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
www.cnds.jhu.edu/archipelago
Overview
Problem:
Route selection in multi-rate ad hoc network
Traditional Technique:
Minimum Hop Path
New Technique:
Medium Time Metric (MTM)
Goal:
Maximize network throughput
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, 802.11a, 802.11g, HiperLAN2…)
Virtually every wireless card in use today
employs multi-rate
Advantage of Multi-Rate?
1 Mbps
2 Mbps
5.5 Mbps
11 Mbps
Lucent Orinoco 802.11b card ranges using
NS2 two-ray ground propagation model
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
Ad hoc Network Single Rate
Example
Destination
Source
Which route to
select?
Ad hoc Network Single Rate
Example
Destination
Source
Which route to
select?
Source and
Destination are
neighbors! Just route
directly.
Multi-rate Network Example
Varied Link Rates
Destination
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
Multi-rate Network Example
Varied Link Rates
Destination
Throughput = 1.04 Mbps
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
Multi-rate Network Example
Varied Link Rates
Destination
Throughput = 1.15 Mbps
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
Multi-rate Network Example
Destination
Varied Link Rates
Min Hop Selects
Direct Link

Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
0.85 Mbps
Multi-rate Network Example
Destination
Varied Link Rates
Min Hop Selects
Direct Link

0.85 Mbps effective
Highest Throughput
Path
Source
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps

2.38 Mbps effective
Multi-rate Network Example
Destination
Under Mobility
Min Hop

Path Breaks
High Throughput Path

Source

11 Mbps
5.5 Mbps
2 Mbps
1 Mbps

Reduced Link Speed
Reliability Maintained
More “elastic” path
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
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
How can we achieve high
throughput?
Throughput depends on several factors



