There are two different types of links:
A link is either used to send L-data messages to the KNX network and receive
L-data messages from the network, the other type of link is to
monitor a KNX network by receiving monitor indication messages.
A link implementation uses a specific underlying protocol to access the KNX network,
hiding its particular type of connection and protocol interaction.
Also, a link is configured with the type of KNX network medium used
to interconnect KNX devices, to enable a transparent handling of KNX medium
specific message parts.