id in bold (in all figures), while Figure 12.3 shows the type of packet-switched WAN that is implemented. The WAN switches are located at Local Exchanges (LEs) and can multiplex together signals from different sites and customers to provide a lower price point. They are interconnected via leased circuits, themselves pro- visioned over an SDH/SONET core transmission network. Ideally connections to the WAN switches are provisioned directly over the local loop at both local and remote customer locations, terminating at the local exchange, where they are fed into the WAN overlay switch. The switch in the LE multiplexes data from several customers, and the data is then carried across the overlay network from one WAN switch to another. Often these networks were sold at a loss, while customers were being added during platform rollout, until enough data was being multiplexed and "oversold" to provide profit to the telecom operator. Overlay Network Concepts CPE A Data Switching Equipment STM-4 STM-4 STM-4 STM-16 STM-4 STM-16 CPE B Fig. 12.2 WAN network (bold) with its underlying SDH/SONET structure (shaded). Customer Premises Customer Equipment Premises Equipment DTE A Customer Premises Equipment E0/E1/T1 STM-1/4 Connections Switching Switching Equipment Equipment Customer Customer Premises Premises Data Switching Equipment Equipment Equipment Fig. 12.3 WAN packet-switched network. Chapter 12 Legacy Packet-Switched Data Overlay Networks The first stage in the connection is from the Customer Premises Equipment (CPE) to the LE. The interface from the CPE to the WAN switch is often designated a User Network Interface (UNI). The CPE end is the Data Terminal Equipment (DTE), and the LE end is the Data Communication Equipment (DCE). If there is a WAN switch present in the LE, local customers can be connected by an existing copper pair. For higher rates, fiber terminating at the LE might have to be run up to the CPE from a junction box in the street. However, not all LEs have a WAN switch. Tail Circuits If there is no WAN switch