\chapter{{\tt l\_in}: the low-level interconnect}\label{chap:l-in}

\index{low-level interconnect}

\section{Introduction}

The {\tt l\_in}, or {\em low-level interconnect} abstracts the network and the

idiosyncracies of specific network protocols from the rest of NoTA, in

particular from the {\tt h\_in} that is discussed in chapter

\ref{chap:h-in}. 

The {\tt l\_in} consists of two parts:

\begin{itemize}

  \item the {\tt l\_in-down} implements the network-protocol specific

    handling; there are different {\tt l\_in-down}-modules, for example for

    TCP, Bluetooth and USB.

  \item the {\tt l\_in-up} is situated on top of these modules, and provides a

    generic interface to the higher levels, ie. to the {\tt h\_in}.

\end{itemize}

Application programmers usually do not have to use the {\tt l\_in} directly,

but instead use it through the {\tt h\_in} or layers on top of that. However,

we feel that it is still important to understand the {\tt l\_in} to some

extent, as it is invaluable to understand what really happens when you send

some bytes from one node to another.

Another reason for discussing the {\tt l\_in} is that programmers might be

interested to implement new {\tt l\_in-down}-modules to support other network

types. We won't discuss that in too much detail in this guide, but we feel

that some understanding of the {\tt l\_in} together with the examples of the

existing {\tt l\_in-down}s will go a long way.