- •Introduction
- •Increasing Demand for Wireless QoS
- •Technical Approach
- •Outline
- •The Indoor Radio Channel
- •Time Variations of Channel Characteristics
- •Orthogonal Frequency Division Multiplexing
- •The 5 GHz Band
- •Interference Calculation
- •Error Probability Analysis
- •Results and Discussion
- •IEEE 802.11
- •IEEE 802.11 Reference Model
- •IEEE 802.11 Architecture and Services
- •Architecture
- •Services
- •802.11a Frame Format
- •Medium Access Control
- •Distributed Coordination Function
- •Collision Avoidance
- •Post-Backoff
- •Recovery Procedure and Retransmissions
- •Fragmentation
- •Hidden Stations and RTS/CTS
- •Synchronization and Beacons
- •Point Coordination Function
- •Contention Free Period and Superframes
- •QoS Support with PCF
- •The 802.11 Standards
- •IEEE 802.11
- •IEEE 802.11a
- •IEEE 802.11b
- •IEEE 802.11c
- •IEEE 802.11d
- •IEEE 802.11e
- •IEEE 802.11f
- •IEEE 802.11g
- •IEEE 802.11h
- •IEEE 802.11i
- •Overview and Introduction
- •Naming Conventions
- •Enhancements of the Legacy 802.11 MAC Protocol
- •Transmission Opportunity
- •Beacon Protection
- •Direct Link
- •Fragmentation
- •Traffic Differentiation, Access Categories, and Priorities
- •EDCF Parameter Sets per AC
- •Minimum Contention Window as Parameter per Access Category
- •Maximum TXOP Duration as Parameter per Access Category
- •Collisions of Frames
- •Other EDCF Parameters per AC that are not Part of 802.11e
- •Retry Counters as Parameter per Access Category
- •Persistence Factor as Parameter per Access Category
- •Traffic Streams
- •Default EDCF Parameter Set per Draft 4.0, Table 20.1
- •Hybrid Coordination Function, Controlled Channel Access
- •Controlled Access Period
- •Improved Efficiency
- •Throughput Improvement: Contention Free Bursts
- •Throughput Improvement: Block Acknowledgement
- •Delay Improvement: Controlled Contention
- •Maximum Achievable Throughput
- •System Saturation Throughput
- •Modifications of Bianchi’s Legacy 802.11 Model
- •Throughput Evaluation for Different EDCF Parameter Sets
- •Lower Priority AC Saturation Throughput
- •Higher Priority AC Saturation Throughput
- •Share of Capacity per Access Category
- •Calculation of Access Priorities from the EDCF Parameters
- •Markov Chain Analysis
- •The Priority Vector
- •Results and Discussion
- •QoS Support with EDCF Contending with Legacy DCF
- •1 EDCF Backoff Entity Against 1 DCF Station
- •Discussion
- •Summary
- •1 EDCF Backoff Entity Against 8 DCF Stations
- •Discussion
- •Summary
- •8 EDCF Backoff Entities Against 8 DCF Stations
- •Discussion
- •Summary
- •Contention Free Bursts
- •Contention Free Bursts and Link Adaptation
- •Simulation Scenario: two Overlapping QBSSs
- •Throughput Results with CFBs
- •Throughput Results with Static PHY mode 1
- •Delay Results with CFBs
- •Conclusion
- •Radio Resource Capture
- •Radio Resource Capture by Hidden Stations
- •Solution
- •Mutual Synchronization across QBSSs and Slotting
- •Evaluation
- •Simulation Results and Discussion
- •Conclusion
- •Prioritized Channel Access in Coexistence Scenarios
- •Saturation Throughput in Coexistence Scenarios
- •MSDU Delivery Delay in Coexistence Scenarios
- •Scenario
- •Simulation Results and Discussion
- •Conclusions about the HCF Controlled Channel Access
- •Summary and Conclusion
- •ETSI BRAN HiperLAN/2
- •Reference Model (Service Model)
- •System Architecture
- •Medium Access Control
- •Interworking Control of ETSI BRAN HiperLAN/2 and IEEE 802.11
- •CCHC Medium Access Control
- •CCHC Scenario
- •CCHC and Legacy 802.11
- •CCHC Working Principle
- •CCHC Frame Structure
- •Requirements for QoS Support
- •Coexistence Control of ETSI BRAN HiperLAN/2 and IEEE 802.11
- •Conventional Solutions to Support Coexistence of WLANs
- •Coexistence as a Game Problem
- •The Game Model
- •Overview
- •The Single Stage Game (SSG) Competition Model
- •The Superframe as SSG
- •Action, Action Space A, Requirements vs. Demands
- •Abstract Representation of QoS
- •Utility
- •Preference and Behavior
- •Payoff, Response and Equilibrium
- •The Multi Stage Game (MSG) Competition Model
- •Estimating the Demands of the Opponent Player
- •Description of the Estimation Method
- •Evaluation
- •Application and Improvements
- •Concluding Remark
- •The Superframe as Single Stage Game
- •The Markov Chain P
- •Illustration and Transition Probabilities
- •Definition of Corresponding States and Transitions
- •Solution of P
- •Collisions of Resource Allocation Attempts
- •Transition Probabilities Expressed with the QoS Demands
- •Average State Durations Expressed with the QoS Demands
- •Result
- •Evaluation
- •Conclusion
