MAC Protocols that use Directional Antennnas

1

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 

Directional Antenna Type

 Switched Antenna : Select One   Omni mode / Directional Mode Steerable/Steered Antenna Adaptive Array Antenna 2

A B X Red

nodes cannot communicate presently

Y Omni-Directional Antenna X

Not Possible using Omni

A B Y Directional Antenna

  

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 3

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)

Omni-directional BT vs Directional BT   

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 BT is not collision-free !!

C  X may cause collision 4

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 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 5

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 6

Multichannel MAC Protocols for Data Transmission

7

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 802.11 when network load is high

8

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

9

Power Control MAC

10

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 A B transmits to A B C B’s transmission is overheard by C which causes unnecessary power consumption 

Power Control

 Vary Transmit Power suitably to reduce power consumption. 11

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. 12

Power Control Schemes

 

Power Control in the IEEE 802.11: 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 A B C D

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 

PCM (Power Control MAC)

 Fix the shortcomings of the IEEE 802.11’s Power Control 13

BASIC Scheme in IEEE 802.11

 

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 802.11 without power control

14

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

15