A method of operating an optical communications system, the system comprising: a plurality of terminals; a main network arm comprising a main head end unit for communicating with the plurality of terminals in an operational state; a back-up network arm with a back-up head end unit arranged to allow communication with the plurality of terminals in the event that the main network arm is faulty; the method comprising the steps of: a) performing a ranging procedure at the main head end unit so as to acquire synchronisation information for the scheduling of the transmissions from the plurality of terminals; characterised in that the main and back-up network arms have respectively a main repeater and a back-up repeater, at least the back-up repeater being responsive to signals from the back-up head end unit so that the back-up repeater can be ranged, the main head-end unit is located at a first location and the back-up head end unit is located at a second location, wherein the first and second locations are different geographical locations, and that the method comprises the further steps of: b) performing a ranging procedure at the back-up head end unit in order to acquire synchronisation information in respect of the back-up repeater; c) if the main network arm is detected to be faulty, using at least part of the synchronisation information acquired in step a) together with the synchronisation information acquired in step b) to schedule transmissions from the plurality of terminals to the back-up head end unit. a plurality of terminals; a main network arm comprising a main head end unit for communicating with the plurality of terminals in an operational state; a back-up network arm with a back-up head end unit arranged to allow communication with the plurality of terminals in the event that the main network arm is faulty; the method comprising the steps of: a) performing a ranging procedure at the main head end unit so as to acquire synchronisation information for the scheduling of the transmissions from the plurality of terminals; characterised in that the main and back-up network arms have respectively a main repeater and a back-up repeater, at least the back-up repeater being responsive to signals from the back-up head end unit so that the back-up repeater can be ranged, the main head-end unit is located at a first location and the back-up head end unit is located at a second location, wherein the first and second locations are different geographical locations, and that the method comprises the further steps of: b) performing a ranging procedure at the back-up head end unit in order to acquire synchronisation information in respect of the back-up repeater; c) if the main network arm is detected to be faulty, using at least part of the synchronisation information acquired in step a) together with the synchronisation information acquired in step b) to schedule transmissions from the plurality of terminals to the back-up head end unit. a) performing a ranging procedure at the main head end unit so as to acquire synchronisation information for the scheduling of the transmissions from the plurality of terminals; characterised in that the main and back-up network arms have respectively a main repeater and a back-up repeater, at least the back-up repeater being responsive to signals from the back-up head end unit so that the back-up repeater can be ranged, the main head-end unit is located at a first location and the back-up head end unit is located at a second location, wherein the first and second locations are different geographical locations, and that the method comprises the further steps of: the main and back-up network arms have respectively a main repeater and a back-up repeater, at least the back-up repeater being responsive to signals from the back-up head end unit so that the back-up repeater can be ranged, the main head-end unit is located at a first location and the back-up head end unit is located at a second location, wherein the first and second locations are different geographical locations, and that the method comprises the further steps of: b) performing a ranging procedure at the back-up head end unit in order to acquire synchronisation information in respect of the back-up repeater; c) if the main network arm is detected to be faulty, using at least part of the synchronisation information acquired in step a) together with the synchronisation information acquired in step b) to schedule transmissions from the plurality of terminals to the back-up head end unit.