- •Definition and Objective of the Nash Equilibrium
- •Bargaining Domain
- •Core Behaviors
- •Available Behaviors
- •Strategies in MSGs
- •Payoff Calculation in the MSGs, Discounting and Patience
- •Static Strategies
- •Definition of Static Resource Allocation Strategies
- •Experimental Results
- •Scenario
- •Discussion
- •Persistent Behavior
- •Rational Behavior
- •Cooperative Behavior
- •Conclusion
- •Dynamic Strategies
- •Cooperation and Punishment
- •Condition for Cooperation
- •Experimental Results
- •Conclusion
- •Conclusions
- •Problem and Selected Method
- •Summary of Results
- •Contributions of this Thesis
- •Further Development and Motivation
- •IEEE 802.11a/e Simulation Tool “WARP2”
- •Model of Offered Traffic and Requirements
- •Table of Symbols
- •List of Figures
- •List of Tables
- •Abbreviations
- •Bibliography
5.1 HCF Contention-based Channel Access |
67 |
Considering all these modifications, the stationary probability distribution b0,0 of an idle system is calculated as
b0,0 |
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The probability τ that a backoff entity is transmitting in a generic slot is calculated by the summation of all stationary distributions bi ,0 , as in Bianchi’s legacy 802.11 model, given by
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In 802.11e, with the controlled channel access, p =1. In this case, all slots are busy at any time, and thereforeτ =1 .
All the rest of the analysis of the saturation throughput can be taken from Appendix D. Note that a generic slot is different to a backoff slot in this thesis. A generic slot may be an idle generic slot during the contention phase, or a busy generic slot during which a frame exchange is completed, or, alternatively, during which a collision occurs. It is referred to as generic slot to differentiate it from the backoff slots, because a generic slot can be a backoff slot or a busy phase with a longer duration than the backoff slot duration.
5.1.2.2Throughput Evaluation for Different EDCF Parameter Sets
The saturation throughput of a number of backoff entities that operate all according to the same AC can be approximated by modifying Bianchi’s legacy 802.11 analysis as explained in the previous Section 5.1.2.1. These approximations are evaluated in this section and compared to WARP2 simulation results. In
68 |
5. Evaluation of IEEE 802.11e with the IEEE 802.11a Physical Layer |
addition to the legacy EDCF parameters, two other EDCF parameter sets are defined in the following. One is referred to as the “higher priority AC” and the other as the “lower priority AC,” as they will allow higher and lower priority in channel access than the legacy priority, respectively. The legacy priority AC is also referred to as “medium priority AC.” The saturation throughput for the medium priority AC can be found in Appendix D, and the results for the higher and lower priority AC are shown in this section. Table 5.2 summarizes the EDCF parameter sets selected for the three ACs. The medium priority AC follows the legacy DCF protocol. The higher priority AC operates with a smaller CWmin [AC ] and a smaller PF [AC ], the lower priority AC operates with a larger CWmin [AC ] and a larger PF [AC ] than what is defined for the legacy DCF. All other EDCF parameters remain equal to the medium priority EDCF parameters.
The contention window sizes for the three priorities for the first four backoff stages are presented in Figure 5.4. It can be seen that the parameter PF has a considerable impact on the resulting contention window sizes. The relative large value of AIFSN for the lower priority ( AIFSN [lower ]= 9 ) together with the larger contention window sizes will give this AC only a very limited priority in channel access.
Larger contention window sizes have also positive effects on the saturation throughput. shows the probability τ that a backoff entity transmits in a generic slot versus the number of backoff entities.
Furher, Figure 5.5 illustrates the probability p that transmission attempts at a particular slot are unsuccessful due to collision, as function of the number of backoff entities. The probabilities are shown for the three known priorities. As expected, the larger the number of backoff entities, the larger the collision probability. Further, the probability that a backoff entity is transmitting at a generic slot decreases with increasing number of backoff entities.
Table 5.2: EDCF parameter sets for the three ACs, as selected for the analysis. The TXOPlimit per AC is not used in this thesis; one value is used for all ACs. Note that the legacy DCF backoff is assumed.
