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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

(01329004)

Guide:

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

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.



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


    Conclusion
    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



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