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Maintaining Performance while Saving Energy on Wireless LANs. Ronny Krashinsky 6.929 Term Project 12-7-2001. 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
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Maintaining Performance while Saving Energy on Wireless LANs Ronny Krashinsky 6.929 Term Project 12-7-2001
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
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
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
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
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 }
802.11 PSM Measured Slowdown • Conclusion: 802.11 PSM is too coarse-grain to maintain network performance
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
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
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
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 (1W5ms)
Performance and Energy Results energy per page (PSM off = 54 J) slowdown (vs. PSM off) PSM basic StayAlive LI-Backoff: 2x Max %delay
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