- •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
42 |
4. IEEE 802.11e Hybrid Coordination Function |
dard referred to as IEEE 802.11e. In this thesis, the drafts 3.3 and 4.0 (IEEE 802.11 WG, 2002a, 2002c) are considered, together with some newer notations of TGe working documents.
The main element of the QoS enhancements of 802.11 is the new coordination function called Hybrid Coordination Function (HCF). This HCF is described in this chapter.
After an overview in Section 4.1, the contention-based channel access of the HCF is described in detail in Section 4.2. The contention-based channel access is also referred to as Enhanced Distributed Coordination Function (EDCF) and can be understood as enhancement of the legacy DCF. In Section 4.3, the controlled channel access of the HCF is described in detail. This controlled channel access relies on the contention-based channel access (the EDCF). It can be interpreted as enhancement of the legacy PCF, which relies on the DCF. In the last part of this chapter, Section 4.4, additional enhancements that are considered to be included in the 802.11e standard for improving the efficiency of the protocol are described.
It is emphasized that this thesis does not provide a complete description of the 802.11e standard, as the standardization process is still ongoing at the time this thesis is written. In this and the following chapter, MAC enhancements are described and evaluated, which are not necessarily part of the standard. For details about the standard, the reader is referred to the original documentation, see IEEE 802.11 WG (2002a, 2002c).
4.1Overview and Introduction
The MAC enhancements of 802.11e enable the support of QoS for a wide variety of applications. However, non-802.11e conformant stations, in this thesis referred to as legacy stations, can operate in parallel to 802.11e stations. 802.11e stations and legacy stations can exchange data under the rules of the legacy coordination functions DCF and PCF (if available). All frames of the 802.11e standard are defined in a way that they do not violate the operation of the legacy stations operating in parallel to the new 802.11e stations.
4.1.1Naming Conventions
A station that operates according to 802.11e is referred to as 802.11e station in this thesis. Other stations are referred to as legacy stations. If the difference between 802.11e stations and legacy stations is not important in a discussed context, or if it is obvious which kind of station is discussed, an 802.11e station is referred to
4.1 Overview and Introduction |
43 |
as station in this thesis. In the 802.11e standard, an 802.11e station is denoted as QoS supporting Station (QSTA), and a legacy station is denoted as Station (STA). These abbreviations are not used in this thesis.
An 802.11 station, which may optionally work as the centralized coordinator for all stations in the QoS supporting BSS (QBSS), including the legacy stations, is referred to as Hybrid Coordinator (HC). A QBSS is an infrastructure-based BSS, which includes an 802.11e-compliant HC and stations. The HC will typically reside within an 802.11e AP. Such an AP is referred to as 802.11e-AP in this thesis only if it is necessary to highlight that it is 802.11e-compliant, otherwise AP is used. In the 802.11e standard, an 802.11e AP is referred to as QoS supporting Access Point (QAP). This abbreviation is not used in this thesis.
802.11e stations comprise multiple backoff entities in parallel, as explained in detail later in this chapter. For this reason, in this thesis it is often referred to a transmitting backoff entity instead of transmitting station. Legacy stations comprise one backoff entity; therefore, legacy backoff entity and legacy station may be used as synonym for each other.
The HCF incorporates two access mechanisms, namely, the contention-based channel access and the controlled channel access. The contention-based channel access is also referred to as EDCF:
“The contention-based channel access mechanism of the HCF is referred to as the »EDCF«. It is closely related to the DCF channel access mechanism being viewed as an enhanced version of that mechanism. Despite the presence of »CF« in its name, the EDCF is part of the HCF and is not a separate coordination function.” (IEEE 802.11 WG, 2002c)
The EDCF is part of the HCF and is not a separate coordination function.
A QBSS is an 802.11-compliant infrastructure-based BSS, which includes the HC. The 802.11-compliant independent BSS is referred to as QoS supporting IBSS (QIBSS).
With 802.11e, there may still be two phases of operation within the superframes, i.e., a Contention Period (CP) and a Contention Free Period (CFP). The contentionbased channel access, i.e., EDCF, is used in the CP only, while the controlled channel access is used in both phases. This is the reason why the new coordination function is called hybrid. Because the controlled channel access can be used during CP, the CFP is not necessary in 802.11e for QoS support.