Synchronous Two-Phase Rate and Power Control in 802.11 WLANs - PowerPoint PPT Presentation

Synchronous Two-Phase Rate and Power Control in 802.11 WLANs Kishore Ramachandran, Ravi Kokku, Honghai Zhang, and Marco Gruteser WINLAB, Rutgers University and NEC Laboratories America

Towards All-Wireless Enterprises Last-mile network access predominantly 802.11 � � High bandwidth, low/no cost Voice and data converging onto mobile devices � 100 million WiFi handsets over the next 5 years. Source: IDC, Frost & Sullivan, Infonetics 802.11 success => changing requirements � Increased Greater density Higher mobility battery life

Why Power and Rate Control? Integral Components in Overall Solution Adaptive transmit power control (TPC) can: Improve spatial reuse and network capacity � Reduce energy consumption for mobile devices � Compensate for link changes due to mobility � Hard problem in WLANs due to distributed operation and unlicensed spectrum

Symphony: A new approach to transmit power control (TPC) in 802.11 WLANs ~50 ft. ~20 ft. Dense 802.11 Enterprise WLANs � Goals: increase network capacity and improve battery life �

Related Work Channel Hidden Dom ain Solution Granularity Realization Deployability Energy Capacity Rate Access Nodes Asym m . [Sheth02] Per-link √ √ √ √ √ X X [Qiao03] Per-link X √ √ X √ X √ W LANs [Akella05] Per-link √ √ X √ √ X X [Chevillat05] Per-link X √ √ √ √ X X [Mhatre07] Per-cell √ √ X √ X √ X [Monks01] Per-link X X X √ X X √ √ [Jung02] Per-link X √ √ X X X Ad-hoc [Muqattash03] Per-link X X √ √ X X √ Nets [Muqattash05] Per-link X X √ √ X X √ [Sheth05] Per-link X X √ √ X √ √ [Kim06] Per-link X X √ √ √ X X [Shah07] Per-link X X √ √ X √ √ [Narayanaswa Per-network √ √ X √ X √ √ my02] No solution is both comprehensive and realizable �

Outline Introduction � Challenges to TPC in WLANs � Symphony Design � Experimental Results � Summary �

Challenge #1: Receiver-side Interference and Asymmetric Channel Access AP2 AP1 AP1 C B AP2 B A A Receiver-side interference Asym m etric channel access Receiver-side interference: Incorrect power reduction results � in increased packet error rate Asymmetric channel access: Incorrect power reduction results � in increased channel access time

Challenge #2: Interaction with Rate Adaptation Rate maxRate +ve -ve maxPower Power (minRate, minPower) Measuring SINR at fine time scales --- infeasible with mobility � Use delivery ratio over a window of packets [Wong06] � � Selecting rate and power non-trivial Incorrect power reduction results in reduced rate �

Challenge #3: Mobility With mobility, challenges #1 and #2 can occur intermittently � Observations: Performance at maximum power --- key reference point � Mobility => Periodic reference measurement needed � Some form of co-ordination between transmitters --- needed �

Outline Introduction � Challenges to TPC in WLANs � Symphony Design � Experimental Results � Summary �

Symphony Design: Two-phase Execution X 1 ms have passed REFERENCE OPERATIONAL Track performance Reduce power at max. power if possible X 2 ms have passed Goal: Link’s performance should be at least as good as in the � baseline maximum power network

Symphony Design: Synchronous Operation At TX1 Phase X 1 X 1 X 1 X 1 REF X 2 X 2 X 2 OPT Time At TX2 Phase X 1 X 1 X 1 X 1 REF X 2 X 2 X 2 OPT Time All transmitters cycle through the two phases in synchrony �

Outline Introduction � Challenges to TPC in WLANs � Symphony Design � Experimental Results � Summary �

Symphony Evaluation Does it deal well with the challenges? � Does Symphony increase spatial reuse? � Does it reduce energy consumption? � Is it incrementally deployable (with non-compliant nodes)? �

Platform: Indoor Office Testbed and ORBIT Static Nodes Problem scenarios Mobile clients Hardware: Dell laptops or ORBIT nodes (with Atheros NICs) � Software: Linux with MadWifi driver (v0.9.3.x) � Synchronization: NTP between APs, broadcast msg. for clients � Traffic: Mix of emulated VoIP, UDP @ 200pps, and TCP �

Results – Synopsis Challenges Goals Channel Access Battery life Hidden Spatial Reuse Terminals Asymmetry Improvement 300% Up to 50% Up to 46% Detect and throughput correct in 1 sec. improvement improvement improvement From a deployment perspective: Symphony opportunistically reduces the transmit power by � 6dB even in the presence of non-compliant nodes

Summary Symphony is Effective in addressing TPC challenges � Easy to realize in a WLAN � Readily deployable (even with non-compliant nodes) �

Thanks! For more information: http://www.winlab.rutgers.edu/~kishore/symphony-tech.pdf

Extra Slides

Traditional approach in WLANs: Static or coarsely dynamic per-cell TPC AP2 AP1 B A D C AP1 � A interferes with AP2 � B Strategy 1: Fixed TPC � � For all clients, use maximum power (18-20dBm) Strategy 2: Per-cell TPC [Mhatre07] � � For all clients in a cell, use power level for weakest client � Adjust power at coarse time intervals (10s of seconds)

Symphony: Dynamic per-link TPC AP2 AP1 B A D C AP1 � A and AP2 � B can “talk” simultaneously Choose power on a per-client (per-link) basis � Adjust power at fine-grained time intervals � Goal: Use “as much power as needed” and adapt to mobility �

Symphony: Implementation Network Layer Rcv. pkts Snd. pkts Device driver Symphony Power adaptation SET Transmit path Receive path Power R O R O R O Rate RTS Asymm. Rate RX-side access tx_pkt() tx_pkt_complete() Interface card Implemented in MadWifi v0.9.3.1

Symphony: Enables up to 18dB power reduction Blah blah �

Symphony: Negligible R-score degradation Average R-score difference: 2 � Worst case R-score difference: 3.4 �

Symphony: Up to 46% more battery life Active Mode Battery life I m provem ent Setup Symphony ORBIT Up to 46% (relative to max. power) Indoor Office Up to 33% (relative to max. power)

Symphony: 30-50% throughput improvement Tw o links Three links

Results – Summary Improves throughput of asymmetry-affected links by 300% � Is responsive to receiver side interference even at short � timescales of 1 second. For mobile VOIP clients, opportunistically reduces transmit � power by up to 97%, with negligible voice quality degradation Increases network throughput by up to 50% � Increases battery life by up to 46% (relative to max. power) �

Evaluation Details Hardware: Dell laptops or ORBIT nodes (with Atheros NICs) � Software: Linux with MadWifi driver (v0.9.3.x) � Synchronization: NTP between APs, broadcast msg. for clients � Traffic: Mix of emulated VoIP, UDP @ 200pps, and TCP � Traditional Symphony Power: 18dBm fixed Power: 3dB step size Rate: Variable (RRAA+) Rate: Variable (RRAA+)

Future Work