FTF 2014 – NET-F0374

Andrew Ross Embedded Wireless FAE

Silex Technology America, Inc.

E-mail: aross@silexamerica.com

Web: www.silexamerica.com

How do you save power?

Description of Power Save Modes: ◦ Active Mode

◦ PS Poll Mode

◦ Automatic Power Save Delivery

◦ Wake-on-Wireless

Wireless LAN Security and Power Save

New device options in WLAN Technologies

Q&A

Physical ◦ Sending information determined by physics of RF

transmission

◦ 802.11 specification is defined and limited by regulatory limits

◦ Existing Transmit power management in proprietary hardware

Transmit efficiency ◦ Increasing the % of real data in the information consuming

energy

Turning things off ◦ Minimizing power draw while waiting for information to

send

Power save modes have always been part of 802.11 specification

◦ Included in original 1999 specification

Originally designed for wirelessly connecting laptops to networks

◦ Large batteries

Only recently with the addition of WiFi to mobile devices have smaller batteries been used with WiFi enabled devices

Greater focus on power save within WLAN

Latest applications are the motivation

◦ IP phones

◦ IP video distribution (Netflix, Hulu, etc.)

◦ Internet radio (Milk, Slacker, etc.)

Power vs. distance ◦ Wired is a linear relationship

◦ Wireless is an inverse square law relationship

Constant listening ◦ Wired can react to level change on cable allowing the

receiver to be off.

◦ Wireless, without synchronization, requires the receiver to be on all the time

Even if you do everything right..

Conflict between security and power save ◦ Need to support security standards imposes maintenance

of authentication that uses power.

Part of the original 802.11 standard

Constrained by the infrastructure (AP and STA)

Looking to address the demands of the modern platforms (1999)

What is PSM? ◦ It utilizes the beacon period of the AP

◦ A Power save bit in the packet from the client

◦ The Traffic Indication map (TIM)

◦ A listening period called DTIM

◦ A PS-Poll packet to initiate information delivery

Why use it? ◦ Provides a method by which the STA can sleep when

information is not being transferred

Latency vs. band width vs. power ◦ Bound to AP beacons

◦ Trade-off between latency and power

◦ Leads to a trade-off between bandwidth and power

Compatibility with AP , too long a DTIM and data can be discarded and even stability of association to AP’s can suffer.

PSM is not suited to high data demand due to PS-Poll packet.

Applications like IP phones and video streaming impacted by the PSM process.

Difficult to balance network performance and power save demands ◦ AP’s set conditions

◦ STA are driven by applications

New power save approach needed to address application demands.

Solution: ◦ Automatic Power Save Delivery (APSD)

Originally part of WiFi Multi Media (WMM) now referred to as WMM Power Save

Not constrained by infrastructure ◦ Not bound by beacons periods or DTIM

Data driven and client initiated

Two modes ◦ Unscheduled

◦ Scheduled

Attempt to deal with power save due to the application level changes in the use of WiFi

Unscheduled Automatic Power Save Delivery

Similar to PSM but prompts delivery of data from the STA by sending a packet called a trigger frame.

The use of the trigger frame is not bound by AP beacon period or DTIM schedule.

Allows STA Wake-Send-Receive-Sleep cycle to be based upon application data period, using trigger frame.

Targeted at bidirectional communication e.g. IP phones

Scheduled Automatic Power Save Delivery

Closely related to 802.11e Quality of Service (QoS).

Utilizes a unit of data called a ‘stream’

Stream interval is requested by the client

Allows STA Wake-Send-Receive-Sleep cycle to function on application data period, without using PS-Poll or trigger frame.

Targeted at stream type communication e.g. video and audio streaming

Power Save Multi-Poll ◦ 802.11n extension that adds ability to reserve a time slot

for Tx and Rx between the STA and AP

◦ Allows for direct control of the time the STA is off.

◦ Both Scheduled and Unscheduled versions defined.

Dynamic MIMO Power Save ◦ Defined to allow radio to turn off unnecessary transmitters

when application demand doesn’t require them

◦ Allows 3x3 11n radio to drop to a 2x2 or 1x1

Energy to send/receive information does not change

How do you save it? ◦ Doing it as efficiently as possible

Lower percentage of management content ◦ Turning on and off

Fast on-off transitions ◦ Knowing when to save power

Where do you save it? ◦ Radio ◦ Host ◦ Both.....

Can the WLAN interface manage power for the whole system?

