Tackling Exposed Node
Problem in IEEE 802.11 Mac
Deepanshu Shukla
(01329004)
Guide:
Dr. Sridhar Iyer

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

 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

Transmission from 1

2 will fail

 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?



C
B
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 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

•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

 Node hears
 RTS
 corresponding DATA

 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

 Is Exposed Node?
 Does network Layer has data?
 Check the Size of DATA
YES
YES
 Is it less than the ongoing data size?
YES
 Check the Destination of DATA
 Is it Broadcast?
NO
 Addressed to already busy node?
 Calculate the time of transmission of ACK
(from RTS duration and propagation delay)
NO
 Schedule DATA packet transmission after the calculated delay

 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

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

Multi hop Scenario – 5 Nodes Single hop Scenario – 6 Nodes

 Reverse exposed node
 Effect of interference range

 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

 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.