a tool for simulating ieee 802.11e contention-based access

A Tool for Simulating IEEE 802.11e A Tool for Simulating IEEE 802.11e Contention-based AccessContention-based Access

Andreas Floros, Theodore Karoubalis

55thth International Symposium on Communication Systems, Networks and Digital Signal Processing International Symposium on Communication Systems, Networks and Digital Signal Processing

IntroductionIntroduction

• WLAN QoS support through the IEEE802.11e specification

– Contention-based access mechanism (EDCA)

– Controlled-access mechanism (HCCA)

• In this work, an EDCA simulation tool is presented

• Basic simulation targets

– Backoff mechanism

– Traffic differentiation scheme efficiency

– Admission control scheme

– Advanced mechanisms for service adaptation

• i.e. upon heavy link degradation

– Optional mechanisms

• Power-save, direct link etc


EDCA overviewEDCA overview

• Traffic differentiation scheme– Maps 8 user priorities to 4 Access Categories

(ACs)• AC_BE, AC_BK, AC_VI, AC_VO

– Each AC can be considered as an independent backoff entity

• contends for medium access, using a set of EDCA parameter values

TXOP Limit AC CWmin CWmax AIFSN

802.11b 802.11a/g AC_BK 31 1023 7 0 0 AC_BE 31 1023 3 0 0 AC_VI 15 31 2 6.016ms 3.008ms AC_VO 7 15 2 3.264ms 1.504ms


Simulation platform descriptionSimulation platform description

• MS-Windows based application

• Basic design-phase requirements

– User friendly application interface

• No special description language required (such as TCL/TK)

– Hybrid event and time driven simulation core

• Efficient handling of protocol signaling

• Improved simulation execution times

– Support of all mandatory EDCA mechanisms

– TSPEC-based traffic source modeling

– Performance evaluation using legacy and advanced networking criteria


Simulation platform description (cont’d)Simulation platform description (cont’d)

• Main application window


Defining simulation scenariosDefining simulation scenarios

• Number and properties of QSTAs– Radio type (802.11b or .11g)– Local scheduler type

• Number and TSPECs of active TS per QSTA– Maximum 4

• QAP properties– Radio type (802.11b or .11g)– Global EDCA parameters– Admission Control Scheme

• Simulation duration• Channel error model• Other simulation parameters

– i.e. TXOP continuation


Defining simulation scenarios (cont’d)Defining simulation scenarios (cont’d)

• Traffic Specifications definition



• QAP properties



• Wireless channel model and QSTA properties


Scenario executionScenario execution

• Scenario validation through a pre-processor stage• Very fast execution times

– Hybrid simulation core– Less than 10 seconds for 100 seconds scenarios


Simulation results and outputSimulation results and output

• Simulation analysis– Represents a detailed analysis of all EDCA-related events

occurred– Can be exported in .html format


Simulation results and output (cont’d)Simulation results and output (cont’d)

• Advanced statistics– Organized by AC, TS or QSTA

• Plot window– Displays throughput, medium occupancy as a function

of time


Simulator accuracy validationSimulator accuracy validation

• Verification using WFA EDCA tests

– Used for ensuring different vendor products interoperability

– Included in the WME pre-standard

• Traffic differentiation tests

• AC fairness tests

• All tests performed for

– Ideal wireless channel

– Non-ideal wireless channel (30% packet loss probability)

– TSPECs defined by WFA

• Optional TXOP continuation was also checked


Traffic differentiation testTraffic differentiation test

• Test description

– two streams belonging to different ACs and QSTAs

– the high priority traffic does not exceed the link

capacity

– the low priority stream provides enough traffic load,

saturating the wireless link

– PHY transmission rate equal to 1Mbps

• Aim of the test is to confirm differentiation of

streams belonging to different ACs


Traffic differentiation test (cont’d)Traffic differentiation test (cont’d)

• Due to channel saturation, the total number of TXOPs is almost constant in both wireless link cases

• The high priority stream gets the bandwidth needed, while the low priority traffic gets only the bandwidth left

• The high priority traffic packet delay is very low, even in the case of non-ideal wireless channel

• Traffic differentiation is achieved!

