1. End-to-End Aware Association in Mesh Networks: Performance Study
September 2007
doc.: IEEE /2432r0
September 2007
End-to-End Aware Association in Mesh Networks: Performance Study
Date:
Authors:
Hang Liu, Thomson
Steve Emeott, Motorola

2. September 2007
doc.: IEEE /2432r0
September 2007
Abstract
Association in mesh networks should be more intelligent to take into account the end-to-end resource availability for optimal performance. This submission provides simulation results to show the importance for a STA to have knowledge of path quality inside the mesh when it select a mesh access point to associate with. Certain signaling is needed in the BSS beacon and probe response of a MAP in order to support this feature.
Hang Liu, Thomson
Steve Emeott, Motorola

3. Outline Introduction E2E Mesh Access Point Association
September 2007
Outline
Introduction
E2E Mesh Access Point Association
Routing Metrics and Access Link Metrics Used in the Study
Simulations Setup and Results
Summary
Straw Poll
Hang Liu, Thomson

4. Wireless Mesh Networks
September 2007
Wireless Mesh Networks
Internet
MAP
Portal
MPs and MAPs form a wireless multi-hop backhaul for routing and data forwarding.
STAs (e.g., laptops and dual-mode smart phones) associate themselves with nearby MAPs to access the network.
A STA does not participate in packet relay and routing process.
Hang Liu, Thomson

5. September 2007
MAP Selection
Traditionally, signal strength is the criterion for a STA to select an AP to associate with in WLANs
Does NOT work well in multi-hop environment.
Unlike single-hop WLANs, the communication quality of a STA in a mesh depends on
the access link performance between the STA and the associated MAP
the wireless connection quality inside the mesh backhaul
Problem: How to select a MAP to associate with in the mesh network
Assume that most of data traffic is between the STAs and the portal
Hang Liu, Thomson

6. Joint MAP Selection Criterion
September 2007
Joint MAP Selection Criterion
Take into account both the quality of the access link and the multi-hop path inside the mesh backhaul from the associated MAP to the portal
Joint MAP Selection Metric
M(a): access link cost
M(p): mesh backhaul path cost between the MAP and the portal.
b: a tunable parameter (0<=b<=1) to balance between the access link quality and mesh path quality.
Hang Liu, Thomson

7. MAP Selection Protocol
September 2007
MAP Selection Protocol
A STA discovers MAPs in its vicinity through scanning
Passive scanning: listens to MAP beacons
Active scanning: sends probes and listens to the probe responses from MAPs.
The beacons and probe responses of a MAP carry
Path cost between the MAP and the portal
The STA evaluate both the access link quality and the path quality for each of its candidate MAPs and determine a MAP to associate with
Hang Liu, Thomson

8. Routing Metrics Used for Study
September 2007
Routing Metrics Used for Study
Hop Count
Airtime or Expected Transmission Time (ETT) link metric
Reflects the amount of channel resources consumed by transmitting the frame over a particular link.
O: Channel access overhead
Bt: Number of bits in test frame
r: Data rate
ef: frame error rate
Hang Liu, Thomson

9. Radio and Load Aware (RALA) Metric
September 2007
Radio and Load Aware (RALA) Metric
ρ= tbusy/tp: channel utilization/load measured by channel busy time (802.11k)
W(r): weight function of channel load r
links with channel load less than r0are weighted equally.
Links with channel load between r0 and rmax are given weights increasing with their channel load.
Links with channel load greater than rmax are not considered in the path selection
Hang Liu, Thomson

10. Route Stability September 2007
The link quality and traffic load varies so the value of RALA may frequently change
Cause route instability.
To maintain the route stability , use a quantized version of link metric
M: the number of quantization levels
Q: the quantization factor
Characteristics of QuantizedRALA
Select a path with not only higher link data rate and less frame loss rate but also lower traffic load and higher available bandwidth
Prevent congestion
Quantization of metric can improve route stability while achieving quick responses to link state and network topology changes
Hang Liu, Thomson

11. Access Link Metric Used in the Study
September 2007
Access Link Metric Used in the Study
RSSI
Contention Aware Airtime or Contention Aware Expected Transmission Time (CAETT)
Expected transmission time for a STA to send a frame to the associated AP while contending with other STAs associated to this AP.
DCF tends to give equal long-term channel access opportunities to all contending STAs associated to an AP
All STAs will achieve approximately equal throughput on average irrespective with their link data rates.
C: the set of STAs currently associated with an AP
n: the new STA to associates to the AP
rj the link data rate between the STA j and the AP
Load Aware Airtime or Load Aware Expected Transmission Time (LAETT)
Take into account link load and contention
Hang Liu, Thomson

12. Simulation Results September 2007
Performance of Joint MAP Association with respect to b
30 mesh access points (MAPs) and 120 STAs in a 750m×500m area
1 portal at a corner
802.11a for mesh and g for access
HWMP tree building mode
b = 0: MAP selection is only based on the access link cost
b = 1: MAP selection is only based on the backhaul path quality
Hang Liu, Thomson

13. Comparison of Joint MAP selection and traditional AP selection
September 2007
Comparison of Joint MAP selection and traditional AP selection
Aggregate throughput and average end-to-end delay under different offered load for several combinations of routing metrics and access link metrics
Hang Liu, Thomson

14. September 2007
Summary
Certainly, the end-to-end performance of STAs can be greatly improved by taking into account both the access link quality and the mesh backhaul path quality between the MAP and portal at association process
The end-to-end performance improvement may not be that significant if we just improve the back-haul routing without jointly considering the access link quality and mesh backhaul path quality.
Information of the path cost between a MAP and the portal is needed in the BSS beacon and probe response of the MAP in order to support end-to-end aware association. We have two approaches
Vendor specific IE in beacon and probe response
The disadvantage of this approach is that each vendor may use different IE ID and format
For interoperability, s may specify an optional IE
Legacy STAs that does not recognize this IE can discard it for backward compatibility
Hang Liu, Thomson

15. September 2007
Straw Poll
Is there interest to investigate the inclusion in the TGs draft of an optional IE in the mesh AP beacon and probe response that provides the path cost information between the MAP and the portal so a STA can make better decision in association process.
Yes:
No:
Abstain:
Hang Liu, Thomson