Does it make sense that the radio controls the PM for the system? ◦ Sometimes

It is aware of when it needs to be on and when it can be off.

Can this awareness be communicated to the host? ◦ Yes

Based upon the Wake-on-LAN ◦ An Ethernet standard that allows systems to be awakened

by a network message

◦ Allows the host to go to sleep

◦ LAN controller stays active and wakes host when it detects network message (magic package)

WoW allows the WLAN to stay active, detect TIM and send PS-POLL

WLAN wakes the host via an interrupt when a magic packet is detected

Allows the host to minimize power consumption by entering a low power mode, while the WLAN listens for a magic packet

The magic packet is defined by filters ◦ Targeted packet for STA

◦ Key rotation

◦ Group message

◦ etc....

Usually the WLAN packet is encrypted, it is not always possible for the radio to decrypt without using an application on the host – security supplicant. ◦ Requires the radio to wake the host in order to process

Wake-Process-Sleep cycle for host initiated by WLAN

Overall there are two parts to saving power ◦ Optimize radio on time

PSM (PS-POLL)

APSD

◦ Minimize host on time

WoW

Turn it OFF!

The world is good.....but is it?

Trouble lurks in competing demands...

A quick review: ◦ WEP

◦ WPA

◦ WPA2

Security standard 802.11i defines the requirements

Saving power and maintaining security are conflicting requirements. ◦ Security is important

◦ Requires transfer of information that is not linked to application

◦ Impacts efficiency of information transfer

◦ Requires the radio to be on

◦ In most cases requires the host is on

Power save has the radio and host in active mode when there is data

But...

Security (802.11i) requires maintenance of the link encryption that cannot be handled just by the radio ◦ Initial connection

◦ Renewing keys

◦ Roaming

This means you need to use energy to be secure. ◦ Turning on the radio and the host to maintain the link

Why can’t we all just get along?

Remove security?

WLAN device manufacturers develop chipsets for specific purposes

For instance Silex supplies the SX-SDMAN based upon the Atheros AR6233 SIP.

The Atheros 6K3 family of products were developed for low power mobile applications ◦ Low stand-by power ◦ Fast, efficient switching of Tx/Rx circuits (on and off) ◦ Ability to automatically switch from PSM to CAM and back

based upon data delivery demands ◦ Support for WoW ◦ Reduced host based demand

Atheros 9K family uses a PCIe interface for high performance ◦ High host based demand

What are these mythical things ◦ Do they have a magic way of transmitting data that defies

the laws of physics?

New classification of devices referred to as: ◦ Low Power WiFi

◦ Ultra Low Power WiFi

◦ Internet-of-Things

◦ Internet-of-Everything

Supplied by ◦ Silex

◦ Gainspan

◦ Redpine Signals

◦ Connect One

◦ Roving Networks

They use the same radios They require all the same pieces

◦ Host (running OS or not, networking, radio driver, etc) ◦ 802.11 Baseband ◦ RF Transceiver ◦ FEM ◦ Switches ◦ Power Supply....

No surprises...so what makes them different?

The integration of the components is greater than we have seen with the more traditional WiFi solutions ◦ Integration equals more efficient hardware, smaller footprints, lower level

control of functions and hardware, faster on/off times, lower voltages, etc..

A reduced set of capabilities (subset of traditional solutions) ◦ Reducing the throughput of the devices. ◦ Not supporting all WiFi standards/options ◦ Limiting security to reduce the use of host based applications

Increased

hardware

efficiencies

Not magic just

good

engineering...

There are a number of Power Save modes that can... ◦ Improve the efficiency of the data transferred

◦ Minimize the devices ‘ON’ time

Use the PSM mode the suites your application

Account for power usage not directly linked to your application demands

Understand the deployment demands and address these as part of the development of your solution

Look at the different WLAN device technologies and pick the one that is best for your long term application requirements

STA – Station, this is the wireless client radio in infrastructure mode.

AP –Access Point this is the master radio in infrastructure Infrastructure – A wireless network that is established and

maintained by an AP and allows STA’s to connect to it. WPA – Wireless Protected Access a security standard that

uses TKIP or AES encryption and the use of a pre-shared common key.

PSM – Power Save Mode (PS-Poll) APSD – Automatic Power Save Delivery CAM – Constantly Active Mode EDCA – Enhanced Distributed Channel Access. Part of WMM

allows for prioritization of packets and contention free periods.

WMM – Wireless Multi Media TIM – Traffic Indication Map DTIM – Delivery Traffic Indication Message