stab- lishment of connections, referred to as Switched Virtual Circuits (SVCs). Other WAN ser- vices, such as Frame Relay, do not include an NNI standard or signaling services for circuit establishment. These connections are normally manually provisioned between the various WAN switches and are hence not dynamic, being referred to as Permanent Virtual Circuits (PVCs). The PSTN as an Overlay Network The SDH/SONET network also provides the PSTN (SS7) bearer bandwidth (Figure 12.5). Although often considered as a whole, the SDH/SONET and SS7 Digital Exchange PSTN switches are separate; hence the telephone network could itself be characterized as an "overlay network." The Local Exchange A key component in any local exchange is the SDH/SONET equipment, which terminates the physical fibers and constitutes the lowest layer of transmission (Figure 12.6). On a fiber- by-fiber basis, more capacity can be gained by using DWDM. Although Optical Cross- Chapter 12 Legacy Packet-Switched Data Overlay Networks SDH/SONET Transport Voice and Data Switches Customer Premises Equipment Plain Old Local Loop Telephone Service STM-16 2.5 GIG Local Loop ISDN Data/Fax/ Voice Services STM-16 10 GIG E1/T1 Fiber Provisioned CPE Routers as Tail Circuit for Leased Line Local Loop E0 Leased Lines STM-1 Legacy PDH Access STM-64 10 GIG Local Loop CPE Frame Local Loop (E0) Provisioned Relay/X.25 STM-16 as Tail Circuit Devices ATM-Enabled CPE STM-64 34/155 M Devices 10 GIG Local Loop Customer Broadband STM-64 DSL Moderns 10 GIG E1 Used as Tail Circuit IP Routers Access SDH Fig. 12.6 Example local exchange. Connects (OXCs) using DWDM and optical switching technology have been researched, these have not as yet proved to be cost effective, and presently DWDM is always employed merely as a point-to-point capacity multiplier. The SDH/SONET network provides all of the bearer bandwidth utilized by both the SS7 digital exchange-switched network and all of the other overlay networks. Access from customer premises to the LE takes a number of forms. Existing 