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MAC Considerations for Mesh

This document provides an overview of the existing 802.11 MAC architecture and analyzes its applicability for mesh networks. It discusses QoS support, mesh-specific MAC issues, practical considerations, and possible MAC architecture alternatives for 802.11s.

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MAC Considerations for Mesh

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  1. MAC Considerations for Mesh L. Lily Yang (Intel Corp.) Akira Yamada (NTT DoCoMo) lily.l.yang@intel.com yamadaakir@nttdocomo.co.jp Lily Yang, Akira Yamada

  2. Overview • Existing 802.11 MAC Architecture • Preliminary Analysis on 11e for 11s • 11s MAC Considerations: • Level of QoS Support • Examples of Mesh Specific MAC issues • Practical Considerations • Possible MAC Architecture Alternatives for 11s • Summary and Call to actions Lily Yang, Akira Yamada

  3. Context: Major Functional Components for 802.11s Mesh Points * Internetworking Configuration/ Management Interfaces IEEE802.11s Amendment .11s Mesh Network Measurement Layer 2 Mesh Routing and Forwarding .11s Mesh Security MAC/MLME enhancement for .11s Mesh IEEE802.11 MAC IEEE802.11 a/b/g/j/n IEEE802.11PHY *[1] slide#3 Lily Yang, Akira Yamada

  4. Existing 802.11 MAC Architecture Overview Point Coordination Function (PCF: optional) HCF Controlled Access (HCCA) HCF Contention Access (EDCA) 11n MAC Enhancement (based on 11e?) MAC Distributed Coordination Function (DCF) 11a/11b/11g/11j PHY 11n PHY * Based on 802 11e D8.0 Clause 9.1, Figure 47 Lily Yang, Akira Yamada

  5. APSD Block Ack EDCA HCCA DLP 11e Components and Their Relevance to 11s • EDCA • Prioritized QoS support and performance enhancement • Important to 11s • HCCA • Parameterized QoS support, need HC • Not directly applicable in mesh • Block Ack • Performance optimization, optional for QSTAs • Useful but not essential to mesh • DLP and APSD • Optimization for QSTAs • Orthogonal to mesh *courtesy of Duncan Kitchin Most Relevant Component to Mesh Points: EDCA Lily Yang, Akira Yamada

  6. EDCA: Important Enhancements over DCF • Prioritized QoS support by separate Channel Access Functions for four ACs • Separate QoS parameters: CWmin, CWmax, TXOP duration, AIFSN • Provide differentiated services • Optional: admission control (by QAP) • Performance enhancement over basic 11 MAC (e.g., TXOP) • Reduce channel access overhead • Fix “slow client” problem Lily Yang, Akira Yamada

  7. EDCA: Not Sufficient for 11s • Designed for QBSS: • Assume one hop traffics between QAPQSTA • Assume QAP responsible for QSTA settings: BSS QoS parameters are set by QAP • how to apply that to mesh points? • Admission control administered by QAP • NOT designed for WLAN Mesh (infrastructure network of Mesh Points)! • None of the Mesh Point has complete visibility into the whole WLAN Mesh => Really need distributed coordination between mesh points • Not designed for multi-hop: issues like hidden/exposed terminals worsen the performance • No end-to-end considerations at all: cause network inefficiency. Lily Yang, Akira Yamada

  8. MAC Considerations (1): QoS Support • From Usage Models: • Multi-media applications requires QoS support • But we need to carefully determine the type of QoS support for 11s: • Is prioritized QoS sufficient? • EDCA can help. • Do we need parameterized QoS support like what HCCA provides? • HCCA not directly applicable due to distributed nature of mesh • Much harder problem • The level of QoS support required will have great impact on the 11s standard completion time Need to determine the level of QoS support in 11s Functional Requirements Lily Yang, Akira Yamada

  9. MAC Considerations (2):Mesh Specific Issues • Fundamentally different network assumptions from 11/e/n: • Distributed data plane: peer-to-peer (no Coordinator) • Multi-hop & end-to-end aspects • Much more complicated traffic pattern in the network than the “star” pattern traffic in the BSS • Mesh points are used to build infrastructure network: • Mesh Point can be co-located with AP or application end points • A mixture of traffic: from own BSS (or own applications), and from WDS • Example of mesh specific issues that are NOT addressed in 11, 11e or 11n: • Hidden/exposed nodes • Lack of flow control in congested network • No prioritization consideration between source traffic and relay traffic Lily Yang, Akira Yamada

