1. Wireless Networking & Mobile Computing CS 752/852 - Spring 2012 Tamer Nadeem Dept. of Computer Science Lec #7: MAC Multichannel

2. Page 2 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver * Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver * (Jungmin So and Nitin Vaidya) * Slides adapted from J. So

3. Page 3 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Multiple Channels available in IEEE 802.11 3 channels in 802.11b 12 channels in 802.11a Utilizing multiple channels can improve throughput Allow simultaneous transmissions Motivation 1 defer 1 2 Single channelMultiple Channels

4. Page 4 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Problem Statement Using k channels does not translate into throughput improvement by a factor of k Nodes listening on different channels cannot talk to each other Constraint: Each node has only a single transceiver Capable of listening to one channel at a time Goal: Design a MAC protocol that utilizes multiple channels to improve overall performance Modify 802.11 DCF to work in multi-channel environment 1 2

5. Page 5 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 802.11 Power Saving Mechanism Time is divided into beacon intervals All nodes wake up at the beginning of a beacon interval for a fixed duration of time (ATIM window) Exchange ATIM (Ad-hoc Traffic Indication Message) during ATIM window Nodes that receive ATIM message stay up during for the whole beacon interval Nodes that do not receive ATIM message may go into doze mode after ATIM window

6. Basics 802.11 Power Saving Mechanism Multi-Channel Hidden Terminals

7. Page 7 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 802.11 Power Saving Mechanism A B C Time Beacon ATIM Window Beacon Interval

8. Page 8 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 802.11 Power Saving Mechanism A B C Time Beacon ATIM ATIM Window Beacon Interval

9. Page 9 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 802.11 Power Saving Mechanism A B C Time Beacon ATIM ATIM-ACK ATIM Window Beacon Interval

10. Page 10 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 802.11 Power Saving Mechanism A B C Time Beacon ATIM ATIM-ACK DATA Doze Mode ATIM Window Beacon Interval

11. Page 11 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 802.11 Power Saving Mechanism A B C Time Beacon ATIM ATIM-ACK DATA ACK Doze Mode ATIM Window Beacon Interval

12. Page 12 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Multi-Channel Hidden Terminals Consider the following naïve protocol Static channel assignment (based on node ID) Communication takes place on receiver’s channel Sender switches its channel to receiver’s channel before transmitting

13. Page 13 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Multi-Channel Hidden Terminals A B C RTS A sends RTS Channel 1 Channel 2

14. Page 14 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Multi-Channel Hidden Terminals A B C CTS B sends CTS Channel 1 Channel 2 C does not hear CTS because C is listening on channel 2

15. Page 15 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Multi-Channel Hidden Terminals A B C DATA C switches to channel 1 and transmits RTS Channel 1 Channel 2 Collision occurs at B RTS

16. Related Work Previous work on multi-channel MAC

17. Page 17 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Nasipuri’s Protocol Assumes N transceivers per host Capable of listening to all channels simultaneously Sender searches for an idle channel and transmits on the channel [Nasipuri99WCNC] Extensions: channel selection based on channel condition on the receiver side [Nasipuri00VTC] Disadvantage: High hardware cost

18. Page 18 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Wu’s Protocol [Wu00ISPAN] Assumes 2 transceivers per host One transceiver always listens on control channel Negotiate channels using RTS/CTS/RES RTS/CTS/RES packets sent on control channel Sender includes preferred channels in RTS Receiver decides a channel and includes in CTS Sender transmits RES (Reservation) Sender sends DATA on the selected data channel

19. Page 19 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Wu’s Protocol (cont.) Advantage No synchronization required Disadvantage Each host must have 2 transceivers Per-packet channel switching can be expensive Control channel bandwidth is an issue Too small: control channel becomes a bottleneck Too large: waste of bandwidth Optimal control channel bandwidth depends on traffic load, but difficult to dynamically adapt

20. Protocol Description Multi-Channel MAC (MMAC) Protocol

21. Page 21 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Proposed Protocol (MMAC) Assumptions Each node is equipped with a single transceiver The transceiver is capable of switching channels Channel switching delay is approximately 250us Per-packet switching not recommended Occasional channel switching not to expensive Multi-hop synchronization is achieved by other means

22. Page 22 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing MMAC Idea similar to IEEE 802.11 PSM Divide time into beacon intervals At the beginning of each beacon interval, all nodes must listen to a predefined common channel for a fixed duration of time (ATIM window) Nodes negotiate channels using ATIM messages Nodes switch to selected channels after ATIM window for the rest of the beacon interval

