plished through the insertion of an add/drop multiplexer (ADM) into the point-to-point network configuration. An example of a point- to-multipoint network configuration is shown in the lower portion of Figure 6.10. Note that the ADM can be a node on the network or can be routed to another multiplexer via a point-to-point network configu- ration. The advantage of the ADM is that it allows DS and E carrier sig- nals to be added or removed without having to first demultiplex, then add or drop channels and then remultiplex data. This capability results from the fact that SONET and SDH frames include pointers that denote the location of signals within the frame. In addition to point-to- point and point-to-multipoint network configurations, both SONET SDH support hub and star network configurations. Thus, let's take look at those two network configurations. HUB A hub network configuration in a SONET or SDH environment is similar to that of a LAN hub, in which the hub represents the focal point or star where a collection of point-to-point connections terminate. However, unlike a LAN hub, a SONET or SDH hub employs a digital access and cross-connect system (DACCS) to route data between individ- ual point-to-point lines. Figure 6.11 provides an example of a SONET or SDH hub-based network configuration. Because all data flow through the DACCS, this system provides the capability to monitor network per- formance and utilization from a single location. Because the DACCS functions as both a switch and an add/drop multiplexer, it is ideal for supporting unexpected network growth as well as affording the capabili- ty to restructure the configuration of a network to meet changing sub- scriber requirements. Now that we are acquainted with the hub or star configuration, we will conclude our examination of SONET and SDH network configura- tions by turning our attention to the ring. RING If you pick up a trade publication, you will probably encounter one or more communications carrier advertisements that discuss their ability to alm