Post on 11-Jan-2016
Delay Analysis of IEEE 802.11 in Single-Hop Networks
Marcel M. Carvalho, J.J.Garcia-Luna-Aceves
Outline
The Distributed coordination function mechanism
Service time characterization Channel probabilities Model validation Performance evaluation of DSSS and FHSS Conclusion
The Distributed coordination function(DCF) backoff
DCF describes two techniques for packet transmissiom Basic access mechanism RTS/CTS access mechanism
The backoff time counter is decremented if the channel is sensed idle
Otherwise it is frozen in its current state until the channel is sensed idle more than a DIFS
Notation Three possible events a node can sense during its backoff
= {successful transmission} , ={idle channel} , ={collision} , Let denote the backoff stage, Let , , and denote the number of idle slots, collision
slots and successful transmission slots respectively, and , and denote their probabilities respectively.
and is the average time the channel is sensed busy due a collision and a successful transmission respectively. is the time used when the channel is sensed idle (one backoff time)
The average backoff step size is
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ct st
Service time characterization At k-th backoff stage, the number of backoff steps is chosen
uniformly from . The average number of backoff steps is The average time a node spends at k-th stage is Let the probability of collision. The probability of successful
transmission at k-th stage is The cumulative delay of a node at k-th backoff stage is
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Service time characterization The average single-hop delay considering the frame
retry limit is given by the following expression
Where
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Channel Probabilities This model is applicable whenever , , and are known These probabilities are computed for saturated, single hop ad
hoc network under ideal channel condition (i.e, no hidden terminals and capture), with fixed number of nodes
This analysis is based on Bianchi framework, the probability that a node transmits in a randomly chosen slot time is
Where p is the probability of a collision experienced by a transmitted packet on the channel
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Channel Probabilities Let then
Using the Taylor series expansion of at p=0 , the first order approximation of is
is approximated the following
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Channel Probabilities
Channel Probabilities
Channel Probabilities
Probability that there is one transmission in the considered time slot is
Let is the probability that a transmission occurring on the channel is successful
The probability of collision is
The probability of successful transmission is
The probability of idle slot is
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Model Validation Ns-2 simulator Network size from 8 to 56
nodes Packet size 1500 bytes Nodes randomly distributed Network area 20*20 m No mobility Performance metrics:
service time and jitter
Model Validation
Performance Evaluation
Performance evaluation
Performance Evaluation
Conclusion DSSS performs better than FHSS in term of delay and
jitter The higher the initial contention window size, the
smaller the average service time and jitter are, especially for large networks
The binary exponential backoff algorithm has negative impact if both the maximum backoff stage and the number of nodes in the network are large