1. Performance Implications of 802.11 DCF to ESS Mesh Networks
Month 2002
doc.: IEEE /xxxr0
May 2004
Performance Implications of 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.
John Doe, His Company

2. Motivation Performance is critical to ESS mesh
May 2004
Motivation
Performance is critical to ESS mesh
Home network (PCs, CEs…)
Provide QoS for multimedia applications
Wireless data access network
Provide infrastructure equivalent performance
DCF has significant impact
In addition to routing
X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

3. Presentation overview
May 2004
Presentation overview
ESS mesh functional with existing 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
Month 2002
doc.: IEEE /xxxr0
May 2004
Issues in 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.
John Doe, His Company

5. Co-channel interference (CCI)
Month 2002
doc.: IEEE /xxxr0
May 2004
Co-channel interference (CCI)
CCI cause performance degradation in general multi-cell 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
Per-cell throughput (Mbps)
# of co-channel cell
X. Guo, W.S. Conner, L.L. Yang, Intel Corp.
John Doe, His Company

6. Interference mitigation
May 2004
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 Based Ad Hoc Networks, Globecom 2003
[2] J. Zhu, et. al, Leveraging Spatial Reuse in 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]
May 2004
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 Distributed Coordination Function, IEEE JSAC, vol. 19, no. 3, March 2000.
[2] C. Liu, et. al., Adaptive Contention Control for IEEE Wireless LANs, submitted to ACM MobiCom 2004
X. Guo, W.S. Conner, L.L. Yang, Intel Corp.

8. Packet-based fairness
May 2004
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
May 2004
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 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
Month 2002
doc.: IEEE /xxxr0
May 2004
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.
John Doe, His Company

11. Summary Performance critical to ESS mesh
Month 2002
doc.: IEEE /xxxr0
May 2004
Summary
Performance critical to ESS mesh
Well-documented performance issues of using DCF in mesh
New MAC offers opportunities and risks
Designed for non-mesh networks
Should TGs investigate enhancement to the MAC/MLME focusing on ESS mesh performance
X. Guo, W.S. Conner, L.L. Yang, Intel Corp.
John Doe, His Company