Traditionally, longer reach optical transport networks have been highly optimised for each specific application to minimise network equipment costs and maximise utilisation. Different vendors have often applied different approaches to achieve the ultimate performance for their target market segments. This approach has also provided competitive advantages to these system vendors and reduced their interest in working towards compatible standard solutions.
More recently, with the introduction of coherent optical interfaces at transmission rates 100 Gbps and above, which are often easier to deploy and manage than conventional direct-detect transport networks operating at below 100 Gbps, system vendors have further built on their proprietary solutions, both for the physical layer transmission and the link management functionality.