\documentclass[a4paper,12pt]{scrartcl}
\usepackage[ngerman]{babel}
\usepackage{graphicx} %BIlder einbinden
\usepackage{amsmath} %erweiterte Mathe-Zeichen
\usepackage{amsfonts} %weitere fonts
\usepackage[utf8]{inputenc} %Umlaute & Co
\usepackage{hyperref} %Links
\usepackage{ifthen} %ifthenelse
\usepackage{enumerate}
\usepackage{stmaryrd}

\usepackage{color}
\usepackage{algpseudocode} %Pseudocode
\usepackage{dsfont} % schöne Zahlenräumezeichen
\usepackage{amssymb, amsthm} %noch stärker erweiterte Mathe-Zeichen
\usepackage{tikz} %TikZ ist kein Zeichenprogramm
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\topmargin-50pt


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{\bf #4}\\
#5
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\vspace{1cm}
\begin{center}
{\Large\bf Sheet #1}

{(Hand in #3)}
\end{center}
}



%counts the exercisenumber
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%Kommando für Aufgaben
%\Aufgabe{AufgTitel}{Punktezahl}
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\begin{document}
%\header{BlattNr}{Tutor}{Abgabedatum}{Vorlesungsname}{Namen}{Semester}{Anzahl Aufgaben}
\header{11}{}{2016-01-20}{Kommunikationsnetze}{\textit{Jan-Peter Hohloch, 3908712}\\\textit{Jonas Jaszkowic, 3592719}}{WS 15/16}{4}
\vspace{1cm}
\Aufgabe{Voice over IP (VoIP)}{5+10+5+5+5}
\begin{enumerate}
  \item SIP (\textit{Session Initiation Protocol}) is used to create, manage and terminate multimedia communication sessions over an IP based network.
  \item \begin{itemize}
    \item A SIP redirect server checks the client request if the recipient is reachable via the particular route. If the recipient has moved from its original position (may it be temporarily or permanently), the server \textbf{redirects} the request back to the client indicating that a different route must be taken.
    \item The exact address of the recipient is not known in advance when the request is generated. Therefore the client sends the request to a SIP proxy server, which forwards the request either to another proxy server or to the recipient itself. A \textbf{stateful} creates a state upon reception of a request and keeps it until the transaction finishes.
  \end{itemize}
  \item Once the connection is established, the participants know the exact address of each other. Therefore they are communicating \textbf{directly}. Since the proxy servers are not used any more, nothing will happen when the proxy server fails at this point. The exact address is saved in the \textbf{contact} field of the header of the response message.
  \item The real-time protocol (RTP) is used to stream audio and video in a video conference.
  \item The real-time control protocol (RTCP) can synchronize different media streams (e.g. audio and video for lip-syncing) within a RTP session. It is realized via timestamps in RTP packets tied to the video and audio sampling clocks. The receiver is responsible for using this information to truly synchronize the playback of audio and video.
\end{enumerate}\ok
\Aufgabe{Mobile Communication - Frequency Assignment}{10+10+20}
\begin{enumerate}
  \item A big problem is the limited number of frequencies to be used. Therefore frequencies have to be reused which bares the risk of inference.\\ For getting high reuse distances cells have to be arranged specifically. %TODO: maybe more?
  \item Microcells are used to add network capacity in areas with very dense phone usage, e.g. train stations, hotels, city centers etc. Sometimes they are deployed temporarily during large events when extra capacity is known to be needed at a specific location in advance.\\
  Advantages:\begin{itemize}
      \item system capacity increases
      \item less power is used by mobiles and base stations
  \end{itemize}
  Disadvantages:
  \begin{itemize}
      \item Covered area is small
      \item[$\Rightarrow$] more handovers when moving
      \item[$\Rightarrow$] handoff-risk increases
  \end{itemize}
  \item \ \\\includegraphics[width=.7\textwidth]{hexcells.png}\\
        Reuse-distance is the length of one edge plus the diameter of the hexagon.
\end{enumerate}\ok
\Aufgabe{2nd Generation Cellular Mobile Radio}{5+5}
\begin{enumerate}
  \item The assigned frequency for a voice communication session is used for the entire time of the session because there is continuous data which has to be transferred. For arbitrary data we only need some packets of data, after these packets are send the frequency can be used by some other user. We do not need to reserve the assigned frequency over a longer period of time. Instead we'd like to have a higher bandwidth when transferring a packet.
  \item The GPRS (General Packet Radio Service) protocol was introduced to solve this problem. It features joint utilization of one channel by several users, higher data rates by using several channels in parallel, flexible utilization of the air interface resources.
\end{enumerate}\ok
\Aufgabe{Satellite Communication}{15+5}
\begin{enumerate}
  \item We use Kepler's Law $\texttt{Period} = C \cdot \texttt{distance}^{1.5}$ with $C=1/100$ to calculate the periods.
  \begin{itemize}
    \item Period of a GPS satellite orbiting the earth at a distance of $18000km + 6378km$ over the center of the earth: $\frac{1}{100} \cdot (18000 + 6378)^{1.5} = 38062s \approx 10.57h$. The script states a period of within 6-8 hours.
    \item Period of a Globalstar satellite orbiting the earth at a distance of $1400km + 6378km$ over the center of the earth: $\frac{1}{100} \cdot (1400 + 6378)^{1.5} = 6860s \approx 1h 54min$.
    \item Period of a Iridium satellite orbiting the earth at a distance of $750 + 6378km$ over the center of the earth: $\frac{1}{100} \cdot (750 + 6378)^{1.5} = 6018 \approx 1h 40min$.
  \end{itemize}
  \item Globalstar satellites do not relay data. The satellites use ground station (gateway) and possibly PSTN to forward traffic to destinations outside own cell. Iridium satellites forward data from handheld over several satellite hops before reaching destination. This satellite relaying eliminates the need for many terrestrial stations.
\end{enumerate}\ok
\end{document}