A network system comprises a physical transport network (120) for transporting data for a session between two terminals, and an access controller (170) connected between a source terminal and the physical transport network, wherein the access controller is configured for controlling a start of a first session in a first virtual network and a start of a second session in a second virtual network using a data record (171) comprising, for each virtual network, information on an available bandwidth for the virtual network and for each quality of service class of the virtual network. Each virtual network comprises different quality of service classes. The controller can be implemented as an Openflow controller and the physical network can be implemented as an MPLS network.
