MAC Considerations for Mesh July 2004 doc.: IEEE 802.11-04/760r1 July 2004 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 Lily Yang, Akira Yamada
Overview Existing 802.11 MAC Architecture 11s MAC Considerations: July 2004 doc.: IEEE 802.11-04/760r1 July 2004 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 Lily Yang, Akira Yamada
Context: Major Functional Components for 802.11s Mesh Points * July 2004 Context: Major Functional Components for 802.11s Mesh Points * Configuration/ Management Internetworking 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 PHY IEEE802.11 a/b/g/j/n *[1] slide#3 Lily Yang, Akira Yamada
Existing 802.11 MAC Architecture Overview July 2004 doc.: IEEE 802.11-04/760r1 July 2004 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) In a non-QoS STA, HCF is not present. In a QSTA implementation, both DCF and HCF are present. PCF is optional in all STAs. PCF is required for contention-free services for non-QoS STA. EDCA (enhanced distributed channel access) is required for prioritized QoS services. HCCA (HCF controlled channel access) is required for parameterized QoS services. 11a/11b/11g/11j PHY 11n PHY * Based on 802 11e D8.0 Clause 9.1, Figure 47 Lily Yang, Akira Yamada Lily Yang, Akira Yamada
11e Components and Their Relevance to 11s July 2004 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 APSD Block Ack EDCA HCCA DLP *courtesy of Duncan Kitchin Most Relevant Component to Mesh Points: EDCA Lily Yang, Akira Yamada
EDCA: Important Enhancements over DCF July 2004 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
EDCA: Not Sufficient for 11s July 2004 EDCA: Not Sufficient for 11s Designed for QBSS: Assume one hop traffics between QAPQSTA 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
MAC Considerations (1): QoS Support July 2004 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
MAC Considerations (2): Mesh Specific Issues July 2004 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
Hidden Node and Exposed Node Problem * July 2004 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
Hidden Nodes: Example AP1 AP2 AP3 AP4 AP1 AP2 AP3 AP4 AP1 AP2 AP3 AP4 July 2004 Hidden Nodes: Example AP1 AP2 AP3 AP4 Case (a) Case (c) AP1 AP2 AP3 AP4 Case (b) Case (d) Case (b) AP1 AP2 AP3 AP4 Case (a) Case (c) AP1 AP2 AP3 AP4 Case (d) Lily Yang, Akira Yamada
Lack of Flow Control: Example July 2004 Lack of Flow Control: Example Individual flow throughput (b/s) N0 N4 N1 N2 N3 Flow 1 Flow 2 Load (mb/s) Lily Yang, Akira Yamada
Lack of Flow Control: Example (cont.) July 2004 Lack of Flow Control: Example (cont.) Actual Scheduling Result when load=700kb/s e2e throughput 674k 449k 355k 235k Congested nodes 238k 347k 450k 697k 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 860k 860k 860k 860k Tx: 430k 860k 860k 860k 430k Lily Yang, Akira Yamada
Traffic Delivery Prioritization July 2004 Traffic Delivery Prioritization Relay traffic Mesh AP STA N4 N0 N1 N2 N3 BSS traffic 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, Lily Yang, Akira Yamada
MAC Considerations (3): Practical Matters July 2004 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
MAC Considerations (4): Architectural Alternatives July 2004 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
MAC Considerations (4): Architectural Alternatives (cont.) July 2004 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
MAC Considerations (4): Architectural Alternatives July 2004 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
July 2004 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
July 2004 doc.: IEEE 802.11-04/760r1 July 2004 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 Lily Yang, Akira Yamada
July 2004 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