23. Page 23 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Preferred Channel List (PCL) Each node maintains PCL Records usage of channels inside the transmission range High preference (HIGH) Already selected for the current beacon interval Medium preference (MID) No other vicinity node has selected this channel Low preference (LOW) This channel has been chosen by vicinity nodes Count number of nodes that selected this channel to break ties

24. Page 24 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Channel Negotiation In ATIM window, sender transmits ATIM to the receiver Sender includes its PCL in the ATIM packet Receiver selects a channel based on sender’s PCL and its own PCL Order of preference: HIGH > MID > LOW Tie breaker: Receiver’s PCL has higher priority For “LOW” channels: channels with smaller count have higher priority Receiver sends ATIM-ACK to sender including the selected channel Sender sends ATIM-RES to notify its neighbors of the selected channel

25. Page 25 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Channel Negotiation A B C D Time ATIM Window Beacon Interval Common ChannelSelected Channel Beacon

26. Page 26 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Channel Negotiation A B C D ATIM ATIM- ACK(1) ATIM- RES(1) Time ATIM Window Beacon Interval Common ChannelSelected Channel Beacon

27. Page 27 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Channel Negotiation A B C D ATIM ATIM- ACK(1) ATIM- RES(1) ATIM- ACK(2) ATIM ATIM- RES(2) Time ATIM Window Beacon Interval Common ChannelSelected Channel Beacon

28. Page 28 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Channel Negotiation A B C D ATIM ATIM- ACK(1) ATIM- RES(1) ATIM- ACK(2) ATIM ATIM- RES(2) Time ATIM Window Beacon Interval Common ChannelSelected Channel Beacon RTS CTS RTS CTS DATA ACK DATA Channel 1 Channel 2

29. Performance Evaluation Simulation Model Simulation Results

30. Page 30 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Simulation Model ns-2 simulator Transmission rate: 2Mbps Transmission range: 250m Traffic type: Constant Bit Rate (CBR) Beacon interval: 100ms Packet size: 512 bytes ATIM window size: 20ms Default number of channels: 3 channels Compared protocols 802.11: IEEE 802.11 single channel protocol DCA: Wu’s protocol MMAC: Proposed protocol

31. Page 31 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Wireless LAN - Throughput 30 nodes64 nodes MMAC DCA 802.11 MMAC shows higher throughput than DCA and 802.11 802.11 DCA MMAC Packet arrival rate per flow (packets/sec) 1 10 100 1000 2500 2000 1500 1000 500 Aggregate Throughput (Kbps) 2500 2000 1500 1000 500

32. Page 32 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Multi-hop Network – Throughput 3 channels4 channels MMAC DCA 802.11 DCA MMAC Packet arrival rate per flow (packets/sec) 1 10 100 1000 Packet arrival rate per flow (packets/sec) 1 10 100 1000 Aggregate Throughput (Kbps) 1500 1000 500 0 2000 1500 1000 500 0

33. Page 33 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Throughput of DCA and MMAC (Wireless LAN) DCA MMAC 3 channels 802.11 MMAC shows higher throughput compared to DCA 6 channels 802.11 3 channels 6 channels Aggregate Throughput (Kbps) 4000 3000 2000 1000 0 4000 3000 2000 1000 0 Packet arrival rate per flow (packets/sec)

34. Page 34 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Analysis of Results DCA Bandwidth of control channel significantly affects performance Narrow control channel: High collision and congestion of control packets Wide control channel: Waste of bandwidth It is difficult to adapt control channel bandwidth dynamically MMAC ATIM window size significantly affects performance ATIM/ATIM-ACK/ATIM-RES exchanged once per flow per beacon interval – reduced overhead Compared to packet-by-packet control packet exchange in DCA ATIM window size can be adapted to traffic load

35. Page 35 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Partially Overlapped Channels Not Considered Harmful * Partially Overlapped Channels Not Considered Harmful * (Arunesh Mishra, Vivek Shrivastava, Suman Banerjee, William Arbaugh) * Slides adapted from Ashwin Wagadarikar, Duke

36. Page 36 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Spectral Bands and Channels Wireless communication uses electromagnetic signals over a range of frequencies FCC has split the spectrum into spectral bands Each spectral band is split into channels Example of a channel

37. Page 37 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Typical usage of spectral band Transmitter-receiver pairs use independent channels that don’t overlap to avoid interference. Fixed Block of Radio Frequency Spectrum Channel AChannel BChannel CChannel D

