the 802.11 mac protocol & quality of service

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

Wireless Networking DivisionWireless Networking DivisionIntel Intel ConfidentialConfidential

Agenda

• 802.11 MAC Overview• QoS Objectives & Applications• Important Questions• 802.11e Details• Future Developments & Summary


802.11 MAC Overview802.11 MAC Overview


802.11 Logical Architecture

Physical

Data link

Network

Transport

Session

Presentation

Application

PHY

MAC

LLC (802.2)


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


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


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


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


QoS Objectives & QoS Objectives & ApplicationsApplications


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


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


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


New Usage Models

BBGateway

PC

PC

TV

Tablet


Applications

• Video

• Audio

• Voice

• Gaming

• Videoconferencing


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


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


Voice & videoconferencing

• Low latency–< 50ms required

• Lower bandwidth requirements–32kb/s and lower for voice–128kb/s for videoconferencing

• Higher tolerance to frame losses


Gaming

• Lowest latency–< 10ms required

• Lower bandwidth requirements–32kb/s – 128kb/s?

• Low tolerance to frame loss


Applications SummaryL

ate

nc

y to

lera

nc

e

Bandwidth

Voice

Videoconference

VideoAudio

Gaming


Important QuestionsImportant Questions


The Protocol Stack

PHYPHY

DLC (MAC + LLC)DLC (MAC + LLC)

NetworkNetwork

TransportTransport

ApplicationApplication

IPIP

TCP/UDPTCP/UDP


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


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


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


802.11e Details802.11e Details


802.11e features

EDCF/WME

Core functionality

Point coordinated mode

Group acknowledge Direct link


802.11e Features

• CSMA

• Direct link

• Block acknowledge

• Point coordinated mode


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


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


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


Direct Link

AP

StationStation

the 802.11 mac protocol & quality of service

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