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\chapter{Network Connections}   \label{hw-con}

In this chapter the network connections between the ACPC groups will 
be discussed. Some basic knowledge about the capacity of these hardware 
connections is very important to understand how the interactive work on
a remote machine will look like. 

First of all, the capacities of the long range connections are stated:

\begin{itemize}
 \item Vienna -- Linz\footnote{This connection will be upgraded to {\it 256
kb/s\/} at the end of May 1993.}: {\it 128 kb/s}.
 \item Linz -- Hagenberg: {\it 64 kb/s}.
 \item Linz -- Salzburg: {\it 64 kb/s}.
 \item Vienna -- Salzburg: {\it 64 kb/s}.
\end{itemize}

Apart from the direct connections between the institutes there is also 
a connection between Linz and Graz. This connection is important, as
the logical connection between Linz and Vienna may be routed over
this connection\footnote{Note that this also affects the machines in
Hagenberg and probably those in Salzburg when they set up a connection to
a machine in Vienna.}. However, currently the routing between
Linz and Vienna is done dynamically either over the direct connection or
over Graz, depending on how much load is on each of these 
connections.
Therefore, it can't be said beforehand which way a packet from Linz to
Vienna will take. On various machines in the ACPC Network a tool called
{\tt traceroute} is installed that gives, when it is invoked with the target 
machine as its parameter, the way of the connection to the target machine
by printing each intermediate machine or router. It is strongly recommended to
use this tool (if available) if there are problems with the connection
between two machines in the ACPC Network.

The capacities of the connections from and to Graz are:

\begin{itemize}
 \item Vienna -- Graz: {\it 128 kb/s}.
 \item Linz -- Graz: {\it 64 kb/s}.
\end{itemize}


\begin{figure}

 \horiz

 \vspace{1cm}

 \input{general/acpc-pic}

 \vspace{1cm}

 \caption{`Map' of the ACPC Network} \label{map}

 \horiz

\end{figure}

To give the reader a global overview of the whole ACPC Network, 
Figure~\ref{map} shows a `map' of this network containing all 
machines currently available. The quarter arc between the lines
to Vienna and to Graz originating in Linz denotes the dynamic routing over
the connection with the smallest load. Furthermore, the capacities of the long 
range connections are also shown in this figure. For a more detailed 
information on the various computers in the network see Chapter~\ref{fac}.

However, for an exact characterization of the speed of a connection the
information about its theoretical capacity is not
sufficient. Better information is provided by the \verb+ping+ command
that allows the user to check the time  
required for sending a packet of a certain size to another machine. 
Some tests with this command have been performed
on most of the machines in the ACPC Network. The results of these tests can be
seen in Figures~\ref{ping-risc} to \ref{ping-unisalz}. 

However, these times must not be taken as absolute values for the general
quality of a certain connection! It must be
emphasized that {\it these times may vary very much according to the daytime\/}
when it is tested. But these timings should give the reader a rough
feeling how fast a certain connection would be.

\input{general/ping-tab}


\subsection*{Experience Reports}

The rest of this chapter contains some experience reports about the
work on remote machines.

\subsubsection*{Connection RISC -- University Vienna}

The interactive work on this connection is quite slow. At least it is
significantly slower than on the connection between RISC and the 
TU-Vienna. Hence, if some serious work should be done over this connection it
is recommended to do the editing of files locally and then to transfer 
the files to the remote computer. This style of work may be a little 
inconvenient but as soon as one gets accustomed to this way of working
there should be no bigger problems with it.

\subsection*{Connection RISC -- TU-Vienna}

This connection is faster than the connection to the University Vienna.
This can also be seen from Table~\ref{ping-tuvie}. With the speed on this
connection it is very well possible to do a remote editing of files. 
Although, there will be a little delay this should not hinder the work.

\subsection*{Connection RISC -- University Linz}

This connection is the fastest one from RISC within the ACPC Network. If 
one works from RISC remotely on a machine at the University Linz there is
hardly any delay recognizable. Therefore, the work could be done as if one
would work locally.

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