EVOLUTION OF DIGITAL OPTICAL-TRANSMISSION NETWORKS

The drivers for today's telecommunications transmission networks are improved performance and reliability at reduced operating costs. Evolution to meet these requirements incorporates greater control, monitoring and flexibility of the network with increased use of international standards to ease interworking between operators. The combination of these factors has led to the development of synchronous digital hierarchy managed transmission networks (SDH/MTN) to replace the existing plesiochronous digital hierarchy (PDH) during the 1990s. Optical transmission and switching technology offers further advantages by removing the 'electro-optic bottle-neck' and releasing the bandwidth inherent in optical systems. It should be possible to combine the increased functionality of the SDH/MTN with the 'infinite' bandwidth of transparent optical networks (TONs) to improve plant utilisation and reduce future upgrade costs. This can be achieved simply using an optical layer, in addition to the SDH transport layer, and employing techniques such as wavelength division multiplexing and wavelength routeing. In this paper an evolutionary path for transmission is traced from PDH to SDH to SDH/TONs and beyond, including indications of where cost savings should occur.