  10. Hidden Node and Exposed Node Problem * • Expected to be more common and profound in mesh and hence have more severe negative impact on performance • Virtual carrier sensing by RTS/CTS: is supposed to solve this problem, but not really effective, esp. for mesh • Main consequence: NAV is not set correctly • Collision becomes severe (hidden nodes) • Waste wireless bandwidth (exposed nodes) * See [2] and other literature for more details. Lily Yang, Akira Yamada

  11. AP1 AP2 AP3 AP4 AP1 AP2 AP3 AP4 AP1 AP2 AP3 AP4 AP1 AP2 AP3 AP4 Hidden Nodes: Example Case (c) Case (a) Case (b) Case (d) Case (b) Case (a) Case (c) Case (d) Lily Yang, Akira Yamada

  12. Lack of Flow Control: Example Individual flow throughput (b/s) N4 N0 N1 N2 N3 Flow 1 Flow 2 Load (mb/s) Lily Yang, Akira Yamada

  13. Lack of Flow Control: Example (cont.) Actual Scheduling Result when load=700kb/s e2e throughput 674k 449k 355k 235k Congestednodes 238k 347k 697k 450k Rx: 238k 1020k 899k 1056k 235k Tx: 674k 687k 702k 685k 697k Wasted TX Ideal Scheduling, when the network is overloaded 430k 430k 430k 430k 430k 430k 430k 430k e2e throughput Rx: 430k 860k860k 860k 860k Tx: 430k 860k 860k 860k 430k Lily Yang, Akira Yamada

  14. Traffic Delivery Prioritization Relaytraffic Mesh AP • When Mesh Point co-located with AP on one RF interface: • How to coordinate the relay traffic for mesh point and the BSS traffic for AP? • Does it make sense to prioritize the traffic streams depends on type of traffic (relay vs source), how far they’ve traveled, STA N4 N0 N1 N2 N3 BSS traffic Lily Yang, Akira Yamada

  15. MAC Considerations (3): Practical Matters • Best if compatible with existing 802.11 MAC architecture (DCF, EDCA etc.) • Hard to adopt if completely different MAC • Simplicity & low cost are always desirable • Encourage implementation flexibility • Mesh AP • Two radio interfaces (one for AP, one for Mesh Point) • Same radio interface for both AP and Mesh Point • Mesh Point being application end points (clients) • Mesh Point only as relay • We want to get the standard out in a reasonable time frame • Address the most important requirements first: Good enough • TGs need to specify such a prioritized list in its functional requirements for MAC Lily Yang, Akira Yamada

  16. MAC Considerations (4): Architectural Alternatives Point Coordination Function (PCF: optional) HCF Controlled Access (HCCA) HCF Contention Access (EDCA) Mesh Coordination Function 11n MAC MAC Distributed Coordination Function (DCF) 11a/11b/11g/11j PHY 11n PHY Option#1: Mesh Coordination Function builds on top of DCF, independent of 11e Lily Yang, Akira Yamada

  17. MAC Considerations (4): Architectural Alternatives (cont.) Point Coordination Function (PCF: optional) HCF Controlled Access (HCCA) Mesh Coordination Function 11n MAC MAC EDCA Distributed Coordination Function (DCF) 11a/11b/11g/11j PHY 11n PHY Option#2: Mesh Coordination Function builds on top of EDCA (could still enhance DCF as well) Lily Yang, Akira Yamada

  18. MAC Considerations (4): Architectural Alternatives Point Coordination Function (PCF: optional) HCF Controlled Access (HCCA) HCF Contention Access (EDCA) Mesh Coordination Function 11n MAC MAC Distributed Coordination Function (DCF) 11a/11b/11g/11j PHY 11n PHY Option#3: Mesh Coordination Function independent of DCF (very different MAC) Lily Yang, Akira Yamada

  19. Summary • Only a very preliminary analysis on 11e and its implications to 11s • Clearly there are MAC performance issues beyond multi-hop routing that 11s must address • Lets start discussing MAC issues and agree on functional requirements • Ad Hoc Group? • Focus on problems and requirements, not solutions (yet) Lily Yang, Akira Yamada

  20. References [1] 11-04-0749-00-000s-suggested-major-functional-components-802-11s.ppt [2] 11-04-0732-01-000s-outdoor-802-11-mesh-mac-protocol-issues-and-considerations.ppt Lily Yang, Akira Yamada

  21. Thank you Send me email if you are interested in ad hoc group discussion on MAC issues: lily.l.yang@intel.com Lily Yang, Akira Yamada

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