Severin Kaderli
2c66a18f0e
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Signed-off-by: Severin Kaderli <severin@kaderli.dev>
50 lines
2.6 KiB
TeX
50 lines
2.6 KiB
TeX
\chapter{Solution}
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The solution consists of two separate applications: a website used for transmitting data and a smartphone application used for receiving data. Transmission between the two application happens using a simple protocol on top of signals made using magnetic induction.
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\section{Protocol}
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\subsection{Signal}
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The main communication protocol works over magnetic induction. By putting load on a CPU the resulting magnetic field can be changed. Using a magnetometer in a smartphone these changes can be measured and interpreted.
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For simplification purposes the protocol in this project is used to transmit ASCII text but in reality any bit stream can be transmitted.
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To indicate the start of a message in the signal, a preamble is used. The preamble used in this protocol is 3 high signals, followed by 1 low signal, followed by 3 high signals.
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\subsection{Packet Format}
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What follows the preamble is the actual contents of the message packet. The packet content is a bitstream encoded using a manchester code.
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For the structure of the packet I
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\cite{LoRaPacketFormat}
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The first part of the message is the header. The header is \SI{8}{\bit} long and contains the length of the payload in bytes.
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Next is the actual payload. As the length of the payload is specified in the header, the payload is limited to a size of $2^8$ bytes.
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For verifying the integrity of the payload, a checksum of the payload is calculated and placed at the end of the packet. The algorithm used is the CRC-16-CCITT algorithm with the polynomial $x^{16} + x^{12} + x^{5} + 1$ and an initial value of $\mathtt{0x0000}$. This algorithm is a 16-bit CRC used in many applications such as Bluetooth. \cite{Bluetooth}
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\begin{figure}
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\begin{bytefield}[bitwidth=.125\linewidth, bitheight=11mm]{4}
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\bitbox[]{1}[]{1 Byte} & \bitbox[]{5}[]{n Bytes} & \bitbox[]{2}[]{2 Bytes} \\
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\bitbox{1}[bgcolor=red!30]{Payload Length} & \bitbox{5}[bgcolor=blue!30]{Payload} & \bitbox{2}[bgcolor=green!30]{CRC-16-CCITT}
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\end{bytefield}
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\caption{Packet Structure}
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\end{figure}
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\subsection{Sending}
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\begin{enumerate}
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\item{Convert the text to an ASCII encoded bit stream}
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\item{Encode the ASCII bit stream using manchester encoding}
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\item{Start the transmission by sending the preamble}
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\item{Transmit the manchester encoded bit stream}
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\item{Repeat from step 3 until transmission is stopped}
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\end{enumerate}
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\subsection{Receiving}
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\begin{enumerate}
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\item{Detect the preamble}
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\item{Decode the received manchester encoded bit stream}
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\item{Convert the ASCII bit stream to text}
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\end{enumerate}
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\section{Website}
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\section{App}
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