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Wireless Networking & Mobile Computing CS 752/852 - Spring 2012. Lec #7: MAC Multichannel . Tamer Nadeem Dept. of Computer Science. Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver * ( Jungmin So and Nitin Vaidya ).

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Wireless Networking & Mobile Computing CS 752/852 - Spring 2012


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    1. Wireless Networking & Mobile ComputingCS 752/852 - Spring 2012 Lec #7: MACMultichannel Tamer Nadeem • Dept. of Computer Science

    2. Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver *(Jungmin So and NitinVaidya) * Slides adapted from J. So

    3. 1 1 2 defer Motivation • 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 Single channel Multiple Channels

    4. 1 2 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

    5. 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. 802.11 Power Saving Mechanism Beacon Time A B C ATIM Window Beacon Interval

    8. 802.11 Power Saving Mechanism Beacon Time ATIM A B C ATIM Window Beacon Interval

    9. 802.11 Power Saving Mechanism Beacon Time ATIM A B ATIM-ACK C ATIM Window Beacon Interval

    10. 802.11 Power Saving Mechanism Beacon Time ATIM DATA A B ATIM-ACK Doze Mode C ATIM Window Beacon Interval

    11. 802.11 Power Saving Mechanism Beacon Time ATIM DATA A B ATIM-ACK ACK Doze Mode C ATIM Window Beacon Interval

    12. 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. A C B Multi-Channel Hidden Terminals Channel 1 Channel 2 RTS A sends RTS

    14. A C B Multi-Channel Hidden Terminals Channel 1 Channel 2 CTS B sends CTS C does not hear CTS because C is listening on channel 2

    15. A B Multi-Channel Hidden Terminals Channel 1 Channel 2 DATA RTS C C switches to channel 1 and transmits RTS Collision occurs at B

    16. Related Work Previous work on multi-channel MAC

    17. 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. 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. 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. 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. 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. 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. 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. Channel Negotiation Common Channel Selected Channel A Beacon B C D Time ATIM Window Beacon Interval

    26. Channel Negotiation Common Channel Selected Channel ATIM- RES(1) ATIM A Beacon B ATIM- ACK(1) C D Time ATIM Window Beacon Interval

    27. Channel Negotiation Common Channel Selected Channel ATIM- RES(1) ATIM A Beacon B ATIM- ACK(1) ATIM- ACK(2) C D ATIM Time ATIM- RES(2) ATIM Window Beacon Interval

    28. Channel Negotiation Common Channel Selected Channel ATIM- RES(1) RTS DATA Channel 1 ATIM A Beacon Channel 1 B CTS ACK ATIM- ACK(1) ATIM- ACK(2) CTS ACK Channel 2 C Channel 2 D ATIM DATA RTS Time ATIM- RES(2) ATIM Window Beacon Interval

    29. Performance Evaluation Simulation Model Simulation Results

    30. 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. Wireless LAN - Throughput 2500 2000 1500 1000 500 2500 2000 1500 1000 500 MMAC MMAC DCA DCA Aggregate Throughput (Kbps) 802.11 802.11 1 10 100 1000 1 10 100 1000 Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec) 30 nodes 64 nodes MMAC shows higher throughput than DCA and 802.11

    32. Multi-hop Network – Throughput 2000 1500 1000 500 0 1500 1000 500 0 MMAC MMAC DCA DCA Aggregate Throughput (Kbps) 802.11 802.11 1 10 100 1000 1 10 100 1000 Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec) 3 channels 4 channels

    33. Throughput of DCA and MMAC(Wireless LAN) 4000 3000 2000 1000 0 4000 3000 2000 1000 0 6 channels 6 channels 3 channels Aggregate Throughput (Kbps) 3 channels 802.11 802.11 Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec) MMAC DCA MMAC shows higher throughput compared to DCA

    34. 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. Partially Overlapped Channels Not Considered Harmful *(Arunesh Mishra, VivekShrivastava, SumanBanerjee, William Arbaugh) * Slides adapted from AshwinWagadarikar, Duke

    36. 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. Typical usage of spectral band • Transmitter-receiver pairs use independent channels that don’t overlap to avoid interference. Channel A Channel B Channel C Channel D Fixed Block of Radio Frequency Spectrum

    38. Channel A Channel B Channel C Channel D Power Frequency Ideal usuage of channel bandwidth • Should use entire range of freqs spanning a channel • Usage drops down to 0 just outside channel boundary

    39. Wastage of spectrum 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 Channel A Channel B Channel C Channel D Power Real Usage

    40. 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. Channel A Channel B Channel A’ Focus of paper • Paper examines approaches to use partially overlapped channels efficiently to improve spectral utilization

    42. Link A Ch 1 Link B Ch 3 Link C Ch 6 Amount of Interference Empirical proof of benefits of partial overlap Ch 1 Ch 3 Ch 6 • Can we use channels 1, 3 and 6 without interference ?

    43. Link A Ch 1 Link B Ch 3 Link C Ch 6 Virtually non-overlapping Empirical proof of benefits of partial overlap • Typically partially overlapped channels are avoided • With sufficient spatial separation, they can be used Ch 1 Ch 3 Ch 6

    44. 6 5 UDP Throughput (Mbps) Link A Ch 1 4 Link B Ch X 3 0 10 20 30 40 50 60 Distance between the 2 links (meters) LEGEND Non-overlapping channels, A = 1, B = 6 5 Partially Overlapped Channels, A = 1, B = 3 2 Partially Overlapped Channels, A = 1, B = 2 1 0 Same channel, A = 1, B = 1 Channel Separation Empirical proof of benefits of partial overlap • Partially overlapped channels can provide much greater spatial re-use if used carefully!

    45. I-factor(i,j) = Pi Pj Interference factor • To model effects of partial overlap, define: • Interference Factor or “I-factor” • Transmitter is on channel j • Pj denotes power received on channel j • Pi denotes power received on channel i

    46. Channel B Channel A -30 dB -50 dB -22 Mhz -11 Mhz FcA FcB Theoretical Estimate for I-Factor • Theoretically, I-factor = Area of intersection between two spectrum masks of transmitters on channels A and B

    47. Estimating I-Factor at a receiver on channel 6 1 I(theory) 0.8 I(measured) 0.6 Normalized I-factor 0.4 0.2 0 0 2 4 6 8 10 12 Receiver Channel

    48. 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. 60 60 60 60 60 Why use partial overlap? Partial overlap 5 channels, 60 APs each Non-overlap 3 channels, 100 APs each Consider a case where you have 300 APs 100 100 100 Worst case Interference by all 60 APs on same channel + some interference from POV channels Worst case Interference by all 100 APs on same channel

    50. 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