Physical configuration of the nodes
Fundamental properties of wireless
communication
MAC protocol
Wireless Shared Medium
Carrier Sense Range
Carrier Sense Range
1
2
Transmission blocks
all nearby activity to
avoid collisions
MAC protocol
provides channel
arbitration
Rate (Mbps)
Transmission Duration
4.55 Mbps
11.0
MAC Overhead
Data
3.17 Mbps
5.5
2.0
1.54 Mbps
0.85 Mbps
1.0
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14
Medium Time (milliseconds)
Medium Time consumed to transmit 1500 byte packet
Hops vs. Throughput
Since the medium is
shared, adjacent
transmissions
compete for medium
time.
Throughput
decreases as number
of hops increase.
1
2
3
Effect of Transmission
Source
1
X
2
X
3
X
4
X
5
X
6
Request
Clear to
DATA
ACK
Send (RTS)
(CTS)
X
7
X
Destination
8
Throughput (Mbps)
Multi-Hop Throughput Loss (TCP)
4.0
1.0 Mbps
3.5
2.0 Mbps
5.5 Mbps
3.0
11.0 Mbps
2.5
2.0
1.5
1.0
0.5
11.0 Mbps
5.5 Mbps
2.0 Mbps
1.0 Mbps
0.0
1
2
3
4
Hops
5
6
7
8
9
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
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
MTM Example
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11
1
2.5ms
= 2.5
4.55 Mbps
13.9ms
= 13.9
0.85 Mbps
MTM Example
11 Mbps
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11 + 11
1
2.5ms 2.5ms
= 5.0
2.36 Mbps
13.9ms
= 13.9
0.85 Mbps
MTM Example
11 Mbps
11 Mbps
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11 + 11 + 11
1
2.5ms 2.5ms 2.5ms
= 7.5
1.57 Mbps
13.9ms
= 13.9
0.85 Mbps
MTM Example
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11 + 11 + 11 + 11
1
2.5ms 2.5ms 2.5ms 2.5ms
= 10.0
1.18 Mbps
13.9ms
= 13.9
0.85 Mbps
MTM Example
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11 + 11 + 11 + 11 + 11
1
2.5ms 2.5ms 2.5ms 2.5ms 2.5ms
= 12.5
0.94 Mbps
13.9ms
= 13.9
0.85 Mbps
MTM Example
11 Mbps
Source
Destination
1 Mbps
Path Medium Time Metric (MTM)
Path Throughput
Link Rate
11 + 11 + 11 + 11 + 11 + 11
1
2.5ms 2.5ms 2.5ms 2.5ms 2.5ms 2.5ms = 15
0.78 Mbps
13.9ms
0.85 Mbps
= 13.9
MTM Example
Medium Time Usage
Destination
Link Throughput
11 Mbps
2.5ms
4.55 Mbps
5.5 Mbps
3.7ms
3.17 Mbps
2 Mbps
7.6ms
1.54 Mbps
1 Mbps
13.9ms
0.85 Mbps
Source
Path Medium Time Metric (MTM)
Path Throughput
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
1
13.9ms
= 13.9 ms
0.85 Mbps
MTM Example
Medium Time Usage
Destination
Link Throughput
11 Mbps
2.5ms
4.55 Mbps
5.5 Mbps
3.7ms
3.17 Mbps
2 Mbps
7.6ms
1.54 Mbps
1 Mbps
13.9ms
0.85 Mbps
Source
Path Medium Time Metric (MTM)
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
5.5 + 2
1
3.7ms
13.9ms
7.6ms
Path Throughput
= 11.3 ms
= 13.9 ms
1.04 Mbps
0.85 Mbps
MTM Example
Medium Time Usage
Destination
Link Throughput
11 Mbps
2.5ms
4.55 Mbps
5.5 Mbps
3.7ms
3.17 Mbps
2 Mbps
7.6ms
1.54 Mbps
1 Mbps
13.9ms
0.85 Mbps
Source
Path Medium Time Metric (MTM)
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
11 + 2
2.5ms 7.6ms
5.5 + 2
3.7ms
1
13.9ms
7.6ms
Path Throughput
1.15 Mbps
= 10.1 ms
= 11.3 ms
= 13.9 ms
1.04 Mbps
0.85 Mbps
MTM Example
Medium Time Usage
Destination
Link Throughput
11 Mbps
2.5ms
4.55 Mbps
5.5 Mbps
3.7ms
3.17 Mbps
2 Mbps
7.6ms
1.54 Mbps
1 Mbps
13.9ms
0.85 Mbps
Source
Path Medium Time Metric (MTM)
11 + 11
11 Mbps
5.5 Mbps
2 Mbps
1 Mbps
2.5ms 2.5ms = 5.0 ms
11 + 2
2.5ms 7.6ms
5.5 + 2
3.7ms
1
Path Throughput
13.9ms
7.6ms
2.38 Mbps
1.15 Mbps
= 10.1 ms
= 11.3 ms
= 13.9 ms
1.04 Mbps
0.85 Mbps
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
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
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
Performance Increase vs. Node
Density in Static Random Line
Average Throughput Increase
(vs. Min Hop)
1600 m
3200 m
4800 m
6400 m
250%
200%
150%
100%
50%
0%
0
5
10
15
20
25
Density (nodes per radius)
30
35
Average Throughput Increase
MTM Throughput Increase
Under 802.11MAC
60%
50%
40%
30%
20%
10%
0%
20
30
40
50
60
70
Average Density (nodes per radius)
-NS2 Network Simulations
-20 TCP Senders and receivers
-Random Waypoint mobility (0-20m/s)
-DSDV Protocol modified to find MTM path
Average Throughput Increase
MTM + OAR Throughput Increase
over Min Hop + 802.11
200%
180%
160%
140%
120%
100%
80%
60%
40%
20%
0%
20
30
40
50
60
70
Average Density (nodes per radius)
-NS2 Network Simulations
-20 TCP Senders and receivers
-Random Waypoint mobility (0-20m/s)
-DSDV Protocol modified to find MTM path
Thank You!
Questions??
Herb Rubens
herb@cs.jhu.edu
More Information:
http://www.cnds.jhu.edu/networks/archipelago/