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Enhancing Wireless LAN Performance and Energy Efficiency in Mobile Devices

This project investigates ways to maintain performance while reducing energy consumption in mobile devices using Wireless LANs (WLANs). It addresses the challenge of high energy usage due to constant network access and proposes enhancements to the existing IEEE 802.11 Power-Saving Mode (PSM). The focus is on mobile clients communicating with wired base-stations, specifically for request/response applications like web browsing. Key findings include the inefficiencies of current PSM implementations and proposed solutions to optimize energy use without sacrificing performance.

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Enhancing Wireless LAN Performance and Energy Efficiency in Mobile Devices

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  1. Maintaining Performance while Saving Energy on Wireless LANs Ronny Krashinsky 6.929 Term Project 12-7-2001

  2. Motivation • Mobile devices limited by battery weight and lifetime • Wireless network access consumes a lot of energy • Want to disable the network interface card whenever its not in use • Basic problem: data may arrive from the network at any time • Focus of this work: a mobile client communicating with a wired base-station to perform request/response traffic (e.g. web browsing) • Not focusing on: ad hoc networks, mobile servers, real-time communication (voice) • Not relying on high-level knowledge of application state

  3. 802.11 Power-Saving Mode Overview(For Infrastructure Networks) • Network Interface Card power consumption: • Cisco Aironet: 1.7W Tx, 1.2W Rx, 1.1W Idle, 50mW Sleep • Basic idea: sleep to save energy, periodically wakeup to check for pending data • Clients go to sleep after sending or receiving data • Base-station buffers received data while client is asleep • Base-station sends out beacons every 100ms indicating whether or not the Client has pending data • Client wakes up to listen to beacon, then polls Base-station to receive data (ListenInterval can be less than BeaconPeriod) • Client can wake up to send data at any time

  4. Talk Outline • Measured performance of TCP over 802.11 PSM (it’s not good) • Trace analysis for characteristics of client HTTP traffic (how to save energy) • Proposed enhancements to 802.11 PSM to improve performance and minimize energy • Simulation of web browsing traffic over existing 802.11 PSM and alternatives

  5. PSM On PSM Off Mobile Client Base- Station Mobile Client Base- Station Server Server syn syn syn+ack syn+ack RTT sleep 100ms ack queue queue queue request beacon beacon beacon response start ack request sleep response start 100ms RTT RTT sleep 100ms Request/Response Over TCP Over 802.11 RTT +delta

  6. Request/Response Performance Test • Client: • Compaq iPAQ with Enterasys Networks RoamAbout 802.11 NIC • Servers: • Methodology: • repeat tests five times, alternating between PSM on and off, use mean for (N := various sizes) { start timer for (several iterations) { TCP connect to server send request receive N bytes close connection } stop timer }

  7. 802.11 PSM Measured Performance

  8. 802.11 PSM Measured Slowdown • Conclusion: 802.11 PSM is too coarse-grain to maintain network performance

  9. time request Req/Resp 1: response Req/Resp 2: resp Req/Resp 3: response Req/Resp 4: response Client State: wait recv idle wait receive idle Client Network Usage • Analyzed UC Berkeley Home-IP (modem) HTTP Traces: • client ID, request time, response start time, response end time • Classified client state as: {wait, idle, receive} • Discarded incomplete transactions (no timestamp) • Ignored receive and idle times longer than 1000s

  10. Idle Time Wait Time Client Network Usage Characteristics • Most wait time and idle time is spent in a few number of long latency events • These events will therefore account for most of the sleep energy • Conclusion: 802.11 PSM is too fine-grain to reduce energy effectively

  11. 0s 1s 2s 3s wakeup to listen to beacons… stay awake after sending request Stay Alive Listen- Interval Backoff increase ListenInterval if there is no response max = 0.9s 20% delay never sleep for more than 20% of total time since request Proposed Solution: StayAlive and ListenInterval-Backoff request PSM basic

  12. Latency and Energy Comparison

  13. Client Web Browsing Simulation • Modeled 802.11 PSM using ns-2 • Did not model detailed MAC protocol: no channel contention, no node movement, no packet losses • Modified Link C++ code to support sleep mode and send alerts to OTcl , control and beaconing in OTcl • Modeled HTTP traffic using empirical model • Based on study by Bruce Mah • Limited “Think Time” to 1000s • Added “Server Response Time” based on wait time from UCB Home-IP traces (less 100ms to account for network delays). • Updated to use FullTcp • Client  BaseStation: 0.1ms, 5Mbps • BaseStation  Server: 20ms, 10Mbps • Energy: 1W while active, 50mW while sleeping, 5mJ per listened-to beacon (1W5ms)

  14. Performance Results

  15. Performance and Energy Results energy per page (PSM off = 54 J) slowdown (vs. PSM off) PSM basic StayAlive LI-Backoff: 2x Max %delay

  16. Conclusions • Existing 802.11 PSM causes RTTs to be rounded up to the nearest 100ms • This adversely affects short TCP connections which are limited by the RTT • A viable solution is to stay awake for a short period of time after sending a request • When using 802.11 PSM, almost all energy consumption is due to sleep power and listening to beacons • ListenInterval-Backoff can reduce the listen energy • Longer sleep intervals have the potential to enable deeper sleep modes

  17. (backup slides)

  18. Simulation vs. Measured

  19. Actual Values Used in HTTP Simulation

  20. Intersil PRISM Radio Chip Set

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