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Performance Analysis of IEEE 802'11 Distributed Coordination Function DCF

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Title: Performance Analysis of IEEE 802'11 Distributed Coordination Function DCF


1
Performance Analysis of IEEE 802.11 Distributed
Coordination Function (DCF)
  • (By Giuseppe Bianchi, 2000)
  • Presented By
  • Sumegha Bagga
  • 01/18/08

2
Outline
  • Introduction
  • 802.11 DCF
  • Maximum Saturation Throughput Performance
  • Throughput Analysis
  • Model Validation
  • Maximum Saturation Throughput
  • Performance Evaluation
  • Conclusions

3
Introduction
  • 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

4
Basic 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

5
Basic Access Mechanism
6
RTS/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

7
RTS/CTS
8
Saturation Throughput
  • Limit reached by system throughput as offered
    load increases
  • Maximum load system can carry in stable
    conditions

9
Throughput 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

10
Throughput 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

11
Packet 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

12
Markov Chain Model
13
Markov Chain Model
Closed form solution for Markov chain
14
Markov Chain Model
  • Probability t that a station transmits in
    randomly chosen slot time

15
Markov 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

16
Throughput
  • Normalized system throughput S
  • Probability of transmission Ptr
  • Probability of successful transmission Ps

17
Throughput
Normalized system throughput
Specify Ts and Tc to compute throughput for DCF
access mechanism
18
Throughput
  • Considering System via Basic Access mechanism
  • Packet header H PHYhrd MAChrd
  • Propagation delay d

19
Throughput
  • Packet transmission via RTS/CTS Access mechanism

20
Model 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
21
Maximum Saturation Throughput
t depends on n, W, and m
22
Maximum Saturation Throughput
  • Analytical model determines maximum achievable
    saturation throughput

23
Performance Evaluation
  • Greater the network size lower is the throughput
    for basic access

Saturation throughput analysis vs. simulation
24
Performance 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
25
Performance Evaluation
  • Throughput obtained with RTS/CTS mechanism
  • Independent of value of W

Saturation throughput vs. initial window size for
RTS/CTS mechanism
26
Performance Evaluation
  • Number of transmissions per packet increases as W
    reduces network size n increases.

Average number of transmissions per packet
27
Conclusion
  • 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
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