 in the nearest LE, then a leased-line "tail circuit" can be provi- sioned to another LE with a WAN switch to connect it to the CPE (see Figure 12.4). This bandwidth is normally provisioned over the SDH/SONET network in the same way as a standard leased line. Avoiding the costs associated with long-haul dedicated leased lines is a major business driver for this type of WAN cloud. Often the costs of provisioning the access SDH/SONET circuits are not passed directly on to the customer and are considered to be part of the development costs of the WAN service. As more customers join the network from remote locations, the number and density of the WAN switches increase, leading to fewer and shorter SDH/SONET tail circuits. Today most national carriers have WAN services with extensive geographic coverage. Depending on the WAN service involved, the customer is then provisioned some type of virtual circuit or path through the "overlay" WAN cloud between the ingress and the egress WAN switches. This is the service for which the telecom operator bills the customer. The interfaces between the WAN switches are designated Network Node Interfaces (NNIs). Some technologies, such as Frame Relay, do not have a standard NNI at all but Nearest Local Exchange (no WAN Switch) SDH Tail Circuit to Nearest Exchange with WAN Switch STM-4 STM-16 WAN Cloud SDH Access Network Fig. 12.4 SDH tail circuit. Overlay Network Concepts Digital STM-4/16 Exchange STM-4/16 Digital Digital STM-4 Exchange STM-4 Exchange STM-16 Digital STM-4/16 Exchange STM-16 Digital Cross- Connects DCS STM-16/64 STM-4 Digital Exchange STM-4/16 STM-16 Digital Exchange STM-16 STM-4 STM-4 Digital STM-4/16 Exchange Fig. 12.5 Example PSTN bearer bandwidth. merely define the UNI portion of the connection, leaving the equipment vendor free to design the NNI portion of the circuit in whatever way is preferred. The virtual circuits over the WAN cloud can be provisioned in a number of ways. Some WAN technologies such as X.25 include signaling protocols, which allow the dynamic e