  A microscopic description of packet transport in the Internet by using a
simple cellular automaton model is presented. A generalised exclusion process
is introduced which allows to study travel times of the particles ('data
packets') along a fixed path in the network. Computer simulations reveal the
appearance of a free flow and a jammed phase separated by a (critical)
transition regime. The power spectra are compared to empirical data for the RTT
(Round Trip Time) obtained from measurements in the Internet. We find that the
model is able to reproduce the characteristic statistical behaviour in
agreement with the empirical data for both phases (free flow and congested).
The phases are therefore jamming properties and not related to the structure of
the network. Moreover the model shows, as observed in reality, critical
behaviour (1/f-noise) for paths with critical load.
