1. Routing Metrics for Wireless Mesh Networks
CSE 6590
Fall 2010
27 December, 2017

2. Wireless Mesh Networks
Mostly static nodes
Limited bandwidth
Ample energy supply
Possibly multi-radio/multi-channel/multi-rate

3. New Routing Metrics for WMNs
Motivation
Limited bandwidth require efficient routing
Goals
High throughput
Low end-to-end delay

4. Intra Flow Interference
Nodes on the path of the same flow compete with each other for channel bandwidth
Causes throughput to decrease sharply
Increases delay at each hop
Increases bandwidth consumption

5. Inter Flow Interference
A node which transmits also contends for bandwidth with the nodes in the neighbouring area of its path.
Leads to bandwidth starvation
Some nodes may never get to transmit

6. Routing Protocols for Mesh Networks
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7. On Demand Routing Originally designed for mobile ad hoc networks
e.g., DSR, AODV
Flood-based route discovery when source needs to communicate with destination
Good for maintaining network connectivity under frequent changes in topology
High overhead is unnecessary in networks with static nodes
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8. Table-Driven (Proactive) Routing
Proactively maintain and update routing tables
Broadcast route update messages
Periodically
Topology changes
Lower overhead than on-demand routing in static networks
Cannot cope with frequent metrics changes
Route flapping
High message overhead
Two approaches:
Source routing
Hop-by-hop routing
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9. Source Routing Example protocol: LQSR
Source nodes put entire path in packet header
Large packet headers waste network bandwidth
Does not scale
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10. Hop-by Hop Routing Distance-vector routing (slow convergence )
Link-state routing (fast convergence)
Packet only carries destination address
Small overhead
Scalable
Preferable, especially link-state routing
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11. Least Cost Path Routing
Routing protocols route packets along minimum weight paths
Performance of minimum weight paths impact the performance of routing protocols
Characteristics of path
Path length
Link packet loss ratio
Link capacity
Intra-flow interference
Inter-flow interference
Capture as many characteristics as possible
Note: In multi-channel multi-radio networks, channel assignment and routing must work together for optimal performance.
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12. Routing Metrics for WMNs
Hop Count
Expected Transmission Count (ETX)
Expected Transmission Time (ETT)
Weighted Cumulative ETT (WCETT)
Metric of Interference and Channel Switching (MIC)
The metrics evolved, each incorporating features of the previous ones
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13. ETX
Expected number of transmissions required for successfully receiving a packet over that link.
ETX = 1 / (Pf . Pr)
Pf : packet delivery ratio in forward direction
Pr : packet delivery ratio in backward direction
To get Pf and Pr : sending one probe packet per second.
ETX is an additive metric
Path cost = sum of link costs on that path

14. ETT Expected transmission time ETT = ETX x (S / B)
S: average packet size
B: data rate

15. WCETT Weighted cumulative expected transmission time
Addresses the issue of channel reuse along a path

16. WCETT (2)

17. WCETT Example

18. Loop Free Routing - Isotonicity
Definition
The order of the weights of two paths must be preserved when we append or prefix a common third path on the two paths
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19. MIC
Metric of Interface and Channel switching
Improves upon WCETT

20. MIC (2)

21. MIC (3) IRU (Interference-aware Resource Usage)
The aggregated channel time of all the neighbouring nodes (include end points of link l) consumed by the transmission on link l
Captures path length, link capacity, loss ratio and inter-flow interference
CSC (Channel Switching Cost)
Captures intra-flow interference

22. Routing Metrics for WMNs
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23. Performance Evaluation Single Channel
Compare MIC, ETT and hop count
Simulation parameters
One radio per node
All radios configured to the same channel
1000m x 1000m, 100 nodes, 20 flows
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24. Single Channel ─ Results
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25. Performance Evaluation Multiple Channels
Compare MIC, ETT, WCETT and hop count
Simulation parameters
2 radios per node
Each can be configured to 1 of 3 channels
1000m x 1000m, 100 nodes, 20 flows
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26. Multiple Channels ─ Results
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27. References
“Wireless Mesh Networking” book, section 1.8.