Routing Metrics and Protocols for
Wireless Mesh Networks
Speaker :
吳靖緯 MA0G0101
2012.01.05
Outline
• Abstract
• Wireless Mesh Routing
• Routing Metrics
• Routing Protocols
• Mesh Network Performance Analysis
• Number of Hops
• Packet Loss Rate
• Network Delay
• Throughput
• 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.
3
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，也週期性以傳送回
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
• minimum loss(ML)
• The minimum loss (ML) metric also is based on probing to
compute the delivery ratio.
• ML finds the route with the lowest end-to-end loss probability.
• weighted cumulative ETT (WCETT)
• The weighted cumulative ETT (WCETT) changes ETT to also
consider intra-flow interference.
• Unlike ETX and ETT, WCETT is an end-to-end metric.
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Routing Metrics
• The metric of interference and channel-switching (MIC)
• MIC uses virtual nodes to guarantee the minimum-cost routes
computation.
• MIC also calculates its value based on the ETT metric.
• modified ETX (mETX)
• The mETX metric also is calculated by broadcasting probes.
• The difference between mETX and ETX is that rather than
considering probe losses.
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Routing Metrics
• effective number of transmissions (ENT)
• ENT is an alternative approach that measures the number of
successive retransmissions per link considering the variance.
• ENT also broadcasts probes and limits route computation to links
that show an acceptable number of retransmissions according to
upper-layer requirements.
• interference aware (iAWARE)
• The iAWARE metric estimates the average time the medium is
busy because of transmissions from each interfering neighbor.
• The higher the interference, the higher the iAWARE value.
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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.
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Routing Protocols
Controlled-flooding
• Controlled-flooding protocols use algorithms designed to
reduce control overhead.
• We identify two baseline approaches that reduce the routing
overhead as compared.
• Classical flooding (Fig. 2a). In temporal flooding (Fig. 2b), the
periodicity is set according to the distance from the source
router. On the other hand, using spatial flooding (Fig. 2c), the
distant nodes receive less precise or less detailed information
from the source.
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Flooding types: a) classical; b) temporal; c) spatial.
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Routing Protocols
• In practice, most protocols disseminate local-scope routing
information, using the temporal approach.
• 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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Mesh Network Performance Analysis
• Our performance measurements were collected in the ReMesh
mesh network deployed at the Fluminense Federal University
(UFF) campus.
• The mesh network deployed at UFF consists of nine mesh
nodes labeled from ID0 to ID8 deployed at the third and fourth
floors of the engineering building of the university (Fig. 3).
• Wireless links connecting nodes were collected by monitoring
the topology built by OLSR within each router, using a plug-in
for the OLSR daemon.
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• The optimized link state routing(OLSR)
Mesh Network Performance Analysis
• Dashed lines indicate low quality links with loss rates higher
than 50 percent, and continuous lines indicate better quality
links.
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UFF's mesh network
Number of Hops
• Figure 4a shows the average number of hops traversed to reach
each node from node ID0 for each metric.
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average route length
Number of Hops
• It can be observed that on average, using the hop metric, each
node is reached with the lowest number of hops, whereas the
ML metric chooses paths with the highest number of hops.
• ETX and ETT tend to select routes with the same number of
hops, but not necessarily the same route.
• Results are consistent with the physical distance between the
nodes and with the quality of the links between them (Fig. 3).
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Packet Loss Rate
• ure 4b shows the average packet loss rate(PLR) experienced at
each node ID for each metric.
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packet loss rate
Packet Loss Rate
• As the distance to node ID0 grows, the use of the hop metric
results in increasingly high packet-loss rates.
• This behavior is expected because the hop metric does not
consider the quality of the links and tends to forward packets
through long noisy wireless links.
• ETX and ETT metrics converged to packet loss rate in the
order of 19 percent and 30 percent, respectively, regardless of
the distance to node ID0.
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Packet Loss Rate
• The ML metric performed best among the four metrics,
because it is designed to select routes with low loss links.
• The ML metric resulted in PLR in the range of 5 percent for up
to node ID6 and around 10 percent for nodes ID7 and ID8.
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Network Delay
• During the PLR experiment, the average round-trip-time (RTT)
for packets traveling from node ID0 to each other node and
back was also collected (Fig. 4c).
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round trip-time
Network Delay
• As the distance to node ID0 grows, the use of the hop metric
results in high RTTs on the order of two seconds.
• All other metrics achieved RTTs lower than 150 ms for ETX,
75 ms for ML, and 35 ms for ETT.
• The ETT metric is the only one to estimate the transmission
time, and this feature produced the best performance in terms
of RTT.
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Throughput
• Figure 4d shows the average throughput in kb/s experienced at
each node ID for each metric.
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Throughput
Throughput
• typically ETX, ETT, and ML choose paths with a higher
number of hops when compared to the hop metric.
• For short distances, all metrics achieved high throughput with
hop leading to throughputs in the order of 5 Mb/s.
• As the distance increased, the hop metric throughputs dropped
significantly to close to zero, whereas all other metrics
exhibited similar performance resulting on throughputs in the
order of 500 kb/s.
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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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