13-09-2012, 05:05 PM
CoolSpots: Reducing the Power Consumption of Wireless Mobile Devices with Multiple Radio Interfaces
[attachment=33744]
Abstract
CoolSpots enable a wireless mobile device to automatically
switch between multiple radio interfaces, such as WiFi and
Bluetooth, in order to increase battery lifetime. The main
contribution of this work is an exploration of the policies
that enable a system to switch among these interfaces, each
with diverse radio characteristics and different ranges, in
order to save power – supported by detailed quantitative
measurements. The system and policies do not require any
changes to the mobile applications themselves, and changes
required to existing infrastructure are minimal. Results are
reported for a suite of commonly used applications, such as
file transfer, web browsing, and streaming media, across a
range of operating conditions. Experimental validation of
the CoolSpot system on a mobile research platform shows
substantial energy savings: more than a 50% reduction in
energy consumption of the wireless subsystem is possible,
with an associated increase in the effective battery lifetime.
Introduction
The utility of mobile devices is directly impacted by
their operating lifetime before on-board batteries need to be
replaced or recharged. In advanced mobile computing platforms
such as PDAs and smart-phones, the wireless communication
subsystem accounts for a major component of
the total power consumption [1][10] due to the communication
centric usage of these devices. Furthermore, these platforms
are increasingly being equipped with multiple radio
interfaces to handle a variety of connections, ranging from
Bluetooth for personal-area links, WiFi for local-area connectivity,
and GPRS for wide-area data access.
Previous research has explored the idea of switching
among multiple radio interfaces in an attempt to reduce
overall power consumption: By using the appropriate wireless
interface for the current application workload, and
keeping the others effectively turned off.
Mobile Computing Platforms
CoolSpots are motivated by the evolving PDA and
smart phone cellular platforms, which are able to provide
access to an ever expanding list of applications beyond
basic communication and networking needs. These devices
are now able to wirelessly download games, music and
video, and play this multimedia content either locally, or
remotely by wirelessly connecting to proximate environments
such as the Digital Home [21][20]. This usage model
is likely to play a more significant role in daily activities in
future years, as both Microsoft and Intel roll-out the next
generation of Windows Media Center Edition and Media
Center PCs supporting multiple radio interfaces such as
WiFi and Bluetooth.
For the spectrum of applications that need to be considered,
latency and bandwidth requirements vary significantly
depending on the applications in use, e.g., from infrequent
low-bandwidth control messages to highbandwidth
video streaming. Taking into account these
varying bandwidth needs we explore techniques that dynamically
reduce power consumption for mobile devices
without compromising network connectivity to the local
infrastructure, communication range or limit the peak
bandwidth needs of applications.
CoolSpots
CoolSpots uses a hybrid WiFi/BT system to provide
improved communication capabilities when a mobile device
is within a CoolSpot-enabled region (Figure 3). For
example, if a home user is in their living room near the
entertainment center, appropriately enabled as a CoolSpot,
the mobile device would exhibit lower-power consumption
while still supporting the desired application bandwidth.
From a technology perspective, this means that the Cool-
Spot (in this case the entertainment center) is equipped with
a Bluetooth radio, while the entire house is assumed to be
covered by WiFi.
Related Work
Several previous projects have investigated techniques
to reduce the power consumption of the wireless interface
in portable battery powered devices. These techniques
range from protocol optimizations at the various layers of
the networking stack for single-radio systems to techniques
balancing the capabilities of multiple radios to optimize
overall system behavior.
Systems based on WiFi with optimizations for reducing
energy consumption using just that radio include work
done at the application layer [7][9], transport layer [4][2]
and MAC layer [24][8]. The optimizations at the MAC
layer adjust some of the parameters of the 802.11 Power
Save Mode (PSM): for example, a bounded-delay addition
to PSM can drastically reduce incurred delay while maintaining
power savings [8]. CoolSpots, in contrast, augments
WiFi with the lower power Bluetooth radio which has an
order of magnitude less idle power than 802.11 PSM.
There have been a variety of proposals to extend Mobile-
IP to support more efficient localized networking
[6][25], but they have not considered utilizing multiple
wireless interfaces from an energy saving perspective.
Similarly, the handoff between local-area and wide-area
networks can be used to provide ubiquitous coverage
[18][19], but again not considering power consumption.
Switching Policies
As mentioned earlier, managing power used by multiple
network interfaces requires the system to make two
decisions: when to activate the higher-power WiFi interface,
and when to shut off the WiFi interface and switch down to
Bluetooth. Framed in terms of bandwidth, the question becomes
when is there too little bandwidth available on the
Bluetooth channel (switch up), or when is there too much
unused bandwidth available (switch down). The simplest
approach, using statically coded thresholds, does not work
well given the variation in channel characteristics: the optimal
bandwidth threshold changes as the distance between
devices changes.
Conclusion
The CoolSpots model provides a seamless way for
mobile devices to automatically reduce their power consumption
during wireless communication. Without requiring
any application modification, the system utilizes multiple
wireless channels to realize a greater than 50% energy
savings across a representative suite of benchmarks when
compared against standard WiFi-only power-saving techniques.
Several policies form the basis for switching between
the wireless interfaces. The simplest policies, based on
bandwidth monitoring, do very well under constrained
channel conditions but have a difficult time adapting to
greater communication ranges. A more adaptive algorithm
(cap-dynamic), based on active channel measurements, is
very effective at recognizing the appropriate instant to
switch interfaces across a variety of channel conditions,
yielding a robust and energy-efficient solution.
