Manifold differences are also possible on the physical and medium-access layers. Some approaches foresee redundant media (Vnet/IP, TCnet), PROFINET IO uses dedicated builtin switches to reduce the data transmission jitter [14], and
EtherCAT and SERCOS III need dedicated controllers [12].
Ethernet Powerlink uses the traditional shared Ethernet and places a master–slave scheduling system on top of it. Common to many proposed networks is that they employ clock synchronization to support real-time applications. To this end,
the IEEE 1588 standard [15], which originally emerged in the instrumentation area, was also officially adopted by IEC. The specific requirements in the automation domain have led to several suggestions for improvement of the standard [16], [17]
that were taken into account in the recent revision.