  A communication network is called a radio network if its nodes exchange
messages in the following restricted way. First, a send operation performed by
a node delivers copies of the same message to all directly reachable nodes.
Secondly, a node can successfully receive an incoming message only if exactly
one of its neighbors sent a message in that step. It is this semantics of how
ports at nodes send and receive messages that defines the networks rather than
the fact that only radio waves are used as a medium of communication; but if
that is the case then just a single frequency is used. We discuss algorithmic
aspects of exchanging information in such networks, concentrating on
distributed randomized protocols. Specific problems and solutions depend a lot
on the topology of the underlying reachability graph and how much the nodes
know about it. In single-hop networks each pair of nodes can communicate
directly. This kind of networks is also known as the multiple access channel.
Popular broadcasting protocols used on such channels are Aloha and the
exponential backoff. Multi-hop networks may have arbitrary topology and packets
need to be routed hopping through a sequence of adjacent nodes. Distributed
protocols run by such networks are usually robust enough not to expect the
nodes to know their neighbors. These ad-hoc networks and protocols model the
situation when nodes are mobile and do not rely on a fixed infrastructure.
