PDH made little provision for management of the network, and the need to fully de-multiplex a high level carrier to extract a lower level signal meant that increasing the capacity of PDH networks beyond a certain point was not economically viable. The main economic factor was the cost of the equipment required at each cross-connect point within the network where either individual channels or low-level multiplexed data streams might need to be extracted or added. It also added additional latency and increased the possibility of errors occurring, thereby reducing network reliability. Matters were further complicated by the fact that much of the lower-level transmission lines were still copper-based, which meant that optical line terminating equipment (OLTE) had to be used to provide the interface between those parts of the network based on copper and those based on optical fibre. The lack of standardisation, both between PDH networks operating in different parts of the world and between competing equipment manufacturers, severely hampered interoperability and necessitated the use of expensive conversion equipment between networks operating in Europe, North America and Japan. Increasing demand for network bandwidth virtually dictated the use of synchronous optical technologies that were both standardised and scalable, and led to the development of the Synchronous Optical Network (SONET) in North America and the Synchronous Digital Hierarchy (SDH) in Europe and the rest of the world.