Title: Performance Analysis of IEEE 802'11 Distributed Coordination Function DCF
1Performance Analysis of IEEE 802.11 Distributed
Coordination Function (DCF)
- (By Giuseppe Bianchi, 2000)
- Presented By
- Sumegha Bagga
- 01/18/08
2Outline
- Introduction
- 802.11 DCF
- Maximum Saturation Throughput Performance
- Throughput Analysis
- Model Validation
- Maximum Saturation Throughput
- Performance Evaluation
- Conclusions
3Introduction
- Distribution Coordination Function (DCF)
- Fundamental mechanism to access medium
- Based on CSMA/CA
- Describes two techniques
- Basic Access Mechanism
- RTS/CTS Mechanism
- Key Approximation
- Constant and Independent collision probability
regardless of retransmission
4Basic Access Mechanism
- Channel Idle for a period of time Distribution
Inter Frame Space (DIFS) - Station generates random backoff interval
- Employs discrete-time backoff scale
- Transmission take at beginning of time slot
- Backoff time chosen between (0,w-1)
- Backoff time counter decremented to 0
5Basic Access Mechanism
6RTS/CTS
- Waits until channel sensed idle
- Transmits short frame called RTS
- Receiving station send CTS
- Carry information of length of packet
- Solves hidden terminal problems
7RTS/CTS
8Saturation Throughput
- Limit reached by system throughput as offered
load increases - Maximum load system can carry in stable
conditions
9Throughput Analysis
- Analytical Evaluation of Saturation Throughput
- Fixed number of stations having packet for
transmission - Analysis divided into two parts
- Study the behavior of single station with a
Markov model - Study the events that occur within a generic slot
time expressed throughput for both Basic
RTS/CTS access method
10Throughput Analysis
- Two cases
- Packet Transmission Probability
- Stochastic Process b(t) representing backoff time
counter - Stochastic Process s(t) representing backoff
stage - Markov Chain model
- Throughput
11Packet Transmission Probability
- Each packet collide with constant and independent
probability - Results more accurate as long as W gets larger
- Model bi-dimensional process s(t) , b(t) with
discrete-time Markov chain
12Markov Chain Model
13Markov Chain Model
Closed form solution for Markov chain
14Markov Chain Model
- Probability t that a station transmits in
randomly chosen slot time
15Markov Chain Model
- When m 0 no exponential backoff is considered
probability t results independent of p
- In general t depends on conditional collision
probability p
16Throughput
- Normalized system throughput S
- Probability of transmission Ptr
- Probability of successful transmission Ps
17Throughput
Normalized system throughput
Specify Ts and Tc to compute throughput for DCF
access mechanism
18Throughput
- Considering System via Basic Access mechanism
- Packet header H PHYhrd MAChrd
- Propagation delay d
19Throughput
- Packet transmission via RTS/CTS Access mechanism
20Model Validation
- Compared analytical results with that obtained by
means of simulation - Analytical model extremely accurate
- Analytical results (lines) coincide with
simulation results (symbols) in both Basic Access
RTS/CTS cases
Saturation throughput analysis vs. simulation
21Maximum Saturation Throughput
t depends on n, W, and m
22Maximum Saturation Throughput
- Analytical model determines maximum achievable
saturation throughput
23Performance Evaluation
- Greater the network size lower is the throughput
for basic access
Saturation throughput analysis vs. simulation
24Performance Evaluation
- Throughput of Basic Access mechanism depends on W
- W depends on number of terminals
- High value of W gives excellent throughput
performance
Saturation throughput vs. initial window size for
Basic Access mechanism
25Performance Evaluation
- Throughput obtained with RTS/CTS mechanism
- Independent of value of W
Saturation throughput vs. initial window size for
RTS/CTS mechanism
26Performance Evaluation
- Number of transmissions per packet increases as W
reduces network size n increases.
Average number of transmissions per packet
27Conclusion
- Model suited for both Basic Access and RTS/CTS
Access mechanisms - Evaluated the 802.11 throughput performance
- Performance of Basic Access method depends on W
and n - RTS/CTS scheme solved hidden terminal problem