Traffic G.711 Voice

CD-quality Audio

Mean/Total

User priority 6 (VO) 2 (BK) - Mapping AC AC_VO AC_BK - MeanDataRate 166kbps 800kbps 966kbps

Ideal wireless channel TXOPs granted 864 3486 4350 Throughput 165.9kbps 669.4kbps 835kbps ThroughputRatio 99.99% 83.68% 86.48% Packet delay 0.158ms 51.319ms 25.738ms

Non-ideal wireless channel TXOPs granted 3177 1153 4330 Throughput 165.9kbps 449.5kbps 616kbps ThroughputRatio 99.99% 56.19% 63.72% Packet delay 1.265ms 78.520ms 39.892ms


AC fairness testAC fairness test

• Test description

– Two identical traffic flows mapped to equal AC priorities

– The total intended load exceeds the channel

capabilities

– PHY transmission rate equal to 1Mbps

• Aim of the test is to confirm that different streams

belonging to the same AC obtain the same

transmission rights


AC fairness test (cont’d)AC fairness test (cont’d)

Traffic CD-quality

Audio CD-quality

Audio Mean/Total

User priority 5 (VI) 5 (VI) - Mapping AC AC_VI AC_VI - MeanDataRate 500kbps 500kbps 1Mbps

Ideal wireless channel Throughput 420.1kbps 417.9kbps 838kbps ThroughputRatio 84.02% 83.58% 83.80% Packet delay 80.489ms 80.431ms 80.460ms

Non-ideal wireless channel Throughput 308.5kbps 310.0kbps 619kbps ThroughputRatio 61.72% 62.00% 61.86% Packet delay 113.036ms 112.421ms 112.728ms

• Both streams get half of the link effective bandwidth

• The same stands for non-ideal wireless channel

– Although the throughput and packet delay performance is significantly degraded

• AC fairness is achieved!


Case study: TXOP continuation testCase study: TXOP continuation test

• Simulation scenario employed

• PHY rate equal to 11Mbps– Aggregated bandwidth occupancy equal to 98%

Traffic Type Mean

data rate (kbps) User

Priority AC

G.711 Voice 166 6 AC_VO CD-quality Audio 500 6 AC_VO

G.711 Voice 166 5 AC_VI QSTA 1

CD-quality Audio 500 5 AC_VI G.711 Voice 166 6 AC_VO

CD-quality Audio 500 6 AC_VO G.711 Voice 166 5 AC_VI

QSTA 2

CD-quality Audio 500 5 AC_VI SDTV 6000 5 AC_VI

QSTA 3 Video Conference

192 2 AC_BK


Case study: TXOP continuation test Case study: TXOP continuation test (cont’d)(cont’d)

• For ideal wireless links

– The measured overall throughput is increased by nearly 27%• The additional throughput corresponds to lower priority ACs (AC_VI)

– An increment of the packet delay is observed, due to the extended duration of the granted TXOPs

– The total number of medium collisions is significantly decreased

With TXOP Continuation AC_BK AC_VI AC_VO Mean/Total TXOPs granted 780 35820 6931 43531 Time granted (ms) 194.22 8990.82 1739.68 10924.72 Mean bitrate (kbps) 192.00 7332.00 1332.00 8856 Throughput (kbps) 149.89 6878.46 1331.76 8360 ThroughputRatio 78.07% 93.81% 99.98% 94.40% Collisions 30 956 1070 1015 Packet delay (ms) 220.938 4.608 2.709 76.085

Without TXOP Continuation TXOPs granted 999 23239 6935 31173 Time granted (ms) 248.75 5832.99 1740.68 7822.43 Mean bitrate (kbps) 192.00 7332.00 1332.00 8856 Throughput (kbps) 191.98 4462.56 1331.94 5986 ThroughputRatio 99.99% 60.86% 100.00% 67.60% Collisions 13 1622 1605 1601 Packet delay (ms) 2.103 3.481 0.708 2.097


Case study: TXOP continuation test Case study: TXOP continuation test (cont’d)(cont’d)

• For non-ideal wireless links

– Compared to the ideal wireless link case• the throughput increment with TXOP continuation is lower • the packet delay also slightly increases.

With TXOP Continuation AC_BK AC_VI AC_VO Mean/Total

TXOPs granted 1002 32126 8010 41138 Time granted (ms) 249.50 8062.87 2009.89 10322.25 Mean bitrate (kbps) 192.00 7332.00 1332.00 8856 Throughput (kbps) 160.54 5586.33 1292.43 7039 Convergence 83.62% 76.19% 97.03% 79.49% Collisions 40 890 1008 952 Packet delay (ms) 146.941 7.159 5.429 53.176

Without TXOP Continuation TXOPs granted 1198 22482 7948 31628 Time granted (ms) 298.30 5642.04 1994.28 7934.62 Mean bitrate (kbps) 192.00 7332.00 1332.00 8856 Throughput (kbps) 191.78 3827.16 1306.19 5325 Convergence 99.89% 52.20% 98.06% 60.13% Collisions 24 1388 1426 1402 Packet delay (ms) 13.828 6.674 3.252 7.918


ConclusionsConclusions

• A contention-based EDCA simulation tool was presented– realizes a user friendly interface for defining the

simulation scenarios– provides fast simulation execution times– The simulation output is realized using easy-to-

use graphical interfaces and well-established formats

• The accuracy of the tool was verified using WFA interoperability test

• The effect of the TXOP continuation scheme was examined as a case study

QuestionsQuestions;;

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a tool for simulating ieee 802.11e contention-based access

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