eam and add or insert other channels into it. 4.8.4 Digital Cross-Connect Systems The digital cross-connect (DXC) systems are network nodes that can rearrange channels in data streams (Figure 4.32). They make the network configura- tion of the transmission network flexible, because, with the help of these nodes, a network operator is able to control actual transmission paths in the network remotely from the network management center. The basic function- ality of DXC is the same as the functionality of digital exchanges that estab- lish speech or ISDN connections. However, DXC is controlled by the network operator, not by a subscriber, and its configuration is not changed as frequently. Cross-connect systems are available that are able to switch high-order data rates, not just 64 Kbps as ordinary exchanges do. DXC may also contain redundancy functions that automatically change configurations SO as to bypass a faulty transmission section. SDH and SONET networks often use a ring topology like that shown in Figure 4.32 for higher reliability. These standards specify redundancy functions and a node in a ring may switch traffic from a faulty connection to the redundant path as shown in Figure 4.32. 4.8.5 Regenerators or Intermediate Repeaters Intermediate repeaters are needed if the communication distance is very long. They amplify an attenuated signal and regenerate the digital signal into its original form and transmit it further. The operation priciple of a regenera- tor was described in Section 4.5. 4.8.6 Optical Line Systems Optical line systems contain two terminal repeaters at each end of the fiber. They convert an electrical digital signal into an optical one and vice versa. These systems include, as most other transmission systems do, supervisory functions such as fault and performance monitoring. Note that SONET and SDH systems include multiplexing functions as well as the functions needed for optical transmission. In PDH multiplexers, optical line systems are Transmission separate devices that are interconne