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
 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
 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).
C
B
 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.
A
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 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 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 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.