Performance Analysis of the IEEE 802.11 Distributed Coordination
Function Giuseppe Bianchi
2014 YU-ANTL Lab Seminar
April 12, 2014
Yashashree JadhavAdvanced Networking Technology Lab. (YU-ANTL)
Dept. of Information & Comm. Eng, Graduate School, Yeungnam University, KOREA
(Tel : +82-53-810-3940; Fax : +82-53-810-4742http://antl.yu.ac.kr/; E-mail : [email protected])
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Outline (1) Background
MACDCFBasic Access MechanismRTS/CTS Mechanism
Main IdeaContribution
Markov ModelProbabilitiesTwo Dimensional Markov chainPacket Transmission ProbabilityThroughput
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Outline (2)Basic Access Mechanism RTS/CTS Access Mechanism
Model Validation & SimulationModel ValidationMaximizing Saturation ThroughputThroughput vs Number of StationsThroughput vs Initial Window SizeThroughput vs Max. Back‐off StageThroughput vs Packet Length
Conclusion
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MAC (1) IEEE802.11 is a set of standards for wireless local
area network (WLAN) This paper’s interest is in MAC layer
The MAC layer is a set of protocols which is respon-sible for maintaining order in the use of a shared medium
The MAC layer defines two different access methods The Distribution Coordination Function (DCF)
Random access scheme Based on CSMA/CA Protocol
The Point Coordination Function (PCF) Based on TDMA
Paper focus on DCF
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MAC (2) WLAN MAC and PHY Layer
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DCF (1) When a station wants to transmit a new packet Mon-
itor the channel activity
If senses idle for DIFS (Distributed Inter Frame Space), the sta-tion transmits
CSMA/CA
If sensed busy (immediately or during the DIFS),the station per-sists to monitor until it is measured idle for DIFS
The station generates a random back‐off interval before trans-mitting to minimize the collision probability
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DCF (2) It describes two techniques to employ for packet
transmission
Basic access mechanism (two‐way handshaking) Source transmits the packet If destination receives successfully transmits a positive ACK
RTS/CTS mechanism (four‐way handshaking) Source sends RTS If destination receives RTS then sends CTS So the channel reservation is done Source then transmits the packet If destination receives successfully transmits a positive ACK
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DCF (3) IEEE 802.11 DCF
At each packet transmission, the back‐off time is uniformly cho-sen in the range(0,w‐1) where w=contention window
w depends on the number of transmissions failed for the packet At first, w=CWmin (minimum contention window) At each unsuccessful, w is doubled (binary back‐off) up to a
maximum value CWmax=2mCWmin The back‐off time counter is Decremented as long as channel is
sensed idle Frozen when a transmission is detected on the channel Reactivated when the channel is sensed idle for more than a
DIFS The station transmits when the back‐off time reaches zero
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Basic Access Mechanism Basic Access Mechanism
station has to wait for DIFS before sending data receiver acknowledges at once (after waiting for SIFS) if the packet was
received correctly (CRC) automatic retransmission of data packets in case of transmission errors
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RTS/CTS Access Mechanism RTS/CTS Access Mechanism
station can send RTS with reservation parameter after waiting for DIFS ( reservation determines amount of time the data packet needs the medium)
acknowledgement via CTS after SIFS by receiver (if ready to receive) sender can now send data at once, acknowledgement via ACK other stations store medium reservations distributed via RTS and CTS
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802.11 – Slot Time in Bianchi’s Model 802.11 – Slot Time in Bianchi’s Model
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Contribution
Analytical evaluation of the saturation throughput Ideal channel conditions (no hidden terminals and capture) Fixed number of stations where each station having a packet available
for transmission Behavior of single station is studied with a Markov model The packet transmission probability (τ) of a station in randomly chosen
slot time is obtained which is independent of access mechanism The throughput of the both access mechanism is expressed as a func-
tion of τ In saturation, each station has immediately a packet available for
transmission Each packet needs to wait for a random back‐off time before transmit-
ting At each transmission attempt each packet collides with constant and in-
dependent probability (p)
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Markov Model (1) s(t) : stochastic process of back‐off stage of a station
at time t b(t): stochastic process of back‐off time counter for a
