OAR: An Opportunistic AutoRate Media Access Protocol for
Ad Hoc Networks
B. Sadeghi, V. Kanodia, A.
Sabharwal, E. Knightly
Presented by Sarwar A. Sha
802.11b – Transmission rates



Different modulation
methods for transmitting
data.
– Binary/Quadrature Phase
Shift Keying
– Quadrature Amplitude
Modulation
Each packs different
quantities of data into the
modulation.
The highest speed has most
dense data and is most
vulnerable to noise.
Highest energy
per bit
1 Mbps
2 Mbps
5.5 Mbps
11 Mbps
Time
Lowest energy
per bit
Transmission Throughput


Why would a node
ever want to slow
down?
– Longer
transmission
distance
– More robust
modulation
– Moving node
rapidly changes
channel conditions
Must adapt to
channel conditions
based on SNR
Image courtesy of G. Holland
Background
IEEE 802.11 multi-rate
 Support of higher transmission rates in better
channel conditions
 Auto Rate Fallback(ARF)
– Use history of previous transmissions to adaptively
select future rates
– Error free transmissions indicates high channel quality
– Lucent ARF implemention reduces rate after 2 lost
ACKs, then attempts to speed up after a time interval

Receiver Based Auto Rate (RBAR)
– Use RTS/CTS to communicate a transmission rate
based on channel quality. Receiver determines rate.
Motivation

Consider the situation below
– ARF?
– RBAR?
B
A
C
Motivation


What if A and B are both at
56Mbps, and C is often at
2Mbps?
Slowest node gets the most
absolute time on channel?
B
Timeshare
A
B
C
C
A
Throughput Fairness vs Temporal Fairness
Opportunistic Scheduling
Goal
 Exploit short-time-scale channel quality
variations to increase throughput.
Issue
 Maintaining temporal fairness (time share) of
each node.
Challenge
 Channel info available only upon transmission
Coherence Interval



The time duration over which a channel is
statistically likely to remain stable.
This interval ranges from (122ms) - (5ms) based
on node motion at speeds of (1 m/s) - (20 m/s).
OAR was designed such that transmissions do not
exceed the coherence interval “most” of the time.
Coherence Interval
OAR Transmission
Opportunistic Auto Rate (OAR)




Poor connections transmit one data packet per RTS/CTS
connection.
Good connections, hence faster rate, transmit multiple data
packets.
But maintain temporal fairness between good & bad
connections by balancing the time using channel, not the
number of packets.
– i.e. (1 packet@2Mbps ~= 5 fast packets@11Mbps)
OAR: Higher overall throughput, while maintaining temporal
fairness properties of single rate IEEE 802.11
OAR Protocol
Channel Condition
Protocol
BAD
MEDIUM
GOOD
Pkts
Rate
Pkts
Rate
Pkts
Rate
802.11
1
2
1
2
1
2
802.11b
1
2
1
5.5
1
11
OAR
1
2
3
5.5
5
11

Rates in IEEE 802.11b: 2, 5.5, and 11 Mbps

Tx Rate
Number of packets transmitted by OAR ~
Base Rate
OAR Protocol (RBAR Based)
Review: Receiver Based AutoRate (RBAR)
[Bahl’01]


Receiver controls the
sender’s transmission
rate
Control messages sent
at Base Rate
ACK
DATA
CTS
RTS
destination
source
OAR Protocol (Multi-packet)
Channel Condition
Protocol
BAD
MEDIUM
GOOD
ACK
Pkts
Rate
Pkts
Rate
Pkts
Rate
802.11
1
2
1
2
1
2
802.11b
1
2
1
5.5
1
11
OAR
1
2
3
5.5
5
11
DATA
ACK
OAR - Opportunistic Auto Rate
DATA
ACK
DATA
CTS

