OAR: An Opportunistic Auto-Rate 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.

Time

1 Mbps

2 Mbps

5.5 Mbps

11 Mbps

Highest energy per bit

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?

AB 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?

AB C

A

BC

Timeshare

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

Rates in IEEE 802.11b: 2, 5.5, and 11 Mbps

Number of packets transmitted by OAR ~ Rate Base

RateTx

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

Protocol

Channel Condition

BAD MEDIUM GOOD

RTS

OAR Protocol (RBAR Based)

source

destination

ACK

Review: Receiver Based AutoRate (RBAR) [Bahl’01]

CTS DATA

Receiver controls the sender’s transmission rate

Control messages sent at Base Rate

RTS

OAR Protocol (Multi-packet)

source

destination

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

Protocol

Channel Condition

BAD MEDIUM GOOD

OAR - Opportunistic Auto Rate

CTS DATA Once access granted, it is possible to send multiple packets if the channel is good

ACK

DATA

ACK

DATA

Observation I Time spent in contention per packet by RBAR is

exactly equal to the average time per packet spent in contention for single-rate IEEE802.11

Transmitter

Receiver

OAR

Performance Comparison

IEEE 802.11

R

C A

D1Transmitter

Receiver

R

C A

D1Transmitter

Receiver

RBAR

R

C A

D2 R

C A

D3

R

C A

D1

A

D2

A

D3

Observation II The total time in contention by OAR is

approximately equal to total time spent in contention by single-rate IEEE802.11 for an experiment spanning T seconds

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 TopologySetup

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

A B

Low speed (L)

High Speed (H)

#2 Asymmetric Topology Results

OAR maintains time shares of IEEE 802.11 Significant gain over RBAR

#3 Random TopologiesSetup

A pair are moved across a communication range Nodes are uniformly distributed over area similar to

test setup #1

#3 Random TopologiesResults

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