 CPE must have the D- channel to the local switch established prior to communication. There are slight (national) differences between ISDN switch types, but in most cases the signaling D-channel stays connected between the switch and the CPE. Q.921 (ITU-T, 1997) runs over this channel, and a Terminal Endpoint Identifier (TEI) is assigned to the CPE by the switch. Q.931 (ITU-T, 1998) operates over the layer-2 service established between the switch and the CPE and is used by the CPE to initiate a call to the remote desired endpoint. The connection request is carried across the PSTN, and the call request is forwarded to the digital exchange at the remote end. Q.931 running on the D-channel at the remote end forwards the call request to the TEI of the remote CPE, and the CPE responds if the call is allowed. Upon the correct response, the two digital exchanges open up a B-channel for the CPE data to be transmitted. The B-channel can be configured to run standard layer-2 services such as Point-to-Point Protocol (PPP) (Simpson 1994). Troubleshooting the ISDN stack can be followed as in Figure 12.11. ISDN comes in two size bundles. Basic Rate Interface (BRI) is established with a single 16-kbit/s D-channel and two 56/64-kbit/s B-channels referred to as b0 and b1 (ITU-T, 1995). Primary Rate Interface (PRI) is established with a 56- or 64-kbit/s D-channel (ITU-T, 1993c) and 23 or 30 X 64-kbit/s B-channels that correspond to T1/E1 access rates. Future Trends The current trend is for many services to converge on a single platform. In fact, convergence is probably the largest goal in telecommunications operator spending and network design today. As the newer IP/MPLS platforms have been deployed, much of the corporate data has moved over from Frame Relay and legacy services to IP/MPLS VPNs. Clearly data services are moving rapidly to fully converged networks. Newer networks are able to emulate circuits and a host of legacy services, bluring the distinction between them. An X.25 connection, for example, might ride over IP, whic