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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 Page 2 802.11 MAC Overview Copyright © 2002 Intel Corporation. Intel Confidential Wireless Networking Division 802.11 Logical Architecture Application Presentation Session Transport LLC (802.2) MAC Network Data link PHY Physical Page 4 Intel Confidential Wireless Networking Division 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” Page 5 Intel Confidential Wireless Networking Division Standards decoder ring • • • • • • • • • • • • • • 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 Page 6 Intel Confidential Wireless Networking Division 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 Page 7 Intel Confidential Wireless Networking Division 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 Page 8 QoS Objectives & Applications Copyright © 2002 Intel Corporation. Intel Confidential Wireless Networking Division 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 Page 10 Intel Confidential Representation of Requirements Wireless Networking Division • 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 Page 11 Intel Confidential Wireless Networking Division 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 Page 12 Intel Confidential Wireless Networking Division New Usage Models BB Gateway TV PC Tablet PC Page 13 Intel Confidential Wireless Networking Division Applications • • • • • Video Audio Voice Gaming Videoconferencing Page 14 Intel Confidential Wireless Networking Division 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 Page 15 Intel Confidential Wireless Networking Division 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 Page 16 Intel Confidential Wireless Networking Division Voice & videoconferencing • Low latency –< 50ms required • Lower bandwidth requirements –32kb/s and lower for voice –128kb/s for videoconferencing • Higher tolerance to frame losses Page 17 Intel Confidential Wireless Networking Division Gaming • Lowest latency –< 10ms required • Lower bandwidth requirements –32kb/s – 128kb/s? • Low tolerance to frame loss Page 18 Intel Confidential Wireless Networking Division Applications Summary Latency tolerance Audio Video Videoconference Voice Gaming Bandwidth Page 19 Important Questions Copyright © 2002 Intel Corporation. Intel Confidential Wireless Networking Division The Protocol Stack Application Transport Network TCP/UDP IP DLC (MAC + LLC) PHY Page 21 Intel Confidential Wireless Networking Division 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 Page 22 Intel Confidential Wireless Networking Division 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 Page 23 Intel Confidential Wireless Networking Division 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 Page 24 802.11e Details Copyright © 2002 Intel Corporation. Intel Confidential Wireless Networking Division 802.11e features Direct link Group acknowledge EDCF/WME Core functionality Point coordinated mode Page 26 Intel Confidential Wireless Networking Division 802.11e Features • • • • CSMA Direct link Block acknowledge Point coordinated mode Page 27 Intel Confidential Wireless Networking Division 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 Page 28 Intel Confidential Wireless Networking Division 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 Page 29 Intel Confidential Wireless Networking Division 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 Page 30 Intel Confidential Wireless Networking Division Direct Link AP Station Station Page 31