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% SPDX-License-Identifier: CC-BY-SA-4.0
%
% Copyright (c) 2020 Philipp Le
%
% Except where otherwise noted, this work is licensed under a
% Creative Commons Attribution-ShareAlike 4.0 License.
%
% Please find the full copy of the licence at:
% https://creativecommons.org/licenses/by-sa/4.0/legalcode
\phantomsection
\addcontentsline{toc}{section}{Exercise 1}
\section*{Exercise 1}
\begin{question}[subtitle={Shannon-Weaver Model}]
Illustrate the process of sending an e-mail using the Shannon-Weaver model! Describe all nodes and edges shortly!
\end{question}
\begin{solution}
Person A send an e-mail to person B:
\begin{itemize}
\item Information source: Brain of person A
\item Signal: Impulses on nerve cells
\item Transducer: Keyboard, converts keystrokes to electronic signals
\item Signal: Electronic signals
\item Modem: PC, Receives keystrokes and decodes them into the e-mail
\item Signal: Data
\item Transmission channel: Internet
\item Signal: Data
\item Modem: PC, decodes e-mail, generates text
\item Signal: Electronic signals
\item Transducer: Screen, displays text
\item Signal: Impulses on nerve cells
\item Information source: Brain of person B
\end{itemize}
\end{solution}
\begin{question}[subtitle={Classes of Signals}]
Assign the following signals to the categories: time-continuous vs. time-discrete, value-continuous vs. value-discrete!
\begin{tasks}
\task
$\cos\left(2 \pi \cdot \SI{50}{Hz} \cdot t\right)$
\task
Letters: a, B, F, f
\task
\begin{equation*}
f(x) = \begin{cases}
-1 & \qquad \forall \; x \leq 0 \\
1 & \qquad \forall \; x > 0
\end{cases}
\end{equation*}
\end{tasks}
\end{question}
\begin{solution}
\begin{tasks}
\task
time-continuous, value-continuous
\task
time-discrete, value-discrete
\task
time-continuous, value-discrete
\end{tasks}
\end{solution}
\begin{question}[subtitle={Frequency Allocation}]
An LTE (4G cell phone) signal can occupy a bandwidth of up to \SI{20}{MHz}. One of the bands allocated to LTE is, amongst others, the band 1 (uplink: \SIrange{1920}{1980}{MHz}, uplink: \SIrange{2110}{2170}{MHz}). The range of one LTE base station is a few kilometres.
However, the HF band (\SIrange{3}{30}{MHz}) has the advantage that waves are reflected by the ionosphere and can propagate over longer distances or even across the whole world. Mostly, narrow-band services like AM broadcasting or amateur radio are allocated to the HF band.
Why is it pointless to use the HF band for LTE?
\end{question}
\begin{solution}
Reasons:
\begin{enumerate}
\item Band capacity:
\begin{itemize}
\item HF band is only \SI{27}{MHz} wide.
\item In contrast, the UHF band is \SI{2700}{MHz} wide.
\item One LTE base station would occupy the whole HF band.
\item LTE base stations should only have a limited range. One base station can only service a limited number of users.
\item Increasing the cell coverage will decrease the number of users and the data rate.
\item \textbf{That's why high data rate services use higher frequencies.}
\end{itemize}
\item Antenna size
\begin{itemize}
\item The antenna size is proportional to the wave length. For example, a $\lambda/2$-dipole measures the half of the wave length.
\item HF band wave length: \SI{10}{100}{m}
\item UHF band wave length: \SI{0.1}{1}{m}
\item UHF antennas are much smaller. They must fit into a cell phone.
\item \textbf{The higher the frequencies, the more compact the antennas and devices.}
\end{itemize}
\end{enumerate}
\end{solution}
\begin{question}[subtitle={OSI Layers}]
What is the difference between a \emph{digital communication system} and a \emph{service}? To which OSI layers are they associated?
\end{question}
\begin{solution}
\begin{itemize}
\item Digital communication system (in our understanding): collection of technologies for conveying information
\item Implemented in lower layers (1 - 4), we consider with layers 1 and 2
\item Services provide user applications (e.g. video-on-demand, social media, etc.). A service uses communication systems.
\item Services are located in layer 5 - 7.
\end{itemize}
\end{solution}
\begin{question}[subtitle={Networks}]
\begin{tasks}
\task
What is the major difference between OSI layers 2 and 3?
\task
Give one example for each layer!
\task
What is routing?
\end{tasks}
\end{question}
\begin{solution}
\begin{tasks}
\task
\begin{itemize}
\item Layer 2: Connection of two devices
\item Layer 3: Data transfer across multiple nodes (creating a network)
\end{itemize}
\task
\begin{itemize}
\item Layer 2: IEEE\,802.11 (WiFi)
\item Layer 3: Internet Protocol (IP)
\end{itemize}
\task
\begin{itemize}
\item Routing is used in layer 3
\item A network consists of devices interconnected using layer 2 protocols.
\item If one device sends a layer 3 packet, it is wrapped into a layer 2 frame and sent to the next node.
\item The next node unpacks the layer 3 packet.
\item The next node has to decide to which next node the layer 3 packet should be relayed.
\item \textbf{This is routing.} Routing means findig the best way to the destination route.
\item The node packs the layer 3 packet into a layer 2 frame and sends to it the node it has selected.
\end{itemize}
\end{tasks}
\end{solution}
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