1. Routing Metrics and Protocols for Wireless Mesh Networks 2011.12.22 Speaker : 吳靖緯 MA0G0101

2. Outline Abstract Wireless Mesh Routing Routing Metrics Routing Protocols Conclusion 2

3. Abstract WMNs are low-cost access networks built on cooperative routing over a backbone composed of stationary wireless routers. WMNs must deal with the highly unstable wireless medium. Therefore, the design of algorithms that consider link quality to choose the best routes are enabling routing metrics and protocols to evolve. 3

4. Abstract In this work, we analyze the state of the art in WMN metrics and propose a taxonomy for WMN routing protocols. Performance measurements for a WMN, deployed using various routing metrics, are presented and corroborate our analysis. 4

5. Wireless Mesh Routing WMN backbone routers use multihop communication similarly to ad hoc networks. 5

6. Wireless Mesh Routing The backbone routers typically are stationary, which permits routing metrics to model link quality instead of simply using the number of hops. Assuming that the common-case application in WMNs is Internet access, traffic is concentrated on links close to the gateways. 6

7. Routing Metrics Ad hoc networks usually use the hop count as a routing metric. This metric is appropriate for ad hoc networks because new paths must be found rapidly, whereas high-quality routes may not be found in due time. In WMNs, the stationary topology benefits quality-aware routing metrics. 7

8. Routing Metrics Wireless ad hoc network 不需要依賴一個既存的網路架構，像是有線系統的路由器， 或是無線系統的無線網路基地台。 每一個節點，都有能力轉送網路封包給其他節點，所以網路 是由節點與節點間動態連結所形成的。 由於網路中的節點沒有當前網路拓撲結構的先驗知識，通常 在需要通信時才能知道連線品質。 8

9. Routing Metrics Expected Transmission Count(ETX) ETX 衡量節點間週期性的傳送一個訊息，也週期地以傳送回 應給鄰居訊息遺失情形，節點根據這些數據產生新的 ETX 。 節點依據此新的 ETX 產生新的衡量，若訊息或回應訊息遺失 了，節點會將增加衡量，作為反應此遺失的補償。 兩變數相乘之後的倒數即是衡量 ETX 的數值 9

10. Routing Metrics ETX 範例應用 考慮兩個節點，節點 A 與 B 。 A 與 B 每一秒鐘廣播一個訊息。 在十秒鐘的週期內， A 節點接收到 B 節點 7 個訊息，而 B 節點在 十秒鐘的週期內接收到 A 節點 8 個訊息。 B 節點將此資訊以單點廣播的方式回覆給節點 A ，則節點 A 認 知到由 A 傳送到 B 節點成功的機率為： (1 － 0.2) x (1 － 0.3) ＝ 0.56 ，則重新計算產生的衡量 ETX 值為 1 ／ 0.56 ＝ 1.78 。 Expected Transmission Time(ETT) ETT 是預計傳輸時間，使用公式如下 10

11. Routing Metrics Table 1 summarizes the main characteristics of the routing metrics discussed. 11

12. Routing Protocols Ad hoc routing Ad hoc routing protocols are usually proactive, reactive, or hybrid. The proactive strategy operates like classic routing on wired networks. Routers keep at least one route to any destination in the network. Reactive protocols, request a route to a destination only when a node has a data packet to send. If a node does not have data packets to send to a particular destination, the node will never request a route to it. 12

13. Routing Protocols Controlled-flooding Controlled-flooding protocols use algorithms designed to reduce control overhead. The basic assumption is that flooding the network is not efficient because most communication in wireless networks is between nearby nodes. Therefore, there is no need to send control packets to the distant nodes as frequently as to nearby ones. 13

14. Routing Protocols Another way to reduce overhead is to limit the number of nodes responsible for flooding the network. A common approach is to use algorithms that find the minimum set of nodes required to forward routing information to all destinations in the network. 14

15. Routing Protocols Opportunistic Opportunistic protocols improve classical routing based on cooperative diversity schemes. These protocols guarantee that the data is always forwarded whenever there is at least one next hop. In addition, the chosen route likely uses the best quality links, considering short-term variations. 15

16. Routing Protocols WMN protocols and their respective routing metrics. 16

17. Conclusion Routing protocols were classified in four categories: ad hoc- based, traffic-aware, controlled-flooding, and opportunistic. Our results confirm that the hop metric performs poorly because it is not aware of link-quality variations. ML, ETX, and ETT, showed better results, considering the different performance measures in accordance with the design of each metric. 17

18. Conclusion The design of WMNs presents a number of open issues, ranging from routing metrics to security. This is accomplished by better reflecting PHY-layer variations onto routing metrics or by better using the available radio spectrum to directly improve the network throughput. 18