1 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*

2 Motivation for Multi-Channel MAC 3 non-overlapping 20MHz channels available in 2.4 GHz b/g/n 12 non-overlapping 20MHz channel available in 5Ghz of a 9 non-overlapping 40MHz channel in 5GHz of n 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 Ch3 Ch2 Ch1 Time Channel Rts (2,3) Cts (2) Rsv (2) Rts (3) Cts (3) Rsv (3) Data... Ack Data Ack Rendezvous & contention occur on the control channel. Legend: Node 1 Node 2 Note 3 Node 4 Node 1+2 Slide courtesy of H. Wilson So

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

7 Common Hopping Ch2 Ch1 Ch0 Time Channel Idle nodes hop together in “ common channel ” Ch Cts, Data, Ack Enough for one RTS RTS (c to d) Legend: Node a Node b Note c Node d RTS (b to a) Slide courtesy of H. Wilson So

8 Parallel Rendezvous t= Ch 1 Ch 2 Ch 3 Ch 4 Sender needs to know the home channel of the receiver ?? Slide courtesy of H. Wilson So

9 Parallel Rendezvous t= Ch1 Ch2 Original schedule Slide courtesy of H. Wilson So

10 Parallel Rendezvous t= Ch1 Ch2 1. Data arrives 4. Hopping resumes 3. Hopping stopped during data transfer 2. RTS/ CTS/ Data Original schedule Slide courtesy of H. Wilson So

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

12 Yet another MAC? Current MACs are not sufficient: –CSMA of current IEEE 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

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

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 B C E AD F

20 B C E AD F RTS Node hears RTS and backs off

21 B C E AD F CTS Node hears CTS and backs off

22 B C E AD F ATS

23 B C E AD F DATA channel 2 RTS

24 B C E AD F DATA channel 2 CTS

25 B C E AD F DATA channel 2 ATS

26 B C E AD F DATA channel 2 DATA channel 1

27 R B S A ATS Mutual Region Z X Y

28 R B S A ATS Mutual Region Z X Y

29 B X SA YR

30 B X SA YR RTS arrives at B in error. B must back off for RTS

31 B X SA YR CTS arrives at X in error (X is aware of it because CTS is slightly longer than RTS). X backs off CTS RTS

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

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 x400, 1000x1000 topology

34 Wireless LAN 36 nodes in 400x400 Throughput

35 Wireless LAN 36 nodes in 400x400 Delay

36 Wireless LAN 64 nodes in 400x400 Throughput

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