End-to-End Aware Association in Mesh Networks: Performance Study September 2007 doc.: IEEE 802.11-07/2432r0 September 2007 End-to-End Aware Association in Mesh Networks: Performance Study Date: 2007-09-18 Authors: Hang Liu, Thomson Steve Emeott, Motorola

September 2007 doc.: IEEE 802.11-07/2432r0 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

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

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

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

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

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

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

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

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

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

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 802.11g 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

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

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, 802.11s may specify an optional IE Legacy STAs that does not recognize this IE can discard it for backward compatibility Hang Liu, Thomson

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