1. High Throughput Route Selection in Multi-Rate Ad Hoc Wireless Networks
Dr. Baruch Awerbuch, David Holmer, and Herbert Rubens
Johns Hopkins University
Department of Computer Science

2. Overview Problem: Traditional Technique: New Technique: Goal:
Route selection in multi-rate ad hoc network
Traditional Technique:
Minimum Hop Path
New Technique:
Medium Time Metric (MTM)
Goal:
Maximize network throughput
Min has a number of problems in multi-rate networks.
Path throughput and path stability
Today I’m going to be motivating the need for a new routing metric
And discussing techniques for maximizing throughput in multi-rate networks.

3. What is Multi-Rate?
Ability of a wireless card to automatically operate at several different bit-rates
(e.g. 1, 2, 5.5, and 11 Mbps)
Part of many existing wireless standards
(802.11b, a, g, HiperLAN2…)
Virtually every wireless card in use today employs multi-rate

4. Advantage of Multi-Rate?
Direct relationship between communication rate and the channel quality required for that rate
As distance increases, channel quality decreases
Therefore: tradeoff between communication range and link speed
Multi-rate provides flexibility
1 Mbps
2 Mbps
5.5 Mbps
11 Mbps
Add that MAC adaptively sets rate
Add that multi-rate allows both longer range and higher speed (but not at same time)
Multi-rate used by most existing standards (802.11abg HiperLANII, etc.) and likely to continue into the future.
Very little research on the effects of multi-rate on ad hoc networks.
Lucent Orinoco b card ranges using
NS2 two-ray ground propagation model

5. Ad hoc Network Single Rate Example
Which route to select?
Destination
Source

6. Ad hoc Network Single Rate Example
Which route to select?
Source and Destination are neighbors! Just route directly.
Destination
Source

7. Multi-rate Network Example
Varied Link Rates
Destination
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps

8. Multi-rate Network Example
Varied Link Rates
Destination
Throughput = 1.04 Mbps
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps

9. Multi-rate Network Example
Varied Link Rates
Destination
Throughput = 1.15 Mbps
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps

10. Multi-rate Network Example
Varied Link Rates
Min Hop Selects Direct Link
0.85 Mbps
Destination
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps

11. Multi-rate Network Example
Varied Link Rates
Min Hop Selects Direct Link
0.85 Mbps effective
Highest Throughput Path
2.38 Mbps effective
Destination
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps

12. Multi-rate Network Example
Under Mobility
Min Hop
Path Breaks
High Throughput Path
Reduced Link Speed
Reliability Maintained
More “elastic” path
Destination X
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps

13. Challenge to the Routing Protocol
Must select a path from Source to Destination
Links operate at different speeds
Fundamental Tradeoff
Fast/Short links = low range = many hops/transmissions to get to destination
Slow/Long links = long range = few hops/transmissions

14. Minimum Hop Path (Traditional Technique)
A small number of long slow hops provide the minimum hop path
These slow transmissions occupy the medium for long times, blocking adjacent senders
Selecting nodes on the fringe of the communication range results in reduced reliability
Min hop – for any arbitrary configuration, what is the least number of hops required to reach the destination.
By selecting links which operate at the lowest speed, the MAC protocol has no flexibility in dealing with topological changes. This results in paths breaking from mobility.

15. How can we achieve high throughput?
Throughput depends on several factors
Physical configuration of the nodes
Fundamental properties of wireless communication
MAC protocol
How do we know the throughput of a path?
How can we achieve high throughput?

16. Wireless Shared Medium
Transmission blocks all nearby activity to avoid collisions
MAC protocol provides channel arbitration
Carrier Sense Range
Carrier Sense Range 1 2
The longer it takes to transmit each packet the less medium time is available to other senders in the network.
This reduces network throughput.

17. Transmission Duration
4.55 Mbps
3.17 Mbps
1.54 Mbps
0.85 Mbps
Time taken for full expected backoff, RTS, CTS, DATA, ACK exchange. (w/ 1500 byte payload)
“Data” is actual application level payload, “Overhead” is everything else.
Almost constant overhead regardless of link speed.
higher link speed = shorter transmission time
Short transmission time = high ratio of MAC overhead per packet
Gives us an idea of how much medium time a packet sent at each rate consumes
Medium Time consumed to transmit 1500 byte packet

18. Hops vs. Throughput
Since the medium is shared, adjacent transmissions compete for medium time.
Throughput decreases as number of hops increase. 1 2 3
Nodes need to alternate when sending along a path.
Since they can’t send at the same time, throughput is reduced.

