Networks HW4

\documentclass[a4paper,10pt]{article}
\usepackage[utf8]{inputenc}

%opening
\title{Computer Networks HW4}
\author{Vlad Ungureanu}

\begin{document}

\maketitle


\section{What is the purpose of the RTS and CTS frames? When are they used?}
  \indent \indent RTS/CTS (Request to send / Clear to send) is used by 802.11 to implement a virtual carrier sense. When a node wants to send something he issues a RTS to the destination node.
  The destination node issues a CTS if it can receive data. Any other nodes that receive the RTS/CTS will wait a defined time (backoff time) until they start normal communication.
  The RTS/CTS mechanism is used for large packets that will create overhead on the network if they are sent multiple times on the network.\\
  \indent This method clears the hidden node problem where 2 nodes cannot directly speak with eachother but they can reach a common access point.
\section{Why are link-layer acknowledgements used in IEEE 802.11 wireless networks but not in IEEE
802.3 wired Ethernet networks?}
  \indent \indent 802.11 uses LLC and MAC. The LLC is the same as in 802.3 LAN networks but the MAC is unique to 802.11 networks.
  We need the MAC for 802.11 because:
  \begin{itemize}
   \item Near/Far problem: To detect a colision a station must transmit/receive at the same time but when transmitting a station cannot hear for collisions.
   \item Hidden Node problem: There are 2 stations with a common AP, both can see the AP but cannot see eachother.
   \item Power issue: When having portable devices the battery power is drained very fast when you have continous reply to packets.
  \end{itemize}
  MAC solves the problems like following:
  \begin{itemize}
   \item Near/Far problem: When a station wants to send something first listens if there is no traffic on the network, if so it waits a randomly chosen time then transmmits data if the network is still free.
   In this way the chances of collision are reduced but does not imply that they do not happen. To overcome this issue the ACK check has been implemented.
   The receiving station sends an ACK packet meaning that it receives the data. If the sender receives the ACK this ends the process.
   If the ACK is not reiceived by the sender that means that a collision has happened and the data is sent again after a fallback timer has expired.
   \item Hidden Node problem: Here RTS/CTS are implemented. Description on how they work is described above in question 1.
   \item Also MAC implements CRC checksums to check if data is intact. Packet fragmentation is allowed on packets bigger than the treshold *MAC layer assembling them at destination*.
  \end{itemize}


\section{The IEEE 802.11 frame format includes a duration field. What is the purpose of the field?}
  Duration field is 16 bits in length.
  We need to discuss the duration field in case of 2 frames.
\begin{itemize}
 \item RTS Frame: here the field contains the value in uS of time need to transmit data or management+CTS+ACK+SIFS interval.
 \item CTS Frame: here the field contains the value in uS obtained by the previous RTS substracted the time to transmit CTS and it SIFS interval.
\end{itemize}
We have extra rules on how to set the duration:
\begin{enumerate}
 \item Contention-free perios we set duration to 32768.
 \item Frames transmitted to a broadcast or multicast destination have a duration of 0.
 \item If MF bit is not set the duration is SIFS + ACK
 \item if MF bit is set the duration is fragment + 3*SIFS + 2*ACK
\end{enumerate}
\section{Explain the terms BSS, BSSID, SSID, ESS and ESSID (in infrastructure mode) and how they relate to each other.}
  \subsection{SSID}
    SSID = Service set identification is a 1 to 32 byte string that represents the name of the wireless network *from 1 up to 32 bytes*.
  \subsection{BSS}
    BSS = Basic Service Set is the set of all stations that can *speak* with each other. Every BSS has a BSSID which is the MAC of the AP of the BSS.
  \subsection{BSSID}
    BSSID = Basic Service Set Identifier is the MAC address of the access point servicing the BSS.
  \subsection{ESS}
    ESS = Extended Service Set is a set of connected BSS that have the same SSID, credentials and settings.
  \subsection{ESSID}
    ESSID = Extended Service Set Identifier. When in ad-hoc mode the BSSID is used as ESSID but when in infrastructure mode the SSID is uesd as ESSID.
\section{How are IP packets encapsulated in IEEE 802.11 wireless frames?}
  (Frame)-$>$(Radiotap Header)-$>$(IEE 802.11 Data)-$>$(LLC)-$>$(IP)\\
  Radiotap Header is a way to inform the user about the wireless network. This header is injected by the device driver.
  IEEE 802.11 Data is the layer 2 datagram used in wireless communication. They can be management frames, control frames or normal frames that carry IP data. Each frame has MAC header, the payload and a frame check sequence.
  From here on the same principle as in IEEE 802.3 applies with the data.
\section{Load the packet trace into wireshark. How can filter out all beacon management frames? How many wireless BSSs are sending beacons? What are their BSSIDs and SSIDs? Which wireless channels are used?}
  To filter beacon management: wlan.fc.type\_subtype==8.\\
  There are 2 BSS that are sending beacons: freebsd-ap and Broadcast.\\
  'freebsd-ap' has BSSID 06:03:7f:07:a0:16 and SSID freebsd-ap.\\
  'Broadcast' has BSSID 00:00:00:00:00:00 and SSID Broadcast.\\
  The same channel number is used 36 *frequency 5180* but on different antennas.
\end{document}