The 802.11 MAC Protocol & Quality of Service

1. The 802.11 MAC Protocol &Quality of Service Duncan Kitchin Wireless Networking Group Intel Corporation 4/4/2003

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

3. 802.11 MAC Overview

4. 802.11 Logical Architecture Application Presentation LLC (802.2) Session MAC Transport Network PHY Data link Physical

5. 802.11 sublayers 802.11c 802.11F Higher layers MAC 802.11d 802.11e 802.11h 802.11i PHY 802.11a 802.11b 802.11g Extensions are “mix and match”

6. 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

7. 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

8. 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

9. QoS Objectives & Applications

10. 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

11. 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

12. 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

13. TV Tablet New Usage Models BB Gateway PC PC

14. Applications • Video • Audio • Voice • Gaming • Videoconferencing

15. 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

16. 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

17. Voice & videoconferencing • Low latency • < 50ms required • Lower bandwidth requirements • 32kb/s and lower for voice • 128kb/s for videoconferencing • Higher tolerance to frame losses

18. Gaming • Lowest latency • < 10ms required • Lower bandwidth requirements • 32kb/s – 128kb/s? • Low tolerance to frame loss

19. Applications Summary Video Audio Latency tolerance Videoconference Voice Gaming Bandwidth

20. Important Questions

21. The Protocol Stack Application Transport TCP/UDP Network IP DLC (MAC + LLC) PHY

22. 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

23. 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

24. 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

25. 802.11e Details

26. Direct link Group acknowledge EDCF/WME Core functionality Point coordinated mode 802.11e features

27. 802.11e Features • CSMA • Direct link • Block acknowledge • Point coordinated mode

28. 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

29. 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

30. 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

31. Direct Link AP Station Station