2014 YU-ANTL Lab Seminar
Performance Analysis of the IEEE
802.11 Distributed Coordination Function
Giuseppe Bianchi
April 12, 2014
Yashashree Jadhav
Advanced Networking Technology Lab. (YU-ANTL)
Dept. of Information & Comm. Eng, Graduate School,
Yeungnam University, KOREA
(Tel : +82-53-810-3940; Fax : +82-53-810-4742
http://antl.yu.ac.kr/; E-mail : [email protected])
Outline (1)
 Background
 MAC
 DCF
 Basic Access Mechanism
 RTS/CTS Mechanism
 Main Idea
 Contribution
 Markov Model
 Probabilities
 Two Dimensional Markov chain
 Packet Transmission Probability
 Throughput
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Outline (2)
 Basic Access Mechanism
 RTS/CTS Access Mechanism
 Model Validation & Simulation
 Model Validation
 Maximizing Saturation Throughput
 Throughput vs Number of Stations
 Throughput vs Initial Window Size
 Throughput vs Max. Back‐off Stage
 Throughput 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 responsible 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 Monitor the
channel activity
 If senses idle for DIFS (Distributed Inter Frame Space), the station
transmits
 CSMA/CA
 If sensed busy (immediately or during the DIFS),the station persists to
monitor until it is measured idle for DIFS
 The station generates a random back‐off interval before transmitting 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 chosen 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 function 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 transmitting
 At each transmission attempt each packet collides with constant and independent
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‐time
Markov 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, Wi)
 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 subsequent
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‐1
station 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 payload bits]
 Ptr=probability that there is at least one transmission 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 successful
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 depends 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 throughput 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
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