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MAC Protocols that use Directional Antennnas

MAC Protocols that use Directional Antennnas. Directional communication Less Energy in the wrong direction Better Spatial reuse and less multipath More Energy in the right direction Longer ranges more robust links Reduce interference to other neighbor nodes  increase throughput

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MAC Protocols that use Directional Antennnas

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  1. MAC Protocols that use Directional Antennnas

  2. Directional communication Less Energy in the wrong direction Better Spatial reuse and less multipath More Energy in the right direction Longer ranges more robust links Reduce interference to other neighbor nodes  increase throughput Antenna Model Typically, 2 operation mode Omni mode / Directional Mode Directional Antenna Type Switched Antenna : Select One Steerable/Steered Antenna Adaptive Array Antenna Directional Antenna Red nodes cannot communicate presently X A B Y Omni-Directional Antenna Not Possible using Omni X B A Y Directional Antenna

  3. Each node has only 1 radio transceiver A transceiver Can tx or rx only one packet at a given time Equipped with M directional antennas Antennas Each antenna has non-overlapping conical radiation pattern Every antenna individually or all the antennas can be switched to the active or passive modes The transceiver used only the antennas in active mode If all the antennas of the node are active, similar to omni-directional antenna It is assumed that the radio range is the same for all directional antennas of the nodes MNs do not know direction of the sender and receiver nodes Make use of RTC/CTS exchange Direction of the sender is identified by the antenna received with max power sender/receiver node tx/rx data packet through the selected directional antennna MAC Protocol using Directional Antennas

  4. Adapts the DBTMA for use with directional antennas Assumption: Orientation of sectors of each antenna element remains fixed (does not support MNs) Sender: tx RTS in all direction Receiver Determines the antenna on which RTS is received with max gain Turn on BTr in the direction toward the sender Send back a directional CTS Sender: Turn directional BTt to the receiver Tx data packet through the antenna on which the CTS packet was received with max gain Directional Busy Tone-based MAC Omni-directional BT vs Directional BT Directional BT is not collision-free !!CX may cause collision

  5. Young-Bae Ko, V. Shankarkumar, N. Vaidya (2000) Assumption: Each node knows about (via GPS) Location of its neighbors Its own location MAC protocol similar to 802.11, but on a per-antenna basis If a node has overheard an RTS or CTS on a particular antenna, then the antenna is blocked for the transmission duration (NAV) But, remaining antennas of the node can be used for Tx D-MAC-1 Directional RTS (DRTS) / Omni-Directional CTS (OCTS) DRTS from E to A may collide with OCTS or ACK from B to A D-MAC: Directional MAC

  6. DMAC-2 DRTS or ORTS / OCTS Send ORTS if non of antennas are blocked Send DRTS, otherwise Reduce collision between control packets After receiving ORTS from node D, node C would not respond node D: backoff and ReTx Avoid this situation, introduce Directional wait-to-send (DWTS) packet Carries the expected duration of AB D-MAC (Cont’d)

  7. Multichannel MAC Protocolsfor Data Transmission

  8. Multiple channels for data Tx No dedicated control channel Need single transceiver Each node maintains a data structure called Preferable Channel List (PCL) High preference channel (HIGH): has been selected and is being used by the node in the current beacon interval Medium preference channel (MID): is free and is not being currently used by neighbor Lowest preference channel (LOW): already being used by neighbor ATIM (ad hoc traffic indication msg) Is used to negotiate for channels during the current beacon interval Exists at the start of every beacon interval ATIM msgs exchange on the default channel Carries the PCL of the transmitting node May be lost due to collision  back-off Higher throughput than IEEE 802.11 when network load is high MMAC: Multichannel MAC

  9. MCSMA: Multichannel CSMA MAC • Available BW is divided into N channels • A channel BW = BW/N • Channels are created by FDMAor CSMA, but not on TDMA (because it requires global time synchronization) • Idle node continuously monitors and marks IDLE channels if TRSS < ST • TRSS: total received signal strength, ST: sensing threshold • CS • If free channel list is empty, waits for any channel to become IDLE, • i.e. wait for LIFS + random back-off period • Otherwise, select an IDLE channel (check first the most recently successfully transmitted channel) • Before actual transmission • If the selected channel is idle (TRSS < ST) for at least LIFS period, Tx immediately • Otherwise, LIFS + random back-off delay • When N is large or traffic is high, each node tends to reserve a channel  greatly reduce collision

  10. Power Control MAC

  11. Energy / Power Conservation B transmits to A • Power Saving • Go to a doze state by Powering off its wireless network interface • Ex) DEC Roamabout Radio • TX: 5.76 W • RX; 2.88 W • Idle; 0.35 W • Power Control • Vary Transmit Power suitably to reduce power consumption. A B C B’s transmission is overheard by C which causes unnecessary power consumption

  12. Power Saving Schemes • PAMAS: Power Aware Multi-Access protocol with Signaling for Ad Hoc Networks • C. Raghavendra, S. Singh (1998) • Based on the MACA with the addition of a separate signaling channel • Powering off nodes that are not actively transmitting or receiving. • Issues • For how long is a node powered off ? • What happens if a neighbor wishes to transmit a packet to a node that has powered itself off ? • Out-of-Band Signaling Channel • Busy Tone; • To exchange Probe Messages to resolve powering off interval.

  13. When A is transmitting a packet to B, this transmission may not be sensed by C and D. So, when C and D transmit to each other using a higher power, their transmission will collide with the on-going transmission from A to B B C D A Power Control Schemes • Power Control in the IEEE 802.11: BASIC • RTS/CTS are transmitted using the highest power level (Pmax) • Data/ACK are transmitted using the minimum power level (Pdesired) necessary to communicate • Different Transmission Power can lead to increase collision • PCM (Power Control MAC) • Fix the shortcomings of the IEEE 802.11’s Power Control

  14. Pdesired= Pmax/Pr x Rxthresh x c Pr: received power level Rxthresh: min necessary received signal strength Assumption attenuation is same in both direction noise level at the nodes is below a predefined threshold value Drawback X and Y defer their Tx during EIFS period by overhearing RTS and CTS After EIFS period, X and Y may attempt to Tx  collision RTS from X may cause collision with ACK RTS from Y may cause collision with DATA Throughput degradation and higher energy consumption (because of ReTx) than even the IEEE 802.11 without power control BASIC Scheme in IEEE 802.11

  15. PCM: Power Control MAC • Eun-Sun Jung, N. Vaidya (2002) • Based on BASIC scheme • To avoid collision • Source node tx DATA packet at Pmax periodically (every EIFS period) • Duration of each such Tx > time required for physical CS • Achieves throughput very close to that of IEEE 802.11 while using much less energy

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