oar: an opportunistic auto-rate media access protocol for ad hoc networks
DESCRIPTION
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. Highest energy per bit. Lowest energy per bit. 802.11b – Transmission rates. Different modulation methods for transmitting data. - PowerPoint PPT PresentationTRANSCRIPT
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