Rohan Murty Harvard University Jitendra Padhye, Ranveer Chandra, Alec Wolman, and Brian Zill Microsoft Research 1

Increased adoption and usage [Forrester] Culture of mobility: Users tend to use Wi-Fi even when wired connections are available [Gartner, Forrester, Economist] Move towards an all wireless office Users want wire-like performance from wireless networks 2

Corporate WLAN Study: 12 users < 1 Mbps each 3

Focus on coverage Fewer APs than clients Clients talk to APs far away; worsens rate anomaly Clients pick APs to associate with Use RSSI of beacon packets Agnostic to channel load at APs Lack adaptive behavior No load balancing; fixed channel assignments Congestion and hotspots worsen 4

Focus on capacity Lots of APs; densely deployed Clients can talk to APs near by; mitigates rate anomaly Infrastructure picks client-AP associations Global view of network conditions (channel load, interference, etc.) Adaptability Load balance associations; Dynamic channel assignment Redistributes load away from local hotspots 5

No client modifications Works with legacy clients Changes limited to the infrastructure Easy to deploy Self-managing 6

DenseAP Central Controller (DC) Associations Channel Assignments Load Balancing DenseAP Nodes (DAPs) Commands to DenseAP nodes Summarized Data from DenseAP nodes Summarized Data Wired Network Commands Interface with clients Send summaries to DC 7

Controlling Associations Mechanisms Policy Dynamic Channel Assignment Mechanism Policy Load Balancing Mechanism Policy 8

ACL 00:09:5B:5A:1F:4F 9

ACL 00:09:5B:5A:1F:4F Probe Request MAC = 00:09:5B:5A:1F:4F RSSI = 30 Probe Request MAC = 00:09:5B:5A:1F:4F RSSI = 42 Probe Request MAC = 00:09:5B:5A:1F:4F RSSI = 40 10

Accept Client ACL 00:09:5B:5A:1F:4F Client only sees one DAP at any given time Probe Response 11

What is the quality of a connection between a client and a DAP? (rate) How busy is the medium around each DAP? Overall goal: Associate client with a DAP that will yield good throughput Overall goal: Associate client with a DAP that will yield good throughput 12

Expected Transmission-Rate (Mbps) Available Capacity (AC) (Mbps) Free Air Time (%) X = 13

Probe Request Free air time = 0.35 DAP2 DAP1 DAP3 RSSI = 20 RSSI = 10 RSSI = 30 Free air time = 0.45 Free air time = 0.22 DAPFree Air- Time RSSI DAP DAP DAP Accept Client Probe Response DAPFree Air- Time RSSIEx. Tx- Rate AC DAP DAP DAP

Correlation between RSSI of Probe Request packets Avg. throughput between a DAP-client pair Rough approximation - ordering of DAPs Online profiling method that builds RSSI to data- rate estimates 15 Upload and RSSI correlation = 0.71 Download and RSSI correlation = 0.61 Upload and RSSI correlation = 0.71 Download and RSSI correlation = 0.61

Estimate how busy is the medium around at a DAP Technique similar to ProbeGap* Measure time taken to finish a packet transmission Estimates match up closely with offered traffic load 16 *Lakshminarayan et al., 2004 *Vasudevan et al., 2005

Integrated into the association process DAPs not discovered by clients dont need channels A DAP is assigned a channel only when it goes from being passive (no clients) to active (services at least one client) Central controller assigns channel with least load 17

So far, associations when a new client joins the network No association is perfect Client traffic demands change Local hotspots created 18

Central controller monitors load on every DAP When channel load on a DAP crosses a certain threshold Client causing most load is determined Moved to less loaded DAP nearby Ensure client continues to get at least as much available capacity at the new DAP Load balancing achieved via handoffs Use association control; manipulate ACLs on DAPs 19

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21 1 Corp AP 24 DAPs 24 Clients a/bg

Performance Density Channels Intelligent Association Load Balancing 22

Gains due to More channels DAP density Intelligent associations % gain Why?

Put all DAPs on the same channel Factors out Channels Intelligent Associations: same load on all DAPs Single out impact of Density 24

Higher density provides better performance 25

Is intelligent association control necessary? 26

Client-Driven Disable intelligent association control Let clients pick DAP to associate with (conventional WLANs) Compare with DenseAP Factors out Channels Density Single out impact of Intelligent association 27

Intelligent association policy is necessary % gain

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Client 1 moved Client 1 improves Clients 2 & 3 improve Client 2 moved 30

Load balancing algorithm and mechanism Mobility Performance Fewer DAPs Fewer channels g ….. Scalability 31

Plenty of prior work on static channel assignment, power control and associations Each studied each aspect in isolation Require client modifications [Ramani and Savage, Infocom 2005] SMARTA [Ahmed et al., CoNext 2006] Examines channel and power control Increase overall network capacity Does not consider associations, load balancing MDG [Broustis et al., MOBICOM 2007] Identified tuning channel, power and associations Studies the order in which these knobs must be tuned Requires client modifications 32

Practical system How do density, intelligent association, and more channels affect capacity? Adaptive system Future directions Impact of hidden terminals Heterogeneous mix of client traffic patterns Other backhauls: e.g. Wireless, powerline 33