1

Realizing the Full Potential of PSM using Proxying

Ning DingAbhinav PathakY. Charlie Hu

Dimitrios KoutsonikolasClay ShepardLin Zhong

2

Smartphone is Energy Constrained

CPU

Screen

3G/4G

WiFi

GPS

Camera

Battery capacity only doubles in

last 15 years

3

WiFi Energy Consumption

Send

Recv

Idle Listen1000mW

765mW650mW

Time (ms)

WiFi NIC Power(mW)

… …

Phone

AP

Data

Data

Idle

Even in idle, WNIC drains out battery

in 6-10 hrs!

4

Modes of WiFi NIC

• Constant Awake Mode (CAM)– Send, Recv, Idle Listen– High power

• Power Saving Mode (PSM)– Cannot send/recv/idle listen– Very little power

5

WiFi: Power Saving Mode

100ms 100ms 100ms

Beacon

Phone

APBeacon Beacon Beacon

Time (ms)

WiFi NIC Power(mW)

Beacon

6

WiFi: Power Saving Mode (cont’d)

Beacon

Phone

AP

Data

WNIC in PSM, cannot recv

WNIC switch to CAM

PSM Wake-up Delay

PSM does not come for free!

Server

Time (ms)

WiFi NIC Power(mW)

7

PSM

• Energy - performance trade off

• Two implementations of PSM– Static PSM– Dynamic PSM

8

Static PSM

Phone

APSYN

ServerSYNACK

SYNACK

Req

Wake-up Delay

Wake-up Delay

Wake-up Delay

Already in PSM

Time (ms)

WiFi NIC Power(mW)

Static PSM

Flow Time: 300ms

Energy:3 μAh

Data

ACK

Beacon

9

Dynamic PSM

WiFi NIC Power(mW)

Phone

APSYN

ServerSYNACK

Req

PSM Timeout Dynamic PSM

Flow Time: 90ms

Energy:10 μAh

Static PSM

Flow Time: 300ms

Energy:3 μAh

Data

ACK

Beacon

Time (ms)

10

Motivation

Performance Energy

Static PSM

Dynamic PSM

Can we make it ?

11

Understand Dynamic PSM

Phone

AP

Server

RTT

PSM Timeout

Key: PSM Timeout > RTT

Phone PSM TimeoutNokia N900 200ms

HTC Nexus One 200ms

iPhone 4 95ms

12

Percy: Design

AP AP Split-TCP Proxy

PSM Timeout200ms/95ms

PSM Timeout30ms

1.

2.

13

Data

ACK

Beacon

Percy: Short Flows

Phone

AP+Proxy SYN

Server

Req

DataSYN

SYNACK Req

Percy

Flow Time: 110ms

Energy:4 μAh

Static PSM

Flow Time: 300ms

Energy:3 μAh

Dynamic PSM

Flow Time: 90ms

Energy:10 μAh

Time (ms)

WiFi NIC Power(mW)

Percy: Long Flows

14

SYNReq

SYN

SYNACK

Req

Periodical Flush

Time (ms)

WiFi NIC Power(mW)

Data

ACK

Beacon

15

Evaluation

• Trace-driven experiment– 10-phone 1-week trace– 38,069 HTTP flows

• PSM configurations– Percy– Static PSM– Dynamic PSM:

NokiaN900 iPhone 4

HTC Nexus One

16

Result – Energy Consumption

Total energy consumption for different PSM schemes

67%

44%

17

Result – Network Performance

CDF of flow time difference compared to Android PSM configuration

Time (ms)

18

Conclusion

• Existing PSMs have inherent problems

• A simple system: Percy– improve the energy savings– maintain good performance

• Percy saves 44-67% energy while incur minimal flow time elongation