On the Performance Behavior of On the Performance Behavior of IEEE 802.11 Distributed IEEE 802.11 Distributed Coordination Function Coordination Function M.K.Sidiropoulos, J.S.Vardakas and M.D.Logothetis M.K.Sidiropoulos, J.S.Vardakas and M.D.Logothetis Wire Communications Laboratory, Wire Communications Laboratory, Department of Electrical & Computer Department of Electrical & Computer Engineering, Engineering, University of Patras, University of Patras, 265 00 Patras, Greece 265 00 Patras, Greece E-mail: [email protected]E-mail: [email protected]
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On the Performance Behavior of IEEE 802.11 Distributed Coordination Function M.K.Sidiropoulos, J.S.Vardakas and M.D.Logothetis Wire Communications Laboratory,
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On the Performance Behavior of On the Performance Behavior of IEEE 802.11 Distributed IEEE 802.11 Distributed Coordination FunctionCoordination Function
M.K.Sidiropoulos, J.S.Vardakas and M.K.Sidiropoulos, J.S.Vardakas and M.D.LogothetisM.D.Logothetis
Wire Communications Laboratory,Wire Communications Laboratory,Department of Electrical & Computer Department of Electrical & Computer
Engineering,Engineering,University of Patras,University of Patras,
We propose a new Markov model for the DCF of We propose a new Markov model for the DCF of IEEE 802.11IEEE 802.11
based on Bianchi’s, Wu’s and Ziouva’s models.based on Bianchi’s, Wu’s and Ziouva’s models.
and derive an analytical formula for the Saturation and derive an analytical formula for the Saturation ThroughputThroughput
for both Basic and RTS/CTS access schemes.for both Basic and RTS/CTS access schemes.
Simulation StudySimulation Study: :
• Validation of our new Markov model based on Validation of our new Markov model based on throughput results by the NS-2.throughput results by the NS-2.
• Average end-to-end packet delay for both Average end-to-end packet delay for both access schemes.access schemes.
Basic access scheme: A 2-way handshaking technique.
Note that: After a DIFS time interval each station defers for an additional random backoff time. The backoff counter is frozen if a transmission is detected on the channel BACKOFF SUSPENSION
DCF-An Example (cont.)DCF-An Example (cont.)RTS/CTS:
Request-To-Send / Clear-To- Send.
It is a 4-way handshaking technique.
Introduced to tackle the hidden terminal problem.
Improve throughput performance in case of long packets.
by simulation:by simulation:• Saturation throughput.Saturation throughput.• End-to-end average packet End-to-end average packet
delaydelay..
Simulations in NS-2Simulations in NS-2 IEEE 802.11b single-hopIEEE 802.11b single-hop networknetwork..
Network TopologyNetwork Topology: : • No hidden stations, all have No hidden stations, all have
LOSLOS..• CBR traffic over UDP links CBR traffic over UDP links
towards the AP.towards the AP.• No mobilityNo mobility..
Model Validation: Simulation vs. Model Validation: Simulation vs. Analysis:Analysis:
1Mbps. 1Mbps. Basic and RTS/CTSBasic and RTS/CTS Close match of analyticalClose match of analytical model and simulation model and simulation
resultsresults..
Our model is closer toOur model is closer to simulation than Wu’s.simulation than Wu’s.
The RTS/CTS gives higherThe RTS/CTS gives higher throughput than Basic throughput than Basic
due todue to the short RTS framesthe short RTS frames..
( Only exception for n ( Only exception for n =5).=5).
Basic 1Mbps
Number of Stations
0 10 20 30 40 50
Sa
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Basic, simulationBasic, new modelBasic, Wu's model
RTS/CTS, 1Mbps
Number of Stations
0 10 20 30 40 50
Sa
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hro
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0.5
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RTS, simulationRTS, new modelRTS, Wu's model
Model Validation: Simulation vs. Model Validation: Simulation vs. Analysis: Analysis:
5.5 and 11 Mbps.5.5 and 11 Mbps. In both cases analysis andIn both cases analysis and simulation are in simulation are in
satisfactorysatisfactory agreementagreement..
Basic access scheme givesBasic access scheme gives higher throughput thanhigher throughput than RTS/CTS when channel bitRTS/CTS when channel bit rate rate ↑↑. ( RTS, CTS packets. ( RTS, CTS packets are transmitted at 1Mbps).are transmitted at 1Mbps).
Throughput Throughput ↓ ↓ as bit rateas bit rate↑↑ ( DIFS,SIFS, Backoff delay( DIFS,SIFS, Backoff delay remain unchanged)remain unchanged)
Number of Stations
0 10 20 30 40 50
Sa
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hp
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0.0
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0.4
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Basic, SimulationBasic, New ModelRTS, SimulationRTS, New Model
5.5 Mbps
Basic
RTS
Number of Stations
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Basic, SimulationBasic, New ModelRTS, SimulationRTS, New Model
11 Mbps
Basic
RTS
Average Delay SimulationAverage Delay Simulation
As network size As network size ↑↑ delay delay ↑ for both access schemes.↑ for both access schemes. As channel bit rate ↑As channel bit rate ↑ delay ↓.delay ↓. RTS/CTS delay is lower than Basic delay only for RTS/CTS delay is lower than Basic delay only for
1Mbps1Mbps..
Not efficient to use RTS/CTS for high data ratesNot efficient to use RTS/CTS for high data rates
Number of stations
0 10 20 30 40 50
Satu
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Dela
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sec)
0.0
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1.0
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2.0
2.5
1 Mbps
5.5 Mbps
11 Mbps
RTS
Basic
ConclusionConclusion We have developed an analytical model to enhance We have developed an analytical model to enhance
Bianchi’s and Wu’s analytical model for the saturation Bianchi’s and Wu’s analytical model for the saturation throughput of the DCF of the IEEE 802.11 protocol.throughput of the DCF of the IEEE 802.11 protocol.
Our model gives greater throughput results than Wu’s Our model gives greater throughput results than Wu’s model for both access schemes, Basic and RTS/CTS. model for both access schemes, Basic and RTS/CTS.
Via numerous simulations with NS-2 we have shown that Via numerous simulations with NS-2 we have shown that our model is close to simulation, for all network sizes .our model is close to simulation, for all network sizes .
As channel bit rate increases:As channel bit rate increases: throughput decreasesthroughput decreasesAverage delay decreases.Average delay decreases.
Basic vs. RTS/CTSBasic vs. RTS/CTS::In low rates RTS is better than Basic.In low rates RTS is better than Basic.In higher rates Basic is preferable than RTS ( gives In higher rates Basic is preferable than RTS ( gives
greater throughput and lower delay).greater throughput and lower delay).