- •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
5.4 HCF Controlled Channel Access, Coexistence of Overlapping QBSSs |
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5.4.1Prioritized Channel Access in Coexistence Scenarios
Figure 5.34 gives an example where more than one HC operate at the same time, in a coexistence scenario. A typical EDCF frame exchange is shown. One MSDU is delivered through contention-based channel access, during an EDCF-TXOP. Within this EDCF-TXOP, RTS, CTS, the data frame that is carrying the MSDU and the ACK are transmitted. In an isolated QBSS, the length of this EDCFTXOP is limited by the TXOPlimit to guarantee that the channel will be sufficiently often idle to allow the allocation of CAPs with some required delays.
The TXOPlimit is under control of the HC in that isolated QBSS. When QBSSs overlap, however, HCs can only control the duration of the EDCF-TXOPs within their own QBSS. That means that if one HC requires a small TXOPlimit, but the coexisting backoff entities of the overlapping QBSSs follow the larger TXOPlimit of a second HC, then the first HC will not meet its delay requirements.
In addition, after the end of an EDCF-TXOP, two or more HCs may attempt to allocate a CAP to deliver data immediately after the ongoing EDCF-TXOP with highest priority, as indicated in the right hand side of Figure 5.34. In this case, the first frames transmitted in the CAPs will collide, which is a substantial problem for the controlled channel access. In general, the probability of collisions increases with increasing durations of the EDCF-TXOPs, i.e., with an increased TXOPlimit. Note that the two HCs may require different TXOPlimits in their respective QBSSs.
Figure 5.35 illustrates another obvious problem that occurs when more than one HC operate at the same time, in a coexistence scenario. The HC 1 needs to allocate a CAP at a point in time when the other HC 2 allocated a CAP already.
optimal CAP allocation time for HC 1
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delayed start of CAP |
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Figure 5.34: One of the problems in overlapping QBSSs: after the end of an EDCF-TXOP, both HCs may attempt to transmit immediately with highest priority. In this case, frames collide.
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5. Evaluation of IEEE 802.11e with the IEEE 802.11a Physical Layer |
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unwanted by HC 1 |
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optimal CAP allocation |
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time for HC 1 |
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PIFS |
CF-Poll |
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Figure 5.35: One of the problems in overlapping QBSSs: HC 1 needs to allocate resources while the other HC 2 allocates resources through a granted TXOP already. The duration of the ongoing TXOP is not limited by the TXOPlimit. As a result, the HC 1 may observe large delays.
The duration of the ongoing CAP is not limited by the TXOPlimit, and therefore not under control of the HC 1. As a result, the HC 1 observes significant delays when attempting to allocate a CAP, and may even have to give up the CAP allocation attempt due to the increased delays.
No HC operating in overlapping QBSSs can meet its requirements, as soon as coexisting HCs allocate CAPs with durations larger than the TXOPlimit that is individually defined by the respective HC.
5.4.2Saturation Throughput in Coexistence Scenarios
The modified version of Bianchi’s legacy 802.11 model is used here to analyze the identified problems, see Section 5.1.2.1, p. 65. Figure 5.36 shows the collision and transmission probabilities p,τ in a generic slot time for an HC, as a function of the number of HCs in overlapping QBSSs. This figure should be compared to Figure 5.5, p. 69, where the same probabilities are shown for the contentionbased channel access of the HCF, i.e., the EDCF, versus the number of contending backoff entities. As expected, due to the lack of contention in the controlled channel access, the collision probability is p = 0 for one HC, and p =1 if more than one HC allocate CAPs. As slots are never idle when one or more HCs allocate as many CAPs as possible (in saturation), the probability that the HC transmits at a generic slot is τ =1 for any number of HCs.
Figure 5.37 shows the respective probabilities that a generic slot is idle, busy with a collided frame, or busy with a successfully allocated CAP, versus the number of HCs. This figure should be compared to Figure 5.6, p. 70, where the same probabilities are shown for the EDCF. As expected, in saturation, slots are never idle. Further, the channel is always busy with unsuccessful transmissions, as soon as
5.4 HCF Controlled Channel Access, Coexistence of Overlapping QBSSs |
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more than one HC attempt to allocate CAPs. One isolated HC will always successfully allocate its CAPs. Figure 5.38 shows the resulting saturation throughput for different frame body sizes and PHY modes, vs. the number of HCs in overlapping QBSSs. It is assumed that one frame per CAP is exchanged. One HC can achieve different saturation throughput for different frame body sizes and PHY modes, similar to the EDCF. However, in contrast to the EDCF, as soon as the number of HCs increases beyond one, the throughput drops down to zero, since all CAP allocation attempts will fail if two or more HCs operate in saturation in the coexistence scenario.
prob. τ, p
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Figure 5.36: Collision and transmission probability p,τ in a generic slot time for an HC, as functions of the number of HCs in overlapping QBSSs.
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prob(CCAidle) |
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Figure 5.37: Probability that a generic slot is idle, busy with a collided frame, or busy with a successfully transmitted frame, as functions of the number of HCs in overlapping QBSSs.