- •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
44 |
4. IEEE 802.11e Hybrid Coordination Function |
4.1.2Enhancements of the Legacy 802.11 MAC Protocol
4.1.2.1Transmission Opportunity
As part of 802.11e, a station that obtained channel access must not allocate radio resources for durations longer than a specified limit. This important new attribute of the 802.11e MAC is referred to as a Transmission Opportunity (TXOP). A TXOP is an interval of time during which a station has the right to initiate transmissions. A TXOP is defined by a starting time and a maximum duration. TXOPs can be obtained via the contention-based channel access. Such TXOPs are referred to as EDCF-TXOPs. Alternatively, a TXOP can be obtained by the HC via the controlled channel access. In this case it is referred to as Controlled Access Phase (CAP). The duration of an EDCF-TXOP is limited by a QBSS-wide parameter called TXOPlimit. This TXOPlimit is distributed regularly by the HC in an information field of the beacon. The HC broadcasts the beacon at the beginning of each superframe. Legacy stations will only understand the fields known from the legacy standard, whereas 802.11e stations will understand all new information fields. The new information fields are ignored by legacy stations. Therefore, legacy stations may transmit for longer durations than allowed by the TXOPlimit.
4.1.2.2Beacon Protection
As part of 802.11e, no backoff entity is allowed to transmit across the TBTT. Frame exchanges are only to be initiated if they can be completed before the next TBTT. This reduces the expected beacon delay, which gives the HC a better control over the channel, especially if the optional CFP is used after the beacon.
4.1.2.3Direct Link
As part of 802.11e, any backoff entity can directly communicate with any other backoff entity in a QBSS, without communicating first with the AP. In the legacy 802.11 protocol, within an infrastructure based BSS (which is denoted as BSS), all data frames are sent to the AP, and received from the AP. This however consumes at least twice the channel capacity compared to the direct communication. Only in an independent BSS (which is denoted as IBSS), station to station communication is allowed in the legacy protocol, due to the absence of the AP. The direct communication is referred to as Direct Link (DiL) in 802.11e. A set-up procedure, the Direct Link Protocol (DLP) is defined to establish a DiL between 802.11e backoff entities.
4.2 Hybrid Coordination Function, Contention-based Channel Access |
45 |
4.1.2.4Use of RTS/CTS
As part of 802.11e, transmissions of data frames can be protected by RTS/CTS whenever required, without considering any threshold in the frame body size. Hence, even small MSDUs can be delivered with the additional NAV protection using RTS/CTS. It is a local decision taken by the transmitting backoff entity if RTS/CTS is used or not. However, RTS/CTS increases the duration of frame exchanges. This duration must not exceed the TXOPlimit, if the respective frame exchange is initiated in an EDCF-TXOP.
4.1.2.5Fragmentation
As part of 802.11e, fragmentation of an MSDU into multiple MPDUs is allowed with any fragmentation size, and any number of fragments, whenever required by an 802.11e backoff entity. The fragmentation threshold known from the legacy protocol can still be used by 802.11e backoff entities, as part of the implementation. However, the 802.11e standard does not explicitly require that a backoff entity uses this threshold to decide if a MSDU should be fragmented into multiple MSDUs or not. As with RTS/CTS, fragmentation may significantly increase the duration of frame exchanges. This duration must not exceed the TXOPlimit.
4.2Hybrid Coordination Function, Contentionbased Channel Access
The EDCF as the contention-based channel access of 802.11e is the basis of the HCF, as illustrated in Figure 4.1. It is indicated that the contention-based channel access of the HCF, i.e., the EDCF, is part of the HCF. It is used to support differentiated services with priorities. The controlled channel access of the HCF is based on the EDCF and is used for time-bounded services with strict QoS guarantees. The details of the EDCF are described in the following sections.
4.2.1Traffic Differentiation, Access Categories, and Priorities
The QoS support in EDCF is realized with the introduction of Access Categories (ACs) and parallel backoff entities. MSDUs are delivered by multiple parallel backoff entities within one 802.11e station, each backoff entity parameterized with AC-specific parameters, the so-called EDCF parameter sets. There are four different ACs, thus, four backoff entities exist in every 802.11e station, with four priorities AC 0…3. See Figure 4.2 for an illustration of the backoff entities.