\section{Energy Consumption in 3G}

The wireless interface on mobile devices consumes a large part of the total energy of energy and the amount consumed is not proportional to the data transmitted nor the same for all wireless interfaces \cite{rice2010decomposing}.

% text about 3G, get more article to cite for same information
% \cite{schulman2010bartendr,qian2011profiling}
\subsection{3G States and State Transitions}

The third generation of mobile networks (\gls{3g}) is a collection name for several different telecommunication techniques such as Universal Mobile Telecommunication System (\gls{umts}). \gls{umts} devices establish network connectivity through \gls{umts} Terrestrial Radio Access Network (\gls{utran}). The \gls{utran} in turn consists of Base Stations (\gls{bs}s) and Radio Network Controllers (\gls{rnc}s). A \gls{rnc} are responsible for \gls{bs}s and control their network resources. The allocation of these network resources is done per device with Radio Resource Control (\gls{rrc}) state machines, transitions of these state machines are triggered by device network traffic and carrier inactivity timers. The different states can be seen in figure \ref{3g_state_graph}. \cite{lagar2012traffic}

\input{graphs/3g_state_graph}

Figure \ref{3g_state_graph} shows the different transitions and states that are available for the \gls{3g} enabled device. The performance in the x-axis indicates the response time and data rate where higher performance means lower response time and/or increased achievable data rate. Power consumption in the y-axis indicates the power usage between the different states. {\cite{vergara2014energybox}}

In the dedicated state (CELL\_{}DCH), the User Equipment (\gls{ue}) has a dedicated physical channel allocated for it which means higher data rate compared to a shared channel but also a higher energy cost. A shared channel is used in the shared state, (CELL\_{}FACH), it consumes less power than in the dedicated state but in both the dedicated and shared state the \gls{ue} is constantly connected to the \gls{rrc}.  During the idle state the \gls{ue} is unconnected but still able to check for downlink packets. The standby state (URA\_{}PCH or CELL\_{}PCH) is optional for operators to implement and has a power profile similar to the idle state. The main difference between the idle and standby state is that the standby state allow the \gls{ue} to switch faster to higher states since it maintains a connection to the \gls{rnc}.  \cite{vergara2014energybox,ding2013characterizing}


In Figure \ref{3g_state_graph} the arrows indicate all the possible transitions for the \gls{ue}. The \gls{rnc} determines which state a \gls{ue} is in by using inactivity timers for demoting to a lower performance state, shown as T1-T3 in Figure \ref{3g_state_graph}, and promoting depending on the data traffic. The inactivity timers are statically set by the carrier and start counting when there is little or no data traffic. \cite{vergara2014energybox,ding2013characterizing}


The \gls{rnc} uses the Radio Link Control (\gls{rlc}) in order to report the observed traffic volume. The \gls{rlc} implements data buffers to trigger state transitions. These buffers trigger transitions when the occupancy of the buffers exceeds a certain fixed threshold. There are thresholds for both uplink and downlink. The thresholds are statically set and different depending on the current \gls{rrc} state. Once the data has been transmitted the buffers are cleared. \cite{vergara2014energybox,ding2013characterizing}

\subsection{Received Signal Strength and Data Rate}

The energy consumption in \gls{3g} is less when \gls{rss} is strong since the communication time and signal amplification is lower. When the \gls{rss} is low the radio draws more current since the amplifier switches to a high power mode to counter the drop in signal strength. Both transmission and reception make use of amplification, transmission for sending data and the reception part for continuously reporting \gls{rss} back to the \gls{bs}. This feedback is used by the \gls{bs} to set the modulation and data rate for the \gls{ue}. The data rate effect how long it takes to transfer data over the connection, in short this means that the \gls{ue} should spend more time in idle state if the data rate is high. However if the data to send is small enough the data rate will not significantly impact the time spent in higher states. \cite{cripps2006rf,Wang:2010:ECA:1953383.1953397,ding2013characterizing}
