  In this article we study a class of hyperbolic partial differential equations
of order one on the semi-axis. The so-called port-Hamiltonian systems cover for
instance the wave equation and the transport equation, but also networks of the
aforementioned equations fit into this framework. Our main results firstly
characterize the boundary conditions which turn the corresponding linear
operator into the generator of a strongly continuous semigroup. Secondly, we
equip the equation with inputs (control) and outputs (observation) at the
boundary and prove that this leads to a well-posed boundary control system. We
illustrate our results via an example of coupled transport equations on a
network, that allows to model transport from and to infinity. Moreover, we
study a vibrating string of infinite length with one endpoint. Here, we show
that our results allow to treat cases where the physical constants of the
string tend to zero at infinity.
