ate and Primary Rate another ISP's access servers if the user is not a BT customer. This can be performed over layer-2 tunneling between the BT and ISP access servers. Currently all of the United Kingdom's home Internet DSL access is configured in this way. The MSIP ATM network carries the DSL home user Internet traffic over the PVCs to the BT access servers at the POPs. The loading of new DSL broadband services over ATM or IP/MPLS cores became a hallmark of the generic model used by early adopters. ISDN Basic Rate and Primary Rate ISDN provides leased-line services dynamically "on demand." However, it has pricing- point issues and was widely deployed as a backup service for leased lines. For a short time in the 1990s, ISDN service was also marketed as a "high-bandwidth" Bandwidth-on-Demand (BOD) solution, but the commercial datarates of networks have changed with the introduc- tion of DSL. ISDN services resemble a data overlay network. As shown in Figure 12.11, CPEs are connected into a shared cloud. The interface from the CPE to the WAN switch is a defined UNI. In the case of ISDN, though, services are offered within the digital exchange switch. This means that "NNI" signaling is in fact integrated with SS7, and the network nodes are the digital telephone exchanges themselves. Two different types of channel are used from the CPE to the ISDN switch. These are termed bearer channels (B-channels) and data channels (D-channels). Both are time division D-Channel B-Channel B-Channel D-Channel Q.921, Q.931 Q921/LAPD 1430/1431 1430/1 Circuit Establishment Digital Exchange D-Channel B-Channel B-Channel D-Channel Q.921, Q.931 Q921/LAPD 1430/1431 1430/1 Digital Exchange Circuit Establishment Fig. 12.11 ISDN services. Chapter 12 Legacy Packet-Switched Data Overlay Networks multiplexed. In the case of narrowband ISDN service, two B-channels are provided for customer data and one D-channel (ITU-T, 1993b). The D-channel is defined between the CPE and the ISDN switch. ISDN can establish dynamic B-channels across the PSTN. Each