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\chapter{Glossary}
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A summary of a few of the more common terms encountered in this document\ldots

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\begin{multicols}{2}

\begin{description}

    \item[\MI{Bps}:] Bytes per second.  Measure of data transfer
        speed.

    \item[\MI{bps}:] Bits per second.  Measure of data transfer speed.
        Not always the same as baud.  For some simple serial
        communications links such as \MI{RS232} though, 1bps~= 1~baud.

    \item[\MI{baud}:]  Usually a parameter indicating \MI{modem speed};
        the higher the baud (often incorrectly given as ``baud rate''),
        the faster the modem will transfer data.  A modem may transmit
        several bits per baud over the phone wire, so a \MI{V.32} modem
        (for example) is 2400~baud and 9600bps (bits per second) over
        the phone line corresponding to 9600~baud and 9600bps over an
        \MI{RS232} (serial) connection to your computer.  Compression may
        complicate the issue further, getting even more bits per second
        of traffic to/from your computer into each bit sent over the
        phone line.

    \item[\MI{bit}:] The smallest unit of digital information; a single
        yes/no, on/off, 1/0, high/low value.

    \item[\MI{byte}:]  Unit of data, big enough to hold one (ASCII,
        Latin alphabet) character\slash letter.  1~byte~= 8~bits.

    \item[\MI{electronic mail}, \MI{e-mail}, \MI{email}:]  Letters sent
        electronically, a little like \MI{fax}, but without the
        ridiculous step of putting the text onto paper and sending a
        picture of the piece of paper.

    \item[\MI{FTP (File Transfer Protocol)}:]  A protocol used to
        transfer files over the Internet.  Also the name of a program
        that implements the client side (the part to run to do the
        transfer) of the FTP protocol.

    \item[\MI{gigabyte}, \MI{GB}:]  1,024~$\times$ 1,024~$\times$ 1,024
        bytes, about what a single CD-ROM holds.

    \item[\MI{IP (Internet Protocol)}:]  The protocol the various
        machines and networks in the Internet use to talk to one
        another.

    \item[\MI{Internet}:]  An \MI{internet} or \MI{internetwork} is
        a network that interconnects other ones.  The \MI{Internet} as
        it is known, or more correctly the \MI{Connected Internet} is a
        global packet-switching network interconnecting millions of
        users' and machines' networks.  Often the users' own networks
        run the same protocols as the Internet itself, called IP\@.
        Note that an Ethernet runs the Ethernet protocol, on which
        machines may then layer IP, or Novell's IPX, or Apple's
        EtherTalk---if the machines on the Ethernet understand the IP
        protocol, albeit on top of Ethernet, they can be described as
        talking `IP' to all intents and purposes.

        The Internet originated in the US as part of a semi-military
        research project and is now a mix of academic and commercial
        users and providers.

    \item[\MI{Kermit}:]  A simple file-transfer protocol for
        transferring files over serial lines such as modem connections
        and also the name of a program available on many machines that
        implements that protocol.

    \item[\MI{kilobyte}, \MI{kB}:]  1,024 bytes, about one
        \MI{typewritten page} of text.

    \item[\MITOK{man}:]  The program to get at the UNIX \MI{on-line
        manual} pages.  Type `\MITOK{man man}' to see how to use it,
        press the spacebar to page through it.

    \item[\MI{NNTP (Network News Transport Protocol)}:]  A protocol used
        to transport USENET news over Internet connections.

    \item[\MI{NTP (Network Time Protocol)}:]  Used to distribute time
        across the Internet so that the clocks on important machines,
        such as mail routers that put timestamps in mail messages, are
        correct.  NTP can help get a machine's clock to within a few
        milliseconds (thousandths of a second) of international
        standard time (UTC---Universal Coordinated Time).

    \item[\MI{megabyte}, \MI{MB}:]  1,024~$\times$ 1,024 bytes, a long
        novel's worth of text.

    \item[\MI{operating system}:]  The basic software on the machine
        that makes things like serial ports, screens and discs work.
        On ExNet machines we use \MI{UNIX} (\MI{SunOS}) where most
        IBM-PC compatibles use DOS, either \MI{MS-DOS} or \MI{PC-DOS}.

    \item[\MI{packet}:]  A parcel of data passed over a link such as
        a UUCP connection or an Internet IP link.  Packets are often
        re-assembled into continuous streams of data by so-called
        ``transport protocols'' to achieve the transfer of whole files
        or to provide some sort of login service for example.  Packets
        are used because they are small data units (typically
        64--1500~bytes) easily resent in case of some transmission
        problem (eg due to a noisy line).

    \item[\MI{packet-switching}:]  A network is packet-switching if
        the logical unit of data passed from `customer' to customer in
        the network is a packet.  The Internet is a packet-switching
        network.  Other possible types of network are
        \MI{message-switching} and \MI{circuit-switching}.  The
        telephone system is a circuit-switching network in that it
        gives you a dedicated connection between the 'phone you dial
        from and the one you dial to for the duration of the call,
        allocating resources (a circuit) to you whether you use it (by
        speaking or sending a fax or whatever) or not (by keeping
        quiet).  E-mail behaves as if a \MI{message-switching} network
        where whole messages of whatever size, bigger than a typical
        `packet', are sent from end to end.  In fact, many message- and
        circuit-switching networks are built on top of packet-switching
        ones these days, ie the data is actually sent by packet.
        Circuit-switched networks tend to charge by the second or some
        other unit of time and the other two types tend to charge by
        the volume of data you send.

    \item[\MI{routing}:]  The process of getting e-mail messages or
        IP packets to their destinations often involving crossing many
        networks and administrative boundaries.

    \item[\MI{Telnet}:]  A protocol for logging into remote machines
        across the Internet which takes care of many details of
        terminal emulation such as the handling of interrupt characters
        (to kill programs off, for example).  Telnet is also the name
        of a program that implements the client side (the part you run
        to make a Telnet connection to a remote machine) of the Telnet
        protocol.

    \item[\MI{terabyte}, \MI{TB}:]  1,024~$\times$ 1,024~$\times$
        1,024~$\times$ 1,024 bytes, about what is shifted daily all
        over the world to propagate all of USENET's postings for the
        day to all sites that get them.

    \item[\MI{UNIX}:]  The operating system that ExNet runs on its
        computers to support the HelpEx service.

    \item[\MI{USENET}:]  An anarchic collection of sites exchanging
        `\MI{articles}' on a frequent basis on a wide range of topics.
        Often referred to as just `news' or `net news'.

    \item[\MI{UUCP (Unix-to-Unix CoPy)}:]  A suite of programs and
        protocols used to transfer files from machine to machine for
        electronic mail, USENET News and so on.  Originally developed
        to enable UNIX machines to exchange mail and files over slow
        (300 baud) and noisy dial-up phone connections in the days
        before the Internet; now widely used as an efficient transport
        for the same type of traffic between machines of more or less
        any type over dial-up connections of any speed.

\end{description}

\end{multicols}

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