1 MAC Protocols that use Directional Antennnas

2 Directional Antenna  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 A B X Y AB X Y Omni-Directional Antenna Directional Antenna Red nodes cannot communicate presently Not Possible using Omni

3 MAC Protocol using Directional Antennas  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

4 Directional Busy Tone-based MAC  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 Omni-directional BT vs Directional BT Directional BT is not collision-free !! C  X may cause collision

5 D-MAC: Directional MAC  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 , 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

6 D-MAC (Cont’d)  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

7 Multichannel MAC Protocols for Data Transmission

8 MMAC: Multichannel MAC  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 when network load is high

9 MCSMA: Multichannel CSMA MAC  Available BW is divided into N channels  A channel BW = BW/N  Channels are created by FDMA or 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  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. ACB B transmits to A 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 Power Control Schemes  Power Control in the IEEE : BASIC  RTS/CTS are transmitted using the highest power level (P max )  Data/ACK are transmitted using the minimum power level (P desired ) necessary to communicate  Different Transmission Power can lead to increase collision  PCM (Power Control MAC)  Fix the shortcomings of the IEEE ’s Power Control A BCD 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

14 BASIC Scheme in IEEE  P desired = P max /P r x Rx thresh x c  P r : received power level  Rx thresh : 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 without power control

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 while using much less energy