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OAR: An Opportunistic Auto-Rate Media Access Protocol for Ad Hoc Networks

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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 – PowerPoint PPT presentation

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Title: OAR: An Opportunistic Auto-Rate Media Access Protocol for Ad Hoc Networks


1
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

2
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.

1 Mbps
2 Mbps
5.5 Mbps
11 Mbps
Time
3
Transmission Throughput
Image courtesy of G. Holland
  • 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

4
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.

5
Motivation
  • Consider the situation below
  • ARF?
  • RBAR?

6
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?

Throughput Fairness vs Temporal Fairness
7
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

8
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.

9
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_at_2Mbps 5 fast packets_at_11Mbps)
  • OAR Higher overall throughput, while maintaining
    temporal fairness properties of single rate IEEE
    802.11

10
OAR Protocol
  • Rates in IEEE 802.11b 2, 5.5, and 11 Mbps
  • Number of packets transmitted by OAR

11
OAR Protocol (RBAR Based)
  • Review Receiver Based AutoRate (RBAR) Bahl01
  • Receiver controls the senders transmission rate
  • Control messages sent at Base Rate

12
OAR Protocol (Multi-packet)
  • OAR - Opportunistic Auto Rate
  • Once access granted, it is possible to send
    multiple packets if the channel is good

13
Performance Comparison
  • IEEE 802.11

R
D1
Transmitter
C
A
Receiver
14
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

15
Simulations
  • Three Simulation experiments
  • Fully connected networks all nodes in radio
    range of each other
  • Number of Nodes, channel condition, mobility,
    node location
  • Asymmetric topology
  • Random topologies
  • Implemented OAR and RBAR in ns-2 with extension
    of Ricean fading model Punnoose et al 00

16
1 Fully Connected Setup
  • Every node can communicate with everyone
  • Each nodes traffic is at a constant rate and
    continuously backlogged
  • Channel quality is varied dynamically

17
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.

18
2 Asymmetric TopologySetup
Low speed (L)
High Speed (H)
B
A
  • 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

19
2 Asymmetric Topology Results
  • OAR maintains time shares of IEEE 802.11
  • Significant gain over RBAR

20
3 Random TopologiesSetup
  • A pair are moved across a communication range
  • Nodes are uniformly distributed over area similar
    to test setup 1

21
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.

22
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
23
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.)

24
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.

25
Discussion Issues
  • Ad Hoc Networks considerations
  • Needed more variety in the network topology.
    Fully connected isnt very interesting in Ad Hoc
    Networks
  • Data traffic patterns. I.e. short bursts of
    traffic vs continuous traffic.
  • No power considerations studied or mentioned

26
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
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