s maintenance personnel, for example, to replace a faulty optical cable connection with a spare one. A digital distribution frame (DDF) is a cross-connection system to which digital interfaces from line systems and the exchange (or other network equipment) are connected. With the help of a 1.5- or 2-Mbps DDF, an operator may easily change transmission connec- tions between equipment sites. The Telecommunications Network: An Overview DDF may be implemented by digital cross-connect equipment (DXC) to which many high-data-rate systems are connected. DXC is managed via its network management interface and an operator may change its cross- connection configuration from a network management system (NMS) site. From a remote NMS he may, for example, define to which of the 1.5- or 2-Mbps interfaces a certain 64-Kbps channel from one 1.5- or 2-Mbps inter- face is connected. Operation of DXC is discussed in Chapter 4. 2.8 Trunk Network As we saw in Section 2.6, the national switching hierarchy includes multiple levels of switches above local exchanges. Figure 2.16 shows a simplified struc- ture for a network where higher levels than local exchanges are shown as a single level of trunk exchanges. The local exchanges are connected to these trunk exchanges, which are linked to provide a network of connections from any customer to any other subscriber in the country. High-capacity transmission paths, usually optical line systems, with capacities up to 10 Gbps, interconnect trunk exchanges. Note that a trans- port network has alternative routes. If one of these transmission systems fails, switches are able to route new calls via other transmission systems and trunk exchanges to bypass the failed system (Figure 2.16). Connections between local and trunk exchanges are usually not fault protected because their faults affect on a smaller number of subscribers. Digital Connections trunk Digital to other regions Digital exchange trunk local exchange exchange Transmission network Common channel Digital signaling trunk exchange Digital 