n different geographical locations. These point-to-point services are routed over ring and mesh networks constructed from physical fibers, interconnected via Add-Drop Multiplexers (ADMs) and Digital Cross-Connects (DXCs) of various sizes (Chapter 10). In order to realize greater capacity over existing fibers, Dense Wave Division Multiplexing (DWDM) may be employed. Although capable of forming a multiplex layer itself, with the potential for wide use of optical add-drop multiplexers and optical cross- connect switching, DWDM is normally implemented as a point-to-point technology between SDH/SONET switches to increase the fiber's data-carrying capacity. The SDH/SONET transmission network provides the raw bandwidth used by all data and telecommunications circuits. Bandwidth is statically provisioned over the SDH/SONET Chapter 12 Legacy Packet-Switched Data Overlay Networks Leased-Line Circuit STM-0 E1 STM-4 STM-4 CPE A STM-16 STM-64 STM-16 Digital Cross- Connects STM-4 Leased-Line Circuit STM-0 E1 STM-16 STM-4 STM-16 CPE B STM-4 Fig. 12.1 Leased-line circuits over SDH. network between two geographical locations. SDH/SONET cross-cornect manufacturers provide management software to enable this static provisioning to be carried out by opera- tors on proprietary management platforms. Figure 12.1 shows a simplified SDH/SONET network to illustrate this concept. The SDH/SONET ADMs and the larger Digital Cross- connect Systems (DCSs) provide transparent service at rates of T1/E1 and above. Figure 12.1 shows that a leased-line network operating at T1/E1 rates and above can be provisioned directly on the underlying SDH/SONET network. For lower-rate lines, additional multiplexers are used to subdivide the bandwidth down to E0 rates. The older PDH network utilized these lower rates directly. Generic Wide-Area Networks Figures 12.2 and 12.3 illustrate multiplexing of different customer services over a shared network; Figure 12.2 shows the underlying SDH/SONET network with the Data Switching Equipment (DSE) shaded grey and overla