collisions & virtual collisions in ieee 802.11 networks libin jiang ee228a communication...
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Collisions & Virtual collisions in IEEE 802.11 networks
Libin JiangEE228a Communication Networks
Outline
Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion
Problem of Carrier-Sensing in 802.11 Hidden-terminal problem (causing collisions)
T1R1
T2
Basic Mode |T2-R1|<IR;
|T1-T2|<PCSRange
PCSRange
R2
1 2
Link contention Graph(Link: a Transmitter-Receiver pair)
T1 R1T2
VCSRange VCS
Range
R2RTS/CTS mode
|T2-R1|<IR;VCSRange>IR
IR: Interference RangePCSRange: Physical Carrier Sensing RangeVCSRange: Virtual Carrier Sensing Range
RTS/CTS
Problem of Carrier-Sensing in 802.11 Collisions & unfairness still exist
if a receiver can sense “busy” channel but the transmitter can’t
Transmitter does not know when to transmit
Collisions Virtual collisions
T2 can send RTS to R2, but R2 does not reply with a CTS
(May not be a real collision) Information asymmetry
T1 knows Link 3 T3 does not know Link 1,
resulting in collisions Link 3 gets a much lower
throughput than Link 1 Cannot be solved by just using
a large CS Range
Collision Virtual Collision
T1 R1R3
RTS/CTS Mode
VCSRange>IR
VCSRange VCS
Range
T3
T2
R2
Problem of Carrier-Sensing in 802.11 Similar situation exists in "Basic Mode“, if the receiver
cannot “restart” to receive a stronger packet
T1R1
T2
PCSRange
R2Preamble, length MAC Data
Preamble, length MAC Data (T2->R2)
If |T1-R2|<PCSRange, R2 can miss thepacket T2->R2Basic Mode
Packets on Link 2 are often lost, for any PCSRange.
Packet T1->R1
Packet T2->R2
Packets arriving at R2
Outline
Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion
Symptoms
Frequent packet collisions cause many problems [1] Throughput Unfairness Routing Instability
[1] Xu, S.; Saadawi, T., “Does the IEEE 802.11 MAC protocol work well in multihop wireless ad hoc networks?”, Communications Magazine, IEEE, Volume: 39, Issue: 6, June 2001, Pages:130 - 137
Symptoms Throughput Unfairness
0 10 20 30 40 500
0.5
1
1.5
2
2.5
Time (s)
Thr
ou
ghp
ut (
Mb
ps) TCP 1
TCP 2
TCP 1: node 1 node 3, starts earlier at 3.0 sec
TCP 2: node 6 node 4. starts at 10.0 sec.
1 2 3 4 5 6
Tool: Network Simulator 2 Nodes are spaced by 140m No RTS/CTS, PCSRange =
550m. 3 hops<PCSRange<4 hops
Data rate: 11Mbps Packet size: 1460 Bytes Routing protocol: AODV (Ad-
hoc On-demand Distance Vector Routing)
Symptoms Routing Instability
A UDP flow: node 1 node 12 (11 hops).
Routing instability is triggered by excessive packet collisions, which is mistaken for route unavailability
1 2 3 4 5 6 11 12...
0 10 20 30 40 500
0.2
0.4
0.6
0.8
1
1.2
Time (s)
Thr
ough
put
(Mbp
s)
Outline
Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion
The New Design--for IEEE 802.11 Basic Mode
Range Requirement: Transmitter must sense the interfering link(s)
Receiver Requirement: Receiver assumes no role in Carrier-Sensing “Restart”: If a stronger packet arrives later, the receiver switches to
receiving the packet In any case, return ACK if receiving a DATA packet
max max
Each link has an "Interference Range": ( ),
where (.) is an increasing function. ( )
links , are "interfering" if | | , where
{ , }, { , }.
i i
i j i j
i i i j j j
i IR f d
f IR f d
i j X X IR or IR
X T R X T R
max max
max{| |}, interfering links ,
max{| |} 2
i j
i j
PCSRange T T i j
T T d IR
T1 R1 R2
IRmaxdmax dmax
T2
Definitions
The New Design--for IEEE 802.11 RTS/CTS Mode Range Requirement: transmitter must receive the RTS or CTS from
interfering link(s)
Receiver Requirement: Receiver assumes no role in Carrier-Sensing Same as before, except… In any case, return CTS/ACK if receiving a RTS/DATA packet
max max
max{min(| |,| |)}, interfering links ,i j i jVCSRange T T T R i j
VCSRange d IR
T1 R1
R2
VCSRange VCS
Range
T2
RTS/CTS modeIf VCSRange>dmax+IRmax,
then |R1-R3|>IRmax
IRmax
dmax
dmax
T3
R3
Outline
Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion
Performance Evaluation TCP unfairness
0 10 20 30 40 500
0.5
1
1.5
2
2.5
Time (s)
Thr
ou
ghp
ut (
Mb
ps) TCP 1
TCP 2
TCP 1: node 1 node 3, starts earlier at 3.0 sec
TCP 2: node 6 node 4. starts at 10.0 sec.
1 2 3 4 5 6
0 10 20 30 40 500
0.5
1
1.5
2
2.5
Time (s)
Thr
ou
ghp
ut (
Mb
ps)
TCP 1TCP 2
Performance Evaluation Routing Instability
A UDP flow: node 1 node 12 (11 hops).
Routing instability is triggered by excessive packet collisions, which is mistaken for route unavailability
1 2 3 4 5 6 11 12...
0 10 20 30 40 500
0.5
1
1.5
Time (s)
Thr
ough
t (M
bps)
Before
After
Outline
Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion
Throughput-collision tradeoff In the design, CSRange/dmax seems
to be large:
A smaller PCSRange can not remove hidden-terminals, but may give a higher throughput
To study the tradeoff, consider a random network M=4 16 APs, 64 randomly located
clients D/M=175m dmax=175/root(2) PCSRange>468m satisfies
Range Requirement
AP1 AP2
AP3 AP4
D
D/MD
max max
max max
2PCSRange d IR
VCSRange d IR
Throughput-collision tradeoff
Collision Probability vs. PCS Range Total throughput vs. PCS Range
Total Throughput
0
10
20
30
40
50
60
70
80
90
0 100 200 300 400 500
PCS Range (m)
To
tal T
hro
ug
hp
ut
(Mb
ps)
After meeting "Receiver Requirement" IEEE 802.11
Collision Probability
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 100 200 300 400 500PCS Range (m)
Co
llisi
on
Pro
bab
ility
After meeting "Receiver Requirement" IEEE 802.11
Range Requirement met
Throughput-collision tradeoff
The tradeoff always exists
The tradeoff is improved by meeting the Receiver Requirement
Throughput-collision tradeoff
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 20 40 60 80 100
Total Throughput (Mbps)
Co
llis
ion
Pro
bab
ilit
y
After meeting "Receiver Requirement" IEEE 802.11
Conclusion
802.11 does not avoid hidden-terminal-induced collisions & virtual collisions
It is the root of many problems (symptoms) 2 requirements (Range Requirement and Receiver
Requirement) is sufficient to solve the problem Tradeoff between throughput & collisions
Thank you!
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