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Min Kyu Han Multimedia Communications Laboratory Hankuk University of Foreign Studies May 23, 2007

2007. Sensor Network. Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver. Min Kyu Han Multimedia Communications Laboratory Hankuk University of Foreign Studies May 23, 2007. Contents. Overview Related Work Propose a protocol

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Min Kyu Han Multimedia Communications Laboratory Hankuk University of Foreign Studies May 23, 2007

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  1. 2007 Sensor Network Multi-Channel MAC for Ad Hoc Networks: Handling Multi-Channel Hidden Terminals Using A Single Transceiver Min Kyu Han Multimedia Communications Laboratory Hankuk University of Foreign Studies May 23, 2007

  2. Contents • Overview • Related Work • Propose a protocol • Issues in Multi-Channel environment • Simulation • Discussion

  3. Overview • Key Point(1/2) • Utilizes multiple channels dynamically to improve performance • 802.11 Standard • Issue • 802.11 MAC DCF(Distributed Coordinate Function) is designed for sharing a single channel between hosts. • Each of current IEEE 802.11 device is equipped with one half-duplex transceiver. • Transceiver is capable of switching channels dynamically, but it can only transmit or listen on one channel at a time • Due to this, a new type of hidden terminal problem occurs in this multi-channel environment multi-channel hidden terminal problem. MAC Designed only for a single channel PHY Allows for the use of multiple channels available IEEE 802.11 Standard

  4. Overview • Key Point (2/2) • Simulation precondition • network : ad-hoc  non-infrastructure • there is no central authority to perform channel management • Simulation • This paper proposed protocol enables hosts to utilize multiple channels by switching channels dynamically, thus increasing network throughput. • The simulation results show that proposed protocol successfully exploits multiple channels to achieve higher throughput than IEEE 802.11 & another multi-channel MAC protocol. • Main Idea(A similar approach is used in IEEE 802.11’s PSM) • The main idea is to divide time in to fixed-time interval using beacons, • and have a small window at the start of each interval to indicate traffic • And negotiate channels for use during the interval.

  5. RELATED WORK(1) • Dual Busy Tone Multiple Access(D-BTMA) • It is divides a common channel into two sub-channels. • one data channel & one control channel • This scheme uses only one data channel and is not intended for increasing throughput using multiple channels. • Hop Reservation Multiple Access(HRMA) & Channel-Hooping with Dual Polling • Multi-channel protocol for networks using FHSS(Frequency Hopping Spread Spectrum) • The hosts hop from one channel to an other according to a predefined hopping pattern(PRN Code) • They cannot be used in systems using other mechanisms such as DSSS(Direct Sequence Spread Spectrum)

  6. RELATED WORK(2) • Multi-channel CSMA protocol • “soft” channel reservation • If there are N channels, the protocol assumes that each host can listen to all N channels concurrently. • Number of listening channel : N • Preferred channel selection factor : the last successful transmission channel • Extension : best channel based on signal power • N’s Channel  need to N’s Transceiver  very expensive. • Dynamic Channel Assignment(DCA) • They maintain one dedicated channel for control messages and other channels for data. • Each host has two transceivers, so that it can listen on the control channel, and data packets are transmitted on the data channel. • RTS/CTS packets are exchanged on the control channel • In RTS packet, the sender includes a list of preferred channel • On receiving the RTS, the receiver decides on a channel • When the number of channels is small, one channel dedicated for control messages can be costly

  7. Propose a protocol • One transceiver per host • It does not require a dedicated control channel • Provide a clock synchronization among all the hosts • At the start of each interval, I’m Overhead But Don’t through me!!! Beacon Interval Common Channel Data Channel N Z … Ch. 2 B Ch. 1 A

  8. Preliminaries • IEEE 802.11 Distributed Coordination Function(DCF) • Random Backoff • The counter is decrementedby on after each “time slot” • IEEE 802.11 Power Saving Mechanism(PSM) • Using ATIM • A node can save energyby going into doze mode Hey! Wake Up!!!

