodes without being routed through a central switch point, thus eliminating centralized failure and

16、 providing self-healing and self-organization behavior. As shown in Figure 3, several mesh routers act as APs to the mobile clients. The mesh provides a wireless infrastructure to mobile clients using an ad hoc wireless back-bone to route packets to their respective destinations. One or more mesh ro

17、uters can act as gateways for the back-haul traffic to external networks, and may use a wired or wireless connection. An ad hoc wireless network protocol, such as Ad hoc On-demand Distance Vector routing, Optimized Link State Routing, or Dynamic Source Routing, is typically used for routing packets

18、in the network between mesh routers. There exist over 70 po-tential protocols that could be used for routing in a mesh network. Path diversity and redundancy are the key features of a successful mesh network.Two-tier mesh networks, such as that shown in Figure3, avoid the completely self-organized n

19、ature of ad hoc networks. Such networks have two or more radios. Typically one radio is used in the “access tier” as an AP to provide wireless service to mobile clients in the area connected to it （just like in a Wi-Fi access point）. The second radio is used in the “mesh tier” by the mesh nodes to f

20、orm a wireless backbone, using a suitable mesh routing protocol. In this context, the term “mesh routing protocol” is synonymous with the term “ad hoc routing protocol”. Thus, each mesh node in a two-tier wireless mesh network provides the functionality of both an access point as well as router.The

21、back-haul of the mesh node wirelessly relays the traffic from mesh router to mesh router until it reaches a gateway. The gateway node then connects to the Internet or to another private network （through a wired connection）. Another option is to use an alternative technology such as satellite or cell

22、ular network links, to wirelessly connect the mesh network with an external network.We use this two-tier model in Dart-Mesh, our mesh implementation testbed. One benet of this architecture is that for end users the experience of joining such a network is identical to the process for a client joining

23、 a Wi-Fi infrastructure-based WLAN. This process is well supported by all compatible wireless client devices that support the published standards of the Wi-Fi device manufacturers consortium. The clients do not need to run any special routing software or congure the radio to run in “ad hoc mode.” In

24、deed, the “ad hoc mode” is not fully supported by many device manufacturers, device drivers and operating systems, and conguring a wireless card to use this mode is not an easy process.c. Relationship between ad hoc networks and Wireless mesh networksAd-hoc networks and WMNs are often used interchan

25、geably, but there are subtle differences between them on a number of aspects.First, end-user devices in ad-hoc networks also perform routing and configuration functionalities for other nodes. In WMNs these functionalities are performed by mesh routers. In terms of functionalities, mesh client device

26、s are the same as those in IEEE 902.11 WLANs, providing lower energy consumption and high-end application capabilities to mobile users and energy constrained users. Moreover, because end-users do not support routing functionalities the cost of WMN client devices may be reduced. Second, WMNs adopt th

27、e multiple-radio interface technology, which differs from ad-hoc networks. The multi-radio interface technology makes it possible to separate routing （control packets） from data transmission （data packets）. This significantly improves the capacity of the network. On the other hand, routing and data

28、transmission in ad-hoc networks are performed on the same radio interface, which would reduce the performance. Third, because end-user devices in ad-hoc networks provide routing functionalities, the network topology and connectivity depend on the movement of users. This imposes additional challenges

29、 to routing protocols, network configurations, and network deployment. Different from ah-hoc networks, routing is performed by mesh routers that are either stationary or slightly mobile. Mesh routers constitute the infrastructure for WMNs. While providing continuous connectivity throughout the netwo

30、rks, WMNs continue to support mobile end-users without compromising the performance.The differences between ad-hoc networks and WMNs are subtle, making it difficult to draw a clean line between the two types of networks. It may be helpful to view ad-hoc networks as a subset of WMNs or vice versa. Ch

31、allenges and problems to be solved in the research of Wireless mesh networksSeveral challenges need to be overcome to realize a wireless mesh network suitable for First Responders. The overarching challenge is to engineer and build a wireless mesh network that provides all the desired characteristics in a cost effective manner. We enumerate some of the desirable characteristics of a mobile mesh network and their corresponding challenges:a. Robustness:The mesh network should be functional even if a few individual nodes fail. For example, in a hazardous scenario