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Collision-Free Asynchronous Multi-Channel Access in Ad Hoc Networks IEEE Globecom 2009, Hawaii

Collision-Free Asynchronous Multi-Channel Access in Ad Hoc Networks IEEE Globecom 2009, Hawaii. University of California Santa Cruz* Palo Alto Research Center^ Duy Nguyen*, J.J. Garcia-Luna-Aceves*^, and Katia Obraczka*. Motivation for Multi-Channel MAC.

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Collision-Free Asynchronous Multi-Channel Access in Ad Hoc Networks IEEE Globecom 2009, Hawaii

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  1. Collision-Free Asynchronous Multi-Channel Access in Ad Hoc NetworksIEEE Globecom 2009, Hawaii University of California Santa Cruz* Palo Alto Research Center^ Duy Nguyen*, J.J. Garcia-Luna-Aceves*^, and Katia Obraczka*

  2. Motivation for Multi-Channel MAC • 3 non-overlapping 20MHz channels available in 2.4 GHz 802.11b/g/n • 12 non-overlapping 20MHz channel available in 5Ghz of 802.11a • 9 non-overlapping 40MHz channel in 5GHz of 802.11n • Good bandwidth utilization

  3. Challenges • Hidden terminal problems • Using only a single transceiver: can only transmit or receive but not both • How to make sure all neighbors aware of the channel selection • Perception of available channel status is different among nodes: • my neighbors’ views of available channel status is different from mine (multi-hop networks)

  4. Approaches • Dedicated Control Channel (DCA[S.Wu and et]) • Dedicated control radio or channel for all control messages • Split Phase (MMAC[J.So and N. Vaidya]) • Fixed periods divided into (i) channel negotiation phase on default channel & (ii) data transfer phase on negotiated channels • Common Hopping (CHMA[A. Tzamaloukas and J.J Garcia-Luna-Aceves]) • All non-busy nodes follow a common, well-known channel hopping sequence -- the control channel changes. • Parallel Rendezvous (SSCH[P. Bahl et] and McMAC[J. So et]) • Each node publishes its own channel hopping schedule

  5. Dedicated Control Channel Rendezvous & contention occur on the control channel. Channel Data Ch3 Ack Node 1+2 Ch2 Data . . . Ack Ch1 Rts(2,3) Cts(2) Rsv(2) Rts(3) Cts(3) Rsv(3) Time Legend:Node 1Node 2Note 3Node 4 Slide courtesy of H. Wilson So

  6. Split-Phase Channel Channel negotiation on a common channel Ch2 Rts Cts Data Ack Ch1 Ch0 Hello(1,2,3) Rts Cts Data Ack Ack(1) Rsv(1) Time Hello(2,3) ... Data Transfer Phase Control Phase Legend:Node 1Node 2Note 3Node 4 Slide courtesy of H. Wilson So

  7. Common Hopping Idle nodes hop together in “common channel” Channel Ch3 Ch2 RTS (b to a) Ch1 Cts, Data, Ack Ch0 RTS (c to d) 1 2 3 4 5 6 7 8 9 10 11 Time Enough for one RTS Legend:Node aNode bNote cNode d Slide courtesy of H. Wilson So

  8. Parallel Rendezvous ? ? • Sender needs to know the home channel of the receiver Slide courtesy of H. Wilson So

  9. Parallel Rendezvous Original schedule Slide courtesy of H. Wilson So

  10. 1. Data arrives 2. RTS/ CTS/ Data 3. Hopping stopped during data transfer 4. Hopping resumes Parallel Rendezvous Original schedule Slide courtesy of H. Wilson So

  11. CSMA vs TDMA IDEAL CSMA Channel Utilization TDMA # of Contenders

  12. Yet another MAC? • Current MACs are not sufficient: • CSMA of current IEEE 802.11 MAC performance can be seriously degraded by the hidden terminal problems. • Many current multi-channel MACs rely on synchronization • Goal: To design a simple, asynchronous, and collision-free MAC with very minimal modifications to 802.11