Once access granted, it
is possible to send
multiple packets if the
channel is good
RTS
destination
source
Performance Comparison
IEEE 802.11
Transmitter
Receiver
R
D1
C
A
Observation II
RBAR
The total
Transmitter
R
time
by OARRis
D1 in contention
R
D2
D3
approximately equal to total time spent in
Receiver
C by single-rate
A
C IEEE802.11
A
Cfor an
contention
experiment spanning T seconds
A
OAR
Observation I
spent
in
by RBAR is
R
D1contention
D2 per packet
D3
exactly equal to the average time per packet spent
Receiver
C
A IEEE802.11
A
in contention
for A
single-rate
Transmitter
Time
MAC Access Delay Simulation



Back to back packets in
OAR decrease the average
access delay
Increase variance in time
to access channel
Figure
– On the left is 2Mbps
– On the right is 5.5 Mbps
Simulations

Three Simulation experiments
1. Fully connected networks: all nodes in radio
range of each other

Number of Nodes, channel condition, mobility, node
location
2. Asymmetric topology
3. Random topologies

Implemented OAR and RBAR in ns-2 with
extension of Ricean fading model [Punnoose et al
‘00]
#1 Fully Connected
Setup



Every node can communicate with everyone
Each node’s traffic is at a constant rate and
continuously backlogged
Channel quality is varied dynamically
#1 Fully Connected
Throughput Results




OAR has 42% to 56% gain over RBAR
Increase in gain as number of flows increases
Note that both RBAR and OAR are significantly better than
standard 802.11 (230% and 398% respectively)
Variation in line of sight (K), mobility, and location
distribution throughput all showed improvements with OAR.
#2 Asymmetric Topology
Setup
Low speed (L)
High Speed (H)
A


B
Asymmetric topology simulated above in 4 different
combinations of channel conditions
– A and B are simulated at slow (2Mbps) and fast (11Mbps)
– Each combination of slow/fast i.e. LL, HL, LH, HH compared
between A & B concurrently communicating
Sender of Flow B hears A and knows when to contend for
channel, but sender in A has to discover a time slot
#2 Asymmetric Topology
Results


OAR maintains time shares of IEEE 802.11
Significant gain over RBAR
#3 Random Topologies
Setup


A pair are moved across a communication range
Nodes are uniformly distributed over area similar to
test setup #1
#3 Random Topologies
Results


Gains are similar as before despite changes
Throughput is 40-50% improved as compared to
RBAR despite motion of a node pair.
Integration with IEEE 802.11

Options to hold the channel and send multiple
packets
– Fragmentation*



A mechanism in IEEE 802.11 to send multiple frames
Each frame/ACK acts as virtual RTS/CTS
Use of more-fragment-flag in Data packets
– Contention window set to zero
– Packet bursting (802.11e)

Transmit as many frames as you like up to threshold
*Method used in study
Discussion Issues

Not enough packets to fill a slot
– If running at “Good” 11Mbps with 5 packets
allowed, but only have 2 packets to send. Then
other nodes NAV tables are wrong (silent for 5
instead of 2).


Authors Fix: “More Fragments” indicator in the data
packet. Upon hearing, nodes revert to RBAR.
Problem: Hidden terminals would still have incorrect
NAV tables, and would remain silent longer than needed.
(Unless the data ACK has a “More Fragments ACK.”)
Discussion Issues

Channel condition changes during multi-packet
transmission.
– Channel gets worse

Later packets get corrupted
– Channel gets better

Wasted channel capacity waiting for packets to finish
– Authors propose adding RSH messages to notify
receiver of these updates and adapt the rate.

The RSH is in the header of the data packet, and would
allow changing speed mid transmission.
Discussion Issues

Ad Hoc Networks considerations
– Needed more variety in the network topology.
Fully connected isn’t very interesting in Ad Hoc
Networks
– Data traffic patterns. I.e. short bursts of traffic
vs continuous traffic.
– No power considerations studied or mentioned
Discussion Issues

Increase variance in time to access channel
– Real-time traffic (like voice) is impacted.
Sometimes there would be more delay before
you hear “something.”
– Short term fairness gets worse!
– Trade throughput for a higher worst case time to
access channel