station Defines W=CWmin m=maximum back‐off stage such that CWmax=2mW Wi= 2iW where i Є(0,m) is the back‐off stage It is possible to model the bi‐dimensional process {s(t),b(t)} with the
discrete‐timeMarkov chain
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Markov Model (2) Probabilities P{i, k |i, k+1}=1 k Є (0,Wi ‐2) and i Є (0, m)
At the beginning of each slot time the back‐off time is decremented P{0, k |i, 0}=(1-p)/W0 k Є (0,W0 ‐1) and i Є (0, m)
New packet following a successful transmission (probability=1‐p) and starts with back‐off stage 0.The back‐off is initially chosen between (0, W0‐1)
P{i, k |i-1, 0}=p/Wi k Є (0,Wi ‐1) and i Є (1, m) Unsuccessful transmission (probability=p) occurs at back‐off stage i-
1,The new back‐ off is uniformly chosen between (0, W i) P{m, k |m, 0}=p/Wm k Є (0,Wm ‐1)
Once the back‐off stage reaches the value m, it is not increased in sub-sequent packet transmission
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Markov Model (3) Two Dimensional Markov chain
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Markov Model (4) Packet Transmission Probability bi, k= lim t-> ∞ P{s (t)=i, b(t)=k} , k Є (0,Wi ‐1) and
i Є(0,m) Stationary distribution of the chain Closed‐form solution is needed
All the bi, k values can be expressed as functions of the values b0,0 and p
τ = probability that a station transmits in a randomly chosen slot time
transmission occurs when back‐off counter=0 regardless of the back‐off stage
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Markov Model (5) Packet Transmission Probability
When m=0 (no exponential back‐off)
One station transmits, collision occurs when at least one of the other n‐1station transmits
Using the two equations it can be derived that
τ (p) Can be shown to be a monotone decreasing function that Starts from ,reduces up to
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Throughput (1) S=Normalized system throughput [fraction of time
the channel is used to successfully transmit pay-load bits]
Ptr=probability that there is at least one transmis-sion in the considered slot time=p=1‐(1‐ τ)n
Ps=probability that a transmission in the channel is successful =
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Throughput (2) E[P]=average packet payload size PtrPs=probability of successful transmission in a slot
time 1-Ptr=probability of the empty slot time Ptr (1-Ps)=probability of collision Ts =average time the channel is busy due to success-
ful transmission Tc =average time the channel is busy during a collision σ=duration of an empty slot time
S depends mainly on Ts and Tc
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Basic Access Mechanism H=packet header=PHYhdr + MAChdr
δ=propagation delay
E[P* ]=Average length of the longest packet payload involved in a collision
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RTS/CTS Access Mechanism H=packet header=PHYhdr + MAChdr δ=propagation delay
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Model Validation & Simulation (1) Used event‐driven custom simulation program in C+
+ It closely follows all the 802.11 protocol details for
each in dependent transmitting station The analytic model is extremely accurate The analytic results (lines) practically coincide with
the simulation results (symbols) in both basic and RTS/CTS access
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Model Validation & Simulation (2) Model Validation
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Model Validation & Simulation (3) Maximizing Saturation Throughput
Max throughput achievable by Basic is very close to by RTS/CTS Throughput of RTS/CTS is less sensitive on τ RTS/CTS throughput has a much lower dependence on the system
engineering parameters
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Model Validation & Simulation (4) Throughput vs Number of Stations
The greater the network size, the lower is the throughput [Except W=32]
For Basic Access it varies with the values of n For RTS/CTS it is almost independent of n
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Model Validation & Simulation (5) Throughput vs Initial Window Size
For both Basic Access and RTS/CTS , a high value of W de-pends on the n
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Model Validation & Simulation (6) Throughput vs Max. Back‐off Stage
For both Basic Access and RTS/CTS , with W=32 and n=10 –50
Choice of m doesn’t practically affect the system through-put as long as is m is greater than 4 or 5
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Model Validation & Simulation (7) Throughput vs Packet Length
RTS/CTS mechanism is effective when packet size increases
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Conclusion Simple but extremely accurate analytical model to
study 802.11 DCF Covers both Basic Access and RTS/CTS mechanism as
well as the hybrid one Provides good simulation results with comparison The best analytical model so far for DCF Finite number of terminals No hidden terminal Fixed Data Rate