1. Routing in Multi-Radio, Multi-Hop Wireless Mesh Networks
CS598JH
Routing in Multi-Radio, Multi-Hop Wireless Mesh Networks
Richard Draves, Jitendra Padhye,
and Brian Zill
Microsoft Research
Presented by Zahid Anwar

2. Overview Core Idea and Contributions of Paper
Design of the framework and WCETT metric
Experimental Results
Cons

3. Background Shortest-path is not always the best path
Link characteristic is not ON-OFF
ETX metric for a link = expected number of retransmissions on that link
Successful packet transmission = DATA + ACK
Measure packet loss rate pf, pr using broadcast packets
Then,
ETX = 1/[1 – (1- pf).(1-pr)]
ETX metric for a path = sum of ETX metrics for links along the path
lossy link
good link

4. Core Idea ETX and multiple radios
ETX does not consider bandwidth while selecting paths, so it will choose b over a if the loss rates are the same (longer range b links).
ETX does not give any preference to channel-diverse paths (more on this later)
Idea: if we use multiple radios at every node,
node can simultaneously transmit and receive
node can simultaneously transmit on multiple channels
“self-interference” in routes can be reduced
can improve robustness to channel variation, noise by using different parts of the spectrum (802.11a/b)

5. Contributions New routing metric for multi-radio mesh networks
Weighted Cumulative Expected Transmission Time (WCETT)
Implementation of the metric in a link-state routing protocol called Multi-Radio Link-Quality source routing (MR-LQSR)
Evaluation on a 24-node, multi-radio mesh testbed
WCETT makes judicious use of two radios
Over 250% better than HOP
Over 80% better than ETX
Gains more prominent over shorter paths and in lightly-loaded scenarios

6. Multi-Hop Networks with Single Radio
With a single radio, a node can not transmit and receive simultaneously.

7. Multi-Hop Networks with Multiple Radios
With two radios tuned to non-interfering channels, a node can transmit and receive simultaneously.
Increased robustness due to frequency diversity
e.g. 2.4GHz (802.11b) and 5GHz (802.11a) have different fading characteristics
Possible tradeoff between range and data rate
Advantages of multiple radios for improving capacity are
+ allows simultaneous transmit and receive (otherwise with only one radio the capacity of relay nodes is halved)
+ the network can utilize more of the radio spectrum
+ radios operate on different freq bands have different bandwidth, range and fading characteristics
+ radios are off the shelf commodity parts with diminishing prices

8. Design of Routing Metric
Model
Nodes have one or more radios tuned to non-interfering channels
No power constraints
Little or no node mobility (Relatively stable links)
Design goals for a new path metric
Takes bandwidth and loss rates on each link into account
Adding a link to the path does not decrease the path metric
Explicitly accounts for reduction in throughput due to interference among links that operate on the same channel

9. Implementation Framework
Implemented in a source-routed, link-state protocol
Multi-Radio Link Quality Source Routing (MR-LQSR)
Node discovers links to its neighbors; Measure quality of those links
Link information floods through the network
Each node has “full knowledge” of the topology
Sender selects “best path”
Packets are source routed using this path

10. Components of a Routing Metric
Link Metric: Assign a weight to each link
Path Metric: Combine metrics of links on path
ETX: Prefer low-loss links
WCETT: Prefer high-bandwidth, low-loss links
HOP: Each link has weight 1
WCETT: Prefer short, channel-diverse paths
HOP: Path Metric = Sum of Link Metrics
ETX: Prefer short, low-loss paths

11. Link Metric: Expected Transmission Time (ETT)
Link loss rate = p
Expected number of transmissions
Packet size = S, Link bandwidth = B
Each transmission lasts for S/B
Lower ETT implies better link
ETX formula bigger, no highlight

12. ETT: Illustration ETT : 0.77 ms ETT : 0.89 ms ETT : 0.77 ms
11 Mbps
5% loss
18 Mbps
50% loss
18 Mbps
10% loss
1000 Byte Packet
ETT : 0.77 ms
ETT : 0.89 ms
1000 Byte Packet
ETT : 0.77 ms
ETT : 0.40ms

13. Combining Link Metric into Path Metric Proposal 1
Add ETTs of all links on the path
Use the sum as path metric
SETT = Sum of ETTs of links on path
(Lower SETT implies better path)
Pro: Favors short paths
Con: Does not favor channel diversity

14. SETT does not favor channel diversity
6 Mbps
No Loss
6 Mbps
No Loss
1.33ms
1.33ms
1.33ms
1.33ms
6 Mbps
No Loss
6 Mbps
No Loss
Path
Throughput
SETT
Red-Blue
6 Mbps
2.66 ms
Red-Red
3 Mbps
2.66 ms

15. Impact of Interference
Interference reduces throughput
Throughput of a path is lower if many links are on the same channel
Path metric should be worse for non-diverse paths
Assumption: All links that are on the same channel interfere with one another
Pessimistic for long paths

16. Combining Link Metric into Path Metric Proposal 2
Group links on a path according to channel
Links on same channel interfere
Add ETTs of links in each group
Find the group with largest sum.
This is the “bottleneck” group
Too many links, or links with high ETT (“poor quality” links)
Use this largest sum as the path metric
Lower value implies better path
“Bottleneck Group ETT” (BG-ETT)

17. BG-ETT Example BG-ETT favors high-throughput, channel-diverse paths.
6 Mbps
6 Mbps
6 Mbps
6 Mbps
6 Mbps
6 Mbps
1.33 ms
1.33 ms
1.33 ms
1.33 ms
1.33 ms
1.33 ms
Path
Throughput
Blue Sum
Red Sum
BG-ETT
All Red
1.5 Mbps
5.33 ms
1 Blue
2 Mbps
1.33 ms
4 ms
Red-Blue
3 Mbps
2.66 ms
BG-ETT favors high-throughput, channel-diverse paths.