  13. Asynchronous Multi-Channel MAC (AM-MAC or “I’m MAC!”) • Asynchronous Split Phase Approach • Allows nodes to switch to rendezvous channel immediately once arrangement is made • New and unique handshake is introduced to eliminate hidden terminal problems and guarantee collision freedom

  14. AM-MAC: Assumptions • N available orthogonal channels are of the same bandwidth. • A single transceiver, can either transmit or receive but not both. • Transmission time of RTS, CTS, ATS is • Maximum end-to-end propagation delay is • Switching delay is

  15. AM-MAC: Basic mechanisms • Borrow RTS/CTS and carrier sensing mechanism from 802.11 • Introduce ATS packet (Announce to Send) • Additional fields: • RTS: available channel list, data time • CTS: selected channel, data time • ATS: selected channel, data time

  16. Conditions for collision-free AM-MAC provides correct data channel acquisition provides that and Let be the maximum channel observation time be the maximum data transmission time AM-MAC is collision-free if

  17. RTS/CTS/ATS Based Access DIFS RTS Data on channel n Src ATS Dest CTS ATS Ack on channel n Other NAV NAV Defer Access RTS/CTS/ATS exchange continues • Duration field in RTS, CTS, ATS frames distribute Medium Reservation information which is stored in a Net Allocation Vector (NAV). • Defer on either NAV or "CCA" indicating Medium Busy.

  18. AM-MAC Summary • A sends RTS with available channels to B, assumes A had already met the observation time requirement. • B replies with a CTS with the selected data channel to A, starts a timer for CTS so that upon expiration sends ATS • On receiving CTS, A prepares to send ATS • Both A and B broadcast ATS with their intention on data channel concurrently • A begins sending data to B on selected channels

  19. C A D B F E

  20. C A D RTS B F E Node hears RTS and backs off

  21. C A D CTS B F E Node hears CTS and backs off

  22. C A D ATS B F E

  23. C RTS DATA channel 2 A D B F E

  24. C CTS DATA channel 2 A D B F E

  25. C ATS DATA channel 2 A D B F E

  26. DATA channel 1 C DATA channel 2 A D B F E

  27. Z A X ATS S R B Y Mutual Region

  28. Z A X ATS S R B Y Mutual Region

  29. X A S B R Y

  30. X A S RTS RTS B R Y RTS arrives at B in error. B must back off for

  31. X A S RTS CTS B R Y CTS arrives at X in error (X is aware of it because CTS is slightly longer than RTS). X backs off

  32. X A S CTS ATS B ATS R Y X stays on the control channel. Y later switches to the data channel and, simply, times out and returns to the control channel.

  33. Simulation Models • Simulation parameters from MMAC • Wireless LAN and Multi-hop scenarios • Channel bit-rate 3mb with CBR traffic • Transmission range approximately 250m • 3 or 4 channels where stated • Packet size of 512 bytes; Drop tail queue length 50 • 400x400, 1000x1000 topology

  34. Wireless LAN 36 nodes in 400x400 Throughput

  35. Wireless LAN 36 nodes in 400x400 Delay

  36. Wireless LAN 64 nodes in 400x400Throughput

  37. Wireless LAN 64 nodes in 400x400 Delay

  38. Multi-hop 121 nodes in 1000x1000 Throughput

  39. Multi-hop 121 nodes in 1000x1000 Delay

  40. Multi-hop 121 nodes in 1000x1000 Throughput

  41. Multi-hop 121 nodes in 1000x1000 Delay

  42. Analytical Analysis Assumptions • A finite population of N nodes • Arrival of RTS is Poisson distributed • Network is fully connected with the same number of neighbors • Successful RTS and DATA occurs as a single event • Packet length are independent and geometrically distributed

  43. Analytical Throughput • 3Mbps per-channel capacity • propagation delay = 1/1000 of packet length

  44. Conclusion • We presented AM-MAC a novel solution to multi-channel medium access for single-transceiver nodes. • AM-MAC employs a simple, yet efficient approach to collision-free data transmission over multiple channels without the need of temporal synchronization among nodes

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