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On Optimizing Backoff Counter Reservation and
Classifying Stations for the IEEE 802.11 Distributed
Wireless LANs
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IEEE 802.11 (MAC)
• DCF - Distributed Coordination Function
• PCF - Point Coordination Function
• CSMA/CA with binary exponential backoff
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DCF Enhancements
• Many enhancements to improve performance, models with hiddent terminal
• Baldwin – Transmission deadline– Stations next backoff value - Enhanced Collision
Avoidance (ECA)– Decrease # collisions under constant backoff
window size
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Proposal
• These studies show: Increase # competing stations --> performance sharp decrease
• BCR-CS Backoff Counter Reservation and Classifying Stations– Reduce collisions– Improve performance
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BCR-CS
• Main reason for collision in DCF is that other station do not know other station’s info such as backoff counter
• If known, unecessary collisions and wasted waiting time can be avoided
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BCR-CS
• Backoff counters of next frames generated in advance and sent in frame transmissions.
• Random backoff counter generated and embedded into header info of next frame
• Classify stations into 3 groups– Idle - no frame to transmit– Reserved - frames ready and backoff counters
announced success through previous frames– Contentious - frames ready and not success
announced
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BCR-CS
• Frames in reserved group do not collide• Frames in Contentious group do collide
because of unknown backoff counter
• BCR-CS subschemes - based upon # stations in contentious group– BCR-CS-b - original binary backoff– BCR-CS-p - psuedo-p-persistent
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Backoff Counter Table
• Store other station’s backoff counter’s inside table.
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BCR-CS-p (pseudo-p-persistent)
• Goal is to avoid choosing conflicted slots already reserved by other stations
• Contentious Group1) Choose smallest available backoff counter2) Xmit when counter reaches zero3) If frame xmit fails, repeat 1 & 2
• Reserved Group– CW values are doubled if there is a collision– Collisions only possible if there are hidden nodes– Initial window size = NR + NC
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BCR-CS-b (exponential backoff)
• Ordinary Exponential backoff
• Initial window size = NR + NC
• Reserved Group– Since collisions are very rare, use min CW– Throughput maximized when CW = 1– Min CW may cause starvation for
contentious group
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Estimation of NR + NC
• NR is approx the # of BCT values != -1
• NI is approx the # of BCT values = -1
• NR + NC + NI varies as nodes move and power down
• We can analyze historic NC
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Estimate by Time (EBT)
• Modify MAC headers to include time spent by the transmitting station in each state– TR(j) + TC (j) + TI (j) = 1
• Delete stations from BCT have CW = -1 for long periods bc they have moved away
• Approximate NC by summing TC (j) • Exponential smoothing can improve
estimate
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Estimate by Probability (EBP)
• Modify MAC headers to include probability that the transmitting station is in each state– Reserved frames / total frames
• Sum probabilities
• Exponential smoothing can improve estimate
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Utilization vs p
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Optimal p vs M (# of stations)
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Optimal U vs M (# of stations)
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Effects of Estimating M
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Performance evaluation under two classes of traffic
• comprehensive evaluation for the proposed schemes
• comparison with the DCF and the ECA
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• frame payload : 500 bytes• beacon interval : 100 ms• DIFS time : 34 s• SIFS time : 16 s• slot time : 9s• physical preamble : 16 s• physical header time : 4s• symbol time : 4 s• control rate : 24Mbps• data rate : 54Mbps• backoff minimal window size :32
• maximum backoff window size : 1,024
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• type A station : always has at least a frame ready to send in the queue at any time
• type B station : a frame only arrives after the previous frame is just transmitted
• NA : number of type A stations • NB : number of type B stations
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Pseudo-p-Persistent
a)Throughput with different p values )No of collisions with different p values
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Comparison over Simulation Time
c) Throughput versus simulation time d) No of collisions versus simulation time
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Comparison over traffic pattern
a) Total throughput b) Number of collisions
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Comparison over traffic pattern
c) idle time d) collision time
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PERFORMANCE EVALUATION UNDER q
• new metric q - defined as the probability that an outgoing frame arrives when the queue is not empty in a station
We study
• performance of the proposed schemes on different traffic situations
• performance over the mean
• performance over the variance
• Two extreme cases of q
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E(q) = 0
E(q) = 1
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Comparison of Schemes under q
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Effects of q Distributions
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Comparison of NC Estimated Methods andReal Value
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Conclusion
• New scheme for contention based protocol : BCR-CS
• Two different back off schemes
• Three key aspects- reservation, classification, and optimality
• scheme outperforms the DCF and ECA
• Two estimation methods of the number of contentious stations are proposed
• Simulation studies are performed to compare the new protocol with the DCF and ECA