Clients can run virtually unlimited bandwidth over the link because it is only limited by the equipment installed on each end of the service. Through utilising Wave Division Multiplexing (WDM) equipment you can run multiple 1Gbps or 10Gbps services over the same link.
Dark Fibre is a Layer 1 physical fibre-optic service linking two end points. It is provided as an unlit dual-core service and has no bandwidth or protocol limitations. Clients are responsible for lighting the fibre at each end via their equipment.