Tackling Exposed Node Problem in IEEE 802.11 Mac

Deepanshu Shukla(01329004)

Guide:

Dr. Sridhar Iyer

Wireless LAN

Physical layer impactImpact of Interference range

Hidden Terminal ProblemExposed 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

BC

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

AB 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

•Timing info contained in RTS

3*SIFS+Packet Time+2*RTS_TX_Time

Proposed Solution

Maintain neighborhood informationIdentify Exposed NodeProcess timing informationSynchronize 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 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

4-6-8 Node, single hop 5 Node, multi hop

Simulation Results

Multi hop Scenario – 5 Nodes Single 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 TACK, TDATA 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.