\chapter{Networks}

A \textit{network} is a system involving two or more computers that are
connected, allowing them to \textbf{communicate}. A computer not
connected to a network is a \textit{stand alone computer}.

Networks allows the easy sharing of data, file, backups, and
peripherals (such as printers). Networks also allow for efficient and
high volume communication and can reduce the cost of computing.
However, a network can also allow hacking and the spread of viruses.
Networks require additional security measures and potentially
specialist equipment or staff at additional expense.

We typically identify three classes of network:

\begin{itemize}
    \item A personal area network (PAN) exists in the range of a
        single person. This may include an individuals devices that
        are networked such as a computer, mobile phone, or tablet and
        other devices connected by network such as Bluethooth
        headphones or USB devices.
    \item A large area network (LAN) covers a small geographical area.
        This could be anything from a single home to a university and
        thereby can vary in complexity and scale. Twisted pair cables
        (Ethernet) and Wi-Fi are commonplace in a LAN.
    \item A wide area network (WAN) covers a large geographical area.
        The Internet is a very large WAN through the use of fibre
        optic cables, often undersea, reaching every continent other
        than Antarctica
        \footnote{\url{https://en.wikipedia.org/wiki/Submarine_communications_cable}}
        (which is nonetheless served by satellite Internet links).
        Organisations with significant sums of money may also
        create a WAN for the transfer of large amounts of data or
        speed critical transfer.
\end{itemize}

\section{Wired networks}

In a wired network the \textit{communication media} is a physical
cable that transmits a signal. A \textit{coaxial cable} is a single
copper cable with shielding. A twisted pair cable is also copper but
can transmit data at a higher \textit{bandwidth} and is flexible
whilst coaxial cable is not. Fibre optic cables are made of glass and
transmit light.

Copper cables are cheap and generally compatible with existing
hardware, but are easy to \textit{eavesdrop} on and cannot transmit
data at longer distances. Fibre optic cables are better for long
distance communications and are much higher bandwidth than copper.
They are also lightweight and hard to eavesdrop on. However fibre
optic cables are very expensive and often require new infrastructure
in order to use.

\section{Wireless networks}

Wireless networks send data by transmitting and receiving
\textit{electromagnetic radiation} (generally radio waves). A wireless
access point (WAP) transmits and receives this data for many devices
and is often built into modern household routers. Wi-Fi removes the
need for cables which are expensive and inconvinient. However the
range, speed, and bandwidth of Wi-Fi are limited. Transmission is 
limited by interference (such as thick walls or metal construction)
and the signal can be easily intercepted causing the need for further
security measures.

\section{Network topology}

The topology of a network describes the layout of network devices and
allow the planning of suitable network infrastructure.

\subsection{Bus topology}

\begin{center}
\begin{tikzpicture}
    \draw (0,0) edge[*-*] (12,0);
    \draw [-o] (2,0) -- (2,-2);
    \draw [-o] (4,0) -- (4,2);
    \draw [-o] (6,0) -- (6,-2);
    \draw [-o] (8,0) -- (8,2);
    \draw [-o] (10,0) -- (10,-2);
\end{tikzpicture}
\end{center}

In the bus topology, nodes (devices or computers) are connected along
the \textit{backbone}, each sending and receiving data.
\textit{Terminators} are positioned at each end of the backbone to
prevent the signal \textit{bouncing} and transmitting to the devices
again.

A bus topology is cheap and scalable, but the backbone is a single
point of failure and can be eavesdropped on, leading to the compromise
of all data. Data sent by one device is received by all other devices
thus compromising privacy and only one device can send data on the
backbone at a time, thus slowing the network.