Routing Metrics and Protocols for
Wireless Mesh Networks
Speaker :
吳靖緯 MA0G0101
2011.12.22
Outline
• Abstract
• Wireless Mesh Routing
• Routing Metrics
• Routing Protocols
• Conclusion
2
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.
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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.
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Wireless Mesh Routing
• WMN backbone routers use multihop communication similarly
to ad hoc networks.
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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.
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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.
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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。
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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。
E𝑇𝑋 =
1
𝑑𝑓 × 𝑑𝑟
𝑑𝑓 ：週期時間內成功收到訊息封包的比率
𝑑𝑟 ：週期時間內成功的回應訊息封包比率
• 兩變數相乘之後的倒數即是衡量ETX的數值
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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
E𝑇𝑇 = 𝐸𝑇𝑋 ×
𝑆
𝐵
S：表示封包大小
B：代表鏈結頻寬
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Routing Metrics
• Table 1 summarizes the main characteristics of the routing
metrics discussed.
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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.
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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.
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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.
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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.
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Routing Protocols
• WMN protocols and their respective routing metrics.
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Conclusion
• Routing protocols were classified in four categories: ad hocbased, 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.
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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.
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