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Tackling Exposed Node Problem in IEEE 802.11 Mac. Deepanshu Shukla (01329004) Guide: Dr. Sridhar Iyer. Wireless LAN. Physical layer impact Impact of Interference range Hidden Terminal Problem Exposed Node. Physical Layer. Medium

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Tackling exposed node problem in ieee 802 11 mac

Tackling Exposed Node Problem in IEEE 802.11 Mac

Deepanshu Shukla



Dr. Sridhar Iyer

Wireless lan

Wireless LAN

Physical layer impact

Impact of Interference range

Hidden Terminal Problem

Exposed Node

Physical layer
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
Effect of Interference Range

Transmission from 1  2 will fail

Hidden terminal problem




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.

Exposed terminal problem
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

Effect of rts cts on nav
Effect of RTS/CTS on NAV

  • Timing info contained in RTS

    • 3*SIFS+Packet Time+2*RTS_TX_Time

Proposed solution

Proposed Solution

Maintain neighborhood information

Identify Exposed Node

Process timing information

Synchronize ACK

Schedule DATA transmission

Identify exposed nodes
Identify Exposed Nodes

  • Node hears

    • RTS

    • corresponding DATA

Process timing info
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
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
    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

    Simulation topologies
    Simulation Topologies

    4-6-8 Node, single hop

    5 Node, multi hop

    Simulation results
    Simulation Results

    Multi hop Scenario – 5 Nodes

    Single hop Scenario – 6 Nodes

    Limitation of algorithm
    Limitation of algorithm

    • Reverse exposed node

    • Effect of interference range

    Related work macap
    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


    • 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