18. BG-ETT does not favor short paths
6 Mbps
6 Mbps D
6 Mbps
1.33 ms
1.33 ms
1.33 ms
6 Mbps
1.33 ms
4 ms
2 Mbps S D
Path
Throughput
Blue Sum
Red Sum
BG-ETT
3-Hop
2 Mbps
4 ms
4-Hop
2 Mbps
4 ms

19. Path Metric: Putting it all together
SETT favors short paths
BG-ETT favors channel diverse paths
Weighted Cumulative ETT (WCETT)
WCETT = (1-β) * SETT + β * BG-ETT
β is a tunable parameter
Higher value: More preference to channel diversity
Lower value: More preference to shorter paths

20. How to measure loss rate and bandwidth?
Loss rate measured using broadcast probes
Similar to ETX
Updated every second
Bandwidth estimated using periodic packet-pairs
Updated every 5 minutes

21. Mesh Testbed 23 nodes running Windows XP.
Two a/b/g cards per node: Proxim and NetGear (Autorate)
Diameter: 6-7 hops.

22. Experiment Setting
2-Minute TCP transfer between 100 randomly selected node pairs (Out of 23x22 = 506)
Only one transfer active at a time
Performance metric:
Median throughput of 100 transfers
Baseline (Single Radio)
NetGear on a (Channel 36), Proxim OFF
Two Radio
NetGear on a (Chan 36), Proxim on g (Chan 10)
(802.11g radios have longer range, lower bandwidth)

23. Median Throughput (Baseline, single radio)
Increase in performance is a result of the fact that wcett takes link bandwidth into account
This sometimes leads it to select longer paths than ETX, however these longer path result in better throughput
WCETT provides performance gain even with one radio.

24. Median Throughput (Two radios)
Outperformed ETX because it doesn’t account for bandwidth or channel diversity
WCETT takes better advantage of the additional capacity provided by the 2nd radio
Shortest –path simply selects g which have lower available bandwidth than a
WCETT makes judicious use of 2nd radio: 86% gain over baseline
Performance of HOP worsens with 2nd radio!
ETX can not take full advantage of 2nd radio

25. Do all paths benefit equally with WCETT?
Group connections by their path length, and plot the improvement in their media throughput, when compared to their throughput in the single radio baseline case
Does not provide any improvement for single-hop connections because it does not strip packets over multiple links between neighbouring nodes.
For multi-hop connections, the performance improvement drops with increase in path length
Surprising because benefits of channel diversity should be more evident on longer paths
Authors attribute this to poor performance of TCP
WCETT gains are more prominent for shorter paths

26. Channel diversity is important; especially for shorter paths
Impact of β value
WCETT metric is a weighted average of 2 quantities: 1st the sum of ETTs of all hops along a path and the 2nd the sum of ETTs on the bottleneck channel.
Metric selects paths with less channel-diversity when B is low
On paths of length 4 or more, channel diversity does not provide significant benefit
Channel diversity is important; especially for shorter paths

27. Performance of Two Simultaneous Flows
2-Minute TCP transfer between 100 randomly selected node pairs
Two transfers active at a time
Two radios: Netgear: 36-a, Proxim: 10-g
Performance metric: 2 x Median throughput
Repeat for ETX and WCETT (β = 0, 0.1, 0.5, 0.9)

28. Two simultaneous flows
β = 0: No weight to diversity
β = 0.9: High weight to diversity
WCETT performs better than ETX for all values of B in this scenerio
At high load levels, the total network throughput is maximized by using lower values of B
Throughput better for lower values of β
WCETT Performs better than ETX
Channel Diversity is important

29. Cons Captures intra-flow interference but what about inter-flow?
Unrealistic Test bed: didn’t consider virtual carrier sensing, > 2 radios scenarios and > 2 flows
Authors suggest WCETT might not work with other other routing protocols like AODV
How to select β ?
Maybe automatic selection of based on load levels

30. Cons continued WCETT is not isotonic like shortest path and ETX
Doesn’t work well with algorithms like Bellman-Ford and Dijkstra's
Requires algorithms of exponential complexity
Incorrectly chooses
S1  S2  C  D  T
over S1 B  T
Correctly chooses
S2  S1  B  T
Causes forwarding holes in
link state routing protocols

31. For more information http://research.microsoft.com/mesh/
Source code, binaries, tech reports, …

32. Backup Slides

33. Impact of Interference
Intuition 1
Impact of interference can be captured by adding ETTs of interfering links
6 Mbps, No Loss
6 Mbps, No Loss
1.33 ms
1.33 ms
Path throughput: 3 Mbps Sum of ETTs: 2.66 ms

34. Why does ETX not do well?
ETX does not take bandwidth and channel diversity into account