AC (priority): |
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medium(=legacy) |
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CWmax[AC]: |
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5.1 HCF Contention-based Channel Access |
69 |
backoff stage
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CW[AC](stage=4) =499 |
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AIFSN[AC]=9 |
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CW[AC](stage=4) =26 |
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position and size of contention windows of three parallel backoff entities |
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Figure 5.4: EDCF QoS-parameter setting for the three priorities, contention windows of the first four backoff stages.
Remarkably, with any number of backoff entities, the collision probability is higher for the higher probability AC than for the legacy and lower probability ACs, which is a result of the large contention window sizes. With a small number of backoff entities, the probability that a particular backoff entity is transmitting at a generic slot is higher for the higher priority AC than for the other ACs.
Figure 5.6 illustrates three other probabilities of the modified model as functions of the number of backoff entities. The probability that a generic slot is idle is referred to as prob(CCAidle), the probability that a collision occurs, if a generic slot is busy is referred to as prob(collision|CCAbusy) and the probability that a frame exchange is successful, provided that a generic slot is busy, is referred to as
prob(success|CCAbusy). |
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τ = prob(backoff |
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entity transmits) |
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Figure 5.5: Collision and transmission probability (p, τ ) for a single backoff entity in a generic slot, as functions of the number of backoff entities.
70 |
5. Evaluation of IEEE 802.11e with the IEEE 802.11a Physical Layer |
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Figure 5.6: Probability that a generic slot is idle, busy with an collided frame, or busy with a successfully transmitted frame, as functions of the number of backoff entities.
The probabilities are defined in Appendix D.2 as PCCAidle , Pcoll , and Psuccess , respectively. The index “CCA” refers to Clear Channel Assessment (CCA), which is the
carrier sense process in the 802.11 protocol. It can be observed from Figure 5.6 that with increasing number of backoff entities the probability that a generic slot
is idle, PCCAidle , decreases, as expected. In addition, the collision probability Pcoll increases with increasing number of backoff entities, which is again an expected
result. An interesting observation is that the probability Psuccess shows maxima for all ACs, which are at different numbers of backoff entities for the different ACs.
The higher the priority, the smaller the number of backoff entities that define the unique maximum, which is an expected result.
In the next two sections, Section 5.1.2.2.1 and Section 5.1.2.2.2, the saturation throughput calculated for the two priorities “lower” and “higher” are discussed and evaluated. The figures can be compared also to the figures that show the results for the legacy AC, see Figure D.3, p. 237, and Figure D.4, p. 238.
5.1.2.2.1Lower Priority AC Saturation Throughput
Figure 5.7 and Figure 5.8 illustrate the resulting saturation throughput obtained through simulation and analytical approximation with the modified model for the lower priority AC. Shown is the saturation throughput for different PHY modes, and a varying number of backoff entities for the frame body sizes 48, 512, 1514, and 2304 byte. The EDCF parameters as defined in Table 5.2 are used. Figure 5.7 shows the saturation throughput for scenarios without use of RTS/CTS, Figure 5.8 shows results for the same scenarios, with the use of RTS/CTS. The results show the expected characteristics.
5.1 HCF Contention-based Channel Access |
71 |
saturation thrp. (norm.)
1
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BPSK1/2 (6 Mbit/s)
16QAM1/2 (24 Mbit/s) 64QAM3/4 (54 Mbit/s)
thrp. increases with increasing number of backoff entities
with address 4, w/o WEP encrypt.
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(a) 48 byte frame body size.
saturation thrp. (norm.)
1 BPSK1/2 (6 Mbit/s)
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saturation thrp. (norm.)
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lines: modified Bianchi approx.; markers: WARP2 simulation results
Figure 5.7: Normalized saturation throughput for different PHY modes, and a varying number of backoff entities, for lower priority (AC=”lower”). EDCF parameters as defined in Table 5.2. RTS/CTS are not used. The respective legacy saturation throughput is illus- trated in Appendix D.4, Figure D.3, p. 237.
The throughput increases with increasing frame body sizes. The higher the number of backoff entities, the lower the saturation throughput. The higher the PHY mode, the smaller the efficiency of the carrier sense protocol.
RTS/CTS increase the saturation throughput for long frame body sizes, but not for short frame body sizes. For small numbers of backoff entities, the saturation throughput increases with increasing number of backoff entities.
This is an expected result for the lower priority AC with its large initial contention window: as long as the collision probability is not too high, more contending backoff entities result in shorter idle phases and thus higher saturation throughput. Comparing the figures to the results of the legacy priority AC (Figure D.3, p. 237, and Figure D.4, p. 238), the saturation throughput is higher for the lower priority than for the legacy priority, which again is an effect resulting from the lower collision probability at the lower priority AC.