An embedded system is some combination of computer \gls{hw} and \gls{sw}, either fixed in capability or programmable, that is specifically designed for a particular function \cite{ganssle2008embedded}. Industrial machines, automobiles, medical equipment, cameras, household appliances, airplanes, vending machines and toys (as well as the more obvious cellular phone and \gls{pda}) are among the myriad possible hosts of an embedded system. Embedded systems that are programmable are provided with programming interfaces, and embedded systems programming is a specialized occupation.
The term embedded system is quite broad, so there is no surprise that the spectrum of used processing units is also wide. Since the general purpose microprocessors require external components, namely memories and peripherals, they tend to consume extra power and a board space. Since the design limitations of an embedded systems are most of the time low physical size, low power consumption and/or long uptime and ruggedness (more components mean more parts could fail), microprocessors are seldom used. However, most of the commonly used architectures and word lengths are covered. Due to aforementioned reasons, microcontrollers are favored over microprocessors.
Today's state of chip integration allows production costs of a complex system on chip devices to be relatively low, thus making \gls{soc} attractive choice for embedded systems. \glspl{soc} could be described as an \gls{ic} that integrates all components of a computer or other electronic system into a single chip. It may contain digital, analog, mixed-signal, and often radio-frequency functions - all on a single chip substrate \cite{flynn2011computer}. \glspl{soc} are very common in the mobile electronics market because of their low power consumption.
An \gls{os} is a computer program that supports a computer's basic functions, and provides services to other programs (or applications) that run on the computer. The applications provide the functionality that the user of the computer wants or needs. The services provided by the operating system make writing the applications faster, simpler, and more maintainable.
Over time, a lot of \textit{embedded}\glspl{os} suited for embedded systems were developed. An embedded \gls{os} is a type of \gls{os} that is embedded and specifically configured for a certain \gls{hw} configuration. \Gls{hw} that uses embedded \gls{os} is designed to be lightweight and compact, forsaking many other functions found in non-embedded (i.e. desktop) computer systems in exchange for efficiency at resource usage \cite{holt2014embedded}. This means that they are made to do specific tasks and do them efficiently. Notable embedded \glspl{os} currently in use by consumers include:
\begin{itemize}
\item\textbf{Symbian} - used in cell phones, mainly ones made by Nokia
\item\textbf{Embedded Linux} - used in many other devices like printers, routers or smart TVs; Android \ref{f:android_scr} is a subset of embedded Linux
\item\textbf{BlackBerry \gls{os}} - for BlackBerry phones
\item\textbf{iOS} - subset of Mac \gls{os} X, used in Apple’s mobile devices Palm \gls{os}
A \gls{rtos} is just a special purpose \gls{os}. The \emph{real time} part of the name does not mean that the system responds quickly, it just means that there are rigid time requirements that must be met. If these time requirements are not met, the results can become inaccurate or unreliable\cite{jean2002microc}. Embedded systems frequently posses the real time requirement. There are two kinds of \gls{rtos}:
\item[Hard Real Time]- system delays are known or at least bounded. Said to be operating correctly if the system can return results within any time constraints.
\item[Soft Real Time]- critical tasks get priority over other tasks and will retain priority until the task is completed. This is another way of saying that real time tasks cannot be kept waiting indefinitely. Soft real time makes it easier to mix the system with other systems.
Linux itself is a kernel, but \emph{Linux} in day to day terms rarely means so. Embedded Linux generally refers to a complete Linux distribution targeted at embedded devices. There is no Linux kernel specifically targeted at embedded devices, the same Linux kernel source code can be built for a wide range of devices, workstations, embedded systems, and desktops though it allows the configuration of a variety of optional features in the kernel itself. In the embedded development context, there can be an embedded Linux system which uses the Linux kernel and other software or an embedded Linux distribution which is a prepackaged set of applications meant for embedded systems and is accompanied by development tools to build the system\cite{hallinan2010embedded}.
With the availability of consumer embedded devices, communities of users and developers were formed around theses devices: Replacement or enhancements of the Linux distribution shipped on the device has often been made possible thanks to availability of the source code and to the communities surrounding the devices. Due to the high number of devices, standardized build systems have appeared like OpenEmbedded, Buildroot, OpenWrt, and LTIB.
The \emph{kernel} is the essential center of a computer \gls{os}, the core that provides basic services for all other parts of the \gls{os}\cite{bovet2005understanding}. It has complete control over what happens in the system. A kernel can be contrasted with a \emph{shell}, the outermost part of an \gls{os} that interacts with user commands. Kernel and shell are terms used more frequently in Unix or Unix-like operating systems than in IBM mainframe or Microsoft Windows systems.
\caption{The simplified view on the Linux system structure}\label{f:linuxbl}
\end{figure}
The simplified view on the Linux system structure can be seen on \ref{f:linuxbl}. It does not include device drivers, compilers, deamon, utilities, commands, library files and such, but should be enough for a demonstration.
OpenWrt is an \gls{os} (in particular, an embedded \gls{os}) based on the Linux kernel, primarily used on embedded devices to route network traffic. The main components are the Linux kernel, util-linux, uClibc and BusyBox. All components have been optimized for size, to be small enough for fitting into the limited storage and memory available in home routers.
OpenWrt is configured using a command-line interface (ash shell), or a web interface (LuCI). There are about 3500 optional \gls{sw} packages available for installation via the opkg package management system.
GL.inet Smart Router is a remake of a common TP-Link router TL-WR703N. The board changes include, but are not limited to, increased \gls{ram} and Flash memory, custom firmware and what is the most important - 5 usable \gls{gpio} pins exposed to the 2cm pin header for utility. Whole thesis is revolving around taking advantage of this fact. The \gls{cpu} frequency is 400 \gls{mhz} and it is suited for running Linux distributions for embedded devices, preferably OpenWrt or DD-Wrt. The board provides \gls{lan} and \gls{wan} connection, as well as other interfaces defined in \gls{ieee}. The information about the board are summed up in the table \ref{t:charact}.
The Atheros AR9331 is a highly integrated and cost effective \gls{ieee} 802.11n 1x1 2.4 G\gls{hz}\gls{soc} for \gls{wlan}\gls{ap} and router platforms. The block diagram of the chip can be seen on figure \ref{f:ar_block}. Features of this \gls{soc} are following:
TP-Link TL-WR703N router is a popular choice among hacker community because of it's cheap price tag compared to processing power and usage of a full-grown Linux distribution. People have figured out how to upgrade \gls{ram} / Flash memories or to make use of not used \gls{gpio} / UART ports for their own needs. These solutions however were mostly crude and expensive to replicate. The GL.inet team saw an opportunity to grasp this public knowledge and rolled out their own improved board clone to the marked.
Whole printed circuit board of TL-WR703N was remade by the GL.inet team to expose the unused \gls{gpio} ports on the \gls{soc}, utilize two Ethernet port instead of one and utilize the \gls{usb} 2.0 port. Memory chips were replaced by their higher capacity alternatives.
