ECE 475: Computer Networks
Homework # 2. The Distributed Coordination Function (DCF) of 802.11
Due Oct. 7, 11:59 pm
1 Preliminaries
• Read the description. When you are finished reading it, read it again.
• Due date: Oct 7, 2016 11:59 pm.
• You are free to use a programming language of your choice. You are also welcomed to use existing
discrete-time event network simulators such as NS-3 (https://www.nsnam.org/). Several wireless
modules are readily available.
2 Description
You are to study the performance of multiple access protocols in a wireless setting. Consider the network
shown in Figure 1. The circles denote the communication range R of each station. We are interested in
the following two scenarios:
A. Concurrent Communications: Stations A,B,C, and D of Figure 1(a) are within the same
collision domain (any transmission is received by all). Communication takes place between pairs A→ B
and C → D. Traffic is generated at A and C according to a Poisson distribution with parameters λA and
λC , respectively.
B. Hidden Terminals: stations A,B,C, and D of Figure 1(b), belong to separate collision domains.
Communication takes place between pairs A→ B and C → D. Traffic is generated at A and C according
to a Poisson distribution with parameters λA and λC , respectively.
For each scenario, compute relevant performance metrics for the following multiple access protocols.
A time-slotted system is assumed.
1. CSMA with Collision Avoidance (CSMA/CA) according to the 802.11 DCF function.
(a) A station Tx ready to transmit (when a frame has arrived for transmission from the upper
layers of the network stack), senses the channel for an initial period of DIFS time.
(b) If the channel is busy, Tx (and every other station with a frame for transmission) monitors
the channel until it becomes idle. When the channel becomes idle for DIFS time, Tx selects a
random backoff value in [0, CW − 1]. Tx decrements his counter by one with every idle slot. If
the channel becomes busy, Tx freezes its backoff counter.When the counter reaches zero, Tx
transmits its frame.
1
A B
C D
A B C D
R
(a) (b)
Figure 1: (a) Topology for parallel transmissions within the same collision domain, (b) topology for
parallel transmissions when A and C are hidden terminals.
(c) If the frame is successfully received (no collision) by Rx, the station Rx replies with an ACK
frame after SIFS time. This completes the transmission round and the protocol repeats for
the next transmission. For successive transmissions, the station has to sense for DIFS time
before starting the countdown.
(d) If a collision occurs, the stations that collided double their contention window CW and repeat
the backoff process by selecting a backoff value in [0, CW − 1]. The CW value cannot exceed
threshold CWmax.
2. CSMA/CA with virtual carrier sensing enabled: RTS and CTS frames are exchanged before the
transmission of a frame. If RTS transmissions collide, stations invoke the exponential backoff
mechanism outlined in 1(c). Otherwise, stations that overhear an RTS/CTS message defer from
transmission for the time indicated in the NAV vector.
3 Simulation parameters
Parameter Value Parameter Value
Data frame size 1,500 bytes ACK, RTS, CTS size 30 bytes
Slot duration 20 µs DIFS duration 40 µs
SIFS duration 10 µs Transmission rate 6 Mbps
CW0 4 slots CWmax 1024 slots
λ {50, 100, 200, 300, 400, 500} frames/sec Simulation time 10 sec
4 Performance Metrics
Evaluate the protocol performance with respect to the following metrics:
Throughput T : The individual station’s throughput as a function of λ.
Collisions N : The number of collisions (data and RTS/CTS) as a function of λ.
Delay D: The average frame delay as a function λ.
Fairness Index FI: The fraction of time that the channel is occupied by pair A→ B over the fraction
of time that the channel is occupied by pair C → D as a function of λ.
Your experiments must be repeated for two different scenarios: (a) λA = λC = λ and (b) λA =
2λ, λC = λ. Assume no losses due to the imperfections of the wireless medium.
2
5 Report
Include with your report:
• A brief introduction describing the Homework
• A description on how you developed your simulations.
• Graphs for each of the simulated scenarios.
1. Throughput T
(a) Node A: Throughput T (Kbps) vs. rate λ (frames/sec) for scenarios A and B, and CSMA
implementations 1 and 2 (four lines in total).
(b) Node C: Throughput T (Kbps) vs. rate λ (frames/sec) for scenarios A and B, and CSMA
implementations 1 and 2 (four lines in total).
(c) Node A: Throughput T (Kbps) vs. rate λ (frames/sec) for scenarios A and B, and CSMA
implementations 1 and 2, when λA = 2λC (four lines in total).
(d) Node C: Throughput T (Kbps) vs. rate λ (frames/sec) for scenarios A and B, and CSMA
implementations 1 and 2, when λA = 2λC (four lines in total).
