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

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

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

Effect of Interference Range Transmission from 1  2 will fail

Hidden Terminal Problem A B C  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.

Exposed Node Scenario

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 Refer figure 3.2

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

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

Identify Exposed Nodes  Node hears RTS corresponding DATA

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

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

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

Simulation Topologies Node, single hop5 Node, multi hop

Simulation Results Multi hop Scenario – 5 NodesSingle hop Scenario – 6 Nodes

Limitation of algorithm  Reverse exposed node  Effect of interference range

Related Work - MACAP  Introduce a ‘Control Phase’  Use additional control packet RTS’  Add 2-byte fields T ACK, T DATA to RTS & CTS and “inflexible bit” to RTS

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

Thank you.