the 802.11 mac protocol & quality of service
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The 802.11 MAC Protocol & Quality of Service. Duncan Kitchin Wireless Networking Group Intel Corporation 4/4/2003. Agenda. 802.11 MAC Overview QoS Objectives & Applications Important Questions 802.11e Details Future Developments & Summary. 802.11 MAC Overview. - PowerPoint PPT PresentationTRANSCRIPT
Copyright Copyright © 2002 Intel Corporation.© 2002 Intel Corporation.
The 802.11 MAC Protocol &The 802.11 MAC Protocol &Quality of ServiceQuality of Service
Duncan KitchinDuncan KitchinWireless Networking GroupWireless Networking GroupIntel CorporationIntel Corporation4/4/20034/4/2003
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Agenda
• 802.11 MAC Overview• QoS Objectives & Applications• Important Questions• 802.11e Details• Future Developments & Summary
Copyright Copyright © 2002 Intel Corporation.© 2002 Intel Corporation.
802.11 MAC Overview802.11 MAC Overview
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802.11 Logical Architecture
Physical
Data link
Network
Transport
Session
Presentation
Application
PHY
MAC
LLC (802.2)
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802.11 sublayers
PHY
MAC
Higher layers
Extensions are “mix and match”Extensions are “mix and match”
802.11a802.11b802.11g
802.11d802.11e802.11h802.11i
802.11c802.11F
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Standards decoder ring• 802.11a 5GHz OFDM PHY• 802.11b 2.4GHz CCK PHY• 802.11c 802.11 bridging• 802.11d International roaming• 802.11e QoS/efficiency enhancements• 802.11F Inter AP protocol• 802.11g 2.4GHz OFDM PHY• 802.11h 5GHz regulatory extensions• 802.11i Security enhancements• 802.11j Japan 5GHz band extensions• 802.11k Radio resource measurement• 802.11l Skipped (typographically unsound)• 802.11m Maintenance• 802.11n High throughput PHY
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Origins of the 802.11 MAC
• Derived from Ethernet (CSMA/CD) philosophy
• Developed into present form 1990-1994
• Required much modification to fit wireless medium– CSMA/CA
• Widely regarded at the time as a kludge
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New 802.11 MAC developments
• 802.11e is the “new” MAC– evolution to base 802.11– adds differentiated QoS…– …but also enhanced efficiency
• Core components represent a simple evolution
• Optional extensions may be widely implemented in the future, subject to market demand
Copyright Copyright © 2002 Intel Corporation.© 2002 Intel Corporation.
QoS Objectives & QoS Objectives & ApplicationsApplications
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What Does QoS Mean?
• We limit the definition to mean “delivering traffic for real-time applications”
• Each application has a requirements tuple– max latency– min data rate– max packet drop probability
• The set of tuples define points that delimit the requirements curve
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Representation of Requirements• Define a set of applications first
– voice– gaming– real-time video (videoconferencing)– “CD like” audio– “Television/VCR like” video
• Each of these applications defines a point on the data rate/latency/drop rate requirements curve
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Why 802.11e and 802.11a• Home wireless network usage model
shift
• 802.11b in home networks was driven by broadband Internet connection sharing
• 802.11a in home networks will be driven by high bandwidth multimedia streams between devices in the home
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New Usage Models
BBGateway
PC
PC
TV
Tablet
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Applications
• Video
• Audio
• Voice
• Gaming
• Videoconferencing
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Video
• Key motivation for multimedia home networks
• High quality, streaming video
• Focus on MPEG-2, MPEG-4, wmv
• Lowest mean rate 2Mb/s (SD)
• Highest mean rate 20Mb/s (HD)
• Variable data rate requirements
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Audio
• High quality, streaming audio has distinct requirements from voice
• Key formats MP3, wma, PCM
• Bandwidth range 64kb/s up to 1.5Mb/s
• Relatively high latency tolerance
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Voice & videoconferencing
• Low latency–< 50ms required
• Lower bandwidth requirements–32kb/s and lower for voice–128kb/s for videoconferencing
• Higher tolerance to frame losses
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Gaming
• Lowest latency–< 10ms required
• Lower bandwidth requirements–32kb/s – 128kb/s?
• Low tolerance to frame loss
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Applications SummaryL
ate
nc
y to
lera
nc
e
Bandwidth
Voice
Videoconference
VideoAudio
Gaming
Copyright Copyright © 2002 Intel Corporation.© 2002 Intel Corporation.
Important QuestionsImportant Questions
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The Protocol Stack
PHYPHY
DLC (MAC + LLC)DLC (MAC + LLC)
NetworkNetwork
TransportTransport
ApplicationApplication
IPIP
TCP/UDPTCP/UDP
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The Protocol Stack
• Before defining the link layer (MAC) must decide what the higher layers are
• If we assume TCP/IP based higher layers, that imposes restrictions on what we can do
• We don’t have latitude to rewrite TCP/IP, or the interface to it
• We also don’t have latitude to rewrite the applications or the OS
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What We Must Do
• Define 802.11 MAC as providing a set of services
• Those services are defined by the 802.2 service primitives, incorporating 802.1D
• Deliver packets, each of which is tagged with a 3-bit priority
• Consider each service request packet-by-packet– we have no mechanism to tell us about connections
from the higher layers
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What the Services Will Look Like in QoS Terms• Each packet, dependent on priority, will
have a latency probability distribution
• If the higher layers (or the MAC) imposes a timeout, there will be a drop probability against timeout curve
• Need to revisit requirements to see what the bounds for the curve should be
Copyright Copyright © 2002 Intel Corporation.© 2002 Intel Corporation.
802.11e Details802.11e Details
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802.11e features
EDCF/WME
Core functionality
Point coordinated mode
Group acknowledge Direct link
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802.11e Features
• CSMA
• Direct link
• Block acknowledge
• Point coordinated mode
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CSMA Strategy
• Use 802.1D tags to classify traffic into groups with widely differing requirements
• 8 priority levels grouped into four classes– best effort– video/audio probe– video/audio– voice/gaming
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Applying to different classes
• Priority access improves chances of getting access to the medium quickly
• Long burst duration provides high bandwidth access, but at the expense of latency
• Set appropriately:– voice/gaming has very high access priority, small
burst size– video/audio has much lower access latency (but
better than best effort) but large burst sizes
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802.11e Direct Link
• 802.11-1997 specification permits traffic in an AP-based network between clients and AP only
• 802.11e adds capability for clients to send traffic directly to each other– improves bandwidth efficiency, particularly
in home networks
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Direct Link
AP
StationStation