19. Effect of Transmission
Source X X X X X X X
Destination 1 2 3 4 5 6 7 8
Request to Send (RTS)
Clear to Send (CTS)
DATA
ACK

20. Multi-Hop Throughput Loss (TCP)
Inverse relationship = 1, 1/2, 1/3, 1/4, 1/5, etc.

21. Analysis General Model of ad hoc network throughput
Multi-rate transmission graph
Interference graph
Flow constraints
General Throughput Maximization Solution is NP Complete
Derived an optimal solution under a full interference assumption

22. New Approach: Medium Time Metric (MTM)
Assigns a weight to each link proportional to the amount of medium time consumed by transmitting a packet on the link
Existing shortest path protocols will then discover the path that minimizes total transmission time

23. MTM Example 11 Mbps Source Destination 1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate 11
2.5ms
= 2.5
4.55 Mbps 1
13.9ms
= 13.9
0.85 Mbps

24. MTM Example 11 Mbps 11 Mbps Source Destination 1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
2.5ms
2.5ms
= 5.0
2.36 Mbps 1
13.9ms
= 13.9
0.85 Mbps

25. MTM Example 11 Mbps 11 Mbps 11 Mbps Source Destination 1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
2.5ms
2.5ms
2.5ms
= 7.5
1.57 Mbps 1
13.9ms
= 13.9
0.85 Mbps

26. MTM Example 11 Mbps Source Destination 1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
2.5ms
2.5ms
2.5ms
2.5ms
= 10.0
1.18 Mbps 1
13.9ms
= 13.9
0.85 Mbps

27. MTM Example 11 Mbps Source Destination 1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
2.5ms
2.5ms
2.5ms
2.5ms
2.5ms
= 12.5
0.94 Mbps 1
13.9ms
= 13.9
0.85 Mbps

28. MTM Example 11 Mbps Source Destination 1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
2.5ms
2.5ms
2.5ms
2.5ms
2.5ms
2.5ms
= 15
0.78 Mbps 1
13.9ms
= 13.9
0.85 Mbps

29. MTM Example Medium Time Usage Destination Source
Link Throughput
Destination
11 Mbps
2.5ms
4.55 Mbps
3.17 Mbps
5.5 Mbps
3.7ms
1.54 Mbps
2 Mbps
7.6ms
0.85 Mbps
1 Mbps
13.9ms
Source
Path Medium Time Metric (MTM)
Path Throughput
11 Mbps
5.5 Mbps 1
13.9ms
= 13.9 ms
0.85 Mbps
2 Mbps
1 Mbps

30. MTM Example Medium Time Usage Destination Source
Link Throughput
Destination
11 Mbps
2.5ms
4.55 Mbps
3.17 Mbps
5.5 Mbps
3.7ms
1.54 Mbps
2 Mbps
7.6ms
0.85 Mbps
1 Mbps
13.9ms
Source
Path Medium Time Metric (MTM)
Path Throughput
11 Mbps
3.7ms
7.6ms
= 11.3 ms
1.04 Mbps
5.5 Mbps 1
13.9ms
= 13.9 ms
0.85 Mbps
2 Mbps
1 Mbps

31. MTM Example Medium Time Usage Destination Source
Link Throughput
Destination
11 Mbps
2.5ms
4.55 Mbps
3.17 Mbps
5.5 Mbps
3.7ms
1.54 Mbps
2 Mbps
7.6ms
0.85 Mbps
1 Mbps
13.9ms
Source
Path Medium Time Metric (MTM)
Path Throughput
11 + 2
2.5ms
7.6ms
= 10.1 ms
1.15 Mbps
11 Mbps
3.7ms
7.6ms
= 11.3 ms
1.04 Mbps
5.5 Mbps 1
13.9ms
= 13.9 ms
0.85 Mbps
2 Mbps
1 Mbps

32. MTM Example Medium Time Usage Destination Source
Link Throughput
Destination
11 Mbps
2.5ms
4.55 Mbps
3.17 Mbps
5.5 Mbps
3.7ms
1.54 Mbps
2 Mbps
7.6ms
0.85 Mbps
1 Mbps
13.9ms
Source
Path Medium Time Metric (MTM)
Path Throughput
2.5ms
2.5ms
= 5.0 ms
2.38 Mbps
11 + 2
2.5ms
7.6ms
= 10.1 ms
1.15 Mbps
11 Mbps
3.7ms
7.6ms
= 11.3 ms
1.04 Mbps
5.5 Mbps 1
13.9ms
= 13.9 ms
0.85 Mbps
2 Mbps
1 Mbps

33. MTM Definition

34. Advantages It’s an additive shortest path metric
Paths which minimize network utilization, maximize network capacity
Global optimum under complete interference
Single flow optimum up to pipeline distance (7-11 hops)
Excellent heuristic in even larger networks
Avoiding low speed links inherently provides increased route stability
Can be applied to existing link state and distance vector protocols.

35. Disadvantages MTM paths require more hops More transmitting nodes
Increased contention for medium
Results in more load on MAC protocol
Only a few percent reduction under the simulated conditions
Increase in buffering along path
However, higher throughput paths have lower propagation delay

36. Sounds great but… Do faster paths actually exist?
There needs to be enough nodes between the source and the destination to provide a faster path
Therefore performance could vary as a function of node density
When density is low: MTM = Min Hop

37. Performance Increase vs. Node Density in Static Random Line

38. MTM Throughput Increase Under 802.11MAC
-NS2 Network Simulations
-20 TCP Senders and receivers
-Random Waypoint mobility (0-20m/s)
-DSDV Protocol modified to find MTM path

39. Reference
Baruch Awerbuch, David Holmer, Herbert Rubens, The Medium Time Metric: High Throughput Route Selection in Multi-rate Ad Hoc Wireless Networks, Technical Report, Johns Hopkins University, October 2004.