A network node for use with a bidirectional communication link, the network node comprising: a filter comprising at least four ports, wherein: a first port is configured to optically communicate with the bidirectional communication link in a first direction; a second port is configured to optically communicate with the bidirectional communication link in a second direction; a third port is configured to optically add/drop at least a wavelength in/from the first direction; and a fourth port is configured to optically add/drop at least a wavelength in/from the second direction; and characterised in that the network node is configured to add a first wavelength or a first set of wavelengths to the first direction and the second direction, and configured to drop a second wavelength or a second set of wavelengths from the first direction and the second direction; wherein the first wavelength and second wavelength are transposed compared to a first wavelength and a second wavelength of an adjacent node coupled to the bidirectional communication link; or wherein the first set of wavelengths and second set of wavelengths are transposed compared to a first set of wavelengths and a second set of wavelengths of an adjacent network node coupled to the bidirectional communication link; where the bidirectional communication link forms part of a fronthaul network and the network node forms part of a fronthaul node. a filter comprising at least four ports, wherein: a first port is configured to optically communicate with the bidirectional communication link in a first direction; a second port is configured to optically communicate with the bidirectional communication link in a second direction; a third port is configured to optically add/drop at least a wavelength in/from the first direction; and a fourth port is configured to optically add/drop at least a wavelength in/from the second direction; and a first port is configured to optically communicate with the bidirectional communication link in a first direction; a second port is configured to optically communicate with the bidirectional communication link in a second direction; a third port is configured to optically add/drop at least a wavelength in/from the first direction; and a fourth port is configured to optically add/drop at least a wavelength in/from the second direction; and characterised in that the network node is configured to add a first wavelength or a first set of wavelengths to the first direction and the second direction, and configured to drop a second wavelength or a second set of wavelengths from the first direction and the second direction; wherein the first wavelength and second wavelength are transposed compared to a first wavelength and a second wavelength of an adjacent node coupled to the bidirectional communication link; or wherein the first set of wavelengths and second set of wavelengths are transposed compared to a first set of wavelengths and a second set of wavelengths of an adjacent network node coupled to the bidirectional communication link; where the bidirectional communication link forms part of a fronthaul network and the network node forms part of a fronthaul node. <EOS>