
- •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
5.3 Radio Resource Capture |
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However, this is usually not the case, especially at times of high traffic load. See Figure 5.29 for an illustration of this problem. Three BSSs are shown, where each BSS can only detect transmissions of its respective neighbor BSSs. BSS 1 and BSS 3 are hidden to each other. Hence, BSS 1 and BSS 3 can independently operate at the same time and are uncoordinated without synchronized channel access. Once a station of BSS 1 started a frame exchange, stations of BSS 2 will defer from channel access. However, this implies that a station of BSS 3, which does not detect the ongoing frame exchange in BSS 1, starts its frame exchanges independently during the ongoing frame exchange. This can continue with any following frame exchange as long as BSS 1 and BSS 3 have data to deliver. As a result, the stations of BSS 2 may not be able to transmit for longer time durations, and the channel is captured by stations in the neighborhood of BSS 2. This leads to increased MSDU Delivery delays and reduced throughput in BSS 2.
5.3.2Radio Resource Capture through Channels that Partially Overlap in the Spectrum
Similarly, the same capture effect can occur also when frequency channels overlap in the frequency domain. See Figure 5.30 for an illustration of this problem. Now all stations are in close vicinity to each other such that any transmission is detected by any station, across all BSSs. However, it is here assumed that every BSS operates at a different frequency channel, as indicated in Figure 5.30. This implies that the three BSSs can operate without mutually interfering each other, as long as the used frequency channels are orthogonal, i.e. as long as frequency channels do not overlap in the frequency domain. When frequency channels overlap, the same resource capture as described in the last section may occur. In the example of Figure 5.30, stations of BSS 1 operating at channel 2 and stations of BSS 3 operating at channel 4 can initiate frame exchanges without mutual interference. The two BSSs can operate independently. The BSS 2 operates on channel 3, which overlaps with the two other channels. Therefore, the CCA processes of all stations in BSS 2 may not find an idle channel for longer time durations, for the same reason as explained in the last section. Note that overlapping frequency channels are not standardized for the 5 GHz band.
5.3.3Solution
To reduce the unwanted effects of the resource capture, Benveniste (2001) proposes an efficient and simple solution based on synchronous time division, here referred to as slotting. In the following, a way to introduce such a slotting in 802.11(e) is presented.

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5. Evaluation of IEEE 802.11e with the IEEE 802.11a Physical Layer |
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20 MHz per channel, 0 overlap assumed
OFDM power density spectrum
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Figure 5.30: Resource capture with frequency channels that overlap in the frequency domain. As before, stations of (Q)BSS 2 may not detect an idle channel for undesirable long periods.
5.3.3.1Mutual Synchronization across QBSSs and Slotting
Two modifications of the 802.11e protocols are required to enable coexisting QBSSs or coexisting IBSSs to synchronize the access. Figure 5.31 illustrates the two modifications. One is that beacons must be transmitted in contention by all stations of a QBSS, as is the case in an IBSS. Stations that are associated to an HC also must transmit the beacon. It is not necessary for such stations to deliver all information an HC usually transmits with its beacon. For the purpose of synchronization of neighbored BSSs, only the TSF information is required.
The second modification to the standard is that all stations, regardless of whether they are an AP or a station of a QBSS or IBSS, must update their timers according to the rules of the IBSS. Exactly this will guarantee the synchronization of neighbored BSSs. By applying these two modifications in 802.11, overlapping QBSSs are mutually synchronized without loosing any functionality of the protocol. Synchronous time division, i.e., slotting, is possible by defining a slot dwell time. With a slot dwell time, frame exchanges of a BSS are allowed to start only if they can be finished before the end of the respective slot, i.e., without exceeding the slot dwell time. After expiry of a slot dwell time, stations of the BSSs contend for the next frame exchange in parallel. Because the slot dwell times are synchronized, all stations of all BSSs content at the same time, which provides a fair ac-