framing, error monitoring, section maintenance, and orderwire. The line layer ensures reliable transport of the payload and overhead generated by the path layer. It provides synchronization and multiplexing for multiple paths. It modifies overhead bits relating to quality control. The path layer is SONET's highest level layer. It takes data to be transmitted and transforms them into signals required by the line layer, and adds or modifies the path overhead bits for performance monitoring and protection switching. SONET/SDH network management protocols Overall functionality Network management systems are used to configure and monitor SDH and SONET equipment either locally or remotely. The systems consist of three essential parts, covered later in more detail: Software running on a 'network management system terminal' e.g. workstation, dumb terminal or laptop housed in an exchange/ central office. Transport of network management data between the 'network management system terminal' and the SONET/ SDH equipment e.g. using TL1/ Q3 protocols. Transport of network management data between SDH/ SONET equipment using 'dedicated embedded data communication channels' (DCCs) within the section and line overhead. The main functions of network management thereby include: Network and network-element provisioning In order to allocate bandwidth throughout a network, each network element must be configured. Although this can be done locally, through a craft interface, it is normally done through a network management system (sitting at a higher layer) that in turn operates through the SONET/SDH network management network. Software upgrade Network-element software upgrades are done mostly through the SONET/SDH management network in modern equipment. Performance management Network elements have a very large set of standards for performance management. The performance-management criteria allow not only monitoring the health of individual network elements, but isolating and identifying most network defects or outages. Higher-layer network monitoring and management software allows the proper filtering and troubleshooting of network-wide performance management, so that defects and outages can be quickly identified and resolved. Consider the three parts defined above: Network management system terminal Local Craft interface Local "craftspersons" (telephone network engineers) can access a SDH/ SONET network element on a "craft port" and issue commands through a dumb terminal or terminal emulation program running on a laptop. This interface can also be attached to a console server, allowing for remote out-of-band management and logging. Network management system (sitting at a higher layer) This will often consist of software running on a Workstation covering a number of SDH/SONET network elements