[attachment=33744]
Abstract
CoolSpots enable a wireless mobile device to automatically
switch between multiple radio interfaces, such as WiFi and
Bluetooth, in order to increase battery lifetime. The main
contribution of this work is an exploration of the policies
that enable a system to switch among these interfaces, each
with diverse radio characteristics and different ranges, in
order to save power – supported by detailed quantitative
measurements. The system and policies do not require any
changes to the mobile applications themselves, and changes
required to existing infrastructure are minimal. Results are
reported for a suite of commonly used applications, such as
file transfer, web browsing, and streaming media, across a
range of operating conditions. Experimental validation of
the CoolSpot system on a mobile research platform shows
substantial energy savings: more than a 50% reduction in
energy consumption of the wireless subsystem is possible,
with an associated increase in the effective battery lifetime.
Introduction
The utility of mobile devices is directly impacted by
their operating lifetime before on-board batteries need to be
replaced or recharged. In advanced mobile computing platforms
such as PDAs and smart-phones, the wireless communication
subsystem accounts for a major component of
the total power consumption [1][10] due to the communication
centric usage of these devices. Furthermore, these platforms
are increasingly being equipped with multiple radio
interfaces to handle a variety of connections, ranging from
Bluetooth for personal-area links, WiFi for local-area connectivity,
and GPRS for wide-area data access.
Previous research has explored the idea of switching
among multiple radio interfaces in an attempt to reduce
overall power consumption: By using the appropriate wireless
interface for the current application workload, and
keeping the others effectively turned off.
Mobile Computing Platforms
CoolSpots are motivated by the evolving PDA and
smart phone cellular platforms, which are able to provide
access to an ever expanding list of applications beyond
basic communication and networking needs. These devices
are now able to wirelessly download games, music and
video, and play this multimedia content either locally, or
remotely by wirelessly connecting to proximate environments
such as the Digital Home [21][20]. This usage model
is likely to play a more significant role in daily activities in
future years, as both Microsoft and Intel roll-out the next
generation of Windows Media Center Edition and Media
Center PCs supporting multiple radio interfaces such as
WiFi and Bluetooth.
For the spectrum of applications that need to be considered,
latency and bandwidth requirements vary significantly
depending on the applications in use, e.g., from infrequent
low-bandwidth control messages to highbandwidth
video streaming. Taking into account these
varying bandwidth needs we explore techniques that dynamically
reduce power consumption for mobile devices
without compromising network connectivity to the local
infrastructure, communication range or limit the peak
bandwidth needs of applications.
CoolSpots
CoolSpots uses a hybrid WiFi/BT system to provide
improved communication capabilities when a mobile device
is within a CoolSpot-enabled region (Figure 3). For
example, if a home user is in their living room near the
entertainment center, appropriately enabled as a CoolSpot,
the mobile device would exhibit lower-power consumption
while still supporting the desired application bandwidth.
From a technology perspective, this means that the Cool-
Spot (in this case the entertainment center) is equipped with
a Bluetooth radio, while the entire house is assumed to be
covered by WiFi.
Related Work
Several previous projects have investigated techniques
to reduce the power consumption of the wireless interface
in portable battery powered devices. These techniques
range from protocol optimizations at the various layers of
the networking stack for single-radio systems to techniques
balancing the capabilities of multiple radios to optimize
overall system behavior.
Systems based on WiFi with optimizations for reducing
energy consumption using just that radio include work
done at the application layer [7][9], transport layer [4][2]
and MAC layer [24][8]. The optimizations at the MAC
layer adjust some of the parameters of the 802.11 Power
Save Mode (PSM): for example, a bounded-delay addition
to PSM can drastically reduce incurred delay while maintaining
power savings [8]. CoolSpots, in contrast, augments
WiFi with the lower power Bluetooth radio which has an
order of magnitude less idle power than 802.11 PSM.
There have been a variety of proposals to extend Mobile-
IP to support more efficient localized networking
[6][25], but they have not considered utilizing multiple
wireless interfaces from an energy saving perspective.
Similarly, the handoff between local-area and wide-area
networks can be used to provide ubiquitous coverage
[18][19], but again not considering power consumption.
Switching Policies
As mentioned earlier, managing power used by multiple
network interfaces requires the system to make two
decisions: when to activate the higher-power WiFi interface,
and when to shut off the WiFi interface and switch down to
Bluetooth. Framed in terms of bandwidth, the question becomes
when is there too little bandwidth available on the
Bluetooth channel (switch up), or when is there too much
unused bandwidth available (switch down). The simplest
approach, using statically coded thresholds, does not work
well given the variation in channel characteristics: the optimal
bandwidth threshold changes as the distance between
devices changes.
Conclusion
The CoolSpots model provides a seamless way for
mobile devices to automatically reduce their power consumption
during wireless communication. Without requiring
any application modification, the system utilizes multiple
wireless channels to realize a greater than 50% energy
savings across a representative suite of benchmarks when
compared against standard WiFi-only power-saving techniques.
Several policies form the basis for switching between
the wireless interfaces. The simplest policies, based on
bandwidth monitoring, do very well under constrained
channel conditions but have a difficult time adapting to
greater communication ranges. A more adaptive algorithm
(cap-dynamic), based on active channel measurements, is
very effective at recognizing the appropriate instant to
switch interfaces across a variety of channel conditions,
yielding a robust and energy-efficient solution.