This document presents a Collision Alleviating DCF (CAD) protocol to enhance the performance of the IEEE 802.11 Distributed Coordination Function (DCF) in congested wireless networks. It proposes CAD to reduce contention among nodes and avoid channel capture effects by introducing an inter-packet backoff delay. The document also designs an exact Markov chain model to evaluate the performance of CAD and predicts it will outperform DCF particularly when the number of competing nodes is large, with over a 10% reduction in collision probability.

1. Presented By:
Md. Ahasanul Alam
Roll: 1301
Gottapu, Sasi Bhushana Rao, Madhavi Tatineni, and M. N. V. S. S. Kumar. IET
networks 2.4 (2013): 204-213.

2.  IEEE 802.11 lays down the architecture and specifications of wireless LANs
(WLANs)
 In IEEE 802.11, the basic medium access control (MAC) technique –
Distributed Coordination Function (DCF)
 Markov chain model is used to evaluate the performance of IEEE 802.11 DCF
protocol
 Based on some assumptions and simplification
 Can not accurately predict performance in real network
 This work
 Propose a Collision Alleviating DCF (CAD) to enhance the performance in congested
wireless network
 Design a exact Markov chain model to evaluate and predict the performance of the
protocol
2

3.  MAC protocol based on IEEE 802.11
 Employs a Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA) method
with the binary exponential backoff algorithm
 Data transfer
 2-way handshaking
 Data Packet is Acknowledged by Receiver
 Acks are used to maintain reliability
3
Sender Receiver
Data
ACK
 4-way handshaking
 First, reserve the channel by
Request-to-Send (RTS) and
Clear-to-Send (CTS)
 Then Follow 2-way data
transfer
Sender Receiver
Data
ACK
RTS
CTS

4. 4
No

5.  Can be experienced in saturated networks
 In congested networks
 Collision occurs more frequently
 DCF uses binary exponential backoff
for collision avoidance
 Increase contention window (CW)
 Successful transmission resets the
CW to minimum
 Channel Capture Effect:
 Node with recent successful transmission
may get biased advantage to channel access
5
CW:
16
CW:
16
CW:
8
CW:
32
CW:
16
Access
Channel

6.  Can be experienced in saturated networks
 In congested networks
 Collision occurs more frequently
 DCF uses binary exponential backoff
for collision avoidance
 Increase contention window (CW)
 Successful transmission resets the
CW to minimum
 Channel Capture Effect:
 Node with recent successful transmission
may get biased advantage to channel access
6
CW:
16
CW:
16
CW:
8
CW:
32
CW:
16
Access
Channel

7.  Can be experienced in saturated networks
 In congested networks
 Collision occurs more frequently
 DCF uses binary exponential backoff
for collision avoidance
 Increase contention window (CW)
 Successful transmission resets the
CW to minimum
 Channel Capture Effect:
 Node with recent successful transmission
may get biased advantage to channel access
7
CW:
8
CW:
16
CW:
8
CW:
32
CW:
16
Access
Channel

8.  Can be experienced in saturated networks
 In congested networks
 Collision occurs more frequently
 DCF uses binary exponential backoff
for collision avoidance
 Increase contention window (CW)
 Successful transmission resets the
CW to minimum
 Channel Capture Effect:
 Node with recent successful transmission
may get biased advantage to channel access
8
CW:
8
CW:
16
CW:
8
CW:
8
CW:
16
Access
Channel

9.  Can be experienced in saturated networks
 In congested networks
 Collision occurs more frequently
 DCF uses binary exponential backoff
for collision avoidance
 Increase contention window (CW)
 Successful transmission resets the
CW to minimum
 Channel Capture Effect:
 Node with recent successful transmission
may get biased advantage to channel access
9
CW:
8
CW:
16
CW:
8
CW:
8
CW:
8
Access
Channel

10.  Can be experienced in saturated networks
 In congested networks
 Collision occurs more frequently
 DCF uses binary exponential backoff
for collision avoidance
 Increase contention window (CW)
 Successful transmission resets the
CW to minimum
 Channel Capture Effect:
 Node with recent successful transmission
may get biased advantage to channel access
10
CW:
8
CW:
32
CW:
8
CW:
8
CW:
8
Access
Channel

11.  Provides inter packet backoff (IPB) delay between successive transmission
 Introduces a post-backoff stage
 Reduces contention among nodes and avoid channel capture
 Post-backoff stage window size: (0, W0-1)
 Doesn’t follow binary exponential increment
 Only applicable when the channel is busy and there are more packets in
queue
11

12. 12

13. 13

14. 14
Idle state
Backoff State
Transmitting State
Binary exponential
Backoff
Post Backoff
State

15. State = (i, k) where, i = backoff state, k = backoff delay
q = prob. of at least one packet in node buffer
Pb = prob. of channel is busy
Pe = prob. of transmission error
Pcol = prob. of packet collision
Peq = Pe + Pcol - Pe Pcol = prob. of failed transmission
 Remain idle
 Idle to transition
 Random backoff
 decrease backoff
 Channel busy
 Successful transition,
no new packet
 failed transition,
increase CW
 Successful transition,
post-backoff
 Decrease post backoff
 Channel busy
 Last backoff stage 15

16. 16
bi,k = Probability that a node occupies a given state {i,k} at any descrete time

17. 17
τ = probability with which a node transmits a packet in a randomly
chosen slot time

18. 18
Pe = Frame error probability
Pe = 1 – (success_in_PHY_layer)*(success_in_MAC_layer)
Pe = 1 – (1-PPHY_error) (1-PMAC_error)
PPHY_error = 1 – (1-Pb1)24x8 Pb1 = Physical layer bit error prop.
PLCP preamble and header = 24 bytes
PMAC_error = 1 – (1-Pb2)(28+MSDU)8 Pb2 = MAC layer bit error prop.
MAC header = 28 byte
MSDU = MAC Service Data Unit

19. 19
Performance Analysis
Throughput
End to End Delay

20. 20
Busy and collision probability, Pb & Pcol
τ = node transmitting probability
Ptr = Pb = 1 - prob. of no node transmitting
Ptr = 1 – (1- τ)n
Pcol = 1 - prob. of zero or one node transmitting
Pcol = 1 – prob. of no node transmitting – prob.
of one node transmitting
Pcol = Ptr – (1- τ)n-1
CAD outperforms particularly when the system contains large number of competing nodes.
When the number of nodes is 25, the reduction in collision probability is more than 10%
Fig. Nodes vs Collision Probability

21. 21
s = fraction of time channel is used to transmit
payload bit successfully
s = system throughput
E[St] = per slot time

22. 22
Fig. Throughput vs packet rate, λ when Pe = 10 − 1
Fig. Throughput vs nodes when Pe = 10 − 3 and λ = 50 Pkts/s
2-way
Handshaking
4-way
Handshaking

23. 23
E[Nc] = number of collisions of a frame until its
successful reception
E[D] = End to end delay = mean frame delay
E[BD] = average backoff delay
E[x] = average no. of backoff req. to reach 0
E[S] = average time the counter stop while
listening to other node transmitting
E[Nfon] = average no. of time node detects
transmission from other node

24. End-to-end delay analysis under saturated load conditions
Fig. Nodes against end-to-end delay for various retries Fig. Nodes against end-to-end delay for various data
rates using RTS/CTS mechanism
24

25. End-to-end delay analysis in unsaturated load conditions
Fig. Nodes against end-to-end delay for different
packet arrival rates
25

26. Simulation Parameters
Fig. Simulation parameters using in different experiments
26

27. 27
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