  9. Issues in Multi-Channel environment • Multi-channel hidden terminal problem • If there was only one channel that every node listens to, C would have heard the CTS and thus deferred its transmission A - B C - D Ch .1 : Control Channel time Node c was buys receiving on another channel

  10. Proposed MMAC(Multi-Channel MAC) Protocol • Assumptions • N channels are available for use and all channels have the same bandwidth. • Each host is equipped with a single half-duplex transceiver. • So a host can either transmit or listen, but cannot do both simultaneously. • Nodes are synchronized, so that all nodes begin their beacon interval at the same time. • Proposed scheme in detail • Preferable Channel List(PCL) • Channel Negotiation during ATIM Window • Rules for Selecting the Channel

  11. Preferable Channel List(PCL) • PCL records the usage of channels inside the transmission range of the node. • Based on this information, the channels are categorized into three states. • High preference(HIGH) • Medium preference(MID) • Low Preference(LOW) • There is a counter for each channel in the PCL to record how many source-destination pairs plan to use the channel for the current interval • The channel states are changed in the following way • Reset to MID state : Power up, start of each beacon interval • HIGH state : S-D nodes agree upon a channel • LOW state : if a node overhears an ATIM-ACK or ATIM-RES

  12. Channel Negotiation during ATIM Window • In MMAC, periodically transmitted beacons divide time into beacon intervals. • A small window called the ATIM window is placed at the start of each beacon interval • Similar to IEEE 802.11 PSM(different purpose in proposed protocol) Update channel state Vicinity of S S D Vicinity of D ATIM(include PCL & NAV) Select one channel Based on the Sender’s PCL & own PCL Check Channel & Last decision & Confirm ATIM-ACK(NAV) Ready to Receive ATIM-RES ATIM-RES wait New type of packet

  13. Rules for Selecting the Channel(1/2) • When a node receives an ATIM packet, • It selects a channel and notifies the sender by including the channel information in the ATIM-ACK packet. • The receiver tries to select the “best” channel based on information included in the sender’s PCL and its own PCL. • Best channel : the least scheduled traffic. • Example • Case 1. Selected HIGH state channel • Case 2. Selected LOW state channel I’m free HIGH MID LOW Ch. 1 Ch. 2 Ch. 3 … Counter : 1 Counter : 2 LOW LOW LOW Ch. 1 Ch. 2 Ch. 3 … Counter : 3

  14. Rules for Selecting the Channel(2/2) • Compare to Src.PCL vs Dest.PCL If D.PCL(1:HIGH) Selected Channel No. 1 S D ATIM(include PCL & NAV) A.PCL(1:HIGH) Selected Channel No. 1 Multiple channels in this state ??? ATIM-ACK(Ch.1 , NAV) S.PCL(1:MID) & D.PCL(1:MID) Selected Channel No. 1 ??? Multiple channels in this state ??? S.PCL(1:MID) |D.PCL(1:LOW) S.PCL(1:LOW) |D.PCL(1:MID) Selected Channel No. 1 ??? ATIM-RES S.PCL(*:LOW) & D.PCL(*:LOW) The channel with the least count is selected. OK : ATIM-RES Send ( Channel Selected Confirm) Otherwise : Does not send an ATIM-RES packet Retry Negotiation

  15. Simulation Model • For simulations, • Used ns-2 • Two network scenario : wireless LAN, multi-hop networks • Bit rate for each channel is 2Mbps(Channel Num : 3) • Transmission range of each node : 250m • Beacon interval : 100ms • Each simulation was performed for a duration of 40sec • Packet size : 512 bytes

  16. Simulation(WLAN : Single hop – 1/2) • Inthe simulated wireless LAN, all nodes are within each other’s transmission range. • First, we examine the throughput and packet delay varying the network load • MMAC uses all 3 channels for data exchange. • Single-hop network

  17. Simulation(WLAN : Single hop – 2/2)

  18. Simulation(WLAN : multi hop – 1/3) • Keypoint Parameter • Control channel saturation(DSA) • Packet size • Number of channel • ATIM Window(MMAC)

  19. Simulation(WLAN : multi hop – 2/3)

  20. Simulation(WLAN : multi hop – 3/3) • Keypoint • ATIM window overhead in MMAC does not increase with the number of channels • DCA does not benefit from having one more channel because of control channel saturation.

  21. Simulation(MMAC – 1/2) • Extended ATIM Window • ATIM Window is affects the throughput of MMAC Protocol.

  22. Simulation(MMAC – 2/2) • For simulations, • WLAN Node # : 30 • Packet Size : 512Bytes • Number of Channel : 3 • Keypoint • Hidden Terminal Problem • DCA(Control Channel) vs MMAC(ATIM Window)

  23. Discussion(1/2) • Case 1. Clock Synchronization • Out-of-band solution(GPS) • In-band solution  ATM window overhead • Beaconing mechanism problem • Clocks of (A,B) and (C,D) may drift away, because they never exchange beacons • Case 2. The node might miss the ATIM packets sent by other nodes. Data send Beacon Interval Common Channel Data Channel N Common Channel If (Current Beacon Interval < transmission time of the packet) then nodes refrain from Transmitting packet Common Channel CTS RTS AITM

  24. Discussion(2/2) • Case 3. 1(Source):N(Destination) Problem • Head of line blocking problem • Starvation problem

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