Why energy is the key factor for mobility
Smartphones and tablets allow
people to carry around more computational power in their hands than most had on
their desktops just a few years ago. However, the usability of these devices is
strongly defined by the energy consumption of mobile applications, and user
reviews of applications reveal many customer complaints related to energy usage
A modern mobile application
brings increasingly complex functionality into mobile devices that are rapidly
overtaking personal computer as a primary computing. Because of the battery/energy
constraints of mobile devices, energy efficiency fitting an energy budget and
maximizing the utility of applications under given battery constraints has
become an important software design and maintenance consideration.
Traditionally concerned with performance and memory usage optimization,
perfective maintenance now has to address the challenges of optimizing energy
consumption, with existing perfective maintenance techniques being mostly
inapplicable
Key factors contribute mobile handset energy consumption
The main factor of mobile
application energy consumption is the mobile network itself. Not like ordinary desktop
applications, Mobile handsets use wireless state machine to manage network
calls. Behaviour of the wireless state machine is totally depending on the
network profile. For example the way that 3G network data calls utilize energy
is not the way that 4G/LTE data calls utilize energy.
How 3G wireless state machine consumes energy
In 3G wireless state machine, it
has three states. They are IDEL State,
CELL-DCH state (High Power Mode) and CELL FACH (Low Power Mode). CELL-DCH state
requires 800mW and CELL FACH require 460mW per second.
When mobile application wants to
initiate 3G data call, first it has to open a 3G network connection. According
to 3G profile, opening connection is taking 2 seconds [1]. When opening a 3G
connection our handset moves from IDEL state to CELL-DECH state [2].
Then connection will be established and request will be send to backend and
handset is waiting for the response [A]. Mobile device remains in DCH
for 5 seconds for the response. Sometimes response [B] takes more than 5
second and this idle time of 5 seconds is called the DCH tail. If no more data
arrives, the wireless state machine switches to a CELL_FACH state [3].
The FACH state can handle signaling packets, but if more data comes in [C],
the radio switches back to the DCH state. If no packets arrive after 12 seconds
in the FACH state, the radio returns to the IDLE state and turns off.
With using above factors we can
calculate actual energy requirement for each mobile transaction by adding
actual time spent for DCH, FACH, IDELà
DCH and FACHàDCH.
Following is a sample of such calculation.
|
3G
Energy Consumption
|
|
|
RCC
State
|
Energy Consumption (in Joules)
|
|
DCH
Active
|
xxxx
|
|
FACH
Active
|
xxxx
|
|
IDELà DCH
|
xxxx
|
|
FACHà DCH
|
xxxx
|
|
Total
Energy
|
xxxx
|
How LTE wireless state machine consumes energy
LTE wireless state machine has
only two states. They are RCC CONNECTED state and RCC IDEL state. RCC CONNECTED state is the high power mode
and it require 1000 to 3500mW and RCC IDEL requires less than 15mW. There are
two sub sates in LTE state machine with in RCC CONNECTED state. They are called
Short DRX and Long DRX Both of these modes has a parent mode call Continuous
Transmission state.
The RCC CONNECTED state that the
wireless state machine uses when it is actively transferring data. This high
power and high bandwidth state is what gives LTE its speed.
While in RCC CONNECTED state the
wireless state machine creates the connection [1}. Then it initiates
the request and waits for its response [A]. When data transmission is completed, the
radio moves to the Short DRX state, but is still using high power while it
waits for more data. If more data arrives, the wireless state machine returns
to the Continuous Transmission state, and if not, it moves to the Long DRX
state. While in the Long DRX state, the wireless state machine prepares to
switch to the IDLE state, but is still using high power and waiting for data.
If more data arrives, the wireless state machine returns to the Continuous
Transmission state, otherwise it goes to low power IDLE state and turns off.
Using above facts we can
calculate LTE energy consumption and following is the sample of LTE energy
consumption calculation.
|
LTE
energy consumption
|
|
|
RCC
State
|
Energy Consumption (in Joules)
|
|
IDELà Continues reception
|
xxxx
|
|
Continues
reception
|
xxxx
|
|
Continues
reception Trail
|
xxxx
|
|
Short
DRX
|
xxxx
|
|
Long
DRX
|
xxxx
|
|
IDEL
|
xxxx
|
|
Total
Energy
|
xxxx |
In the 3G state machine, the
power drain in the DCH state is basically constant for all files. In LTE, the
power drain in the Continuous Reception state varies depending on the
throughput. Small files keep the power low, but as the throughput increases,
the power gets even higher. Therefor LTE state machine drain slightly more
power than 3G, because the tail states (Short DRX and Long DRX) stay at the
higher base power, while much of the 3G tail is in the FACH state which uses
half power.
Modem end uses expectation of a
mobile application response id less than 3 second. This will make usage of LTE
more. But LTE requires more energy than 3G. This requires QA professions to
effectively test there AUT on 3G and LTE and calculate energy consumption
effectively and find ways of optimizing.
Best Practice
Recommendation
Test for insufficiency
due to scattered data transfers
In many mobile applications,
files/data are requested with large gaps. Because of this actual data transfer
through state machine will be through scatted way. Please note following
diagram.
QA should be able determine the
gap between scatted data transfer so development teams can make sure scatted
data are tightly grouped. This will enable less time consumption in 3G or LTE
sate machine.
Test for insufficiencies
from delaying closing connections
In case open connections are not
closed properly and effectively, 3G and LTE state machines are assuming some
more data are on the way and they are in receiving modes.
Please refer to above diagram.
Actual data burst was completed in 315th second and developer
actually closing the connection in 320th second. Because of this,
state machines waits unnecessarily in receiving mode for 5 secounds and it will
utilize energy depending on the type of the connection (3G or LTE). Assume that
end user travels to this page/screen for 3 times during his usage actual energy
waste is three times as your calculation.
Test for content insufficiencies
Mobile screens are comes with different sizes and
resolutions. Images use for each screen size and resolutions are needs to
optimize. Otherwise this will cause unnecessary data transfer and require more
energy.
Image on the left side of above
takes 43 seconds to download. Next image looks identical but takes 3.5 second
to download. This significant gain achieved simply by reducing the image
resolution. 3G or LTE sate machines do
not require additional time to download the second image and this will result
low energy consumption.
Test For periodic transfers
Periodic transfers are specific
packets that are sent repeatedly by a server over an infinite time horizon.
They typically occur at regular intervals.
Periodic transfers ([P1]
[P2] …. [P6]) trying keep the connection for long time but actually
state machine after its time out disable the connection and again create a new
connection ([1] [2] [3] and [4]). This will require additional 2secound
time to create the connection and it use 2 into 800mW of energy. Therefore Periodic
transfers needs to be optimized to minimize no of connections and QA needs to
monitor them effectively to determine following
- Best timing for Periodic transfer so that application uses as long a period as possible between transfers.
- Feasibility of combine Periodic transfer with other data.
- Feasibility of bundle as much data as possible into a single message.
Test for duplicate content
If mobile applications calling
for same content form the back-end over and over then it should be cached. By
doing this we can stop unnecessary data download and save energy.
Test for uncompressed
file transfer
If mobile applications use file
transfers then it has to be compressed. By doing this we can stop unnecessary
bandwidth usage and save energy.
