Diapositiva 1

1
A Software Architecture for Simulating IEEE
802.11e HCCA
Claudio Cicconetti, Luciano Lenzini, Enzo
Mingozzi, Giovanni Stea Dipartimento di
Ingegneria dell'Informazione University of Pisa,
Italy IPS-MoMe 2005 Warsaw, Poland, 14-15th
March 2005
2
Summary

• Introduction
• Software Architecture
• Implementation and preliminary results
• Conclusions

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Introduction

• New (EuQoS) applications require QoS
• videoconference
• VoIP
• online gaming
• ...
• The legacy IEEE 802.11 lacks QoS support
• Distributed Coordination Function
• Point Coordinated Function
• Solution 802.11e
• Enhanced Distributed Channel Access (prioritized
  access)
• HCF Controlled Channel Access (parametrized
  access)

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Legacy 802.11 - DCF

• Distributed protocol, based on CSMA/CA
• listen before transmit
• collisions detected with positive acknowledgment
• binary exponential backoff procedure if collision
• All the stations access the medium with the same
  procedure
• Only suitable for best-effort traffic

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Legacy 802.11 - PCF

• Contention-Free Periods (CFPs) alternates to
  Contention-Periods (CPs) at fixed intervals
• During CFPs the AP cyclically polls the STAs
• Still unsuitable for providing QoS

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802.11e Enhancements

• QoS Access Point (QAP), with
• enhanced scheduling capabilities
• Admission control
• QoS Stations (QSTAs), capable of
• Contention-based access
• DCF, EDCA
• Responding to polls
• PCF, HCCA

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802.11e - EDCA

• Distributed protocol
• Up to four Access Categories
• Based on the differentiation of the CSMA/CA
  parameters
• Achieves relative differentiation of traffic from
  different ACs
• If in infrastructure mode, admission control at
  the QAP

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802.11e HCCA (i)

• Centralized
• Up to 8 Traffic Streams (TSs) for each QSTA
• Traffic specs and requirements (TSPEC)
  negotiation
• Mean data rate, delay bound, etc.
• Admission control of TSs at the QAP
• Absolute QoS guarantees enforced by scheduling
  Controlled Access Phases (CAPs)

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HCCA Scheduling

• The 802.11e does not define a standard scheduling
  algorithm at the QAP
• A reference scheduler is provided
• TDM-like scheduling TXOP of fixed duration at
  fixed time intervals for all admitted TSs
• Different schedulers are currently being proposed
  in the literature
• We need a common framework for evaluation and
  testing

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Software Architecture

• Modular architecture
• MAC fully compliant to the standard HCCA function
• MAC functions separated from HCCA scheduling
• Allows for flexible integration of various
  schedulers

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MAC (i)

• MAC is implemented as a FSM driven by a set of
  events

• CAP_Hand
• Has_Control/Lost_Control
• Transmit

• Receive/Data_Receive
• Success

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MAC (ii)
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HCCA Scheduler (i)

• Listens to a subset of the MAC events
• Has_Control/Lost_Control
• Possibly others, depending on its actual needs
• General interface
• enque() adds a new packet coming from the LL
• deque() pops the HOL packet
• QAP-specific functions
• get_next_cap() returns the expiration time of
  the next CAP
• addTSPEC() requests the admission of a new TS

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HCCA Scheduler (ii)
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Implementation

• The software architecture was implemented in the
  Network Simulator 2 simulation environment
• QAP scheduler reference 802.11e
• QSTA schedulerFIFO with only one TS

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Preliminary Results (i)

• 1 QAP
• 3 QSTAs (bidirectional video streaming session)
• 3 legacy STAs (asymptotic condition)
• 802.11b _at_ 11 Mbps
• Error-free channel
• No RTS/CTS, no MAC fragmentation

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Preliminary Results (ii)
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Preliminary Results (iii)
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Conclusions and future work

• A software architecture for simulating the IEEE
  802.11e HCCA was defined
• The scheduling and MAC functions were decoupled
  by a generic communication interface
• The contributed framework has been implemented
  using ns2
• Future (ongoing) work
• Defining, implementing and comparing different
  HCCA scheduling algorithms for 802.11e

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End of presentation

• Thanks for your attention
• ?? //