1. A data capacity modelling system having: a memory having a data capacity model modelling data capacity in a telecommunications network, the model specifying one or more communications circuits and capacity information associated with the one or more circuits, the capacity information comprising for at least one communications circuit, one or more bandwidth partitions each representing a portion of the bandwidth of the communications circuit; a service provisioning system accessing the data capacity mode, the service provisioning system further receiving a service request for a telecommunications service having a data transmission component with at least one associated requirement, assigning a circuit to the telecommunications service using the capacity model in dependence on the at least one requirement, and provisioning the telecommunications service using the assigned circuit; and the memory storing one or more service definitions, each defining a service available to users of the telecommunications network and specifying one or more capacity requirements associated with the service; wherein the service request specifies one of the stored service definitions; wherein the data capacity modelling system assigns one or more circuits to the service using the capacity model in dependence on the one or more capacity requirement, and provisions the service using the assigned circuit or circuits.