Tackling Exposed Node Problem in IEEE 802.11 Mac

1. Tackling Exposed Node Problem in IEEE 802.11 Mac Deepanshu Shukla (01329004) Guide: Dr. Sridhar Iyer

2. Wireless LAN Physical layer impact Impact of Interference range Hidden Terminal Problem Exposed Node

3. Physical Layer • Medium • Communicate over a medium significantly less reliable than wired PHYs. Medium is a scarce commodity • Lack full connectivity • Transmission • Have time-varying and asymmetric propagation properties • Due to propagation limits, collision may not be ‘sensed’ by some nodes • Fix Range • Have Carrier sense threshold and Receive threshold • If packet below receive threshold, it is marked as in error and passed to MAC. • Packets below carrier sense threshold are ignored by PHY layer

4. Effect of Interference Range Transmission from 1  2 will fail

5. A C B Hidden Terminal Problem • Hidden terminals • A and C cannot hear each other. • A sends to B, C cannot receive A. • C wants to send to B, C senses a “free” medium (CS fails) • Collision occurs at B. • A cannot receive the collision (CD fails). • A is “hidden” for C. • Solution? • Hidden terminal is peculiar to wireless (not found in wired) • Need to sense carrier at receiver, not sender! • “virtual carrier sensing”: Sender “asks” receiver whether it can hear something. If so, behave as if channel busy.

6. Exposed Node Scenario

7. Exposed Terminal Problem • Exposed terminals • A starts sending to B. • S1 senses carrier, finds medium in use and has to wait for AB to end. • D is outside the range of A, therefore waiting is not necessary. • A and C are “exposed” terminals. • A->B and S2->anyNode transmissions can be parallel; no collisions • NOT allowed under IEEE 802.11 Refer figure 3.2

8. Effect of RTS/CTS on NAV • Timing info contained in RTS • 3*SIFS+Packet Time+2*RTS_TX_Time

9. Proposed Solution Maintain neighborhood information Identify Exposed Node Process timing information Synchronize ACK Schedule DATA transmission

10. Identify Exposed Nodes • Node hears • RTS • corresponding DATA

11. Process Timing Info • Upon hearing RTS • Record Recv. Time • Upon hearing DATA • Record Recv. Time • Calculate “Propagation Delay” • Difference of the advertised time and actual time. • Calculate following • expected time of ACK • time required for DATA • Delay required to synchronize ACK

12. Transmit DATA • Is Exposed Node? YES • Does network Layer has data? YES • Check the Size of DATA • Is it less than the ongoing data size? YES • Check the Destination of DATA • Is it Broadcast? NO • Addressed to already busy node? NO • Calculate the time of transmission of ACK (from RTS duration and propagation delay) • Schedule DATA packet transmission after the calculated delay

13. Modification to packets • RTS • Requires no change • CTS / ACK • IEEE 802.11 CTS does not have TA (Transmitter's Address) • Added to maintain neighborhood information • DATA • Add “isExposed ”, 1- bit flag • No such bit in IEEE 802.11

14. Simulation Topologies 4-6-8 Node, single hop 5 Node, multi hop

15. Simulation Results Multi hop Scenario – 5 Nodes Single hop Scenario – 6 Nodes

16. Limitation of algorithm • Reverse exposed node • Effect of interference range

17. Related Work - MACAP • Introduce a ‘Control Phase’ • Use additional control packet RTS’ • Add 2-byte fields TACK, TDATA to RTS & CTS and “inflexible bit” to RTS

18. Conclusion • Exposed nodes play a major role bandwidth underutilization, especially in case of multi hop scenario, as shown by the performance increase. • The Medium lays down various constraints and coming up with optimum PHY layer parameters is important. • The increase in performance is substantial to further investigate this problem in case of “Reverse Exposed” nodes

19. Thank you.