38. Page 38 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Ideal usuage of channel bandwidth Should use entire range of freqs spanning a channel Usage drops down to 0 just outside channel boundary Channel AChannel B Frequency Power Channel CChannel D

39. Page 39 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Realistic usage of channel bandwidth Realistically, transmitter power output is NOT uniform at all frequencies of the channel. PROBLEM: Transmitted power of some freqs. < max. permissible limit Results in lower channel capacity and inefficient usage of the spectrum Real Usage Channel AChannel B Power Channel CChannel D Wastage of spectrum

40. Page 40 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Consideration of the 802.11b standard Splits 2.4 GHz band into 11 channels of 22 MHz each Channels 1, 6 and 11 don’t overlap Can have 2 types of channel interferences: Co-channel interference Address by RTS/CTS handshakes etc. Adjacent channel interference over partially overlapping channels Cannot be handled by contention resolution techniques  Wireless networks in the past have used only non-overlapping channels

41. Page 41 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Focus of paper Paper examines approaches to use partially overlapped channels efficiently to improve spectral utilization Channel AChannel B Channel A’

42. Page 42 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Empirical proof of benefits of partial overlap Can we use channels 1, 3 and 6 without interference ? Ch 1Ch 6Ch 3 Amount of Interference Link A Ch 1 Link C Ch 6 Link B Ch 3

43. Page 43 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Empirical proof of benefits of partial overlap Typically partially overlapped channels are avoided With sufficient spatial separation, they can be used Link A Ch 1 Link C Ch 6 Link B Ch 3 Ch 1Ch 6Ch 3 Virtually non-overlapping

44. Page 44 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Empirical proof of benefits of partial overlap Link A Ch 1 Link B Ch X Partially overlapped channels can provide much greater spatial re-use if used carefully!

45. Page 45 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Interference factor To model effects of partial overlap, define: Interference Factor or “I-factor” Transmitter is on channel j P j denotes power received on channel j P i denotes power received on channel i PiPjPiPj I-factor(i,j) =

46. Page 46 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing FcAFcA-11 Mhz-22 Mhz -50 dB -30 dB FcBFcB Channel A Channel B Theoretical Estimate for I-Factor Theoretically, I-factor = Area of intersection between two spectrum masks of transmitters on channels A and B

47. Page 47 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Estimating I-Factor at a receiver on channel 6 0 0.2 0.4 0.6 0.8 1 0 2 4 6 8 10 12 Normalized I-factor Receiver Channel I(theory) I(measured)

48. Page 48 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing WLAN Case study WLAN comparison between: 3 non-overlapping channels, and 11 partially overlapping channels over the same spectral band WLAN consists of access points (APs) and clients AP communicates with clients in its basic service set on a single channel GOAL: allocate channels to AP’s to maximize performance by reducing interference

49. Page 49 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing 60 Why use partial overlap? Consider a case where you have 300 APs 100 Worst case Interference by all 100 APs on same channel Non-overlap 3 channels, 100 APs each Partial overlap 5 channels, 60 APs each Worst case Interference by all 60 APs on same channel + some interference from POV channels

50. Page 50 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Channel assignment w/ non-overlap Mishra et al. previously proposed “client-driven” approach for channel assignment to APs Use Randomized Compaction algorithm Optimization criterion: minimize the maximum interference experienced by each client 2 distinct advantages over random channel assignment: Higher throughput over channels Load balancing of clients among available APs

51. Page 51 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Channel assignment w/ non-overlap (X,C) = WLAN X = set of APs and C = set of all clients How to assign APs to these 3 channels? MUST LISTEN TO THE CLIENTS! To evaluate a given channel assignment Compute interference for each client: Sum taken over APs on same channel since channels are independent Create vector of cf c ’s (CF) and sort in non-increasing order Optimal channel assignment minimizes CF

52. Page 52 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Each client builds I-factor model using scan operation POV(x,x ch,y,y ch ) = 1 if nodes x and y on their channels interfere with each other To evaluate a given channel assignment Compute interference for each client: Sum taken over APs that interfere on own channel + all POV channels Create vector of cf c ’s (CF) and sort in non-increasing order Optimal channel assignment minimizes CF Channel assignment w/ partial overlap = +

53. Page 53 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Results for high interference topologies 28 randomly generated topologies with 200 clients and 50 APs –14 high interference topologies (average of 8 APs in range for client) –14 low interference topologies (average of 4 APs in range for client)

54. Page 54 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Results for low interference topologies Using partially overlapped channels and I-factor, clients can experience less contention at the link level.  Higher layers have better throughput

55. Page 55 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing Questions