naveen santhapuri, srihari nelakuditi and romit roy choudhury university of south carolina duke...
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Naveen Santhapuri, Srihari Nelakuditiand Romit Roy Choudhury
University of South Carolina Duke University
WCNC 2008
Introduction The optimal network capacity for wireless
networksRelated to spatial reuse
Efficient spatial reuse is inhibited by InterferenceExternal noiseOther physical factorsLimitations of MAC protocols
Introduction The 802.11 protocol with its virtual carrier
sensing has role reversals which Reduce the hidden node problem Introduce the exposed node problem
Introduction If there was no ACK
Two nodes can transmit DATA simultaneouslyExposed sender problem can be solved
partially
Piggybacked ACK mechanism“Piggybacked-Ack-aided Concurrent
Transmissions”N. Santhapuri, J. Wang, Z. Zhong, and S.
NelakuditiICNP Poster Session, 2005
A B CDATART
S
CTS for AACK for C
Introduction -- collision Multiple packets arriving at a receiver are
generally considered to cause packet loss due to the collision at the receiver
A B C
Introduction A sufficiently stronger frame can still be
successfully received by the receiver in spite of a collisionThis phenomenon is called physical layer
capture (PLC)
Related work“Sniffing out the correct physical layer
capture model in 802.11b” A. Kochut, A. Vasan, A. U. Shankar and A.
AgrawalaICNP, Oct. 2004
R1 S1 S2 R2
Related work“An experimental study on the capture effect
in 802.11a networks”J. Lee, W. Kim, S.-J. Lee, D. Jo, J. Ryu, T. Kwon
and Y. ChoiWinTECH, Sept. 2007
A signal is significantly more vulnerable to interference if it starts after the interfering frame than had it started before the interfering frame.
S1R1S3
S1R1S2
interest
interferingX
interest
interferingX
Preamble
DATA
Proposed approachAssumption
Assume two hop signal strength informationS1’s signal strength at R1 will be known by S2
R1 S1 S2100m50
m
Proposed approachCapture-Aware Staggering of Transmissions
(CAST)
S1 ->R1
RTS
DATA
R1 S1 S2 R2 S3100m50
m50m
50m
802.11
CASTS1 ->R1S2 ->R2
ACK
CTS
PWAIT
CASTRTS: S1 -> R1
No other frame > SF hold
S1 ->R1
RTS
DATA
802.11
CASTS1 ->R1
S2 ->R2
ACK
CTS
PWAIT
The SINR value at R1 for a signal from S1in the presence of interference from S2
The SINR value at R1 for a signal from S1in the presence of interference from S2
2
11
2
11 , R
RSSRS SS
CASTCTS: R1 -> S1RTS: S2 -> R2 after one physical preamble
CTS startsMedium is free > SF hold
RTS starts > SLC hold
S1 ->R1
RTS
DATA
802.11
CASTS1 ->R1
S2 ->R2
ACK
CTS
PWAIT
1
22
RRSS
2
11
2
11 , R
SRSSR SS
CASTCTS: R2 -> S2DATA: S1 -> R1 after 2 physical preamble
times
CTS startsMedium is free > SF hold
DATA starts > SLC hold
S1 ->R1
RTS
DATA
802.11
CASTS1 ->R1
S2 ->R2
ACK
CTS
PWAIT
1
22
1
22 , R
SRSSR SS
2
11
RRSS
CASTDATA: S1 -> R1DATA: S2 -> R2
S1 -> R1Medium is free > SF hold
S2 -> R2 > SLC hold
S1 ->R1
RTS
DATA
802.11
CASTS1 ->R1
S2 ->R2
ACK
CTS
PWAIT
2
11
2
11 , R
RSSRS SS
1
22
SRSS
CASTIn 802.11, for two concurrent transmissions
8 signal strength value must be above SLC threshold
In CAST, for two concurrent transmissionsOnly 5 signal strength value must greater than
SLC
1
22
1
22
2
11
1
22
2
11 , , , , R
RSSRS
RRS
SSR
SRS SSSSS
Optimize the protocol
RTS
DATA
CASTPrimary
Secondary
CTS
PWAIT
CASTPrimary
Secondary
SLC or SLC 1
22
2
11 S
RSSRS SS
Conclusions Spatial reuse in wireless networks is limited
by the SINR threshold requirementsThis problem is amplified because of role
reversals in wireless networksPhysical layer capture can improve the spatial
reuse by staggering the transmissionsSimulation results show that the number of
concurrent transmissions can be improved significantly