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Performance Implications of 802.11 DCF to ESS Mesh Networks

Performance Implications of 802.11 DCF to ESS Mesh Networks. Xingang Guo W. Steven Conner L. Lily Yang Intel Corporation. Motivation. Performance is critical to ESS mesh Home network (PCs, CEs…) Provide QoS for multimedia applications Wireless data access network

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Performance Implications of 802.11 DCF to ESS Mesh Networks

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  1. Performance Implications of 802.11 DCF to ESS Mesh Networks Xingang Guo W. Steven Conner L. Lily Yang Intel Corporation X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

  2. Motivation • Performance is critical to ESS mesh • Home network (PCs, CEs…) • Provide QoS for multimedia applications • Wireless data access network • Provide infrastructure equivalent performance • 802.11 DCF has significant impact • In addition to routing X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

  3. Presentation overview • ESS mesh functional with existing 802.11 DCF • Be aware of the performance issues • Large quantity of research activities • General DCF problems aggravated by mesh • Mesh specific problems • Implications of ongoing MAC efforts to mesh • How should TGs proceed with performance issues? X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

  4. Issues in 802.11 DCF • Inefficient handling of co-channel interference • Interference mitigation • Collision avoidance • Lack of spatial reuse hurts mesh performance • Undesirable effects with packet-based fairness • Throughput unfairness in BSS • Unfairness in BSS degrades E2E throughput in multi-hop route • Mesh-specific issues • Using DCF for multi-hop forwarding • Achieving flow fairness Mesh can magnify the problem X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

  5. Per-cell throughput (Mbps) # of co-channel cell Co-channel interference (CCI) • CCI cause performance degradation in general multi-cell 802.11 networks • ESS mesh may aggravate the problem • Entire ESS uses 1 channel • Mesh APs and their clients • Easier to cause CCI • Easier to spread CCI X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

  6. Interference mitigation • Ineffective virtual carrier sensing • Over-reserve channel usage [1][3] • Protocol messages may not be decodable [1][2] • Worse, Hidden Group – a group of nodes [2] • A single node may not disrupt communication • Collectively a group of nodes can interfere • Poor spatial reuse hurts mesh [1][2] [1] F. Ye, et. al, Improving Spatial Reuse of IEEE 802.11 Based Ad Hoc Networks, Globecom 2003 [2] J. Zhu, et. al, Leveraging Spatial Reuse in 802.11 Mesh Networks with Enhanced Physical Carrier Sensing, ICC 2004 [3] S. Ray, et al, RTS/CTS-Induced Congestion in Ad Hoc Wireless LAN, WCNC 2003 X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

  7. Collision avoidance • Initial backoff window size [1] • The fixed size is not optimal for large (mesh) networks • Contention window backoff algorithm [2] • Reset too fast (reset upon succeed) • Still generate collision in dense network (e.g. mesh) [1] G. Bianchi, Performance Analysis of the IEEE 802.11 Distributed Coordination Function, IEEE JSAC, vol. 19, no. 3, March 2000. [2] C. Liu, et. al., Adaptive Contention Control for IEEE 802.11 Wireless LANs, submitted to ACM MobiCom 2004 X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

  8. Packet-based fairness slow client joins Uplink traffic dominates cell capacity • Throughput unfairness within a BSS • w/ slow client [1], mix of uplink and downlink • Impact to the E2E throughput of a path • What if a node is also on a multi-hop path? [1] B. Sadeghi, et al. , Opportunistic media access for multirate ad hoc networks, 8th ACM MobiCom, 2002. X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

  9. Flow forwarding/scheduling • Indiscriminate packet dropping lowers efficiency of multi-hop path • Loss of forward packet “wastes” more bandwidth • Links/flows get different share of bandwidth [1] • Interfacing with high-layer (e.g. TCP) rate control [2] [1] J. Li, et al. , Capacity of Ad Hoc Wireless Networks, 7th ACM MobiCom, 2001. [2] S. Xu, et.al, Does the IEEE 802.11 MAC Protocol Work Well in MultiHop Wireless Ad Hoc Networks, IEEE Comm. Mag. , June 2001. X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

  10. New MAC mechanisms? • Current MAC enhancement efforts • EDCA (TGe), High Throughput (TGn) – new mechanisms offered • ESS mesh not the targeted environment • How can ESS Mesh benefit from new MAC? • How much improvement will it provide out-of-box? • Will it help in solving mesh-specific issues? • How should the MAC parameters be configured for mesh? • Will new issues be introduced? • Future enhancement for ESS mesh should anticipate the likely wide adoption of EDCA-like MAC in devices X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

  11. Summary • Performance critical to ESS mesh • Well-documented performance issues of using DCF in mesh • New MAC protocols (.11e/n) offer opportunities and risks • Designed for non-mesh networks • Should TGs investigate enhancement to the 802.11 MAC/MLME focusing on ESS mesh performance X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

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