Opportunistic media access for multirate ad hoc networks
Download
1 / 15

Opportunistic Media Access for Multirate Ad Hoc Networks - PowerPoint PPT Presentation


  • 71 Views
  • Uploaded on

Opportunistic Media Access for Multirate Ad Hoc Networks. B.Sadegahi, V.Kanodia, A.Sabharwal and E.Knightly Presented by Matti Raustia. Contents. WLAN Rate Adaptation ARF RBAR OAR Simulation Results Conclusion. 802.11 WLAN. Specified by IEEE 802.11b (1999) 2.4 GHz DSSS

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Opportunistic Media Access for Multirate Ad Hoc Networks' - yitta


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Opportunistic media access for multirate ad hoc networks

Opportunistic Media Access for Multirate Ad Hoc Networks

B.Sadegahi, V.Kanodia, A.Sabharwal and E.Knightly

Presented by

Matti Raustia


Contents
Contents

  • WLAN

  • Rate Adaptation

  • ARF

  • RBAR

  • OAR

  • Simulation Results

  • Conclusion


802 11 wlan
802.11 WLAN

  • Specified by IEEE

  • 802.11b (1999)

    • 2.4 GHz DSSS

    • Multirate: 1, 2, 5.5, 11 Mbit/s

    • Proprietary modes up to 22 Mbit/s

  • 802.11a

    • 5 GHz OFDM

    • Multirate: 6, 12, 24, 36, 48, 54 Mbit/s

  • 802.11g

    • 2.4 GHz OFDM, 2.4GHz DSSS

    • Compatible with 802.11b devices

    • Multirate: 802.11b data rates + OFDM data rates up to 54 Mbit/s


Why is rate adaptation needed
Why is Rate Adaptation Needed?

  • According to IEEE standards we can scale the data rate according to channel conditions

  • Radio channel conditions are changing if TX, RX or something between them is moving

    • this causes that maximum achievable data rate in the channel is changing also

    • if we can use higher data rate when channel is good we can increase the throughput

      • Higher data rates overall

      • More traffic or users

  • We must have some way to control the data rate adaptation


How to implement it
How to Implement It?

  • First we have to have some knowledge about the channel so we can adapt the data rate

    • 802.11 uses time division (TDMA/TDD) and radio channel is reciprocal

    • Physical layer (PHY) of destination or sender can measure the channel and inform MAC


Auto rate fallback arf
Auto Rate Fallback (ARF)

  • ARF is the first commercial implementation that exploits multi-rate capability

  • Senders use history of previous transmission error rates to select future transmission rates

    • If no errors, increase the data rate

    • If errors, decrease the data rate

  • Achieves performance gain over plain 802.11


Receiver based auto rate rbar
Receiver Based Auto Rate (RBAR)

  • Idea: Why not let the receiver decide which data rate to use? He knows the channel much better than sender because of RTS message

    • Well, due the CTS message the sender knows the channel also but then sender would have to signal the data rate used to the receiver

  • Physical Layer (PHY) of the receiver analysis the channel conditions from RTS message and inserts orders of data rate to the CTS message

    • Other nodes hear the CTS also and can adapt their NAV accordingly

  • Sender adapts the data rate

  • Performance gain over IEEE 802.11 and ARF


802 11 with rbar
802.11 with RBAR

DIFS = Distributed InterFrame Spacing

SIFS = Short InterFrame Spacing

RTS = Ready to Send

CTS = Clear to Send

ACK = Acknowledge

RSH = Reservation Sub-

Header

NAV = Network Allocation

Vector


Opportunistic auto rate
Opportunistic Auto Rate

  • Source sends Ready to Send (RTS) message at base date rate

  • Receiver’s PHY measures channel condition from it

  • Receiver sends Clear to Send (CTS) message to sender with orders to use specific data rate

  • Source sends as many packets that can be sent in base rate time window (for example 11 Mbit/s / 2 Mbit/s = 5 packets)

  • Time window is the same all the time regardless of channel conditions

    • Fairness


Fragmentation
Fragmentation

  • Long packets can be fragmented according 802.11

  • What if channel changes during transmission of long packet?

    • OAR receiver monitors channel conditions during transmission and if channel quality decreases, receiver can inform sender with additional RSH message to decrease data rate

  • What if sender doesn’t have enough packets in queue?

    • In this case OAR sender can revert to RBAR without throughput loss


Rbar and oar a comparison
RBAR and OAR: A Comparison

  • RBAR causes channel contention after each packet

  • It seems like the Node 2 doensn’t get any channel access at all...



Simulation results1
Simulation results

  • OAR and RBAR have performance nearly independent from the velocity

    • Slow velocity -> large channel coherence times...


Conclusion
Conclusion

  • Nodes with good channel conditions are allowed to transmit multiple packets

  • OAR ensures that all the nodes get equal time share

    • fair

  • OAR obtains throughput gains up to 50 % if compared to RBAR

  • OAR method is simple


References
References

  • B.Sadegahi, V.Kanodia, A.Sabharwal and E.Knightly: Opportunistic Media Access for Multirate Ad Hoc Networks. MOBICOM’02


ad