Transcript The 802.11 MAC Protocol & Quality of Service

The 802.11 MAC Protocol &
Quality of Service
Duncan Kitchin
Wireless Networking Group
Intel Corporation
4/4/2003
Copyright © 2002 Intel Corporation.
Intel Confidential
Wireless Networking Division
Agenda
• 802.11 MAC Overview
• QoS Objectives & Applications
• Important Questions
• 802.11e Details
• Future Developments & Summary
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802.11 MAC Overview
Copyright © 2002 Intel Corporation.
Intel Confidential
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802.11 Logical Architecture
Application
Presentation
Session
Transport
LLC (802.2)
MAC
Network
Data link
PHY
Physical
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802.11 sublayers
Higher layers
802.11d
802.11e
802.11h
802.11i
802.11c
802.11F
MAC
PHY
802.11a
802.11b
802.11g
Extensions are “mix and match”
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Standards decoder ring
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802.11a
802.11b
802.11c
802.11d
802.11e
802.11F
802.11g
802.11h
802.11i
802.11j
802.11k
802.11l
802.11m
802.11n
5GHz OFDM PHY
2.4GHz CCK PHY
802.11 bridging
International roaming
QoS/efficiency enhancements
Inter AP protocol
2.4GHz OFDM PHY
5GHz regulatory extensions
Security enhancements
Japan 5GHz band extensions
Radio resource measurement
Skipped (typographically unsound)
Maintenance
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
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QoS Objectives &
Applications
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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
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• 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
BB
Gateway
TV
PC
Tablet
PC
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Applications
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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 Summary
Latency tolerance
Audio
Video
Videoconference
Voice
Gaming
Bandwidth
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Important Questions
Copyright © 2002 Intel Corporation.
Intel Confidential
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The Protocol Stack
Application
Transport
Network
TCP/UDP
IP
DLC (MAC + LLC)
PHY
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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-bypacket
– 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
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802.11e Details
Copyright © 2002 Intel Corporation.
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802.11e features
Direct link
Group acknowledge
EDCF/WME
Core functionality
Point coordinated mode
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802.11e Features
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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
Station
Station
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