- •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 |
105 |
cess. As a result, the channel cannot be captured for time durations longer than a slot duration. In the simulation discussed in the following, a slot duration of 4 Time Units (TUs), i.e., 4.096 ms, is used. An immediate drawback of the slotting is the throughput degradation in each BSS. To mitigate this, it is here proposed to apply CFBs as explained in the next section.
5.3.4Evaluation
A symmetric scenario is selected to investigate the effects of slotting and CFBs in 802.11e. A circle of 12 stations forms a QBSS where all stations operate according to EDCF, see Figure 5.32. All frames are transmitted with the 12 Mbit/s PHY mode. Each station generates the same offered traffic comprising three single hop data streams, labeled as high, medium and low, according to their priorities.
By means of these streams, stations deliver MSDUs with three different priorities to one direct neighbor station, and also receive three streams from the other neighbor station. In total, 12 3 = 36 data streams are simulated. All MSDUs arrive with negative-exponentially distributed inter-arrival times.
The three priorities are always offered the same traffic. At the different backoff entities in the simulated system model, MSDUs with a size of either 600 byte or 128 byte arrive such that the queues are always full. Another traffic load is derived from an Ethernet traffic trace file that offers 1 Mbit/s with typical Ethernet frame sizes of up to 1514 byte as traffic source for each data stream.
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NEW: QBSSs apply distributed TSF as originally defined for IBSS only
NEW: co-located QBSSs accept timer information from other BSS
Figure 5.31: Two modifications in 802.11e to allow mutual synchronization for the reduction of the radio resource capture and to support multi-hop traffic.
106 |
5. Evaluation of IEEE 802.11e with the IEEE 802.11a Physical Layer |
Table 5.4 shows the relevant EDCF parameters selected for the three priority classes, summarizing the parameters that are mainly used. RTS/CTS is used for MSDUs larger than 512 byte. MSDUs are delivered in single MAC Protocol Data Units (MPDUs), i.e., MSDUs are not fragmented. The TXOPlimit is larger than the slot dwell time.
5.3.4.1Simulation Results and Discussion
The three simulated protocol configurations are the standard configuration labeled “with (resource) capture” being the protocol specified in the standard, the slotted configuration labeled “slotted,” and the slotted configuration with CFBs, labeled “bursted.” Table 5.5 presents the maximum achievable throughput for a load, where one station transmits to its neighbor and all other stations remain idle. Thus, no collisions occur at the radio channel. The results are given for the three configurations and three different types of traffic load. It can be seen that slotting reduces the throughput compared to non-slotting, whereas the CFBs has a better throughput. For shorter frame body sizes, CFB achieves even the highest throughput.
Table 5.6 shows the maximum achievable throughput in the hidden station scenario of Figure 5.32. It can be seen how severe the throughput is degraded in 802.11 by hidden stations without using slots plus CFBs.
Figure 5.33 (a) and (b) show the resulting backoff delays for the three ACs as CCDF. Here the MPDU frame body size is 600 byte. All stations together are evaluated, as the scenario is symmetric. The advantage of the slotting is clearly visible. For all ACs, the backoff times are reduced. As expected, CFBs increase the delay compared to slotting. The shapes of the curves in both figures are modulated by the slotting with a duration of 4.096 ms.
Figure 5.33 (c) and (d) show the resulting backoff delay for the Ethernet traffic trace files. Here, the positive effect of the slotting is again visible, although the advantage is not as high as before. In general, the longer the MPDUs, the more probable are channel captures, and thus, the more efficient is the slotting.
Table 5.4: Used EDCF parameters for the three ACs.
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5.3 Radio Resource Capture |
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Table 5.5: Achievable throughput (all ACs) with two isolated stations [kbit/s].
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*) although labeled as “with capture,” captures do not occur here, as this is an isolated scenario where no hidden stations exist.
Table 5.6: Max. achievable throughput saturation throughput (all ACs) [kbit/s].
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5.3.4.2Conclusion
The results indicate that resource channel capture by coexisting BSSs can be efficiently reduced when applying the slotting scheme as described in Benveniste (2001) together with CFBs. This modified protocol may especially have a potential for multi-hop communication and the efficient forwarding of MSDUs across multiple (Q)BSSs under QoS constraints.
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Figure 5.32: Symmetric hidden station scenario. Each station detects its two neighbors. Each station carries three parallel streams of data with high, medium, low priority.
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5. Evaluation of IEEE 802.11e with the IEEE 802.11a Physical Layer |
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(d) “Bursted”, with CFBs, Ethernet trace. |
Figure 5.33: Backoff delays for different offered traffics, standard 802.11 (“with capture”), with slotting, and with slotting incl. CFBs.
5.4HCF Controlled Channel Access, Coexistence of Overlapping QBSSs
The controlled channel access of the HCF provides the highest possible priority in channel access to a backoff entity. Usually, the HC makes use of the controlled channel access by periodically polling various backoff entities during the contention period. A time interval during which the channel is allocated with the controlled channel access is referred to as Controlled Access Phase (CAP).
At any time during the contention free period (which is optional) or during the contention period, the HC can allocate a CAP, as soon as the channel is idle for a duration of PIFS. This works sufficiently only as long as one HC operates at the channel. When two or more HCs of overlapping QBSSs operate at the same time in a coexistence scenario, the controlled channel access cannot provide the required QoS, as will be discussed in the following.