Anand Prabhu Subramanian and Samir R. Das {anandps, samir@cs.sunysb}

1. Addressing Deafness and Hidden Terminal Problem in Directional Antenna Based Wireless Multi-hop Networks Anand Prabhu Subramanian and Samir R. Das {anandps, samir@cs.sunysb.edu} Computer Science Department Stony Brook University, NY, USA

2. Outline • Motivation – Why Directional Communication? • Deafness • Directional Hidden Terminal Problem • Antenna Model • CW-DMAC Design • Performance Evaluation • Summary

3. Motivation - Capacity Problem in Multi-hop Wireless Networks • Wireless Multi-hop Networks – Ad hoc Networks, Mesh Networks • Data packets forwarded over multiple hops • Wireless channel is a shared medium • Capacity of multi-hop networks limited by wireless interference • Directional communication can reduce interference

4. Directional Antenna - Basics • A Directional/Beam forming antenna has certain preferred transmit and receive directions • Steerable beam vs switched beam Omni-directional Beam

5. Directional Antenna - Benefits • Directional Communication • Less energy in the undesired directions • Better spatial reuse • More energy in the desired direction • Longer Ranges • More robust links • Both higher spatial reuse and longer range can be simultaneously obtained

6. F F E E B B C C A A D D Directional Communication Omni- Directional Communication Directional Communication

7. Directional Communication - Challenges • Just adding a directional antenna is not enough • New Challenges: • Direction to transmit • Deafness • Directional Hidden Terminal Problem • Broadcasting • And more …

8. 1 1 1 2 2 2 Deafness 4 4 4 3 3 3 B A X Deafness – Type I • Destination engaged in communication

9. Deafness 1 1 1 1 2 2 2 2 4 4 4 4 3 3 3 3 B D A S Deafness – Type II • Precautionary Deafness at the Receiver

10. Collision 1 1 1 1 2 2 2 2 4 4 4 4 3 3 3 3 D S A B Directional Hidden Terminal Problem - Due to unheard RTS/CTS packets

11. Current Literature • Many proposals in the past to solve deafness • ko00infocom, choudhury02mobicom, choudhury04icnp, elbatt03wcnc, gossain04globecom, nasipuri00wcnc, sundaresan03mobihoc, takai02mobihoc • These approaches use additional resources such as additional channels, radios or busy tones • Directional hidden terminal problem not addressed

12. Our Goal • Solve both deafness and directional hidden terminal problem using • Single Channel • Single Radio Interface

13. Antenna Model • Switched beam antenna with N beams covering the entire 360 degrees • Two modes of operation • Omni mode • Directional Mode • Directional Gain = Omni Gain A

14. Antenna Model • 8 phased-array antenna elements • Can form both omni-directional and directional beams • Customizable beam pattern • Beam switch time around 150 μs • Directional gain = 15dBi Commercially available Directional Antenna

15. Antenna Model • Packet Transmission – either in omni or directional mode • Packet Reception • When Idle – omni mode • When it detects a packet, does an azimuthal scan and goes to directional mode

16. Assumptions • Nodes are fairly static – (e.g. routers in Wireless Mesh Networks) • Each node knows the direction (beam index) to its neighbor – Simple neighbor discovery protocol • Nodes need not have an aligned axis

17. CW-DMAC Design • Type I deafness can be solved if transmitter/receiver can inform neighbors about their impending transmission. • Type II deafness can be solved if the blocked receiver can somehow inform the transmitter that their transmission cannot take place without disturbing an ongoing transmission. • Directional hidden terminal problem can be solved if the nodes do not miss any RTS/CTS packets in the neighborhood

18. CW-DMAC Design • RTS/CTS packets sent omni-directionally • DATA/ACK packets sent directionally • RTS/CTS packets are overloaded with the beam index of the intended DATA/ACK transmission • Neighboring nodes set their DNAV tables appropriately depending on the beam index in the RTS/CTS packets • Each neighboring node record this transmission in their neighborhood transmission table

19. 1 1 1 1 2 2 2 2 4 4 4 4 3 3 3 3 A B Y X CW-DMAC Design Aware of A’s transmission

20. 1 1 1 1 2 2 2 2 4 4 4 4 3 3 3 3 A D S B CW-DMAC Design • There is a possibility of collision between omni-directionally sent RTS/CTS packets and DATA/ACK packets. • We separate transmission of data packets and control packets in time • Control window – added in RTS/CTS packets • Prevents collision between data and control packets • Allows multiple simultaneous transmissions in the neighborhood in different directions

21. CW-DMAC Design • Adjustable control window • Small control window – less parallelism • Large control window – poor channel utilization • To solve – deafness of type II • Negative CTS sent by blocked receivers • So the transmitter can cancel its transmission

22. Performance Evaluation • Simulation – Qualnet 3.7 • 8 beam directional antenna (45 degrees) • 802.11b physical layer • 11 Mbps data rate • Comparison between CW-DMAC and DMAC • 30 nodes in an area of 1500m x 1500m

23. Performance Evaluation - Deafness

24. Deafness Ripple Effect

25. Directional Hidden Terminal Problem

26. Random Network • - 30 nodes in 1500m x 1500m • 5 simultaneous flows • Deafness and directional hidden terminal problem cause performance degradation

27. Summary • Directional communication can reduce interference in multi-hop networks and improve capacity • Studied various scenarios in which deafness and directional hidden terminal problem could occur • Proposed a directional MAC protocol that solves both the problems using a single radio and single channel • Simulations show the improvement when the both the problems are solved

28. Thank you Questions ???