2. Collisions N
(a) Node A: Number of collisions N vs. rate λ (frames/sec) for scenarios A and B, and
CSMA implementations 1 and 2 (four lines in total).
(b) Node C: Number of collisions N vs. rate λ (frames/sec) for scenarios A and B, and
CSMA implementations 1 and 2 (four lines in total).
(c) Node A: Number of collisions N vs. rate λ (frames/sec) for scenarios A and B, and
CSMA implementations 1 and 2, when λA = 2λC (four lines in total).
(d) Node C: Number of collisions N vs. rate λ (frames/sec) for scenarios A and B, and
CSMA implementations 1 and 2, when λA = 2λC (four lines in total).
3. Delay D
(a) Node A: Average delay D vs. rate λ (frames/sec) for scenarios A and B, and CSMA
implementations 1 and 2 (four lines in total).
(b) Node C: Average delay D vs. rate λ (frames/sec) for scenarios A and B, and CSMA
implementations 1 and 2 (four lines in total).
(c) Node A: Average delay D vs. rate λ (frames/sec) for scenarios A and B, and CSMA
implementations 1 and 2, when λA = 2λC (four lines in total).
(d) Node C: Average delay D vs. rate λ (frames/sec) for scenarios A and B, and CSMA
implementations 1 and 2, when λA = 2λC (four lines in total).
4. Fairness Index FI
(a) Fairness Index FI vs. rate λ (frames/sec) for scenarios A and B, and CSMA implemen-
tations 1 and 2 (four lines in total).
(b) Fairness Index FI vs. rate λ (frames/sec) for scenarios A and B, and CSMA implemen-
tations 1 and 2, when λA = 2λC (four lines in total).
• Justification for the results shown in your graphs.
3
Appendix
Generating Poisson-distributed traffic: To generate Poisson-distributed traffic, it is sufficient to
generate a series of exponentially-distributed inter-arrival times. Such times can be generated using the
inverse CDF transformation method.
Step 1: Generate a series of uniformly distributed numbers U = {u1, u2, . . . , un} with ui ∈ (0, 1), ∀i.
Step 2: Compute series X = {x1, x2, . . . , xn} of exponentially distributed numbers with λ, as
X = −
1
λ
ln(1− U) (1)
Using X, you can determine the time of each frame arrival at each station. For instance, frame 1
arrives at time x1, frame 2 arrives at time x1 + x2, etc. The inter-arrival time generation process has to
be repeated for each transmitting station.
Plotting tips:
1. Label your axes and use appropriate units
2. Make sure the scales on both axes are appropriate. If you are to use the same variable on multiple
plots (e.g. throughput) use the same scale on all plots so they can be compared
3. Do not superimpose more than 4-5 plot lines on the same plot.
4. If more than one plot lines are present in the same plot make sure to individually label each one
5. For individual plot lines use different marker shapes so they can be distinguishable.
6. Keep in mind that colors do not show on a black and white printout. So if you color code your
lines, use some other discernable labeling such as dashed lines to differentiate between plot lines.
MATLAB code for generating good figures
close all; % closes all open figure windows
set(0,’defaulttextinterpreter’,’latex’); % allows you to use latex math
set(0,’defaultlinelinewidth’,2); % line width is set to 2
set(0,’DefaultLineMarkerSize’,10); % marker size is set to 10
set(0,’DefaultTextFontSize’, 16); % Font size is set to 16
set(0,’DefaultAxesFontSize’,16); % font size for the axes is set to 16
figure(1)
plot(X, Y1, ’-bo’, X, Y2, ’–rs’, X); % plotting three curves Y1, Y2 for the same X
grid on; % grid lines on the plot
legend(’CSMA’, ’CSMA w. virtual Sensing’);
ylabel(’$T$ (Kbps)’);
xlabel(’$λ$’ (frames/sec));
4
Student Name:
GRADE
CSMA/CA
1 Throughput /6
2 Number of Collisions /6
3 Delay /6
4 Fairness Index /5
Virtual Carrier Sensing
5 Throughput /6
6 Number of Collisions /6
7 Delay /6
8 Fairness Index /5
CSMA/CA
9 Throughput /6
10 Delay /6
11 Number of Collisions /6
12 Fairness Index /5
Virtual Carrier Sensing
13 Throughput /6
14 Delay /6
15 Number of Collisions /6
16 Fairness Index /5
Presentation /8
/100
Implementation of CSMA
Total
Implementation of virtual carrier sensing
Grading rubric
Scenario A – One Collision Domain Topology
Implementation of CSMA
Scenario B – Hidden Terminal Topology
Implementation of virtual carrier sensing
project1_2012hwrk1
rubric