Slots (311) for transmission of data of a particular transmission type over an optical network are allocated by selecting a first available slot (313_2) at an ordinal position corresponding to a multiple of n and allocating the selected first available slot and the next n−1 consecutive slots (313_4, 313_5) from the selected first available slot (313_3), if all n−1 consecutive slots (313_4, 313_5) are available, for transmission of data of the particular transmission type. The data is transmitted over an optical network comprising a plurality of nodes (305, 327) interconnected by optical sections (301, 309, 329, 331) the nodes (305, 327) supporting a plurality of transmission types, wherein transmission of data of the particular transmission type requires a predetermined number n of consecutive slots. Alternatively the slots may be divided in groups (333, 335, 337) and slots are allocated to a group in which all slots are available.