1. COE-541 LAN / MAN Simulation & Performance Evaluation of CSMA/CA
Presented by
Muhamad Khaled Alhamwi

2. Outline Introduction & Background Simulation Model Proposed Approaches
Event-Driven Approach
Implementation
Simulation Results
Conclusions
Q & A

3. Introduction IEEE 802.11 is the standard for Wireless LANs
Has two modes of operations
Point Coordination Function (PCF)
Distributed Coordination Function (DCF)
CSMA/CA Protocol
Contention-based

4. CSMA/CA
CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance
Differs from CSMA/CD: CSMA with Collision Detection
Collision can not be detected
Inter Frame Space (sorted by length - priority)
Short (SIFS)
PCF (PIFS)
DCF (DIFS)

5. CSMA/CA If a station has data to transmit
Sense medium, and if it is idle for DIFS, transmit
Otherwise, defer transmission until the medium is sensed idle
Invoke the backoff algorithm

6. Backoff Procedure
A station invokes its backoff algorithm when it defers its transmission
A backoff timer is chosen randomly from [0 .. CW]
This timer is decremented while the medium is sensed idle
When the timer reaches zero, the station transmits its frame
After each unsuccessful transmission CW is doubled up to CWmax

7. CSMA/CA Example Station A Station B Station C Station D Station E
Frame
Defer
Station B
Frame
Defer
Station C
Frame
Defer
Station D
Frame
Defer
Station E
DIFS
DIFS
DIFS
Remaining Backoff
Backoff

8. Simulation Model Event-driven approach Multi-threaded approach
Single thread that performs all operations
All operations are performed using events
An event processing results in changing system state, and can result in generation of multiple other events
Multi-threaded approach
Each station has two threads
One for packets generation
The other one for packets transmission applying CSMA/CA

9. Event-Driven Approach (1)
Event is defined in terms of
Station Id
Event Type
Occurrence Time
Example
An event of packet generation for station 1 at time 500
{1, Generate_Packet, 500}
Maintain an ordered list of events according to their occurrence (time)

10. Event-Driven Approach (2)
Start
Initialize Variables
Create “Generate_Packet” event for every station
Select Next Event(s)
Check Event Type
Process Event
Update System Variables
Is Simulation
Over?
Generate Report
End No
Yes

11. Event-Driven Approach (3)
Available events types
GENERATE_PACKET
START_CSMA
DEFER
START_BACKOFF
DEC_BACKOFF
STOP_BACKOFF
TRANSMIT_FRAME
TRANSMIT_ACK
FREE_MEDIUM
RECV_ACK
NO_ACK

12. Event-Driven Approach (4)
Condition / Next Event Time
Event_Type (1)
Actions to be done
Event_Type (2)
Actions to be done
Queue is empty / +1
Generate_Packet
Insert packet in q …
Start_CSMA
Check medium …
When processing “Generate_Packet” event, “Start_CSMA” event will be generated if “Queue is empty”
+1 means that the event will be fired in the next time slot

13. Event-Driven Approach (5)
GENERATE_PACKET
Enqueue packet
Check queue length
START_CSMA
Check Medium
DEFER
Check Medium
Initialize backoff
q = 0 / +1
Busy / Fin
/ + Inter-arrival
Idle / +DIFS
Idle / +DIFS
Backoff > min / +1
Retry < limit/+1
q > 0 / Fin+1
TRANSMIT_FRAME
Medium = Busy
Fin = cur + Frame len
Check queue len
Check collision
DEC_BACKOFF
Dec Backoff
Compare Backoff
START_BACKOFF
Check Medium
Counter++
Backoff = min / +1
/ +min
No collision/ Fin+SIFS
/ Fin
TRANSMIT_ACK
Medium = Busy
Fin = cur + ACK len
RECV_ACK
Remove frame
Decrease CW
Collision / Fin + Timeout
/ Fin
FREE_MEDIUM
Medium = Idle
/ Fin
NO_ACK
Increase CW
Increase Retry

14. Implementation MATLAB is used as development tool
Events are maintained in a sorted list
List is kept sorted while adding
Multiple scenarios can be created and run
Results can be plotted and multiple scenarios can be compared
Simulation results can be saved
Simulation can be resumed later

15. Simulator Snapshot (1)
Main GUI

16. Simulator Snapshots (2)
Plot Results

17. Simulation Parameters
Packet Inter-arrival Time (Exponential): 2 msec
Slot Time: 20 usec
SIFS: 10 usec
DIFS: 50 usec
Data Rate: 1Mbps
CWmin: 31
CWmax: 1023
Propagation Delay: 0
Retry Limit: 7
ACK Size: 38 Bytes
ACK Timeout: 500 usec
Station Queue Size: 300 packets

18. Performance Measures LAN Throughput Collision Ratio
Successful Transmission Time / Simulation Time
Collision Ratio
Number of collided transmissions / Overall transmissions
Average Queue Length
Average of queue lengths of all stations
Average Queuing Time
Average queuing times of all stations

19. Simulation Results (1)
WLAN Throughput

20. Simulation Results (2)
Collision Ratio

21. Simulation Results (3)
Average Queue Length

22. Simulation Results (4)
Average Queuing Time (microseconds)

23. Conclusions
Simulator efficiency is very important to get reasonable results in a limited time period
Event-Driven simulation can be very efficient when it is well implemented
Multi-threaded approach is not scalable
When increasing the number of stations

24. References (main)
IEEE Std , 1999 Edition, Information technology – Telecommunications and information exchange between systems – Local and metropolitan area networks – Specific requirements. Part 11: MAC and PHY specifications
A. Al-Akeel, "Optimizing Backoff Procedure for Enhanced Throughput and Fairness in Wireless LANs", MS Thesis, King Fahd University of Petroleum & Minerals 2007

25. Thank you
Q & A