1. 2014 YU-ANTL Lab Seminar
Performance Analysis of the IEEE 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 : ; Fax :

2. 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

3. 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

4. MAC (1)
IEEE 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

5. MAC (2)
WLAN MAC and PHY Layer

6. 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

7. 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

8. DCF (3)
IEEE 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

9. 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

10. 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

11. 802.11 – Slot Time in Bianchi’s Model

12. 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)

13. 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

14. 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

15. Markov Model (3)
Two Dimensional Markov chain

16. 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

17. 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

18. 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 =

19. 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

20. Basic Access Mechanism
H=packet header=PHYhdr + MAChdr
δ=propagation delay
E[P* ]=Average length of the longest packet payload involved in a collision

21. RTS/CTS Access Mechanism
H=packet header=PHYhdr + MAChdr
δ=propagation delay

22. Model Validation & Simulation (1)
Used event‐driven custom simulation program in C++
It closely follows all the 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

23. Model Validation & Simulation (2)

24. 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

25. 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

26. 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

27. 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

28. Model Validation & Simulation (7)
Throughput vs Packet Length
RTS/CTS mechanism is effective when packet size increases

29. Conclusion
Simple but extremely accurate analytical model to study 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